Ti 3 C 2 T x MXene with High Pseudocapacitive Activity and Large Potential Window in a Mild AlCl 3 Aqueous Electrolyte.

MXenes have been extensively explored as supercapacitor electrodes, especially in acidic aqueous electrolytes, where ultrahigh specific capacitance can be achieved; however, their narrow working potential window (≤ 1.0 V) limits the acquisition of high energy. Neutral and alkaline electrolytes can be used to extend the working potential window but MXenes in these electrolytes are less pseudocapacitive active, which leads to reduced charge storage. In this study, it is shown that Ti 3 C 2 T x MXene in a mild AlCl 3 aqueous electrolyte can operate at a wide potential range from 0 to -1.3 V versus Hg/Hg 2 SO 4 and retain high pseudocapacitive activity. Thus, a high capacity of up to 85 mAh g -1 is achieved, surpassing its performance in H 2 SO 4 electrolyte of 78 mAh g -1 . More interestingly, most of the capacity is released at a more negative potential range than that in acidic electrolytes, making it more suitable as a negative electrode material. In situ electrochemical quartz crystal microbalance results suggest that the high capacity originates from the pseudocapacitive intercalation/deintercalation of H + instead of Al 3+ , providing the possibility of coupling MXene anodes with proton redox active cathodes to achieve high-energy and high-power devices.