Recent Progress in MXene-Based Supercapacitors: From Synthesis and Surface Engineering to Next-Generation Applications

The emergence of MXenes, 2D layered transition metal carbides, nitrides, and carbonitrides, has unlocked different possibilities in supercapacitor (SC) electrode design. The metallic electronic conductivity, hydrophilic surfaces, adjustable surface chemistry, and open interlayer spacing enable efficient charge storage via pseudocapacitance and rapid ion intercalation, distinguishing them from traditional carbon-based and metal-oxide materials. The different synthesis methods, which included selective etching with hydrofluoric acid (HF), alkali, electrochemical, molten salts, and non-etching techniques, as well as chemical vapor deposition (CVD), lithiation-expansion, ammonization, and mechanical delamination, improved researchers' ability to control MXene morphology, functional groups, and interlayer spacing, which resulted in changes to ion mobility and charge storage behavior. Structural engineering techniques, through their application of surface functionalization, interlayer modification, graphene, CNT, TMD, transition metal oxide, and conductive polymer integration, provided solutions to three main challenges, which included oxidation, restacking, and slow ion accessibility. The MXene-based electrodes reached outstanding electrochemical performance because they achieved specific capacitances (C_sp) above 1500 F/g, energy densities close to 80 Wh/kg, power densities greater than 10 kW/kg, and capacitance retention over 90% after 10,000 cycles. Mechanistic insights show that electric double-layer capacitor (EDLC) and pseudocapacitive ion intercalation and surface redox reactions function together to control charge storage and self-discharge characteristics. This review integrates synthesis methods, charge storage mechanisms, and performance correlations, which emphasize new applications in SCs and wearable electronics. MXenes demonstrate their potential as a platform for advanced energy storage systems (ESSs) through their future development capabilities, which enable multifunctional device integration, structural stabilization, and scalable fabrication.