Design and Mechanisms of Asymmetric Supercapacitors | Chemical

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Cite This: Chem. Rev. 2018, 118, 9233−9280

Design and Mechanisms of Asymmetric Supercapacitors Yuanlong Shao,*,†,‡,§ Maher F. El-Kady,‡ Jingyu Sun,∥ Yaogang Li,⊥ Qinghong Zhang,† Meifang Zhu,† Hongzhi Wang,*,† Bruce Dunn,*,#,∇ and Richard B. Kaner*,‡,#,∇

Chem. Rev. 2018.118:9233-9280. Downloaded from pubs.acs.org by NEW MEXICO STATE UNIV on 01/07/19. For personal use only.

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State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Material Science and Engineering, Donghua University, Shanghai 201620, China ‡ Department of Chemistry and Biochemistry and #Department of Materials Science and Engineering, University of California, Los Angeles (UCLA), Los Angeles, California 90095, United States § Cambridge Graphene Center, Department of Engineering, University of Cambridge, Cambridge CB3 0FA, United Kingdom ∥ College of Energy, Soochow Institute for Energy and Materials InnovationS (SIEMIS), Soochow University, Suzhou 215006, People’s Republic of China ⊥ Engineering Research Center of Advanced Glasses Manufacturing Technology, Ministry of Education, Donghua University, Shanghai 201620, China ∇ California NanoSystems Institute, UCLA, Los Angeles, California 90095, United States ABSTRACT: Ongoing technological advances in diverse fields including portable electronics, transportation, and green energy are often hindered by the insufficient capability of energy-storage devices. By taking advantage of two different electrode materials, asymmetric supercapacitors can extend their operating voltage window beyond the thermodynamic decomposition voltage of electrolytes while enabling a solution to the energy storage limitations of symmetric supercapacitors. This review provides comprehensive knowledge to this field. We first look at the essential energystorage mechanisms and performance evaluation criteria for asymmetric supercapacitors to understand the wide-ranging research conducted in this area. Then we move to the recent progress made for the design and fabrication of electrode materials and the overall structure of asymmetric supercapacitors in different categories. We also highlight several key scientific challenges and present our perspectives on enhancing the electrochemical performance of future asymmetric supercapacitors.

CONTENTS 1. Introduction 2. Brief Historic Overview of Supercapacitors 3. Basic Background, Charge Storage Mechanisms, Theory, Key Composition, and Operation of Supercapacitors 3.1. Background for Supercapacitors and Their Differences from Batteries 3.2. Electric Double Layer Capacitors 3.3. Pseudocapacitors 3.4. Capacitive Asymmetric Supercapacitors Vs Hybrid Capacitors 3.5. Electrolytes 3.6. Thermodynamic and Kinetic Considerations for Potential Window of Pseudocapacitive Materials 3.7. Full Cell Voltage 4. Principles and Methods of Experimental Evaluation 4.1. Calculating the Capacitance for a Single Electrode 4.2. Evaluation of the Capacitance and Energy Density of Asymmetric Supercapacitors

4.3. Charge-Balancing Principles between Two Electrodes 4.4. Power Density and Equivalent Series Resistance (ESR) 4.5. Some Concerns Regarding Performance Evaluations 5. Aqueous Electrolyte-Based Asymmetric Supercapacitors 5.1. Aqueous Capacitive Asymmetric Supercapacitors 5.1.1. RuO2-Based Asymmetric Supercapacitors 5.1.2. MnO2-Based Asymmetric Supercapacitors 5.2. Faradaic Materials-Based Aqueous Hybrid Capacitors 5.2.1. Metal Oxide-Based Electrodes for Aqueous Hybrid Capacitors 5.2.2. Metal Hydroxide-Based Electrodes for Aqueous Hybrid Capacitors 5.2.3. Mixed Transition-Metal-Based Electrodes for Aqueous Hybrid Capacitors

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9244 Received: April 18, 2018 Published: September 11, 2018

© 2018 American Chemical Society

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DOI: 10.1021/acs.chemrev.8b00252 Chem. Rev. 2018, 118, 9233−9280

Chemical Reviews 5.3. Faradaic Materials/Carbon Composite Electrodes for Aqueous Hybrid Capacitors 5.4. All Redox Electrodes-Based Capacitive Asymmetric Supercapacitors 6. Redox-Active Electrode-Based Nonaqueous Asymmetric Supercapacitors 6.1. Li-Ion Capacitors 6.2. Na-Ion Capacitors 6.3. Intercalation Pseudocapacitive MaterialsBased Electrodes for Nonaqueous Capacitive Asymmetric Supercapacitors 7. Other Asymmetric Supercapacitors 7.1. EDLC-Based Asymmetric Supercapacitors 7.2. Asymmetric Supercapacitors Based on Different Redox Functional Groups 7.3. Surface Charge Optimization 8. Redox-Active Electrolyte-Based Hybrid Capacitors 8.1. Charge-Storage Mechanisms in Hybrid Capacitors Containing Redox Electrolytes 8.1.1. Charge Storage by Sorption and Redox Reactions of Anionic Species on the Electrode Surfaces 8.1.2. Charge Storage Enhanced by Sorption and Redox Reactions of Metal Cations 8.2. Redox Electrolyte-Based Hybrid Capacitors with Two Redox Couples 9. Summary and Perspectives Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments References

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Figure 1. Ragone plot illustrating the performances of specific power vs specific energy for different electrical energy-storage technologies. Times shown in the plot are the discharge time, obtained by dividing the energy density by the power density.

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cycles).13,14,28−33 As a result, they have triggered a growing interest where fast charging, great cycling stability, and high power density is required. For example, supercapacitors are now used in heavy-duty vehicles, hybrid platforms for trucks and buses, load-leveling systems for intermittent renewable energy sources, and storing the regenerative braking energy of electric vehicles and light rail.10 Although commercially available supercapacitors can deliver much higher energy density (∼5 Wh kg−1) than traditional solid-state electrolytic capacitors, this is still significantly lower than batteries (up to 200 Wh kg−1) and fuel cells (up to 350 Wh kg−1). Thus, the widespread use of supercapacitors has been limited, and extensive research efforts are underway with the goal of achieving the energy-storage capability of batteries without sacrificing their power density and cycling stability.34−38 One means by which the supercapacitor field has tried to address the challenge of relative low energy density is developing asymmetric supercapacitors. In this context, asymmetric supercapacitors cover a wide range of device configurations.35,39−44 As shown in Figure 2, generally supercapacitors can be classified into three different types including electric double-layer capacitors (EDLCs), pseudocapacitors, and asymmetric supercapacitors.45 Generally, asymmetric supercapacitors are classified into two types, namely, systems with two capacitive electrodes38,40,46−48 or hybrid capacitors.41,49−52 Hybrid capacitors have come to be identified as devices in which one electrode stores charge by a battery-type Faradaic process while the other electrode stores charge based on a capacitive mechanism. During the charge and discharge processes, asymmetric supercapacitors can take full advantage of the different potential windows of the two electrodes to maximize the operating voltage of the full device. For example, while the voltage of an aqueousbased symmetric system is limited to ∼1.2 V, the operating voltage of an asymmetric supercapacitor can be extended beyond 2.0 V.38,53 Energy density is one of the key parameters used to evaluate the electrochemical performance of supercapacitors. The energy

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1. INTRODUCTION The potential exhaustion of fossil fuels and the environmental impacts associated with greenhouse gas emissions have led to increasing global demands in developing sustainable energy supplies throughout the world.1−3 Renewable sources, such as solar, hydro, and wind energy, are the most promising solutions for addressing these concerns.4−8 However, due to the large fluctuations in generation, electricity produced from these renewable energy sources must be efficiently stored to supply the world with energy on demand.9−11 Among the various energy-storage devices, batteries and supercapacitors represent the two leading electrochemical energy-storage technologies as illustrated in a graph of energy density in Wh/kg vs power density in W/kg known as a Ragone plot (Figure 1).10,12−23 With their high energy density (approaching 180 Wh kg−1), lithium-ion batteries are currently widely used in consumer electronics.15,17 However, due to a variety of resistive losses from sluggish electron and ion transport, batteries give rise to heat generation and dendrite formation when operated at high power which can lead to serious safety issues.24,25 This has resulted in some well-publicized failures including the electric car made by Tesla and the Dreamliner airplane made by Boeing.26,27 On the other hand, supercapacitors, also known as electrochemical capacitors, can complement or even replace batteries in some applications, because they can safely provide high power and rapid charging with extremely long cycle life (>100 000 9234


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Figure 2. Classification of supercapacitors.

Figure 3. Historic timeline for the development of supercapacitors. The schematics of the Helmholtz model, the Gouy−Chapman model, and the Stern model were reproduced with permission from ref 14. Copyright 2009 Royal Society of Chemistry. The schematic of the first EC patented by GE was reproduced from ref 66. Copyright General Electric Co.

density of a supercapacitor, E, is proportional to voltage squared according to eq 113 E=

1 2 CV 2

supercapacitors and our perspectives in future research are also described and identified in the last section.

2. BRIEF HISTORIC OVERVIEW OF SUPERCAPACITORS The developmental history of supercapacitors is the story of discovering charge-storage mechanisms. The demonstraction of the first capacitor dates back to the middle of the 18th century.45 The first capacitor, named a “Leyden jar”, was invented separately by a German cleric Ewald Georg von Kleist in 1745 and a Dutch scientist Pieter van Musschenbroek in 1746. It consisted simply of two pieces of metal foils, water, and a conductive chain inside a glass jar, as shown in the Leyden Jar schematic of the timeline in Figure 3. Static electricity could be produced by rotating the glass jar. On the basis of this design, the concept of storing static electricity at the interface of a solid electrode and a liquid electrolyte developed. This established the initial concept of an electric double layer over 100 years earlier than the invention of the battery in 1880. The nature of static electricity was still poorly understood until the 19th century. In 1853, von Helmholz first studied the electrical charge-storage mechanism in capacitors and built the first electric double-layer model by investigating colloidal suspensions.61During the 19th and early 20th centuries, some pioneering interfacial electrochemists, including Gouy,62 Chapman,63 Stern,64 and Grahame,65 developed the modern theory of

(1)

where C is the capacitance of the device and V is the operating voltage window for the cell. On the basis of this expression, a 2fold increase in voltage could result in a 4-fold increase in energy density for the same value of capacitance. Thus, a properly designed asymmetric supercapacitor offers the prospect of increasing the energy density for applications where energy needs to be stored and delivered with high power. Recently, several reviews have summarized the latest progress in the supercapacitor field toward different directions, such as state-of-the-art progress in charge-storage mechanisms,54,55 in situ techniques,56 microsupercapacitors,57,58 Li-ion/Na-ion capacitors,59,60 and novel electrode materials, device designs for supercapacitors.55 In this review, we focus on an asymmetric supercapacitor systems, with emphasis on the design, mechanisms, electrode materials fabrication, and their electrochemical performance evaluation. In addition to presenting the background, development history, and electrochemical features, we also review recent advances in asymmetric supercapacitors and analyze the relationship between microstructure and electrochemical performance. The challenges facing asymmetric 9235


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Figure 4. Comparison of the electrochemical behavior for a typical supercapacitor and a typical battery: (a and b) cyclic voltammogram curves and (c and d) galvanostatic charge−discharge curves. ESR: equivalent series resistance.

Pinnacle Research Institute (PRI) started a project in the 1980s to develop a high-performance supercapacitor based on ruthenium/tantalum oxide pseudocapacitance, which named it the PRI Ultracapacitor. However, due to the high price of the noble metal, ruthenium, the PRI ultracapacitor was only used for military applications, such as laser weapons and missile launch systems. In 1989, the U.S. Department of Energy (DOE) began supporting a long-term supercapacitor study targeting high energy density supercapacitors for use in electric drivelines as a part of their Electric and Hybrid Vehicle Program.71 Afterward, a now world-leading supercapacitor manufacturing company, Maxwell Technologies. Inc., contracted with the DOE to develop high-performance supercapacitors. This included an application for the energy load leveling system in electric or hybrid vehicles, where supercapacitors function together with batteries or fuel cells to collect energy from braking and releasing the electric energy for acceleration. From then on a large variety of supercapacitors have become available, including EDLCs, pseudocapacitors, and asymmetric supercapacitors. Each type of supercapacitor has its own significant characteristics and target applications, such as backup energy for portable electronics or uniterrupted power supplies (UPS) and high power energy sources for heavy loading trucks or cranes.72,73 Supercapacitor companies from all around the world such as Nesscap (Korea), ELTON (Russia), Nippon Chemicon (Japan), and CAP-XX (Australia) have been developing and delivering different types of supercapacitors for various applications.74 Since 2000, the amount of research related to supercapacitors has continuously and significantly increased according to the emerging increased demand for high-power, high-reliability, and safe energy-storage devices.75 On the other hand, based on the

electric double-layer capacitance at the interfaces between two metal electrodes and liquid electrolytes. Although the general concept of electric double-layer capacitance has been known since the beginning of 20th century, the first electrochemical capacitor patent was not applied for until 1954 by H. I. Becker at General Electric.66 This patent described for the first time an energy-storage device containing porous carbon electrodes immersed in an aqueous electrolyte, which stored electric energy at the interfacial electric double layer. Unfortunately, this patent was never commercialized. The first nonaqueous electrolyte-based electrochemical capacitor was invented by Robert Rightmire at the Standard Oil Co. of Ohio (SOHIO).67 According to the wider decomposition voltage of nonaqueous electrolytes, the system described in this patent could provide much higher operating voltages (3.4−4.0 V) and energy densities than Becker’s aqueous electrochemical capacitor. In 1978, a Japanese company, Nippon Electric Corp. (NEC), after licensing SOHIO’s technology, first commercialized an electrochemical capacitor, with the name of “SuperCapacitor”. Then NEC successfully developed the electrochemical capacitor application market for back-up power for clock chips and complementary metal−oxide−semiconductor (CMOS) memories inside electronics, which is still one of the main applications for current supercapacitors. In 1971, a new class of electrochemical capacitor, termed a pseudocapacitor, which involves Faradaic processes, was discovered based on RuO2.68,69 While the nature of the RuO2 thin film electrode charge-storage mechanism is Faradaic in nature, the cyclic voltammograms (CVs) of RuO2 exhibit a retangular shape, which demonstrates its typical capacitive behavior.70 The discovery of pseudocapacitance opened up a new approach to enhance the charge-storage capability of electrochemical capacitors. On the basis of this discovery, the 9236


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Figure 5. Schematics of charge-storage mechanisms for (a) an EDLC and (b−d) different types of pseudocapacitive electrodes: (b) underpotential deposition, (c) redox pseudocapacitor, and (d) ion intercalation pseudocapacitor. (b−c) reproduced with permission from ref 128. Copyright 2014 Royal Society of Chemistry.

illustrated in Figure 4. In response to potential scanning, supercapacitors usually show a potential-independent capacitance. Thus, the CV curve of a supercapacitor should maintain a rectangular shape, while the current is nearly constant during charge/discharge processes. Batteries, on the other hand, show prominent and separated peaks with significant Faradaic reactions. The galvanostatic charge/discharge (GCD) curve of a supercapacitor exhibits a sloping shape with a constant slope value. In comparison, a battery typically exhibits a relatively flat charge/discharge plateau at a constant voltage stage. At the same time, for applications requiring a constant output voltage, supercapacitors need to be integrated with a DC−DC converter to regulate and stabilize the output voltage. This difference in the charge-storage mechanism also results in a different definition and unit (capacitance vs capacity) of the energy stored in these two types of electrodes. Capacitance is evaluating the capability of a capacitive electrode to store charge, which is a constant number over a given voltage and can be estimated using eq 2

dramatic development of nanoscience and advanced characterization techniques, many novel physical and electrochemical phenomena have been discovered for both EDLC and pseudocapacitive systems. The charge-storage mechanism of EDLC and pseudocapacitance are clearly in need of further study.

