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 (ηSTC). 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 ηSTC 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 O2 bubbles generated on the anodes from approaching the cathodes.