The Rise of MXene - the Two-dimensional Material Revolution from Graphene to Supercapacitors

The Beginning of A New Era of Two-dimensional Materials

Research of two-dimensional materials has intensified since graphene was discovered in 2004 because they exhibit unique physical and chemical properties. Scientists considered graphene a revolutionary energy storage material because it measures only a single atomic layer thick while exhibiting both ultra-high conductivity and mechanical strength. The strong hydrophobic nature of the material combined with its tendency to stack easily creates defects which restrict the specific surface area and electrolyte wettability thus causing the real capacitance performance to fall significantly below theoretical predictions. The synthesis of MXene started through etching MAX phases with hydrofluoric acid which initiated a new chapter for two-dimensional materials. MXene maintains graphene's exceptional conductivity and exhibits hydrophilicity along with adjustable surface functional groups and pseudocapacitance features which result in superior volume capacity and cycle stability in supercapacitors over traditional carbon-based materials. The new discovery signifies a shift from one-dimensional carbon-based two-dimensional materials to diverse metal compound forms.

MXene vs. Graphene: Generational Transition of Two-Dimensional Materials

Structural Differences:

Graphene: It is composed of pure carbon six-membered rings, and the interlayers rely only on van der Waals forces to bond. It is highly hydrophobic (contact angle>100°), which makes it difficult for electrolyte to penetrate.

MXene: The chemical formula is Mₙ₊₁XₙTx (M=transition metal, X=C/N, Tx=surface functional group). Its layered structure is bonded by metal bonds and covalent bonds. The rich polar functional groups on the surface give it excellent hydrophilicity (contact angle 25°–45°), which significantly improves the ion transmission efficiency.

Performance Comparison:

Conductivity: The theoretical electron mobility of MXene is as high as 10⁶ cm²/(V·s), which exceeds that of graphene (2×10⁵ cm²/(V·s)). This is due to the high electron density of the transition metal d orbital and the synergistic effect of the metal-carbon bond in the layer.

Energy storage mechanism: Graphene mainly relies on double-layer energy storage (EDLC), while MXene has both EDLC and pseudocapacitance properties. For example, the -O functional group of Ti₃C₂Tx y can undergo a reversible redox reaction, contributing additional capacity (pseudocapacitance accounts for 30%–50%).

Synthesis and Structural Optimization of MXene

MAX phase etching technology:

HF solution etching: The traditional method etches the Al layer of the MAX phase through HF, but it is highly toxic, the byproducts are difficult to handle, and the residual -F group may reduce the electrochemical activity.

Salt-based etching: The LiF/HCl system achieves mild etching through an intercalation-stripping mechanism, reducing toxicity and increasing yield (such as Ti₃C₂Tx yield>90%), and the proportion of surface -O/-OH functional groups is higher, which is conducive to improving capacitance performance.

Functional modification strategy:

Surface functional group regulation: The introduction of macromolecules such as lignin sulfonate through post-treatment can expand the interlayer spacing, expose more active sites, and inhibit oxidation.

Composite structure design: Composite with carbon nanotubes (CNT), graphene or conductive polymers (such as PEDOT:PSS) to construct a three-dimensional porous network and solve the stacking problem. For example, the specific capacitance of Ti₃C₂Tx/rGO composite aerogel is increased to 426 F/g (pure MXene is 245 F/g), and the capacity retention rate is >95% after 5000 cycles.

Dimensional Engineering of MXene/graphene Composites

1D fiber structure

MXene/graphene composite fibers prepared by wet spinning technology show excellent performance. Graphene oxide liquid crystals act as templates to guide the orderly arrangement of MXene nanosheets to form highly conductive fibers. At the same time, the pseudocapacitive properties of MXene and the mechanical flexibility of graphene work together to make it an ideal choice for flexible electrodes. The introduction of 1D reduced graphene oxide (rGO) nanoribbons as spacer materials can effectively inhibit the interlayer stacking of MXene and improve the ion transport kinetics. The specific capacitance reaches 397.4 F/g at a scan rate of 5 mV/s, and the cycle stability is excellent.

2D film structure

Electrostatic self-assembly technology forms a free-standing composite film by alternately stacking positively charged rGO and negatively charged MXene. This structure not only inhibits the interlayer stacking of MXene, but also achieves a volume capacitance of 1040 F/cm³ by expanding the interlayer spacing (such as M/G-5% film), and the capacity retention rate reaches 61% at a high scan rate of 1 V/s. rGO as a conductive bridge enhances the diffusion efficiency of electrolyte ions. At the same time, the porous MXene film doped with nitrogen and phosphorus end groups further optimizes the wettability and active site density, increasing the energy density to 32.6 Wh/L.

3D aerogel structure

MXene/rGO porous aerogels prepared by in-situ reduction or γ-ray radiation, combined with three-dimensional interconnected pores and high specific surface area (such as lignin sulfonate-modified MXene aerogels), have a specific capacitance of 386 F/g, and a capacity retention rate of more than 99% after 5000 cycles. This type of structure achieves an energy density of 15.5 mWh/cm³ and a tensile strength of 126.9 MPa in solid-state supercapacitors by inhibiting MXene oxidation and enhancing mechanical strength (such as introducing aramid nanofibers), combining flexibility with high energy storage performance.

Performance Breakthroughs in Supercapacitors

High volume energy density: MXene/rGO hybrid films achieve an energy density of 32.6 Wh/L by optimizing interlayer structure and surface chemistry, which is 2-3 times that of traditional devices.

Fast charge and discharge capability: The porous design and heterogeneous interface accelerate ion diffusion, with a capacity retention rate of 61% and a power density of 12,780 W/kg at a scan rate of 1 V/s.

Extreme environmental adaptability: MXene-based devices maintain stable capacitance output under deep-sea high pressure (>30 MPa) and high temperature (80°C) conditions, attributed to their hydrophilic surface and oxidation resistance.

Diversified Application Scenarios of MXene

Transportation: The composite electrode supports instantaneous high power output of electric vehicles (such as fast start and stop), the energy density of the all-solid-state device reaches 10.6 Wh/kg, and the performance is intact after bending 2500 times.

Smart grid: For wind energy storage and microgrid peak regulation, the MXene/rGO electrode has a charge and discharge efficiency of 84% and a cycle life of more than 20,000 times.

Flexible electronics: Self-healing supercapacitors achieve a volume capacitance of 345.2 F/cm³ through MXene-graphene composite fibers, which is suitable for the multi-angle bending requirements of wearable devices.