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.

Two-dimensional (2D) copper halides represent an emerging class of semiconductors with tunable electronic and optical properties, yet their structural stability and alloying behavior remain largely unexplored. Alloying is an effective strategy for enhancing the thermoelectric response of two-dimensional materials. Using first-principles density functional theory, we investigate the structural, electronic, and thermoelectric properties of monolayer CuI–CuBr alloys. The calculations predict thermodynamically stable CuIxBr1−x solid solutions across the full compositional range, as evidenced by uniformly negative mixing enthalpies for all considered crystal symmetries. Halide substitution induces systematic lattice contraction and symmetry-dependent distortions of the Cu-centered coordination environment, leading to a monotonic reduction of the electronic bandgap with increasing Br content. Within the constant relaxation time approximation, alloying significantly enhances the electronic contribution to the thermoelectric figure of merit, leading to high calculated ZT values within the constant relaxation time approximation framework, which should be interpreted as electronic upper-limit estimates at elevated temperatures for all investigated polymorphs. These results demonstrate that alloying provides a robust and controllable route to improve thermoelectric performance in two-dimensional materials.