MXene Electrocatalyst: Advanced Two-Dimensional Materials For Sustainable Energy Conversion

Fundamental Structure And Physicochemical Properties Of MXene Electrocatalyst Materials

MXene electrocatalysts belong to the family of two-dimensional transition metal carbides, nitrides, or carbonitrides with the general formula Mn+1XnTx, where M represents early transition metals (Ti, V, Cr, Mo, Nb, Ta, W), X denotes carbon and/or nitrogen, n = 1-3, and Tx represents surface termination groups including hydroxyl (-OH), oxygen (=O), fluorine (-F), and chlorine (-Cl) 123. These materials are synthesized through selective etching of the A-layer (typically Al, Si, or Ga) from layered MAX phase precursors (Mn+1AXn) using acidic etchants, resulting in accordion-like multilayered structures that can be further exfoliated into few-layer or monolayer nanosheets 412.

The unique structural characteristics of MXene electrocatalyst materials include:

• Metallic conductivity: Electrical conductivity ranging from 6,000 to 20,000 S/cm for Ti3C2Tx, significantly higher than graphene oxide (0.05-5 S/cm) and comparable to metallic conductors, facilitating rapid electron transfer during electrocatalytic reactions 359
• High specific surface area: Theoretical surface area up to 400-600 m²/g for fully delaminated single-layer MXene, though practical values typically range 50-150 m²/g due to restacking, providing abundant active sites for catalytic reactions 14
• Hydrophilic surface chemistry: Contact angle <5° for water on Ti3C2Tx surfaces due to dense coverage of oxygen-containing functional groups, enabling excellent electrolyte accessibility and ion transport 26
• Tunable electronic structure: Work function adjustable from 4.1 to 5.6 eV depending on surface termination composition, allowing optimization of hydrogen adsorption free energy (ΔGH*) for HER catalysis 816

The surface termination groups play a critical role in determining electrocatalytic performance. Oxygen and hydroxyl terminations generally provide more favorable hydrogen binding energies compared to fluorine terminations, which are electronegative and reduce catalytic activity 16. Recent advances in fluorine-free synthesis using alternative etchants such as deep eutectic solvents or molten salt methods have demonstrated improved HER performance with overpotentials reduced by 150-200 mV compared to HF-etched counterparts 219.

Synthesis Methodologies And Structural Engineering Of MXene Electrocatalyst

Precursor Preparation And Etching Strategies For MXene Electrocatalyst

The synthesis of MXene electrocatalyst materials begins with the preparation of MAX phase precursors, typically through solid-state reaction of elemental powders at 1300-1600°C under inert atmosphere 13. Common MAX phases include Ti3AlC2, Ti2AlC, Nb2AlC, Ta4AlC3, and V2AlC, with Ti3AlC2 being the most extensively studied due to its commercial availability and relatively straightforward etching process 111.

Selective etching methods for MXene electrocatalyst synthesis include:

1. Hydrofluoric acid (HF) etching: Direct immersion of MAX powder in 10-50% HF solution at room temperature for 18-72 hours, achieving complete Al removal but introducing fluorine terminations that reduce catalytic activity (typical HER overpotential η10 = 400-600 mV at 10 mA/cm²) 412

2. In-situ HF generation: Mixing MAX powder with LiF and HCl (molar ratio 7.5:1) at 35-55°C for 24-48 hours, producing HF in situ while simultaneously intercalating Li+ ions for easier delamination, resulting in larger lateral flake sizes (5-15 μm) and improved conductivity 16

3. Fluorine-free etching: Employing acidic deep eutectic solvents (DES) composed of hydrogen bond donors (e.g., oxalic acid, citric acid) and acceptors (e.g., choline chloride) with metal precursors at 120-180°C for 12-24 hours, eliminating fluorine contamination and enabling direct single-atom metal loading with enhanced HER performance (η10 = 180-250 mV) 219

4. Molten salt etching: Treating MAX phases with eutectic salt mixtures (e.g., NaF-KF-LiF) at 550-750°C under inert atmosphere, producing MXene with minimal defects and controlled surface chemistry, particularly effective for tungsten-based MXenes (W2TiC2Tx) with near-thermoneutral hydrogen adsorption (η10 = 89 mV at 10 mA/cm²) 13

Delamination And Morphology Control For Enhanced Electrocatalytic Performance

Following etching, multilayered MXene must be delaminated to maximize accessible surface area and active site density. Delamination strategies include:

