Cheng Zhan1 Michael Naguib2 Maria Lukatskaya3 Paul Kent2 Yury Gogotsi3 Jiang De-en1

1, University of California, Riverside, Riverside, California, United States
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.