Electrolyte Study with in Operando pH Tracking Providing Insight into the Reaction Mechanism of Aqueous Acidic Zn//MnO2 Batteries

The reaction mechanisms (RM) during cycling of aqueous rechargeable Zn//MnO2 batteries (ARZIBs) are still controversially discussed. The present study of different acidic electrolyte compositions (0.9 mM H2SO4, 0.5 M MnSO4, 2 M ZnSO4, 2 M ZnSO4+0.5 M MnSO4) and their pH behaviour is therefore designed as an alternative approach to investigate the RM. In operando pH tracking during cycling shows periodic pH changes for each electrolyte, highlighting the role of the pH-relevant ions OH− and H+ in the chemical processes, the major influence of MnO2 deposition/dissolution mechanisms and the buffering behaviour of the zinc hydroxide sulphate (ZHS) precipitation. Innovative coupled cyclic voltammetry (CV) and pH measurements can link CV redox peaks to a pH change and a corresponding chemical reaction. It was found that a Zn2+ (de-)intercalation has little or no influence on the capacity. The cycling of the SO42−-free electrolyte 2 M Zn(CF3SO3)2 underlines the pH-dependant behaviour of the chemical processes. The results can contribute to the debate of RMs in ARZIBs and other aqueous battery chemistries by introducing a novel measurement technique.