Recent Advances in Two-Dimensional Nanomaterials for

Recent Advances in Two-Dimensional Nanomaterials for Supercapacitor Electrode Applications Kowsik Sambath Kumar,†,‡ Nitin Choudhary,† Yeonwoong Jung,†,‡,§ and Jayan Thomas*,†,‡,⊥ †

NanoScience Technology Center, University of Central Florida, Orlando, Florida 32816, United States Department of Materials Science and Engineering, University of Central Florida, Orlando, Florida 32816, United States § Department of Electrical and Computer Engineering, University of Central Florida, Orlando, Florida 32816, United States ⊥ CREOL, College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, United States ‡

ABSTRACT: Supercapacitors represent a major technology to store energy for many applications including electronics, automobiles, military, and space. Despite their high power density, the energy density in supercapacitors is presently inferior to that of the state-ofthe-art Li-ion batteries owing to the limited electrochemical performance exhibited by the conventional electrode materials. The advent of two-dimensional (2D) nanomaterials has spurred enormous research interest as supercapacitor electrode materials due to their fascinating electrochemical and mechanical properties. This Review discusses cutting-edge research on some of the key 2D supercapacitor electrode materials including transition metal dichalcogenides, transition metal oxides and hydroxides, MXenes, and phosphorene. Various synthetic approaches, novel electrode designs, and microstructure tuning of these 2D materials for achieving high energy and power densities are discussed. high surface area and specific pore sizes.9,10 Another group of electrochemical capacitors (ECs), broadly known as pseudocapacitors, utilize fast and reversible faradaic reactions on the electrode surface (or near-surface) for charge storage. Supercapacitors with pseudocapacitive materials can achieve significantly higher energy density compared to EDL capacitors as they have a variety of oxidation states for redox charge transfer reactions. However, relatively low electrical conductivity and poor cycle stability due to slow ion transfer rates are limitations, hampering their widespread commercial realization.11,12 The aforementioned limitations of the traditional ECs drive significant research interests for the development of novel electrode materials exhibiting excellent electrochemical performances to achieve high energy density. In recent years, twodimensional (2D) materials have drawn considerable attention as supercapacitor electrode materials. 2D materials are ultrathin layered crystals that show unusual physiochemical properties at single- or few-atom thickness. 2D materials offer several key advantages for next-generation electrochemical devices: (i) atomically thin 2D nanosheets (NSs) provide a larger surface area due to complete exposure of the surface atoms, (ii) the edge

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he world’s appetite for energy generation/storage from renewable sources like solar, wind, and hydro is directly driven by an exhaustive use of depleting fossil fuels. Unfortunately, the intermittent nature of these renewable energy sources makes them incompetent for the ever-increasing demand for energy.1,3 The development of sustainable energy generation/storage systems and technologies is critical to reliably cater the commercial/residential energy requirements and to reduce the severe economic impacts of intermittent energy sources.2 Rechargeable lithium-ion batteries (LIBs) have already cemented their position as dominant energy sources powering almost all forms of consumer electronics and electric vehicles (EVs).4,5 Albeit the advantage of being a high energy density provider, LIBs have the limitations of poor cycle life, low power performance, and fire hazards.6 Supercapacitors, also known as ultracapacitors, are currently being considered as an alternative technology to LIBs because they are safer, pack ∼10 times more power density, and last for several tens of thousands to millions of charge/discharge cycles.7,8 Supercapacitor technology has been improving at a fast pace, and they are highly reliable, as evidenced by their use in the emergency exits of Airbus 380 aircrafts.3 One of the major drawbacks of supercapacitors is their intrinsically low energy density. The limitation is mainly attributed to the purely electrostatic storage of charges in electrodes driven by electrical double layer (EDL) formation, which strictly requires © 2018 American Chemical Society

Received: November 23, 2017 Accepted: January 22, 2018 Published: January 22, 2018 482

DOI: 10.1021/acsenergylett.7b01169 ACS Energy Lett. 2018, 3, 482−495

Review

Cite This: ACS Energy Lett. 2018, 3, 482−495

ACS Energy Letters

Review

(ASC) device configuration utilizing two dissimilar electrodes having dissimilar voltage windows has enabled a significant boost in the energy density of supercapacitors in the past few years.26,27

sites in 2D NSs are chemically more reactive than basal planes and the open van der Waals gaps enable the intercalation of electrolyte ions, and (iii) the high mechanical strength and flexibility at atomic dimensions allow them to be used in nextgeneration wearable electronics. For example, graphene, a 2D sheet of carbon, has remained a major source of scientific fascination for energy storage over the past decade.13 Beyond graphene, a large family of 2D materials have been emerging as viable electroactive components for next-generation energy storage devices while taking over the functions initially assigned to LIBs. 2D transition metal dichalcogenides (TMDs) are considered as major postgraphene contenders, which display unique properties of large surface area and variable oxidation states (e.g., +2 to +6 in MoS2), making them viable for both EDL and faradaic charge storage mechanisms.14,15 The most interesting attribute of 2D TMDs is their different polytype (1T and 2H phase) structures, which show extraordinary electrochemical performance and high operational voltage windows.16 Transition metal oxides and hydroxides (TMOs and TMHs), pseudocapacitive materials commonly used in bulk forms, have intrinsically low electronic conductivities, limiting fast ion diffusions, which makes them incompetent for high rate performances. However, developing TMOs and TMHs in the form of 2D nanostructures as nanofilms, NSs, nanoflakes, nanoplatelets, nanopetals, nanobelts, etc. significantly alter their inherent properties compared to their bulk counterparts. These nanostructures often provide high conductivity, easy ion diffusion, and improved mechanical integrity.17−20 It should be noted that 2D TMOs and TMHs are not layered materials like graphene and 2D TMDs, but a similar “2D” terminology is used because of their ultrasmall (a few atom) thickness, exhibiting extraordinary properties different from their bulk counterparts. 2D carbides and nitrides of transition metals, i.e., MXenes have recently emerged as potential candidates for supercapacitor electrodes due to their large surface-to-volume ratio, high intrinsic conductivity, and abundant electrochemically active sites, leading to significant intercalation or pseudocapacitance through their transition metal chemistry.21 Phosphorene, an analogue of graphene is emerging as a new 2D material, possessing a puckered lamellae structure with weakly bonded layers of phosphorus atoms.22 Despite its high electrical conductivity, thermodynamic stability, and fast ion diffusivity, phosphorene still remains largely unexplored as a supercapacitor electrode material compared to other similar 2D materials. Hence, an array of novel electrode materials exhibiting an EDLC or pseudocapacitive charge storage mechanism are currently being developed for high-performance supercapacitors. Beyond materials development, synthesis of hybrid electrode materials comprising of two or more electrochemically active 2D nanomaterials (EDLC or pseudocapacitive) in the form of composites, core/shell structures, and heterostructures have been simultaneously pursued.23−25 These hybrid designs introduce more electrochemical active sites, improved rate capability, and enhanced cycle stability in the supercapacitors owing to the synergistic effects enabled by high-quality heterointerfaces, tunable electronic/chemical properties, and high structural stabilities. Nevertheless, the construction of a novel supercapacitor device configuration using these electrodes is gaining significant attention as the energy density of the devices relies not only on the capacitive performance of the electrode but also on the higher operational voltage window (i.e., energy density = (1/2)CV2, where C is the capacitance and V is the operation voltage).8 For example, an asymmetric supercapacitor

