In Situ Hydrogen Evolution Monitoring During the Electrochemical Formation and Cycling of Pressed‐Plate Carbonyl Iron Electrodes in Alkaline Electrolyte

The hydrogen evolution reaction (HER) on iron is a parasitic side reaction for the reduction of iron (hydr)oxide in alkaline electrolyte, which lowers the Coulombic efficiency of iron-based batteries. Tackling this issue, here we investigate the HER on iron electrodes by in situ gas chromatography, allowing for a quantitative correlation of the applied electrode potential and the resulting hydrogen evolution. As a result, it is shown that the HER follows a distinctive profile corresponding to the electrode potential and changes depending on the state of the iron electrode formation. Moreover, it is shown that the charging efficiency of the iron electrode can be increased by an alteration of the charging procedure, i. e., a more negative cut-off potential for the discharge and a potential limitation for the recharge. In this study, a charging efficiency of 96.7 % is achieved, using an optimized charging procedure for a formed carbonyl iron electrode containing 8.5 wt.% of Bi2S3.