Solar Fuel Production from CO2 Using a 1 m-Square-Sized Reactor with a Solar-to-Formate Conversion Efficiency of 10.5%

Solar-driven electrochemical (EC) reduction of CO2 combined with photovoltaic (PV) cells is a promising means for carbon neutrality. However, scale-up of the EC reactors for practical realization often lowers the solar-to-chemical energy conversion efficiency (eta(STC)). Here, we constructed an EC reactor for CO2 reduction to formate as large as 1 m-square in size powered by a single-crystalline silicon PV module and achieved a high eta(STC) of 10.5% with a formate production rate as high as 1167 mmol/h. We used Ru-complex polymer for the cathode catalyst and IrOx nanocolloids for the anode catalyst. The advantageous feature of this combination of a low threshold voltage for the formate production was fully exploited using low-resistive Ti plates for the anode/cathode substrates, resulting in a large operating current of 65 A at a low operating voltage of around 1.65 V (overpotential of 0.22 V). A well-suppressed crossover reaction that is another feature of these catalysts enabled the use of a simple reactor configuration of a single-compartment type suitable for scale-up, with the help of nanoporous separators made of hydrophilic polyethylene that blocked O-2 bubbles generated on the anodes from approaching the cathodes.

Automated summary

In this study, a large-scale EC reactor for CO2 reduction to formate powered by a single-crystalline silicon PV module was constructed, achieving a high solar-to-chemical energy conversion efficiency of 10.5%. The use of efficient catalysts and low-resistive materials for the reactor components resulted in a high operating current of 65 A at a low voltage of around 1.65 V. By effectively suppressing crossover reactions and utilizing nanoporous separators, a simple and scalable reactor configuration suitable for large-scale CO2 reduction was achieved.