Insight into the activity and mechanism of FeNiB LDH electrocatalysts in alkaline OER via operando DRIFTS

Green H₂ shows significant promise as an energy carrier capable of replacing fossil fuels and meeting global energy demands. Understanding the oxygen evolution reaction (OER) mechanism is crucial for the development of efficient electrocatalysts for green H₂ production. In this study, we investigated the activity and stability of FeNiB layered double hydroxide (LDH) catalysts with varying Fe/Ni ratios (0.9/1, 1/1, 1/0.9) using steady-state polarisation, electrochemical impedance spectroscopy (EIS), electrochemical kinetic modelling, and surface characterisation techniques, including X-ray photoelectron spectroscopy (XPS) and operando diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS). Tafel slope and EIS analysis revealed that FeNiB 0.9/1 exhibited the best stability among the three catalysts. In the low Tafel region slopes of 34, 23, and 32 mV dec⁻¹ were obtained for FeNiB 0.9/1, 1/1, and 1/0.9, respectively, whereas 80, 102, and 100 m Vdec⁻¹ were obtained in the high Tafel region. Kinetic studies indicated that FeNiB 0.9/1 and 1/1 followed the first step of Bockris' pathway as the rate-determining step, whereas FeNiB 1/0.9 proceeded through the second step. XPS evidenced the superior performance of FeNiB 0.9/1 in the high overpotential region, attributed to its enhanced stability from the higher Ni content. Operando DRIFTS provided further mechanistic insights, showing that at lower Fe concentrations, OH⁻ bound to Ni plays a dominant role in OER, while Fe-bound OH⁻ governs the reaction in the Fe-rich catalysts.