• Intercalation-assisted exfoliation: Treating etched MXene with organic intercalants (DMSO, TBAOH, isopropylamine) or inorganic cations (Li+, Na+, K+) followed by sonication (200-400 W, 30-120 min) or manual shaking, achieving monolayer yields of 60-90% with lateral dimensions 0.5-5 μm 111
• Freeze-thaw cycling: Rapid freezing of MXene dispersion in liquid nitrogen followed by lyophilization at -40 to -50°C, creating foam-like three-dimensional architectures with hierarchical porosity and preventing restacking, particularly effective when combined with spacers like C3N4 or carbon nanotubes 14
• Electrochemical delamination: Applying constant voltage (5-15 V) in aqueous electrolyte to intercalate cations and generate gas bubbles that mechanically separate layers, producing high-quality nanosheets without organic solvent contamination 17

Morphology engineering approaches to overcome MXene restacking and enhance electrocatalytic performance include:

1. Fiber-templated scaffolding: Incorporating electrospun polymer nanofibers (PAN, PVA, PVP) as structural supports during MXene assembly, followed by carbonization at 600-900°C under N2, creating interconnected 3D networks with specific surface areas 180-320 m²/g and preventing layer collapse during cycling 4

2. Foam structure construction: Mixing delaminated MXene with precursors (melamine, urea) followed by freeze-drying and thermal treatment at 500-600°C, generating foam-like MXene/C3N4 composites with hierarchical macro-meso-microporous structures, increasing CO2 adsorption capacity by 3-5 times and improving Faradaic efficiency for CO2 reduction from 80% to >95% 1

3. Nanodot-shell architecture: Fragmenting MXene nanosheets into quantum dots (2-8 nm) through prolonged sonication or chemical oxidation, then encapsulating in carbon shells via hydrothermal carbonization of glucose at 180-220°C, enhancing stability in acidic/alkaline media while maintaining high edge site density 20

Composite Strategies And Heterostructure Engineering For MXene Electrocatalyst

Metal And Metal Compound Loading On MXene Electrocatalyst Substrates

The intrinsic catalytic activity of pristine MXene for HER and OER is often insufficient for practical applications, necessitating the incorporation of catalytically active species. MXene's abundant surface functional groups and high conductivity make it an ideal support for various active materials:

Single-atom catalysts: Anchoring isolated metal atoms (Pt, Pd, Ru, Ni, Co, Fe) on MXene surfaces through coordination with oxygen/hydroxyl groups, maximizing atomic utilization efficiency while maintaining high activity. Acidic deep eutectic solvent treatment enables high single-atom loading (up to 8-12 wt%) without aggregation, with Pt single atoms on Ti3C2Tx achieving HER performance comparable to commercial Pt/C (η10 = 30-45 mV) at 1/10th the Pt loading 2

Nanoparticle decoration: Depositing metal nanoparticles (Ag, Au, Ni, Co) on MXene through solvothermal, hydrothermal, or electrodeposition methods. For CO2 reduction, Ag nanoparticles (5-15 nm) uniformly distributed on foam-structured MXene/C3N4 demonstrate Faradaic efficiency >95% for CO production at current densities 150-300 mA/cm² with stability >100 hours 1. NiFeOOH nanoparticles grown on Ti3C2Tx via hydrothermal synthesis (120-180°C, 6-12 hours) exhibit OER overpotentials of 280-320 mV at 10 mA/cm² and maintain current density 500-1000 mA/cm² for >20 hours without degradation 3

Transition metal phosphides: Forming MXene-supported metal phosphides (Ni2P, CoP, FeP, MoP) through phosphidation of metal precursors at 300-450°C under H2/Ar atmosphere. The heterogeneous bonding between MXene and phosphides creates interfacial charge redistribution, optimizing hydrogen adsorption energetics. Bimetallic phosphides (NiFeP, CoMoP) on MXene demonstrate bifunctional activity with HER overpotentials 120-180 mV and OER overpotentials 260-310 mV at 10 mA/cm², enabling overall water splitting at cell voltages 1.52-1.65 V 5910

