Activation of NiFe oxide-based OER catalysts: Iron determines the growth of the catalytically active phase in NiO

05 June 2025, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Mixed nickel iron hydr-/oxides are nowadays the best performing alkaline O2 evolution electrocatalysts. It has been assumed that the activity of these catalysts is determined by surface species but that a conditioning of the bulk is necessary to achieve optimal performances. Only recently, research delved into the surface properties and the composition near the solid-liquid interface of such catalysts. Using sputter depth-profiling XPS, we compared pure nickel oxides with mixed nickel iron oxides and found that electrochemical transformation to (oxy-)hydroxides in the mixed oxide systems is limited to just a few nanometers at the surface, unlike pure nickel oxides, which transform deeper into the bulk. This suggests that non-surface-sensitive techniques may overlook the true active phase in mixed nickel iron oxide catalysts. Our results indicate that bulk transformation may not be essential for catalytic activity. We propose a model in which iron limits nickel oxidation and promotes charge transfer directly to the oxygen evolution reaction.

Keywords

alkaline water electrolysis
nickel oxide
active phase
X-ray photoelectron spectroscopy

Supplementary materials

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Description
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Electronic Supporting Information
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additional information on CV fitting, XPS spectra
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