Rapid Room-Temperature Sulfidation of Commercial Fenico Alloy for Efficient Oxygen Evolution Reaction

Alkaline water electrolysis (AWE) remains the most mature and scalable electrolysis technology for large-scale hydrogen production, owing to its compatibility with earth-abundant transition-metal-based electrodes. [1] However, practical anode design for the oxygen evolution reaction (OER) continues to face durability challenges, particularly when externally synthesized catalysts are immobilized onto conductive substrates using polymer binders. Such binders can block active sites and suffer from mechanical detachment under prolonged operation, especially at high current densities. [2] As a result, industrial AWE systems often rely on binder-free Ni-based electrodes, despite their relatively modest intrinsic OER activity compared to multi-metal systems incorporating Fe and Co. [3] In this work, we demonstrate a simple and effective room-temperature surface sulfidation strategy to transform a commercial FeNiCo alloy (Kovar) into a highly active, stable, and binder-free OER anode. The proposed surface treatment involves brief immersion of Kovar in an aqueous solution containing sodium persulfate and thiourea, followed by electrochemical stabilization in 1 M KOH. This approach requires no elevated temperature, external catalysts, or complex fabrication steps. Electrochemical measurements reveal that surface-sulfidized Kovar exhibits a marked enhancement in OER performance compared to untreated and electrochemically stabilized-only samples. The optimized electrode achieves an overpotential of 261 mV at 10 mA/cm 2 , significantly reduced from 345 mV for untreated Kovar, along with a Tafel slope of ~40 mV /dec. Notably, the electrode maintains stable operation at 100 mA/cm 2 for over 120 hours with minimal performance degradation, highlighting its robustness under industrially relevant conditions. Morphological and compositional analyses indicate that the surface sulfidation step produces a thin modified surface layer without compromising mechanical integrity or adhesion to the bulk alloy. While microcracks form during sulfidation, electrochemical impedance spectroscopy shows that performance improvements cannot be attributed solely to increased electrochemically active surface area (ECSA). X-ray photoelectron spectroscopy provides insight into the origin of this enhanced activity. First, electrochemical stabilization induces preferential Fe leaching, enriching the surface with Ni and shifting the Fe/Ni ratio toward a composition known to be favorable for OER in ternary Ni–Co–Fe systems. Second, surface sulfidation promotes higher fractions of Ni 3+ and Fe 3+ oxidation states, which have been widely associated with improved OER activity due to enhanced metal–oxygen hybridization and charge transfer. Third, sulfur incorporation into lattice oxygen facilitates the formation of OER-active (oxy)hydroxide phases during electrochemical stabilization. Kinetic analysis further reveals that both untreated and surface-sulfidized Kovar share the same rate-determining step, corresponding to the *OH to *O conversion in alkaline OER, as indicated by similar Tafel slopes. However, the exchange current density of the surface-sulfidized electrode increases by nearly three orders of magnitude, indicating substantially accelerated reaction kinetics. This enhancement is attributed to sulfur-induced modulation of *OH binding energy, which lowers the activation barrier for the rate-determining step without altering the overall reaction pathway. To assess the generality of this surface sulfidation strategy, we extended the treatment to pure Ni and a commercial FeNi alloy (Invar). While performance improvements were observed for Ni and Invar, the most pronounced enhancement occurred for Fe- and Co-containing alloys, underscoring the importance of multi-metal synergy. Overall, this study introduces a rapid, room-temperature surface sulfidation method that enables commercial FeNiCo alloys to function as efficient, durable, and binder-free OER anodes for alkaline water electrolysis. By simultaneously tuning surface composition, oxidation states, and intermediate binding energetics, this approach offers a scalable pathway for deploying low-cost commercial alloys in practical hydrogen production systems. References [1] L. Han et al., Adv. Mater. 28 (2016). [2] S. Yuan et al., Prog. Energy Combust. Sci. 96 (2023). [3] W. J. F. Gannon et al., Electrochim. Acta 322 (2019).