The need of a sustainable society to advance towards a circular economy, in which there is a balance between the emission and capture of anthropogenic gases like carbon dioxide (CO₂), is of utmost scientific and technological importance to ensure the increase or rather preservation of the current prosperity for future generations. The growing global population and increase in worldwide prosperity is tightly connected to a rise in power consumption. Traditionally, this energy demand is supplied by the combustion of fossil fuels, resulting in growing emission of greenhouse gases. In this, CO₂ is a key issue which climatic influences and general mitigation is rigorously disused not only in scientific field, but in politics as well. Here, the electrochemical CO₂ reduction reaction (CO₂RR) is posing as one potential technology, to address questions of power storage for renewable energies and sustainable usage of natural resources.

The CO₂RR shows a diverse spectrum of products ranging from liquid fuels, as ethanol and propanol, to gaseous building blocks for the chemical industry, as ethylene and CO. The selectivity of this reaction is highly dependent on the nature of the catalyst and is always competing with the hydrogen evolution reaction (HER), due to the aqueous conditions. Recently, very selective and active catalysts have been developed for the production of CO and formic acid. Both compounds are comparably easy to produce, as the reduction only involves the transfer of 2 electrons. Unfortunately, the production of valuable hydrocarbons is much more difficult and suffers from losses in selectivity due to the highly complex reaction mechanism. So far, only copper showed acceptable production rates for hydrocarbons, owing to the favorable binding energies of intermediates. Recent studies have been focused on fundamental parameters to control the selectivity of this complex reaction on copper for C₂₊ species, ranging from catalyst design to electrolyte selection. On the catalyst side, many effects as the presence of (100) facets¹, grain boundaries² and oxides³ have been suggested to be beneficial, whereas influences of buffer capacity and local electric field brought additional advantages by choice of the electrolyte. While promising results were shown, many studies only focused on low-current density, which are far from an industrial application. Here, at least 200 mA cm^-2 are needed for a functional electrolyzer. This poses as a discrepancy between research and application and raises the question if the results from fundamental studies can be transferred to full size electrolyzers.⁴

For this, we are presenting a cubic Cu₂O catalyst, which we investigate in a comprehensive study, moving from initial tests in an H-Cell towards high currents on a Gas Diffusion Electrode (GDE) in a Flow-Cell setup. To deconvolute the parameters dictating CO₂RR selectivity, we used X-Ray-Diffraction (XRD), quasi in-situ X-Ray Photon Spectroscopy (XPS) and operando X-ray Absorption Spectroscopy (XAS) to trace phase changes during reaction, in addition to morphological investigations by Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). This complementary analysis depicts a highly dynamic system, in which the reduction of oxidized Cu progresses from the surface towards deeper layers, resulting in a purely metallic, defect-rich material. We further focus on performance tests of our Cu₂O catalyst at high current density of up to 700 mA cm^-2 in a flow-electrolyzer. By varying system parameters as mass loading and nafion content we observe strong changes in selectivity and activity during CO₂RR, which we correlate to accessibility of the active copper sites and issues of mass transport. We further discuss the role of surface pH at high current density by varying electrolyte concentration and therefore buffer capacity.

Our study suggests distinct differences between CO₂RR in electrochemical cells for fundamental studies at low currents compared to tests at high currents in a flow-electrolyzer. Furthermore, we show how recent results from literature translate to performance at high current density and comment on the importance of electrode preparation.

1. Y. Hori, I. Takahashi, O. Koga and N. Hoshi, Journal of Molecular Catalysis A: Chemical, 2003, 199, 39-47.
2. A. Verdaguer-Casadevall, C. W. Li, T. P. Johansson, S. B. Scott, J. T. McKeown, M. Kumar, I. E. L. Stephens, M. W. Kanan and I. Chorkendorff, Journal of the American Chemical Society, 2015, 137, 9808-9811.
3. H. Mistry, A. S. Varela, C. S. Bonifacio, I. Zegkinoglou, I. Sinev, Y.-W. Choi, K. Kisslinger, E. A. Stach, J. C. Yang, P. Strasser and B. R. Cuenya, Nature Communications, 2016, 7, 12123.
4. T. Burdyny and W. A. Smith, Energy & Environmental Science, 2019, DOI: 10.1039/c8ee03134g.