Electrocatalytic Oxygen Evolution by Pyridyl-Thiosemicarbazone-Based Ni(II) and Cu(II) Complexes: Synthesis, Characterizations, and Crystal Structures

In order to lower the reaction overpotential and energy input during the water electrolysis process, it is crucial to design and develop novel, low-cost, and effcient electrocatalysts for the oxygen evolution reaction (OER) in an alkaline environment. Here, we report the synthesis of 2-formylpyridine-derived ligand, namely, 1( E )- N -(4-nitrophenyl)–2-(pyridin-2-ylmethylene)hydrazine-1-carbothioamide ( HNPhPyHCT ) and its Ni(II) ( [Ni(NPhPyHCT) 2 ] ) and Cu(II) ( [Cu(NPhPyHCT) 2 ] ) complexes. The synthesized compounds were thoroughly characterized by elemental analysis, spectroscopic methods, single-crystal X-ray diffraction, and cyclic voltammetry. Further, the electrochemical characterization of Ni(II) and Cu(II) confirmed distinct redox behaviors in aqueous solution for both complexes, with surface-confined processes dominating over diffusion-controlled processes, which is advantageous for catalytic applications. Among the two complexes, [Ni(NPhPyHCT) 2 ] exhibited superior OER activity, achieving a low overpotential of 360 mV at a current density of 10 mA cm –2, whereas [Cu(NPhPyHCT) 2 ] showed low OER activity. When benchmarked against commercial RuO 2, [Ni(NPhPyHCT) 2 ] demonstrated better catalytic performance, with a lower Tafel slope value (41 mV dec –1 ) compared to RuO 2 (114 mV dec –1 ) and a reduced overpotential (360 and 390 mV, respectively), indicating more favorable reaction kinetics.