3. BASIC BACKGROUND, CHARGE STORAGE MECHANISMS, THEORY, KEY COMPOSITION, AND OPERATION OF SUPERCAPACITORS 3.1. Background for Supercapacitors and Their Differences from Batteries

Supercapacitors are a category of energy-storage devices based on high-speed electrostatic or Faradaic electrochemical processes. The charge is mainly stored at the electrode− electrolyte interface of the active materials, such as high surface porous carbons, metal oxides, or conducting polymers. They consist of one positive electrode and one negative electrode soaked in an electrolyte and separated by an ion-permeable, electronically insulating separator. Although the general chargestorage mechanism and performance principles are similar to conventional capacitors, the specific capacitance and energy density of the initial supercapacitor are increased by a factor of 100 000 or even greater than that of regular capacitors. This is accomplished by incorporating active electrode materials with 1000 times higher surface areas, nanoscale dielectric distances, and additional pseudocapacitance by fast Faradaic reactions. Thus, supercapacitors can even store several thousand Farads per single device, much higher than the micro- or milli-Farads stored by conventional capacitors. Compared with batteries, supercapacitors can supply much faster charge and discharge rates within seconds or minutes time scales but lower specific energy. Besides the high power densities, supercapacitors also have some other advantages over batteries, such as high operating safety, long cycling life, high efficiency, and high performance stability. Another major electrochemical feature that differentiates supercapacitors from batteries is that there is always a linear voltage increase upon constant current charge (or decrease upon discharge), with the charge stored (released) from supercapacitor electrodes,76 as

C=

ΔQ ΔU

(2)

where ΔQ is the charge stored and ΔU is the voltage applied on the electrode. The capacitance, C, can be calculated by evaluating the charge-storage capability within a specific voltage window, which is given by the unit of one Farad (F). Meanwhile, the capacity is the charge-storage amount for a battery electrode based on a Faradaic reaction, in Coulombs (C), or mAh. Recently, several perspectives and tutorials have been published with the aim of discussing the differences of electrochemical behavior and performance evaluation between supercapacitors and batteries.77−79 The categorization of the charge-storage modes between supercapacitors and batteries is critical to study the energy-storage mechanism and report on the performances correctly. As illustrated in Figure 5, the energy-storage mechanism of capacitive electrodes can be generally classified into two main types: (1) electric double-layer capacitors (EDLCs) and (2) pseudocapacitors.13,28,31,80 9237


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transfer.127−130 Ruthenium dioxide (RuO2) was the first electrode material reported to exhibit pseudocapacitive behavior.29,68 Even though the charge storage from a chargetransfer reaction on an RuO2 thin film electrode is a type of Faradaic reaction, the CV curve exhibits close to a rectangular shape, which is a typical capacitive feature.128 The term pseudocapacitance is formally used to identify electrode materials whose electrochemical signatures are capacitive but charge storage occurs by charge-transfer Faradaic reactions across the double layer.131 This process is Faradaic in origin from the fast and reversible surface redox thermodynamic processes, but capacitance arises from the linear relationship between the extent of charge (ΔQ) and the potential change (ΔU). The active centers that contribute to the pseudocapacitance are located near the surface of the metal oxides, at a distance, ≪(2Dt)1/2, where D is the diffusion coefficient for charge-compensating ions (cm2/s) and t is the diffusion time range (s).132 Energy storage involving pseudocapacitance exhibits the intermediate electrochemical behavior between pure electrostatic EDLCs and solid-state diffusion dominated by Faradaic reactions in bulk battery-type materials. Different Faradaic mechanisms can result in electrochemical capacitive features as shown in Figure 5b−d:29,128,129 (1) underpotential deposition,133 which occurs when ions deposit on a 2-dimensional metal−electrolyte interface at potentials positive to their reversible redox potential (e.g., H+ on Pt or Pd2+ on Au);134,135 (2) redox pseudocapacitance, where some extent of conversion of reduced species is electrochemically absorbed onto the surface or near surface of oxidized species (or vice versa) in a Faradaic redox system (e.g., RuO2136−139 or MnO 2 , 129,140−142 as well as some conducting polymers129,143−146); (3) intercalation pseudocapacitance, where ion intercalation into a redox-active material occurs with no crystallographic phase change and in a time scale close to that of an EDLC (e.g., Nb2O5147−151). Underpotential deposition is well known for the adsorption of hydrogen atoms on catalytic noble metals including Pt, Rh, Ru, and Ir, along with the electrodeposition of metal cations at potentials less negative than their equilibrium potential for cation reduction. This process can be described by eq 4

3.2. Electric Double Layer Capacitors

When an electronically conductive electrode is immersed in an ion-conductive electrolyte solution, a double layer spontaneously forms due to the organization of charges at the electrode−electrolyte interfaces. An EDLC is the simplest and most commercially available supercapacitor, where the charge is physically stored by electrostatic charge adsorption at the interface between electrode and electrolyte.14,30,31,80−86 One of the significant features of EDLCs is that no charge transfer occurs between the interfaces of electrodes and electrolytes, i.e., no Faradaic processes occur. The specific capacitance of an EDLC is strongly dependent on the accessible surface area of the electrode materials and surface properties of the carbon materials. The capacitance of an EDLC electrode can be generally estimated according to eq 314 C=

εrε0 A d

(3)

where εr is the relative permittivity related to the liquid electrolyte used, ε0 is the permittivity of vacuum, A is the effective surface area of the electrode materials which are accessible to the electrolyte ions, and d is the effective charge separation distance between the electrical double layers, i.e., the Debye length. According to the physical electrostatic processes, the formation and relaxation of the electric double layer occurs in a very short time range, on the order of ∼10−8 s, which is smaller than that of redox reactions for pseudocapacitance which is in the range of 10−2−10−4 s.12 The process of charge/ discharge of EDLCs only involves charge rearrangement, without any Faradaic reactions; thus, the electric double layer can respond immediately to potential changes. EDLCs can store much more electric energy than conventional dielectric capacitors because of their several orders of magnitude higher effective surface area and the nanoscale charge separation distance. Carbon-based materials ranging from commercial activated carbons,82,87−90 carbon aerogels,91,92 templated carbon,93,94 to carbon nanomaterials,95,96 such as carbon nanotubes50,82,97−105 and graphene,96,106−126 are the most widely used and studied active materials for EDLCs owing to their high specific surface areas, desirable electrochemical stabilities, and open porosity to electrolyte ions. The effective thickness of the electrical double layer is in the range of 5−10 Å, which depends on the concentration and size of the electrolyte ions and the solvation shell.31 On the basis of the relative permittivity of the electrolyte medium and the effective electrical double-layer capacitance for a carbon-based system, the specific capacitance is considered to be between 10 and 21 μF cm−2.81,116,117 Thus, the high specific surface area (1000− 3000 m2/g) of carbon-based materials can, in principle, lead to double-layer capacitances of 300−550 F g−1.109 However, experimentally, due to the finite conductivity and unavailability of all of the surface sites, the specific capacitance of pure carbonbased EDLCs practically achieved has generally been limited to ∼100−250 F g−1.14 As a result, commercial supercapacitors based on EDLC electrode materials can store energy in the range of 3−10 Wh kg−1.

M + xC Z + + xze− ↔ C·M

(4)

where C is the absorbed atoms (e.g., H or Pd), M is the noble metal (e.g., Pt or Au), x is the number of absorbed atoms, z is the valence of the absorbed atoms, and thus zx is the number of transferred electrons.134 The applied voltage should be below the cation redox potential. For example, the potential should be positive to the reversible hydrogen electrode potential when a hydrogen underpotential is used to deposit on Pt. In this case, a very high specific capacitance (∼2200 μF cm−2) can be achieved.29 The good reversibility of the charge and discharge kinetics is a prime factor in attaining favorable power densities for pseudocapacitive materials. However, the operational potential ranges are usually small, only 0.3−0.6 V, and the capacitance values are potential dependent for the underpotential deposition process. Thus, the energy density is limited compared with other pseudocapacitive systems. Pseudocapacitance based on redox reactions, uses electron transfer between an oxidized species Ox (e.g., RuO2,129 MnO2,142 or p-doped conductive polymers152,153) and a reduced species, Red (e.g., RuO2−z(OH)z, MnO2−z(OH)z, or n-doped conductive polymers). These reactions are usually described as electrochemical adsorption of cations on the

3.3. Pseudocapacitors

In contrast to EDLCs, pseudocapacitive electrode materials store charge via Faradaic processes that involve fast, reversible redox reactions at the surface or near surface of the active materials. This mechanism is associated with a valence state change of the electrode material as a result of electron 9238


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Figure 6. Dependence of the electrochemical behavior of energy storage materials as a function of the particle size. (a) b values for 10 nm size TiO2 particles calculated at different potentials of the CV curves. (b) Relationship between the cathodic current and scan rates of 7 nm size TiO2 electrode at different potentials: (1) 2.0, (2) 1.9, (3) 1.8, (4) 1.7, and (5) 1.6 V. Scan rates of the CV curves changed from 0.5 to 10 mV s−1. (c) CV curves for three different electrodes with varied TiO2 paricle size at a scan rate of 0.5 mV s−1. (d) Battery-type intercalation and capacitive charge-storage contributions comparison for different TiO2 particle sizes electrodes. Reproduced with permission from ref 154. Copyright 2007 American Chemical Society.

transport kinetics, short charge time, high rate capability, and long cycling stability, whereas battery materials are limited by solid-state diffusion, resulting in lower power density. According to its relatively high capacitance, wide potential window, and good rate capability, ion intercalation pseudocapacitive materials have attracted increasing attention since they have been discovered. In an attempt to identify the origin of ion intercalation pseudocapacitance, researchers have begun to distinguish between “intrinsic” and “extrinsic” pseudocapacitive materials. The pseudocapacitive charge-storage properties of intrinsic pseudocapacitive materials are not related to their crystalline grain sizes or morphologies. On the other hand, some battery-type materials (such as LiCoO2 and bulk V2O5) that only show diffusion-controlled capacity in the bulk state could also demonstrate a pseudocapacitive behavior upon decreasing the particle dimensions to nanosizes.128 Such materials are considered to be extrinsic pseudocapacitive materials. Therefore, even the same type of material could be endowed with pseudocapacitive or battery-type energy-storage characteristics depending on its particle size. Overall, for the ion intercalation type charge-storage systems, the totally stored charge can be divided into three parts: (i) the Faradaic contribution from the bulk solid-state diffusiondominated ion intercalation process, (ii) the Faradaic chargetransfer process at the surfaces and/or the surface of the active materials with fast ion diffusion dynamics, referred to as pseudocapacitance, and (iii) a non-Faradaic EDLC contribution from electrostatic ion adsorption/desorption.154 Therefore, it is

oxidized species surface accompanying fast and reversible electron transfer cross the interface between the electrode and the electrolyte,13 as illustrated in eq 5 Ox + zC + + z e− ↔ RedCz

(5)

where C is the surface-absorbed electrolyte cation C+ (H+, K+, Na+, ...) and z is the number of transferred electrons. The maximum capacitance achievable in such redox pseudocapacitor systems is ∼5000 F cm−3 depending on the reactant ions and reactant site density.29 This is much larger than ∼825 F cm−3, the maximum achievable theoretical capacitance with a doublelayer capacitor from 1 cm3 of a compact high surface area carbon.122 Pseudocapacitance can also occur in the case of ion insertion/ intercalation into layered crystalline materials as illustrated in eq 6 MA y + x Li+ + x e− ↔ LixMA y

(6)

where MAy is the layer−lattice intercalation host material (e.g., Nb2O5149,150) and x is the transferred electrons number. The intercalation is accompanied by a change of metal valence to maintain electric neutrality. For a cation intercalation pseudocapacitor, the electrochemical performance is described as “transitional” behavior between Li-ion batteries and supercapacitors.150 The main difference between these two processes is that intercalation pseudocapacitance is characterized by several capacitor-like electrochemical signatures, such as fast ion 9239


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necessary to distinguish the charges stored between capacitive contributions and the bulk Li-ion intercalation process. Athough the charge-storage processes in pseudocapacitive materials are Faradaic in nature, capacitive features such as a linear dependence of charge storage over the potential window are important kinetic characteristics for distinguishing pseudocapacitive charge-storage processes from battery-type behavior. To distinguish the pseudocapacitive charge-storage mechanism from that of battery materials, low scan rate CV measurements as the dependence of the current response on the sweep rate provides insights into the charge-storage mechanism according to eq 7155 i = avb

i(V ) v1/2

where i is the current obtained at a specific sweep rate v and a and b are adjustable parameters. For strictly battery-type Faradaic reactions, ion intercalation is limited by solid-state ion diffusion processes. Thus, the voltammetric current, i, is expected to be proportional to the square root of the scan rate. The detailed relationship can be described as in eq 8155,156 1/2

π 1/2χ (bt )

(8)

Comparing eqs 7 and 8, when the slope b = 1/2 then Cottrell’s equation (eq 9) is satisfied i = av1/2

(9)

so that the current response is indicative of semi-infinite diffusion-dominated charge storage as occurs in bulk battery materials. The capacitive current response follows a linear dependence on the scan rate (b = 1.0) according to eq 10157 i = vCdA

(10)

where Cd is the capacitance. In this case charge storage is characterized by capacitor-like behavior. Wang et al. discussed the capacitive contributions of the anatase phase of TiO2 by utilizing this analysis.154 Figure 6a shows the calculated b value obtained from the CV curves for the fabricated 10 nm TiO2 film. The equation of log i = log a + b log v can be derived from eq 7, so that the value of b for different potentials always evaluates from the slope of the straight line of log i vs log v, as shown in the inset to Figure 6a. The b value is 0.55 at the peak potential 1.70 V of the CV curve, indicating that the main contribution of current comes from a solid-state diffusion-controlled intercalation. At other potentials, the b values vary in the range of 0.8−1.0, which illustrates that charge storage is mainly contributed to by ion intercalation pseudocapacitive processes. In a related analysis based on previous studies and CV results, the current at a given potential point, V, is the sum of two types of charge-storage contributions arising from a capacitive contribution and a solid-state-dominated Li-ion intercalation.157 The voltammetric current, i, can be divided into two parts by following eq 11 i(V ) = k1v + k 2v1/2

(12)

Experimentally, current values at different potentials can be collected from CV measurements taken with varied scan rates (v). Then we can obtain the k1 and k2 values from the slope and the y axis intercept using straight plots of i(V)/v1/2 vs v1/2 at different potentials points, as shown in Figure 6b. In this case, the potential range is between 1.6 and 2.0 V, which covers the regime of the redox reaction peaks. Afterward, it is easy to distinguish the voltammetric current between the capacitive (k1v) and the diffusion-controlled (k2v1/2) contributions as a function of varied potentials. The gray areas shown in Figure 6c indicate how the capacitive current contribution compares with the total current in the CV curve. As shown in Figure 6d, the capacitive currents contributed 55%, 35%, and 15% of total electric charge stored in 7, 10, and 30 nm sized TiO2-based electrodes. It is obvious that smaller sized TiO2 nanoparticles produce a higher ratio of capacitive charge contributions than larger particle size active materials. This demonstrates that a more exposed active surface area and short ion diffusion distances lead to faster ion intercalation processes and more capacitive charge contributions in the charge-storage system. The three pseudocapacitive mechanisms (Figure 5 b−d) proceed based on different electrochemical interfacial processes. However, the electrochemical properties exhibit similar features due to the same relationship between potential (E) and the fractional coverage (X) of the electrolyte ions reacting on the surface and near surface of the active material during the charge/ discharge processes.128 The resulting charge-storage mechanism, pseudocapacitance, could arise at the electrode surfaces that are 10−100 times larger than that of the EDLC process. As a result, it has attracted a lot of attention by using pseudocapacitive materials in asymmetric supercapacitor devices that are based on either a redox pseudocapacitance or an intercalation pseudocapacitance. It is worth noting that these discussions related to pseudocapacitive behavior should only be applied to an individual electrode. When combining a capacitive electrode with a Faradaic electrode, it is hard to distinguish whether the Faradaic electrode belongs to a pseudocapacitive or a batterytype mechanism. Even when a battery-type electrode is combined with a capacitive electrode, the CV curve could still possibly exhibit a nearly rectangular shape.

(7)

i αn F y i = nFAC·D1/2v1/2jjj a zzz k RT {

= k1v1/2 + k 2

3.4. Capacitive Asymmetric Supercapacitors Vs Hybrid Capacitors

Different from the definition of pseudocapacitive behavior, the discussion between “asymmetric” and “hybrid” only refers to devices, not electrodes. It is generally accepted that a “hybrid capacitor” describes the case where the two electrodes have two different charge-storage mechanisms: one capacitive and one battery-type Faradaic.35,39,78,159−161 Asymmetric supercapacitors theoretically cover a wider range. These devices include two different electrode materials (which means that they can contain a hybrid device if the active materials exhibit different chargestorage mechanisms or have a varied ratio of redox-active sites on the electrode material), different redox-active electrolytes, or even if they contain the same EDLC carbon material with different surface functional groups. In any case, there is no question that hybrid devices are a specific category of asymmetric devices. Moreover, devices consisting of a Faradaic electrode (such as Ni(OH)2 or Co3O4) and a carbon electrode represent a typical type of hybrid capacitor device.

(11)

where k1v and k1v1/2 correspond to the current contributions from the capacitive process (including both ion intercalation pseudocapacitance and EDLC) and the solid-state diffusiondominated Li+ intercalation processes. Then this equation can be slightly rearranged to eq 12 9240


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Figure 7. Schematic illustration of the typical CV (up) and GCD curves (bottom) characteristics of (a and d) a battery, (b and e) a capacitive asymmetric supercapacitor, and (c and f) a hybrid capacitor. (a−c) Reproduced with permission from ref 158. Copyright 2015 The Electrochemical Society.

accessible surface area of smaller aqueous electrolyte ions, supercapacitors with aqueous electrolytes can achieve higher capacitances than those using the same electrodes in nonaqueous electrolytes. However, there is still a restriction of voltage range due to the low thermodynamic stability window of water and moderate stability and durability during long-term cell operation. In addition, extra care needs to be taken for the selection of current collectors to avoid corrosion when choosing alkali or acidic electrolytes. While the low voltages of aqueous supercapacitors can only deliver limited capability to achieve high energy densities, they can do a very good job in the field of high power, low cost, excellent safety, and large-scale energystorage applications. Recently, some research has reported that neutral aqueous electrolytes can exhibit a wide potential window of ∼1.6−1.9 V, which significantly exceeds the theoretical limit of the water decomposition voltage, 1.23 V.163−168 Such a high operating voltage is due to the high hydrogen (H2) evolution overpotential and OH− ion generation potential in a neutral aqueous electrolyte. According to the Nernst equation (Ered = −0.059 pH), the potential will shift to lower values when the pH is increased. Although early supercapacitors used aqueous electrolytes, organic electrolytes and ILs-based supercapacitors are currently dominating the commercial market owing to their higher operating potential windows typically in the range of 2.5−2.8 and 3.5−4.0 V, respectively.162,169−172 The increased potential window can provide a significant improvement in energy density. Another advantage of nonaqueous supercapacitors with high output voltage is that a number of energy-storage applications require operations with high output voltages. The high intrinsic cell voltage can reduce the required number of devices needed to be placed in series. In fact, the electrochemically stable potential windows of organic electrolytes and ILs depend on several key factors, including solvents, the type of conducting salts (i.e., cation and anion), and impurities, especially trace amounts of water.

