Redox Additive-Improved Electrochemically and Structurally Robust

Subscriber access provided by UNIV OF SCIENCES PHILADELPHIA

Article

Redox Additive-improved Electrochemically and Structurally Robust Binder-free Nickel Pyrophosphate Nanorods as Superior Cathode for Hybrid Supercapacitors Kalimuthu Vijaya Sankar, Youngho Seo, Su Chan Lee, and Seong Chan Jun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19357 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Redox Additive-improved Electrochemically and Structurally Robust Binder-free Nickel Pyrophosphate Nanorods as Superior Cathode for Hybrid Supercapacitors Kalimuthu Vijaya Sankara,‡, Youngho Seoa,‡, Su Chan Leea, Seong Chan Juna,* a

Nano-Electro Mechanical Device Laboratory, School of Mechanical Engineering, Yonsei

University, Seoul 120-749, South Korea ABSTRACT For several decades, one of the great challenges for constructing a high-energy supercapacitor has been designing electrode materials with high performance. Herein, we report for the first time to our knowledge, a novel hybrid supercapacitor composed of battery-type nickel pyrophosphate one-dimensional (1D) nanorods and capacitive type N-doped reduced graphene oxide as the cathode and anode, respectively, in an aqueous redox-added electrolyte. More importantly, ex situ microscopic images of the nickel pyrophosphate 1D nanorods revealed that the presence of the battery-type redox additive enhanced the charge storage capacity and cycling life as a result of the microstructure stability. The nickel pyrophosphate 1D nanorods exhibited their maximum specific capacitance (8120 mF cm-2 at 5 mV s-1) and energy density (0.22 mWh cm-2 at a power density of 1.375 mW cm-2) in 1 M KOH + 75 mg K3[Fe(CN)6] electrolyte. On the other side, the N-doped reduced graphene oxide delivered an excellent electrochemical performance, demonstrating that it was an appropriate anode. A hybrid supercapacitor showed a high specific capacitance (224 F g-1 at a current density of 1 A g-1) and high energy density (70 Wh kg-1 at a power density of 750 W kg-1), as well as a long cycle life (a coulombic efficiency of 96% over 5000 cycles), which was a higher performance than most of those in recent reports.

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Our results suggested that the materials and redox additive in this novel design hold great promise for potential applications in a next-generation hybrid supercapacitor. KEYWORDS: Redox additive, Structurally robust, Anode, High performance, Hybrid supercapacitor 1. Introduction Over the last few years, research has been conducted on a hybrid supercapacitor as a state-of-the-art or next-generation energy storage device in the supercapacitor family. A hybrid supercapacitor consists of two distinct electrodes, which have capacitive (as the power source) and battery-type charge storage (as the energy source) functions. Fundamentally, the electrode materials are classified according to their charge storage process. A pseudocapacitive (PS) electrode stores a charge based on rapid continuous surface redox reactions within the potential range, where multiple redox states exist in the metal oxides/hydroxides (MnO2, RuO2, MnOOH) used as electrodes. Further, battery-type electrodes also store a charge based on redox reactions. Nonetheless, the redox reaction occurs at a precise potential instead of an extended redox reaction. Over the last few decades, battery-type electrodes have enormously boosted the specific capacitance and energy density of supercapacitors as a result of fast and reversible redox reactions compared to a PS electrode. Different cobalt- and nickel-based materials have been extensively investigated for supercapacitors. However, a battery-type material decreases the cycle life and rate capability.1,2 Hitherto, many approaches have been broadly considered to resolve the problems with battery-type materials.3–5 From the results, we can understand that the controlled synthesis of well-defined nanostructures not only offers basic building blocks in materials science, but also


Page 2 of 38

Page 3 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


supports day-to-day down-to-earth applications. Therefore, the innovative idea of a onedimensional material on a conductive substrate with nanostructures has appealing features. The direct growth of one-dimensional nanostructured materials on a conductive substrate is more favorable for fast electron transport and easy ion accessibility within the electrode, even at a high current. Metal pyrophosphates are the hottest materials in the field of catalysts and energy storage applications because of their extensive working potential window, layered structure, good reversibility, high conductivity, and low cost.6,7 However, very few reports are available on the use of metal pyrophosphates for energy storage applications. Pang et al. prepared and reported amorphous nickel pyrophosphate microrods for a flexible supercapacitor, which exhibited a specific capacitance of 145.8 mAh g-1 at 0.5 A g-1.6 Senthilkumar et al. prepared Ni2P2O7 grainlike particles using a co-precipitation technique and explored their use as a cathode material for a hybrid supercapacitor, which delivered a maximum specific capacitance of 1893 F g-1 at a current density of 2 A g-1.7 Likewise, Co2P2O7 with different nanostructures has been prepared. Among these, hierarchical Co2P2O7 exhibited a high specific capacitance of 367 F g-1 at 0.625 A g-1.8 Khan et al. prepared Co2P2O7 nanosheets and enhanced their electrochemical performance using a redox additive. The maximum specific capacitance obtained was 580 F g-1 at a current density of 1 A g-1.9 Sodium-doped Ni2P2O7 hexagonal tablets were prepared using a calcination method, which delivered a maximum specific capacitance of 557.7 F g-1 at a current density of 1.2 A g-1.10 Amorphous Ni-Co pyrophosphate microplates delivered a maximum specific capacitance of 1259 F g-1 at a current density of 1.5 A g-1.11 In these studies, the metal pyrophosphates were prepared in the form of a powder instead of using the direct growth of different nanostructures, and only one report was available on enhancing the electrochemical 3 ACS Paragon Plus Environment


performance of pyrophosphates using a redox additive. These inadequate studies stimulated us, for the first time, to develop binder-free nickel pyrophosphate one-dimensional (1D) nanorods on Ni foam and improve their electrochemical performance using a battery-type redox additive. An electric double layer (EDL) electrode stores a charge in a double layer formation at the electrode/electrolyte interface as a result of physical adsorption or a non-faradaic reaction, where high surface area carbon allotropes are employed as electrodes. Graphene has been treated as a novel material for energy storage because of its specific benefits compared to other carbon allotropes.12,13 N-doped graphene offers superior physical and electrochemical properties compared to graphene. Likewise, N-doped graphene is chemically inert and has been shown to have high thermal and electrical conductivity, good mechanical strength, and a large surface area.14–17 According to the doping configuration, N-doped graphene shows excellent electrochemical performance. Thus, we decided to dope N in different configurations like pyridinic, pyrrolic, and graphitic nitrogen in reduced graphene oxide, which enhanced the overall performance of the material. Herein, for the first time, we report a binder-free, high-performance pyrophosphate for a hybrid supercapacitor, along with the improvements in its specific capacitance achieved by utilizing a battery-type redox additive. Single-phase nickel pyrophosphate 1D nanorods were directly grown on Ni foam using the hydrothermal method. The electrochemical performances (specific capacitance, energy density, cycling life, and rate capability) of these nickel pyrophosphate 1D nanorods rapidly improved when using K3[Fe(CN)6] as a redox additive in 1 M KOH electrolyte and revealed that they were a suitable cathode. Curiously, a microscopic analysis proved that the redox additive inhibited the fracture of particles, which led to a large cycle life. In addition, N-doped reduced graphene oxide (rGO) was examined for use as an anode 4 ACS Paragon Plus Environment