2D TMOs and TMHs are not layered materials like graphene and 2D TMDs, but a similar “2D” terminology is used because of their ultrasmall (a few atom) thickness, exhibiting extraordinary properties different from their bulk counterparts. This Review emphasizes the recent developments of novel 2D electrode materials such as TMDs, TMOs and TMHs, MXenes, and phosphorene and their electrochemical performances for developing high energy density supercapacitor electrodes and devices. The impact of crystal structures, crystalline phases, and electrical/chemical properties inherent to these materials on the supercapacitor electrode performances is discussed in detail. We also deliberate how the characteristic properties of these materials such as specific surface area, electrical conductivity, and porosity can be tuned by engineering their nanostructures into different form factors such as quantum dots, nanospheres, nanowires, and nanorods. Design principles for rationally integrating 2D materials with other electroactive materials to achieve hybrid electrodes (composites, core/shell, and threedimensional (3D) nanostructures) are also highlighted. (1) 2D Transition Metal Dichalcogenide (TMD) Electrodes. 2D TMDs are layered materials in which a unit cell is composed of a transition metal (M) layer sandwiched between two chalcogen (X) layers in the form of MX2 (where M = Mo or W and X = S, Se, or Te).28,29 The large surface area and variable oxidation states in TMDs allow electrical double layer and fast/reversible redox charge storage mechanisms. In addition, 2D TMDs exhibit high electrochemical activity derived from their edge sites, which offer large energy storage capability in supercapacitors.23,30−32 Despite a plethora of research conducted on monolithic 2D TMDs based supercapacitor electrodes, their electrochemical performance is often limited by poor cycle life, inherently low electrical conductivity, large volume change during cycling, and restacking.33,34 For example, sheet-like morphology of MoS2 proposed by Soon et al.32 provided a large surface area for double layer storage; however, owing to its poor electrical conductivity, it exhibited a low specific capacitance of ∼100 F g−1 at a scan rate of 1 mV s−1. To circumvent these issues, 2D TMDs have been mixed, wrapped, or deposited with highly conductive/electroactive materials such as carbonaceous materials and conducting polymers (CPs) using various top-down/bottom-up synthetic techniques and their combinatorial approaches.35−42 Most notably, the development of metallic TMD nanomaterials, as discussed later in this section, are revolutionizing supercapacitor research by offering exceptionally high electronic/ionic transport and unprecedented charge storage ability.16,43,44 (1.1) TMDs/Carbonaceous Materials Hybrids. Typically, carbonaceous materials like graphene, carbon nanotubes (CNTs), carbon aerogel, etc. have high electrical conductivity and surface area. Developing TMD hybrids or composite materials with carbon-based materials provides a synergistic effect of the two materials; carbon offers conductive channels and enhancing interfacial contact, while TMDs contribute a short ion diffusion path and successive short electron transport path, thus 483

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Figure 1. Different hybrid designs for high-performance TMDs supercapacitors. SEM images of (a) aligned MWCNT sheets, (b) MWCNT/MoS2 hybrids, and (c) tightly knotted MoS2/MWCNT and rGO/MWCNT fibers. (d) CV curves of rGO/MWCNT (cathode) and MoS2-rGO/ MWCNT (anode) in different voltage windows. (e) Cycle test of the fiber-based asymmetric device at a current density of 0.55 A cm−3. Adapted with permission from ref 48, Copyright 2015 Wiley Online Library. (f) SEM and TEM images showing the formation of PANI nanoneedles on MoS2 NSs. (g) Ragone plot showing the energy and power density at different voltage windows. (h) Schematic representation of facile H+ ion intercalation into a MoS2/PANI composite. Adapted with permission from ref 24, Copyright 2011 Wiley Online Library.

enhancing overall electrochemical performance.45,46 A 3D nanocomposite of MoS2/MWCNT (multiwall carbon nanotube) prepared by a facile one-pot L-cysteine-assisted hydrothermal process offered a large surface area and fast ionic transport properties and delivered a high specific capacitance of 452.7 F g−1 with 95.8% retention after 1000 cycles.47 Almost three times increase in capacitance (149.6 to 452.7 F g−1) was observed in the composite compared to bare MoS2. Another interesting electrode was designed by incorporating 2D MoS2 and reduced graphene oxide (rGO) NSs into aligned MWCNT fibers.48 Figure 1a−c shows the scanning electron microscopy (SEM) images of the aligned MWCNT fibers and their hybrids with MoS2 and rGO. The solid-state supercapacitor fabricated using a MoS2-rGO/MWCNT fiber electrode operates at a stable potential window of 1.4 V, as shown by the CV curves in Figure 1d. Moreover, the device yields 100% Coulombic efficiency in bending state even after 7000 cycles (Figure 1e). A high energy density of 78.9 Wh kg−1 and an operational voltage window of 2.0 V have been reported in an ASC based on flower-like MoS2 grown on graphene NSs (GNSs) and MnO2/graphene hybrid electrodes.49 This is an impressive achievement because the energy density of the supercapacitor is approaching closer to thin film LIBs (∼100 Wh kg−1).5

(1.2) TMDs/Conductive Polymers (CPs) Hybrids. CPs are another promising additive that significantly enhances the capacitive performance of 2D TMDs owing to their high redox-active capacitance, moderately high electrical conductivity, low cost, and high intrinsic flexibility.50,51 In situ polymerization methods have been mainly employed to synthesize TMDs/CPs hybrids as this method yields a better dispersion of TMD flakes in CPs while inhibiting restacking of TMDs layers.52 Controlled growth of polyaniline (PANI) nanowires on internal and external faces of the 3D tubular MoS2 have been reported via oxidative polymerization of aniline monomers. The hybrid electrode exhibited a faradaic behavior with a high specific capacitance of 552 F g−1 at a current density of 0.5 A g−1. The improved electrochemical performance was attributed to the unique 3D tubular design that facilitates access of electrolyte ions to the active sites of the electrode surface.53,54 Following a similar approach, the morphological tuning of 2D MoS2 NSs was achieved24 in which 1D PANI nanoneedle arrays vertically stand on either side of the exfoliated 2D MoS2 NSs, as shown in SEM and transmission electron microscopy (TEM) images in Figure 1f. This highly electrochemically active 2D MoS2@PANI composite with large surface area and good electrical conductivity yields a high specific capacitance of 853 F g−1 at a 484

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Figure 2. (a) SEM side view of the stacked 1T-MoS2 layers. (b) Volumetric capacitance of the 1T-MoS2 electrode at different scan rates and in different electrolytes. (c) Cyclic stability test of 1T-MoS2 in different electrolytes. Adapted with permission from ref 16, Copyright 2015, Nature Publishing Group. (d) SEM image of the VS2·3NH3 precursor with the inset showing a flake thickness of about 110 nm. (e) HR-TEM image of the exfoliated VS2 NSs along (001) facets. (f) Specific capacitance and Coulombic efficiency of the metallic VS2 electrode. Adapted with permission from ref 43, Copyright 2011, American Chemical Society.

