Role of Catalyst Preparation on the Electrocatalytic Activity of Ni_1–<i>x</i>Fe_<i>x</i>OOH for the Oxygen Evolution Reaction

Ni&lt;sub&gt;1–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Fe&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;OOH thin films prepared via cathodic electrodeposition have been demonstrated to be highly active catalysts for the oxygen evolution reaction (OER) in basic media. Integration of these catalysts with light-absorbing semiconductors is required for photoelectrochemical fuel generation. However, the application of cathodic potentials required for typical electrochemical catalyst deposition limits the library of compatible photoanode materials. Sputter deposition of catalysts circumvents this limitation by enabling facile catalyst layering without cathodic potentials. In this work, we compare the structure and OER activity of sputter-deposited and electrodeposited Ni&lt;sub&gt;1–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Fe&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;OOH thin films. Electrochemical cycling converts sputtered Ni&lt;sub&gt;1-x&lt;/sub&gt;Fe&lt;sub&gt;x&lt;/sub&gt; metallic films to the desired oxides/(oxy)hydroxides. Both film preparation methods give catalysts with similar electrochemical behavior across all compositions. Additionally, OER activity is comparable between the deposition methods, with maximum activity for films with ~20% Fe content (320 mV overpotential at j = 10 mA cm&lt;sup&gt;-2&lt;/sup&gt; geometric). Electrochemical cycling to convert sputtered metallic Ni&lt;sub&gt;1-x&lt;/sub&gt;Fe&lt;sub&gt;x&lt;/sub&gt; films to metal oxides/(oxy)hydroxides is found to lower the Fe/Ni ratio, while the electrodeposited films exhibit comparable Fe/Ni ratios before and after electrochemical cycling and characterization. Structurally, Fe is found to incorporate within the Ni(OH)&lt;sub&gt;2&lt;/sub&gt;/NiOOH lattice for films formed through both sputter-deposition and electrodeposition. Layered films were also compared to codeposited 1:1 Fe/Ni films. It is found that, for layered films, an Fe top layer inhibits the electrochemical conversion of metallic Ni to Ni(OH)&lt;sub&gt;2&lt;/sub&gt;/NiOOH, thus reducing the amount of Ni&lt;sub&gt;1–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Fe&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;OOH OER-active phase formed. In contrast, migration of metals within Ni-on-top films occurs readily during electrochemical cycling, resulting in films that are structurally and electrochemically indistinguishable from codeposited Ni&lt;sub&gt;1–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Fe&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;OOH. These findings enable direct application of Ni&lt;sub&gt;1–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Fe&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;OOH sputtered films to a wider library of photoanodes for light-driven water-splitting applications.