High Volumetric Capacitance Research Articles

Interfacial synthesis of crystalline quasi-two-dimensional polyaniline thin films for high-performance flexible on-chip micro-supercapacitors

Quasi-two-dimensional (q2D) conducting polymer thin film synergizes the advantageous features of long-range molecular ordering and high intrinsic conductivity, which are promising for flexible thin film-based micro-supercapacitors (MSCs). Herein, we present the high-performance flexible MSCs based on highly ordered quasi-two-dimensional polyaniline (q2D-PANI) thin film using surfactant monolayer assisted interfacial synthesis (SMAIS). Owing to high electrical conductivity, rich redox chemistry, and thin-film morphology, the q2D-PANI MSCs show high volumetric specific capacitance (ca. 360 F/cm3) and energy density (17.9 mWh/cm3), which outperform the state-of-art PANI thin-film based MSCs and promise for future flexible electronics.

Nitrogen-Doped and Carbon-Coated Activated Carbon as a Conductivity Additive-Free Electrode for Supercapacitors

The development of supercapacitors with high volumetric capacitance and high-rate performance has been an important research topic. Activated carbon (AC), which is a widely used material for supercapacitor electrodes, has different surface structures, porosities, and electrochemical properties. However, the low conductivity of the electrode material is a major problem for the efficient use of AC in supercapacitors. To tackle this challenge, we prepared conductive, additive-free electrodes for supercapacitors by a simple one-pot treatment of AC with melamine (nitrogen source), pitch, and sucrose (both carbon source). Nitrogen-doped and carbon-coated AC was successfully generated after high-temperature heat treatment. The AC was doped with approximately 0.5 at.% nitrogen, and coated with carbon leading to a decreased oxygen content. Thin carbon layers (~10 nm) were coated onto the outer surface of the AC, as shown in TEM images. The modification of the AC surface with a sucrose source is favorable, as it increases the electrical conductivity of AC up to 3.0 S cm−1, which is 4.3 times higher than in unmodified AC. The electrochemical performance of the modified AC was evaluated by conducting agent-free electrode. Although the obtained samples had slightly reduced surface areas after the surface modification, they maintained a high specific surface area of 1700 m2 g−1. The supercapacitor delivered a specific capacitance of 70.4 F cc−1 at 1 mA cm−1 and achieved 89.8% capacitance retention even at a high current density of 50 mA cm−2. Furthermore, the supercapacitor delivered a high energy density of 24.5 Wh kg−1 at a power density of 4650 W kg−1. This approach can be extended for a new strategy for conductivity additive-free electrodes in, e.g., supercapacitors, batteries, and fuel cells.

A Redox-Mediator-Integrated Flexible Micro-Supercapacitor with Improved Energy Storage Capability and Suppressed Self-Discharge Rate.

To effectively improve the energy density and reduce the self-discharging rate of micro-supercapacitors, an advanced strategy is required. In this study, we developed a hydroquinone (HQ)-based polymer-gel electrolyte (HQ-gel) for micro-supercapacitors. The introduced HQ redox mediators (HQ-RMs) in the gel electrolyte composites underwent additional Faradaic redox reactions and synergistically increased the overall energy density of the micro-supercapacitors. Moreover, the HQ-RMs in the gel electrolyte weakened the self-discharging behavior by providing a strong binding attachment of charged ions on the porous graphitized carbon electrodes after the redox reactions. The micro-supercapacitors with HQ gel (HQ-MSCs) showed excellent energy storage performance, including a high energy volumetric capacitance of 255 mF cm−3 at a current of 1 µA, which is 2.7 times higher than the micro-supercapacitors based on bare-gel electrolyte composites without HQ-RMs (b-MSCs). The HQ-MSCs showed comparatively low self-discharging behavior with an open circuit potential drop of 37% compared to the b-MSCs with an open circuit potential drop of 60% after 2000 s. The assembled HQ-MSCs exhibited high mechanical flexibility over the applied external tensile and compressive strains. Additionally, the HQ-MSCs show the adequate circuit compatibility within series and parallel connections and the good cycling performance of capacitance retention of 95% after 3000 cycles.