Figure 3. (a) Specific capacitance vs scan rate of the 2D MnO2 nanosheets; the inset is the capacitance versus discharging current density. Photographs of the (b) “panda” asymmetric MS/GA supercapacitor lighting up a red light-emitting diode. (c) Electrodes demonstrating mechanical flexibility. Adapted with permission from ref 18, Copyright 2013 Springer Nature Publishing Group. (d) SEM image of Co3O4 nanoarrays. (e) Specific capacitance vs charge and discharge current density. Adapted with permission from ref 76, Copyright 2013 Springer Nature Publishing Group.

(1.3) Metallic TMDs. Thus, far, supercapacitors based on TMDs have mainly relied on the thermodynamically stable 2H phase, which shows semiconducting yet poorly conducting

current density of 1 A g−1. A very impressive energy density of 106 Wh kg−1 (Figure 1g,h) was also achieved. 485

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Figure 4. (a) TEM image of single-layer β-Co(OH)2 and colloidal ethanol dispersion displaying the Tyndall effect. (b) Schematic illustration of the hydrogen adsorption/desorption process in the single-layer β-Co(OH)2 electrode. Adapted with permission from ref 90, Copyright 2014 Wiley Online Library. (c) Schematic illustrating synthesis of α-Ni(OH)2 NSs. (d) FESEM image of α-Ni(OH)2 NSs. (e) Specific capacitance as a function of current densities. Adapted with permission from ref 20, Copyright 2014 Nature Publishing Group.

their intrinsically rich redox activity but also due to enlarged surface areas endowing surface sites for EDL charge storage.55−62 Moreover, the thin/flexible TMO electrodes with high mechanical and chemical stabilities provide the opportunity to integrate them with next-generation flexible devices.63−66 Among various TMOs, manganese dioxide (MnO2) has been extensively investigated owing to its low cost, nontoxicity, and high theoretical capacitance (1370 F g−1).56,67,68 Ultrathin MnO2 NSs (MSs) (thickness ≈ 2 nm) synthesized using a soft template method exhibited a specific capacitance of 774 F g−1 at a current density of 0.1 A g−1 (Figure 3a).18 The ASCs fabricated using MSs and graphene electrodes (GAs) exhibited a very high energy density of 97.2 Wh kg−1 with 97% capacitance retention after 10000 cycles. These electrodes could be designed into various pictures and patterns via screen printing (e.g., a panda design in Figure 3b) and were made flexible (Figure 3c). In a recent attempt,69 defects/vacancies were created in birnessite MnO2 (i.e., δ-MnO2, a 2D MnO2 allotrope with excellent capacitive properties) in a controlled manner to achieve additional cation intercalation sites. The exfoliated and reassembled δ-MSs form 3D macroporous electrodes, which provided a pseudocapacitance of ≥300 F g−1 attributed to the synergistic effects between the defect content and Mn redox, which worked together to lower the charge transfer resistance and promote the ion intercalation. Despite some successful demonstrations, 2D MnO2-only electrodes suffer from intrinsically poor electronic conductivities, resulting in low power density and rate performance. Thereby, MnO2 hybrids with conductive yet capacitive materials such as graphene and CNTs have been proposed.70−73 For example, a planar supercapacitor developed by using a 2D δ-MnO2/ graphene hybrid exhibits a high rate capability of ∼208 F g−1 at 10 A g−1 and capacitance retention of about 92% after 7000 charge/discharge cycles.63 Iron oxide (Fe3O4) has also been used as an active 2D TMO component with carbon NSs and GNSs to form hybrid composites.74,75 A 2D sandwich-like graphenesupported Fe3O4 electrode formed by electrochemical method exhibited a capacitance of 329 F g−1 at 0.5 A g−1 and an excellent

characteristics. Recently, the introduction of a metallic 1T phase in 2D TMDs is revolutionizing their potential for device applications. The high electrical conductivity and excellent ion intercalation capability in 1T-TMDs facilitate electron/ion diffusion and dramatically enhance the specific capacitance.44 The Chhowalla research group16 demonstrated ∼107 times higher electrical conductivity in 1T-MoS2 films as compared to that of their semiconducting (2H-MoS2) counterparts. The electrode assembled by stacking exfoliated 1T- 2D MoS2 NSs, as shown in the SEM image of Figure 2a, yields a high capacitance in the range of ∼400−700 Fcm−3 (Figure 2b) in various aqueous/ organic electrolytes. This electrode can operate at 3.5 V in organic electrolytes and provides a Coulombic efficiency of ∼95% over 5000 charge/discharge cycles (Figure 2c). 2D vanadium disulfide (VS2), an emerging metallic TMD, has been reported to form in-plane supercapacitors.43 Figure 2d is an SEM image of the ammonia-assisted VS2 flakes in bulk VS2 precursor, where the thickness of an individual flake is ∼110 nm (inset of Figure 2d). The 2D VS2 NSs extracted from these ammoniated VS2 flakes were 4−5 atomic layers thick, as shown in Figure 2e. Electrochemical characterizations revealed a specific capacitance of 4760 μF cm−2 without any capacitance loss even after 1000 charge/discharge cycles (Figure 2f). (2) 2D Transition Metal Oxides and Hydroxides Electrodes. 2D oxides and hydroxides of transition metals are promising supercapacitor electrode materials for their high chemical stability/compatibility, high specific capacitance, and environmental friendliness. In contrast to the bulk TMOs and TMHs, which show poor rate performances due to intrinsically low electronic conductivity impeding their fast ion diffusivity; their 2D counterparts provide an alternate route for compensating the aforementioned shortcomings by offering short diffusion paths, high electronic conductivity, and large surface areas. In the following sections, we describe the development of 2D TMOand TMH-based supercapacitor electrode materials. (2.1) 2D Transition Metal Oxides (TMOs). 2D TMOs (e.g., MnO2, Fe3O4, Co3O4, NiO, etc.) offer exciting properties as promising supercapacitor electrode materials not only because of 486