Figure 7 schematically illustrates the respective CV and GCD curves of a battery, a capacitive asymmetric supercapacitor, and a hybrid capacitor. In Figure 7a and 7d, the CV and GCD curves for a battery using both electrodes and a full device exhibit obvious Faradaic peaks and charge/discharge plateaus. In comparison, for the capacitive asymmetric supercapacitor, the two electrodes both display capacitive properties, resulting in an ideal rectangular-shaped CV curve and a triangular-shaped GCD curve for the full device (Figure 7b and 7e). The electrochemical performance of the capacitive asymmetric supercapacitor based on fully capacitive electrodes can be assessed in terms of capacitance as derived from the ΔQ/ΔU ratio. For the hybrid capacitor, both the capacitive and the battery-type electrodes have been combined into one device. Although one electrode performs as a battery electrode with obvious anodic and cathodic peaks, the CV and GCD curves of the full device can exhibit more capacitive-like behavior with obvious deviation from ideal capacitive characteristics as shown in Figure 7c and 7f. In such a case, evaluating the electrochemical performance of the hybrid capacitor using eq 2 can lead to a disproportionate misestimation of the ΔQ/ΔU ratio. In section 4 we will describe the difference of electrochemical performance evaluation for asymmetric supercapacitors in detail. 3.5. Electrolytes

Generally, supercapacitors use electrolytes that are usually classified into three types:162 (i) aqueous, i.e., ions in water, (ii) organic, salts in organic solvents, and (iii) ionic liquid (IL), pure liquid salts. Electrolyte is one of the key parameters to determine the operating voltage of a supercapacitor. The rate capability and cycling performance also can be affected by the electrolyte ionic conductivity and electrochemical stability. Aqueous electrolytes can be classified into acids (e.g., H2SO4), alkali (e.g., NaOH), and neutral (e.g., Na2SO4). In addition, they usually exhibit the advantages of high ionic conductivity (up to ∼1 S cm−1), low cost, ease of handling, and nonflammability. Due to their better electronic conductivity, higher dielectric constant, and larger 9241


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Figure 8. (a) Schematic representation of the electrochemical stability range of water and potential windows versus the standard hydrogen electrode (SHE) for different pseudocapacitor materials in an aqueous electrolyte. These potential data are summarized from the following references: MnO2,142 RuO2,174 MoO3,175 Bi2O3,176 Fe3O4,177 TiO2,178 WO3−x,179 V2O5,180 PANi,153 PPy,181 and In2O3.46 Note the following potentials were used for normalizing the potential differences of common reference electrodes with respect to SHE: mercury/mercury oxide (Hg/HgO, E = +0.098 V), silver/ silver chloride electrode (Ag/AgCl, saturated, E = +0.197 V), and saturated calomel electrode (SCE, E = +0.241 V). (b) Schematic diagram of the cell potential window of an aqueous system, illustrating the relationship between the cell working voltage, the electrochemical stable potentials, the electron energies of electrolytes, and the positive or negative electrodes.

decomposition and electrode material oxidation may occur. This will lead to some gas evolution, such as CO2 and/or CO from the positive electrode and H2, propylene, CO2, and/or CO from the negative electrode. These reactions will accelerate the aging of electrolytes and active materials and may even destroy the supercapacitor. Furthermore, in order to ensure the normal operation of a supercapacitor, an organic electrolyte often requires complicated purification and assembly procedures in a strictly controlled environment with limited amounts of oxygen and moisture impurities.

Among different types of organic electrolytes, acetonitrile (ACN) and propylene carbonate (PC) are the two most commonly used solvents. ACN is a very good electrolyte solvent because it can dissolve larger amounts of electrolyte salt and provide better ionic conductivity than almost any other organic solvent. Unfortunately, it also leads to environmental problems and toxicity during battery manufacturing and recycling. PC solvent is much friendlier than ACN. It can also provide a wider stable electrochemical window, a larger operating temperature range, and acceptable ionic conductivity. However, for an organic electrolyte there is always a very strict requirement to keep the moisture level below 3−5 ppm.162 Otherwise, this will lead to severe performance degradation, an operating voltage drop, and even safety issues. An ionic liquid (IL) is a pure salt whose melting point is below a specific temperature range, where the environment could provide enough heat to the system to counter balance the lattice energy of the salt. ILs exhibit a series of distinguishing properties, including low vapor pressure, low flammability, high chemical and thermal stability, and a wide electrochemical potential range from 2 to 6 V, typically 4.5 V.173 The ionic conductivity is lower than aqueous and organic systems, but it still could achieve a reasonable number with a value of ∼10 mS cm−1. These features make ILs very good candidates for electrolytes in high-performance supercapacitors. Because it is a solvent-free electrolyte system, IL does not include any solvation shell. Thus, ILs can offer a very well identified ion size for ion dynamics studies and EDLC charge-storage mechanism research. There are still some particular concerns that should be considered when using organic electrolytes or ILs for supercapacitors. Compared with aqueous electrolytes, organic electrolytes and ILs are usually characterized by lower ionic conductivity and higher viscosity that hinders ion penetration. In addition, since the ionic conductivity of nonaqueous electrolytes is at least 1 order of magnitude lower than that of aqueous electrolytes, this leads to higher internal resistances of the supercapacitors. Thus, the capacitance of a supercapacitor using an organic- or an IL-based electrolyte usually does not exceed 200 F g−1 and exhibits relatively poor power density. Such supercapacitors also often suffer from high costs for ILs and safety concerns for organic liquids related to their high flammability, volatility, and toxicity.162 When the applied voltage is higher than the stable potential window, electrolyte

3.6. Thermodynamic and Kinetic Considerations for Potential Window of Pseudocapacitive Materials

An appropriate starting point for characterizing pseudocapacitance is to consider the relationship between the potential, E, and the charge that develops at the electrode/electrolyte interface12,128 E ≈ E0 −

RT ji X zy zz lnjj nF k 1 − X {

(13)

here E0 is the standard potential of the redox couple, n is the number of transferred electrons, F is Faraday’s constant, X is the fractional charge coverage, and R and T take on their usual meanings of the ideal gas constant and temperature, respectively. Thus, the real-time potential of the pseudocapacitive electrode changes as a function of the ratio of charge coverage (X) at the active electrode surface during the charge/discharge process. By following eq 14, capacitance can be defined over the potential range where E is linear with X128 i nF y X C = jjj zzz k m {E

(14)

where m is the active material’s molecular weight. The curve of E vs X may not be linear. The theoretical capacitance of pseudocapacitive materials can be calculated based on this equation when ones assumes X is 1. Although the thermodynamic analysis of potential and pseudocapacitance is important, kinetic considerations are equally important for determining the potential window and electrochemical performance. Because pseudocapacitance involves Faradaic reactions in much the same way as battery electrode materials, several kinetic effects must be considered that lead to polarization in battery materials to also take place with pseudocapacitor materials. Three different polarization 9242


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Table 1. Summary of Redox Reactions, Theoretical Capacitance, and Potential Windows for Selected Pseudo-Capacitive Materials materials MnO2 RuO2·nH2O V2O5 MoO3 VN

theoretical capacitance (F/g)

redox reaction equations MnO2 + xC+ + xe− ↔ MnOOCx (Max(x) = 1, C could be H+, Li+, Na+, K+) RuOx(OH)y + zH+ + ze− ↔ RuOx−z(OH)y+z (Max(z) = 2) V2O5 + xM+ + xe− ↔ MxV2O5 (Max(x) = 4, M could be H+, Li+, Na+, K+) MoO3 + xM+ + xe− ↔ MxMoO3 (Max(x) = 1, M could be H+, Li+, Na+, K+) VNxOy + OH− ↔ VNxOy||OH− + VNxOy−OH + e−

effects that occur are12 (1) activation polarization accompanied by Faradaic charge-transfer processes, (2) ohmic polarization related to the electronic and ionic resistance of cell components and the contact resistance between current collector and active material, and (3) concentration polarization which arises from mass transport during the electrochemical process. On the basis of the influence of these polarization process and/or surface redox reactions, a passivation layer always forms at the interface of the electrode material and electrolyte. This passivation layer could have a significant influence on the potential window and specific capacitance of the active materials. To provide some insights for future research on asymmetric supercapacitors, we summarize the redox potential windows in Figure 8a and the theoretical capacitance and redox reaction equations of some commonly studied pseudocapacitive materials in Table 1. This will help researchers identify appropriate electrode materials and proper electrolytes for building high-performance asymmetric supercapacitors. Note that the potential ranges could change with different electrolytes (pH or ion types) and active material structures (crystal phases or particle sizes). The theoretical capacitance may not be the maximum capacitance, especially for some porous, high surface area Faradaic electrode materials. It is well known that carbon supercapacitors can exhibit perhaps 1−5% of their capacitance as pseudocapacitance due to the Faradaic reactivity of oxygen functional groups. Some active materials can be even higher depending on the amount of their oxygen functional groups. Analogously, pseudocapacitors always exhibit some EDLC component, usually about 5−10%, proportional to their electrochemically accessible interfacial area.29 Therefore, the practical capacitance of supercapacitor materials should be the sum of contributions from both pseudocapacitance and EDLC.

potential window (V)

ref

1380 (0.8 V) 1233 (0.9 V) 1450 (1.0 V) 2122 (1 V)

0−0.8 (Ag/AgCl) Na2SO4 0−0.9 (Ag/AgCl) Na2SO4 0.2−1.2 (RHE) 0.5 H2SO4 0−1.0 (Ag/AgCl) 0.5 K2SO4

141,142,188 7 189 190

670 (1 V)

0−1.0 (Ag/AgCl) 1 M H2SO4

175

1238 (1.2 V)

−1.2−0.0 (Hg/HgO) 1 M KOH

191−193

significance for exploring the stable voltage of a supercapacitor. Figure 8b schematically illustrates the potential window of an aqueous asymmetric cell and its relationship with relative electron energies of the electrode active materials and electrolyte. The energy separation Eg between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) is the thermodynamically stable range of an aqueous electrolyte.182 As previously discussed, the positive and negative electrodes have their own standard electrode potentials, μp and μn, respectively, which are related to their Fermi energies, εF. The μp or μn may correspond to the Fermi energy in an electron band for a positive or a negative electrode material, for example, the energy level of a carbon material or a redox couple for a transition-metal oxide. For a Faradaic reaction, depending on the charge-transfer direction, energy levels at or near the Fermi level of the active materials must be matched with a suitable vacant (LUMO) or occupied (HOMO) orbital in the reactant. At the same time, an applied potential, V, is required to modify the electron work function Φ to some value Φ ± n eV to achieve the condition of balance required for facile electron transfer to take place at the potential V. This potential V is the standard electrode potential μ for the specific active materials. If the positive electrode has a μp higher than the HOMO, the electrolyte will be oxidized unless it forms a passivation layer to mitigate the electron transfer from the HOMO of the electrolyte to the positive electrode. In addition, when the negative electrode has applied a potential lower than the LUMO, the electrolyte will be reduced unless a passivation layer hinders the electron transfer from the negative material to the electrolyte LUMO.183 Thus, without any passivation layer, the electrode electrochemical potentials μp and μn should locate within the theoretically stable window of the electrolyte. In an aqueous electrolyte, the operating potential window range is generally limited by the water decomposition voltage. However, for actual electrochemical systems, a passivation layer is commonly formed at the electrode/ electrolyte interface, which can give a kinetic stability to enlarge the voltage window of the full cell. In other words, if there is a passivating layer formed, the potential window of the full cell could exceed the limit of the thermodynamically stable electrolyte window. In an aqueous electrochemical system the operating voltage can be extended further depending on the overpotentials for H2 and O2 evolution at the negative and positive electrodes, respectively.165,167,184−186 For example, hydrous RuO2 exhibits remarkably reversible cyclic voltammograms at potentials ranging from 0.05 to 1.4 V (vs RHE (the reversible hydrogen electrode)), which exceeds the limitation of the standard

3.7. Full Cell Voltage

In addition to the restriction of the intrinsically stable redox potential window of active electrode materials, the electrolyte is the other key component to determine the stable potential window of a supercapacitor. The electrolyte provides sufficient cations and anions as well as the ionic transport pathways needed for electrochemical cell operation. Each electrolyte can be stably operated only within a certain potential range. The stable potential ranges of electrolytes depend on several factors, including the type of conducting salts (cations and anions), solvent, and their purity level.162 As shown in Figure 8a, the potentials of standard pseudocapacitive materials always cross a wide potential range, which is different from the very narrow Faradaic electron transfer potential range for individual battery anodes and cathodes. However, the general principles to determine the overall operating voltage of a battery still have some guiding 9243


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decomposition voltage of water.129 The potential window of the overall supercapacitor is given by eq 15 Edevice = Epositive − Enegative

standardized characterization metrics and clear rules for reporting performance need to be established and widely executed, which would be helpful for researchers, scientists in this field as well as engineers, and investors who are interested in supercapacitor applications. Here, in order to eliminate the existing inconsistencies and confusion, we select some confusing key parameters and try to clearly elaborate their meanings and evaluation methods. CV, GCD, and electrochemical impedance spectroscopy (EIS) are the three most commonly used electrochemical techniques to characterize the supercapacitor energy-storage performance for a single electrode or a full device.

(15)

Overall, the operating voltage of a supercapacitor is determined by three key factors: the stable potential window of the electrolyte, the standard potential range of each electrode, and the passivation layer formed at the electrode/electrolyte interface. For a supercapacitor device, the potentials of separated electrodes extend in opposite directions during charging until each electrode reach roughly the same capacity. Thus, the potential window of the whole cell depends on the upper potential limit of the positive electrode and the lower potential limit of the negative electrode. Conventional symmetric supercapacitors consist of electrodes with the same type of electrode materials and the same mass loading, which means the stable potential window of the supercapacitor only covers a narrow potential range of a single type of active material. The concept of an asymmetric supercapacitor was considered as one way to solve the issues of the narrow potential window and the relative low energy density of a symmetric cell.38−40 As illustrated in Figure 8b, in an asymmetric configuration, the overpotentials of ΔE1 for O2 evolution at the positive electrode (such as RuO2 or MnO2) and ΔE2 for H2 evolution at the negative electrode (such as carbon, VN or Fe3O4), respectively, extend the practical potential windows of a supercapacitor beyond the thermodynamic limit, even up to 2 V.184,187 This can result in a significant increase in energy density compared to a conventional symmetric supercapacitor in an aqueous electrolyte with the same weight, area, or volume.

4.1. Calculating the Capacitance for a Single Electrode

For a single electrode, the capacitance is a key parameter, which is reflected in the electrical charge stored under a given voltage, specifically, the total charge-storage capability. The capacitance is defined by eq 2, ΔQ/ΔU, i.e., the charge-storage capability of a supercapacitor within a specific voltage. Generally, this value can be calculated from the results for all three types of electrochemical measurements. The working behavior of GCD is more closely related to the electrochemical behavior when an external load is typically applied to a supercapacitor for a majority of applications. Thus, the GCD test is the most commonly used and accurate method to determine the capacitance. It can be calculated by applying eq 16 C=

I (dV /dt )

(16)

where discharge current I and dV/dt are derived from the slope of the GCD discharge. Due to the internal resistance, the initial portion of a discharge curve exhibits an IR voltage drop. For more accurate results the potential range of V is the voltage drop upon discharge excluding the IR drop. As shown in Figure 6, capacitive and Faradaic electrodes exhibit different characteristics of GCD curve shapes, linear or nonlinear. However, according to the definition of capacitance, one cannot use the same equation to calculate the “capacitance” for Faradaic materials based on their nonlinear GCD curves. In other words, for a nonlinear GCD curve, eq 16 cannot be applied. The capacitance can also be calculated by integration of the CV curves using eq 17

4. PRINCIPLES AND METHODS OF EXPERIMENTAL EVALUATION When characterizing the electrochemical performance of supercapacitors, one must be very careful to use the appropriate tests and evaluation methods. For example, there was one reported work which attempted to derive a claimed “capacitance” from an obvious nonlinear GCD plot.194 This incorrect calculation led to a significant overestimation of the supercapacitor performance. Generally, supercapacitor performance can be characterized by some essential parameters, such as specific capacitance (C), operating voltage (V), and equivalent series resistance (ESR), cycling stability, time constant, energy density, and power density.10,195−200 The researchers in academia and industry demonstrated various methods to evaluate these performances. For commercial products where the parameters for materials, fabrication, and cell design are all fixed, it is feasible to compare the active materials performance with benchmarked characterization methods. However, a lot of novel materials, advanced fabrication methods, and new cell designs are still being developed by researchers. Thus, considering the multiple characterization metrics, measurement methods, affecting factors, and multifaceted relationships among them, inconsistencies become inevitable for the test results of the same active materials in different laboratories by using different electrode fabrication, cell assembly, and evaluation methods between academic institutes and/or industrial concerns. Some confusion caused by these inconsistencies not only hinders rational comparisons of the latest research results with standard cells but also creates a hurdle for transferring novel research achievements to commercial applications.199,201,202 Therefore,

C=

∫ I dV vΔV

(17)

where I is the discharge current, i.e., the current below the X axis, v is the scan rate, and ΔV is the operating discharge potential range. 4.2. Evaluation of the Capacitance and Energy Density of Asymmetric Supercapacitors

For a full device, the key parameters, such as capacitance, ESR, operating voltage, and subsequently, the time constant, energy density, and power density, can be also estimated from the CV, GCD, and EIS measurements. On the basis of these calculation methods for capacitance, the specific capacitance for a device or active material from a two (or three) electrode system can be calculated using eq 18 Cs =

C Π

(18)

where Π is the effective weight, area, or volume. If Cs refers to the capacitance of a single electrode then Π is based on the active 9244


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Figure 9. (a) CV curves of the separate graphene and graphene/MnO2 electrodes performed in a three-electrode cell in 1 M Na2SO4 electrolyte at a scan rate of 10 mV s−1. (b) CV curves of a graphene//graphene/MnO2 asymmetric supercapacitor operating at voltages of 1.5, 1.8, 2.0, and 2.1 V recorded at a scan rate of 10 mV s−1. Reproduced with permission from ref 204. Copyright 2010 American Chemical Society.

materials of the electrodes. If Cs is calculated for a full device, Π should be based on the total value of the full device, including electrodes, electrolyte, and separator, using a two-electrode system. The electric energy stored in supercapacitors, i.e., the energy density, can be evaluated through the integration of the GCD curves. For EDLCs and pseudocapacitors with linear charge/ discharge curves, the integration turns into the calculation of triangle areas as shown in Figure 7e; therefore, the energy density can be calculated by E=

∫ Q dV = ∫0

V0

CV dV =

1 2 CV 2

with a platinum foil as the counter electrode and a saturated calomel electrode (SCE) as a reference. As exhibited in Figure 9a, the CV curves of both the graphene/MnO2 electrode and the pure graphene electrode exhibit relatively rectangular shapes and near mirror-image current responses over potential windows in the ranges from 0.0 to 1.0 V (vs SCE) and −1.0 to 0.4 V (vs SCE), thus indicating ideal capacitive behavior and stable potential windows for both electrodes. As shown in Figure 9b, the stable operating voltage of this asymmetric supercapacitor has been extended to 2.0 V as the sum of the potential ranges for positive and negative electrodes, with the graphene/ MnO2 electrode going up to +1.0 V and the graphene electrode going down to −1.0 V. Note that there was no evolution of gas observed. When the operational voltage is moved to greater than 2.0 V (such as 2.1 V), a sharp peak occurs at the end of the CV curve, indicating the evolution of oxygen during the electrochemical measurement. In addition to achieving optimal performance, there should be a charge balance calculation between the positive and the negative electrodes. The total charge stored in each electrode is determined by the specific capacitance (Celectrode), the active mass, and the potential window (ΔE) of each electrode. It can be estimated according to eq 2146

(19)

However, for a hybrid capacitor with a nonlinear GCD curve as shown in Figure 7f the energy density cannot be calculated simply by using eq 19 due to the nonlinear change of V. In this case, the equation should be modified E=

∫ Q dV = ∫t

1

t2

IV(t ) dt

(20)

All of the discharge times (tdischarge) and discharge voltages (V(t)) are taken into account for the calculation after the initial IR drop. In eq 20, t1 is the time after the initial IR drop, t2 is the moment that the discharge is finished, and I is the constant current applied to the supercapacitor. In many literature reports, the ΔQ/ΔU ratio has been used to calculate the “capacitance” of Faradaic materials and energy for hybrid capacitors with nonlinear GCD curves. Direct use of eqs 2 and 16 could lead to an overestimation or underestimation of the charge-storage capability of the Faradaic materials and hybrid capacitors. In addition, it is worth noting that the parameter of energy density should be calculated based on twoelectrode systems, not just using a single electrode.

