A prototype reactor for highly selective solar-driven CO reduction to synthesis gas using nanosized earth-abundant catalysts and silicon photovoltaics

The conversion of carbon dioxide (CO 2 ) into value-added chemicals and fuels, preferably using renewable energy and earth-abundant materials, is considered a key priority for future energy research. In this work, a bias-free reactor device for the solar-driven conversion of CO 2 to synthesis gas (syngas) has been developed. The integrated fluidic device consists of a cathode made of copper foam coated with low-cost nanosized zinc flakes as catalyst to perform the CO 2 reduction reaction (CO 2 RR) to syngas, an adapted silicon heterojunction solar cell structure as photoanode with nickel foam as catalyst to facilitate the oxygen evolution reaction (OER), and a bipolar membrane separating the respective catholyte and anolyte compartments. The membrane allows for the operation of the catholyte and anolyte at different pH values. Stable and tunable hydrogen-to-carbon monoxide (H 2 : CO) ratios between 5 and 0.5 along with high CO Faradaic efficiencies of up to 85% and CO current densities of 39.4 mA cm −2 have been demonstrated. Under photoelectrolysis conditions, the photovoltage of the photoanode was varied between 0.6 V and 2.4 V by connecting up to four heterojunction solar cells in series, and thus reducing the overall cell voltage solely by solar energy utilization. Bias-free operation of the integrated device has been achieved under ambient conditions with active areas for CO 2 RR and OER, respectively, of 10 cm 2 . An operation current density of 5.0 mA cm −2 was measured under 100 mW cm −2 illumination of the complete device, which corresponds to a solar-to-syngas conversion efficiency of 4.3%. Bias-free syngas production from solar CO 2 conversion employing a Cu foam cathode coated with Zn nanoflakes as catalyst and integrated in a scalable solar flow-cell reactor.