Probing the electrochemical capacitance of MXene nanosheets for high-performance pseudocapacitors

Abstract

Pseudocapacitors, which can store more energy at high charge/discharge rates, have attracted considerable attention. The performance of a pseudocapacitive material mainly depends on the interaction between electrode materials and the electrolyte ions. However, the understanding of the interaction is still limited. Here, the performance of Ti₂CT₂ (T = O, F, and OH) nanosheets as pseudocapacitor electrode materials has been investigated through a novel first-principles approach. The results suggest that O-terminated Ti₂C nanosheets are shown to be pseudocapacitive cathode materials. The pseudocapacitance is attributed to the large intrinsic capacitance of Ti₂CO₂ nanosheets and contact adsorbed cations. The former mainly decides the capacity of charges and the latter reduces the change of potential in the electrode. The integral capacitance and Na-ion capacity of Ti₂CO₂ nanosheets are simulated to be 291.5 F g⁻¹ with a broad potential window range from 0 to 2.80 V (versus Na/Na⁺). Low diffusion energy barriers on Ti₂CO₂ and Ti₂CF₂ nanosheets indicate fast transportation and high charge and discharge rate for Na-ions. Our results provide insight into the origin of pseudocapacitance on stacked two-dimensional materials.