Two water soluble copper(ii) complexes containing amine-pyridine ligands were found to be catalytically active for water oxidation reactions in basic solutions, with observed rate constants of 13.1-18.7 s. These complexes are robust in basic solutions and displayed good stability over 5 h of electrolysis in pH 11.5 phosphate buffer at 1.4 V vs. NHE.

Herein, we review the recent advances and challenges in the field of visible-light-absorbing semiconductor/molecular catalyst photoelectrodes for H2 and O2 evolution.

Two copper complexes, [(L1)Cu(OH2)](BF4)2 [1; L1=N,N′‐dimethyl‐N,N′‐bis(pyridin‐2‐ylmethyl)‐1,2‐diaminoethane] and [(L2)Cu(OH2)](BF4)2 [2, L2=2,7‐bis(2‐pyridyl)‐3,6‐diaza‐2,6‐octadiene], were prepared as molecular water oxidation catalysts. Complex 1 displayed an overpotential (η) of 1.07 V at 1 mA cm−2 and an observed rate constant (kobs) of 13.5 s−1 at η 1.0 V in pH 9.0 phosphate buffer solution, whereas 2 exhibited a significantly smaller η (0.70 V) to reach 1 mA cm−2 and a higher kobs (50.4 s−1) than 1 under identical test conditions. Additionally, 2 displayed better stability than 1 in controlled potential electrolysis experiments with a faradaic efficiency of 94 % for O2 evolution at 1.58 V, when a casing tube was used for the Pt cathode. A possible mechanism for 1‐ and 2‐catalyzed O2 evolution reactions is discussed based on the experimental evidence. These comparative results indicate that fine‐tuning the structures of tetradentate N4 ligands can bring about significant change in the performance of copper complexes for electrochemical water oxidation.