Element-Selective Surface Reconstruction of Stainless Steel Anodes Governed by the Electrolyte Environment: Insights from High-Throughput Experiments

In neutral and mildly alkaline water electrolysis, electrolytes are primarily viewed as media controlling macroscopic solution properties such as pH and ionic conductivity. Beyond this conventional role, electrolyte species can interact with metal ions and dynamically reconstruct electrode surfaces through dissolution and reformation. Although such reconstruction is widely recognized, the extent to which operating surfaces retain the initial oxidation history or are governed by the electrolyte environment remains unclear. Here, a high-throughput electrochemical platform evaluated cell voltages under 108 conditions using 304 stainless steel anodes, independently varying anodic pretreatment, electrolyte pH (9.5, 10.5, and 11.5), phosphate concentration (1 and 3 M), and electrolysis regime. Analysis of variance showed that pH accounted for 70–81% of cell-voltage variation, whereas anodic pretreatment accounted for only 6–7%, demonstrating the dominant influence of the electrolyte environment on the electrochemical response. Surface analyses revealed widespread nickel depletion and heterogeneous chromium-rich films under lower-pH, high-phosphate conditions. Iron behavior was consistent with the equilibrium solubility trend, whereas nickel depletion did not follow the equilibrium solubility trend, indicating the involvement of processes beyond equilibrium thermodynamics. Together, these results reveal that multicomponent electrode surfaces do not reconstruct through a uniform pathway. Instead, the operating surface emerges from distinct element-specific dissolution and retention processes. The surface formed during operation should therefore be treated as a materials-design target rather than considering the as-prepared electrode and electrolyte independently.