Solar-Driven CO Reduction Using a Semiconductor/Molecule Hybrid Photosystem: From Photocatalysts to a Monolithic Artificial Leaf.

The synthesis of organic chemicals from HO and CO using solar energy is important for recycling CO through cyclical use of chemical ingredients produced from CO or molecular energy carriers based on CO. Similar to photosynthesis in plants, the CO molecules are reduced by electrons and protons, which are extracted from HO molecules, to produce O. This reaction is uphill; therefore, the solar energy is stored as the chemical bonding energy in the organic molecules. This artificial photosynthetic technology mimicking green vegetation should be implemented as a self-standing system for on-site direct solar energy storage that supports CO recycling in a circular economy. Herein, we explain our interdisciplinary fusion methodology to develop hybrid photocatalysts and photoelectrodes for an artificial photosynthetic system for the CO reduction reaction (CORR) in aqueous solutions. The key factor for the system is the integration of uniquely different functions of molecular transition-metal complexes and solid semiconductors. A metal complex catalyst and a semiconductor appropriate for a CORR and visible-light absorption, respectively, are linked, and they function complementary way to catalyze CORR under visible-light irradiation as a particulate photocatalyst dispersion in solution. It has also been proven that Ru complexes with bipyridine ligands can catalyze a CORR as photocathodes when they are linked with various semiconductor surfaces, such as those of doped tantalum oxides, doped iron oxides, indium phosphides, copper-based sulfides, selenides, silicon, and others. These photocathodes can produce formate and carbon monoxide using electrons and protons extracted from water through potential-matched connections with photoanodes such as TiO or SrTiO for oxygen evolution reactions (OERs). Benefiting from the very low overpotential of an aqueous CORR at metal complexes approaching the theoretical lower limit, the semiconductor/molecule hybrid system demonstrates a single tablet-formed monolithic electrode called "artificial leaf." This single electrode device can generate formate (HCOO) from HO and CO in a water-filled single-compartment reactor without requiring a separation membrane under unassisted or bias-free conditions, either electrically or chemically. The reaction proceeds with a stoichiometric electron/hole ratio and stores solar energy with a solar-to-chemical energy conversion efficiency of 4.6%, which exceeds that of plants. In this Account, the key results that marked our milestones in technological progress of the semiconductor/molecule hybrid photosystem are concisely explained. These results include design, proof of the principle, and understanding of the phenomena by time-resolved spectroscopies, synchrotron radiation analyses, and DFT calculations. These results enable us to address challenges toward further scientific progress and the social implementation, including the use of earth-abundant elements and the scale-up of the solar-driven CORR system.