Q electrode = Celectrode × m × ΔE

(21) +

−

To achieve the charge balance Q = Q , a mass balance following eq 22 is needed48 m+ C × ΔE− = electrode − m− Celectrode + × ΔE+

(22)

Thus, the optimal mass ratio between the two electrodes m+/m− can be adjusted to get the optimal performance of an asymmetric supercapacitor.

4.3. Charge-Balancing Principles between Two Electrodes

4.4. Power Density and Equivalent Series Resistance (ESR)

In order to evaluate the properties in an accurate way, several principles need to be kept in mind during the fabrication and testing of an asymmetric supercapacitor. In determining the electrochemical properties of an asymmetric supercapacitor it is necessary to carry out CV measurements using a three-electrode system first to estimate the stable potential ranges and specific capacitances of positive and negative electrodes.46,48,203 An example of this analysis applied to a graphene/MnO2// graphene asymmetric supercapacitor is shown in Figure 9.204 Both the MnO2/graphene electrode and the pure graphene electrode have been measured in a three-electrode configuration

Power density is an important parameter widely used to characterize how much power can be delivered within a specific unit. The power density (Pmax) is calculated according to eq 23205 Pmax =

2 Vcharged

4R s

(23)

where Vcharged is the charged potential of the supercapacitor and Rs is ESR. The ESR is one of the key factors in determining the power density and the overall performance of a supercapacitor. 9245


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Figure 10. (a) Impedance spectrum recorded in a two-electrode configuration. Reproduced with permission from ref 121. Copyright 2016 John Wiley and Sons. (b) Potential drop associated with the ESR (IR drop) of a supercapacitor vs different discharge current densities. Reproduced with permission from ref 48. Copyright 2011 John Wiley and Sons.

A number of components contribute to ESR:81,206 (1) the intrinsic electronic resistance of the active material and current collector; (2) the contact resistance between the active material and the current collector; (3) the electrolyte ionic resistance, especially the transport resistance of ions inside confined pores; and (4) the ions transport resistance across the separator. Generally, there are two methods for determining ESR: (a) use EIS and (b) measure the IR drop at the initation of a constant current discharge curve. Figure 10a shows a typical Nyquist plot and the equivalent circuit for impedance analysis based on characterization by using a symmetric supercapacitor with 3D porous graphene as the active material.121 The Nyquist plot exhibits a semicircle at the high-frequency range and shows a sloping line at the lowfrequency region. As illustrated in the equivalent circuit (inset, Figure 10a), these resistor and capacitor elements in the equivalent circuit are associated with specific parts of the Nyquist plot. To evaluate the values for the various elements, the measured Nyquist plot can be fit based on an equivalent Randles circuit by using eq 24121,207 Z = Rs +

1 jωCdl +

1 R ct + Wo

+

curves with different current densities. This behavior is shown in Figure 10b, according to eq 25205 IR drop = a + bI

where a is the voltage difference between the applied volatge (Vapplied) and the real voltage of the device or electrode (Vcharged) and b represents twice the value of the ESR. From this analysis, the maximum power density can be calculated from eq 26205 Pmax =

1 R leak

2 Vcharged

4R s

=

(Vapplied − a)2 2b

(26)

4.5. Some Concerns Regarding Performance Evaluations

The widespread interest in electrochemical capacitors over the last several years has led to significant growth in the field. Meanwhile, there have been some inconsistencies because of the different materials and metrics used to characterize and evaluate electrochemical capacitors. A few publications have addressed this issue and identified guidelines with regard to the measurements, device configurations, and presenting the data in a more consistent framework.84,198,201 One recurring problem in characterizing the performance of all electrochemical materials is the low mass loading of the active material. Frequently, this value is not even given, but it is a very important parameter. When low mass loading is used (7

Å), which allows for the reversible intercalation of hydrated electrolyte ions.128 The charge-storage mechanism of Ni(OH)2 is based on a reversible redox reaction of the Ni2+/Ni3+ couple in an alkaline electrolyte. Senthilkumar and co-workers fabricated a cable-type hybrid capacitor by using plate-like β-Ni(OH)2 as the positive electrode and activated carbon as the negative electrode.250 As shown in Figure 15b, the β-Ni(OH)2 electrode works at a positive potential range, from 0 to 0.6 V (vs Hg/ HgO), while activated carbon works at a negative potential window, from −0.8 to 0 V (vs Hg/HgO). The recorded CV and GCD curves presented in Figure 15c and 15d show that the hybrid capacitor stable operating voltage can be extended up to 1.4 V. Similarly, Huang et al. developed a hybrid capacitor by decorating β-Ni(OH)2 on Ni foam as the positive electrode and using activated carbon as the negative electrode.252 This βNi(OH)2/Ni foam hybrid electrode exhibited a very high specific capacity of 790.3 C g−1 in an aqueous electrolyte. In addition to crystalline Ni(OH)2, some amorphous phase of Ni(OH)2 can offer an even higher specific capacity.253,254 Li et al. demonstrated that the specific capacity of an amorphous Ni(OH)2 nanosphere electrode slightly exceeds the theoretical capacitance of Ni(OH)2 (1040 C g−1).254 Co(OH)2 is another promising candidate for use as a positive electrode material in hybrid capacitors due to its high theoretical capacity and natural abundance.251,255 Gao et al. fabricated an asymmetric supercapacitor consisting of a single-layer βCo(OH)2-based positive electrode and a N-doped graphene negative electrode.251 As illustrated in Figure 15e, the thin layer structure of β-Co(OH)2 sheets not only enhances electrolyte ion diffusion but also provides completely exposed electroactive sites. As a result, the single-layer β-Co(OH)2-based electrode exhibited a very high capacity of 1420 C g−1. 5.2.3. Mixed Transition-Metal-Based Electrodes for Aqueous Hybrid Capacitors. Mixed transition-metal spinels such as NiCo2O4,256,257 NiMoO4,258 and CoNi2S4259 represent another category of positive electrode materials for hybrid capacitors. NiCo2O4 exhibits a particularly high theoretical capacity value (estimated at 780 C g−1) via a three-electron redox reaction with a typical potential window of 0.65 V in an 9252


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Table 3. Comparisons of Positive Electrodes, Negative Electrodes, Potential Windows, and Retention for Aqueous Hybrid Capacitors positive electrode

negative electrode

voltage window

CuO NiO Ni@NiO Ni(OH)2 Ni(OH)2 Ni(OH)2 Ni(OH)2@Ni Co(OH)2 Co(OH)2 Co3O4 Co3O4 NiCo2O4 NixCo3−xO4 NiMoO4 CoNi2S4 CoO@PPy Co3O4@MnO2 CuO@MnO2

AC reduced graphene oxide reduced graphene oxide AC AC AC AC AC N-doped graphene AC graphene AC AC AC AC AC activated graphene activated graphene

1.4 V (3 M KOH) 1.7 V (1 M KOH) 1.5 V (6 M KOH) 1.3 V (1 M KOH) 1.6 V (6 M KOH) 1.4 V (PVA/KOH) 1.3 V (1 M KOH) 1.6 V (6 M KOH) 1.8 V (2 M KOH) 1.4 V (6 M KOH) 1.5 V (6 M KOH) 1.6 V (2 M KOH) 1.6 V (2 M KOH) 1.7 V (2 M KOH) 1.7 V (2 M KOH) 1.8 V (3 M NaOH) 1.6 V (1 M LiOH) 1.8 V (1 M Na2SO4)

retention (cycles)

ref

96% (3000) 95% (3000) 97% (5000), 81% (10 000) 92% (1000) 88% (1000) 96% (3000) 86.3% (5000) 93.2% (10 000) 97% (2000) 84% (1000) 80% (2000) 82.8% (2000) 85.7% (10 000) 78% (3000) 91.5% (20 000) 95% (5000)

272 247 248 254 252 250 253 255 251 249 273 257 256 258 259 271 274 229

redox reactions but also have higher electronic conductivity. As an example, NiCo2S4 displays much higher specific capacitance than the corresponding monometal sulfides (NiSx and CoSx) and exhibits an electrical conductivity about 2 orders of magnitude higher than that of NiCo2O4.268 Benefiting from its high electronic conductivity, rich redox chemistry, and synergistic effects of two metal ions, mixed metal sulfides have emerged as promising electrode materials for hybrid capacitors. Guan et al. developed a sequential ion-exchange strategy to prepare onion-like NiCo2S4 particles with unique hollowstructured shells using onion-like metal oxide particles as the precursor.269 The onion-like Co3O4 particles were first transformed into onion-like Co4S3 hollow-structured particles. Afterward, these onion-like Co4S3 particles were further converted into NiCo2S4 particles through a subsequent cationexchange reaction with Ni2+ ions. A hybrid capacitor was fabricated by using onion-like NiCo2S4 particles as the positive electrode and activated carbon as the negative electrode, which provides excellent cycling stability with enhanced rate capability. The same group then developed a sequential chemical etching and sulfurization strategy to prepare well-defined double-shelled zinc−cobalt sulfide (Zn−Co−S) rhombic dodecahedral cages.270 Owing to the structural and compositional benefits, the obtained double-shelled Zn−Co−S rhombic dodecahedral cages exhibit enhanced performance with high specific capacity (696.3 C g−1 at 1 A g−1) and long-term cycling stability (91% retention after 10 000 cycles) for a hybrid capacitor. One approach for enhancing the conductivity of a metal oxide electrode is by combining it with a conducting polymer. Zhou et al. integrated polypyrrole (PPy) with a CoO nanowire array to achieve a high-performance positive electrode for a hybrid capacitor.271 The relatively high electrical conductivity of PPy (10−100 S cm−1) signifcantly improved the electron transport of CoO nanowires compared to the low conductivity of CoO (∼10−2 S cm−1). As a result, the CoO@PPy 3D electrode exhibited a high capacity of 1556 C g−1 at a currrent density of 1 mA cm−1. Figure 16d schematically illustrates the configuration of the hybrid capacitor. Both the CV curves at different scan rates (Figure 16e) and the GCD curves with varied current

aqueous electrolyte. These distinguished electrochemical characteristics can be attributed to the full utilization of the redox behavior of both nickel and cobalt.260,261 Wang et al. fabricated a porous NixCo3−xO4 nanowire array on Ni foam for the assembly of a supercapacitor.256 The NixCo3−xO4 electrode showed a gravimetric specific capacity of 740 C g−1 at 1 A g−1. With activated carbon as the negative electrode, the hybrid capacitor exhibited a wide potential window of 1.6 V. Coordination polymers have been shown to be a general synthetic precursor to highly complex mixed metal oxides. Guan et al. synthesized an onion-shaped nanostructured Ni−Co mixed oxide by using Ni−Co coordination polymer spheres as the precursor.262 This could be a potential approach to produce ternary or even quaternary metal oxides with tunable crystal structure size and metal elements composition. Lou and co-workers developed a type of metal molybdates, such as NiMoO4, and studied their electrochemical performance as positive electrodes for hybrid capacitors. For example, they developed a calcination treatment method to grow hierarchical NiMoO4 nanosheets on conductive substrates and then incorporated them as the positive electrode in a hybrid capacitor.258 The growth of well-defined hierarchical nanosheets homogeneously formed on Ni foam is shown in Figure 16a and 16b. A high-resolution TEM (HR-TEM) image (Figure 16c) shows the highly crystalline nature of the NiMoO4 nanosheets with an interplanar distance of 0.698 nm that matches well with the spacing of the (001) plane of NiMoO4. The optimized asymmetric supercapacitor can deliver a relatively high working voltage of 1.7 V. Recently, metal sulfides have attracted increased attention as promising electrode materials for hybrid capacitors according to their better electrical conductivity, mechanical and thermal stability, and higher electrochemical activity than their corresponding metal oxide counterparts.263−265 Compared with monometal sulfides, mixed metal sulfides exhibit significantly enhanced electrochemical performances,266,267 which are largely related to their increased conductivity and enhanced redox reactions. Moreover, mixed metal sulfides also present superior electrochemical performance when compared to mixed metal oxides because they not only possess comparable 9253


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Figure 17. CV curves of a Ni(OH)2/graphite//graphite foam hybrid capacitor: (a) CV curves at varied operating voltages. (b) CV curves at different scan rates with an operating voltage of 1.8 V. (c) GCD curves at different current densities. Reproduced with permission from ref 276. Copyright 2013 American Chemical Society. (d) SEM of the C/CoNi3O4 network. Reproduced with permission from ref 277. Copyright 2014 John Wiley and Sons. (e) Schematic illustration of the design for a Ni−Co−S electrode-based hybrid capacitor. Reproduced with permission from ref 280. Copyright 2014 American Chemical Society.

method in which nanoporous Ni(OH)2 thin films were grown on conductive ultrathin graphite foam.276 This 3D porous conductive carbon structure provided electrolyte ion reserviors and extra space for ion transport. A hybrid capacitor was fabricated by using a Ni(OH)2/graphite foam as the positive electrode and a microwave-reduced GO film as the negative electrode. As shown in Figure 17a, a series of CV curves with varied set potential ranges was performed to get the maximum operating voltage for this hybrid capacitor. Some obvious oxygen evolution polarization started at voltage extended to 2.0 V. Thus, 1.8 V was selected as the working voltage for further electrochemical investigations. Figure 17b and 17c shows the CV curves under different scan rates and GCD curves with varied current densities for the designed hybrid capacitor tested under an operating voltage of 1.8 V. The interconnected 3D porous architecture of carbon foam makes it a promising conductive support for hybrid capacitor electrodes.277−279 Afterward, a 3D hierarchical carbon-supported CoNi3O4 nanostructured array electrode was reported by Zhu et al.277 As shown in an SEM image (Figure 17d), the C/CoNi3O4 electrode possessed an integrated 3D network architecture, which consists of interconnected nanowalls woven together. The porous C/CoNi3O4 electrode exhibited a relatively high surface area of 128.1 m2 g−1 and a large gravimetric capacity of 1299 C g−1. Sulfide-based redox systems have also been investigated. Shen et al. grew NiCo2S4 nanosheets on nitrogen-doped carbon foams and used them as a flexible positive electrode for building hybrid capacitors.279 The NiCo2S4 composite electrode exhibited a doubling of capacity to 614 C g−1 at a current density of 20 A g−1. Carbon fibers are commonly employed as the substrate and current collectors for supercapacitors because of their good conductivity, high chemically stability, and high mechanical strength.230,231,281−283 Moreover, the inherent flexibility of fibers has enabled carbon fiber-based electrodes to be fabricated

densities (Figure 16f) demonstrate that the hybrid capacitor can be operated reversibly with a potential window up to 1.8 V. By introducing pseudocapacitive or battery-type materials (metal oxides and/or conductive polymers) as positive electrode active materials, the resulting hybrid capacitors can exhibit exciting electrochemical properties such as extended potential windows and improved specific capacitance and energy density when compared to conventional symmetric supercapacitors. Table 3 summarizes the active materials used for positive and negative electrodes and the aforementioned electrochemical properties for aqueous hybrid capacitors. These systems clearly show the opportunity to realize the high performance of hybrid capacitors through the judicious selection of active materials and the rational design of nanostructures. 5.3. Faradaic Materials/Carbon Composite Electrodes for Aqueous Hybrid Capacitors

As discussed in the prior sections, both pseudocapacitive and battery-type materials have been used as the positive electrode in asymmetric supercapacitor devices. These materials are frequently wide band gap semiconductors whose modest levels of conductivity constrain the charge transport properties and lead to low power density. To overcome this limitation, there has been a significant amount of research directed at creating electrode structures with better electrical conductivity and improved kinetics for redox reactions. The classic approach is to combine the redox-active material with conductive carbon black.275 The research reviewed here are electrodes involving conducting scaffolds comprised of carbon materials in different forms such as carbon fibers, carbon nanotubes, and graphene. In all cases, the conductive component is combined with a batterytype material to form a composite which serves as one of the electrodes for a hybrid capacitor. Foam structures have been widely used as scaffolds as these architectures couple a conductive framework with a porous morphology. Ruoff and co-workers reported a hydrothermal 9254


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Figure 18. (a) N 1s and O 1s core level XPS spectra fitting of highly functionalized activated carbons. (b) Schematic illustration of the typical Ncontaining groups in the precursor egg white protein and of N-functionalities in functionalized activated carbon. (c) CV curves of commercially activated carbon, functionalized activated carbon, and NiCo2O4/graphene tested in a three-electrode setup at 5 mV s−1. (d) CV curves of the two hybrid full cells for NiCo2O4/graphene//commercial activated carbon and NiCo2O4/ graphene//functionalized activated carbon. (e) GCD curves of the two hybrid cells at 0.5 A g−1 and the tracked variation of the potential in each individual electrode. Reproduced with permission from ref 291. Copyright 2014 Royal Society of Chemistry.

