High‐Performance Ultrathin Flexible Solid‐State Supercapacitors Based on Solution Processable Mo1.33C MXene and PEDOT:PSS

MXenes, a young family of 2D transition metal carbides/nitrides, show great potential in electrochemical energy storage applications. Herein, a high performance ultrathin flexible solid‐state supercapacitor is demonstrated based on a Mo_1.33C MXene with vacancy ordering in an aligned layer structure MXene/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) composite film posttreated with concentrated H₂SO₄. The flexible solid‐state supercapacitor delivers a maximum capacitance of 568 F cm^?3, an ultrahigh energy density of 33.2 mWh cm^?3 and a power density of 19 470 mW cm^?3. The Mo_1.33C MXene/PEDOT:PSS composite film shows a reduction in resistance upon H₂SO₄ treatment, a higher capacitance (1310 F cm^?3) and improved rate capabilities than both pristine Mo_1.33C MXene and the nontreated Mo_1.33C/PEDOT:PSS composite films. The enhanced capacitance and stability are attributed to the synergistic effect of increased interlayer spacing between Mo_1.33C MXene layers due to insertion of conductive PEDOT, and surface redox processes of the PEDOT and the MXene.     

Bimetal Organic Framework–Ti3C2Tx MXene with Metalloporphyrin Electrocatalyst for Lithium–Oxygen Batteries

The gravest oxidation of MXenes has become a critical problem due to the formation of metal oxides, leading to the loss of their intrinsic properties. Herein, bimetallic cobalt–manganese organic framework (CMT) directly grown on a Ti₃C₂T_x MXene sheet via solvothermal treatment to obtain strong oxidation resistance in an open structured application and to enhance electrocatalytic properties for oxygen evolution and reduction reaction is reported. Inspired by ligand chemistry, the carboxyl acids in tetrakis(4–carboxyphenyl)porphyrin acting as an organic linker are grafted with the surface terminators of Ti₃C₂T_x MXene through the Fischer esterification and substitution reaction of fluorine, thereby greatly enhancing the antioxidation stability. Furthermore, the as-formed metalloporphyrin structure and unpaired electrons, produced between CMT and Ti₃C₂T_x MXene during solvothermal treatment, improve their electrocatalytic activity, durability, and electrical conductivity through an electron hopping mechanism. Consequently, the CMT@MXene demonstrates high stability as a bifunctional electrocatalyst at a fixed specific capacity of 1000 mAh g⁻¹ and a current density of 500 mA g⁻¹ for 247 cycles in lithium–oxygen (Li O₂) battery. This approach suggests new strategies for the synergistic coupling of MXenes and MOFs for future open structured applications.     

Advances in Manganese‐Based Oxides Cathodic Electrocatalysts for Li–Air Batteries

Li–air batteries, characteristic of superhigh theoretical specific energy density, cost‐efficiency, and environment‐friendly merits, have aroused ever‐increasing attention. Nevertheless, relatively low Coulomb efficiency, severe potential hysteresis, and poor rate capability, which mainly result from sluggish oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) kinetics, as well as pitiful cycle stability caused by parasitic reactions, extremely limit their practical applications. Manganese (Mn)‐based oxides and their composites can exhibit high ORR and OER activities, reduce charge/discharge overpotential, and improve the cycling stability when used as cathodic catalyst materials. Herein, energy storage mechanisms for Li–air batteries are summarized, followed by a systematic overview of the progress of manganese‐based oxides (MnO₂ with different crystal structures, MnO, MnOOH, Mn₂O₃, Mn₃O₄, MnOx, perovskite‐type and spinel‐type manganese oxides, etc.) cathodic materials for Li–air batteries in the recent years. The focus lies on the effects of crystal structure, design strategy, chemical composition, and microscopic physical parameters on ORR and OER activities of various Mn‐based oxides, and even the overall performance of Li–air batteries. Finally, a prospect of the research for Mn‐based oxides cathodic catalysts in the future is made, and some new insights for more reasonable design of Mn‐based oxides electrocatalysts with higher catalytic efficiency are provided.     