Page 4 of 38

Page 5 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and delivered a good electrochemical performance. The foregoing properties prompted us to assemble a hybrid supercapacitor, which exhibited an excellent electrochemical performance. Our results showed that well-designed 1D nanostructures, heteroatom doping, and a redox additive are the most favorable methods for improving the performance for real-world applications. 2. Experimental procedures 2.1. Material synthesis 2.1.1. Synthesis of nickel pyrophosphate 1D nanorods Nickel pyrophosphate 1D nanorods were grown on Ni foam using a simple hydrothermal method. The Ni foam (300 × 300 mm, with a thickness of 1.6 mm and porosity of 95%) was purchased from Sigma-Aldrich and cleaned using the following procedure. The Ni foam was cleaned with a 3M HCl solution under ultrasonication for 15 min. Then, it was cleaned with deionized (DI) water and acetone several times and dried in a vacuum oven for 12 h. In a typical synthesis, stoichiometric amounts of Ni(NO3)2·6H2O and NH4H2PO4 were dissolved in 50 ml of DI water using a stirrer until a clear solution was obtained. Urea (4.5 mmole) was added to this solution under vigorous stirring to form a transparent solution. The above solution with the Ni foam substrate was transferred to a Teflon-lined stainless steel autoclave and maintained at 180 °C overnight, followed by a natural cool down to room temperature. The sample was washed with DI water and ethanol several times using ultrasonication to remove the redundant ions and then dried in a furnace at 60 °C overnight. 2.1.2. Synthesis of N-doped rGO The N-doped rGO was prepared using thermal reduction with the help of melamine without any toxic reducing agents. This demonstrated a simple approach for preparing doped 5 ACS Paragon Plus Environment


rGO. In the typical synthesis, graphene oxide (GO) was synthesized from graphite powder using the modified Hummers method.12,13 A GO suspension (4 mg of GO/1 mL of water) was produced by ultrasonication until it a complete dispersion was obtained. The GO (40 ml) solution was added to the Teflon-lined autoclave, and a melamine sponge was immersed in the GO solution. The reaction was carried out at 120 °C for 5 h. After the reaction finished, it was allowed to cool to room temperature and air dried for 3 days at room temperature. Then, the product was ground well and pyrolysed at 800 °C for 2 h in an Ar atmosphere. 2.2. Characterizations The Bruker D8 Advance with Cu Kα radiation X-ray diffraction (XRD) instrument was used to examine the phase purity and crystallinity of the samples. The size and morphological features of the samples were analyzed using a field emission scanning electron microscope (FESEM, JEOL-7001F) and transmission electron microscope (TEM, JEOL JEM-2010). Further, the electronic states of the elements were analyzed using X-ray photoelectron spectroscopy (XPS, VG Scientifics ESCALAB250). All of the electrochemical testes were carried out using a ZIVE SP2 LAB multichannel electrochemical workstation. 2.3. Electrochemical measurements 2.3.1. Half-cell The Ni foam containing the nickel pyrophosphate 1D nanorods was cut into 1 cm × 1 cm pieces and used as working electrodes. Cyclic voltammetry (CV) and galvanostatic chargedischarge (GCD) analyses were used to examine the electrochemical performance of each sample in 1 M KOH (100 ml) electrolyte. The same procedure was followed to prepare the KOH and redox additive-added electrolytes in 100 ml volumes containing 50 mg (050 mg K3[Fe(CN)6]), 75 mg (075 mg K3[Fe(CN)6]), and 100 mg (100 mg K3[Fe(CN)6]). For 6 ACS Paragon Plus Environment

Page 6 of 38

Page 7 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrochemical analyses, we used 20 ml of the prepared electrolyte solution. Here, Pt foil and Hg/HgO acted as the counter and reference electrodes, respectively. The areal capacitance of the nickel pyrophosphate 1D nanorods was calculated using CV and GCDs analyses and the following equations: CA =

∫ iVdV 2 A∆Vs

(1)

mF cm− 2

i∆t mF cm −2 A∆V

CA =

(2)

where ∫ iVdV , A, s, ∆V, i, and ∆t represent the integrated area of the CV (A V), active area (cm2

), scan rate (mV s-1), potential window (V), current density (mA cm-2), and discharge time (s),

respectively. Subsequently, the energy (EA) and power (PA) density of the nickel pyrophosphate 1D nanorods were calculated using the following equations: EA =

1 C AV 2 Wh cm -2 2

(3)

PA =

EA W cm -2 ∆t

(4)

2.3.2. Full-cell The anode was prepared by mixing N-doped rGO (30 mg), carbon black (6 mg), and polyvinylidene difluoride (PVDf; 2 mg) with N-methyl 2-pyrrolidine (NMP; 0.4 mL) to produce a homogeneous slurry. The slurry was coated onto the Ni foam and dried at 60 °C overnight in a vacuum oven. Coin-cell-type hybrid supercapacitors were fabricated using a 1 M KOH electrolyte. The cathode and anode were separated by a polypropylene film. Before undergoing the electrochemical investigation, the electrodes and separator were immersed in aqueous



electrolyte for 1 h. The electrochemical performances of the assembled device were studied using CV and GCD analyses. 3. Results and discussions 3.1. Cathode material The overall strategy for the material preparation and device assembly is schematically represented in Fig. 1. To fabricate a high-performance supercapacitor, battery-type nickel pyrophosphate 1D nanorods were grown over nickel foam without any templates or surfactants using the simple hydrothermal method. Afterward, the N-doped rGO was prepared using two steps: a hydrothermal step and pyrolysis (800 °C). The graphene sheet and nitrogen doping were supports to boost the EDL and pseudocapacitive process. Beyond any doubt, using the nickel pyrophosphate 1D nanorods and N-doped rGO as the cathode and anode, respectively, led to a wider potential window and increased the energy density of the device. Fig. 2a shows a schematic representation of the nickel pyrophosphate 1D nanorods grown on the Ni foam as a binder-free electrode. This morphology favored rapid electrolyte ion diffusion in the electrode and faster electron transport even at high rates, which led to a good rate capability. The morphology of the nickel pyrophosphate 1D nanorods was proven using FESEM. As shown in Figs. 2b and c, the nickel pyrophosphate 1D nanorods have a uniform hexagonal shape with diameters and lengths in the ranges of 60–135 nm and 22–28 µm, respectively. Closer inspection proved that the hexagonal nanorods had smooth faces. Additionally, the nanorods were uniformly grown and had good contact with the Ni foam. Each nanorod had a uniform space, which was expected to produce easy electrolyte ion diffusion, restrict the volume expansion/shrinkage during charge-discharge cycling, and afford fast electron transfer during the redox reaction. It may have produced the high electrochemical performance of the nickel 8 ACS Paragon Plus Environment

Page 8 of 38

Page 9 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pyrophosphate 1D nanorods. In addition, a TEM analysis was performed, and the results are shown in Fig. S1. The TEM image is very consistent with FESEM images (Fig. S1a). High magnification images (Figs. S1b, c) revealed that the surface and edges of the nanorods are smooth and clear. No other particle impurities were observed on the surface of the nanorods. The high-resolution TEM (Fig. S1d) image disclosed the low crystallinity of the nickel pyrophosphate 1D nanorods. The calculated d-spacing between fringes was 0.287 nm, which corresponded to the (12-2) lattice plane of the nickel pyrophosphate 1D nanorods. Such a construction enabled faster ion accessibility and rapid charge transport during a Faradaic reaction. It potentially improved the energy storage capability of the material. The structure and phase purity of the sample were examined using XRD, and the results are shown in Fig. 2d. It shows three well-defined diffraction peaks with d-spacing values of 2.036, 1.763, and 1.246 Å, corresponding to the Ni foam. Apart from the Ni foam, the diffraction peaks are broader, which infers the low crystallinity of the sample. The observed diffraction peaks at dspaces of 5.6, 4.9, 4.2, 3.4, 2.9, 2.6, and 2.3 Å correspond to the diffraction planes of (11-1), (110), (10-2), (120), (12-2), (130), and (102), respectively. In addition, the calculated d-spacing values confirm that the nickel pyrophosphate 1D nanorods have a layer structure. The layer structure was expected to favor the fast and easy accessibility of ions. The diffraction peaks are well matched with the standard card for nickel pyrophosphate 1D nanorods (JCPDS 00-0390710). The chemical environment and valence states of the elements were investigated using XPS analysis. Fig. 2e shows the XPS survey spectra of the sample, which include various elements such as Ni 2p (856.84 eV), P 2p (134.14 eV), and O 1s (532.02 eV). However, the absence of other elements confirms the sample purity. The high-resolution spectra (Fig. 2f) of Ni 2p consist of two peaks at binding energies of 856.10 eV and 874.52 eV, with satellite peaks that 9 ACS Paragon Plus Environment


correspond to Ni 2p3/2 and Ni 2p1/2, respectively. These confirm that the nickel had a divalent oxidation state. Likewise, the P 2p (Fig. 2g) shows a single peak at 134.14 eV, which is characteristic of (PO4)3-. This reveals that the P atom had +5 oxidation states in the sample. A broader O 1s (Fig. 2h) spectrum consists of lattice oxygen (531.8 eV) and chemically adsorbed hydroxyl (533.2 eV). The above observation infers the single phase and phase purity of the nickel pyrophosphate 1D nanorods.7,11 To verify the suitability of nickel pyrophosphate 1D nanorods for potential use in nextgeneration supercapacitors, the electrochemical performances were studied using three-electrode systems. A creative method was developed to utilize a redox additive in the electrolyte to enhance the energy density and specific capacitance of battery-type electrode materials via additional redox reactions.18,19 Fig. 3a shows the CVs of the bare nickel foam and nickel pyrophosphate 1D nanorods with and without the redox (K3[Fe(CN)6]) additive at a scan rate of 5 mV s-1. The nickel foam covers a much smaller current area than the nickel pyrophosphate in the 1 M KOH electrolyte. The CV profile significantly deviates from the ideal capacitive behavior. Oxidation and reduction peaks are noticed that correspond to the transition of Ni2+ and Ni3+, which demonstrates the dominance of the battery-type charge storage process. The peak broadening and peak separation potential are high, which indicates the decrease in electrochemical reversibility due to the high electrode resistance. Luckily, when adding the redox additive until 75 mg of K3[Fe(CN)6], the peak current increased and peak separation potential decreased, which revealed the high electrochemical reversibility and enhanced areal capacitance. Unluckily, beyond 75 mg of K3[Fe(CN)6], the shape of the CV changed into an oval, and the peak currents decreased, which led to a decrease in the areal capacitance. However, the CVs enclosed greater current areas than the pristine 1 M KOH electrolyte. Overall, the electrode 10 ACS Paragon Plus Environment