Table 4. Comparison of Positive Electrodes, Negative Electrodes, Potential Windows, and Retention (after cycling) for Selected Advanced Hybrid Capacitor Electrodes positive electrode

negative electrode

voltage window

retention (cycles)

ref

NiCo2S4/N-doped carbon foams CoNi3O4/3D C Co−Ni−S/carbon fiber MoO3−x/CNTs Ni3S2/CNTs Ni(OH)2/CNTs/carbon fiber NiS/CNTs V2O5/reduced graphene oxide NiO/3D graphene Ni(OH)2/graphene NiCo2O4/graphene NiCo2O4/graphene

ordered mesoporouscarbon/N-doped carbon foams AC grapehen/carbon fiber CNTs AC AC graphene reduced graphene oxide AC graphene AC AC

1.6 V (6 M KOH) 1.8 V (3 M KOH) 1.8 V (1 M KOH) 1.9 V (LiCl/PVA) 1.6 V (2 M KOH) 1.8 V (KOH) 1.4 V (6 M KOH) 1.6 V (1 M KCl) 1.5 V (1 M NaOH) 1.6 V (6 M KOH) 1.4 V (6 M KOH) 1.55 V (1 M KOH)

70.4% (10 000) 99% (5000) 82.2% (20 000) 90% (10 000) 90% (5000) 83% (3000) 92% (1000)

279 277 280 297 298 289 290 294 295 293 292 291

into flexible supercapacitors.104,284−287 Recently, Chen et al. reported the electrodeposition of CoNi2S4 nanosheet arrays on conductive carbon cloth as an electrode for hybrid capacitors.280 By pairing this electrode with porous graphene films as the negative electrode, the resulting hybrid capacitor was fabricated (Figure 17e) and exhibited a high claimed energy density. Due to their high surface area, good electrical conductivity, and strong mechanical strength, carbon nanotubes (CNTs), a 1D form of carbon, have been explored extensively as active materials or conductive substrates for supercapacitors.50,98,100,103,288 Compared with carbon fibers, the high aspect ratio, small diameter, and low density makes the CNTbased electrodes convenient for the chemical modification of Faradaic materials. There are several published research reports related to CNT-based electrodes for hybrid capacitors.225,234,289,290 For example, Singh et al. decorated NiS nanoparticles on multiwalled CNTs by using a hydrothermal synthesis.290 With graphene as the negative electrode, the resulting hybrid capacitor exhibited a specific capacitance of ∼250 C g−1 at 1 A g−1.

85% (5000) 94.3% (3000) 98% (5000)

Similarly, graphene, a single layer of 2D pure carbon material, has also received a lot of attention according to its impressive properties including high electrical conductivity, excellent mechanical strength, and high theoretical specific surface area. Recently, researchers have integrated graphene nanosheets into macroscopic materials to create composite pseudocapacitive or battery-type materials that serve as the positive electrode in capacitive asymmetric or hybrid capacitors.48,147,235,237,238,240,243,292,293 Nagaraju et al. reported the synthesis of reduced graphene oxide (rGO)/V2O5 nanosheet electrodes,294 which can store almost twice the energy of pure 2D V2O5 electrodes. Wang et al. reported a hydrothermal method to grow NiO nanoflakes on 3D graphene scaffolds.295 These NiO−graphene composite electrodes exhibit a capacity of ∼1097 C g−1. Yan et al. prepared an electrode composed of hierarchical flowerlike Ni(OH)2 decorated on graphene sheets.293 A 1.6 V hybrid capacitor was successfully fabricated by combining the Ni(OH)2/graphene electrode with graphene to serve as the positive and negative electrodes, respectively. Li et al. demonstrated a template-free method to prepare 9255


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Table 5. Comparison of Positive Electrodes, Negative Electrodes, Potential Windows, Energy Densities, Power Densities, and Retention (after cycling) for All Redox Electrode-Based Asymmetric Supercapacitors positive electrode

negative electrode

voltage window

energy density

power pensity

retention (cycles)

ref

MnO2/carbon fiber MnO2/carbon fiber MnO2/graphene MnO2/CNTs/graphene MnO2/CNTs CoOx/Ni(OH)2 VOx/carbon fiber MnO2/graphene MnO2/graphene

Fe2O3/carbon fiber Fe2O3/carbon fiber FeOOH/CNTs/graphene PPy/CNTs/graphene In2O3/CNTs Fe3O4/reduced graphene oxide VN/carbon fiber MoO3/graphene VOS/C

1.6 V (LiCl/PVA) 1.6 V (LiCl/PVA) 1.7 V (1 M Li2SO4) 1.6 V (0.5 M Na2SO4) 2 V (1 M Na2SO4) 1.7 V (6 M KOH) 1.8 V (LiCl/PVA) 2 V (1 M Na2SO4) 1.8 V (5 M LiCl)

0.41 mWh cm−3 0.55 mWh cm−3 30.4 Wh/kg 22.8 Wh/kg 25.5 Wh/kg 45.3 Wh/kg 0.61 mWh cm−3 42.6 Wh/kg 45.1 Wh/kg

0.1 W cm−3

81.6% (6000) 84% (5000) 89% (1000) 83.5% (10 000)

311 299 302 301 46 300 306 203 312

0.85 W cm−3

91.8% (5000) 87.5% (10 000) 91% (5000)

those systems reviewed above (Table 2) in which charge storage in the negative electrode was based on an EDLC. Some pseudocapacitive materials can work in a negative potential range including MoO3,175,203,304,305 VN,306 Bi2O3,176 V2O5,307 and Fe3O4.177 In general, these materials exhibit higher specific capacitances than carbon-based materials. An early example to build a capacitive asymmetric supercapacitor with all pseudocapactive electrodes used MnO2/CNTs as the positive electrode and SnO2/CNTs as the negative electrode.308 In such a design, it is important to determine the capacitive potential ranges for the positive and negative electrodes. For the MnO2 positive electrode, both protons and alkali cations are involved in the redox transition between Mn4+ and Mn3+. Although Sn is not a transition metal, it has two stable valences, Sn4+ and Sn2+, which are the basic states for the redox chemistry of SnO2. CV curves with different potential windows were studied to optimize the potential range of the SnO2/CNTs electrode. Overall, the asymmetric system can reach a wide potential range from −0.8 to 0.9 V vs Ag/AgCl while maintaining a fairly rectangular CV curve for separate positive and negative electrodes. As a result, the full cell achieved a stable operating voltage of 1.7 V in 2.0 M KCl without any noticeable electrolyte decomposition. The extension of the potential window is likely due to the relatively high hydrogen evolution overpotential on the SnO2/CNTs negative electrode. The full cell reached a high specific energy of 20.3 Wh/kg and a specific power density of 143.7 kW/kg in an aqueous electrolyte. Afterward, Tang et al. prepared MoO3 nanoplates that exhibited a high specific capacitance of 280 F g−1, which could be operated reversibly at a potential range from −0.1 to −0.85 V vs SCE in an aqueous electrolyte of 0.5 M Li2SO4.305 The CV curves of a MoO3 nanoplate electrode exhibited two sets of typical redox peaks at −0.39/−0.32 V and −0.75/−0.59 V. The redox reactions at these peaks are related to the Li-ions reversible intercalation/deintercalation of the lattice interspace of MoO3. Chang and co-workers demonstrated an asymmetric supercapacitor based on a MnO2/ rGO composite as the positive electrode and a MoO3/rGO hybrid film as the negative electrode.203 In this work, the good conductivity of the rGO coupled with the high pseudocapacitive metal oxides produced a very good energy density of 42.6 Wh kg−1. According to its wide potential window and various vanadium oxidation states (II−V), V2O5 is another promising candidate for a negative electrode material in asymmetric supercapacitors. However, due to its poor electronic conductivity and high dissolution in aqueous electrolytes, it is challenging for pure V2O5 to achieve high-rate and long-term cycling performance. Qu et al. developed a core−shell structured PPy/V2O5 hybrid material to increase the conductivity and restrain the dissolution of V2O5.307 The PPy/V2O5 hybrid material exhibited a much

macroscopically monolithic activated carbons that possess a high nitrogen content and high surface area (∼1405 m2 g−1).291 Figure 18a shows the high-resolution N 1s and O 1s XPS spectra of the highly functionalized activated carbon with a maximum 7.6 wt % nitrogen. Figure 18c shows the CV curves of functionalized activated carbon, commercial activated carbon, and NiCo2O4/graphene electrodes in a 3-electrode setup. The fabricated hybrid capacitor can be reversibly charged−discharged with an operating voltage of 1.5 V. Recently, Liang et al. developed a sea urchin-like NiCoO2/C hybrid electrode.296 On the basis of this compact porous hybrid electrode structure, a NiCoO2/C composite electrode could result in both high electronic conductivity and fast redox processes due to the shortened ion path lengths. In conclusion, depositing Faradaic materials on conductive carbon is an effective approach to enhance the performance of hybrid capacitors. This approach improves the electrical conductivity and results in a significant enhancement in power density and cycling stability of the hybrid capacitors. Table 4 provides a summary of the compositions and properties of these composite electrodes and the performance in hybrid capacitors. 5.4. All Redox Electrodes-Based Capacitive Asymmetric Supercapacitors

Another category of aqueous-based asymmetric supercapacitors is where both electrodes are pseudocapacitive but the redox reactions are different. The total capacitance (CT) for an asymmetric supercapacitor is calculated according to eq 27 1 1 1 = + CT Cp Cn

50.3 Wh/kg

(27)

where Cp and Cn stand for the capacitances of the positive and negative electrodes, respectively. Therefore, the total capacitance of an asymmetric supercapacitor is mainly limited by the electrode with the lower value of the specific capacitance. As described above in section 5.2 for aqueous electrolyte hybrid capacitors, the negative electrodes used for all of the abovereviewed hybrid capacitors are mainly carbon-based EDLC electrodes, whose specific capacitance is generally much lower than that of the positive electrode. Thus, if one can replace the carbon-based EDLC negative electrode of low specific capacitance with a Faradaic system of higher specific capacitance, this represents a promising approach to increase the total energy density of a capacitive asymmetric supercapacitor. Table 5 summarizes some of the capacitive asymmetric supercapacitors reported which involve different electrode compositions and redox reactions.300−303 In these systems the negative electrode undergoes redox reactions in contrast to 9256


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Figure 19. (a) Schematic illustration of the fabrication procedure for MnO2 and Fe2O3 on carbon cloth in an asymmetric supercapacitor device. Reproduced with permission from ref 299. Copyright 2014 American Chemical Society. (b) Comparative three-electrode CV curves of an MnO2/ single-walled CNT hybrid electrode and an In2O3/single-walled CNT hybrid electrode. (c) CV curves of asymmetric supercapacitors under different working voltages. Reproduced with permission from ref 46. Copyright 2010 American Chemical Society.

better rate capability and higher specific capacitance (308 F g−1) than that of V2O5 itself (162 F g−1). With activated carbon as the positive electrode material, a full asymmetric supercapacitor was assembled in 0.5 M L−1 K2SO4 electrolyte that could be cycled 10 000 times with only 5% capacitance decrease, which is much better than the 17.5% capacitance loss found in a V2O5/activated carbon asymmetric supercapacitor. The state and viscosity properties of the electrolyte are another key factor affecting the flexibility of a supercapacitor. As illustrated in Figure 19a, Yang et al. fabricated a quasi-solid-state flexible asymmetric supercapacitor by growing MnO2 nanowires and Fe2O3 nanotubes on flexible carbon fabric serving as the positive and negative electrodes, respectively.299 The resulting flexible asymmetric supercapacitor exhibited a relatively high volumetric energy density of 0.55 mWh cm−3 with an extended potential window of 1.6 V. Another flexible system was reported by Chen et al., who fabricated an asymmetric supercapacitor based on flexible singlewall CNT films. In this work, MnO2 nanowires served as the positive pseudocapacitive electrode material and In2O3 as the negative pseudocapacitive electrode material (Figure 19b). On the basis of this design, the potential window of the asymmetric supercapacitor can be extended up to 2.0 V (Figure 19c). The energy density can be improved to 25.5 Wh kg−1 with a Coulombic efficiency of ∼90%. The conducting polymer, polyaniline, can also serve as a negative electrode.309,310 This was demonstrated in an asymmetric device in which RuO2 served as the positive electrode. The fabricated asymmetric supercapacitor exhibited a good energy density of 26.3 Wh kg−1.

magnitude lower than that of aqueous electrolytes. However, as previously mentioned, these nonaqueous electrolytes can reversibly operate over much wider stable electrochemical working voltages and temperature ranges (e.g., from −30 to 70 °C) than those of an aqueous system. This wide range of voltage stability is a key asset in developing next-generation high energy density asymmetric supercapacitors. Currently, the maximum voltage of a normal EDLC, consisting of activated carbon for both electrodes, is roughly 2.7 or 3.5 V by using an organic electrolyte or an IL, respectively. Hybridizing battery-type and pseudocapacitive materials can overcome the capacitance limitation of the existing EDLC system. The hybrid materials are mainly charged by a Faradaic reaction, including lithium/sodium hosts, nickel/cobalt hydroxide, and pseudocapacitive materials.11,39 Usually the nonaqueous hybrid capacitor system needs to combine a batterytype active material with an EDLC capacitive material. To integrate the high-speed electrochemical processes of an EDLC with the high energy capacity of a battery-type material, the microstructure, capacity, and potential balance between these two types of materials should be carefully designed. In particular, the battery Faradaic reaction rates should be improved to match a similar level of electrochemical dynamic speed with EDLC process. Evans Capacitor Co. patented the term “hybrid capacitor” to describe an electrolytic capacitor using a pseudocapacitive negative electrode.313 We will next discuss nonaqueous electrolyte-based asymmetric supercapacitors by classification of metal-ion capacitors and intercalation pseudocapacitance-based capacitive asymmetric supercapacitors.

6. REDOX-ACTIVE ELECTRODE-BASED NONAQUEOUS ASYMMETRIC SUPERCAPACITORS Nonaqueous electrolytes, including organic electrolytes and ILs, have an ionic conductivity that is typically 1−2 orders of

6.1. Li-Ion Capacitors

Among the various nonaqueous asymmetric supercapacitors, Liion capacitors have attracted the most attention.35,37,41,49,160,314 A typical Li-ion capacitor consists of a Li-ion battery-type anode 9257


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Figure 20. (a) GCD curve of the negative (blue) and positive (black) electrodes and the full cell (dotted line) for the designed hybrid capacitor. (b) Cycle life of hybrid capacitors cycled between 1.5 V and different maximum cutoff voltages (Umax). Reproduced with permission from ref 170. Copyright 2008 Elsevier Ltd.

