In situ transformed binuclear Co(II) and Ni(II) complexes as electrocatalysts for the water oxidation reaction

The metal (Co and Ni) coordinated three cationic complexes formed from a transformed ligand, namely N,N-(amino(methoxy)(pyrimidin-2-yl)methyl)pyrimidine-2-carboxamide, which was produced from 2-cyanopyrimidine and hydroxyl-amine hydrochloride under in situ conditions and 3d metal mediation, are the first reported dimeric water oxidation catalysts (WOCs). From the single crystal X-ray data it was inferred that, both the cobalt complex (1) and nickel complex (3) are homologous dinuclear cationic complexes with octahedral geometries with two chloride counter anions, but the cobalt complex (2) has a similar dimeric cationic complex structure to its monomeric tetrahedral [CoCl₄]^2- anionic counterpart for balancing the overall charge of complex 2. Electrochemical studies were conducted for the three complexes, and comparisons were made. However, the Ni-analogue (3) exhibited a specific activity of 104.4 mA cm^-2 at 1.60 V (vs. RHE), which was much greater than that of the Co-analogues, 2 (54.79 mA cm^-2) and 1 (37.95 mA cm^-2). Meanwhile, the mass activity of 3 at 1.6 V (vs. RHE) was 74.73 mA cm^-2, which was higher than those of 2 (32.7 mA cm^-2) and 1 (17.81 mA cm^-2). It was also established that the Ni analogue (3) exhibits superior oxygen evolution reaction (OER) activity compared to its Co analogues, with a lower overpotential, smaller charge-transfer resistance, higher electrochemically active surface area, higher turnover frequency, and superior faradaic efficiency.