(667a) Synthesis, Activity Testing, and Soft X-Ray Spectroscopy of Nickel-Iron (Oxy)Hydroxide Oxygen Evolution Electrocatalysts

Many electrocatalytic processes of contemporary interest, including H₂ evolution and CO₂ reduction, must be coupled with an anodic oxidation reaction to create a working device. The anodic oxidation reaction must be efficiently performed at scale over an earth­abundant anode electrocatalyst to ensure commercial viability. The oxygen evolution reaction (OER) in alkaline media is a promising candidate reaction, since it can be performed over earth-abundant nickel-iron (oxy)hydroxide (NiFe) electrocatalysts with activity comparable to what is observed over Ir and Ru oxides in acidic media. However, driving OER over NiFe still requires an excessive overpotential, limiting the overall efficiency of many electrocatalytic processes. There has been considerable interest in understanding the structural and electronic evolution of NiFe during OER and identifying the role of Fe in promoting electrocatalytic activity. Nevertheless, many questions regarding the structure of this catalyst and the electronic state of the constituent metals of this electrocatalyst remain, despite these efforts. These ambiguities have hindered the rational design of superior electrocatalysts. One of the challenges has been to precisely control the electrocatalyst composition in the low iron concentration regime (1 to 20 at. % Fe) and identifying the oxidation state of Fe during OER. In this study, we discuss the cause of Fe enrichment observed in electrodeposited NiFe electrocatalysts and leverage this knowledge to design a new synthesis procedure that enables exceptional control over electrocatalyst composition throughout the entire composition range. We use this approach to investigate the role of both catalyst composition and thickness on electrocatalytic activity, coupled with synchrotron-based soft x-ray spectroscopy to investigate the oxidation state of Fe.