Tuning Surface State in CoFe (Oxy)Hydroxide for Improved Oxygen Evolution Electrocatalysis

CoFe-based catalysts have shown excellent activity for the oxygen evolution reaction (OER), with the oxidation states of the active sites playing a crucial role in determining catalytic performance. However, how to effectively increase the oxidation state of these active sites remains a key challenge. In this work, a facile treatment with NaBH4 solution was employed to modulate the surface state of CoFeOxHy catalysts, inducing an enhanced OER activity. The overpotential at 10 mA cm−2 for the NaBH4-treated CoFe catalyst was reduced to 270 mV, indicating improved OER activity. X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) results reveal that NaBH4 treatment induced a phase reconstruction of the CoFe oxalate framework, a critical step in enhancing its catalytic properties. The strong reducing ability of NaBH4 strengthened the Co-Fe interaction, allowing the retention of low-valence Co species while facilitating the formation of high-valence Fe sites. This dual modulation of Co and Fe oxidation states significantly accelerated charge transfer kinetics, ultimately boosting OER performance. These findings highlight the importance of improving the oxidation states of active sites in CoFe-based catalysts, providing insights for developing efficient catalysts for electrochemical water splitting.