Capacitance changes associated with cation-transport in free-standing flexible Ti3C2Tx (TO, F, OH) MXene film electrodes

Free-standing Ti₃C₂T_x MXene films with flexible feature are considered to be promising electrodes for the application in flexible energy devices. Exploring the different cation effect and each transport mechanism will help us to identify the optimized cation for special applications. Herein, we investigate the relationship between cation transports and capacitance behavior under different potential windows. In addition, we uncover intercalation mechanisms for Li⁺, Na⁺, and K⁺-containing electrolytes. As a result, we found that capacitive intercalation mechanism of characteristic cation can be influenced by different potential windows. Importantly, after 5000 charge-discharge cycles from −1 V to 0 V, the cation-intercalation behavior is demonstrated by significant electrode expansion in LiCl, NaCl, and KCl electrolytes with the increased d-spacing of 1.344 nm, 1.313 nm, and 1.305 nm, respectively. Additionally, larger size of K⁺ with fast ion transport is involved in fast electrochemical surface adsorptions, which explains higher capacitance of 346 F cm⁻³ in KCl than 218 F cm⁻³ in NaCl electrolyte. In contrast, small ion size of Li⁺ is vulnerable to adsorb and permeate the spacing between layers of Ti₃C₂T_x MXene electrode, which delivers volumetric capacitance of 320 F cm⁻³. The present study provides direct experimental evidence for electrochemical mechanism of cation intercalation between Ti₃C₂T_x MXene layer sheets.