Page 10 of 38

Page 11 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


covered a greater current area with the 1 M KOH + 0.075 mg K3[Fe(CN)6] electrolyte than with the other electrolytes. From the CVs, the decreasing order of areal capacitances was 8120, 7573, 7322, and 5264 mF cm-2 for the 1 M KOH + 075 mg K3[Fe(CN)6], 1 M KOH + 050 mg K3[Fe(CN)6], 1 M KOH + 100 mg K3[Fe(CN)6], and 1 M KOH electrolytes, respectively. The possible charge storage process of the nickel pyrophosphate 1D nanorods in 1 M KOH + K3[Fe(CN)6] can be explained (Fig. 3b) as follows. The Fe(CN)64- from the electrolyte adsorbed on the surface of the nanorods and was subjected to a redox reaction at a particular potential, which enhanced the overall capacitance of the electrode. In particular, the K3[Fe(CN)6] supported electron shuttling during the charge-discharge process,18 which improved the rate capability of the electrode material. The possible redox reaction contribution from the electrode and redox additive can be expressed as follows:7,18 Ni2 P2O7 + 2OH- ↔ Ni 2 P2O7 (OH- ) 2 + 2e− 3−

Fe(CN)6 + e − ↔ Fe(CN)6

4−

(5) (6)

The CVs of the nickel pyrophosphate 1D nanorods at different scan rates with and without the redox additive are given in Figs. 3c and S2a–c. It can be seen that the oxidation and reduction peaks shifted in the positive and negative directions with respect to the scan rates due to the electrode polarization or internal resistance of the electrode. However, the nickel pyrophosphate 1D nanorods clearly show a well-defined redox peak even at 10 mV s-1 in the 1 M KOH + 075 mg K3[Fe(CN)6]. Additionally, the linear fit of the square root of the scan rates with peak currents (inset Fig. 3c) confirmed the good electrochemical reversibility as a result of fast electron transport supported by the 1D nanostructure and K3[Fe(CN)6]. Again, even at 10 mV s-1, the nickel pyrophosphate 1D nanorods exhibited a high areal capacitance (Fig. 3d) in the 1 M KOH + 075 mg K3[Fe(CN)6] compared to the other electrolytes. 11 ACS Paragon Plus Environment


Page 12 of 38

The CV indicated that the nickel pyrophosphate 1D nanorods stored the maximum amount of charge based on a diffusion-controlled battery-type process rather than a capacitive process. To obtain a better understanding, the power law was used to distinguish the relative contributions from the capacitive and diffusion-controlled battery-type processes to the total charge storage.1,3,5 The scan rate-dependent peak current can be expressed as follows: i p = aυ b

(7)

where ip represents the current (A), ʋ is the scan rate (V s-1), and a & b are adjustable parameters. The slope of the linear fit of the log (i) versus log (ʋ) at the peak potential (V) gives the b-value. The b-values are 0.5 and 1, which represent the diffusion-controlled battery-type process and capacitive process, respectively. Fig. 4a shows a plot of the log (i) versus log (υ) at the peak potential (V). The calculated b-values are 0.43, 0.50, 0.56, and 0.57, which correspond to the 1 M KOH, 1 M KOH + 050 mg K3[Fe(CN)6], 1 M KOH + 075 mg K3[Fe(CN)6], and 1 M KOH + 100 mg K3[Fe(CN)6], respectively, indicating the dominance of the diffusion-controlled batterytype charge storage process. It is clear that the redox additive also enhances the diffusioncontrolled battery-type process. Hence, the maximum amount of charge is stored by the diffusion-controlled battery-type process rather than the capacitive process at the redox peak potential. In order to quantify the charge stored by the capacitive and diffusion-controlled battery-type processes, the power law could be modified into the following equation:1,3,5 i p = s1υ + s2υ1/ 2

(8)

This can be changed into i p /υ1/ 2 = s1υ1 / 2 + s2

(9)


Page 13 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where ip represents the peak current (A), and s1u and s2u1/2 represent the current contributions from the capacitive and diffusion-controlled battery-type processes. The slope and intercept value of the linear (Fig. 4b) fit of the plot between ip/u1/2 and u1/2 correspond to the s1 and s2 values, respectively. The capacitive and diffusion-controlled battery-type currents were calculated using equation 8 and are presented in Figs. 4c–f. In all cases, the diffusion-controlled battery-type current rapidly increases with the scan rate compared with the capacitive current. This indicates the good electrochemical performance of the material in the electrolytes. Even at a high scan rate, it was found that the current contribution from the diffusion-controlled batterytype process was greater than that of the capacitive process, which originated with the active material and redox additive. Further, a GCD analysis was performed determine the role of the redox additive in enhancing the electrochemical performance of the material. Fig. 5a shows the GCD results for the nickel pyrophosphate 1D nanorods at 5 mA cm-2. The non-linearity of the charge-discharge plot confirms that the charge storage was based on the faradaic or battery-type process rather than the ideal capacitive process.14 The plateau regions are connected to the reversible redox reactions contributed by the active material and redox additive. The charge or discharge time is direct evidence for the charge storage capability of the electrode material. In this regard, the nickel pyrophosphate 1D nanorods showed a large discharge time in the 1 M KOH + 075 mg K3[Fe(CN)6] electrolyte compared to the other electrolytes, which confirmed that the redox additive played an important role in enhancing the capacitance. The calculated areal capacitance or specific capacity values were 3609 or 461, 4809 or 615, 5354 or 684, and 5272 mF cm-2 or 674 C g-1 at 5 mA cm-2 for the 1 M KOH, 1 M KOH + 050 mg K3[Fe(CN)6], 1 M KOH + 075 mg K3[Fe(CN)6], and 1 M KOH + 100 mg K3[Fe(CN)6], respectively. The GCD results at 13 ACS Paragon Plus Environment


different current densities are shown in Figs. 5b and S3a–c. The discharge time decreased with the current density because of the limited interaction time of the electrolyte ions with the electrode material. Even at a high current density, the clear plateau region was retained, which illustrated the good electrochemical reversibility of the redox reactions contributed by the electrode material and redox additive. Surprisingly, the nickel pyrophosphate 1D nanorods exhibited a larger plateau region and also worked up to a current density of 50 mA cm-2 in the 1 M KOH + 075 mg K3[Fe(CN)6] compared to the other electrolytes. This confirmed that the redox (K3[Fe(CN)6]) additive not only improved the electrochemical performance but also enhanced the working current of the electrode. The calculated capacitance retention values of the nickel pyrophosphate 1D nanorods were 48.4%, 31.7%, 75.7%, and 49.6 % in the electrolytes of 1 M KOH, 1 M KOH + 050 mg K3[Fe(CN)6], 1 M KOH + 075 mg K3[Fe(CN)6], and 1 M KOH + 100 mg K3[Fe(CN)6], respectively. The low capacitance retention of the electrode in the 1 M KOH + 100 mg K3[Fe(CN)6] electrolyte was due to the larger polarization effect. Fig. 5c illustrates the variation of the energy density with the power density, which is called a Ragone plot. As seen in this figure, the nickel pyrophosphate 1D nanorods exhibit a maximum energy density of 0.224 mWh cm-2 at a power density of 1.375 mW cm-2 in the 1 M KOH + 075 mg K3[Fe(CN)6], which is relatively higher than the values in the other electrolytes. It can be seen that the energy density of the electrode increased by up to 33% with the K3[Fe(CN)6] redox additive in the 1 M KOH electrolyte. In addition, the obtained energy density was relatively higher than those in recent reports such as for PPy/GO (12.9 µWh cm-2; 954.3 µW cm-2),21 PPy/GO (61.3 µWh cm-2; 1.2 mW cm-2),22 MnO2/CNT (138 µWh cm-2; 630 mW cm-2),23 and CoNi2S4/NiSe (nearly 10 µWh cm-2; 6 mW cm-2).24 The capacitance durability is an essential factor when selecting a material for a supercapacitor, and was tested over 5000 cycles. The cycling life of the nickel 14 ACS Paragon Plus Environment

