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.
Supplementary materials
Title
Electronic Supporting Information
Description
additional information on CV fitting, XPS spectra
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