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energy density of 85 Whkg−1 at a power density of 2.4 kWKg−1.75 Uniformly distributed Fe3O4 on graphene sheets prevents aggregation of Fe3O4 nanoparticles, offering a cycle life of 1000 cycles with 95% capacitance retention. Ultrathin cobalt oxide (Co3O4) is another potential pseudocapacitive electrode material offering low environmental impact, low cost, and great redox activity. A uniform NS array composed of ∼5 μm long, ∼10 nm thick Co3O4 NSs (Figure 3d) developed by a two-step hydrothermal reaction exhibited a specific capacitance of 1782 F g−1 at a current density of 1.8 A g−1 (Figure 3e) and retained >90% of initial capacitance after 2000 cycles.76 Porous 2D TMObased nanostructures play a critical role in boosting its surface area, offering short pathways for charge transport and better contact with electrolyte and accommodating substantial volume expansion during cycling processes.77 Novel 3D porous Co3O4 NS structure exhibited a remarkable specific capacitance of 1500 F g−1 at 1 A g−1 and provided a high energy density of 15.4 Whkg−1 in ASC device configuration.78 In another report, binder-free synthesis of mesoporous Co3O4 NS arrays on the Ni foam using a two-step strategy of cobalt hydroxide (Co(OH)2) electrodeposition followed by calcination witnessed a high specific capacitance of 2735 F g−1 at a current density of 2 A g−1 in 2 M potassium hydroxide (KOH).79 Besides Co3O4, porous hollow spheres enabled by randomly oriented 2D nickel oxide (NiO) NS electrodes achieved a high specific capacitance of 600 F g−1 at 10 A g−1 and an excellent energy density of 19.44 Wh kg−1.80 The good electrochemical performance in these electrodes is attributed to the enhanced ionic transport of electrolyte ions in the porous NiO NS network. (2.2) 2D Transition Metal Hydroxides (TMHs). Similar to TMOs, TMHs have been widely employed as high-performance pseudocapacitive electrode materials in supercapacitors.81−85 However, their electrochemical performance is mainly dictated by their morphology, and in bulk electrode structures, the shortcoming was found to be due to the inefficient utilization of active materials.27,86,87 Specifically, scaling of bulk TMHs to their 2D counterparts enables facile ion/electron transport, large strain accommodation during cycling, and a reduced ion diffusion barrier due to structural changes, which provides excellent charge storage as well as high rate capability.20,88,89 TMHs of Co (i.e., Co(OH)2) and Ni (i.e., nickel hydroxide (Ni(OH)2) have been mainly explored in their 2D nanostructures. For example, a solid-state ASC consisting of single layers of β-Co(OH)2 (Figure 4a) as the cathode and N-doped graphene as the anode delivered a high operation voltage of 1.8 V and an exceptionally high energy/power density of 98.9 Wh kg−1/17981 W kg−1.90 Moreover, the device retained 93.2% capacitance after 10000 cycles. Performance akin to that of LIBs is attributed to the 100% exposure of surface hydrogen atoms by five-atom thick Co(OH)2 single layers to serve as electroactive sites for significant faradaic redox reactions, as demonstrated in the schematic of Figure 4b. 2D Ni(OH)2 is another cost-effective material that presents excellent electrochemical properties for high energy density supercapacitors. Ultrathin, free-standing 2D NSs (thickness < 2 nm) of α-Ni(OH)2 were prepared using a microwave-assisted liquid-phase growth method.20 Figure 4c,d shows the growth process and SEM image of the as-synthesized α-Ni(OH)2 NSs. These ultrathin 2D nanostructures exposed the majority of the atoms for surface-dependent electrochemical reaction processes, exhibiting a maximum specific capacitance of 4172.5 F g−1 at a current density of 1 A g−1. Even at a higher rate of 16 A g−1, the specific capacitance was maintained at 2680 F g−1 with 98.5% retention after 2000 cycles (Figure 4e).

Despite remarkable capacitive performance exhibited by single-component 2D TMHs like Co(OH)2 and Ni(OH)2, these may suffer significant capacitance decay after prolonged cycling at high current densities. To circumvent this issue, hybrids of 2D TMHs are being developed by growing/mixing them with highly conductive materials, metal oxides, or other 2D TMHs. For instance, single-crystalline Ni(OH)2 nanoplates were grown on 3D graphene sheet networks.91 The composite delivered high specific and rate capacitances of ∼1335 F g−1 at 2.8 A g−1 and ∼953 F g−1 at 45.7 A g−1, respectively. The energy density was estimated to be ∼37 Wh kg−1 at a power density of ∼10 kW kg−1. 2D TMH/TMH hybrid electrodes present advanced supercapacitors due to their enhanced redox activity bestowed by multiple redox reactions in each participating TMH. 2D porous Ni(OH)2-Co(OH)2 hybrids were facilely prepared in ethylene glycol−water solution.92 These electrodes exhibit an excellent specific capacitance of 1537 F g−1 at 0.5 A g−1 and retained ∼1181 F g−1 even at a high current density of 10 A g−1. The ASCs fabricated with carbonaceous electrodes achieved a high energy density of 33.7 Wh kg−1 at a power density of 551 W kg−1 and an operating voltage of 1.5 V. Moreover, an excellent cycling stability with 109% capacitance retention after 10000 cycles was observed. A novel Ni(OH) 2 /Co(OH) 2 NS architecture composed of electrodeposited Ni(OH)2 and Co(OH)2 consecutive layers was recently developed.93 These electrodes showed a specific capacitance of 1524 F g−1 at a specific current of 1 A g−1. An ASC using Ni(OH)2/Co(OH)2 as the positive electrode and carbon nanofoam paper as the negative electrode yielded a specific energy density of 101.3 Wh g−1. TMH−TMO hybrids have also been explored to achieve high energy densities in supercapacitors. A hydrothermally grown ultrathin Ni(OH)2−MnO2 hybrid NS array on 3D macroporous nickel foam exhibits an ultrahigh specific capacitance of 2628 F g−1.94 The highly hydrophilic and ultrathin nature of hybrid NSs with high synergetic effects between Ni(OH)2 and MnO2 was responsible for the outstanding electrochemical performance. Nevertheless, the ASCs assembled using a Ni(OH)2−MnO2 cathode and rGO as the anode provided a very high energy density of 186 Wh kg−1 and a power density of 778 W kg−1. (3) MXene Electrodes. The major limitation of low electrical conductivity in TMOs/TMHs needs modification of their structural morphology or development of their composites with highly conductive additives to effectively use them as supercapacitor electrodes. However, a material with inherently high electrical conductivity on its own holds great promise as an excellent energy storage electrode. MXenes, an emerging class of 2D supercapacitor electrode material, possesses a high intrinsic electronic conductivity, good hydrophilic nature, and excellent mechanical stability.95,96 MXenes are few atoms thick transition metal carbides, nitrides, or carbonitrides derived from the MAX phase having a chemical composition of Mn+1AXn, where n = 1, 2, or 3, M is an early transition metal, A is mostly group 13 and 14 elements of the periodic table, and X is C and/or N.97 MXenes, developed by extracting A from the MAX phase, is denoted as Mn+1XnTx, where T represents surface termination groups (−O, −OH, and −F) left over from the etching process and x is the number of terminating groups. Here, we describe several significant developments in the MXene-based supercapacitors. (3.1) Synthesis, Surface Modif ication, and Mass Loading. Gogotsi’s group at Drexel University pioneered the development of MXene-based supercapacitors.21,98 They exfoliated titanium aluminum carbide (Ti3AlC2) in the presence of hydrofluoric acid (HF) to produce 2D NSs of titanium carbide (Ti3C2) exhibiting 487