Page 14 of 38

Page 15 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pyrophosphate 1D nanorods were studied in 1 M KOH (30 mA cm-2) and 1 M KOH + 075 mg K3[Fe(CN)6] (50 mA cm-2) over 5000 cycles, as shown in Fig. 5d. The nickel pyrophosphate 1D nanorods retained 87% of the initial areal capacitance with the redox additive, which was relatively higher than that (30%) for the 1 M KOH electrolyte. This longer lifetime was relatively higher than those in recent reports such as for Co(OH)2/3D graphene (retention of 75% after 1000 cycles),25 Co3S4/NiS (retention of 81.7% after 1000 cycles),26 and Co3O4/RGO/Co3O4 (retention of 81.6% after 2000 cycles).27 For further insight into the influence of the morphology on the longer lifetime, an FESEM analysis was first carried out for the samples after cycling (Figs. 5e, f) in the 1 M KOH and 1 M KOH + 075 mg K3[Fe(CN)6] electrolytes, respectively. After cycling in the 1 M KOH (Fig. 5e), the nanorods retained almost smooth but slightly rough surfaces as a result of the limited interaction of the electrolyte ions with the electrode. However, the edges of the nanorods were seriously injured in the 1 M KOH electrolyte, which may be the reason for the rapid loss of areal capacitance within particular cycles. Further, the elemental mapping (Figs. S4a–d) images confirm the uniform distribution of the Ni, P, and O elements in the sample. In particular, Fig. 5f shows that the nanorods have a rough surface, and the presence of particles is noticed on the surface without any damage to the nanorods, which shows the good structural robustness in the presence of the redox additive. Finally, this leads to a high areal capacitance and offers wonderful long lifetimes. Further, the Ni, P, O, Fe, C, and N elements are evenly distributed in the samples (Figs. S4e–k). This more clearly shows that the redox additive has good interaction with the electrode material and supports a large charge storage and structural robustness. This effectively shows the good electrochemical life of the nickel pyrophosphate 1D nanorods in the 1 M KOH + 075 mg K3[Fe(CN)6] electrolyte. In order to better appreciate the structural robustness 15 ACS Paragon Plus Environment


and interaction of the redox additive with the electrode, a TEM analysis of the nanorods was conducted and the results are shown in Fig. S5. Figs. S5a–c correspond to the TEM image of the nanorods after cycling in the 1 M KOH electrolyte. Figs. S5b and c show that the edges of the nanorods are seriously injured after cycling in the 1 M KOH, which reduces the cycling life. It can be noticed that the diffusion length of the ions into the nanorod is nearly 100 nm. A high magnification image (Fig. S5c) clearly distinguishes the injured and rigid parts of the nanorod. In addition, the injured part has a nanoflake shape with good transparency compared to the rigid part. More surprisingly, the nanorods were not injured (Figs. S5d–f) in the 1 M KOH + 075 mg K3[Fe(CN)6] electrolyte. However, the presence of particles on the surface of the nanorods was observed, which may be a result of the adsorption of the redox additive (revealed from an elemental mapping by FESEM). The high-magnification image also shows that no crack or fracture is observed in the nanorod, which enables a long cycling life in the 1 M KOH + 075 mg K3[Fe(CN)6] electrolyte. Overall, the improved areal capacitance, good rate capability, low electrode resistance, and longer lifetime of the nickel pyrophosphate 1D nanorods in the 1 M KOH + 075 mg K3[Fe(CN)6] electrolyte could be associated with the following aspects. First, the 1D nanorods boost the charge transfer even at a high current density. Second, the empty space between 1D nanorods supports the fast access of electrolyte ions, lowers the diffusive resistance, enhances the rate capability, and supports a longer cycling life. Third, the binder-free electrode greatly reduces the electrical resistance, and contributes good contact between the nanorods and substrate. Fourth, the stable morphology of the material provides a longer lifetime. Fifth, the good interaction of the redox additive with the nanorods is favorable for a large charge storage. Moreover, the vertical alignment of the nanorods supports fast ion and electron transport during the charge16 ACS Paragon Plus Environment

Page 16 of 38

Page 17 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


discharge process. There is no doubt that the above mentioned positive features boosted the electrochemical performance of the nickel pyrophosphate 1D nanorods. Therefore, the results suggested that 75 mg K3[Fe(CN)6] was the optimal quantity to improve the electrochemical performance of the binder-free cathode material for a hybrid supercapacitor. 3.2. Anode material N-doped rGO has emerged as a good candidate for energy storage and conversion applications because of its unique properties. In order to better understand the microstructure of N-doped rGO, we conducted a TEM analysis. Fig. 6 shows TEM images of the N-doped rGO before and after pyrolysis at different magnifications. The optical transparency of the N-doped rGO sheets reveals that it consists of a few layers. Before pyrolysis (Figs. 6a and S6a), the surface of the sheet is rough and has more wrinkles or folds at the edges. The observed wrinkles or folds are due to the presence of more oxygen functional groups. The selected area (Fig. S6b) electron diffraction pattern (SAED) shows the low intensity spots, indicating the non-crystalline nature of the sheets.12 After pyrolysis (Figs. 6b and S6c), the surface of the N-doped rGO sheet becomes smoother, with fewer wrinkles or folds at the edges. This may be due to the reduction of the oxygen functional groups and nitrogen doping in the graphene sheets. Further, the SAED (Fig. S6d) pattern infers the lower crystallinity of the sample.13,28,29 The elemental mapping images indicate (Figs. S6e–h) the uniform distributions of the N, C, and O elements in the entire sheet and confirm the nitrogen doping in the rGO. Furthermore, XRD was conducted, and the results are shown in Fig. 6c. The disappearance of a diffraction peak at 11° suggests the removal of oxygen functional groups in the rGO sheets. A broader peak seen around 20–30° confirms the reduction of oxygen functional groups and exfoliation of sheets. The shift in the diffraction peak to a higher angle after pyrolysis reveals the reduction of the interlayer spacing of the rGO sheets. 17 ACS Paragon Plus Environment


The prominent peaks observed at 26° and 42° correspond to the (002) and (110) reflections of turbostratic carbon, respectively.30 Further, the structural changes caused by doping and pyrolysis (Fig. 6d) were investigated using Raman analysis. The Raman spectra of the samples show the characteristic D (disorder carbon) and G (sp2 hybridized graphitic carbon) bands at nearly 1360 cm-1 and 1598 cm-1, respectively. The ID/IG ratios are 1.17 and 1.13 before and after the pyrolysis of the N-doped rGO sheets, respectively. The decrease in the ratio may be the result of the reorientation of the graphene sheets, along with decreases in the doping and oxygen functional groups. For further insight and a better understanding of the elemental composition, an XPS analysis was performed, and the results are shown in Fig. 6e. Before pyrolysis, there are five peaks noticed at binding energy values of 164, 228.6, 284.37, 399.04, and 531.84 eV, corresponding to the elements of S 2p, S 2s, C 1s, N 1s, and O 1s, respectively. The existence of sulfur is due to the bisulfite form in melamine.31 After pyrolysis, the sulfur peak completely vanishes, and the intensities of the N 1s and O 1s peaks decrease. The decrease in the nitrogen content may originate from the unstable nitrogen moieties at 800 °C, which reveal the selfarrangement.32 This infers that the melamine is decomposed during pyrolysis and N atom doping in the rGO sheets. Additionally, the oxygen functional groups are almost gone compared with those before pyrolysis. The controlled N atom doping and removal of the oxygen functional groups reduces the defect sites and may improve the electron transport, which is supported by the Raman analysis results. To evaluate the type of doping in the rGO sheets, a high-resolution N 1s spectrum is shown in Fig. 6f. Before pyrolysis, it can be seen that the N 1s peak is broader, with a binding energy range of 398–405 eV. It contains the nitrogen functional groups of nitrogen oxide, graphitic nitrogen, and pyrrolic nitrogen at binding energies of 402.8 eV, 401.2 eV, and 18 ACS Paragon Plus Environment

