Cooperative Fe sites on transition metal (oxy)hydroxides drive high oxygen evolution activity in base.

Fe-containing transition-metal (oxy)hydroxides are highly active oxygen-evolution reaction (OER) electrocatalysts in alkaline media and ubiquitously form across many materials systems. The complexity and dynamics of the Fe sites within the (oxy)hydroxide have slowed understanding of how and where the Fe-based active sites form-information critical for designing catalysts and electrolytes with higher activity and stability. We show that where/how Fe species in the electrolyte incorporate into host Ni or Co (oxy)hydroxides depends on the electrochemical history and structural properties of the host material. Substantially less Fe is incorporated from Fe-spiked electrolyte into Ni (oxy)hydroxide at anodic potentials, past the nominally Ni2+/3+ redox wave, compared to during potential cycling. The Fe adsorbed under constant anodic potentials leads to impressively high per-Fe OER turn-over frequency (TOFFe) of ~40 s-1 at 350 mV overpotential which we attribute to under-coordinated "surface" Fe. By systematically controlling the concentration of surface Fe, we find TOFFe increases linearly with the Fe concentration. This suggests a changing OER mechanism with increased Fe concentration, consistent with a mechanism involving cooperative Fe sites in FeOx clusters.

Nickel Site Modification by High-Valence Doping: Effect of Tantalum Impurities on the Alkaline Water Electro-Oxidation by NiO Probed by Operando Raman Spectroscopy.

In an effort to support the large-scale implementation of clean hydrogen in industry and society, the electrolytic decomposition of water is considered a realistically enticing prospect, provided the guarantee of affordable and durable material components. Within alkaline systems, earth-abundant electrocatalysts could provide both these requirements. However, a continued exploration of the reactivity and the causes behind different behaviors in performance are necessary to guide optimization and design. In this paper, Ta-doped NiO thin films are prepared via DC magnetron sputtering (1-2-4 at % Ta) to demonstrate the effect of surface electronic modulation by non-3d elements on the catalysis of the oxygen evolution reaction (OER). Material properties of the catalysts are analyzed via Rutherford backscattering spectrometry, X-ray diffractometry, photoelectron spectroscopy, and Raman spectroscopy. Ta impurities are shown to be directly responsible for increasing the valence state of Ni sites and enhancing reaction kinetics, resulting in performance improvements of up to 64 mV at 10 mA cm-2 relative to pristine NiO. Particularly, we show that by applying operando Raman spectroscopy, Ta enhances the ability to create high-valence Ni in γ-NiOOH at a lower overpotential compared to the undoped sample. The lowered overpotentials of the OER can thus be attributed to the energetically less hindered advent of the creation of γ-NiOOH species on the pre-catalyst surface: a phenomenon otherwise unresolved through simple voltammetry.