High-density three-dimensional graphene cathode with a tailored pore structure for high volumetric capacity zinc-ion storage

Zinc-ion hybrid supercapacitors (ZHCs), based on the multivalent ion storage principle, are considered to be a promising candidate for large-scale energy storage devices. However, with the increasing demands for portable and miniaturized energy storage devices, how to obtain sufficient energy output in a limited volume is particularly critical, so that requires high volumetric energy density with no sacrifice of power density. Herein, we designed a compact three-dimensional porous graphene (3D-PG-1) cathode for ZHCs to balance the porous structure and high density by drying the graphene hydrogel with a short period of capillary evaporation. As for 3D-PG-1, the pore size range is 0.6–10 nm, where the micropore is for high capacity and the mesopore helps Zn2+ rapid transport, and the high density of 1.38 g cm−3 guaranteed its high volumetric capacitance. Therefore, the ZHC based on this 3D-PG-1 cathode delivered an excellent volumetric performance of 299 F cm−3 at 0.1 A g−1, excellent long cycle stability (225 F cm−3 at 5 A g−1, 85% retention after 30000 cycles) and extremely high volumetric energy density up to 118 Wh L−1 at 116 W L−1, which retained 61 Wh L−1 at the power density as high as 23.2 kW L−1.

High Performance 2D Micro-Supercapacitor Electrode Composed of Graphene with Polydopamine As Inserts

Over the last years, graphene is deemed as one of the most promising electrode materials for EDLCs due to its large specific surface area (2600 m2·g-1), high theoretical specific capacitance (550 F·g-1) and high conductivity.1 Among all the reported approaches to achieve a graphene-like material, the most favorable route is the reduction of GO and notably the electrochemical reduction of GO that is viewed as an economic, simple, fast and eco-friendly method with production possibility at a relatively large scale.2,3 However, reduced graphene oxide (rGO) suffers from the small surface area due to the partial re-stacking of graphene sheets.4 Therefore, it is necessary to develop an effective and feasible route to avoid rGO re-stacking to obtain graphene-based electrodes with a high specific capacitance and a good stability.Herein, we report graphene-polydopamine (PDA) composites as electrodes for micro-supercapacitor devices fabricated by a facile electrochemical approach. 5 We particularly address the rGO re-stacking issue by exploiting the PDA as a “chemical insert” between the graphene sheets but also explore the impact of PDA on the interfacial charge storage properties and the cycling performance. The optimized ERGO-PDA electrode possesses combined features of excellent capacitive behavior: high gravimetric and volumetric capacitances (178 F·g-1 and 297 F·cm-3, respectively, at 10 mV·s-1). Even at a scan rate of 1V/s, its volumetric capacitance is still as high as 94 F/cm3 (versus only 39 F/cm3 for ERGO) with an excellent cycling stability. Additionally, the electrochemical quartz crystal microbalance analyses demonstrate a dominant cationic charge compensation and a very efficient interfacial transfer characteristics, since a totally reversible mass response during charge/discharge was observed for the optimized ERGO-PDA electrode. Here the favourable impact of PDA is shown to tackle rGO restacking issue, shedding light on the design of graphene based composite electrodes and can also be extended to other 2D materials for high performance electrochemical energy storage electrodes. Keywords: EDLC, reduced graphene oxide, polydopamine, composite, microsupercapacitor.

High-volumetric capacitance and high-rate performance in liquid-mediated densified holey MXene film

Achieving both high-volumetric capacitance and high-rate performance in thick pseudocapacitive film materials plays a critical role in facilitating the realization of large-scale electrochemical energy storage. However, MXene (Ti3C2Tx), as a two-dimensional material with a high aspect ratio, is blocked by the industry problem of nanosheets self-stacking when assembled into dense films, which would induce extremely poor ionic mobility and significant attenuation of thick electrode performance. Herein, we create uniform in-plane nanopore defects in MXene nanosheets, and introduce nonvolatile liquids with different water contents, in which the interlaminar structure of holey MXene film can be accurately controlled by the capillary compression generated during evaporation. More importantly, the residual nonvolatile liquids effectively maintain the wettability of MXene surface, making it is easy to obtain compact pseudocapacitive film materials with pore structure connectivity, and allows ions to rapidly transport. The results demonstrate that the liquid-mediated densified holey MXene film can still present high-volumetric capacitance and high-rate performance at the actual mass loading level (>10 mg cm−2), which is of great significance for promoting the advancement of MXene materials from the theoretical elucidation to practical application.