Page 18 of 38

Page 19 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


399.6 eV, respectively. After pyrolysis, the N 1s peak intensity and doping sites are drastically changed. These results suggest that the nitrogen atom is successfully doped in the form of pyridinic (398.1 eV), pyrrolic (399.6 eV), and graphitic nitrogen (401.2 eV). The pyridinic, pyrrolic, and graphitic nitrogen doping refer to one p electron and sp2 hybridization, two p electron and sp3 hybridization, and a nitrogen atom located in a hexagonal ring, respectively. This enhances the overall capacitance and conductivity of the N-doped rGO. The pyridinic and pyrrolic nitrogen supports the electrochemical performances, and the graphitic nitrogen enhances the electronic conductivity.33–36 The above characterization results demonstrated that the pyrolysis treatment provided controlled nitrogen doping with no defects. A three-electrode system was used to validate the pyrolysis treatment to enhance the electrochemical performance. Fig. S7a shows the CVs before and after pyrolysis of the N-doped rGO in the 1 M KOH + 075 mg K3[Fe(CN)6] electrolyte at 5 mV s-1. It can be found that the CVs cover a larger current area after pyrolysis, suggesting a large energy storage capability. The quasi-rectangularity of the CVs demonstrates the electric double layer charge-discharge capacitive process. An observed broad hump in the CVs reveals the presence of a pseudocapacitive behavior due to oxygen functional groups and nitrogen doping in the rGO.12,37 Figs. S7b and c show the CVs of the electrodes at various scan rates. It can be noticed that, after pyrolysis, the quasi-rectangularity of the CVs is retained even at a high scan rate, which infers the low electrode resistance. The current area under the CVs corresponds to the amount of charge stored in the electrode material. The calculated areal capacitance values before (after) the pyrolysis of the graphene are 22 (375), 20 (354), 19 (339), 18 (328), 16 (319), 15 (312), 10 (263), 8 (229), 7 (197), and 6 (175) mF cm-2 at scan rates of 5, 6, 7, 8, 9, 10, 20, 30, 40, and 50 mV s-1, respectively. After the pyrolysis, the N-doped rGO exhibited a 17-fold higher areal 19 ACS Paragon Plus Environment


capacitance than before the pyrolysis. Moreover, a GCD analyses were performed before and after the pyrolysis of the N-doped rGO at 1 mA, and the results are shown in Fig. S7d. The nonlinear charge-discharge profiles infer the co-presence of the electric double layer and pseudocapacitive nature of the electrodes. After pyrolysis, a larger discharge time leads to a higher specific capacitance than before the pyrolysis. Consequently, after the pyrolysis, the Ndoped rGO shows a small IR drop during discharge, which indicates the low electrode resistance and excellent electrochemical performance of the electrode. This may be due to the reduction of the oxygen functional groups and controlled nitrogen doping in the rGO sheets. On the other hand, a very larger IR drop is noticed, which leads to a high electrode resistance, due to a large atomic percentage of oxygen functional groups. Surprisingly, after the pyrolysis, the N-doped rGO works up to a high current of 5 mA, which is five times that before the pyrolysis (Figs. S7e and f). The discharge time is directly proportional to the specific capacitance of the electrode. The variation in the areal capacitance with the current was calculated and is shown in Fig. S8. It can be seen that the areal capacitance value decreases with the current as a result of the limited interaction of the electrolyte ions with the electrode material. Overall, the considerably enhanced performance can be ascribed to the reduction of the oxygen functional groups and controlled doping of nitrogen in the rGO sheets after the pyrolysis. Hence, the N-doped rGO is a suitable anode material for fabricating a high-energy hybrid supercapacitor. 3.3. Hybrid supercapacitors These interesting results demonstrated that it would be suitable to use the nickel pyrophosphate 1D nanorods (battery type to enhance the energy density) as a cathode and Ndoped rGO (a wider potential window due to the over-potential of the reversible hydrogen electrosorption) as an anode with a battery-type redox additive-containing (1 M KOH + 0.75 mg 20 ACS Paragon Plus Environment

Page 20 of 38

Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


K3[Fe(CN)6]) electrolyte for a hybrid supercapacitor, which is schematically illustrated in Fig. 7a.38 Based on the potential window of the electrodes, the operating potential window of the hybrid supercapacitor was determined to be 1.6 V. Initially, the CV was measured to understand the electrochemical performance of the device. As shown in Fig. 7b, the CV has a quasirectangular shape with a distinct sharp peak, which proves that the device stores the charge using battery-type (contributed by the cathode and redox additive) and capacitive-type (originating from the graphene) processes. A negligible shift in the redox peaks with the scan rate illustrates the superior rate capability. The redox peaks were preserved even at a high scan rate, which inferred the good electrochemical reversibility of the device. Fig. 7c shows the GCDs of the device at various current densities. These plots are nonlinearly related to the potential, suggesting the presence of both capacitive and battery-type processes in the device. Even at a high current density, the nearly symmetric charge-discharge profile indicates the good electrochemical reversibility and fast charge-discharge capability of the device. The specific capacitance of the device was calculated using the total mass of the cathode and anode. The device delivered a maximum specific capacitance of 224 F g-1 at a current density of 1 A g-1 (Fig. 7d). Notably, this performance is superior to those in recent reports (Table 1). From the discharge plot, the energy and power densities of the device were calculated and are given in Fig. 7e. The device delivered a maximum energy density of 70 Wh kg-1 at a power density of 750 W kg-1, which were relatively larger or comparable to those in recent reports (Table 1). The cycling life is one of the main tools for determining the practical application of a fabricated device. The cycling life (Fig. S9) of this device was tested over 5000 cycles at a current density of 5 A g-1. The device retained 73% of its initial specific capacitance over 5000 cycles. The coulombic efficiency is one of the key points to understand the electrochemical reversibility of a device. The coulombic efficiency 21 ACS Paragon Plus Environment


is a measure of the ratio of the discharge and charge times, multiplied by 100.60 Initially, the coulombic efficiency of this device was low. However, it increased after some cycles, and remained at 96% over 5000 cycles. This suggested the good electrochemical reversibility of the redox reaction and long-term life of the device. The superior electrochemical performance of the fabricated hybrid supercapacitor may have been the result of various factors, including the synergistic effect of the electrodes, significantly enhanced energy density as a result of the high specific capacitance and working potential window, additional capacitance contributed by the redox additive, and structural robustness of the redox additive, which improved the cycling life. Overall, the proposed device configuration and electrochemical properties demonstrate a new method for attaining high energy storage. 4. Conclusions In summary, we successfully developed binder-free nickel pyrophosphate 1D nanorods and N-doped rGO as cathode and anode materials, respectively, for a high-performance hybrid supercapacitor. We showed that the 1D nanostructure, redox additive, and nitrogen doping enhanced the electrochemical performance. More importantly, the redox additive not only improved the specific capacitance, energy density, reversibility, and cycle life, but also prevented fracturing of the microstructure during cycling. The N-doped rGO also showed a good electrochemical performance after pyrolysis as a result of a reduction in the oxygen functional groups and the controlled doping of nitrogen, which was explored for the anode. The hybrid supercapacitor was assembled using these two electrodes and delivered a high specific capacitance (224 F g-1 at 1 A g-1) and high energy density (70 Wh kg-1 at a power density of 750 W kg-1), with improved coulombic efficiency over 5000 cycles. Hence, the results demonstrated


Page 22 of 38

Page 23 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


useful and valuable points for constructing a high-performance, next-generation hybrid supercapacitor. ASSOCIATED CONTENT Supporting Information TEM and HRTEM images of nanorods; CV and GCDs curves of nanorods with and without the redox additive; elemental mapping and TEM images after cycling; TEM image, SAED pattern, and elemental mapping of N-doped rGO; CV and GCDs curves of N-doped rGO before and after pyrolysis; current-dependent areal capacitance; and cycle life of hybrid supercapacitor AUTHOR INFORMATION Corresponding author *E-mail: [email protected]; Tel: +82-2-2123-5817 Author Contributions The manuscript was written based on contributions from all authors, who have approved the final version of the manuscript. ‡ These authors contributed equally. ACKNOWLEDGEMENTS This research was partially supported by the Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF-2017M3A7B4041987); the Priority Research Centers Program through the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology, (2009-0093823); and a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIP) (no. 2015R1A5A1037668).



