Green hydrogen (H2) is a key energy carrier for global sustainability, and its large-scale production heavily relies on alkaline water electrolysis (AWE), one of the most mature and promising techniques. In most industrial alkaline water electrolyzers, iron (Fe)—an omnipresent impurity—profoundly influences the oxygen evolution reaction (OER) on Ni-based anodes and overall cell performance, yet understanding of this effect remains poor across testing scales, especially at practical operating conditions. Here, we systematically investigate trace Fe addition in diverse setups—from conventional H-cell tests to high-performance capillary-fed electrolyzer (CPE)[1] under industrially relevant conditions (6 M KOH, 85 °C, 500 mA cm⁻²)—revealing critical discrepancies that underscore the need for device-level optimization beyond half-cell assumptions.
In laboratory-scale H-cell experiments at room temperature, trace Fe addition (0.1 – 10 ppm) significantly promotes the OER by lowering its overpotential by up to ~200 mV. In contrast, the experiment in a CPE operating under comparable conditions showed much smaller improvements, with only a ~50 mV reduction in OER potential at 500 mA cm⁻², underscoring the strong dependence of Fe promotion on cell configuration. At elevated temperature, the OER overpotential decreased by 80 mV at 500 mA cm⁻² when the operating temperature reached 85 °C under similar [Fe] (~10 ppm). Although 3-electrode measurements suggested improved OER performance with increasing [Fe], full-cell operation reveals an opposite trend: the best-performing capillary-fed cell is obtained at minimal Fe levels (0.1–0.2 ppm). This was attributed to the deactivation effects of Fe on the noble-metal (Pt/C) cathodes, which can be exacerbated at high temperatures. These findings highlight a critical gap between conventional H-cell testing and realistic electrolyzer operation. Thus, for the efficient application of trace Fe in electrolytes, this concentration should be optimized carefully at the device level—rather than in half-cells alone—to balance OER promotion against deactivation of the hydrogen evolution reaction (HER) in practical AWE.