Semiconductor/metal-complex Hybrid Systems for Solar CO2 Reduction Utilizing Water as an Electron Donor and a Proton Source

Recycling of CO₂ is regarded as a technology necessary for next generation beyond solar hydrogen production by water splitting reaction, because organic chemicals are considered to be more useful than hydrogen if they were synthesized from CO₂. In this presentation, I will introduce our research results on the solar CO₂ reduction in an aqueous solution conducted around ambient temperature and normal pressure.

We have proposed the novel concept of selective CO₂ photoreduction using a hybrid photocatalyst comprising of p-type N-doped Ta₂O₅ semiconductor photosensitizer linked with a Ru-complex electrocatalyst ([Ru(dcbpy)₂(CO)₂]²⁺), in which conduction band electrons in photoexcited N-Ta₂O₅ transferred to the Ru-complex within tens of picoseconds. Then we have proven that a photocahode of zinc-doped indium phosphide (InP) coated with a ruthenium complex polymer ([Ru{4,4’-di(1H-pyrrolyl-3-propylcarbonate)2,2’-bipyridine}(CO)₂]_n, RuCP) electrocatalyst can reduce CO₂ to formate with high selectively (>70%) under visible light irradiation in water. By wiring the InP/[RuCP] photocathode for CO₂ reduction with a TiO₂ or SrTiO_3-x photoanode for H₂O oxidation in two compartment cell separated with proton exchange membrane, solar formate production by the Z-scheme reaction utilizing only sunlight, CO₂ and H₂O was successfully realized at solar-to-chemical conversion efficiencies (SCE) of 0.04-0.14 % without an external electrical bias application. Development of new metal-complex catalysts was also conducted, and the semiconductor/metal-complex system has recently been integrated to a monolithic tablet-shaped device of [RuCP]/carbon/SiGe-jn/IrOx. By applying functions of selective reduction /oxidation reactions to both catalytic sites, the device operated in a single-compartment reactor in phosphate buffer at pH =6.4 (Fig.1), and it generated formate with a very high SCE of 4.6 %.

Fig. 1 Schematic illustration of a monolithic tablet-shaped device for CO₂ photoreduction in an aqueous solution.