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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 Ni(2+/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 (TOF(Fe)) 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 TOF(Fe) 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.
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This content will become publicly available on March 19, 2027
Spectroelectrochemical Studies of Oxygen Evolution Reaction Kinetics for Surface-Incorporated Iron in Nickel Oxyhydroxide Electrocatalysts
NixFe1−xOyHz is the state-of-the-art catalyst for the oxygen evolution reaction (OER) in alkaline water electrolyzers; however, understanding the impact of Fe on the active sites, reaction mechanism, and consequently intrinsic activity has been under intense debate. In this work, operando UV−vis spectroscopy was used to investigate Fe-free NiOxHy and NiOxHy with Fe selectively incorporated onto the surface. At oxygen-evolution potentials, similar oxidized nickel states were present before and after the Fe incorporation, with negligible changes in their redox potentials. However, the discharge kinetics of the Ni states show a substantial acceleration after the introduction of Fe, consistent with an increase in OER kinetics upon Fe incorporation and formation of active Ni−Fe species. Using optical spectroscopy, we determined the intrinsic reaction time constant per surface Fe site is <0.1 s, which is 2 orders of magnitude faster than Ni sites not in proximity to surface Fe sites (∼10 s), and also an order of magnitude faster than Ni sites in pure NiOxHy (∼1 s). Consequently, we propose that the OER occurs via charge accumulation primarily on Ni centers in these catalysts, followed by hole transport to the surface Fe species where oxygen evolution occurs.
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- Award ID(s):
- 2400195
- PAR ID:
- 10685057
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- ACS Catalysis
- Volume:
- 16
- Issue:
- 7
- ISSN:
- 2155-5435
- Page Range / eLocation ID:
- 6749 to 6757
- Subject(s) / Keyword(s):
- Water Oxidation, Catalysis, Spectroelectrochemistry, Kinetics, Mechanism, Nickel Oxide
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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