A Critical Study of the Electrochemical Performance of MXenes for Energy Storage

A systematic comparative study of Ti3C2Tx, Nb2CTx, Nb4C3Tx, and V4C3Tx MXenes synthesized via a fluoride–salt (HCl + LiF) etching route is presented to elucidate the influence of MXene phase and stoichiometry on their electrochemical behavior in sulfuric acid. Structural and surface characterizations confirmed the successful exfoliation of the MXenes, revealing distinct morphologies and varying degrees of functionalization. The operating potential window was rigorously determined by combining cyclic voltammetry, galvanostatic charge–discharge, chronoamperometry, and electrochemical impedance spectroscopy. Ti3C2Tx exhibited enhanced cathodic stability (−0.5 V vs Ag/AgCl), whereas Nb- and V-based phases showed slightly narrower negative limits (∼−0.4 V vs Ag/AgCl). In addition, M4C3 structures demonstrated improved anodic tolerance. At 0.5 A g−1, specific capacitances of 225.3, 140.1, 190.6, and 131.5 F g−1 were obtained for Ti3C2Tx, Nb2CTx, Nb4C3Tx, and V4C3Tx, respectively. Dunn’s analysis revealed predominantly capacitive charge storage at 10 mV s−1 (94−85%), with diffusion contributions becoming more significant at low scan rates, particularly for the M4C3 phases. Therefore, the results demonstrate that MXene electrochemical performance is strongly influenced by structural stoichiometry, while also reflecting the coupled effects of surface chemistry, morphology, and interlayer structure, which collectively modulate stability limits and charge-storage kinetics in sulfuric acid.