2, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
3, Drexel University, Philadelphia, Pennsylvania, United States
MXenes have attracted great attention as the next-generation capacitive energy-storage material, but the mechanism underlying its pseudocapacitive behavior remains elusive. Here we provide an overall understanding on the surface redox process of Ti3C2Tx (T=O, OH), a prototypical MXene, in H2SO4 electrolyte based on joint density functional theory and the analysis of Gibbs free energy under the constant electrode potential. From the dependence of the O/OH ratio and configuration and the surface charge on the applied potential, we obtain a complete capacitive energy-storage picture of Ti3C2Tx that shows very good agreement with previous experimental findings in terms of the electron transfer number, capacitance, and redox peak position. We find a voltage-dependent redox/double-layer co-charging behavior that provides new insight into the capacitive energy storage mechanism of MXenes.