KMnO4-assisted synthesis of oxygen-containing porous graphene with high gravimetric and volumetric performances for supercapacitor

• The samples use KMnO 4 as activator precursor. • The samples display high gravimetric and volumetric capacitance. • The optimized electrode shows good electrochemical performance. • The symmetric supercapacitor shows high energy density and cycling stability. Graphene becomes a promising electrode material for supercapacitor because of their high theoretical specific surface area and good conductivity. Nevertheless, the main disadvantage of graphene is the irreversible agglomeration because of the strong interlayer van der Waals force, which is significantly decreases the surface area, thus resulting in poor electrochemical performance. Herein, we report a new and facile strategy to synthesize rich oxygen-containing porous graphene (OPG) using KMnO 4 as activator precursor. The optimized OPG-600 samples possess crumpled porous framework with a suitable surface area and massive oxygen-containing functional groups. Benefiting from their synergistic influence, the OPG-600 electrode displays a high gravimetric and volumetric capacitance of 363.3 F g −1 and 342.5 F cm −3 at 0.5 A g −1 and good rate performance (240 F g −1 and 226.3 F cm −3 at 30 A g −1 ). More interestingly, the OPG-600 symmetric supercapacitor displays a high volumetric energy density of 19.2 Wh L −1 and superior electrochemical stability.

High performance stretchable fibrous supercapacitors and flexible strain sensors based on CNTs/MXene-TPU hybrid fibers

In order to improve the practicability of fibrous supercapacitors, stretchability is a significant characteristic for the devices, which puts forward high requirements for electrode materials. In this work, highly stretchable and conductive CNTs/MXene-TPU hybrid fiber electrodes with porous structure were prepared by wet spinning technique. The mechanical and electrical properties of the fiber electrodes remained stable during the tensile process when the amount of CNTs/MXene is 10 wt%. Meanwhile, the electrodes also showed a high volumetric specific capacitance of 3.9 F cm−3 even at the scan rate of 1 V s−1. The all-solid-state fibrous supercapacitors based on CNTs/MXene-TPU fiber electrodes also exhibited superior electrochemical performance and can be stretched by more than 50% strain. The specific capacitance of the devices changed with the tensile strain and reached 3.1 F cm−3 while the stain was 50%. Based on these characteristics, the fibrous supercapacitors were further applied to flexible strain sensors, which can convert action signals to electrochemical signals rapidly. The stretchable fibrous supercapacitor can be used not only for energy storage, but also as a strain sensor to monitor human movements, making it very promising for the next generation smart textiles.

Facile synthesis and high volumetric capacitance of holey graphene film for supercapacitor electrodes with optimizing preparation conditions

ABSTRACT For miniaturized portable energy storage devices with limited space, electrode materials are needed to be developed in the direction of reducing the occupied volume without compromising high specific capacitance and stable cycling ability. Herein, a facile strategy was developed to fabricate a free-standing binder-free holey graphene film as a supercapacitor electrode. In this material mechanism, holey structure in graphene sheets provides abundant ion transport channels and accelerates ion diffusion efficiency, which could guarantee that this film performs a high packing density without compromising electrochemical performance. After being properly optimized on the amount of H2O2 added and the mechanical pressure while preparing, this electrode film could consequently perform a high volumetric capacitance of 406 F cm−3 and excellent cycle stability with a packing density of 1.34 g cm−3. Therefore, this kind of densely packed electrode film with excellent electrochemical performance is of great significance for the development of miniaturized portable energy storage devices.

Wafer-scale quasi-layered tungstate-doped polypyrrole film with high volumetric capacitance

Layered materials are particularly attractive for supercapacitors because of their unique physical, electrical and chemical properties. Here, we demonstrate a facile and scalable electrochemical deposition method for wafer-scale synthesis of quasi-layered tungstate-doped polypyrrole films (named TALPy) with controllable thickness and size. The as-prepared TALPy film exhibits a high gravimetric density and excellent volumetric capacitance, exceeding many high-performing carbon- and polymer-based film electrodes. Based on combined results of ex-situ X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS), it is determined that TALPy stores charge through an ion intercalation process accompanied by change in oxidation states of polypyrrole backbone, which is referred as intercalation pseudocapacitance. All these results suggest the great promise of electrochemical deposition as a scalable and controllable bottom-up approach for synthesizing quasi-layered conductive organic-inorganic hybrid films for electrochemical energy storage applications with high volumetric performance.