Transition metal carbides: In-situ growth of secondary carbides (Mo2C, WC, Co3C) on MXene surfaces through carbothermal reduction of metal salts at 600-900°C, creating carbide-carbide heterointerfaces with enhanced electron transfer. Carbon-encapsulated carbide nanoparticles (3-8 nm) uniformly distributed on MXene exhibit exceptional stability in alkaline seawater electrolysis, maintaining >90% activity after 200 hours at current densities 100-500 mA/cm² 12

Layered Double Hydroxide And Metal-Organic Framework Integration With MXene Electrocatalyst

MXene/LDH composites: Growing layered double hydroxides (NiFe-LDH, CoFe-LDH, NiCo-LDH) on MXene substrates through co-precipitation or hydrothermal methods creates synergistic interfaces where Ti sites in MXene act as electron donors to stabilize LDH and modulate OER intermediates. Defect engineering through selective etching or ion exchange (replacing Fe with Co2+, V5+, Cr3+) increases reactive site density. NiFe-LDH/Ti3C2Tx composites demonstrate OER overpotentials 240-280 mV at 10 mA/cm² with Tafel slopes 35-45 mV/dec, approaching noble metal performance 36

MXene/MOF hybrids: Uniformly nucleating metal-organic frameworks (Ni-MOF, Co-MOF, NiFe-MOF) on MXene nanosheets through room-temperature coordination assembly addresses MOF's poor conductivity while preventing MXene restacking. MOF nanoparticles (10-100 nm) with loading >75 wt% on MXene create hierarchical porous structures with specific surface areas 400-800 m²/g. Subsequent thermal treatment (300-500°C) converts MOFs to metal oxides/hydroxides while maintaining structural integrity, yielding OER catalysts with overpotentials 270-320 mV and stability >100 hours in 1 M KOH 14

Heteroatom Doping And Surface Functionalization Of MXene Electrocatalyst

Modifying MXene's electronic structure through heteroatom incorporation optimizes catalytic site activity:

• Nitrogen and sulfur co-doping: Annealing MXene with thiourea at 400-600°C under N2/H2 atmosphere introduces electron-donating N and S atoms into the MXene lattice and surface terminations, reducing Gibbs free energy for hydrogen adsorption (ΔGH* approaching 0 eV). B,S-Ti3C2Tx demonstrates HER overpotentials 150-200 mV at 10 mA/cm² with Tafel slopes 55-70 mV/dec, representing 3-4 fold improvement over pristine MXene 816

• Oxygen vacancy engineering: Controlled reduction of MXene-supported metal oxides (MnO2, Co3O4) through H2 treatment or electrochemical activation creates oxygen vacancies that serve as active sites and enhance electrical conductivity. Oxygen-vacancy-rich MnO2/MXene composites exhibit 2-3 times higher specific capacitance (450-650 F/g at 1 A/g) and improved OER activity compared to stoichiometric counterparts 15

• Surface termination engineering: Replacing fluorine terminations with oxygen/hydroxyl groups through alkaline treatment (NaOH, KOH) or thermal annealing in oxidizing atmosphere improves hydrogen binding energetics and electrolyte wettability, reducing HER overpotentials by 100-180 mV 216

Performance Metrics And Mechanistic Insights For MXene Electrocatalyst Applications

Hydrogen Evolution Reaction Performance Of MXene Electrocatalyst

MXene-based electrocatalysts have demonstrated substantial progress in HER across acidic, alkaline, and neutral electrolytes:

Acidic HER: Tungsten-based W2TiC2Tx MXene synthesized through modified precursor etching exhibits near-thermoneutral hydrogen adsorption with overpotential η10 = 89 mV at 10 mA/cm² in 0.5 M H2SO4, Tafel slope 48 mV/dec, and exchange current density 0.18 mA/cm², approaching Pt/C performance (η10 = 30-40 mV, Tafel slope 30 mV/dec) 13. B,S-co-doped Ti3C2Tx achieves η10 = 165 mV with Tafel slope 62 mV/dec and maintains >95% activity after 5000 cycles, demonstrating price competitiveness and stability advantages over noble metals 8

Alkaline HER: MXene-supported bimetallic phosphides (NiFeP/Ti3C2Tx, CoMoP/Ti3C2Tx) demonstrate η10 = 120-180 mV, Tafel slopes 45-65 mV/dec, and stability >200 hours at 100 mA/cm² in 1 M KOH. The strain-controlled heterointerface between phosphides and MXene induces electronic structure rearrangement, lowering water dissociation barriers and optimizing H* binding 910.