increase the capacity by Li-ion intercalation/deintercalation, while the positive electrode was activated carbon to accelerate the charge-storage dynamics. Replacing the metal oxide negative electrode with a graphite positive electrode can provide several advantages. As shown in Figure 20a, the positive potential window of the activated carbon electrode can reach 4.7 V vs Li+/ Li0, whereas the negative part of the graphitic carbon can reach as low as 0.1 V vs Li+/Li0. The weight ratio of activated carbon to graphitic carbon had significant effects on the capacitance, voltage, and energy density. After carefully balancing the respective masses of the two electrodes, this Li-ion capacitor could achieve a high operating voltage of 4.5 V and maintain over 85% of its initial discharge capacitance after 10 000 charge/ discharge cycles (Figure 20b). In addition, this hybrid device exhibited ultrahigh gravimetric and volumetric energy densities of 103.8 Wh kg−1 and 111.8 Wh L−1, respectively. The flat discharge profile of the graphite negative electrode enables improvements to the Li-ion capacitor by allowing higher utilization of the positive electrode within an expanded voltage window. However, this graphite/activated carbon hybrid system still suffers from a drawback of the lack of a lithium-ion source. Thus, the prelithiation of the graphite was thought to be a necessary step prior to charging the capacitor.51,320 Recently, Brousse and co-workers innovatively demonstrated a unique approach by using a sacrificial lithiated organic salt, namely 3,4dihydroxybenzonitrile dilithium (Li2DHBN) as the Li-ion source.321 In their work they demonstrated that this lithiated salt can supply Li ions to intercalate into the graphite at the first charging step, although Li2DHBN is an almost insoluble Li-ion salt. However, after its removal of Li ions, the formed product 3,4-dioxobenzonitrile (DOBN) is soluble. Then the Li2DHBN particles can be mixed with carbon materials and form Li2DHBN hybrid material to serve as the positive electrode for a Li-ion capacitor. This Li-ion capacitor with the combination of a sacrificial lithium salt and activated carbon could sufficiently maintain high energy density (40−60 Wh kg−1) as well as longterm cycling stability (5000 cycles). In addition, this method not only eliminates the complicated prelithiation process for the graphite electrode but also provides the opportunity to develop a full carbon-organic-based Li-ion capacitor while avoiding the loss of potentially scarce and costly metal-containing chemicals, thus forming a cell comprised entirely of recyclable components. All of the above-discussed Li-ion capacitors are based on liquid organic electrolytes, which often result in leakage of electrolyte and other safety issues. In this regard, replacing the liquid electrolyte by a gel quasi-solid-state electrolyte without

as the negative electrode and an EDLC-type material as the positive electrode in a Li-containing organic electrolyte, such as LiPF6 or LiClO4 in organic solvents.159 Li-ion capacitors usually take advantage of both Faradaic and capacitive mechanisms to attain higher energy densities than EDLCs and higher power densities than Li-ion batteries.160 Specifically, they combine the energy-storage mechanisms of a supercapacitor with anion physical adsorption/desorption onto/from carbon electrode surfaces for the positive electrode and Li-ion intercalation/deintercalation into/from Li-ion acceptors for the negative electrode. Examples include hydroxides (e.g., β-FeOOH51), transition-metal oxides (e.g., V2O5,315 α-MnO2,188 bronze TiO2,316 or Li4Ti5O12),317 and polyanions (e.g., TiP2O7 or LiTi2(PO4)3).41 In 2001, Amatucci et al. replaced the activated carbon negative electrode in a symmetric EDLC by a Li4Ti5O12 electrode with a standard Li-ion organic electrolyte battery to build the first Li-ion capacitor.318 Because the typical Li-ion intercalation potential of a Li4Ti5O12 electrode is limited to 1.5 V vs Li+/Li0 and the maximum stable potential of the positive activated carbon can reach 4 V vs Li+/Li0, the fabricated Li-ion capacitor can result in a full cell operating voltage of 2.5 V. The use of a lower voltage negative electrode such as a transition-metal nitride, a conventional oxide, or a low-voltage intercalation oxide could increase the energy density significantly. An example of the improvements that can be achieved is based on the intercalation compound WO2.318 WO2-based lithiation occurs at approximately 0.7 V vs Li+/Li0. A properly balanced Li-ion capacitor was fabricated by using an activated carbon positive electrode and a WO2 negative electrode. Compared with a typical Li4Ti5O12/activated carbon Li-ion capacitor, the Li ions intercalate into the WO2 at 1 V more negative than Li4Ti5O12. This decrease in negative electrode potential results in an output voltage of 3 V. The use of a WO2 negative electrode increases the energy density of the Li-ion capacitor by 30% compared to the Li4Ti5O12 negative electrode. Another good Li-ion capacitor example is to replace a graphitic carbon electrode with an activated carbon electrode, which results in the switch from a symmetric activated carbon// activated carbon EDLC to an activated carbon//graphite hybrid capacitor in a nonaqueous Li salt electrolyte.51,319,320 Khomenko et al. designed a high energy density Li-ion capacitor by using commercial grades of graphite and activated carbon serving as the negative and positive electrode materials, respectively, in a LiPF6/EC/DMC organic electrolyte.170 Graphitic carbon was chosen as the negative electrode to 9258


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conductivity (σ ≈ 10−3−10−5 Ω−1 cm−1), CNTs were introduced to form an entangled V2O5/CNT nanowire composite. The electrochemical Na+ intercalation/deintercalation process of the V2O5 electrode can be expressed by eq 28

sacrificing the Li-ion capacitor performance would be a promising approach. Wang et al. reported a quasi-solid-state Li-ion capacitor by using a Li-ion conducting polymer gel, poly(vinylidene difluoride-co-hexafluoropropylene) [P(VDFHFP)], as the electrolyte.322 Combined with an electrochemically exfoliated graphene positive electrode and a graphenewrapped porous TiO2 hollow microspheres composite material as the negative electrode, the resulting quasi-solid-state Li-ion capacitor can achieve a high energy density of 72 Wh kg−1 based on the total mass of the positive and negative electrodes. It can also reach a rapid charge/discharge capability within 1 min and relatively long cycle life (1000 cycles) using a Li-ion conducting gel polymer electrolyte with a high ionic conductivity (0.3 mS cm−1). Lithium vanadate (Li3VO4) is another promising negative material for Li-ion batteries, which exhibits a safer discharge plateau than graphite and a lower potential plateau as well as a higher capacity than those for Li4Ti5O12 and TiO2. Its operating potential occurs at 0.5−0.8 V vs Li+/Li. On the basis of this operating potential, Li3VO4 could avoid the formation of lithium dendrites and achieve a high operating voltage and high energy density when assembled into full cells. The same group also developed another quasi-solid-state Li-ion capacitor with Li3VO4/carbon nanofibers as the negative electrode.323 The obtained Li-ion capacitor can achieve an energy density of 110 Wh kg−1 and a maximum power density of 3.9 kW kg−1 with the same Li-ion conducting gel polymer electrolyte.

V2O5 + x Na + + x e− ↔ NaxV2O5

(28)

The reversible charge storge of the V2O5/CNT composite is realized by cycling the insertion and extraction of Na+ at a potential of 1.5 V. The interpenetrating structure of CNT and V 2 O 5 nanowires could lead to the formation of an interconnected network, which provides effective ions transport channels and abundant spaces for accessing redox-active sites. In addition, the conductive CNT network can enhance the electronic conductivity. On the basis of the short ion intercalation/deintercalation distance of the V2O5 nanowires and the fast electron transport, this V2O5/CNT hybrid electrode exhibited a fast ion intercalation pseudocapacitive behavior without phase transformations during the Na-ions insertion/ extraction. However, the intrinsic instability of the organic Naion electrolyte could result in safety issues due to flammability, the possibility of leakage, and internal short circuits. Wang et al. developed a Na-ion conducting gel polymer electrolyte-based quasi-solid-state Na-ion hybrid capacitor.329 A 3D macroporous graphene was prepared by microwave-assisted electrochemical exfoliation in an aqueous electrolyte combined with a hard template for use as the positive electrode. Disordered carbon nanoparticles with a large amount of nanoporosity were obtained by the carbonization−activation of polypyrrole with KOH and served as the negative electrode. In the quasi-solidstate Na-ion capacitor, Na ions can intercalate/deintercalate into/out of the disordered carbon, while the ClO4− anions are absorbed/desorbed on/from the surface of the modified graphene. The full device can operate at a voltage of 4.2 V, achieve an energy density of 168 W h kg−1 based on the total mass of the positive and negative electrodes, and retain 85% of the initial discharge capacitance even after 1200 cycles.

6.2. Na-Ion Capacitors

Due to the increasing prices of lithium and cobalt often used in commercial Li-ion batteries, other metals used for ions intercalation energy storage have attracted increasing attention in recent years. Due to several advantges, such as the large natural abundance of sodium (Na), the low cost of sodium carbonate (Na2CO3) and sodium salt electrolytes, and similar intercalation chemistry to that of the Li-ion system, Na-ion batteries have been considered an emerging technology to partially replace some applications where low cost is needed. Moreover, the standard Na/Na+ redox potential (2.71 V) is fairly close to that of the Li/Li+ (3.02 V) system.324 Additionally, since the solvated sodium cation is slightly smaller it has a relatively faster ion diffusion speed with lower viscosity and higher ionic conductivity than that of solvated Li ions.60 The first Na-ion capacitor was developed in 2012 based on a V2O5 nanowire/CNT hybrid electrode.325 Generally, Na-ion capacitors have a similar configuration to Li-ion capacitors. With the successful application of graphite as the Li-ion battery negative electrode there is some expectation that carbonaceous materials could be used for the negative electrode of Na-ion batteries or Na-ion capacitors.60 However, it has been reported that sodium can hardly intercalate into graphite due to the unfavorable mismatch between the graphite lattice structure and the size of the solvated Na ions,324 which has hindered its application as a negative electrode for Na-ion systems. Thus, the Na-ion capacitor negative electrode must be designed to be compatible with Na-ion insertion, for example, using crystalline structures with large tunnels, such as the layered oxide V2O5. Many layered metal oxides possess large tunnels that are favorable for ion insertion leading to promising capacity values. For example, Dunn et al. reported a high-rate Na-ion capacitor based on an α-V2O5 nanocomposite electrode.325 V2O5 is a layered transition-metal oxide with a large lattice spacing along (001) (lattice distance ≈ 9.5 Å) that can accommondate various metal ions.326−328 To overcome its disadvange of low

6.3. Intercalation Pseudocapacitive Materials-Based Electrodes for Nonaqueous Capacitive Asymmetric Supercapacitors

In this section, we will discuss asymmetric supercapacitors in which the Faradaic electrode is comprised of an ion intercalation pseudocapacitive material rather than a Li-ion or Na-ion battery material. Such materials could also exhibit pseudocapacitance based on structures supporting fast ion insertion in a nonaqueous electrolyte. Transition-metal oxides (such as orthorhombic-Nb2O5 (T-Nb2O5),150,330 molybdenum oxide (MoO3),331,332 vanadium oxide (V2O5),156,315 and titanium oxide (TiO2)157,316) could exhibit ion intercalation pseudocapacitive behavior in a nonaqueous lithium-ion electrolyte.128 With these materials, the rate at which ion intercalation occurs is comparable to the rate of redox pseudocapacitive reactions, leading to pseudocapacitive electrochemical features but with charge storage arising from Li-ion intercalation and no phase transitions. The layered molybdenum and vanadium oxides have been studied for over 40 years as possible cathode materials for Li-ion batteries.333 The layered structures of molybdenum and vanadium oxides contain predominantly MO6 octahedra. These octahedra normally share corners and/or edges to form two-dimensional (2D) sheets, which could be accessible for fully reversible ion intercalation. For example, molybdite (α-MoO3) is a layered material, which could provide Li-ion storage to 1.5 Li 9259


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use as a pseudocapacitive electrode.148 This composite electrode exhibited a high T-Nb2O5 loading (74.2%) and had an excellent volumetric capacitance of over 960 F cm−3. A nonaqueous electrolyte asymmetric supercapacitor prepared with the TNb2O5/rGO composite film displayed good energy density of 47 Wh kg−1 and power density of 18 kW kg−1. V2O5 is an example of an extrinsic pseudocapacitive material as it exhibits capacitorlike energy-storage properties when fabricated into nanostructure with mesoporous morphologies.156,315,325 Chen et al. fabricated a Na-ion-based asymmetric supercapacitor which operated at a voltage of 2.8 V and had a maximum energy density of ∼40 Wh kg−1. The energy density of supercapacitors can be improved significantly by using nonaqueous high-voltage electrolytes and the selective use of the appropriate existing or new negative and positive electrode materials. In addition, the EDL capacitive electrode can also increase the rate performance and cycling stability.

ions per Mo. In 2010, Brezesinski et al. fabricated a mesoporous MoO3 thin film with iso-oriented nanosize crystalline domains by using a block polymer template-based evaporation-induced self-assembly process to provide increased Li-ion intercalation kinetics and cycling stability.332 They discovered that the isooriented nanosized crystalline structure can lead to good intercalation pseudocapacitive charge-storage capabilities. The charge/discharge rates of this ion intercalation pseudocapacitive process are comparable to traditional redox pseudocapacitive electrodes because of their shortened ion diffusion lengths. In 2017, the same group reported that the reduced form of αMoO3−x, which contains oxygen vacancies, exhibits a larger interlayer spacing along the b axis that promotes faster chargestorage kinetics331 This increased interlayer spacing enables the α-MoO3 structure to be retained during the insertion and removal of Li ions. At the same time the reduced α-MoO3−x exhibits a higher specific capacity and improved rate capability as a result of the formation of oxygen vacancies. The high specific capacity in reduced α-MoO3−x can be explained by increased Mo4+ formation following lithiation, a process that occurs reversibly without conversion to monoclinic MoO2. V2O5 is another active material that can undergo intercalation pseudocapacitance in nonaqueous electrolytes. Crystalline V2O5 is known to form an orthorhombic layered structure wherein VO5 square pyramids share adjacent corners and edges. The V2O5 usually comprises bilayers separated by a characteristic interlayer distance that can serve as a location for intercalating guest species. Sathiya et al. coated a 4−5 nm thick layer of V2O5 onto multiwalled CNTs, resulting in a V2O5/CNT composite.156 At the rate of 1 C, the capacitive behavior dominates the charge-storage mechanism, in which 2/3 of the overall discharge capacity is contributed by an ion intercalation pseudocapacitive process. The researchers confirmed the value with the Trasatti theory and studied the valence states of vanadium during charge and discharge, demonstrating that 85% of the vanadium was in the +4 oxidized state after discharging at the 1 C rate. The high capacity value of this V2O5/CNTs hybrid electrode is attributed to the 4−5 nm very thin thickness of the V2O5 film, which only contains 8−10 stacked V2O5 layers. Such a nanostructure provides very short diffusion pathways for ion intercalation and provides easy access for Li-ion insertion, enabling a fast ion intercalation pseudocapacitance associated with the reduction from V5+ to V4+. There are only a few reports for asymmetric supercapacitors with nonaqueous electrolytes that are comprised of an intercalation pseudocapacitive negative electrode in combination with a carbon-based EDLC material. In view of the kinetic behavior of the pseudocapacitive energy-storage process, it is likely that nonaqueous asymmetric supercapacitors with pseudocapacitive electrodes will generally exhibit better rate capability and cycling stability than Li-ion capacitors. Wang et al. prepared a layered H2Ti6O13 nanowire pseudocapacitive electrode, which exhibited a high capacitance of 828 F g−1.334 A nonaqueous asymmetric supercapacitor was fabricated based on using this material as the negative electrode and an ordered mesoporous carbon as the positive electrode. The device was cycled at operating voltages up to 3.5 V, where it exhibited a maximum energy density of 90 Wh kg−1 and achieved a highest average power density of 11 kW kg−1. A second pseudocapacitive material, T-Nb2O5, is also known for its rapid Li-ion intercalation properties and represents another stable, high-rate pseudocapacitive electrode material.147,151,335 Kong et al. developed a free-standing T-Nb2O5/rGO composite film for

7. OTHER ASYMMETRIC SUPERCAPACITORS 7.1. EDLC-Based Asymmetric Supercapacitors

Asymmetric supercapacitors can also use the same type of active materials with different loading mass ratios, different functional groups, and/or different active ions between the two terminals to extend the voltage range for the full cell. The capacitive potential ranges of the positive and negative electrodes are crucial in determining the overall voltage and electrochemical performance. Within the capacitive potential range, the electrochemical processes of the active materials are highly electrochemically reversible and capacitive in nature. This potential range is before the onset of any irreversible reaction of the electrode and/or decomposition of the electrolyte. As mentioned previously, the charge amount stored, Q, in each positive and negative electrode in an asymmetric supercapacitor must be balanced, as described in eq 31, which is derived from eq 21 Q = CpEp = CnEn

(31)

Thus, En = EpCp/Cn. On the basis of eq 15 Edevice = Ep − En Edevice = Ep(1 − Cp/Cn)

(32)

where Cp and Cn are the capacitances and Ep and En are the potentials of the positive and negative electrodes. On the basis of eq 32, the cell operating voltage Edevice can be optimized by adjusting the capacitance ratio between two electrodes. The Chen group discovered this interesting principle and adjusted the capacitance ratio of the electrodes to optimize the overall energy density for asymmetric supercapacitors.44,186,336,337 For example, they control the capacitance ratio between two Cabot Monarch 1300 pigment black (CMPB) carbon electrodes to optimize the operating voltage.336 For the symmetric cell, the maximum operating cell voltage was ∼1.60 V, with a capacitance ratio of 1 to 1. Above this voltage they observed a continuous capacitance decay during the cycling measurement. After a loading mass ratio adjustment to achieve a corresponding capacitance ratio of Cp/Cn = 4:3, the potential range of the negative electrode was successfully extended toward a more negative direction. Meanwhile, the overall full cell operating voltage was increased from 1.60 to 1.90 V. With the 1.9 V operating voltage, 85% of the initial capacitance was retained even after 10 000 cycles. After optimizing the operating voltage to 1.9 V, the energy density was increased by over 38% in 9260


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Figure 21. (a) Schematic drawing showing the setup used for testing the electrolyte-based asymmetric supercapacitors. (b) Example of corresponding CV curves for the entire cell (black), counter electrode (red), and working electrode (blue). (c) CV curves for cells assembled with mixtures of EMI− TFSI and EMI−BF4 with a volumetric 20% of EMI−BF4; operating potential window in all cases is 2.5 V, and three scan rates are shown (5, 10, and 20 mV s−1). (d) Models of ions contained in the ionic-liquid electrolytes, including EMI+, TFSI−, and BF4−. Reproduced with permission from ref 341. Copyright 2013 John Wiley and Sons.

electrodes, as the ions forming the electrochemical double layer would interact with less solvent molecules. Another interesting method for extending the operating potential window of a supercapacitor is to use a mixed electrolyte system and adjust the different ratios of cations and anions.341 Van Aken et al. used onion-like carbon as the active materials for both electrodes and two commercially available ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI) and EMI-BF4, for the electrolyte. A test cell was assembled (Figure 21b), and the potential windows for pure or mixed electrolytes with different ratios of TFSI− and BF4− were studied in both full cell and halfcell configurations (Figure 21c and 21d). This asymmetric device in a 20% EMI−BF4 mixed electrolyte showed a much larger operating potential window and better cycling stability than a pure ionic liquid electrolyte.

comparison with a similar cell structure and equal electrode capacitances at 1.60 V. In some state-of-the-art EDLC systems, activated carbon is used as the electrode material with a typical organic electrolyte, such as tetraethylammonium tetrafluororoborate (Et4NBF4) in PC as the electrolyte.338,339 This supercapacitor can lead to an operating voltage of 2.8 V. However, when the voltage is increased over 2.8 V, the EDLC exhibits a significant decrease in cycle stability. Such a decrease is mainly the result of degradation of the electrolyte components. With the same organic solvent, the maximum operating voltage of the electrolyte strongly depends on the selection of the conductive anion/cation salt and their composition ratio. The physical-chemical properties, electrochemical and chemical stability, and interaction with carbon surfaces of these ions can have a dramatic impact on the stable potential window of the EDLCs.340 Pohlmann et al. extended the maximum operating voltage of a PC-based electrolyte by selecting different anions and cations.169 The final operating voltage was increased up to 3.5 V by using Pyr14+ and TFSI−. A possible explanation of the higher electrochemical stability observed for the new electrolyte system could be that the new anion/cation combination displays a high distribution of charge and exhibits less interaction with the polar solvent molecules. This would result in a decreased concentration of solvent molecules directly at the charged surfaces of the

