Unveiling the Pseudocapacitance of Ti2C Monolayer for High Performance Electrochemical Capacitor: a First-Principles Study

Two-dimensional (2D) transition-metal (TM) compound nanomaterials have been widely investigated for electrochemical capacitor due to their high-surface-area and large potential of providing pseudocapacitance. Very recently, a new family of 2D materials called MXenes were synthesized and have been considered as one of the most promising pseudocapacitor electrode materials with respect to both power density and energy density. In this work, we employ first-principles calculations to study the electronic structures and redox potential and diffusion of Li on Ti₂C monolayer, a representative MXene, based on density functional theory (DFT). The electronic structures indicate a significant redox pseudocapacitance characteristics as an anode with the redox potential at about 0.25 V and 0.75 V refer to the standard hydrogen electrode. Moreover, the calculated metallic behavior and low Li ions diffusion barriers suggest that Ti₂C monolayer would manifest a low resistance in charging/discharging process. Our findings provide an evidence that the Ti₂C monolayer is a promising electrochemical capacitor electrode material.

Figure a shows the crystal structure of Ti₂C monolayer in the top view and side view. The Ti and C are revealed by yellow and blue spheres, respectively. On the top view, the lighter yellow spheres consist of the first layer with the third layer formed by dim ones. Relative dispositions of PDOS and Integral DOS of d-orbitals of Ti atoms in Ti₂C monolayers are shown in Figure b both on the vacuum scale and with respect to the SHE reference. The dot dashed red lines indicate the Fermi level positions of Ti₂C monolayer. The light blue region represents the electrolyte window. To deliberate the diffusion of Ti₂C monolayer, three pathways (Figure c) and corresponding energy barriers (Figure d) are considered.