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

TLDR

Fe plays a critical but not yet understood role in enhancing Ni‑based oxygen evolution reaction electrocatalysts. The study investigates how Fe inclusion alters the electronic properties, OER activity, and structure of Ni₁₋ₓFex(OH)₂/Ni₁₋ₓFexOOH thin films using electrochemical, in situ electrical, photoelectron spectroscopy, and X‑ray diffraction measurements, and introduces a simple KOH purification method that employs bulk Ni(OH)₂ to absorb Fe impurities. Fe‑free Ni(OH)₂/NiOOH exhibits new Ni redox features and negligible OER activity until >400 mV, β‑NiOOH is less active than γ‑NiOOH because of Fe impurities, Fe addition boosts film conductivity >30‑fold yet does not fully explain the activity increase, and Fe exerts a partial‑charge‑transfer activation effect on Ni, making Ni₁₋ₓFexOOH the fastest OER catalysts under basic conditions.

Abstract

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, in situ electrical, photoelectron spectroscopy, and X-ray diffraction measurements on Ni1–xFex(OH)2/Ni1–xFexOOH 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)2 to absorb Fe impurities. Cyclic voltammetry on rigorously Fe-free Ni(OH)2/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 in situ 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 Ni1–xFexOOH OER electrocatalysts, which are the fastest measured OER catalysts under basic conditions.