Activation of NiFe Oxide-Based OER Catalysts: Iron Determines the Growth of the Catalytically Active Phase in NiO

Mixed nickel iron hydr-/oxides are nowadays the best performing alkaline O 2 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 X-ray photoelectron spectroscopy, 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.