Single‐Atomic‐Co Electrocatalysts with Self‐Supported Architecture toward Oxygen‐Involved Reaction

Single‐atomic electrocatalysts (SACs) have shown great promise in electrocatalysis fields owing to their theoretical maximum atom utilization (100%). Yet still, it is far from expectation in practical applications due to entrapping within supports and blocking by aggregation. Herein, self‐supported carbon nanosheet arrays consisting of single‐atomic Co electrocatalyst (SS‐Co‐SAC) toward oxygen‐involved reaction and zinc–air batteries are reported. Impressively, the as‐synthesized SS‐Co‐SAC gives a markedly enhanced utilization of active sites (≈22.3%@2.3 wt%) as a result of single‐atomic dispersion of Co within a unique nanosheet arrays architecture, which is the largest value among other reported results. Benefiting from the high utilization of active sites, the SS‐Co‐SAC electrode exhibits outstanding electrocatalytic performance for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Notably, the turnover frequency value for ORR is determined to be ≈9.26 s^?1, which stands for the highest level among noble metal‐free electrocatalysts reported previously. Moreover, as an air‐cathode for zinc–air batteries with SS‐Co‐SAC, a power density of 195.1 mW cm^?2 and a robust durability are achieved. It is believed that this study would guide the future design of highly active and durable single‐atom catalysts for both fundamental research and practical applications.     

Recent Advances in Non‐Noble Bifunctional Oxygen Electrocatalysts toward Large‐Scale Production

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial reactions in energy conversion and storage systems including fuel cells, metal–air batteries, and electrolyzers. Developing low‐cost, high‐efficiency, and durable non‐noble bifunctional oxygen electrocatalysts is the key to the commercialization of these devices. Here, based on an in‐depth understanding of ORR/OER reaction mechanisms, recent advances in the development of non‐noble electrocatalysts for ORR/OER are reviewed. In particular, rational design for enhancing the activity and stability and scalable synthesis toward the large‐scale production of bifunctional electrocatalysts are highlighted. Prospects and future challenges in the field of oxygen electrocatalysis are presented.     

Surface‐Modified Metallic Ti3C2Tx MXene as Electron Transport Layer for Planar Heterojunction Perovskite Solar Cells

MXenes are a large and rapidly expanding family of 2D materials that, owing to their unique optoelectronic properties and tunable surface termination, find a wide range of applications including energy storage and energy conversion. In this work, Ti₃C₂T_x MXene nanosheets are applied as a novel type of electron transport layer (ETL) in low‐temperature processed planar‐structured perovskite solar cells (PSCs). Interestingly, simple UV‐ozone treatment of the metallic Ti₃C₂T_x that increases the surface Ti? O bonds without any change in its bulk properties such as high electron mobility improves its suitability as an ETL. Improved electron transfer and suppressed recombination at the ETL/perovskite interface results in augmentation of the power conversion efficiency (PCE) from 5.00% in the case of Ti₃C₂T_x without UV‐ozone treatment to the champion PCE of 17.17%, achieved using the Ti₃C₂T_x film after 30 min of UV‐ozone treatment. As the first report on the use of pure MXene layer as an ETL in PSCs, this work shows the great potential of MXenes to be used in PSCs and displays their promise for applications in photovoltaic technology in general.     

Single‐Atom Catalysts for Electrocatalytic Applications

The recent advances in electrocatalysis for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), carbon dioxide reduction reaction (CO₂RR), and nitrogen reduction reaction (NRR) are thoroughly reviewed. This comprehensive review focuses on the single‐atom catalysts (SACs) including Sc, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Sn, W, Bi, Ru, Rh, Pd, Ag, Ir, Pt, and Au with single‐metal sites or dual‐metal sites. The recent development of single‐atom electrocatalysts with novel configurations and compositions is documented. The understanding of the process–structure–property relationships is highlighted. For the SACs, their electrocatalytic performance and stability in fuel cells, zinc–air batteries, electrolyzers, CO₂RR, and NRR are summarized. The challenges and perspectives in the emerging field of single‐atom electrocatalysis are discussed.     

