Local Potential Indicators for Alkaline Oxygen Electrodes (Supporting Information)

Measuring the local potential distributions in oxygen electrodes, such as bifunctional air electrodes for metal-air secondary batteries and cathodes for fuel cells, is crucial for understanding reaction processes and for developing electrode design guidelines. In this study, we explore indicators for optically visualizing the local potential in alkaline oxygen electrodes. Ni, Ag, and Cu are selected as potential indicators, and their brightness changes depending on applied potentials were measured by using a confocal optical microscope in aqueous solutions of 1.0 mol dm−3 and 8.0 mol dm−3 KOH. The brightness of Ni, Ag, and Cu foil electrodes (thickness: 30 µm) decreases/increases with oxidation/reduction at around the oxygen evolution reaction potential (1.22–1.42 V vs. RHE), the equilibrium potential (1.12–1.22 V vs. RHE), and the oxygen reduction reaction potential (0.62–0.77 V vs. RHE) regions, respectively, demonstrating that these materials can function as potential indicators for alkaline oxygen electrodes. Furthermore, Ni and Ag microwire electrodes (diameter: 50 µm), which are supposed to have less effect on mass transport than the metal foil electrodes, also show brightness changes similar to those of the metal foils in 8.0 mol dm−3 KOH.