Phosphate Buffer-Induced Depletion of Nickel Redox Sites Limits Oxygen Evolution on Stainless Steel Anodes

Abstract

Although water electrolysis at neutral to mildly alkaline pH (7-12) expands the range of applicable cell components, pH fluctuations and mass transport limitations can hinder high-efficiency operation at high current densities. Phosphate buffer systems mitigate pH variations and therefore are considered promising electrolytes for such systems; however, their unrecognized chemical impact on electrode materials remains a critical bottleneck. Here, we systematically investigate the oxidation of stainless steel (SUS304) anodes and its impact on the oxygen evolution reaction (OER) in phosphate-buffered electrolytes. The data demonstrate that the electrode surface generated in phosphate electrolytes is fundamentally different from that formed in KOH. The SUS surface formed in phosphate electrolytes leads to an irreversible decrease in OER activity due to the depletion of nickel-based electrochemically redox-active sites. In contrast, KOH electrolytes readily form and maintain an active layer primarily comprising nickel species, enabling the full recovery of both redox capacity and OER activity even for electrodes previously deactivated in phosphate electrolytes. Notably, rather than adsorption, phosphate in the electrolytes induces the selective dissolution of nickel and iron to a depth of approximately four to six monolayers, resulting in the depletion of redox-active sites. These findings reveal intrinsic limitations of phosphate-based electrolytes for OER catalysts relying on nickel redox-active sites. The present results also highlight the importance of considering complexation-induced metal dissolution in electrolyte design and assessing the interfacial chemistry of redoxdriven transition metal catalytic systems in general.