MXenes earned their name from their layered MAX-phase parents, with M standing for transition metals such as titanium, vanadium, niobium, or molybdenum, X for carbon or nitrogen, and the suffix echoing graphene to signal a two-dimensional structure. First synthesized in 2011, when Ti3AlC2 particles were submerged in hydrofluoric acid to strip out the aluminum layers, these accordion-like multilayer sheets can be delaminated into thin, transparent nanosheets with enormous surface area. Unlike graphene or carbon nanotubes, which rely mainly on electrostatic charge storage and often require extra functionalization, MXenes combine intrinsic metallic conductivity with hydrophilic surfaces decorated with oxygen, hydroxyl, and fluorine terminations. That combination enables pseudocapacitive charge storage, rapid ion transport, and strong interfacial bonding with metals, sulfides, phosphides, and semiconductor partners.

The review devotes considerable attention to synthesis, because the production route largely dictates surface chemistry and electrochemical behavior. Hydrofluoric acid etching remains the classical method, but safer in situ etching using fluoride salts with hydrochloric acid, hydrothermal etching in sealed autoclaves, solvothermal processing, electrochemical etching, and fluorine-free molten salt routes have all expanded the toolkit. Lewis acidic molten salt etching with zinc chloride, for example, can produce chlorine-terminated MXenes with controlled surface chemistry, while microwave-assisted approaches can deliver MXenes in minutes. The authors stress that no single fabrication strategy satisfies every requirement for practical electrodes, so rational matching of synthesis technique to application remains essential.

On the energy-storage front, MXene-based supercapacitors have progressed dramatically since the first demonstration in 2013. A binder-free Ti3C2Tx paper electrode produced by vacuum filtration achieved a volumetric capacitance of 350 F cm-3 in potassium hydroxide electrolyte, matching advanced graphene systems, and later LiF/HCl-etched clay films pushed volumetric capacitance to roughly 900 F cm-3. Composite engineering has multiplied these gains. Manganese dioxide nanowhiskers grown directly on MXene surfaces delivered around 212 F g-1 with 87.7% retention after 10,000 cycles, while MXene/PANI hybrids fabricated through oxidant-free in situ polymerization reached approximately 503 F g-1 and 1682 F cm-3, retaining 98.3% of their capacitance after 10,000 cycles even at thick, high-mass-loading electrodes. Sandwich-like heterostructures combining bimetallic sulfides with Ti3C2Tx have reported specific capacitances as high as 2637 F g-1 and device-level energy densities near 80 Wh kg-1.

These performance leaps rest on a charge-storage mechanism that uniquely blends electric double-layer capacitance with fast, reversible surface redox reactions. Electrolyte ions adsorb onto the conductive MXene sheets while the terminal groups mediate Faradic processes involving protons or alkali-metal cations. In acidic media, proton-coupled surface redox dominates; in neutral and alkaline electrolytes, hydrated cation intercalation between expanded interlayers becomes the key factor. Strategies such as interlayer pillaring, which can widen the spacing to 3.2 nanometers, heteroatom doping, defect engineering via plasma-assisted mechanochemistry, and deliberate vacancy creation in vanadium-based MXenes all enhance ion accessibility and active-site density, though excessive defects can compromise structural stability.

The second half of the review turns to electrocatalytic water splitting, where water is decomposed into hydrogen and oxygen through the hydrogen evolution reaction at the cathode and the more sluggish four-electron oxygen evolution reaction at the anode. Theoretical work indicates that oxygen-terminated MXenes approach the ideal near-zero hydrogen adsorption free energy required for efficient HER. Experimental systems have validated this promise: a platinum-anchored vanadium carbide MXene achieved a 27 mV overpotential at 10 mA cm-2 with a Tafel slope of 36.5 mV dec-1, while ruthenium-incorporated cobalt phosphide on MXene delivered overpotentials of just 9 mV in alkaline media and about 97% voltage retention over 200 hours. Molybdenum disulfide quantum dots on Ti3C2Tx, Ru single atoms and clusters on molybdenum carbide MXene, and nitrogen-doped Ti2CO2 designed through density functional theory all illustrate how MXene supports accelerate charge transfer and water dissociation.

For oxygen evolution, MXenes excel primarily as conductive, stabilizing supports for catalytically active species. High-entropy oxides grown on MXene nanosheets by rapid microwave heating produced a Tafel slope of 71 mV dec-1 and strong photoelectrochemical activity, whereas bimetallic NiFe-Ti3C2Clx MXene hybrids formed through molten salt synthesis reached 310 mV overpotential with Ni centers identified as the dominant active sites. Metal-organic-framework-derived nickel-cobalt sulfide on ultrathin MXene sheets and oxygen-vacancy-engineered FeOOH arrays on Ti3C2Tx, the latter outperforming commercial iridium oxide in stability tests exceeding 100 hours, underscore how heterostructure and doping strategies convert MXene limitations into synergistic advantages.

