Nickel–Iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: The Role of Intentional and Incidental Iron Incorporation

Fe plays a critical, but not yet understood, role in enhancing the activity of the Ni-based oxygen evolution reaction (OER) electrocatalysts. We report electrochemical, <i>in situ</i> electrical, photoelectron spectroscopy, and X-ray diffraction measurements on Ni<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>(OH)<sub>2</sub>/Ni<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>OOH thin films to investigate the changes in electronic properties, OER activity, and structure as a result of Fe inclusion. We developed a simple method for purification of KOH electrolyte that uses precipitated bulk Ni­(OH)<sub>2</sub> to absorb Fe impurities. Cyclic voltammetry on rigorously Fe-free Ni­(OH)<sub>2</sub>/NiOOH reveals new Ni redox features and no significant OER current until >400 mV overpotential, different from previous reports which were likely affected by Fe impurities. We show through controlled crystallization that β-NiOOH is less active for OER than the disordered γ-NiOOH starting material and that previous reports of increased activity for β-NiOOH are due to incorporation of Fe-impurities during the crystallization process. Through-film <i>in situ</i> conductivity measurements show a >30-fold increase in film conductivity with Fe addition, but this change in conductivity is not sufficient to explain the observed changes in activity. Measurements of activity as a function of film thickness on Au and glassy carbon substrates are consistent with the hypothesis that Fe exerts a partial-charge-transfer activation effect on Ni, similar to that observed for noble-metal electrode surfaces. These results have significant implications for the design and study of Ni<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>OOH OER electrocatalysts, which are the fastest measured OER catalysts under basic conditions.