Ultralong and Millimeter-Thick Graphene Oxide Supercapacitors with High Volumetric Capacitance

The fabrication of a high energy density supercapacitor with electrodes bearing ultrahigh aspect ratio active materials is still a big challenge. Here, we successfully developed ultralong and millimeter-thick supercapacitors, which enable practical applications due to the large electrochemical volumetric capacitance overcoming the limits of previous carbon-based materials. The gel graphene oxide fibers (GOFs) with an ultrahigh aspect ratio of over 20,000,000 were completely reduced at room temperature and biscrolled with a conductive Korean traditional paper used as the matrix material for the supercapacitors without any post-treatment. The average cell capacitance value of the yarn-type supercapacitors containing 80 reduced GOFs is maximized at a scan rate of 100 mV/s. The capacitance measured at a scan rate of 1000 mV/s is over 75% of that measured at 100 mV/s.

Surface-charge regulated TiO2 nanotube arrays as scaffold for constructing binder-free high-performance supercapacitor

Although TiO2 with outstanding photocatalytic activity has wide applications as a photo-to-electricity conversion material, the electrochemical energy storage of TiO2-based electrodes is commonly restricted by inherent limitations such as intrinsic low theoretical specific capacitance and poor electroconductivity. In this study, we introduce highly uniformed spaced TiO2 nanotube arrays (NTA) as a robust ultrathin scaffold and current collector in a potassium ion electrochemical storage capacitor. By integrating a simple surface-charge regulation with successive ionic adsorption technique, we construct both the cathodic and anodic electroactive materials on a TiO2 NTA based ~5 μm film. Hierarchical MnO2 nanosheets modified TiO2 NTA cathode exhibited a high volumetric capacitance of 1051 F cm−3, and the Fe2O3 nanospindles anchored TiO2 NTA anode exhibited a volumetric capacitance of 608.2 F cm−3. Based on the as-prepared cathode and anode, the assembled asymmetric supercapacitor with a thickness of only 0.4 mm exhibits a high energy density of 2.67 mWh cm−3 in K2SO4 electrolyte with 91.7% capacitance retention after 5000 cycles. The deformable Ti-substrate, excellent stability, good biocompatibility, and satisfactory capacitance in a neutral aqueous system enable the as-proposed supercapacitor a promising energy-storage system, thus expanding the application potentials of TiO2 photocatalysis based energy conversion devices.

Designing Donor-Acceptor Copolymers for Stable and High-Performance Organic Electrochemical Transistors.

Organic electrochemical transistors (OECTs) are oft-used for bioelectronic applications, and a variety of OECT channel materials have been developed in recent years. However, the majority of these materials are still limited by long-term performance and stability challenges. To resolve these issues, we implemented a next-generation design of polymers for OECTs. Specifically, diketopyrrolopyrrole (DPP) building blocks were copolymerized with propylene dioxythiophene-based (Pro-based) monomers to create a donor-acceptor-type conjugated polymer (PProDOT-DPP). These PProDOT-DPP macromolecules were synthesized using a straightforward direct arylation polymerization synthetic route. The PProDOT-DPP polymer thin film exhibited excellent electrochemical response, low oxidation potential, and high crystallinity, as evidenced by spectroelectrochemical measurements and grazing incidence wide-angle X-ray scattering measurements. Thus, the resultant polymer thin films had high charge mobility and volumetric capacitance values (i.e., μC* as high as 310 F cm-1 V-1 s-1) when they were used as the active layer materials in OECT devices, which places PProDOT-DPP among the highest performing accumulation-mode OECT polymers reported to date. The performance of the PProDOT-DPP thin films was also retained for 100 cycles and over 2000 s of ON-OFF cycling, indicating the robust stability of the materials. Therefore, this effort provides a clear roadmap for the design of electrochemically active macromolecules for accumulation-mode OECTs, where crystalline acceptor cores are incorporated into an all-donor polymer. We anticipate that this will ultimately inspire future polymer designs to enable OECTs with both high electrical performance and operational stability.

Development and Applications of MXene-Based Functional Fibers.

The increasing interest toward wearable and portable electronic devices calls for multifunctional materials and fibers/yarns capable of seamless integration with everyday textiles. To date, one particular gap inhibiting the development of such devices is the production of robust functional fibers with improved electronic conductivity and electrochemical energy storage capability. Recent efforts have been made to produce functional fibers with 2D carbides known as MXenes to address these demands. Ti3C2Tx MXene, in particular, is known for its metallic conductivity and high volumetric capacitance, and has shown promise for fibers and textile-based devices when used either as an additive, coating or the main fiber component. In this spotlight article, we highlight the recent exciting developments in our diverse efforts to fabricate MXene functionalized fibers, along with a critical evaluation of the challenges in processing, which directly affect macroscale material properties and the performance of the subsequent prototype devices. We also provide our assessment of observed and foreseen challenges of the current manufacturing methods and the opportunities arising from recent advances in the development of MXene fibers and paving future avenues for textile design and practical use in advanced applications.