7.2. Asymmetric Supercapacitors Based on Different Redox Functional Groups

V. Khomenko et al. created an aqueous asymmetric supercapacitor based on different activated carbon sources with varying amounts of oxygen groups and different pore size distributions.342 As shown in Figure 22a, the two types of activated carbon, labeled as Aox and Box, can be optimized for their electrochemical performance in different potential windows due to the two kinds of surface functional groups with different pseudocapacitive redox reactions. Consequently, 9261


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Figure 22. Asymmetric supercapacitors based on carbons functionalized with different redox functional groups. (a) CV curve comparisons based on a three-electrode cell with activated carbons Aox and Box in 1 mol L−1 H2SO4. (b) GCD curves of the asymmetric Aox/Box capacitor in 1 mol L−1 H2SO4 and potential variation for the Aox positive and Box negative electrodes with an Hg/Hg2SO4 reference electrode. (c) Ragone plots for this asymmetric Aox/Box supercapacitor with an operating voltage of 1.6 V and for the Aox/Aox and Box/Box symmetric supercapacitors with a voltage of 0.8 V. (d) Illustration of the specific discharge capacitance vs the cycling number of the Aox/Box asymmetric supercapacitor at 1.6 V and for the Aox/Aox symmetric supercapacitor at a voltage of 0.8 V. Reproduced with permission from ref 342. Copyright 2008 Elsevier Ltd.

deviation of the zero voltage potential (E0 V) from the middle of the electrolyte stable potential window. The E0 V is the potential position when the potential distributions between negative and positive electrodes are equal. The E0 V has a significant influence on the Coulombic efficiency and maximum charging voltage of a supercapacitor. Dai et al. illustrated the relationship between the capacitive potential range and the potential of zero voltage and demonstrated their effects on the maximum charging voltage.337 If one of the potential windows of the two electrodes exceeds the limit of stable potential, the total device will suffer electrolyte decomposition. In the example illustrated in Figure 23, the initial potential of both electrodes measured at E0 V is about 3.0 V vs Li+/Li0, which is close to the

combining these two activated carbon materials in an asymmetric cell allowed the designed device to have a stable operating voltage up to 1.6 V (Figure 22b) without any destructive oxidation of the activated carbon or electrolyte decomposition. After an additional adjustment of potential and capacitance by mass balancing the two electrodes, the aqueous electrolyte-based asymmetric supercapacitor achieved an energy density of ∼30 Wh kg−1 (Figure 22c). This value is much higher than that of 14.3 and 21.7 Wh kg−1 obtained from symmetric devices of A/A and B/B. Figure 22d shows the cycle life of the asymmetric supercapacitor at a current density of 1 A g−1 for 10 000 cycles. A small decrease (15%) in the discharge capacitance was observed during the first 2000 cycles. Afterward, the specific capacitance remained almost constant during the subsequent 8000 cycles, indicating a high stability for this asymmetric supercapacitor. 7.3. Surface Charge Optimization

The theoretically stable potential windows of many carbonatebased electrolytes and ionic liquids are in the range of 4.5−6 V when carried out with Pt or glassy carbon electrodes.343 However, the maximum potential window of supercapacitors is always lower than this expected voltage window and rarely extends beyond 4.0 V. As previously mentioned, one of the main reasons for the narrower operating voltage is parasitic redox reactions due to impurities in the electrolyte and active materials. In addition, during operation of a supercapacitor, the charge (Q) between the positive and the negative electrodes has to be balanced. Thus, if the capacity of one electrode is lower than that of the counter electrode, this electrode will compensate by operating over a higher potential window to balance the charge between the two electrodes. This results in an uneven distribution of the total window across the two electrodes.186 This common phenomenon will lead to a

Figure 23. Positive and negative electrochemical potential variation during tuning E0 V. Adjusting the E0 V can increase the specific capacity and working voltage simultaneously to enhance the energy density. Reproduced with permission from ref 344. Copyright 2013 John Wiley and Sons. 9262


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Figure 24. Hybrid capacitors based on redox active electrolytes. (a) Schematic illustration of the device structure and charge-storage mechanisms of a supercapacitor with a redox-active electrolyte. Redox couples functionalized at the surface of the positive electrode (which is oxidized on charging) are highlighted in blue (catholyte), and couples working at the negative electrode (which is reduced on charging) are highlighted in red (anolyte). (b) Redox potentials of various candidate redox couple highlighting regions of the thermodynamic stability window of water (pH 13). FMN, flavin mononucleotide; TEMPOL, 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl; DHAQ, 2,6-dihydroxyanthraquinone; MV, methyl viologen; Fe(CN)6, ferrocyanide. Reproduced with permission from ref 352. Copyright 2016 Springer Nature.

8. REDOX-ACTIVE ELECTROLYTE-BASED HYBRID CAPACITORS

upper limiting potential for electrolyte decomposition. Weng et al. demonstrated that controlled electrochemical charge injection could influence the surface charge density of the carbon active materials and thereby adjust the potential ranges of the electrodes.344 As shown in Figure 23, by tuning the surface chemical structure, the E0 V moved to a lower potential position and the operating voltage of the supercapacitor increased up to 4.5 V. Theoretically, optimizing the surface charge of each electrode is another way to extend each end of the stable voltage range for both negative and positive electrodes. A similar method has also been applied in an aqueous system to maximize the operating voltage by optimizing the surface charge.345

In addition to developing high-performance active electrode materials, the improvement of capacitance can also be achieved by introducing a redox-active electrolyte into the supercapacitor system. After replacing the inert electrolyte by a redox-active electrolyte, extra capacity will be contributed by high-speed solution-phase Faradaic reactions and maintain the high rate capability and cycling stability. In this case, the redox-active electrolyte will mainly contribute to the charge-storage capability, based on the redox electrolyte species, as shown in 9263


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Figure 24b, such as Cu2+,194,346 iodine,347,348 bromine,349 hydroquinone,350 VOSO4,347,351 or flavin mononucleotide.352 In order to realize stable pseudocapacitive charge storage and maximize the operating voltage and energy density based on the redox-active electrolyte hybrid capacitors, there are some requirements for the redox-active electrolytes: (1) The redox species should have a high solubility with high concentrations, ideally >1 M, to guarantee a charge-storage capacity contributed to by the interfacial redox electrolyte reaction; (2) the electrolyte should have sufficient electrolyte ion transport kinetics in the chosen electrolyte solvent to guarantee good rate capability and power density; (3) the redox couple should be oxidized and reduced separately on the positive and negative electrodes to avoid the shuttle effect, cross diffusion, and selfdischarge between the reducing and the oxidizing species; (4) the redox species should be reversibly and stably reacted to achieve long cycle; and (5) the individual redox potentials of the two species in the designed system should reach boundaries of the electrolyte solvent decomposition potential to maximize the full cell operating voltage. Although the redox species in the electrolyte are the active materials that contribute to the main charge-storage capacity of redox-active electrolyte hybrid capacitors, the electrochemical performance, such as capacitance, energy density, and power density, are mainly evaluated based on solid electrodes (mass, volume) rather than the liquid redox-active electrolyte for most published literature. This calculation principle makes the performance comparable with other supercapacitor systems.

3I−1 ↔ I−3 1 + 2e−

(33)

2I−1 ↔ I 2 + 2e−

(34)

2I−3 1 ↔ 3I 2 + 2e−

(35)

As shown in the equations, one interesting aspect of these polyiodide electrochemical redox reactions is that both I− and In− are negatively charged. In this case, these iodide ions can not only serve as a redox-active source for enhanced charge capacity but also keep electrostatically absorbed onto the surface of the electrode to contribute more EDLC. The development of this iodide electrolyte hybrid capacitor opens up a novel approach to extend the energy density of an asymmetric supercapacitor. However, the stable potential window for the iodine redox reaction is in a very narrow potential range, only 0.17 V. Although it could supply a high capacity of 313 C/g in such a narrow potential range, the overall energy density of the full device will still be limited by the restricted operating voltage. In addition, this iodide species only provides the redox-active species at the terminal of the positive electrode. Thus, the metal ions with redox activities should also be developed to improve the capacity of the negative electrode, Figure 25.

8.1. Charge-Storage Mechanisms in Hybrid Capacitors Containing Redox Electrolytes

The general device structure and charge-storage process of a redox-active electrolyte hybrid capacitor is illustrated in Figure 24a. O and R stand for the oxidized and reduced electrolyte redox-active species. The metal cations are highlighted in red, and the anionic species are highlighted in blue. During the charging process, the redox-active electrolyte ions are driven by the applied voltage to become firmly absorbed into the micropores of the high surface carbon materials. Then the electron transfer via redox reactions of the oxidized and reduced species will occur across the EDL and significantly increase the charge-storage capacity compared with traditional EDLC systems with an inert electrolyte. The surface properties of the active carbon materials, such as microstructure and the surface functional groups, will also have a significant influence on the chemical adsorption capability and wettability of the redox electrolytes. In the following sections, we separately discuss the charge-storage mechanism of redox-active anionic and metal ion species in detail. 8.1.1. Charge Storage by Sorption and Redox Reactions of Anionic Species on the Electrode Surfaces. Anionic species, such as iodide, bromide, and hydroquinone, have been successfully demonstrated as active species for redox electrolyte hybrid capacitors. Take iodide for example, where M represents one type of alkali metal, an activated carbon/MI hybrid electrode was claimed to exhibit an extremely high capacitance of >1000 F/g.347 This high-efficiency chargestorage process is based on the adsorption and stable redox reactions of polyiodide ions, In−, which has various oxidation states. In this system, the iodide is not only playing a key role in the Faradaic reactions but also providing a high ionic conductivity media. The redox capacity can be provided according to eqs 33−35353

Figure 25. (a) CV curves of the positive and negative electrodes of activated carbon in 1 mol L−1 KI redox-active electrolyte. (b) GCD curves for the same system with a current density of 0.5 A g−1. Reproduced with permission from ref 353. Copyright 2009 Elsevier Ltd.

8.1.2. Charge Storage Enhanced by Sorption and Redox Reactions of Metal Cations. Compared to electrosorption and redox reaction of anionic species, metal cations (such as cupric ion, Cu2+, ferric ion, Fe3+, and vanadyl ion, VO2+) can also be adsorbed onto a negative electrode and used as redox-active species on the negative side.347,351 The VO2+ ions can be applied as anionic species and be oxidized to VO2+ by a solution redox reaction. In addition, it has been shown that the vanadyl ion can also react with the oxygen functional groups on a 9264


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Figure 26. Electrochemical properties of hybrid capacitors based on two redox electrolytes. (a and b) GCD curves for 1 M KBr/0.1 M MVCl2 and 0.1 M HVBr2 hybrid capacitors with cell potential between 0.8 and 1.4 V, respectively. (c) Energy retention ratio for the different redox-active electrolytebased hybrid capacitors, illustrating the self-discharge performances. (d) Self-discharge performance for inert electrolyte EDLCs. Reproduced with permission from ref 349. Copyright Rights 2015 Springer Nature.

with a 1 mol L−1 vanadyl sulfate (VOSO4) solution to provide redox-active species for the negative and positive electrodes, respectively.347 In this bifunctional redox electrolyte system a high surface area-activated carbon (∼2520 m2 g−1) was used as the active electrode material and a Nafion proton exchange membrane was applied as the separator. It is worth noting that neutral aqueous electrolytes have been applied in this system to avoid using high-cost corrosion-resistant current collectors. The average unit price for 1 kJ received from this capacitor is $1.60, and the price for 1 F is below $0.01. Boettcher and co-workers developed a system using viologen and bromide as the “catholyte” and “anolyte”, respectively.349 The device provides a maximum energy density of ∼14 Wh kg−1. Note that this energy density was calculated based on the total mass of the active materials including the electrolyte. Additionally, there is no ion-selective membrane separator needed for this system. Self-discharge is a big concern for a redox-active electrolyte hybrid capacitor. In this work, they also studied the self-discharge mechanism by comparing the redox-active behavior with various redox-active electrolyte species. They concluded that physical adsorption is found to be the main mechanism to prevent the redox-active ions from cross diffusion and relative self-discharge rather than electrostatic effects. The researchers carried out a systematic study on the stable voltage windows and self-discharge mechanism of separate “catholytes” and “anolytes”. For the redox-active electrolyte system, all redoxactive species should be stable in the electrolyte medium. Because iodide exhibited slower self-discharge behavior compared with bromide, the MVCl2/KI system was first studied. Unfortunately, the mixed redox-active electrolyte formed MV+−I− precipitates during charging, which led to an

carbon electrode surface to form −COOVO+ to contribute extra capacity.347 Apart from vanadyl cations, CuCl2 can also contribute extra charge-storage capacity in an acid solution. The significant capacity increase was attributed by strong chemical adsorption of Cu2+ onto surface functional groups of carbon materials. The redox reaction can be described following eqs 36 and 37194,346 + Cu 2 + + e− ↔ Cu(adsorption)

(36)

+ Cu(adsorption) + e− ↔ Cu(deposition)

(37)

The potentials for these reactions are Cu2+ concentration dependent. Thus, the actual operating voltage of this system should be optimized based on the redox-active electrolyte concentration to maximize the chemical adsorption of Cu2+ and also avoid the irreversible Cu metal electrochemical deposition. Otherwise, some researchers found that adding Fe2+ into the Cu2+/Cu+ redox couple can also increase the charge-storage capacity by forming an Fe2+/Fe3+ redox couple.346 8.2. Redox Electrolyte-Based Hybrid Capacitors with Two Redox Couples

As mentioned in section 8.1.1, although anionic species can offer a very good capacity at the positive electrode, the low capacitance of a pure carbon negative electrode will still hinder the full cell from achieving high charge-storage capacity. In order to make up the disadvantage of the low-capacitance negative electrode, a conjugated redox couple electrolyte has been developed by mixing two different types of electrolytes including both reduced and oxidized redox-active species. Frackowiak et al. mixed together a 1 mol L−1 potassium iodide (KI) solution 9265


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Figure 27. Ionic liquid electrolytes with two redox active species for asymmetric supercapacitors with improved charge storage. (a) Molecular structure of the BMImTFSI IL and the biredox IL include AQ−PFS− and MImC−TEMPO·. (b) CV curves with 0.5 M biredox IL in BMImTFSI and pure IL at a scan rate of 5 mV s−1, respectively. (c) Schematic illustration of the GCD voltage response of the EDLC and redox reaction in the biredox IL. Reproduced with permission from ref 357. Copyright Rights 2016 Springer Nature.

high power density of supercapacitors. The combination of different Faradaic active and capacitive materials should be a viable approach, but a deeper understanding of the synergistic interactions is still required to ensure optimized capacitive performance. This review has summarized the insights into the design, mechanism, fabrication, and evaluation of asymmetric supercapacitors, where the term “asymmetric” is broadly referring to the difference between positive and negative electrodes. Within this definition, the hybrid capacitor has been classified into a specific category of asymmetric supercapacitor, which contains the negative and positive electrodes with different charge storage mechanismsone is capacitive and one is battery type. We have introduced the fundamental considerations related to the energy-storage mechanisms in electrode materials and the performance of asymmetric supercapacitors. Despite the aforementioned impressive progress, innovative approaches are still needed to develop asymmetric supercapacitors with wide operational voltages and sufficient energy density while maintaining high power density and long cycle life, especially for large-scale energy-storage applications. There is still more work that needs to be done to better understand the charge-storage mechanism(s) and achieve higher electrochemical performance for asymmetric supercapacitors. We summarize some perspectives and opportunities for future research directions here. (1) The charge-storage mechanisms need further understanding, including both EDLC and pseudocapacitance. Fundamental studies related to the charge-storage mechanism of EDLCs and pseudocapacitance will be helpful in understanding the physical and chemical processes at the electrolyte/electrode interfaces within an asymmetric supercapacitor and enhance the electrochemical performance. Notably, many significant breakthroughs have been made in the past decade,54,358 especially with regard to the fundamental understanding of the electrolyte ion dynamics inside confined pores 3 5 9 , 3 6 0 and intercalation pseudocapacitance.128,150,331,332 For a long time, researchers thought that the best approach to enhance the capacitance of EDLCs was to increase the specific surface area of the active materials. Yet, if the pore size was made smaller than the size of the solvated ions, the capacitance should decrease. However, this “common sense” understanding was negated by experimental research in 2006 that showed the capacitance increased significantly when the active material pore size approached the size of the desolvated ions.358 Since then many research efforts,

irreversible capacity loss. Figure 26a shows the GCD curves of the cell in 1 M KBr/0.1 M MVCl2 mixed electrolyte at different cell voltages. With a 1 V potential window, all positive, negative, and total curves showed a typical triangular shape, indicating capacitive charge-storage behavior. When the voltage was increased up to 1.4 V, all three curves showed an initial capacitive (linear) process followed by a redox reaction response corresponding to the Br− and MV2+ redox couples. Figure 26c and 26d exhibits the self-discharge measurements for the 1 M KBr/0.1 M MVCl2 hybrid capacitor charged to 1.4 V. Heptyl viologen (HV), another type of viologen, can form strong bonds to electrodes leading to a decrease in the self-discharge rate. Figure 26b and 26c indicates a similar reversible redox behavior for HVBr2 with the KBr/MVCl2 couple but with a lower selfdischarge rate. In addition to aqueous redox electrolyte systems, ionic liquid electrolytes with redox-active species have been developed recently and had high energy density with large potential windows.354−357 As demonstrated in Figure 27a, Fontaine and co-workers reported a biredox-active ionic liquid electrolytebased supercapacitor, where anions and cations are decorated with anthraquinone (AQ) and 2,2,6,6-tetramethylpiperidinyl-1oxyl (TEMPO).357 Figure 27b shows the CV curves for PICAactivated carbon with pure BMImTFSI IL and biredox IL in BMImTFSI. The cell with the biredox IL displayed a similar CV shape but with significantly increased specific capacitances (from 100 to 200 F g−1). Figure 27c schematically illustrates the GCD curve divided by the EDLC and redox reaction processes contributions. This biredox IL system achieved an energy density of 70 Wh kg−1. Although the field of redox electrolyte-based asymmetric supercapacitors is promising, with significant recent progress, it still has not attracted as much attention as other types of asymmetric supercapacitors.

