Surface terminals reconstruction: The way to widen the output voltage of MXene-based aqueous symmetrical microsupercapacitors

The low anodic oxidation potential severely suppresses the output voltage (<= 0.6 V) of MXene-based symmetrical aqueous microsupercapacitors (MSA-MSCs) employing acidic electrolytes. Herein, a surface terminals reconstruction mechanism on cathode of MSA-MSCs adopting aqueous neutral electrolyte (1 M Na2SO4) is first revealed by systematical electrochemical experiments and in/ex-situ spectral analysis, which indicates that: the -O terminals on Ti3C2T, flakes of cathode can combine with intercalated Na* cations during charging process to reconstruct into -0Na units to (i) inhibit the splitting reaction of adjacent water molecules, decreasing cathodic hydrogen evolution potential, and more significantly, (ii) lower the potential of zero voltage (P-ov) between the symmetrical electrodes to avoid anode oxidation, enabling full use of the unexploited potential range of cathode. Thus, the output voltage of the MSA-MSCs tremendously expanded from 0.6 V in acidic polyacrylamide (PAM)/1 M H2SO4 hydrogel electrolyte to 1.5 Vin neutral polyacrylamide/1 M Na2SO4 hydrogel electrolyte, boosting the corresponding areal energy density from 9,9 to 34.6 mu W.h-cm(-2). The demonstrated deep insight on the surface terminals reconstruction mechanism for synchronously modulating the P-ov between symmetrical electrodes and hydrogen evolution potential on cathode provides critical guidance for widening the cell voltage of MSA-MSCs with safer and more inexpensive neutral electrolytes.