REFERENCES 1. Kim, H-S.; Cook, J. B.; Lin, H.; Ko, J. S.; Tolbert, S. H.; Ozolins, V.; Dunn, B. Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO3−x Nat. Mater. 2017, 16, 454-460. 2. Akinwolemiwa, B.; Wei, C.; Chen, G. Z. Mechanisms and Designs of Asymmetrical Electrochemical Capacitors. Electrochim. Acta 2017, 247, 344-357. 3. Guo, X. L.; Liu, X. Y.; Hao, X. D.; Zhu, S. J.; Dong, F.; Wen, Z. Q.; Zhang, Y. X. NickelManganese Layered Double Hydroxide Nanosheets Supported on Nickel Foam for Highperformance Supercapacitor Electrode Materials. Electrochim. Acta 2016, 194, 179-186. 4. Guan, B.; Shan, Q. Y.; Chen, H.; Xue, D.; Chen, K.; Zhang, Y. X. Morphology Dependent Supercapacitance of Nanostructured NiCo2O4 on Graphitic Carbon Nitride. Electrochim. Acta 2016, 200, 239–246. 5. Chen, H.; Qi, X. Q.; Kuang, M.; Dong, F.; Zhang, Y. X. Hierarchical copper/nickel-based manganese dioxide core-shell nanostructure for supercapacitor electrodes. Electrochim. Acta 2016, 212, 671–677. 6. Pang, H.; Zhang, Y-Z.; Run, Z.; Lai, W-Y.; Huang, W. Amorphous nickel pyrophosphate microstructures for high-performance flexible solid-state electrochemical energy storage devices Nano Energy 2015, 17, 339–347. 7. Senthilkumar, B.; Khan, Z.; Park, S.; Kim, K.; Ko, H.; Kim, Y. Highly porous graphitic carbon and Ni2P2O7 for a high performance aqueous hybrid supercapacitor J. Mater. Chem. A, 2015, 3, 21553–21561. 8. Pang, H.; Yan, Z.; Ma, Y.; Li, G.; Chen, J.; Zhang, J.; Du, W.; Li, S. Cobalt pyrophosphate nano/microstructures as promising electrode materials of supercapacitor J Solid State Electrochem 2013, 17, 1383–1391. 9. Khan, Z.; Senthilkumar, B.; Lim, S.; Shanker, R.; Kim, Y.; Ko, H. Redox-AdditiveEnhanced High Capacitance Supercapacitors Based on Co2P2O7 Nanosheets Adv. Mater. Interfaces, 2017, 4, 1700059-1700066. 10. Wei, C.; Cheng, C.; Wang, S.; Xu, Y.; Wang, J.; Pang, H. Sodium-Doped Mesoporous Ni2P2O7 Hexagonal Tablets for High Performance Flexible All-Solid-State Hybrid Supercapacitors Chem. Asian J. 2015, 10, 1731–1737. 24 ACS Paragon Plus Environment

Page 24 of 38

Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11. Chen, C.; Zhang, N.; He, Y.; Liang, B.; Ma, R.; Liu, X. Controllable Fabrication of Amorphous

Co-Ni

Pyrophosphates

for

Tuning

Electrochemical

Performance

in

Supercapacitors ACS Appl. Mater. Interfaces 2016, 8, 23114−23121. 12. Sankar, K. V.; Selvan, R. K.; Vignesh, R. H.; Lee, Y. S. Nitrogen-doped reduced graphene oxide and aniline based redox additive electrolyte for a flexible supercapacitor RSC Adv. 2016, 6, 67898-67909. 13. Sankar, K. V.; Selvan, R. K. Improved electrochemical performances of reduced graphene oxide based supercapacitor using redox additive electrolyte Carbon 2015, 90, 260-273. 14. Yu, A.; Sy, A.; Davies, A. Graphene nanoplatelets supported MnO2 nanoparticles for electrochemical supercapacitor Synthetic Metals, 2011, 161, 2049-2054. 15. Singh, A.; Roberts, A. J.; Slade, R. C. T.; Chandra, A. High electrochemical performance in asymmetric supercapacitors using MWCNT/nickel sulfide composite and graphene nanoplatelets as electrodes J. Mater. Chem. A, 2014, 2, 16723-16730. 16. Beidaghi, M.; Wang, Z.; Gu, L.; Wang, C. Electrostatic spray deposition of graphene nanoplatelets for high-power thin-film supercapacitor electrodes J. Solid State Electrochem. 2012, 16, 3341-3348. 17. Han, J.; Zhang, L. L.; Lee, S.; Oh, J.; Lee, K-S.; Potts, J. R.; Ji, J.; Zhao, X.; Ruoff, R. S.; Park, S. Generation of B-Doped Graphene Nanoplatelets Using a Solution Process and Their Supercapacitor Applications ACS Nano 2013, 7, 19-26. 18. Hwang, J. Y.; El-Kady, M. F.; Li, M.; Lin, C-W.; Kowal, M.; Han, X.; Kaner, R. B. Boosting the capacitance and voltage of aqueous supercapacitors via redox charge contribution from both electrode and electrolyte Nano today 2017, 15, 15-25. 19. Zhao, C.; Zheng, W.; Wang, X.; Zhang, H.; Cui, X.; Wang, H. Ultrahigh capacitive performance from both Co(OH)2/graphene electrode and K3Fe(CN)6 electrolyte Sci. Rep. 2013, 3, 2986-2992. 20. Wang, R.; Yan, X.; Lang, J.; Zheng, Z.; Zhang, P. A hybrid supercapacitor based on flowerlike Co(OH)2 and urchin-like VN electrode materials J. Mater. Chem. A, 2014, 2, 12724– 12732. 21. Zhou, H.; Han, G.; Xiao, Y.; Chang, Y.; Zhai, H-J. Facile preparation of polypyrrole/graphene

oxide

nanocomposites

with

large

areal

capacitance

electrochemical codeposition for supercapacitors J. Power Sources 2014, 263, 259-267. 25 ACS Paragon Plus Environment

using


Page 26 of 38

22. Shu, K.; Wang, C.; Zhao, C.; Ge, Y.; Wallace, G. G. A Free-standing Graphene-Polypyrrole Hybrid Paper via Electropolymerization with an Enhanced Areal Capacitance Electrochim. Acta 2016, 212, 561–571. 23. Wang, K.; Gao, S.; Du, Z.; Yuan, A.; Lu, W.; Chen, L. MnO2-Carbon nanotube composite for high-areal-density supercapacitors with high rate performance J. Power Sources 2016, 305, 30-36. 24. Tang, Z.; Jia, C.; Wan, Z.; Zhou, Q.; Ye, X.; Zhu, Y. Facile preparation of CoNi2S4@NiSe nano arrays on compressed nickel foam for high performance flexible supercapacitors RSC Adv., 2016, 6, 112307–112316. 25. Yin, T.; Zhang, W.; Yin, Y.; Yan, Y.; Zhan, K.; Yang, J.; Zhao, B. Co(OH)2 nanoflakes grown on 3D graphene foam as a binder-free hybrid electrode for high-performance supercapacitors J Mater Sci: Mater Electron 2017, 28, 7884-7891. 26. Wang, X.; Xia, H.; Wang, X.; Shi, B.; Fang, Y. A super high performance asymmetric supercapacitor based on Co3S4/NiS nanoplates electrodes RSC Adv., 2016, 6, 97482-97490. 27. Wang, S.; Ju, P.; Zhu, Z.; Zhao, C. Co3O4/RGO/Co3O4 pseudocomposite grown in situ on a Co foil for high-performance supercapacitors RSC Adv., 2016, 6, 99640-99647. 28. Shi, C.; Hu, L.; Guo, K.; Li, H.; Zhai, T. Highly Porous Carbon with Graphene Nanoplatelet Microstructure Derived from Biomass Waste for High-Performance Supercapacitors in Universal Electrolyte Adv. Sust. Syst. 2017, 1, 1600011-1600021. 29. Peng, L-Q.; Yi, L.; Yang, Q-C.; Cao, J.; Du, L-J.; Zhang, Q-D. Graphene nanoplatelets based matrix solid-phase dispersion microextraction for phenolic acids by ultrahigh performance liquid chromatography with electrochemical detection Sci. Rep. 2017, 7, 7496-7508. 30. Hou, Y.; Wen, Z.; Cui, S.; Ci, S.; Mao, S.; Chen, J. An Advanced Nitrogen-Doped Graphene/Cobalt-Embedded Porous Carbon Polyhedron Hybrid for Efficient Catalysis of Oxygen Reduction and Water Splitting Adv. Funct. Mater. 2015, 25, 872-882. 31. Lee, J-S.; Park, G. S.; Kim, S. T.; Liu, M.; Cho, J. A Highly Efficient Electrocatalyst for the Oxygen

