Low-spin state of Fe in Fe-doped NiOOH electrocatalysts

Doping with Fe boosts the electrocatalytic performance of NiOOH for the oxygen evolution reaction (OER). To understand this effect, we haveemployed state-of-the-art electronic structure calculations and thermodynamic modeling. Our study reveals that at low concentrations Fe exists in a low-spin state. Only this spin state explains the large solubility limit of Fe and similarity of FeO and Ni-O bond lengths measured in the Fe-doped NiOOH phase. The low-spin state renders the surface Fe sites highly active for the OER. The low-to-high spin transition at the Fe concentration of similar to 25% is consistent with the experimentally determined solubility limit of Fe in NiOOH. The thermodynamic overpotentials computed for doped and pure materials, eta = 0.42V and 0.77 V, agree well with themeasured values. Our results indicate a key role of the low-spin state of Fe for the OER activity of Fe-doped NiOOH electrocatalysts.

Automated summary

The doping of Fe at low concentrations enhances the electrocatalytic performance of NiOOH for the oxygen evolution reaction (OER). The existence of Fe in a low-spin state explains the large solubility limit of Fe and similarity of FeO and Ni-O bond lengths in the Fe-doped NiOOH phase. The transition from low-spin to high-spin state at around 25% Fe concentration is consistent with the experimentally determined solubility limit of Fe in NiOOH. The computed thermodynamic overpotentials for doped and pure materials are in good agreement with the measured values, indicating the key role of the low-spin state of Fe in the OER activity of Fe-doped NiOOH electrocatalysts.