9. SUMMARY AND PERSPECTIVES Supercapacitors represent one of the key elements in energystorage systems having the potential to complement or even replace batteries in a variety of applications. The supercapacitor market is driven by their unique electrochemical characteristics such as high power density, extremely short charging time, and excellent low-temperature performance. Given the rapid improvements of this technology, asymmetric supercapacitors are expected to play a key role in the energy-storage industry. The future of asymmetric supercapacitors looks promising with opportunities in transportation, power tools, and consumer electronics. Research in this area can be expected to flourish with the goal of increasing energy density without compromising the 9266


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(3) Electrolyte optimization is also of equal significance to optimize the overall electrochemical performance of asymmetric supercapacitors. Apart from optimizing the active materials for positive and negative electrodes, the stable potential range of the electrolyte is another key factor in determining the electrochemical performance of asymmetric supercapacitors. Both the electrode and the electrolyte can affect the cell operating voltage. For aqueous electrolytes, the pH, i.e., the concentration of H+ and OH− in the electrolyte, plays a crucial role in determining the final stable potential range of electrolytes.377 This is mainly because the pH has a great influence on the redox reactions and electrolyte decomposition including the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). A neutral electrolyte could lead to much higher overpotentials for both the HER and the OER than acidic or alkaline electrolytes. As a consequence, carbon-based supercapacitors always exhibit higher operating voltages of ∼2.0 V in neutral aqueous electrolytes than the ∼1.0 V available in acidic or alkaline electrolytes. Some groups have even developed an aqueous electrolyte-based supercapacitor with a voltage window extended to 2.0 V.165,167 In addition to neutral aqueous electrolytes, a super highly concentrated electrolyte has demonstrated a highly stable potential window. A large amount of lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) (more than 20 M) was dissolved in water to form a “water-insalt” electrolyte.378 This electrolyte exhibited a wide stable potential window of 3 V with stainless steel working electrodes by pushing the onset potentials of both the HER and the OER beyond their thermodynamic limits. In order to significantly increase the energy density of an asymmetric supercapacitor it is also necessary to develop organic electrolytes or ionic liquids with higher operating potential windows and good ionic conductivity, which can be matched with appropriate pseudocapacitive materials. In addition, developing organic electrolytes and ionic liquids with redox-active species, high electrochemical stability, and a wide operating voltage is another promising approach to enhance the energy density of redox electrolyte-based hybrid capacitors. (4) There remains a strong impetus to explore more sustainable metal-ion capacitors based on earth-abundant metals besides Li and Na. An attractive alternative is potassium (K)-ion-based energy-storage systems. K metal has a lower redox potential (−2.93 V for K+/K vs SHE) compared to Na (−2.71 V for Na+/Na vs SHE). Like Liion and Na-ion capacitors, finding suitable electrode materials holds the key for developing high-performance K-ion capacitors. Recently, a graphite−polyacrylate electrode was proposed as a negative electrode for 4 V K-ion capacitors.379 Sodium polyacrylate was utilized as the binder to improve the solid electrolyte interphase (SEI) passivation. The electrolyte used in this K-ion capacitor was 1 M potassium bis(fluoroslufonyl)imide (KFSI) dissolved in EC/DEC. The high conductivity is very important for ion transport in the electrolyte and at the electrode/electrolyte interface, especially in the application of metal-ion hybrid capacitors. K ions can be intercalated into graphite to form KC8 with a capacity of 279 mA h g−1. However, at a low potential (close to 0− 0.2 V vs K+/K), both K metal plating and intercalation of

including theoretical studies and experimental work, have been dedicated to this field to understand the ion dynamics inside the confined carbon spaces. On the basis of these research efforts, it has been demonstrated that the capacitance and ion dynamics can be significantly affected by charge screening,361,362 ionic rearrangements363 and confinement,364 and pore surface properties,365 such as ionophilic and ionophobic pores. In addition, researchers have demonstrated different ion dynamics during the charging processes,359,360 such as counterion adsorption, ion exchange, and co-ion desorption, and also discovered some novel phenomena, such as initially ions fill pores before the voltage is applied,366 and partial breaking of the ions due to Coulombic ordering.367 However, further research efforts are still needed to provide more insights into these new findings. For example, there is still a lack of solid, systematic experiments to illustrate the influence of pore surface properties on the capacitance and ion dynamics. It is still not clear what are the specific factors that determine the different ion dynamic processes (such as confinement, pore surface properties, or electrolyte properties). What are the key factors that influence the ion Coulombic ordering breakdown and polarization inside confined nanocarbon pores? Pseudocapacitive materials can lead to much higher levels of charge storage than EDLCs, according to their redox reactions and intercalation pseudocapacitance. Pseudocapacitive behavior can also exhibit high energystorage capability even at high discharge/charge rates. Recently developed intercalation pseudocapacitive materials, such as Nb2O5150 and MoO3−x,331 have been recognized as promising for achieving high energy density. Although recent studies have shed some light on the main features of the various mechanisms involved in surface redox reactions and intercalation pseudocapacitance, their molecular level processes still need further study especially at the interfaces of the active material/ electrolyte. More attention should be paid to the study of passivation layer formation at the surface of traditional pseudocapacitive materials, such as RuO2, MnO2 in aqueous electrolytes, or intercalation pseudocapacitive materials in organic electrolytes. These studies will be crucial to enhance stability and extend the potential window of asymmetric supercapacitors. (2) Searching for new materials is essential to develop advanced asymmetric supercapacitors with enhanced electrochemical performance. In the past few years new 2D materials such as MXenes,161,368−370 metal nitrides,306,371 metal organic frameworks (MOFs),372 covalent organic framework (COFs),373 black phosphorus,374 and transition-metal dichalcogenides375,376 have been examined for their potential for high energy density systems. The use of these redox-active electrodes coupled with controllable pore structures offers a great opportunity to create optimized asymmetric supercapacitors. In addition, the selection of positive and negative electrode materials should be optimized to extend the operating voltage of the final device. For hybrid Li-ion capacitors, it is necessary to develop new negative electrode materials with lower potentials and new positive electrode materials with higher capacity to replace activated carbon. 9267


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K ions into the graphite will occur on the negative electrode. In such a scenario the emerging major problem will be the growth of K−dendrites at such low potentials, leading to real safety concerns. Beyond the monovalent metal-ion (Li+, Na+, and K+) systems, multivalent metal-ion (Mg2+ , Al 3+ , etc.) capacitors are of interest due to their potentially higher energy density. Few confirmed instances of multivalent ion storage currently exist. Magnesium is a unique active metal for negative electrodes for batteries, with a volumetric capacity of 3833 mA h cm−3 that far exceeds that of lithium (2046 mA h cm−3).380 Mg deposition, when the right electrolytes are used, does not form dendrites, which is a major issue regarding the use of lithium metal anodes in rechargeable batteries. Owing to its great abundance in the earth’s crust (the eighth most abundant element), Mg can serve as an active element for sustainable energy storage. Yoo et al. developed a prototype hybrid capacitor by using activated carbon as the positive electrode and Mg foil as the negative electrode with a complex electrolyte of ether with a magnesium organo-halo-aluminate (0.25 M Mg2Cl3+− Ph2AlCl2−).380 This hybrid capacitor system demonstrated a stable cycling performance with a specific capacitance of 90 F g−1 with a potential range of 2.4 V. However, for Mg-ion storage there are still some fundamental challenges related to the large desolvation energy of Mg ions in nonaqueous electrolytes, slow solidstate diffusion of Mg ions in intercalation compounds, and moderate compatibility between high-voltage cathodes and electrolyte solutions. Aluminum is another earth-abundant element, which has attracted increasing attention in batteries and hybrid capacitors because of its trivalent ion intercalation reaction and small ionic radius. Reversible Al-ion storage behavior in aqueous electrolytes has been developed for Ti3C2 and the anatase form of TiO2.369,381 The redox reactions of Ti3+/Ti4+ at or near the surface leads to a pseudocapacitive charge-storage process. The first Al-ionbased asymmetric capacitor prototype was demonstrated with a Prussian blue analog as host.382 After this MoO3coated polypyrrole383 and W18O49 nanowires288 were prepared as the negative electrode for Al-ion-based aqueous asymmetric supercapacitors. However, although the cycling stability of the Al-ion-based asymmetric capacitor is excellent, the overall energy density is still limited and needs further improvement by developing a better Al-ion intercalation host. (5) Advanced techniques and in situ experiments are essential to study the complex interfacial processes of EDLC and pseudocapacitance. The development of new techniques has recently improved our understanding of EDLC. Carbon active materials have been explored with more realistic structural models by a combination of coupling modeling and some advanced scattering approaches, such as neutron scattering (SANS)384,385 or small-angle X-ray scattering (SAXS).386,387 Nuclear magnetic resonance (NMR)360,388,389 experiments and molecular dynamics (MD) simulations can give the ion distribution inside the confined carbon pores with or without an applied potential, which greatly improves our understanding of the electrochemical behavior and ion dynamics. Advanced in situ/operando experimental approaches, such

as electrochemical quartz crystal microbalance (EQCM),390,391 in situ X-ray diffraction, spectroscopy characterization, such as NMR360,388,392 or infrared,393 and scattering techniques387,394 can provide a deeper understanding of electrolyte distribution and ion dynamics in carbon-confined pores with/without applied voltages. For pseudocapacitive material, in situ synchrotron X-ray diffraction analysis is a powerful technique to study the structural behavior during charge storage.395 The use of in situ characterization techniques will be a crucial and unique technique to understand the microscopic pseudocapacitive mechanisms derived for oxide materials and for improving their energy density. (6) Theoretical modeling and computational simulation could provide an effective way to understand the active material structures and the ion-wetting properties and transport dynamics. The specific surface area and pore size distributions are thought to be two key parameters to determine the EDLC electrochemical performance of porous carbon materials, which are usually estimated by gas adsorption techniques and theoretical models. However, the real nanoporous structure of a high surface area carbon is more complicated beyond our previous understanding. It is almost impossible to resolve such complex structures using only a single approach, experimental or computational. For instance, highintensity synchrotron X-ray diffraction and SAXS can provide detail on short- and long-domain morphology of carbon materials. Afterward, a hybrid reverse Monte Carlo simulation can transform 1D structural information obtained from scattering techniques into 3D structures.386 NMR spectroscopy results can indicate the ions and solvent dynamic transport between the micropores and the outside of the pores.359 Molecular dynamic simulations could reveal an overall picture of electrolyte ions absorbed into the nanoporous carbon electrode that spontaneously fills the highly confined nanopores.363,364,396,397 This information is critical to precisely study the ion dynamics and charge-storage mechanisms inside the microsize-confined carbon pores. In addition, theoretical analysis and calculations can provide more valuable insights into the charge screening and its influence on the charge-storage mechanism.362,365,398 (7) Device innovation and integration with multifunctionality will attract more attention for future supercapacitor designs. Despite the significant progress made in the preparation of electrode materials, there has been growing interest in the design of highly flexible and wearable energy-storage devices to meet the increasing power demands of modern electronic devices.121,226,286 The rapid development of wearable technology demands wearable power sources such as fiber-type supercapacitors.399,400 These promising devices are anticipated to be able to be knitted into fabrics and, therefore, eliminate the need for conventional heavy battery packs. Future devices are likely to start with flexible energy-storage units that can be directly printed onto textiles for ultrathin roll-up displays and touch screens. At the same time, according to the wide range of current fluctuations, supercapacitors could also be integrated with other energy-harvesting devices, such as solar cells, piezoelectric devices, thermoelectric devices, and other functional devices, such as 9268


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Biographies

displays, sensors, and electrochromics, to build a multifunctional, self-charging wearable platform. (8) Another important parameter that requires our attention is the self-discharge of supercapacitors. While significant efforts are being made to improve the energy density and power density of supercapacitors, the self-discharge phenomenon has attracted little attention. This phenomenon causes supercapacitors to rapidly lose their charge within days, raising concerns over the use of supercapacitors for long-term charge storage.401,402 The selfdischarge and leakage current of supercapacitors should be monitored as this can have a significant impact on how supercapacitors are used in commercial devices.403 Understanding the self-discharge process will allow for intelligent solutions for moderating such phenomenon and may eventually lead to supercapacitors that can store energy more efficiently. In the last few decades, the field of energy storage has attracted intense attention from many talented chemists, physicists, electrochemists, material scientists, and engineers who have used a variety of characterization techniques and made significant progress due to tackling an intriguing set of diverse physical and chemical scientific phenomenon and questions. While many advances in electrochemical energy-storage materials have been achieved over the past few years and are well appreciated, the diversity in materials and systems also underscores the point that the development of asymmetric supercapacitors is still in its infancy. There is the sense, however, that the proper combination of electrode materials and chargestorage processes has the inherent ability to achieve high energy and power densities that are well beyond what lithium-ion batteries and EDLCs can achieve on their own. The continued maturation of new asymmetric systems will require additional research aimed at understanding electrochemical processes which lead to enhanced electrode properties as well as the development of architectures and configurations that enable high-performance devices. The fact that only a few of the pathways for achieving these goals have been identified makes this field an exciting one that will have a significant impact well into the future.

Yuanlong Shao is a research associate in Cambridge University. He received his B.S. degree from Changsha University of Science and Technology majoring in Inorganic Materials Science and Engineering in 2010. He was a joint Ph.D. candidate in the Department of Chemistry and Biochemistry at UCLA and in the College of Materials Science and Engineering at Donghua University. He joined Cambridge University at the beginning of 2016. He has been working on electrochemical capacitor research for over 8 years, published 25 papers in scientific journals, has 6 issued patents, and has been involved in several research projects related to supercapacitors and other energystorage systems. His current research interests are electrochemical capacitors, Li−air batteries, flexible energy-storage devices, and fundamental studies related to ion dynamics in confined spaces. Yaogang Li obtained his Ph.D. degree in Materials Science and Technology from the Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), in 2003. Prior to earning his Ph.D., he worked as an Associate Professor in Taiyuan University of Technology between 1991 and 2000. In 2003, he joined the College of Materials Science and Engineering, Donghua University, and has been working as a full professor since then. Currently, he is the Vice President of the Shanghai Education Evaluation Institute (SEEI) and the Secretary of the Asia-Pacific Quality Network (APQN). His current research interests include low-dimensional carbon materials (including carbon nanotubes and graphene) and their composites for novel magnetic, electronic, and energy-storage devices. Hongzhi Wang received his Ph.D. degree in 1998 from the Shanghai Institute of Ceramics, Chinese Academy of Science (SICCAS), and joined the State Key Lab of High Performance Ceramics and Superfine Microstructure as an assistant researcher for 2 years. Afterward, he spent 2 years as a postdoctoral researcher in the Kyushu Center of the National Institute of Advanced Science and Technology (AIST) in Japan with a Science and Technology Agency (STA) fellowship. He then worked as an AIST fellow in the Micro-Space Chemistry Lab. Since 2005, he has been working as a full professor and vice-dean of the college of Materials Science and Engineering at Donghua University. He leads a research group of over 30 graduate students, and his current research interests include graphene and its composites for electronic and energy-storage devices, electrochromic devices based on transitionmetal oxides, and conducting polymers and novel functional fibers. Bruce Dunn is the Nippon Sheet Glass (NSG) Professor of Materials Science and Engineering at UCLA. After receiving his Ph.D. degree from UCLA in 1974, he was a staff scientist at the General Electric Corporate Research and Development Center in Schenectady, New York. He joined the Department of Materials Science and Engineering at UCLA in 1980. He has published over 250 papers in scientific and technical journals and has been awarded 13 patents with several pending. Among the honors he has received are a Fulbright research fellowship, invited professorships at the University of Paris, the University of Bordeaux, the University of Picardie (Jules Verne), and the Nanyang Technological University (Singapore), and two awards from the DOE for his research in materials science. In 2003 he was named to the NSG Chair in Materials Science and Engineering.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. Fax: +44-1223-748348. Phone: +44-7562-543093. *E-mail: [email protected]. Fax: +86-021-67792855. Phone: +86-021-67792881. *E-mail: [email protected]. Fax: +01-310 2067353. Phone: +01 310 8251519. *E-mail: [email protected]. Fax: +01-310-2064038. Phone: +01-310-8255346. ORCID

Yuanlong Shao: 0000-0002-4950-9911 Jingyu Sun: 0000-0002-9812-3046 Hongzhi Wang: 0000-0002-5469-2327 Bruce Dunn: 0000-0001-5669-4740 Richard B. Kaner: 0000-0003-0345-4924

Richard B. Kaner is the Dr. Myung Ki Hong Endowed Chair in Materials Innovation. After receiving his Ph.D. degree in Inorganic Chemistry from the University of Pennsylvania in 1984, he carried out postdoctoral research at UC Berkeley. He joined UCLA in 1987 as an Assistant Professor, earned tenure in 1991, became a full professor in 1993, and was given the title Distinguished Professor in 2012. He is a Fellow of the American Association for the Advancement of Science, the American Chemical Society, the Materials Research Society, and

Notes

The authors declare no competing financial interest. 9269


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the Royal Society of Chemistry. According to the 2014, 2015, and 2016 Thomson-Reuters rankings, he is among the world’s most highly cited authors. He has received awards from the Dreyfus, Fulbright, Guggenheim, Packard, and Sloan Foundations, as well as the Exxon Fellowship in Solid State Chemistry, the Tolman Medal, and the Award for the Chemistry of Materials from the American Chemical Society along with the MRS Medal from the Materials Research Society for his work on refractory materials, including new synthetic routes to ceramics, superhard metals, conducting polymers, and nanostructured carbon.

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