Locally Resolved Operando pH Measurement Technique in Aqueous Zinc-Manganese Dioxide Batteries

Accurate information about the local pH on electrode surfaces is critical to understand the underlying reaction mechanism and side reactions in Aqueous Zinc-Manganese Dioxide Batteries (AZMBs). Previously reported pH measurements lack sufficient quality and accuracy due to the placement of the pH sensor in the bulk electrolyte and significant excess of electrolyte, which dilutes the pH effects during operation. We present a locally resolved, application‑oriented measurement technique using flat‑tip surface pH sensors placed in direct contact with both electrodes in a setup with minimized electrolyte volumes. The novel pH measurement technique is combined with visual observations from a windowed coin cell operando microscopy setup. In this work, we demonstrate the severity of the cathode-driven pH shifting between 2 and 5.5 during cycling in an unbuffered sulfate electrolyte, which induces hydrogen evolution reaction (HER) on the anode, zinc hydroxy-sulfate (ZHS) precipitation at the anode and incomplete ZHS dissolution upon charge. Also, the pH buffering by an acetate-sulfate electrolyte and its limitations with high capacities are investigated. We demonstrate that local pH phenomena influenced by the apparent current density and buffer concentration determine the cell voltage and the onset of HER and ZHS precipitation. The results show the crucial role of the electrolyte composition - controlling both the local and global pH by developing new buffers, cell concepts or adjusted cycling protocols will be essential to make AZMBs an application-ready battery cell technology.