The Impact of Functionalized MXene Surfaces on Capacitive Energy Storage in Acidic Aqueous Electrolytes: A Density Functional Theory Study

Abstract

The surface chemistry of Ti3C2Tx MXenes strongly influences their electrochemical charge-storage behavior, yet the specific role of residual −F groups in acidic aqueous electrolytes remains insufficiently resolved. Here, density functional theory calculations were performed on 24 Ti3C2OxFy surface models covering 10 O/F compositions and multiple inequivalent termination arrangements. Surface stability, proton adsorption, charge transfer, electrode-potential variation, and electric double-layer and pseudocapacitive contributions were systematically evaluated. The results show that −O groups provide proton-active redox sites, whereas −F groups are comparatively electrochemically inactive and compete with −O for available surface sites. Consequently, increasing the −F coverage progressively suppresses Faradaic charge transfer and reduces the pseudocapacitive contribution. Within the investigated static DFT models, low −F coverages cause only a limited decrease in the calculated equilibrium capacitance because sufficient proton-active −O sites remain accessible. A more pronounced capacitance reduction occurs after the −F coverage exceeds an approximately 4/9 model-dependent crossover. This crossover is specific to the present supercell, termination configurations, and equilibrium thermodynamic framework and should not be regarded as a universal experimental threshold. These results clarify the site-competition mechanism between −O and −F terminations in Ti3C2Tx under acidic conditions, while explicit solvation, ion transport, and rate-dependent effects remain to be examined.