Alternating‐Current MXene Polymer Light‐Emitting Diodes

MXenes (Ti₃C₂) are 2D transition‐metal carbides and carbonitrides with high conductivity and optical transparency. However, transparent MXene electrodes suitable for polymer light‐emitting diodes (PLEDs) have rarely been demonstrated. With the discovery of the excellent electrical stability of MXene under an alternating current (AC), herein, PLEDs that employ MXene electrodes and exhibit high performance under AC operation (AC MXene PLEDs) are presented. The PLED exhibits a turn‐on voltage, current efficiency, and brightness of 2.1 V, 7 cd A^?1, and 12 547 cd m^?2, respectively, when operated under AC with a frequency of 1 kHz. The results indicate that the undesirable electric breakdown associated with heat arising from the poor interface of the MXene with a hole transport layer in the direct‐current mode is efficiently suppressed by the transient injection of carriers accompanied by the alternating change of the electric polarity under the AC, giving rise to reliable light emission with a high efficiency. The solution‐processable MXene electrode can be readily fabricated on a flexible polymer substrate, allowing for the development of a mechanically flexible AC MXene PLED with a higher performance than flexible PLEDs employing solution‐processed nanomaterial‐based electrodes such as carbon nanotubes, reduced graphene oxide, and Ag nanowires.     

Ultrathin Molybdenum Carbide MXene with Fast Biodegradability for Highly Efficient Theory‐Oriented Photonic Tumor Hyperthermia

The explosion of emerging high‐performance 2D MXenes in theranostic nanomedicine is still at the preliminary stage. Despite tremendous efforts devoted to photonic tumor hyperthermia, current photothermal‐conversion nanoagents still suffer from critical issues preventing further clinical translation such as low biodegradability. Here, for the first time, the construction of novel 2D molybdenum carbide (Mo₂C) MXenes for photonic tumor hyperthermia is reported. The structure of both bulk Mo₂Ga₂C ceramic and Mo₂C MXene is fully revealed. Especially, computational simulation, as a novel strategy and a powerful tool for photonic‐performance prediction, is employed to reveal that Mo₂C MXene is featured with intense near‐infrared (NIR) absorption, covering the first and the second biological transparency window (NIR I and II). After further surface engineering with polyvinyl alcohol (PVA), Mo₂C‐PVA nanoflakes exhibit high biocompatibility and fast degradability. Importantly, it is experimentally corroborated that Mo₂C‐PVA nanoflakes possess intriguing broad absorption band spanning NIR in both the I and II regions, and desirable photothermal‐conversion efficiency (24.5% for NIR I and 43.3% for NIR II). This study not only broadens the nanomedical applications of MXene by fabricating novel material members (Mo₂C), but also provides the paradigm of inorganic multifunctional biomedical nanoplatform with desirable biodegradability and high therapeutic performance.     

MXene‐Reinforced Cellulose Nanofibril Inks for 3D‐Printed Smart Fibres and Textiles

Fibre‐based materials have received tremendous attention due to their flexibility and wearability. Although great efforts have been devoted to achieve high‐performance fibres over the past several years, it is still challenging for multifunctional macroscopic fibres to satisfy versatile applications. 2D transition metal carbides/nitrides (MXenes) with intriguing physical/chemical properties have been explored in broad application, and may be able to reinforce synthetic fibres. Inspired by natural materials, for the first time, flexible smart fibres and textiles are fabricated using a 3D printing process with hybrid inks of TEMPO (2,2,6,6‐tetramethylpiperidine‐1‐oxylradi‐cal)‐mediated oxidized cellulose nanofibrils (TOCNFs) and Ti₃C₂ MXene. The hybrid inks display good rheological properties, which allow them to achieve accurate structures and be rapidly printed. TOCNFs/Ti₃C₂ in hybrid inks self‐assemble to fibres with an aligned structure in ethanol, mimicking the features of the natural structures of plant fibres. In contrast to conventional synthetic fibres with limited functions, smart TOCNFs/Ti₃C₂ fibres and textiles exhibit significant responsiveness to multiple external stimuli (electrical/photonic/mechanical). TOCNFs/Ti₃C₂ textiles with electromechanical performance can be processed into sensitive strain sensors. Such multifunctional smart fibres and textiles will be promising in diverse applications, including wearable heating textiles, human health monitoring, and human–machine interfaces.     

