Nickel–Iron\nOxyhydroxide Oxygen-Evolution Electrocatalysts:\nThe Role of Intentional and Incidental Iron Incorporation

Abstract

Fe plays a critical,\nbut not yet understood, role in enhancing\nthe activity of the Ni-based oxygen evolution reaction (OER) electrocatalysts.\nWe report electrochemical, <i>in situ</i> electrical, photoelectron\nspectroscopy, and X-ray diffraction measurements on Ni_1–<i>x</i>Fe_<i>x</i>(OH)₂/Ni_1–<i>x</i>Fe_<i>x</i>OOH\nthin films to investigate the changes in electronic properties, OER\nactivity, and structure as a result of Fe inclusion. We developed\na simple method for purification of KOH electrolyte that uses precipitated\nbulk Ni­(OH)₂ to absorb Fe impurities. Cyclic voltammetry\non rigorously Fe-free Ni­(OH)₂/NiOOH reveals new Ni redox\nfeatures and no significant OER current until >400 mV overpotential,\ndifferent from previous reports which were likely affected by Fe impurities.\nWe show through controlled crystallization that β-NiOOH is less\nactive for OER than the disordered γ-NiOOH starting material\nand that previous reports of increased activity for β-NiOOH\nare due to incorporation of Fe-impurities during the crystallization\nprocess. Through-film <i>in situ</i> conductivity measurements\nshow a >30-fold increase in film conductivity with Fe addition,\nbut\nthis change in conductivity is not sufficient to explain the observed\nchanges in activity. Measurements of activity as a function of film\nthickness on Au and glassy carbon substrates are consistent with the\nhypothesis that Fe exerts a partial-charge-transfer activation effect\non Ni, similar to that observed for noble-metal electrode surfaces.\nThese results have significant implications for the design and study\nof Ni_1–<i>x</i>Fe_<i>x</i>OOH OER electrocatalysts, which are the fastest measured OER\ncatalysts under basic conditions.