Reduction

Reaction:

N-Doped

Ketjenblack

Incorporated

into

Fe/Fe3C-

Functionalized Melamine Foam Angew. Chem. Int. Ed. 2013, 125, 1060-1064. 32. Miao, H.; Li, S.; Wang, Z.; Sun, S.; Kuang, M.; Liu, Z.; Yuan, J. Enhancing the pyridinic N content of Nitrogen-doped graphene and improving its catalytic activity for oxygen reduction reaction Int. J. Hydrogen Energy, 2017, DOI: 10.1016/j.ijhydene.2017.09.138. 26 ACS Paragon Plus Environment

Page 27 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


33. Lee, M. S.; Choi, H-J.; Baek, J-B.; Chang, D. W. Simple solution-based synthesis of pyridinic-rich nitrogen-doped graphene nanoplatelets for supercapacitors Appli. Energy 2017, 195, 1071-1078. 34. Wang, K.; Xu, M.; Gu, Y.; Gu, Z.; Liu, J.; Fan, Q. H. Low-temperature plasma exfoliated ndoped graphene for symmetrical electrode supercapacitors Nano Energy 2017, 31, 486-494. 35. Romero, J.; Rodrigues-San-Miguel, D.; Ribera, A.; Mas-Balleste, R.; Otero, T. F.; Manet, I.; Licio, F.; Abellan, G.; Zamora, F.; Coronado, E. Metal-functionalized covalent organic frameworks as precursors of supercapacitive porous N-doped graphene J. Mater. Chem A, 2017, 5, 4343-4351. 36. Wei, X.; Wan, S.; Gao, S. Self-assembly-template engineering nitrogen-doped carbon aerogels for high-rate supercapacitors Nano Energy 2016, 28, 206-215. 37. Sankar, K. V.; Kumar, N. R.; Lee, Y. S.; Selvan, R. K. Fabrication of Reduced Graphene Oxide Based Ultra–high Cycle Life Flexible Fiber Supercapacitor with Different Modes Chem. Select 2016, 1, 6476-6484. 38. Ji, J.; Zhang, L. L.; Ji, H.; Li, Y.; Zhao, X.; Bai, X.; Fan, X.; Zhang, F.; Ruoff, R. S. Nanoporous Ni(OH)2 Thin Film on 3D Ultrathin-Graphite Foam for Asymmetric Supercapacitor ACS Nano 2013, 7, 6237-6243. 39. He, W.; Wang, C.; Zhuge, F.; Deng, X.; Xu, X.; Zhai, T. Flexible and high energy density asymmetrical supercapacitors based on core/shell conducting polymer nanowires/manganese dioxide nanoflakes Nano Energy 2017, 35, 242-250. 40. Liang, H.; Xia, C.; Jiang, Q.; Gandi, A. N.; Schwingenschlogl, U.; Alshareef, H. N. Low temperature synthesis of ternary metal phosphides using plasma for asymmetric supercapacitors Nano Energy 2017, 35, 331-340. 41. Wang, R.; Xu, C.; Lee, J-M. High performance asymmetric supercapacitors: New NiOOH nanosheet/graphene hydrogels and pure graphene hydrogels Nano Energy 2016, 19, 210-221. 42. Guo, X. L.; Zhang, J. M.; Xu, W. N.; Hu, C. G.; Sun, L.; Zhang, Y. X. Growth of NiMn LDH nanosheet arrays on KCu7S4 microwires for hybrid supercapacitors with enhanced electrochemical performance J. Mater. Chem. A, 2017, 5, 20579-20587. 43. Yang, J.; Yu, C.; Fan, X.; Liang, S.; Li, S.; Huang, H.; Ling, Z.; Hao, C.; Qiu, J. Electroactive edge site-enriched nickel–cobalt sulfide into graphene frameworks for highperformance asymmetric supercapacitors Energy Environ. Sci. 2016, 9, 1299-1307. 27 ACS Paragon Plus Environment


44. Li, S.; Yu, C.; Yang, J.; Zhao, C.; Zhang, M.; Huang, H.; liu, Z.; Guo, W.; Qiu, J. A superhydrophilic ‘‘nanoglue’’ for stabilizing metal hydroxides onto carbon materials for high-energy and ultralong-life asymmetric supercapacitors Energy Environ. Sci. 2017, 10, 1958-1965. 45. Kim, M.; Choi, J.; Oh, I.; Kim, J. Design and synthesis of ternary Co3O4/carbon coated TiO2 hybrid nanocomposites for asymmetric supercapacitors Phys. Chem. Chem. Phys. 2016, 18, 19696-19704. 46. Zhu, Y.; Wu, Z.; Jing, M.; Hou, H.; Yang, Y.; Zhang, Y.; Yang, X.; Song, W.; Jia, X.; Ji, X. Porous NiCo2O4 spheres tuned through carbon quantum dots utilised as advanced materials for an asymmetric supercapacitor J. Mater. Chem. A 2015, 3, 866-877. 47. Li, J-J.; Liu, M-C.; Kong, L-B.; Wang, D.; Hu, Y-M.; Han, W.; Kang, L. Advanced asymmetric supercapacitors based on Ni3(PO4)2@GO and Fe2O3@GO electrodes with high specific capacitance and high energy density RSC Adv. 2015, 5, 41721-41728. 48. Kumar, R.; Rai, P.; Sharma, A. 3D urchin-shaped Ni3(VO4)2 hollow nanospheres for highperformance asymmetric supercapacitor applications J. Mater. Chem. A 2016, 4, 9822-9831. 49. Wang, Y.; Shen, C.; Niu, L.; Li, R.; Guo, H.; Shi, Y.; Li, C.; Liu, X.; Gong, Y. Hydrothermal synthesis of CuCo2O4/CuO nanowire arrays and RGO/Fe2O3 composites for highperformance aqueous asymmetric supercapacitors J. Mater. Chem. A, 2016, 4, 9977-9985. 50. Du, D.; Lan, R.; Xu, W.; Beanland, R.; Wang, H.; Tao, S. Preparation of a hybrid Cu2O/CuMoO4 nanosheet electrode for high-performance asymmetric supercapacitors J. Mater. Chem. A 2016, 4, 17749-17756. 51. Shinde, N. M.; Xia, Q. X.; Yun, J. M.; Singh, S.; Mane, R. S.; Kim, K-H. A binder-free wet chemical synthesis approach to decorate nanoflowers of bismuth oxide on Ni-foam for fabricating laboratory scale potential pencil-type asymmetric supercapacitor device Dalton Trans. 2017, 46, 6601-6611. 52. Wang, X.; Liu, W. S.; Lu, X.; Lee, P. S. Dodecyl sulfate-induced fast faradic process in nickel cobalt oxide–reduced graphite oxide composite material and its application for asymmetric supercapacitor device J. Mater. Chem. 2012, 22, 23114-23119. 53. Niu, L.; Wang, Y.; Ruan, F.; Shen, C.; Shan, S.; Xu, M.; Sun, Z.; Li, C.; Liu, X.; Gong, Y. In situ growth of NiCo2S4@Ni3V2O8 on Ni foam as a binder-free electrode for asymmetric supercapacitors J. Mater. Chem. A 2016, 4, 5669-5677. 28 ACS Paragon Plus Environment

