(Invited) Photocatalytic and Electrochemical CO2 Reduction By Semiconductors, Metal Complexes, and the Combinations
Authors: Arai Takeo , Morikawa Takeshi 
Published: December 2024

The establishment of so-called artificial photosynthetic reactions that use solar energy to drive the synthesis of organic matter using CO2 and H2O as raw materials is significant for reducing CO2 emissions and realizing carriers of abundant solar energy in an environmentally friendly way. Our primary approach to artificial photosynthetic reactions is the combinatorial technologies that effectively utilize the excellent properties of solid semiconductor photocatalysts and molecular metal complex catalysts. 1

We have realized the low overpotential CO2 reduction reaction (CO2RR) combined with the H2O oxidation reaction (WOR), a pair reaction for artificial photosynthesis, in a single aqueous solution at near neutral pH (Figure 1). Simultaneous operation of the CO2RR and the WOR in a single solution is one of the concepts necessary to construct a simplified form of an artificial photosynthetic system that does not require a setup for separation of sites like the thylakoid membrane existing in natural photosynthetic process for CO2RR and WOR. We will explain a photocatalytic system functioning by a two-step photoexcitation (Z-scheme) mechanism in a self-organized manner using a mixture of particulate (CuGa)0.3Zn1.4S2, BiVO4, and Co complex ([Co(4,4’-dimethyl-2,2’-bipyridine)3]2+, ([Co-dmbpy])) having bipyridine ligands. Under visible light irradiation (λ > 420 nm), a photocatalytic reaction produces O2, and yields CO with a product selectivity of 98 % by suppressing competitive H2 generation in an aqueous NaHCO3 solution bubbled with gaseous CO2. 2
3 Experimental studies and DFT calculations suggest that the Co complex acts dual-functionally in synergy with (CuGa)0.3Zn1.4S2 and BiVO4: it behaves as an efficient ionic electron mediator, and also acts as a highly selective CO2RR cocatalyst after a structural change following the acceptance of photoexcited electrons from (CuGa)0.3Zn1.4S2.
We have also constructed a PV-powered electrolyzer system by using catalysts and a photo absorber without precious metal elements toward the future realization of a low-cost and low-lifecycle CO2 emission system. Mn(I) complex polymer cathode4 and β-FeOOH:Ni hyperfine nanorod anode5, 6 were set in a gas diffusion flow reactor. The system demonstrated electrochemical CO2 to CO conversion with 94% selectivity at a very low cell voltage of 1.35 V, originating from a very low overpotential of CO2 reduction approaching the theoretical lower limit at the Mn(I) polymer catalyst. 7 A direct connection of the cathode and anode with a conventional Si solar cell achieved a solar-to-CO conversion efficiency of > 20% using an aqueous alkaline media.
The stability and versatility of molecular catalysts is always a topic of debate. Long-term stability, high energy efficiency, and synthesis of high carbon chemicals by molecular metal complex catalysts will also be presented. In a gas diffusion electrode (GDE)-based gas flow system, a Co-tetrapyridino-porphyrazine complex supported on carbon black together with K salt promoted electrocatalytic CO2 reduction with a current density of 100 mA/cm2 and generated CO over at least one week with a selectivity of approximately 95%. The optimal catalyst gave a turnover number of 3,800,000 and an energy conversion efficiency of more than 62% at 200 mA/cm2. 8 A Br-bridged dinuclear Cu(I) complex electrochemically yielded high-value-added C3 products, C3H7OH, with high robustness during the reaction. 9

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Figure 1