Bulk vs Intrinsic Activity of NiFeO<sub><i>x</i></sub> Electrocatalysts in the Oxygen Evolution Reaction: The Influence of Catalyst Loading, Morphology, and Support Material
Xingyi Deng, Dan C. Sorescu, Iradwikanari Waluyo, Adrian Hunt, Douglas R. Kauffman
Abstract
We used a combination of ultrahigh vacuum surface science techniques, X-ray spectroscopy, electrochemistry, and density functional theory (DFT), to characterize the influence of catalyst morphology, loading/coverage, and substrate material on the bulk (all atoms) and intrinsic (electrochemically accessible atoms) activity of NiFeO<sub><i>x</i></sub> electrocatalysts in the oxygen evolution reaction (OER). NiFeO<sub><i>x</i></sub> catalysts were grown on both Au(111) and highly oriented pyrolytic graphite (HOPG) electrodes. DFT predicted Fe edge-site atoms at the NiFeO<sub><i>x</i></sub>/Au(111) interface to be the most thermodynamically favorable reaction center, and X-ray absorption spectroscopy data indicated small NiFeO<sub><i>x</i></sub> catalyst particles on Au(111) contained a high population of OER active Fe edge-site atoms. However, restructuring of the Au(111) surface due to repeated oxidation and reduction cycles of the OER CV measurements encapsulated small NiFeO<sub><i>x</i></sub> nanoparticles at catalyst loadings below ~1.5 nmol<sub>metal</sub>/cm<sup>2</sup>, passivated catalyst edges and reduced bulk OER activity of Au-supported NiFeO<sub><i>x</i></sub> compared with HOPG-supported ones. Analysis of intrinsic activity revealed that the Au(111) support strongly benefited electrochemically accessible NiFeO<sub><i>x</i></sub> atoms, and we observed a 2–3 fold activity enhancement compared with HOPG-supported catalysts for loadings above ~1 nmol<sub>metal</sub>/cm<sup>2</sup>. Overall, evaluating bulk vs intrinsic activity and identifying loading/coverage-dependent support effects is important for accurately probing fundamental interfacial chemistry, choosing suitable catalyst loadings and supports, and optimizing system parameters to maximize the performance of electrocatalyst systems.