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High mass loading of the active material is an important aspect when it comes to practical application of supercapacitors. An increase in material loading typically increases the resistance and limits electrolyte accessibility, leading to poor capacitive performance. Lin et al.102 discovered that Ti3C2 is exceptional because its layered microstructure helps in the electrochemical utilization of all of the Ti3C2 sheets and exhibits high electronic conductivity. The electrode made from these NSs with a high mass loading of 7.6 mg achieved an areal capacitance of 579 mF cm−2 at 2 mV s−1 compared to 211 mF cm−2 with 1.8 mg of material. Also, the good connectivity between Ti3C2 sheets makes the equivalent series resistance (ESR) value of high mass loading (0.90 Ω, 7.6 mg) Ti3C2 electrode material comparable to that of the low mass loading (0.83 Ω, 1.8 mg) material. This mass loading test opens up the significance of using 2D layered nanomaterials for achieving high energy for practical applications. (3.2) Flexible MXenes. Developing flexible energy storage devices is highly necessary for designing next-generation smart electronics and wearables.103−105 Peng et al.106 fabricated an allTi3C2Tx solid-state interdigital microsupercapacitor (MSC) by solution spray-coating of large size (3−6 mm laterally) HCl/LiF etched Ti3C2Tx as the current collector, which was followed by spray-coating of small size HCl/LiF etched Ti3C2Tx (∼1 mm) with more defects and edges on top as the electroactive layer. The device exhibited areal and volumetric capacitances of ∼27 mF cm−2 and ∼357 F cm−3, respectively, at a scan rate of 20 mV s−1 with 100% capacitance retention after 10000 cycles at a scan rate of 50 mV s−1. Recently, a symmetric solid-state flexible fiberbased MXene supercapacitor107 was developed using Ti3C2Tx flakes loaded on silver-plated nylon fibers as electrodes and a polyvinyl alcohol (PVA)-H2SO4 hydrogel as the electrolyte. The fabricated fiber device exhibited a high areal capacitance of 328 mF cm−2 with excellent cyclability, flexibility, knittability, and cycle stability. The capacitance remained at least above 80% under various deformation modes including bending, twisting, and knotting. At a scan rate of 2 mV s−1, it showed an energy density of 7.3 μWh cm−2 and a power density of 132 μW cm−2. These results promise a plethora of opportunities for powering wearable and flexible electronics applications employing MXenes and their heterostructures. (3.3) MXene-based Composites. Like other 2D materials, restacking and irreversible aggregation of 2D NSs is a potential

graphene-like morphology with good chemical stability, electrical conductivity, and high ductility. A free-standing conductive paper made from these Ti3C2 MXene NSs exhibited a high volumetric capacitance of 340 F cm−3 in KOH.98 Ghidiu et al. produced freestanding Ti3C2 with intercalated water, adopting a much safer route using a solution of lithium fluoride (LiF) and hydrochloric acid (HCl) as an etchant.99 The MXene produced was a clay-like paste that can be molded to yield various shapes with high conductivity and can also be used as an ink to print MXene on various substrates. Electrochemical performance of these Ti3C2 freestanding films in 1 M Na2SO4 exhibited a volumetric capacitance of 900 F cm−3 at 20 mV s−1, a gravimetric capacitance of 245 F g−1 at 2 mV s−1, and magnificent cycle stability (100% capacitance retention after 10000 cycles). The outstanding

The outstanding performance of the LiF + HCl etched Ti3C2 compared to HF etched Ti3C2 is attributed to preintercalated water and the smaller size of the cation (H+) in the electrolyte.

performance of the LiF + HCl etched Ti3C2 compared to HF etched Ti3C2 is attributed to preintercalated water and the smaller size of the cation (H+) in the electrolyte. Another approach to develop Ti3C2Tx films is a simple dropping-mild baking method, which demonstrated high gravimetric capacitances up to 499 F g−1 even with low mass loading.100 To study the effect of surface chemistry on capacitive performance, Simon and co-workers101 modified Ti3C2Tx via (i) delamination of Ti3C2Tx by intercalation of dimethyl sulfoxide (d-Ti3C2) and (ii) chemical intercalation of K+ ion using potassium hydroxide (KOH-Ti3C2) and potassium acetate (KOAc-Ti3C2) (Figure 5a). Chemical intercalation of potassium salts replaced terminal fluorine with oxygen-containing functional groups, leading to a 4-fold increase in capacitance. Delamination led to an increase in surface area, causing the dTi3C2 to exhibit outstanding volumetric capacitance of 520 F cm−3 and a gravimetric capacitance of 325 F g−1 at 2 mV s−1. This work led to a new approach of modifying surface chemistry of various MXenes to achieve better electrochemical performance.

