Counter ion-regulated heterostructured Co@Fe-based core@shell materials: as remarkable bifunctional electrodes for green H₂ production

Understanding the effect of counter ions on Co and Fe-based electrocatalysts is crucial for optimizing electrolyzer electrodes for efficient green H₂ production. Hence, for the first time, we present the synthesis of nickel foam (NF) supported core@shell type Co(OH)₂@FeOOH, Co₃O₄@Fe₃O₄, and Co₂P@Fe₂P materials and their utilization as electrodes for electrochemical water splitting. Anion regulation in these materials can enhance their electronic properties, boosting their electrocatalytic performance. Notably, Co(OH)₂@FeOOH/NF exhibits a remarkably low oxygen evolution reaction (OER) overpotential of 237 mV at 50 mA cm⁻², significantly surpassing the “core” Co(OH)₂ (435 mV) and “shell” FeOOH (266 mV). The recorded OER overpotentials of Co₂P@Fe₂P/NF and Co₃O₄@Fe₃O₄/NF materials are 267 and 378 mV, respectively. Examining the hydrogen evolution reaction (HER) overpotentials, Co(OH)₂@FeOOH/NF, Co₃O₄@Fe₃O₄/NF, and Co₂P@Fe₂P/NF materials demonstrate 175, 255, and 95 mV at 10 mA cm⁻², respectively. Among the fabricated symmetrical alkaline electrolyzers, Co₂P@Fe₂P/NF‖Co₂P@Fe₂P/NF exhibits the highest water splitting performance with a low cell potential of only 1.53 V at 10 mA cm⁻², followed by Co(OH)₂@FeOOH/NF‖Co(OH)₂@FeOOH/NF (1.58 V), FeOOH/NF‖FeOOH/NF (1.65 V), and Co₃O₄@Fe₃O₄/NF‖Co₃O₄@Fe₃O₄/NF (1.7 V). Besides being highly active toward the OER, HER and overall water splitting (OWS), all the synthesized materials along with the fabricated electrolyzers show exceptional durability at a high current density of 50 mA cm⁻², proving their excellent bifunctionality toward the OER and HER.