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.

Capacitive deionization (CDI) is a promising technology to satisfy the global need for fresh water, since it can be both economical and sustainable. While two-dimensional transition metal carbides/nitrides (MXenes) exhibit great characteristics for use as CDI electrode materials, their tightly spaced layered structure renders intercalation inefficiency. In this study, the interlayer distance of MXenes is precisely modulated by inserting different quantity of one-dimensional bacterial fibers (BC), forming freestanding MXene/BC composite electrodes. Among the studied samples, MXene/BC-33% electrode with the interlayer spacing of 15.2 Å can achieve an optimized tradeoff among various desalination performance metrics and indicators. The salt adsorption capacity (SAC), the average salt adsorption rate (ASAR), the energy normalized adsorbed salt (ENAS), and the thermodynamic energy efficiency (TEE) of the MXene/BC-33% electrode are improved by 24%, 46%, 13%, and 66% respectively compared with those of pure MXene electrode. While the insertion of BC improves the ion diffusion pathways and facilitates the intercalation kinetics, the desalination performance decreases when the insertion amount of BC exceeds 40%. This is attributed to the overlarge resistance of the composite and the resulting increased energy consumption. This study reveals the desalination performance tradeoffs of MXene-based electrodes with different interlayer distances and also sheds light on the fundamental ion storage mechanisms of intercalation materials in a CDI desalination system.