Photoelectrochemical CO reduction activity of a hybrid photocathode, based on a Ru(II)-Re(I) supramolecular metal complex photocatalyst immobilized on a NiO electrode (NiO-RuRe) was confirmed in an aqueous electrolyte solution. Under half-reaction conditions, the NiO-RuRe photocathode generated CO with high selectivity, and its turnover number for CO formation reached 32 based on the amount of immobilized RuRe. A photoelectrochemical cell comprising a NiO-RuRe photocathode and a CoO/TaON photoanode showed activity for visible-light-driven CO reduction using water as a reductant to generate CO and O, with the assistance of an external electrical (0.3 V) and chemical bias (0.10 V) produced by a pH difference. This is the first example of a molecular and semiconductor photocatalyst hybrid-constructed photoelectrochemical cell for visibl-light-driven CO reduction using water as a reductant.

Three types of photocatalytic systems for CO2 reduction, which were recently developed in our group, are reviewed. First, two-component systems containing different rhenium(I) complexes having different roles; i.e., redox photosensitizer and catalyst in the reaction solution are described. The mixed system of a ring-shaped rhenium(I) trinuclear complex and fac-[Re(bpy)(CO)3(MeCN)](+) is currently the most efficient photocatalytic system for CO2 reduction (ΦCO = 0.82 at λex = 436 nm). The second is a series of supramolecular photocatalysts, which have units with different functions in one molecule, i.e., redox photosensitizer, catalyst, and bridging ligand. The highest durability and speed of photocatalysis were achieved by using this system (ΦCO = 0.45, TONCO = 3029, and TOFCO = 35.7 min(-1)). The third is a novel type of artificial Z-Scheme photocatalyst for CO2 reduction, of which photocatalysis is revealed by stepwise excitation of both a semiconductor photocatalyst unit and the supramolecular photocatalyst unit. This system has both strong oxidation and reduction powers.

A new Ru(ii)–Re(i)/CuGaO2 hybrid photocathode was developed and combined with a CoOx/TaON photoanode to drive non-biased visible-light-driven CO2 reduction with water oxidation.

A photocathode for CO2 reduction was successfully developed using a hybrid electrode comprising a Ru(II)-Re(I) supramolecular photocatalyst and a NiO electrode. Selective photoexcitation of the Ru photosensitizer unit of the photocatalyst at -1.2 V vs. Ag/AgNO3 selectively afforded CO with high faradaic efficiency.