LAUSR

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 Ti3 C2 Tx MXene in a mild AlCl3 aqueous electrolyte can operate at a wide potential range from 0 to -1.3 V versus Hg/Hg2 SO4 and retain high pseudocapacitive activity. Thus, a high capacity of up to 85 mAh g-1 is achieved, surpassing its performance in H2 SO4 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 Al3+ , providing the possibility of coupling MXene anodes with proton redox active cathodes to achieve high-energy and high-power devices.