Impacts of Dissolved Iron and Dynamic Operation on Nickel-Based Anode Catalysts in Alkaline Water Electrolysis Cells

Alkaline water electrolysis (AWE) is a robust method for generating hydrogen. 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.The stability and efficiency of AWE anodes are impacted by reverse polarization conditions that occur during shutdown or power interruptions. Here we report the effect of reverse polarization on model thin-film Ni(Fe)OOH anodes in conditions that mirror those seen in commercial AWE stacks. Regardless of the extent of the reverse polarization potential, the anodic charge capacity decreases over successive cycles, indicating loss of active Ni²⁺/Ni³⁺ sites with cycling. Electrodes held for a longer period in the reduced state exhibited an accelerated catalyst loss through dissolution and structural reorganization during reverse polarization. Electrolytes spiked with soluble Fe preserved activity throughout repeated reverse current events, whereas suspended Ni(OH)2 preserved Ni²⁺/Ni³⁺ sites that exhibited lower activity for the OER. In-situ Raman spectroscopy and post-polarization X-ray photoelectron spectroscopy revealed a significant decrease in the Fe:Ni surface ratio after reverse polarization, which correlates with the loss of active Fe–Ni oxyhydroxide domains and a measurable decline in oxygen evolution reaction (OER) activity. These findings on model thin-film anodes thus illustrate both the mode of typical anode degradation during shutdown/reverse-polarization conditions and how they can be mitigated by managing the dissolved ions in the recirculating KOH. Stainless steel components were aged both thermally and electrochemically in KOH to produce electrolytes with different extents of corrosion. A thin-film NixFe(1-x)OOH catalyst coated on Pt foil was then subjected to repeated charge–discharge cycling in these electrolytes. The aged electrolytes produced distinct cell-voltage and discharge-capacity responses, demonstrating that catalyst behavior depended on both electrolyte-aging duration and the evolving mixture of dissolved metal species. Detailed analysis of the electrolyte aged during 3 h of OER operation showed that corrosion-derived impurities altered the Ni²⁺/Ni³⁺ redox response, consistent with changes in the local electronic environment and Fe incorporation. Surface analysis further showed that Fe, Cr, and Mn incorporated into the anode during operation but were only partially retained following repeated charge–discharge cycling.