Fe Coordination Environment, Fe-Incorporated Ni(OH)2 Phase, and Metallic Core Are Key Structural Components to Active and Stable Nanoparticle Catalysts for the Oxygen Evolution Reaction

Bimetallic iron–nickel oxide/hydroxide (FeNiO(H)x) nanocatalysts have emerged as nonprecious metal candidates for alkaline oxygen evolution reaction (OER) electrocatalysis. However, there are still significant open questions regarding the role of electrocatalyst synthesis route, and the resulting electrocatalyst morphology and nanoscale structure, in determining the operando atomic-scale structure when subjected to the faradic OER voltage environment. Herein, we report on two nanoparticle FeNiO(H)x electrocatalysts and their different chemical structures using operando X-ray absorption spectroscopy (XAS) studies at relevant OER conditions. The two bimetallic nanoparticle electrocatalysts were synthesized using aqueous (NP-aq) vs oil-based (NP-oil) synthesis routes but resulted in compositionally similar surface chemistry as-synthesized. Operando XAS results suggest that Ni oxidizes from the initial +2 oxidation state to +3/+4 state reminiscent of the transformation of α-Ni(OH)2 to γ-NiOOH; the oxidation state change is voltage-dependent and occurs in both NP-aq and NP-oil nanoparticles. There does not appear to be an oxidation state change for Fe, but the Fe coordination environment does change with voltage. The NP-aq nanoparticles resulted in Fe coordination transitions between Fe3+ Td, observed in as-synthesized and 0.8–0.9 V vs Ag/AgCl conditions, and Fe3+ Oh, observed at 0 V vs Ag/AgCl, while the NP-oil nanoparticles resulted in a largely stable Fe3+ Oh coordination with more subtle changes in the coordination environment. The voltage dependence of this Fe coordination transition is nanoparticle-dependent, with NP-aq nanoparticles transitioning dramatically at 0.7 V vs Ag/AgCl but NP-oil nanoparticles transitioning slowly starting at 0.1 V vs Ag/AgCl. Additionally, a shortening of both the Fe–O and Ni–O bond distances occurs for both nanoparticle materials, but the magnitude of change is different for NP-aq vs NP-oil, suggesting that the nanoparticle structures result in unique changes under applied potential. Extended X-ray absorption fine structure (EXAFS) analysis showed distinct chemical environments for the Fe species of NP-aq vs NP-oil, metallic Fe and Ni character in NP-aq, and Ni largely in a hydroxide phase for both nanoparticles. NP-aq results in improved activity and stability during OER, as compared to NP-oil, suggesting that the Fe3+ Oh → Td transition, metallic core, and a predominant Fe-incorporated Ni(OH)2 phase in the shell are important for OER performance. This study highlights that both the electrochemical environment and the as-synthesized morphology of nanoparticle electrocatalysts are important in determining the operational chemical structures and structure–performance relationships.