Identification of highly active surface iron sites on Ni(OOH) for the oxygen evolution reaction by atomic layer deposition

Atomic layer deposition (ALD) has become a versatile tool in catalysis, allowing for precise synthesis of catalysts useful for gaining fundamental understanding of complex systems. In this work, ALD was used to perform surface-directed modification of Ni(OH)2/Ni(OOH) electrocatalysts for the oxygen evolution reaction (OER) to elucidate the different roles of iron as a surface dopant and of iron distributed throughout the NiOOH structure. Electrochemical and material characterization of FeOx ALD-modified Ni(OH)2 indicates that iron is deposited on the surface of Ni(OH)2 sheets without modifying its bulk properties. Sub-monolayer amounts of iron were deposited on the surface of Ni(OH)2 using low cycle numbers of ALD. This surface iron results in high OER activity, characteristic of Ni-Fe(OOH) catalysts. Ni(OH)2/Ni(OOH) catalysts modified to incorporate iron throughout the structure were prepared by depositing large cycle numbers of FeOx ALD, which results in the deposition of a Fe2O3 overlayer. Corrosion of this Fe2O3 overlayer during electrochemical cycling in alkaline electrolyte leads to the incorporation of iron throughout the structure of Ni(OH)2/Ni(OOH) while leaving some iron at the surface. Incorporation of iron throughout the Ni(OH)2/Ni(OOH) structure was found to increase OER geometric activity for thick, high surface area Ni(OH)2/Ni(OOH) catalysts. Lastly, Ni-FeOx electrocatalysts synthesized fully by ALD were investigated for the OER; these catalysts demonstrated high OER activity and potential for photocatalysis applications.