In this contribution, we report on strategies for high yield syngas production from solar CO₂ recycling. To make CO₂ re-use available at industry level, versatile and cost-effective processes at large-scale are in dire need. Therefore, we focused our study on efficient processes, which are scalable to large areas and compatible with state-of-the-art photovoltaics and electrocatalysts. Efficient processes include low overpotentials for the CO₂ reduction reaction (CO₂RR) and the oxygen evolution reaction (OER), respectively, along with excellent selectivity for the desired products (in particular for the case of CO₂RR). We demonstrate that the coating of three-dimensional metallic foam electrodes with adapted nanosized catalysts resulted in high yield CO₂ conversion to syngas. Furthermore, we provide evidence that photovoltaic structures based on cheap and mature silicon technology can be adapted to provide the required photovoltage to break the CO₂ molecule into the targeted product.

In particular, we have investigated the application of silicon heterojunction photovoltaic cells as photoanodes, which requires meeting challenges, such as increasing the photovoltage without impairing the photovoltaic efficiency; protection of the solar cell by robust coatings to increase the stability in aqueous electrolytes; and the decoration with catalysts ensuring an efficient OER. We show that photovoltages up to 2.5 V with photocurrent densities up to 7.5 mA/cm² can be reached by connecting four HIT cells in series. Furthermore, for the rear contact of the HIT cells we explored the applicability of metallic foams (e.g. Ni and Cu) loaded with metallic particles as OER catalyst. We demonstrate OER overpotentials below 300 mV (for 10 mA/cm²).

The CO₂RR was performed by using large-scale metallic foam electrodes as highly conductive catalyst scaffolds. In this context, we developed a deposition process, which enables tunable coating of Cu and Ni foams, respectively, with highly active nanosized metal catalysts, such as Zn or Ag, for selective syngas production. The performance of the as-produced electrodes has been evaluated in terms of product selectivity, Faradaic efficiency, overpotentials, and stability. The developed electrodes exhibit overpotentials below 400 mV for CO₂RR. Furthermore, stable and tunable H₂:CO ratios between 5 and 1 along with high CO Faradaic efficiencies of up to 96% and CO current densities of 40 mA/cm² were measured. Finally, we demonstrate a bias-free operation of the complete  device (2-electrode configuration) providing a photocurrent density of 5.0 mA/cm² measured under 100 mW/cm² illumination. This corresponds to a solar-to-syngas conversion efficiency of 4.3%.