Perhaps the most commercially consequential advance involves bifunctional electrocatalysts that perform both HER and OER in a simplified two-electrode electrolyzer. Triple-interface Ni3Se4-NiSe2-Co3O4 heterostructures on MXene reached 100 mA cm-2 at a cell voltage of only 1.64 V with 100 hours of stable operation, while L-tryptophan-anchored CoNi-Ti3C2Tx split water at 1.58 V and sustained an industrially relevant 500 mA cm-2 at 1.87 V in an anion-exchange membrane electrolyzer. Ultrafine ruthenium-cobalt nanoparticles on MXene required just 1.52 V with near-100% Faradaic efficiency, and a low-cost multi-hybrid MXene/Mn3O4/CuO composite maintained stability for twelve days. The authors argue these systems bridge the gap between laboratory metrics and real-world hydrogen production.

Significant hurdles remain before MXene technologies reach the market. Oxidation instability remains the foremost concern, as pristine nanosheets degrade in air, water, and light, converting conductive carbides into insulating titanium dioxide and eroding capacitance. Nanosheet restacking suppresses ion transport, hydrofluoric acid etching raises environmental and scalability questions, surface terminations are difficult to control reproducibly, and precursor costs complicate mass production. Countermeasures are maturing rapidly, including edge capping with polyanionic salts, cold storage under inert conditions, polymer encapsulation, and three-dimensional aerogel architectures. Looking ahead, the review calls for green etching chemistry, defect and interface engineering, machine-learning-guided materials discovery, solid-state and wearable supercapacitor designs, and stronger academia-industry collaboration to carry MXenes from the laboratory bench toward sustainable energy storage and industrial-scale clean hydrogen generation.

Subject of Research: MXene nanomaterials for next-generation supercapacitors and electrochemical water splitting

Article Title: Recent advances of MXene as a frontier nanoarchitecture in energy science for shaping the future of next-generation supercapacitor and water splitting technology

Article References: Sau, A., Hassan, T., Mitra, S., Ghosh, S., Basu, R., & Das, S. (2026). Recent advances of MXene as a frontier nanoarchitecture in energy science for shaping the future of next-generation supercapacitor and water splitting technology. Discover Electrochemistry, 3(1), Article 70. https://doi.org/10.1007/s44373-026-00156-9

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00156-9

Keywords: MXene, supercapacitors, water splitting, hydrogen evolution reaction, oxygen evolution reaction, two-dimensional materials, energy storage, electrocatalysis, pseudocapacitance, heterostructures, surface functionalization, green hydrogen

The appeal of TMDs begins with their unusual structure. With the general formula MX2, where M is a transition metal such as molybdenum, tungsten, niobium, or tantalum and X is sulfur, selenium, or tellurium, each monolayer consists of a sheet of metal atoms sandwiched between two sheets of chalcogen atoms. Adjacent layers are held together only by weak van der Waals forces, which means bulk crystals can be peeled into single-atom-thick sheets. This architecture yields enormous surface-area-to-volume ratios, coordinatively unsaturated edge atoms with dangling bonds that serve as natural binding sites for reactive intermediates, and interlayer galleries that can host rapid ion transport. These are precisely the features that electrochemical energy devices demand.

Yet pristine TMDs carry intrinsic handicaps. The thermodynamically stable 2H phase is semiconducting, which limits charge transport; the basal plane is chemically inert and contributes little to catalysis; and narrow interlayer spacing slows ion intercalation. Exfoliated nanosheets also tend to restack during device fabrication, degrading stability and cycling performance. The review’s central message is that these limitations are not dealbreakers but design opportunities, addressable through a toolkit that includes defect engineering, heteroatom doping and alloying, strain engineering, atomic-scale modulation, nanostructure design, interlayer and phase control, and heterostructure fabrication.

Defect engineering has proven especially powerful. Sulfur vacancies in molybdenum disulfide create donor states within the band gap and expose undercoordinated metal atoms that bind hydrogen favorably. One highlighted study used high-throughput density functional theory calculations to identify the optimal vacancy configuration, then developed a hydrogen peroxide chemical etching method to distribute single sulfur vacancies uniformly across MoS2 nanosheets. The resulting catalyst achieved a hydrogen evolution overpotential of just 131 millivolts at 10 milliamperes per square centimeter, with a Tafel slope near 48 millivolts per decade and excellent stability. The authors caution, however, that defects cut both ways: they can also act as scattering centers and trap states that degrade carrier mobility, so passivation of harmful defects must accompany the deliberate introduction of useful ones.

