Bridging Quantum Capacitance and Experimental Electrochemical Performance in 2D Materials for Supercapacitors: From Density of States to Device-Level Interpretation

Two-dimensional (2D) materials, particularly MXenes and transition metal dichalcogenides (TMDs), have attracted intense interest as supercapacitor electrodes due to their high surface area and tunable electronic structure. However, large discrepancies persist between the quantum capacitance values predicted by density functional theory (DFT) calculations and experimentally measured gravimetric capacitances. In this review, we critically analyze DFT methodologies, surface models, normalization strategies, and electrochemical characterization protocols, and compile an extensive dataset of reported MXene and TMD systems to quantify the degree of experimental–theoretical agreement. We show that MXenes typically achieve less than 20% of their predicted capacitance because of restacking, surface terminations, and limited ion accessibility, whereas TMDs exhibit substantially better correspondence, often approaching or exceeding 70% of theoretical values. These results indicate that the theoretical capacitance predicted by DFT is primarily determined by the electronic structure of the material, which defines the upper limit of charge storage, whereas the experimentally achieved capacitance is largely controlled by morphological factors, surface chemistry, and electrode architecture that limit ion accessibility.