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

The low anodic oxidation potential severely suppresses the output voltage (≤ 0.6 V) of MXene-based symmetrical aqueous micro-supercapacitors (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_x flakes of cathode can combine with intercalated Na⁺ cations during charging process to reconstruct into -ONa 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_0V) 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 V in neutral polyacrylamide/1 M Na₂SO₄ hydrogel electrolyte, boosting the corresponding areal energy density from 9.9 to 34.6 μW·h·cm^–2. The demonstrated deep insight on the surface terminals reconstruction mechanism for synchronously modulating the P_0V 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.