Regulating MXene Functionalization and Oxidation for High-Rate, High Mass-Loading Supercapacitors

Unlocking the commercial potential of MXene electrodes in high-energy supercapacitors requires overcoming a fundamental limitation: severe performance decay at increased electrode thickness and mass loading. This work resolves the intrinsic trade-off between surface functionalization and oxidation in MXene chemistry regulation by constructing a controlled redox environment. A urea-assisted hydrothermal process effectively removes inert -F terminations while inducing the formation of a 3D MXene hydrogel enriched with -N active sites. Concurrently, L-ascorbic acid is introduced as an antioxidant to suppress structural oxidation and enhance stability. As a result, the optimized MXene exhibits superior rate capability and long-term cycling stability under high mass loadings. Specifically, at 10.97 mg cm^-2, a high specific capacitance of 597 F g^-1 is achieved at 1 A g^-1, with 69.66% retention at 50 A g^-1, significantly outperforming that of pristine MXene (23.44%) and N-doped MXene (59.03%). Even at an ultrahigh loading of 108.64 mg cm^-2, 44.50% of the capacitance is retained (from 564 F g^-1 to 251 F g^-1) over the current density range of 1 to 20 A g^-1. This work establishes an effective strategy for designing high-performance MXene-based electrodes via reaction pathway regulation, enabling practical operation at commercially relevant mass loadings.