Toward Finding\nthe Role of Surface Iron Ions in Enhancing\nOxygen-Evolution Reaction

The oxygen evolution reaction (OER)\nin alkaline media is crucial\nfor energy conversion technologies, and Fe-based catalysts have garnered\nsignificant attention for their efficacy. In this study, we provide\nan investigation of Fe-based catalysts under OER conditions using\nsome techniques. Our findings reveal minimal structural alterations\nin the bulk FeH<sub><i>x</i></sub>O<sub><i>y</i></sub> framework during OER, indicating that the bulk structure remains\nlargely intact. Instead, the catalytic activity is primarily localized\non the material’s surface. This conclusion is corroborated\nby the measured low electrochemical surface area (ECSA) of FeH<sub><i>x</i></sub>O<sub><i>y</i></sub>, suggesting\nthat its limited OER performance stems from a paucity of active sites\nrather than low intrinsic activity. The superior efficiency of Fe\nions in conductive oxides or on conductive metal substrates (e.g.,\ncopper and gold) in OER is attributed to the increased availability\nof surface-active sites in these systems. To further elucidate this\nphenomenon, we investigated two additional systems: NiFe and VFe (hydr)oxides.\nFor NiFe (hydr)oxides, we demonstrate that only surface Fe ions contribute\nactively to OER. In contrast, in VFe (hydr)oxides, the removal of\nvanadium resulted in a marked increase in ECSA and the generation\nof defect sites, significantly enhancing OER activity. These findings\nprovide critical insights into the surface-specific role of Fe ions\nin catalytic activity and could inform future design strategies for\nmore efficient OER catalysts by optimizing the availability and reactivity\nof surface-active Fe sites.