From Promotion to Poisoning: Temperature- and Coverage-Dependent Effects of Trace Fe on the Hydrogen Evolution Activity of Pt

Abstract

Trace iron impurities in alkaline electrolytes can strongly alter the hydrogen evolution reaction (HER), yet how temperature governs their surface accumulation and kinetic consequences remains poorly understood. Here, the influence of 0–60 ppb Fe ions on polycrystalline Pt was systematically examined in 0.1 M KOH over 280–336 K. Fe ion addition progressively suppressed the hydrogen underpotential-deposition (Hupd) charge, indicating blockage of Pt sites by Fe-derived surface species, while this suppression was substantially mitigated at elevated temperatures. Despite this site blocking, 10–20 ppb Fe enhanced the HER current, whereas further Fe addition caused pronounced deactivation, particularly at 280–298 K. Wide-potential-range voltammetry revealed electrochemical responses consistent with hydroxide-like Fe species undergoing an Fe(II)/Fe(III)-type redox transition, with the Fe-associated anodic charge decreasing markedly at 336 K. Correlation of the Hupd-derived blocked-site fraction with the Tafel slope revealed three coverage-dependent regimes: kinetic promotion at low Fe coverage, predominantly site-number-limited deactivation at intermediate coverage, and severe kinetic inhibition at high coverage. These results demonstrate that the effect of trace Fe on Pt is not intrinsically detrimental but evolves from HER promotion to severe poisoning depending on Fe coverage and electrolyte temperature. The findings establish electrolyte temperature and trace-impurity coverage as coupled parameters controlling alkaline HER kinetics on Pt.