Harnessing the Catalytic Competency of Nonprecious Co–Fe Oxalate Microstructure and Nonstoichiometric Co_0.85Se Toward Alkaline Overall Water Splitting

Designing efficient cobalt-based electrocatalysts for the alkaline oxygen evolution reaction (OER) is a tedious task owing to limited per-site activity. However, the interfacial modulation by the insertion of low-cost iron considerably stimulates both geometric and intrinsic activity of cobalt centers. Here, we have prepared a series of bimetallic cobalt-iron oxalate composites (Co_<i>x</i>Fe_1-<i>x</i>C₂O₄, where <i>x</i> = 1 to 0) via a simple coprecipitation method. The thorough physical studies confirm the development of efficient interfacial junctions between Co and Fe oxalate microstructures. Among the as-prepared Co_<i>x</i>Fe_1-<i>x</i>C₂O₄ (where <i>x</i> = 1 to 0) composites, the Co_0.6Fe_0.4C₂O₄ exhibits the best catalytic proficiency, requiring only 292 ± 4 and 361 ± 5 mV overpotentials to achieve the benchmark 10 and 100 mA cm^-2_geo current density on a carbon paper substrate. The corresponding overpotentials are further reduced to 245 ± 5 and 288 ± 5 mV on nickel foam (NF) substrate. The swift reaction kinetics (Tafel slope ≈ 32 mV dec^-1), low charge-transfer resistance, and enduring 48 h stability make the Co_0.6Fe_0.4C₂O₄ well-suited for alkaline OER. In addition, the Fe incorporation into Co centers induces strong synergistic interactions, improves surface porosity, and enhances the turnover frequency (TOF ≈ 0.95 s^-1). Further, the Co-Fe bimetallic system showcases the notably low activation energy, indicating the excellent catalytic competency of the Co_0.6Fe_0.4C₂O₄ composite at a high operating voltage and current density. The isotope labeling study uncovered the occurrence of the adsorbate evolution mechanism (AEM) within the Co_0.6Fe_0.4C₂O₄ composite during alkaline OER catalysis. Meanwhile, to demonstrate the overall water splitting, the cobalt oxalate-assisted Co₃O₄ was hydrothermally transformed into Co_0.85Se, showcasing 141 mV @ -10 mA cm^-2 towards alkaline hydrogen evolution reaction (HER). Finally, the Co_0.6Fe_0.4C₂O₄ (+) || Co_0.85Se (-) electrolyzer was constructed, delivering 1.68 V cell voltage to accomplish 10 mA cm^-2 current density on carbon paper substrate and 36 h prolonged stability with a negligible performance drop.