A large-sized cell for solar-driven CO2 conversion with a solar-to-formate conversion efficiency of 7.2

• Published in: Joule Vol. 5; no. 3; pp. 687 - 705
• Main Authors: Kato, Naohiko, Mizuno, Shintaro, Shiozawa, Masahito, Nojiri, Natsumi, Kawai, Yasuaki, Fukumoto, Kazuhiro, Morikawa, Takeshi, Takeda, Yasuhiko
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 17.03.2021
• Subjects: electrochemical cells; formate; large-scale; photovoltaic; solar fuels; solar-driven CO2 reduction; electrochemical cells; solar-driven CO2 reduction; formate; solar fuels; large-scale; photovoltaic
• ISSN: 2542-4351; 2542-4351
• DOI: 10.1016/j.joule.2021.01.002

Solar-driven electrochemical (EC) reduction of CO2 to fuel using photovoltaic (PV) cells is a promising CO2 recycling technology. However, the scale-up of EC reactors lowers the solar-to-chemical conversion efficiency (ηSTC) due to the large electric resistance of the electrode catalysts and an insufficient supply of reactants. We designed a large-sized cell to convert CO2 to formate. It consists of five stacked electrodes (electrically parallel connected) and six series-connected single-crystalline Si PV cells (area ∼1,000 cm2). Low-resistivity anodes loaded with IrOx and cathodes loaded with Ru complex polymer on carbon support operate without a membrane between them at a low voltage of 1.85 V. They generate a reaction current of 6.30 A, owing to the homogeneous flow of the CO2-dissolved electrolyte to achieve a sufficient CO2 supply and a good match between the PV and EC performances. The cell yields a conversion efficiency of 7.2% and the high production rate of 93.5 mmol/h. [Display omitted] •Large cells integrating PV and EC operate without membrane and DC-DC converter•ηSTC of 7.2% (∼1,000 cm2 irradiation area) for solar-driven reduction of CO2 to formate•Low operating voltage Vop due to Ru complex polymer catalyst with a low-resistive substrate•Stacked EC electrode catalysts owing to a good match between the PV and EC Solar-driven electrochemical (EC) reduction of CO2 to fuel using photovoltaic (PV) cells is a promising CO2 recycling technology. However, the scale-up of EC reactors lowers the solar-to-chemical conversion efficiency (ηSTC) due to the large electric resistance of the electrode catalysts and the insufficient supply of reactants. In this study, we demonstrated a high-level balance between a high formate production ηSTC of 7.2% and a large cell integrating PV and EC (∼1,000 cm2 irradiation area) without a membrane and direct current to to direct current (DC-DC) converter, resulting in a large formate production rate of 93.5 mmol/h. The high ηSTC is due to the very low Vop = 1.85 V at 6.30 A. The low Vop was realized using the Ru complex polymer cathode catalyst, low-resistive electrode substrates, sufficient CO2 supply, and a good match between the PV cells and EC reactor. Large-sized solar-driven CO2 reduction cells with a high ηSTC that are demonstrated here open the door to solar-driven CO2 recycling plants. We demonstrated a high-level balance between a high formate production ηSTC of 7.2% and a large cell size (∼1,000 cm2 irradiation area), resulting in a large formate production rate of 93.5 mmol/h. Large-sized solar-driven CO2 reduction cells with a high ηSTC that are demonstrated here open the door to solar-driven CO2 recycling plants.