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Commercial membrane electrolyzers rely on acidic fluorocarbon membranes and ionomers, requiring the use of expensive IrOx-based oxygen-evolution catalysts. Anion-exchange-membrane water electrolyzers (AEMWEs) operate in an alkaline environment, enabling the use of non-precious-metal catalysts. Here, we study and engineer CoOx-based catalyst-coated anodes deposited via hydrothermal synthesis directly onto porous transport layers both with and without thermal annealing. Self-supported, nanoneedle-structured Co3O4 anode, formed by annealing the as-synthesized cobalt carbonate hydroxide, Co(CO3)x(OH)y, outperforms the baseline Co3O4 nanoparticle ink-based anode in pure-water-fed AEMWE, due to improved catalyst layer continuity and thus number of electroactive Co species. The as-synthesized and unannealed Co(CO3)x(OH)y), however, appears to undergo substantial conversion to a more-active CoOx(OH)y phase predominantly at the surface, with nominally Co3+ present and higher electrical conductivity, lowering the cell voltage ~200 mV at 1.0 A∙cm-2 in pure-water-fed AEMWE compared to the conventional Co3O4 nanoparticle anodes. We analyze the differences in electrode electrochemical response between pure-water and KOH feed modes finding distinct activation and degradation modes. The Co(CO3)x(OH)y anode shows significant activation and slower degradation linked to the conversion to oxyhydroxide. We propose catalyst layer designs that promote both hydroxide and electron transport, alongside interfacial engineering strategies to obtain high performance while mitigating anode degradation. 

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. 

Dissolved iron (Fe) species are a pre-requisite for the most active catalyst sites for the oxygen evolution reaction in alkaline electrolytes, but the overall effects of dissolved Fe on energy- efficient advanced alkaline water electrolysis cells remain unclear. Here, we systematically control the concentration of Fe in a model zero-gap alkaline water electrolyzer to understand the interactions between Fe and high surface area catalyst coatings. Cells employing a platinum-group- metal-containing cathode and a high surface area, mixed-metal-oxide anode yielded an optimum voltage efficiency at elevated temperatures and in the presence of 6 ppm Fe, which reduced the cell voltage by ~100 mV compared to rigorously Fe-free electrolytes. Increasing concentrations of Fe led to a systematic increase in anode activity towards the oxygen evolution reaction and a reduction in the electrochemically active surface area at both the anode and cathode. Metallic Fe was not observed to electrodeposit at cathodes which operate at overpotentials ≤ 120 mV, but dissolved Fe does reduce the apparent number density of sites available for hydride adsorption. These findings suggest that the energy efficiency of advanced alkaline water electrolysis systems can be improved by managing the Fe concentration in recirculating KOH electrolytes. 

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.