A Type of 1 nm Molybdenum Carbide Confined within Carbon Nanomesh as Highly Efficient Bifunctional Electrocatalyst

Highly efficient platinum‐alternative bifunctional catalysts by using abundant non‐noble metal species are of critical importance to the future sustainable energy reserves. Unfortunately, current electrocatalysts toward hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) are far from satisfactory because of lacking reasonable design and assembly protocols. A type of 1‐nm molybdenum carbide nanoparticles confined in mesh‐like nitrogen‐doped carbon (Mo₂C@NC nanomesh) with high specific surface area is reported here. In addition to the superior ORR performance comparable to platinum, the catalyst offers a high HER activity with small Tafel slope of 33.7 mV dec^?1 and low overpotential of 36 mV to reach ?10 mA cm^?2. Theoretical calculations indicate that the active sites of the catalyst are mainly located at Mo atoms adjacent to the N‐doped carbon layer, which contributes the high HER activity. These findings show the great potential of Mo₂C species in wide electrocatalysis applications.     

Self‐Powered Water‐Splitting Devices by Core–Shell NiFe@N‐Graphite‐Based Zn–Air Batteries

Development of highly efficient and low‐cost multifunctional electrocatalysts for the oxygen evolution reaction (OER), the oxygen reduction reaction (ORR), and the hydrogen evolution reaction is urgently required for energy storage and conversion applications, such as in Zn–air batteries and water splitting to replace very expansive noble metal catalysts. Here, the new core–shell NiFe@N‐graphite electrocatalysts with excellent electrocatalytic activity and stability toward OER and ORR are reported and the Ni_0.5Fe_0.5@N‐graphite electrocatalyst is applied as the air electrode in Zn–air batteries. The respective liquid Zn–air battery shows a large open‐circuit potential of 1.482 V and a small charge–discharge voltage gap of 0.12 V at 10 mA cm⁻², together with excellent cycling stability even after 40 h at 20 mA cm⁻². Interestingly, the all‐solid‐like Zn–air battery thus derived shows a highly desired mechanical flexibility, whereby little change is observed in the voltage when bent into different angles. Using the same Ni_0.5Fe_0.5@N‐graphite electrode, a self‐driven water‐splitting device, which is powered by two Zn–air batteries in‐series, is constructed. The present study opens a new opportunity for the rational design of metal@N‐graphite‐based catalysts of core–shell structures for electrochemical catalysts and renewable energy applications.     

Janus‐Structured Co‐Ti3C2 MXene Quantum Dots as a Schottky Catalyst for High‐Performance Photoelectrochemical Water Oxidation

MXene materials have attracted increasing attention in electrochemical energy‐storage applications while MXene also becomes photo‐active at the quantum dot scale, making it an alternative for solar‐energy‐conversion devices. A Janus‐structured cobalt‐nanoparticle‐coupled Ti₃C₂ MXene quantum dot (Co‐MQD) Schottky catalyst with tunable cobalt‐loading content serving as a photoelectrochemical water oxidation photoanode is demonstrated. The introduction of cobalt triggers concomitant surface‐plasmon effects and acts as a water oxidation center, enabling visible‐light harvesting capability and improving surface reaction kinetics. Most importantly, due to the rectifying effects of Co‐MQD Schottky junctions, photogenerated carrier separation/injection efficiency can be fundamentally facilitated. Specifically, Co‐MQD‐48 exhibits both superior photoelectrocatalysis (2.99 mA cm^?2 at 1.23 V vs RHE) and charge migration performance (87.56%), which corresponds to 194% and 236% improvement compared with MQD. Furthermore, excellent photostability can be achieved with less than 6.6% loss for 10 h cycling reaction. This fills in gaps in MXene material research in photoelectrocatalysis and allows for the extension of MXene into optical‐related fields.     

