Bulk pH-dependent failure mechanisms of transition metal anodes in seawater electrolysis

Abstract

Direct seawater electrolysis for hydrogen production can effectively alleviate freshwater resource constraints, but its anode stability faces severe chloride-induced corrosion. Although alkaline conditions enhance the thermodynamic selectivity of the oxygen evolution reaction (OER), the corrosion mechanism of chloride under complex bulk pH remains unclear. This study systematically investigates the electrochemical performance and failure mechanisms of four typical transition metal electrodes (FeNi36, 304SS, 316SS, NiFeO_xH_y) in chloride-containing electrolytes at different bulk pH (KOH concentrations, [OH⁻]). The results reveal competition between OER and metal dissolution at the anode, with the dominant pathway strongly dependent on applied potential, bulk pH, and electrode material. The galvanostatic polarization results further demonstrate that anodic failure in chloride-containing media is not a continuous dissolution process, but rather follows an “anodic deposition-mediated abrupt failure” mechanism. Cl⁻ first triggers active metal dissolution, followed by precipitation of dissolved metal ions under high bulk pH. This process is governed by the high-exponential dependence of the solubility product (K_sp) on [OH⁻]. At low bulk pH, precipitation primarily occurs in the electrolyte, forming a dense surface precipitate layer only after severe substrate corrosion. At high pH, precipitation occurs at the electrode interface, rapidly forming a dense passivation layer that causes electrode deactivation. Simultaneously, it is confirmed that the intrinsic corrosion resistance of 316SS (Mo element) and the high OER selectivity of the NiFeO_xH_y catalytic layer effectively extend anode service life. Overall, this study elucidates the critical role of pH in chloride-induced corrosion, providing a theoretical basis for designing highly stable seawater electrolysis anodes.