Fig. 2: Local pH evolution at both electrodes in aqueous Zn-Mn batteries.

The schematic illustrates the time-dependent, spatially non-uniform pH distribution near the Mn-based cathode and Zn anode. Different ways of pH evolution at the MnO₂ cathode (top image) are (1) proton intercalating into MnO₂ and the free OH^- increases the local pH, (2) Free H⁺ at the interface reduces the local pH, when Mn²⁺ reacts with H₂O at higher potentials, and (3) At OER regimes, the protons are released, that decreases the local pH. In the case of anode (bottom image), free OH^- are generated during the parasitic HER, thereby increasing the local pH.