Searching General Sufficient‐and‐Necessary Conditions for Ultrafast Hydrogen‐Evolving Electrocatalysis

The development of cost‐effective and high‐performance electrocatalysts for the hydrogen evolution reaction (HER) is one critical step toward successful transition into a sustainable green energy era. Different from previous design strategies based on single parameter, here the necessary and sufficient conditions are proposed to develop bulk non‐noble metal oxides which are generally considered inactive toward HER in alkaline solutions: i) multiple active sites for different reaction intermediates and ii) a short reaction path created by ordered distribution and appropriate numbers of these active sites. Computational studies predict that a synergistic interplay between the ordered oxygen vacancies (at pyramidal high‐spin Co³⁺ sites) and the O 2p ligand holes (OLH; at metallic octahedral intermediate‐spin Co⁴⁺ sites) in RBaCo₂O_5.5+δ (δ = 1/4; R = lanthanides) can produce a near‐ideal HER reaction path to adsorb H₂O and release H₂, respectively. Experimentally, the as‐synthesized (Gd_0.5La_0.5)BaCo₂O_5.75 outperforms the state‐of‐the‐art Pt/C catalyst in many aspects. The proof‐of‐concept results reveal that the simultaneous possession of ordered oxygen vacancies and an appropriate number of OLH can realize a near‐optimal synergistic catalytic effect, which is pivotal for rational design of oxygen‐containing materials.     

Precious‐Metal‐Free Electrocatalysts for Activation of Hydrogen Evolution with Nonmetallic Electron Donor: Chemical Composition Controllable Phosphorous Doped Vanadium Carbide MXene

The insufficient strategies to improve electronic transport, the poor intrinsic chemical activities, and limited active site densities are all factors inhibiting MXenes from their electrocatalytic applications in terms of hydrogen production. Herein, these limitations are overcome by tunable interfacial chemical doping with a nonmetallic electron donor, i.e., phosphorization through simple heat‐treatment with triphenyl phosphine (TPP) as a phosphorous source in 2D vanadium carbide MXene. Through this process, substitution, and/or doping of phosphorous occurs at the basal plane with controllable chemical compositions (3.83–4.84 at%). Density functional theory (DFT) calculations demonstrate that the P? C bonding shows the lowest surface formation energy (ΔG_Surf) of 0.027 eV Å^?2 and Gibbs free energy (ΔG_H) of –0.02 eV, whereas others such as P‐oxide and P? V (phosphide) show highly positive ΔG_H. The P3–V₂CT_x treated at 500 °C shows the highest concentration of P? C bonds, and exhibits the lowest onset overpotential of –28 mV, Tafel slope of 74 mV dec^?1, and the smallest overpotential of ‐163 mV at 10 mA cm^?2 in 0.5 m H₂SO₄. The first strategy for electrocatalytically accelerating hydrogen evolution activity of V₂CT_x MXene by simple interfacial doping will open the possibility of manipulating the catalytic performance of various MXenes.     

Nanoporous Nitrogen‐Doped Graphene Oxide/Nickel Sulfide Composite Sheets Derived from a Metal‐Organic Framework as an Efficient Electrocatalyst for Hydrogen and Oxygen Evolution

Engineering of controlled hybrid nanocomposites creates one of the most exciting applications in the fields of energy materials and environmental science. The rational design and in situ synthesis of hierarchical porous nanocomposite sheets of nitrogen‐doped graphene oxide (NGO) and nickel sulfide (Ni₇S₆) derived from a hybrid of a well‐known nickel‐based metal‐organic framework (NiMOF‐74) using thiourea as a sulfur source are reported here. The nanoporous NGO/MOF composite is prepared through a solvothermal process in which Ni(II) metal centers of the MOF structure are chelated with nitrogen and oxygen functional groups of NGO. NGO/Ni₇S₆ exhibits bifunctional activity, capable of catalyzing both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) with excellent stability in alkaline electrolytes, due to its high surface area, high pore volume, and tailored reaction interface enabling the availability of active nickel sites, mass transport, and gas release. Depending on the nitrogen doping level, the properties of graphene oxide can be tuned toward, e.g., enhanced stability of the composite compared to commonly used RuO₂ under OER conditions. Hence, this work opens the door for the development of effective OER/HER electrocatalysts based on hierarchical porous graphene oxide composites with metal chalcogenides, which may replace expensive commercial catalysts such as RuO₂ and IrO₂.     

