Nickel(II) Complexes of Pentadentate Pyridylalkylamines as Bifunctional Electrocatalysts for Hydrogen Evolution and Water Oxidation

The development of catalysts based on nonnoble transition metals for hydrogen evolution reaction (HER) and water oxidation reaction (WOR) is conducive to the exploitation of sustainable green energy resources. Herein, two nickel(II) complexes of pentadentate pyridylalkylamines with the different alkyl chain lengths, namely, [Ni(tpmen)(CH 3 CN)](ClO 4 ) 2 (Ni‐tpmen, tpmen = N , N , N ′‐tris(2‐pyridylmethyl)‐ N ′‐methylethylenediamine) and [Ni(tpmpn)(CH 3 OH)](ClO 4 ) 2 (Ni‐tpmpn, tpmpn = N , N , N ′‐tris(2‐pyridylmethyl)‐ N ′‐methylpropane‐1,3‐diamine), were prepared and characterized. The electrochemical behaviors and the catalytic activities of two nickel(II) complexes for both HER and WOR were studied. The overpotentials of HER catalyzed by Ni‐tpmen and Ni‐tpmpn in CH 3 CN in the presence of AcOH were 587 and 500 mV, respectively, and the rate constants were 119.5 and 333.8 s −1 , respectively. Meanwhile, Ni‐tpmen and Ni‐tpmpn displayed a catalytic activity for WOR in pH 8.0 0.1 M phosphate buffer solution with rate constants of 0.375 and 0.17 s −1 , respectively, and the Faradaic efficiencies were 84% and 98%, respectively. Ni‐tpmpn with a longer alkyl chain exhibited a better catalytic activity for HER than Ni‐tpmen. In contrast, Ni‐tpmen showed a better catalytic activity for WOR in terms of the overpotential and catalytic rate. This work showed that pyridylalkylamine metal complexes were promising candidates for bifunctional catalysts and that their catalytic performances were affected by the length of the alkyl chains in pyridylalkylamines.