Nanowire Architectures Improve Ion Uptake Kinetics in Conjugated Polymer Electrochemical Transistors.

Organic electrochemical transistors are believed to face an inherent material design tension between optimizing for ion mobility and for electronic mobility. These devices transduce ion uptake into electrical current, thereby requiring high ion mobility for efficient electrochemical doping and rapid turn-on kinetics and high electronic mobility for the maximum transconductance. Here, we explore a facile route to improve operational kinetics and volumetric capacitance in a high-mobility conjugated polymer (poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl)thieno [3,2-b]thiophene)], DPP-DTT) by employing a nanowire morphology. For equivalent thicknesses, the DPP-DTT nanowire films exhibit consistently faster kinetics (∼6-10× faster) compared to a neat DPP-DTT film. The nanowire architectures show ∼4× higher volumetric capacitance, increasing from 7.1 to 27.7 F/cm3, consistent with the porous structure better enabling ion uptake throughout the film. The nanowires also exhibit a small but energetically favorable shift in a threshold voltage of ∼17 mV, making the nanostructured system both faster and energetically easier to electrochemically dope compared to neat films. We explain the variation using two atomic force microscopy methods: in situ electrochemical strain microscopy and nanoinfrared imaging via photoinduced force microscopy. These data show that the nanowire film's structure allows greater swelling and ion uptake throughout the active layer, indicating that the nanowire architecture exhibits volumetric operation, whereas the neat film is largely operating via the field effect. We propose that for higher-mobility materials, casting the active layer in a nanowire form may offer faster kinetics, enhanced volumetric capacitance, and possibly lower threshold voltage while maintaining desirable device performance.

Reassembly of MXene Hydrogels into Flexible Films towards Compact and Ultrafast Supercapacitors

AbstractThe freestanding MXene films are promising for compact energy storage ascribing to their high pseudocapacitance and density, yet the sluggish ion transport caused by the most densely packed structure severely hinders their rate capability. Here, a reassembly strategy for constructing freestanding and flexible MXene‐based film electrodes with a tunable porous structure is proposed, where the Ti3C2Tx microgels disassembled from 3D structured hydrogel are reassembled together with individual Ti3C2Tx nanosheets in different mass ratios to form a densely packed 3D network in microscale and a film morphology in macroscale. The space utilization of produced film can be maximized by a good balance of the density and porosity, resulting in a high volumetric capacitance of 736 F cm−3 at an ultrahigh scan rate of 2000 mV s−1. The fabricated supercapacitor yields a superior energy density of 40 Wh L−1 at a power density of 0.83 kW L−1, and an energy density of 21 Wh L−1 can be still maintained even when the power density reaches 41.5 kW L−1, which are the highest values reported to date for symmetric supercapacitors in aqueous electrolytes. More promisingly, the reassembled films can be used as electrodes of flexible supercapacitors, showing excellent flexibility and integrability.

In Situ Growth of Novel Graphene Nanostructures in Reduced Graphene Oxide Microspherical Assembly with Restacking-Resistance and Inter-Particle Contacts for Energy Storage Devices.

Graphene is extensively investigated for various energy storage systems. However, the very low density (<0.01 g cm-3 ) of graphene nanosheets has hindered its further applications. To solve this issue, a controlled assembly of 2D graphene building blocks should be developed into graphene microspheres with high packing density, and restacking of graphene should be prevented to ensure an electrochemically accessible surface area during the assembly. Furthermore, graphene microspheres should have multiple 1D external conductive architecture to promote contacts with the neighbors. This study reports in situ growth of novel graphene nanostructures in reduced graphene oxide microspherical assembly (denoted as GT/GnS@rGB) with restacking resistance and interparticle contacts, for electrochemical energy storage. The GT/GnS@rGB showed high gravimetric (231.8 F g-1 ) and volumetric (181.5 F cm-3 ) capacitances at 0.2 A g-1 in organic electrolyte with excellent rate capabilities of 94.3% (@ 0.2 vs 10 Ag-1 ). Furthermore, GT/GnS@rGB exhibited excellent cycling stability (96.1% of the initial capacitance after 100 000 charge/discharge cycles at 2 A g-1 ). As demonstrated in the electrochemical evaluation as electrode materials for electrical double-layer capacitors, unique structural and textural features of the GT/GnS@rGB would be beneficial in the use of graphene assembly for energy storage applications.