Novel Electronic and Magnetic Properties of Two‐Dimensional Transition Metal Carbides and Nitrides

Layered MAX phases are exfoliated into 2D single layers and multilayers, so‐called MXenes. Using first‐principles calculations, the formation and electronic properties of various MXene systems, M₂C (M = Sc, Ti, V, Cr, Zr, Nb, Ta) and M₂N (M = Ti, Cr, Zr) with surfaces chemically functionalized by F, OH, and O groups, are examined. Upon appropriate surface functionalization, Sc₂C, Ti₂C, Zr₂C, and Hf₂C MXenes are expected to become semiconductors. It is also derived theoretically that functionalized Cr₂C and Cr₂N MXenes are magnetic. Thermoelectric calculations based on the Boltzmann theory imply that semiconducting MXenes attain very large Seebeck coefficients at low temperatures.     

Visualizing the Oxidation Mechanism and Morphological Evolution of the Cubic‐Shaped Superoxide Discharge Product in Na–Air Batteries

Sodium–air (Na–O₂) batteries have recently developed as a high theoretical energy density energy storage and conversion system. In particular, Na–O₂ batteries with superoxide as the discharge product have a very high round‐trip energy efficiency over lithium–air batteries due to their significantly reduced charging overpotential. However, Na–O₂ batteries yet suffer from limited cycling lives because of the formation and incomplete removal of side products during oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes, while the mechanism of these processes is still not fully understood. Herein, a detailed investigation on tracking the decomposition pathway of cubic‐shaped micrometer‐sized NaO₂ discharge products in Na–O₂ batteries with carbon‐based air electrodes is reported. A detailed electrochemical charging mechanism is revealed during the charging process. The evolution of the chemical compositions of the discharge/side products in air electrode during charging is also verified by synchrotron‐based X‐ray absorption spectroscopy experiments. The formation of these intermediate phases other than NaO₂ during the charging process results in high overpotentials. These new findings can contribute to a better understanding and the rational design of future Na–O₂ batteries.     

From 3D ZIF Nanocrystals to Co–Nx/C Nanorod Array Electrocatalysts for ORR,OER, and Zn–Air Batteries

Designing a highly active electrocatalyst with optimal stability at low cost is must and non‐negotiable if large‐scale implementations of fuel cells are to be fully realized. Zeolitic‐imidazolate frameworks (ZIFs) offer rich platforms to design multifunctional materials due to their flexibility and ultrahigh surface area. Herein, an advanced Co–N_x/C nanorod array derived from 3D ZIF nanocrystals with superior electrocatalytic activity and stability toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) compared to commercial Pt/C and IrO₂, respectively, is synthesized. Remarkably, as a bifunctional catalyst (E_{j = 10} (OER) ? E_1/2 (ORR) ≈ 0.65 V), it further displays high performance of Zn–air batteries with high cycling stability even at a high current density. Such supercatalytic properties are largely attributed to the synergistic effect of the chemical composition, high surface area, and abundant active sites of the nanorods. The activity origin is clarified through post oxygen reduction X‐ray photoelectron spectroscopy analysis and density functional theory studies. Undoubtedly, this approach opens a new avenue to strategically design highly active and performance‐oriented electrocatalytic materials for wider electrochemical energy applications.     

Revealing the Pseudo‐Intercalation Charge Storage Mechanism of MXenes in Acidic Electrolyte

Since the discovery of Ti₃C₂T_x in 2011, the family of two‐dimensional transition metal carbides, carbonitrides, and nitrides (collectively known as MXenes) has quickly attracted the attention of those developing energy storage applications such as electrodes for supercapacitors with acidic aqueous electrolytes. The excellent performance of these MXenes is attributed to a pseudocapacitive energy storage mechanism, based on the non‐rectangular shape of cyclic voltammetry curves and changes in the titanium oxidation state detected by in situ X‐ray absorption spectroscopy. However, the pseudocapacitive mechanism is not well understood and no dimensional changes due to proton insertion have been reported. In this work, in situ X‐ray diffraction and density functional theory are used to investigate the charge storage mechanism of Ti₃C₂T_x in 1 m H₂SO₄. Results reveal that a 0.5 Å expansion and shrinkage of the c‐lattice parameter of Ti₃C₂T_x occur during cycling in a 0.9 V voltage window, showing that the charge storage mechanism is intercalation pseudocapacitance with implication for MXene use in energy storage and electrochemical actuators.