Figure 5. (a) Schematic illustration of the modifications of Ti3C2Tx: delamination and intercalation of K+. Adapted with permission from ref 101, Copyright 2014 Elsevier. (b) Cross-sectional SEM image of sandwich-like Ti3C2Tx/SWCNT. (c) Cycling stability of the sandwich-like Ti3C2Tx/ SWCNT electrode at 5 A g−1. (Inset) First three cycles of GCD. Adapted with permission from ref 114, Copyright 2014 Wiley Online Library. 488

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maintaining 1.43 F cm−3 (4.8 F g−1) of volumetric capacitance at 10 V s−1. Phosphorene electrode, even after prolonged (30000) charge/discharge cycles, exhibits high mechanical stability, maintaining its crystalline structure as investigated under TEM with corresponding capacitance retention of 71.8%. Red phosphorus (RP), one of the allotropes of phosphorus with high theoretical specific capacitance (similar to BP)22 has been used to make a hybrid electrode material composed of BP and RP. A simple sonochemical process118 was followed to develop a single elemental hybrid of BP and RP (BP/RP hybrid), and the hybrid electrode was electrochemically tested in 0.1 M KOH electrolyte. The BP/RP hybrid electrode demonstrated typical EDLC behavior even at higher scan rates and yielded specific capacitances of 60.1 and 41.6 F g−1 at 0.5 and 8 A g−1 current densities, respectively. Hybridization of BP and RP facilitated fast and short electronic/ionic transfer, leading to high reaction kinetics with good cyclic stability. The hybrid cycled for 2000 cycles retained 83.3% of its initial capacitance, higher than that of RP. Microsupercapacitors (MSCs) with a mask-assisted interdigital electrode pattern have been fabricated using layerby-layer stacking of phosphorene NSs and electrochemically exfoliated graphene (PG-MSCs) in ionic liquid electrolyte.119 The simplified mask-assisted development of PG-MSCs is scalable for production of a serially interconnected MSC pack and provides excellent flexibility, as shown in Figure 6a,b. MSCs

problem in MXene electrodes as well, which essentially diminish the interlayer spacing and accessibility of ions for electrochemical utilization. Interlayer spacing can be modulated by developing MXene composites by incorporating species like polymers,108,109 carbon derivatives,110,111 and metal oxides108,112 into the layers. These interlayer spacers prevent restacking of the MXene flakes and thus increasing electrochemical utilization of the entire surface of the material. A MAX phase−polymer composite was first developed by Ling et al. using poly(diallyldimethylammonium chloride) (PDDA) and PVA.113 Ti3C2Tx films on their own possess excellent mechanical and tensile strength (22 ± 2 MPa). Upon introducing PVA, the Ti3C2Tx/PVA film exhibited a four times increase in tensile strength. Introduction of PVA also prevented restacking and increased interlayer spacing of Ti3C2Tx flakes, thus improving the accessibility of ions to deep trap sites. The composite showed maximum capacitance of 530 F cm−3 at 2 mV s−1. Zhao et al.114 prepared sandwich-like MXene/CNT composite electrodes by alternate deposition of Ti3C2Tx flakes and CNT layers (Figure 5b). CNT provided faster diffusion paths and enlarged interlayer spacing between the Ti3C2Tx flakes for cation intercalation. A high volumetric capacitance of 350 F cm−3 at a 5 A g−1 current density was achieved with no degradation even after 10000 cycles for MXene/SWCNT papers (Figure 5c). Incorporation of rGO into Ti3C2Tx flakes increased the interlayer spacing and exhibited a volumetric capacitance of 435 F cm−3 at 2 mV s−1. Despite the high areal and volumetric capacitance, MXenes suffered from poor gravimetric capacitance because of their low surface area (30 m2 g−1). Decorating MXenes with pseudocapacitive materials improve the surface area considerably. The first MnO2/MXene hybrids (MnO2/Ti3C2Tx) fabricated by direct chemical synthesis evidence a large surface area of 183.8 m2 g−1 compared to pristine Ti3C2Tx (21.1 m2 g−1).115 Such an increase in surface area enhances the effective contact of electrolyte and active materials by trapping more electrolyte solution. This provides superior pseudocapacitance and improves overall cycling performance. Specific capacitance values of 77.5 and 210.9 F g−1 are obtained for symmetric supercapacitors of Ti3C2Tx and MnO2/Ti3C2Tx samples, respectively, at a constant scan rate of 10 mV s−1. At a constant power density of 20 kW kg−1, the MnO2/Ti3C2Tx hybrid exhibited an energy density of 12.25 Wh kg−1 with good cyclic stability (88% capacitance retention) after 10000 cycles. Hybrid electrodes of MXene with titanium dioxide (TiO2) and MoO3 have also been developed.108,112 A hybrid of TiO2/Ti3C2Tx108 exhibited a gravimetric capacitance of 143 F g−1, retaining ∼92% of its initial capacitance after 6000 cycles. MoO3, being a highly electrochemically active material, increases the specific surface area and active sites, delivering a maximum capacitance of 151 F g−1 with 93.7% capacitance retention after 8000 cycles of the charge−discharge process.112 (4) Phosphorene Electrodes. Another recently emerging 2D material for energy storage applications is phosphorene, a single atomic layer of black phosphorus (BP) analogous to graphene. Phosphorene, a puckered lamellae structure with weakly bonded layers of phosphorus atoms via van der Waals interactions, shows good electrical conductivity, thermodynamical stability, and fast ion diffusivity. These properties make phosphorene an attractive candidate for supercapacitor applications.22,116 Interestingly, a flexible solid-state supercapacitor117 based on liquid phase exfoliated phosphorene nanoflakes has been recently developed, which exhibits an impressive volumetric capacitance of 17.78 F cm−3 (59.3 F g−1) at 0.1 V s−1 and excellent rate capacitance while

Figure 6. (a) Photograph of nine serially interconnected PG-MSCs. (b) Flexibility and stability demonstration of PG interdigital electrodes at a highly folded state with the inset showing electrodes in the unfolding state. Reprinted with permission from ref 119, Copyright 2017 American Chemical Society.

provided excellent electrochemical performance of 9.8 mF cm−2 and volumetric capacitance of 37.5 F cm−3 at 5 mV s−1 and retained 94% of their initial capacitance under different bending states. The excellent performance of PG-MSCs can be attributed to the strong coupling of phosphorene and GNSs offering high ionic accommodation and fast transport pathways and restricting graphene restacking due to phosphorene. The graphene serves as a mechanical skeleton, providing high-speed electron transfer network. The remarkable electrochemical performance endowed by a phosphorene-based supercapacitor is indicative of its potential for widespread applications. (5) Summary and Outlook. The post-graphene era witnesses explosive development of a breadth of 2D inorganic nanomaterials for many current and future electronic/energy technologies. Electrochemical energy storage via supercapacitors is one of the rapidly evolving thrust areas where these materials show uncanny potential toward high energy density devices, replacing traditional LIBs. In this Review, we summarize the major advances of 2D inorganic materials such as TMDs, TMOs, TMHs, MXenes, and phosphorene for energy storage in supercapacitors. The use of 2D TMDs as supercapacitor electrode materials is enabled by their large surface areas and 489

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Table 1. Various 2D Materials and Their Supercapacitor Performances 2D family TMDs TMDs/ carbonaceous hybrids

TMDs/CPs hybrids

metallic TMDs

TMOs

TMHs

MXene

active material

MXene composites

phosphorene

rate capability

MoS2 nanowall films MoS2/MWCNT nanocomposite

100 F g−1/1 mV s−1 452 F g−1/1 A g−1

MoS2-rGO/MWCNT flower-like MoS2/GNS tubular MoS2/PANI nanowires

∼6.2 F cm−3/0.07 A cm−3 320 F g−1/2 A g−1; ASC: 142 F g−1/2 mV s−1 552 F g−1/0.5 A g−1