Tailoring Ti3CNT MXene via an acid molecular scissor

Abstract MXenes are attracting growing attentions from scientific community owing to their decent electric and ionic conductivity, highly accessible surface area, and the presence of redox-active sites. Herein, an acid molecular scissor is proposed to artificially tailor Ti3CNTx MXene at the atomic scale and create defective nanosheets and redox-active sites. The tailored Ti3CNTx MXene exhibits a significantly improved electrochemical performance with the specific capacitance reaching 376 F g−1 and 230.68 mF cm−2, much higher than that of original Ti3CNTx MXene (237 F g−1 and 168.27 mF cm−2). The tailored Ti3CNTx MXene was further assembled into a micro-supercapacitor, which demonstrates a high volumetric capacitance of 250 F cm−3, a high energy density of 12.46 mW h cm−3 at a power density of 0.43 W cm−3. This work offers a new strategy to reinvent MXenes at the atomic scale with largely enhanced redox-active sites for energy storage, catalysis, solid lubricants and electromagnetic interference shielding.

Opening MXene Ion Transport Channels by Intercalating PANI Nanoparticles from the Self-Assembly Approach for High Volumetric and Areal Energy Density Supercapacitors.

Two-dimensional (2D) MXene materials have attracted great attention as advanced energy storage devices. A Ti3C2 MXene film can be used as a high-performance electrode material for flexible supercapacitors owing to its high specific capacitance, excellent conductivity, and remarkable flexibility. Unfortunately, self-stacking of MXene nanosheets makes them hard to balance the volumetric and areal capacitance performance. Herein, high conductive polyaniline nanoparticles (PANI NPs, ∼10 nm) are proposed as intercalators to regulate the MXene nanosheet interlayer by the self-assembly method. Interlayered PANI NPs not only restrain MXene self-stacking but also enable more ion transport routes, and conductive PANI NPs filled in MXene interlayer are in the form of nanoparticles that can build interconnected conductive channels. Meanwhile, PANI NPs slightly changes the thickness of the MX/PANI NPs hybrid film, thus bringing a high volumetric capacitance. As a result, the freestanding MX/PANI NPs-10% electrode displays an excellent areal capacitance of 1885 mF cm-2 (377 F g-1), meanwhile maintains a high volumetric capacitance of 873 F cm-3 even when the load of MXene reaches 5 mg cm-2. Moreover, the symmetric supercapacitor assembled by MX/PANI NPs hybrid film demonstrates high areal energy density (90.3 μWh cm-2) and volumetric energy density (20.9 Wh L-1) compared to MXene-based symmetric supercapacitors reported in the literature. This rational design balancing areal and volumetric energy densities provides another approach for solving the inherent problems of MXene and further exploiting MXene materials toward application in advanced energy storage devices.

2D titanium and vanadium carbide MXene heterostructures for electrochemical energy storage

Two-dimensional (2D) heterostructured electrodes built from vertical stacking of different 2D materials are among the most promising electrode architectures for electrochemical energy storage devices. These materials offer interesting opportunities for energy storage applications such as versatility in the structural design of electrode, and the possibility to integrate individual 2D building blocks with different properties into heterostructures. These features can potentially enable new materials with improved or new electrochemical features. Here, we report on large-scale liquid phase self-assembly of 2D heterostructures built from two different 2D transition metal carbides (MXenes), Ti3C2Tx and V2CTx. A cation-driven self-assembly process was used to assemble the negatively-charged flakes of the two MXenes into heterolayered flakes. The freestanding and binder-free MXene heterostructure films could deliver a high volumetric capacitance of ~1473 F cm−3 and showed no capacitance loss after 50,000 charge-discharge cycles in 3 M H2SO4 electrolyte. Due to coupling of redox reactions of Ti3C2Tx and V2CTx, the heterostructure electrodes showed a nearly constant current over their entire potential window, which is reminiscent of traditional pseudocapacitive materials. This electrochemical behavior differs from individual MXene electrodes or most other emerging pseudocapacitive materials whose maximum performance is usually achieved in a narrow potential range.