Electrocatalytic and Photocatalytic Hydrogen Evolution by Ni(II) and Cu(II) Schiff Base Complexes

Abstract

Homogeneous catalytic hydrogen evolution reaction (HER) by five-coordinate Schiff base complexes with first-row transition metals (Ni(II), Cu(II) and Zn(II)) were firstly investigated in organic medium. Both nickel and copper complexes revealed efficient electrocatalytic and photocatalytic HER activities. Cyclic voltammetry experiments and foot of wave analysis (FOWA) were applied to assess the electrocatalytic performance and to probe the catalytic mechanism. The Cu(II) and Ni(II) catalysts achieved maximum electrocatalytic HER turnover frequencies (TOFmax) of 70139.5 and 63827.5 s−1, respectively, in MeCN using acetic acid as a proton source. Photocatalytic HER investigation suggested a superior activity of the Cu(II) complex to the Ni(II) counterpart. In an aqueous three-component system consisting of organic Eosin Y (EY2−) photosensitizer, trimethylamine (TEA) sacrificial donor, and the Cu(II) catalyst, a photocatalytic HER turnover number (TON) of 21 was obtained after 4 h irradiation under white LED light. Employing triethanolamine (TEOA) instead of TEA as a sacrificial donor substantially enhanced stability of the photocatalytic HER system, of which deactivation was not observed after 6 h photocatalysis. The photocatalytic HER reaction was monitored by time-resolved UV-vis spectra which suggested faster decomposition of EY2− in the presence of TEA than TEOA.