(406h) Oxidation State Modulation Via Surface Restructuring: The Impact of Fe Incorporation into Ni Catalyst on Activity and Stability in AEM Water Electrolysis

Non-precious metal group (PGM-free) catalysts for the Oxygen Evolution Reaction (OER) have gained significant interest due to their potential to lower the cost of green hydrogen production. In this context, NiFe-based materials represent come of the most promising for applications in electrolyte free membrane-based water electrolyzers; thought few details are known regarding their surface restructuring dynamics and associated changes in the oxidation states of the active metal centers under working conditions.

Here, we synthesized and tested a series of Ni:Fe nanoparticles supported on Ni Foam (NF), with corresponding metal ratios of Ni₃Fe, Ni₃Fe₂, NiFe, and NiFe₃ in both a 3-electrode cell and a continuous-flow AEM electrolyzer to better understand the role of Fe in modulating the OER rates. Our results indicate that Ni₃Fe/NF and NiFe₃/NF exhibit 1.33 and 2.09 times higher current densities, respectively compared to pristine NF. However, NiFe/NF and NiFe₃/NF do not demonstrate significant differences in current density; suggesting the existence of an optimal Fe content in Ni/NF, beyond which further addition of Fe may no longer enhance current density. Our working hypothesis is that exceeding this threshold iron content might result in Fe atoms reducing the adjacent Ni atoms coordinated with oxygen. Such changes would increase the proportion of Ni²⁺ relative to Ni³⁺ within the structure acting as insulated sites, as Ni³⁺ is more electrically conductive than Ni²⁺. Consequently, this could lead to a drop in overall electronic conductivity.

Alternatively, there may be an upper Ni:Fe ratio above which Fe atoms are destabilized, resulting in Fe leaching and a concomitant increase in impurities into the electrolyte. A quantitative understanding of these behaviors, specifically the change in oxidation state of Ni from Ni³⁺ to Ni²⁺ while compensating for the difference in electronic charge by another element, could provide insights into the factors controlling activity and stability in AEMWE.