MoS2 NSs @PANI nanoneedle arrays

853 F g−1/1 A g−1

metallic 1T-WS2 nanoribbons metallic 1T-MoS2 NSs

2813 μF cm−2/0.5 A m−2 ∼400−700 F cm−3

VS2 NSs

δ-MSs δ-MnO2/graphene hybrid

4760 μF cm−2 (317 F cm−3)/0.1 A m−2 774 F g−1/0.1 A g−1; ASC: 175 F g−1 at 0.1 A g−1 ≥300 F g−1 267 F g−1/0.2 A g−1

e-Fe3O4@rGO sheets

329 F g−1/0.5 A g−1

90% from 0.5 to 10 A g−1

ultrathin Co3O4 NSs

51% from 5 to 30 mA cm−2

2D Co3O4 by 3D interconnected nanoflakes Co3O4 NS arrays porous NiO hollow spheres β-Co(OH)2

1782 F g−1/5 mA cm−2; ASC: 108 F g−1/5 mA cm−2 1500 F g−1/1 A g−1 2735 F g−1/2 A g−1 600 F g−1/10 A g−1 2028 F g−1/5 mV s−1

53.78% from 2 to 10 A g−1

α-Ni(OH)2 NSs Ni(OH)2 nanoplates

4172 F g−1/1 A g−1 1335 F g−1/2.8 A g−1

64.23% from 1 to 16 A g−1 71.40% from 2.8 to 45.7 A g−1

Ni50Co50-LDH

76.9% from 0.5 to 10 A g−1

Ti3C2 Ti3C2Tx flakes Ti3C2Tx@ silver-plated nylon

1537 F g−1/0.5 A g−1; ASC−: 107.8 F g−1/1 mA cm−2 762 C g−1/1 A g−1 2628 F g−1/3 A g−1; ASC: 5 38 F g−1/1.4 A g−1 340 F cm−3 900 F cm−3/20 mV s−1; 245 F g−1/2 mV s−1 499 F g−1/2 mV s−1 520 F cm−3 and 325 F g−1/2 mV s−1 579 mF cm−2/2 mV s−1 ∼357 F cm−3/20 mV s−1 328 mF cm−2/2 mV s−1

Ti3C2Tx/PVA film

530 F cm−3/2 mV s−1

1. Ti3C2Tx/SWCNT composite paper; 2. Ti3C2Tx/rGO composite paper MnO2/Ti3C2Tx hybrid

1. 345 F cm−3/5 A g−1; 2. 435 F cm−3/2 mV s−1 210.9 F g−1/10 mV s−1; 212.1 F g−1/1 A g−1 143 F g−1/5 mV s−1 151 F g−1/2 mV s−1 17.78 F cm−3 (59.3 F g−1)/0.1 V s−1 60.1 F g−1/0.5 A g−1 9.8 mF cm−2 or 37.5 F cm−3/5 mV s−1

MSs

Ni(OH)2/Co(OH)2 ultrathin Ni(OH)2-MnO2 hybrid NS arrays layered Ti3C2 Ti3C2 clay Ti3C2 nanoflakes d-Ti3C2

flexible MXene

capacitance/scan rate or current density

TiO2/Ti3C2 nanocomposite MoO3/Ti3C2Tx composite phosphorene nanoflakes BP/RP hybrid phosphorene NSs

energy density

73.71% from 1 to 10 A g−1 61% from 0.07 to 3.3 A cm−3 ∼50% from 2 to 20 A g−1

78.8 Wh kg−1 at 284.1 W kg−1

82% from 0.5 to 30 A g−1 106 Wh kg−1 at 106 kW kg−1 0.016 Wh cm−3 at 0.62 W cm−3

cycle test capacitance retention (%)/cycles

ref

95.8/1000

32 47

∼99/7000 90/5000

48 49

79/6000

52

83/4000

24

∼34/2000 >93−97/5000

44 16

100/1000

43

76% from 2 to 50 mV s−1

97.2 Wh kg−1

>97/10000

18

77.9% from 0.2 to 10 A g−1

18.64 Wh kg−1 at 12.6 kW kg−1 85 Wh kg−1 at 2.4 kW kg−1 134 Wh kg−1 at 1111 W kg−1 15.4 Wh kg−1 at 0.8 kW kg−1

92/7000

69 63

95/1000

75

>90/2000

76

99.3/2000

78

99/3000 93.2/10000

79 80 90

98.5/2000 100/2000

20 91

80.3/1000

92

ASC: 76/3000

93 94

100/10000 100/10000

98 99

100/10000 100/10000

100 101

98/10000 100/10000 ∼100/10000

102 106 107

48.73% from 2 to 100 mV s−1

∼85/10000

113

1. ∼87% from 5 to 10 A g−1; 2. 74% from 2 to 200 mV s−1 83% from 1 to 40 A g−1

1. 100/10000; 2. 109/10000 88/10000

114

92/6000 93.7/8000 71.8/30000

108 112 117

83.3/2000 89.5/2000

118 119

55.2% from 1 to 10 A g−1

19.44 Wh kg−1 98.9 Wh kg−1 at 17.981 kW kg−1

50.53% from 3 to 20 A g−1

37 Wh kg−1 at 10 kW kg−1 33.7 Wh kg−1 at 5.4 kW kg−1 101.3 Wh g−1 186 Wh kg−1 at 778 W kg−1

7.3 μWh cm−2 at 132 μW cm−2

12.25 Wh kg−1 at 20 kW kg−1

82% from 5 to 200 mV s−1 ∼9% from 0.1 to 10 V s−1 69.2% from 0.5 to 8 A g−1 11.6 mW cm−3

115

promising electrochemical performance. 2D nanostructures of TMOs and TMHs not only provide large surface areas but a high percentage of surface atoms enriched with redox activities are exposed to electrolyte ions, leading to enhanced pseudocapaci-

variable oxidation states, which provide them the capability to store charge via EDLs as well as by pseudocapacitance. Moreover, the presence of active edge sites and weak van der Waals gaps between the neighboring layers in 2D TMDs render 490

