Chloride-Induced Corrosion and Mixed-Potential Control of BiHCF Electrodes in Saline Electrolytes

Bismuth hexacyanoferrate (BiHCF), a Prussian blue analogue containing redox-active Fe-CN-Bi coordination motifs, was investigated as a model electrode for cathodic processes in chloride-rich saline and hypersaline electrolytes. Rather than evaluating BiHCF solely as a hydrogen evolution catalyst, this work focuses on the coupled electrochemical and interfacial processes that govern its response in NaCl solutions and natural brines from seawater, reverse osmosis (RO) reject, and high-altitude brine environments. Structural characterization by SEM-EDS, XRD and FTIR confirmed the formation of crystalline BiHCF with rod-like micrometric morphology and preserved cyanide coordination. Linear sweep voltammetry under controlled hydrodynamic conditions revealed a progressive cathodic displacement of the mixed potential with increasing NaCl concentration, together with a marked suppression of oxygen reduction kinetics at high chloride activity. Mixed-potential analysis showed that HER kinetics remain comparatively less sensitive to salinity than ORR, whereas the anodic contribution associated with BiHCF oxidation becomes strongly affected by chloride-induced surface transformation. Post-electrochemical characterization indicates the formation of a BiOCl-rich surface layer when the BiHCF is in contact with a hypersaline electrolyte during the cathodic subprocess (close to 0 mV_SHE), which accounts for the transition from active mixed-control behaviour to a passivated interfacial regime. Density functional theory calculations suggest that elementary water activation and hydrogen-forming steps at Bi sites are intrinsically feasible, implying that the experimentally observed overpotentials originate primarily from transport, interfacial resistance and chloride-driven passivation rather than from an unfavourable molecular reaction pathway. These findings provide a mechanistic framework for understanding Bi-based Prussian blue analogue electrodes in non-purified saline electrochemical systems and highlight the dual role of chloride as both a charge-compensating electrolyte species and a passivating reactant.