Introducing a Concept for Designing an Aqueous Electrolyte with pH Buffer Properties for Zn-MnO₂ Batteries with Mn²⁺/MnO₂ Deposition/Dissolution

For large-scale energy-storage systems, the aqueous rechargeable zinc-manganese dioxide battery (ARZMB) attracts increasing attention due to its excellent advantages such as high energy density, high safety, low material cost, and environmental friendliness. Still, the reaction mechanism and its influence on the electrolyte's pH are under debate. Herein, a pH buffer concept for ARZMB electrolytes is introduced. Selection criteria for pH buffer substances are defined. Different buffered electrolytes based on a zinc salt (ZnSO4, Zn(CH3COO)(2), Zn(CHOO)(2)), and pH buffer substances (acetic acid, propionic acid, formic acid, citric acid, 4-hydrobenzoic acid, potassium bisulfate, potassium dihydrogen citrate, and potassium hydrogen phthalate) are selected and compared to an unbuffered 2 M ZnSO4 reference electrolyte using titration, galvanostatic cycling with pH tracking, and cyclic voltammetry. By adding buffer substances, the pH changes can be reduced and controlled within the defined operating window, supporting the Mn2+/MnO2 deposition/dissolution mechanism. Furthermore, the potential plateau during discharge can be increased from approximate to 1.3 V (ZnSO4) to approximate to 1.7 V (ZnSO4 + AA) versus Zn/Zn2+ and the energy retention from approximate to 30% after 268 cycles (ZnSO4) to approximate to 86% after 494 cycles (ZnSO4 thorn AA). Herein, this work can serve as a basis for the targeted design of long-term stable ARZMB electrolytes.