Doping and alloying offer complementary control over electronic structure. When researchers doped MoS2 with zinc using a fusion heat method, X-ray photoelectron spectroscopy revealed binding energy shifts of roughly 0.47 and 0.40 electron volts for the Mo 3d and S 2p levels, indicating increased electron density that accelerates the hydrogen discharge step. Bimetallic strategies push further: cobalt-doped MoS2 works bifunctionally in both acidic and alkaline water splitting, while ruthenium doping wrapped in carbon nanotubes activates the otherwise inert 2H basal plane. Alloying enables continuous band gap tuning, with CVD-grown MoS2(1-x)Se2x films showing more than ten percent band gap modulation and quaternary alloys spanning 1.60 to 2.03 electron volts. Remarkably, doping can even trigger phase transitions, as rhenium concentrations above 40 percent stabilize the metallic 1T-prime phase of MoSe2.

Strain engineering adds another dimension. Because TMD monolayers can withstand more than 20 percent elastic distortion, mechanical deformation directly reshapes their band structure. Computational work predicted that only 0.3 to 3 percent uniaxial tensile strain converts 1H-MoTe2 into the quasi-metallic 1T-prime phase at room temperature, and experiments confirmed strain-induced band gap tuning in MoS2. The most striking results come from combining strain with vacancies: when sulfur vacancies in monolayer 2H-MoS2 were simultaneously strained, gap states shifted toward the Fermi level, yielding near-optimal hydrogen adsorption free energy. The combined system showed a Tafel slope of 60 millivolts per decade versus 98 for pristine MoS2, and the turnover frequency of its molybdenum atoms exceeded even that of conventional edge sites.

Phase engineering targets the most consequential lever of all. The metallic 1T phase of MoS2 conducts electricity roughly ten million times better than the semiconducting 2H phase and is hydrophilic, both critical for electrochemistry. Chemically exfoliated 1T-MoS2 reaches benchmark hydrogen evolution current densities at around 187 to 195 millivolts versus RHE, compared with more than 300 millivolts for the 2H phase, with Tafel slopes dropping from about 110 to the mid-40s. Because 1T is metastable and reverts to 2H near 92 degrees Celsius, researchers have developed stabilization tricks including sulfur intercalation, metal cation insertion, and palladium doping that partially converts the phase and slashes Tafel slopes from 157 to as low as 62 millivolts per decade.

Heterostructures and single-atom catalysts round out the toolkit. Coupling MoS2 with WTe2 creates a low Schottky barrier at the interface that shortens electron transport paths from micrometers to roughly 700 picometers, dramatically improving charge injection. Covalent 0D-2D hybrids of Co9S8 nanoparticles bonded to MoS2 through Co-S-Mo links render molybdenum sites electron-rich and activate the basal plane across all pH values. Meanwhile, isolated nickel atoms anchored on MoS2 nanofibers cut hydrogen evolution overpotentials from 263 to 161 millivolts, and nickel-oxygen sites engineered onto 1T-MoS2 achieved an onset potential near 0 volts with an overpotential of only 46 millivolts in alkaline media. In photocatalysis, single-layer 1T-MoS2 paired with nitrogen-doped graphene produced hydrogen roughly 600 times faster than comparable 2H systems under visible light.

Energy storage results are equally compelling. Vertically aligned metallic MoS2 on graphene delivered initial lithium-ion capacities near 1700 milliampere-hours per gram, while carbon-free metallic nanotube anodes retained the 1T phase for at least 120 days in air. For sodium-ion batteries, freestanding 1T-MoS2 grown on hollow graphene foam sustained stable capacities around 313 milliampere-hours per gram over 200 cycles. Supercapacitors built from restacked 1T-MoS2 films achieved volumetric capacitances of 400 to 650 farads per cubic centimeter, retaining over 93 percent of initial capacitance after 5000 cycles, and water-coupled metallic MoS2 with nanochannels reached 150 farads per gram even without conductive additives. The review concludes that scalable synthesis, long-term phase stability, and clear structure-property correlations remain the key hurdles, and it points to artificial intelligence-assisted materials discovery as the accelerant that could carry these engineered atomically thin catalysts from laboratory demonstrations to industrial deployment.

Subject of Research: Materials engineering strategies for tuning metallic two-dimensional transition metal dichalcogenides for energy conversion and storage

Article Title: Recent advances in tuning the properties of metallic 2D transition-metal dichalcogenides for energy conversion and storage

Article References: Ha, J., Park, G., Kang, G., Kang, J., Bak, H., Lee, D., Lee, H., Cho, K., & Kim, Y. (2026). Recent advances in tuning the properties of metallic 2D transition-metal dichalcogenides for energy conversion and storage. Advances in Industrial and Engineering Chemistry, 2(1), Article 5. https://doi.org/10.1007/s44405-026-00045-0

Image Credits: AI Generated

DOI: 10.1007/s44405-026-00045-0

Keywords: transition metal dichalcogenides, two-dimensional materials, hydrogen evolution reaction, phase engineering, defect engineering, heteroatom doping, strain engineering, heterostructures, lithium-ion batteries, supercapacitors, electrocatalysis, energy storage