Page 28 of 38

Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


54. Hou, S.; Xu, X.; Wang, M.; Xu, Y.; Lu, T.; Yao, Y.; Pan, L. Carbon-incorporated Janus-type Ni2P/Ni hollow spheres for high performance hybrid supercapacitors J. Mater. Chem. A 2017, 5, 19054-19061. 55. Zhang, S.; Li, D.; Chen, S.; Yang, X.; Zhao, X.; Zhao, Q.; Komarneni, S.; Yang, D. Highly stable supercapacitors with MOF-derived Co9S8/carbon electrodes for high rate electrochemical energy storage J. Mater. Chem. A 2017, 5, 12453-12461. 56. Das, A. K.; Bera, R.; Maitra, A.; Karan, S. K.; Paria, S.; Halder, L.; Si, S. K.; Bera, A.; Khatua, B. B. Fabrication of an advanced asymmetric supercapacitor based on a microcubical PB@MnO2 hybrid and PANI/GNP composite with excellent electrochemical behaviour J. Mater. Chem. A 2017, 5, 22242-22254. 57. Singh, A.; Akhtar, M. A.; Chandra, A. Trade-off between capacitance and cycling at elevated temperatures in redox additive aqueous electrolyte based high performance asymmetric supercapacitors Electrochim. Acta 2017, 229, 291-298. 58. Kim, M.; Yoo, J.; Kim, J. Quasi-solid-state flexible asymmetric supercapacitor based on ferroferric oxide nanoparticles on porous silicon carbide with redox-active p-nitroaniline gel electrolyte Chem. Eng. J. 2017, 324, 93-103. 59. Sankar, K. V.; Lee, S. C.; Seo, Y.; Ray, C.; Liu, S.; Kundu, A.; Jun, S. C. Binder-free cobalt phosphate one-dimensional nanograsses as ultrahighperformance cathode material for hybrid supercapacitor applications J. Power Souces 2018, 373, 211-219. 60. Sankar, K. V.; Shanmugapriya, S.; Surendran, S.; Jun, S. C.; Selvan, R. K. Facile hydrothermal synthesis of carbon-coated cobalt ferrite spherical nanoparticles as a potential negative electrode for flexible supercapattery J. Colloid Interface Sci. 2018, 513, 480-488.



Fig. 1 Schematic illustration of preparation of nickel pyrophosphate 1D nanorods (cathode) and N-doped rGO (anode), along with assembly of hybrid supercapacitor.


Page 30 of 38

Page 31 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 2 (a) Schematic representation of nickel pyrophosphate 1D nanorods grown over Ni foam. (b, c) FESEM images of nickel pyrophosphate 1D nanorods at different magnifications. (d) XRD pattern of nickel pyrophosphate 1D nanorods. XPS (e) survey and (f–h) high resolution spectra of nickel pyrophosphate 1D nanorods.



Fig. 3 (a) CVs of nickel pyrophosphate 1D nanorods with and without redox additive at scan rate of 5 mV s-1. (b) Schematic illustration of charge storage process of nickel pyrophosphate 1D nanorods with redox additive electrolyte. CVs (c) of nickel pyrophosphate 1D nanorods in 1 M KOH + 0.075mg K3[Fe(CN)6] at various scan rates (5–10 mV s-1) (Inset: linear fit of the peak currents with square root of scan rates). (d) The areal capacitance of the electrodes in different quantities of redox additive electrolytes.


Page 32 of 38

Page 33 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4 (a) Determination of b-values from charge peak currents in different electrolytes. (b) Calculation of slope and intercept values of nickel pyrophosphate 1D nanorods from the plot of ʋ vs. i/ʋ1/2 in different electrolytes. Contribution of capacitive and diffusion-controlled battery-type currents of nickel pyrophosphate 1D nanorods in (c) 1 M KOH, (d) 1 M KOH + 050 mg K3[Fe(CN)6], (e) 1 M KOH + 075 mg K3[Fe(CN)6], and (f) 1 M KOH + 100 mg K3[Fe(CN)6] electrolytes.



Fig. 5 (a) GCDs of nickel pyrophosphate 1D nanorods with and without redox additive at 5 mA cm-2. (b) GCDs of the nickel pyrophosphate 1D nanorods in 1 M KOH + 0.075 mg K3[Fe(CN)6] at various current densities. (c) Ragone plot related to energy and power densities of the nickel pyrophosphate 1D nanorods. (d) Cycling life and (e, f) FESEM images of the nickel pyrophosphate 1D nanorods after cycling in 1 M KOH and 1 M KOH + 0.075 mg K3[Fe(CN)6], respectively.


Page 34 of 38

Page 35 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 6 TEM images of N-doped rGO before (a) and after (b) pyrolysis. (c) XRD pattern of Ndoped rGO. (d) Raman spectra of N-doped rGO. (e, f) Survey and N 1s high resolution spectra of N-doped rGO, respectively.



Fig. 7 (a) Schematic illustration of charge storage process of hybrid supercapacitor. (b, c) CV and GCDs of the hybrid supercapacitor at various scan rates and current densities, respectively. (d) The calculated specific capacitance of the hybrid supercapacitor at various current densities. (e) Ragone plot related to energy and power densities of the hybrid supercapacitor. 36 ACS Paragon Plus Environment

Page 36 of 38

Page 37 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Comparison of electrochemical properties of fabricated device and data recently reported in literature. S. Device Specific capacitance Energy density Reference No 1 HPGC//Ni2P2O7 183 F g-1 at 1 A g-1 65 Wh kg-1 at 800 7 W kg-1 2 MnO2@PPy//activated carbon (AC) 57 F g-1 at 1 A g-1 25.8 Wh kg-1 at 39 902 W kg-1 3 NiCoP//graphene 43.8 mAh g-1 at 2 A g-1 32.9 Wh kg-1 at 40 1301 W kg-1 4 H-NiOOH/GS//H-GS 188 F g-1 at 1 A g-1 66.8 Wh kg-1 at 41 800 W kg-1 5 KCu7S4@NiMn LDH//activated 51 F g-1 at 1 A g-1 15.9 Wh kg-1 at 42 graphene nearly 900 W kg-1 6 Ni-Co-S/G//PCNS 122 F g-1 at 1 A g-1 43.2 Wh kg-1 at 43 800 W kg-1 7 CC-NC-LDH//AC 196 F g-1 at 1 A g-1 69.7 Wh kg-1 at 44 800 W kg-1 8 TiO2@C/Co//AC 57.7 F g-1 at 0.5 A g-1 18.54 Wh kg-1 at 45 223 W kg-1 9 AC//CQDS/NiCo2O4 88.9 F g-1 at 0.5 A g-1 27.8 Wh kg-1 at 46 128 W kg-1 10 Fe2O3@GO//Ni3(PO4)2@GO 189 F g-1 at 0.25 A g-1 67.2 Wh kg-1 at 47 200 W kg-1 11 Ni3(VO4)2//AC 114 C g-1 at 0.3 A g-1 25.3 Wh kg-1 at 48 240 W kg-1 12 CuCo2O4/CuO//RGO/Fe2O3 93 F g-1 at 0.25 A g-1 33 Wh kg-1 at 200 49 W kg-1 13 Cu2O/CuMoO4//AC 191 F g-1 at 0.5 A g-1 75.1 Wh kg-1 at 50 420 W kg-1 14 Bi2O3-Ni-F//graphite 37 F g-1 at 1 mA cm-2 11.43 Wh kg-1 at 51 720 W kg-1 15 NiCo2O4/rGO//AC 99.4 F g-1 at 1 A g-1 23.3 Wh kg-1 at 52 325 W kg-1 16 NiCo2S4@Ni3V2O8//AC 150 C g-1 at 0.5 A g-1 42.7 Wh kg-1 at 53 200 W kg-1 17 Ni2P/Ni/C//AC 117 F g-1 at 1 A g-1 32.02 Wh kg-1 at 54 700 W kg-1 18 Co9S8/NS-C//AC 75.59 F g-1 at 1 A g-1 14.85 Wh kg-1 at 55 682 W kg-1 19 PB@MnO2//PG 98 F g-1 at 1 A g-1 16.5 Wh kg-1 at 56 550 W kg-1 20 MWCNTs/ZrO2//MWCNTs/WO3 198 F g-1 at 1 A g-1 65 Wh kg-1 at 966 57 W kg-1 21 SiC//SiC/Fe3O4 95 F g-1 at 0.5 A g-1 49 Wh kg-1 at 464 58 W kg-1 22 AC//Co3(PO4)2 85 F g-1 at 1 A g-1 26.66 Wh kg-1 at 59 750 W kg-1 23 Nickel pyrophosphate 1D 224 F g-1 at 1 A g-1 70 Wh kg-1 at 750 Present Work nanorods//N-doped rGO W kg-1



Table of Content


Page 38 of 38

Redox Additive-Improved Electrochemically and Structurally Robust

Recommend Documents