Substantial research effort has been dedicated to the electrochemical reduction of CO₂ (CO₂R) to higher carbon products throughout the recent years, baring the promise of a production pathway for green fuels and chemicals.¹ However, only little progress has been achieved in the light driven heterogeneous CO₂R catalysis, especially considering selective processes towards C₂₊ products. This arises from the additional complexity of photo-electrochemical reactions, which means that not only the sluggish reaction kinetics, high overpotentials and low selectivity of CO₂R, but also the insufficient voltage and sensitivity towards harsh electrolyte conditions of photoabsorbers have to be encountered. Considering CO₂R as multi-step process with CO as intermediate mitigates some issues of the process, e.g. the necessary overpotential is reduced and a higher selectivity towards valuable products can be achieved.²

Multi-junction solar stacks can provide operating voltages >2 V, which is sufficient for reducing CO₂ to CO with high efficiencies or even produce multi-carbon products from CO while oxidizing water as anode reaction.³

In this work, we designed a process for photo-electrochemical CO reduction with multi-junction photoabsorbers. We start out by showing photo-electrochemical modelling of tandem photoabsorbers that emphasizes the advantages of CO as reactant compared to CO₂. Further, we focus on the preparation of a nano-structured Cu catalyst, the most common material for reducing CO to C₂₊. Therefore, the electrochemical deposition and surface characterization using SEM, EDX and XPS of the catalyst on a dark model electrode coated with a sputter deposited TiO₂ protection layer will be presented. Moreover, the CO reduction performance of the model at different potentials are characterized. In addition, the light transmission of the model electrode is reported, baring the possibility of illuminating the photoelectrode from the catalyst front side in mind. Lastly, the transfer of the model catalyst to a photoabsorber as well as the design of a photo-electrochemical flow-cell are discussed.

1. Nitopi, S. et al. Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte. Rev. 119, 7610–7672 (2019)1.
2. Wang, L. et al. Electrochemical Carbon Monoxide Reduction on Polycrystalline Copper: Effects of Potential, Pressure, and pH on Selectivity toward Multicarbon and Oxygenated Products. ACS Catal. 8, 7445–7454 (2018)
3. Seger, B., Hansen, O. & Vesborg, P. C. K. A Flexible Web-Based Approach to Modeling Tandem Photocatalytic Devices. RRL 1, e201600013 (2017).