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best materials properties? What are the mechanical/chemical binding principles of 2D materials with interacting materials and how does the interface chemistry influence the overall electrochemical performance? Detailed/advanced studies using in situ characterizations techniques are required to unveil the fundamentals of hybrid interfaces. (5.2) Metallic 2D Materials Need Further Development. Although metallic 2D materials such as 1T-TMDs and MXenes show enormous potential toward high energy density supercapacitors owing to their intrinsically high electrical conductivity as well as versatile transition metal chemistry to exhibit pseudocapacitive charge storage, their research is still at the tip of the iceberg. 1T-TMD fabrication is generally accompanied by the formation of 2H counterparts, or an easy phase transition occurs from the 1T to 2H phase in some cases. Therefore, proper fabrication methods are required to achieve a stable/pure 1T phase to access their real performance. MXene growth techniques use toxic acids (e.g., HF), resulting in arbitrary functionalization, which overshadows their real performance. Hence, acid-free growth of MXenes and MXenes without surface functionalization is required to leverage their intrinsic performance as electrode materials. Furthermore, the behavior of metallic TMDs and MXenes with other capacitive materials such as CPs, carbon materials, and TMOs/nitrides in the form of hybrids/ composites is less known. Such hybrids are expected to promote the electron/ion conduction pathways by a manifold to achieve high-performance supercapacitors. (5.3) Need for Novel Electrolytes and Stability Issues. The limited number of aqueous/nonaqueous electrolytes known to the scientific community are becoming obsolete with the advent of an array of novel 2D nanomaterials due to the novel physiochemical properties of the surface atoms of newly developed 2D electrode materials. For example, MXenes are prone to easy oxidization in various aqueous electrolytes, which results in an increase of resistance and capacitance decay during cycling tests.98 Hence, there is a parallel need to develop compatible electrolytes along with the development of novel 2D electrode materials. On the other hand, novel electrodes like phosphorene face stability issues under ambient environments and require an inert environment to assemble supercapacitors.120 Serious research efforts are needed to unveil and mitigate this limitation. Proper encapsulation or functionalization of phosphorene while maintaining its electrochemical entity could be one of the possible approaches to alleviate the stability issues.

tive performance. Despite significant enhancement in the performance of supercapacitors based on the 2D TMDs, TMOs, and TMHs, the major intrinsic bottleneck that keeps them from widespread practical applications is their low electrical conductivity, which results in inferior pseudocapacitance. To compensate the aforementioned demerits, these materials are either incorporated into highly conductive/capacitive materials like carbon materials and CPs or subjected to exotic nanomaterials engineering in various form factors such as porous nanostructures, 1D/2D and 3D/2D morphologies, core/shell designs, etc. For example, conducting graphene layers in these hybrids act as fast electron transfer channels, preventing restacking issues as well as showing high synergic effects with pseudo-materials to exhibit high electrochemical performance. Developing a porous electrode enhances the surface area and interface contact between the electrode and electrolyte, which in turn enhances the specific capacitance/energy density of supercapacitors. In addition to the above approaches, hybrids of 2D TMOs/TMHs, and 2D TMHs/TMHs have also been developed to achieve advanced supercapacitor electrodes with high redox activity and improved cycle life. In recent years, supercapacitor research grew considerably due to the emergence of the metallic phase in the existing 2D materials or the advent of new metallic materials. For example, 2D TMDs in their metallic (1T) phase have shown exceptional supercapacitor properties, which is benefited by the enhanced conductivity of the network (i.e., 107 times greater than that of the semiconducting 2H phase) and high hydrophilicity enhancing its contact with aqueous electrolytes. 2D transition metal carbides/nitrides (MXenes) are another emerging class of novel pseudocapacitive electrode materials with extremely high intrinsic electronic/ionic conductivity. The high surface pseudocapacitive activity in MXenes demonstrates very high intercalation capacitance (∼1000 F cm−3) in aqueous electrolytes. Moreover, MXene fillers are playing a key role in enhancing the mechanical property and chemical stability of polymeric- and metal oxide-based supercapacitor electrodes. Phosphorene, a 2D allotrope of phosphorus, is showing great potential for highperformance supercapacitors owing to its excellent electrical conductivity (∼300 S m−1), large interlayer spacing (∼3.08 A), high thermodynamic stability, and fast ion diffusivity. In addition, the outstanding flexibility in phosphorene makes it viable for flexible energy storage devices. A comparison of the electrochemical performance of all 2D materials discussed above is presented in Table 1. Despite a plethora of research conducted in the development of novel 2D nanomaterials discussed in this Review, there are still many current and foreseen challenges as well as future directions that might be considered to make these materials viable for commercial energy storage technologies: (5.1) Interface−Property and Performance Relationships in 2D Hybrids. To leverage the optimum materials properties from the constituent elements in 2D hybrids, it is imperative to understand their interface relationships that govern the physiochemical properties of the hybrids to exhibit enhanced performances. Even though a large section of the scientific community observes synergistic effects in terms of enhanced electrochemical performances, the legitimate factors responsible for the hybrids’ performance are still unknown. For example, how do the interface atoms adjust their orientation, crystalline structure, and layer number to exhibit high synergistic effects? What are the fundamental atomistic parameters that define the viability of distinct materials to construct highly compatible hybrids and are these factors tunable/adjustable to leverage the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yeonwoong Jung: 0000-0001-6042-5551 Jayan Thomas: 0000-0003-3579-6064 Notes

The authors declare no competing financial interest. Biographies Kowsik Sambath Kumar is a Ph.D. student in the Materials Science and Engineering Department at the University of Central Florida (UCF) working under the supervision of Dr. Jayan Thomas. He received his B.Tech in Chemical and Electrochemical Engineering from CSIRCECRI, India in 2016. His current research focuses on the development of nanostructured materials for energy storage devices. 491

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Nitin Choudhary is a postdoctoral researcher at the NanoScience Technology Center (NSTC) of the University of Central Florida, USA. He graduated from the Indian Institute of Technology (IIT), India in the field of experimental physics/nanotechnology. He is currently working in the field of low-dimensional materials for next-generation electronics and energy applications. Yeonwoong Jung is an assistant professor at the NanoScience Technology Center (NSTC), Materials Science and Engineering, and Electrical and Computer Engineering at the University of Central Florida. He received his Ph.D. in materials science and engineering from the University of Pennsylvania, Philadelphia, USA. He joined UCF in 2015 after completing his postdoctoral training at Yale University. His research group at UCF currently focuses on developing 2D layered materials and their hybrid systems for electronics, energy, and environmental applications. Jayan Thomas is an associate professor at the NanoScience Technology Center (NSTC), College of Optics and Photonics (CREOL) and College of Engineering and Computer Science at the University of Central Florida. After receiving his Ph.D. from Cochin University of Science and Technology in India, he joined the College of Optical Sciences, the University of Arizona in 2001 as a research faculty. He moved to UCF in 2011 and is currently working on the development of energy harvesting devices, wearables energy devices, solar cells, and photorefractive polymers.

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ACKNOWLEDGMENTS

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REFERENCES

J.T. acknowledges the University of Central Florida “Reach for the stars” award for financial support. N.C. acknowledges the partial support from the Preeminent Postdoctoral Program (P3) at UCF.

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