Photocatalytic and electrochemical CO2 reduction by semiconductors, metal complexes, and their hybrid systems

Takeshi Morikawa^a

^aToyota Central R&D Labs., Inc., 41-1 Yokomichi, Nagakute, Aichi, Japan

Proceedings of MATSUS Fall 2026 Conference (MATSUSFall26)

E1 Solar-driven systems for renewable fuels and chemical generation; Towards viable Solar fuels technology

Palma, Spain, 2026 October 26th - 30th

Organizers: Sudhanshu Shukla and Francesca Toma

Invited Speaker, Takeshi Morikawa, presentation 378
Publication date: 22nd July 2026

The development of artificial photosynthetic reactions that harness solar energy to drive the synthesis of organic compounds using CO₂ and H₂O as feedstocks holds significant potential for mitigating CO₂ emissions and enabling the environmentally sustainable conversion of abundant solar energy into chemical energy carriers. Our primary strategy for achieving artificial photosynthesis involves the integration of combinatorial technologies that leverage the advantageous properties of solid semiconductor photocatalysts and molecular metal complex catalysts. ¹ ²

We have achieved a low-overpotential CO₂ reduction reaction (CO₂RR) coupled with the H₂O oxidation reaction (WOR), a key reaction pair for artificial photosynthesis, within a single aqueous solution at near-neutral pH. The simultaneous operation of CO₂RR and WOR in a unified solution represents a crucial concept for the development of a simplified artificial photosynthetic system, operating via a two-step photoexcitation (Z-scheme) mechanism in a self-organized manner. The system utilizes an aqueous suspension of particulate (CuGa)_0.3Zn_1.4S₂(CGZS), BiVO₄, and a water-soluble cobalt complex ([Co(4,4’-dimethyl-2,2’-bipyridine)₃]²⁺, [Co-dmbpy]). Upon irradiation with visible light (λ > 420 nm), the photocatalytic reaction facilitates O₂ evolution while achieving a CO production selectivity of 62-98% by effectively suppressing competitive H₂ generation. This reaction occurs in an aqueous NaHCO₃ solution bubbled with gaseous CO₂, demonstrating efficient and selective photocatalytic CO₂ reduction.³^,⁴ Experimental investigations combined with density functional theory (DFT) calculations indicate that the cobalt complex exhibits dual functionality in synergy with CGZS and BiVO₄. Specifically, it serves as an efficient ionic electron mediator while also functioning as a highly selective cocatalyst for CO₂RR on CGZS. The system continuously generates CO and H₂ accompanied by O₂ evolution, with an electron/hole ratio close to the stoichiometric value of unity.

In the meeting, I will explain a compact photovoltaic-cell (PV)-powered electrolyzer system using catalysts composed of earth-abundant elements, specifically a Mn(I) complex polymer and β-FeOOH, together with a Si photoabsorber, as well as a large-scale PV-powered electrolyzer system employing 1 m² electrodes for the highly selective conversion of CO2 to CO or formate ions.^5-8 I will also discuss the long-term stability, high energy efficiency, and potential applicability of molecular metal complex catalysts for CO2 electrolysis and the synthesis of higher-carbon chemicals. ^9,10

References:

[1] Morikawa, T., Sato, S., Sekizawa, K., Suzuki, T. M. & Arai, T. Solar-Driven CO2 Reduction Using a Semiconductor/Molecule Hybrid Photosystem: From Photocatalysts to a Monolithic Artificial Leaf. Accounts of Chemical Research 55, 933-943 (2022).

[2] Morikawa, T., Suzuki, T. M., Yamaguchi, Y. & Kudo, A. Photocatalytic CO2 reduction with a semiconductor/metal complex hybrid system: Toward visible light and water utilization. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 66 (2026).

[3] Suzuki, T. M. et al. Photocatalytic CO2 reduction by a Z-scheme mechanism in an aqueous suspension of particulate (CuGa)0.3Zn1.4S2, BiVO4 and a Co complex operating dual-functionally as an electron mediator and as a cocatalyst. Applied Catalysis B: Environmental 316, 121600 (2022).

[4] Suzuki, T. M. et al. Highly selective CO2 electrolysis in aqueous media by a water-soluble cobalt dimethyl-bipyridine complex. Chemical Communications 59, 12318-12321 (2023).

[5] Sato, S., Saita, K., Sekizawa, K., Maeda, S. & Morikawa, T. Low-Energy Electrocatalytic CO2 Reduction in Water over Mn-Complex Catalyst Electrode Aided by a Nanocarbon Support and K+ Cations. ACS Catalysis 8, 4452-4458 (2018).

[6] Suzuki, T. M. et al. Highly enhanced electrochemical water oxidation reaction over hyperfine β-FeOOH(Cl):Ni nanorod electrode by modification with amorphous Ni(OH)2. Bulletin of the Chemical Society of Japan 91, 778–786 (2018).

[7] Arai, T., Sato, S., Sekizawa, K., Suzuki, T. M. & Morikawa, T. Solar-Driven CO2 to CO Reduction Utilizing H2O as an Electron Donor by Earth-Abundant Mn–Bipyridine Complex and Ni-Modified Fe-Oxyhydroxide Catalysts Activated in a Single-Compartment Reactor. Chemical Communications 55, 237-240 (2019)

[8] Sekizawa, K., Sato, S., Sakamoto, N., Suzuki, T. M. & Morikawa, T. 20% Efficient Solar CO2 Electrolysis with Earth-Abundant Materials Enabled by Molecular Catalyst. ChemRxiv (2024)

[9] Sato, S., Sekizawa, K., Shirai, S., Sakamoto, N. & Morikawa, T. Enhanced performance of molecular electrocatalysts for CO2 reduction in a flow cell following K+ addition. Science Advances 9, eadh9986 (2023)

[10] Sakamoto, N. et al. Dinuclear Cu(I) molecular electrocatalyst for CO2-to-C3 product conversion. Nature Catalysis 7, 574-584 (2024).

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