Tuning the ligand periphery in homoleptic Co(iii) complexes: a versatile strategy to modulate electrocatalytic oxygen evolution reaction efficiency

Abstract

In the relentless pursuit of sustainable energy solutions, the development of efficient, cost-effective, Earth-abundant molecular electrocatalysts is crucial. In this study, we report the design and synthesis of four new mononuclear Co(III) complexes (C1, C2, C3 and C4) obtained through the reaction of Co(CH₃COO)₂·4H₂O with Schiff bases (L1H₂, L2H₂, L3H₂, and L4H₂) via aerial oxidation. Single-crystal X-ray diffraction (SCXRD) analysis confirms the formation of homoleptic mononuclear anionic Co(III) complexes (L₂Co)⁻, with cationic triethylammonium as the counterion. The synthesized cobalt complexes demonstrate significant potential for catalyzing the oxygen evolution reaction (OER) in a 1 M KOH solution. All four complexes demonstrated promising electrocatalytic activity; however, complex “C4”, comprising 2-amino-4-nitrophenol coupled with a 3-ethoxysalicylaldehyde ligand, exhibited superior performance. Notably, C4 achieved a low overpotential of 340 mV at a current density of 10 mA cm⁻², along with a lower Tafel slope of 63 mV dec⁻¹, signifying rapid reaction kinetics. The outstanding catalytic efficiency of C4 is attributed to a synergistic electronic effect resulting from the coexistence of electron-donating (EDGs) and electron-withdrawing groups (EWGs) in the ligand framework with the Co(III) ion, which enhances the charge transfer efficiency and stabilizes the active species during the catalytic cycle. Post-operational studies, including X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), were performed to evaluate the structural and morphological stability of complex C4 after prolonged OER operation. Furthermore, this work could offer new insights into how electronic modifications on the ligand framework influence the catalytic efficiency of homoleptic Co(III) complexes in the OER.