Recent progress in the electrochemical reduction of CO₂ (CO₂RR) has led to notable breakthroughs in generating C₂ compounds such as ethylene and ethanol. Nevertheless, the direct formation of C₃ products encounters significant limitations due to the C₂–C₁ coupling reaction, posing a considerable challenge to improving their faradaic efficiency. Here, a design for an elevated pressure cascade catalytic reactor to convert CO₂ to C₃ products in a two-step electrochemical process is presented. At 25 bar pressure, by regulating the potential of the cascade system and the electrolyte flow rate, a 40% selectivity for 2-propanol on a copper electrode placed upstream of a silver electrode that converts CO₂ to CO is reported. In cascade mode (with both silver and copper electrodes active), the C₃:C₂ oxygenate ratio significantly increases to 7 compared to the noncascade mode (copper only) with a modest ratio of about 0.6. Therefore, our elevated pressure cascade electrolysis approach demonstrates a notable step forward in CO₂ electroreduction to oxygenated C₃ products.

Electrochemical CO₂ reduction is a promising way of closing the carbon cycle while synthesizing useful commodity chemicals and fuels. One of the possible routes to scale up the process is CO₂ reduction at elevated pressure, as this is a way to increase the concentration of poorly soluble CO₂ in aqueous systems. Yet, not many studies focus on this route, owing to the inherent challenges with high-pressure systems, such as leaks, product quantification, and ease of operation. In this study, we use a high-pressure flow cell setup to investigate the impact of CO₂ pressure on the electrochemical performance of a copper foam electrode for CO₂ reduction within a pressure range of 1 to 25 bar. Our initial findings using a 0.5 M potassium bicarbonate (KHCO₃) electrolyte show a consistent improvement in selectivity towards CO₂ reduction products, with HCOOH being the dominant product. By conducting a systematic exploration of operating parameters including applied current density, applied CO₂ pressure, cation effect, and electrolyte concentration, the selectivity towards formate (HCOOH) is optimized, achieving a remarkable 70 % faradaic efficiency (FE) under moderate conditions of 25 bar in a 0.5 M cesium bicarbonate (CsHCO₃) electrolyte. Additionally, we report the synthesis of isopropanol with a FE of 11 % at the 25 bar in 0.5 M KHCO₃ which is the highest reported selectivity towards isopropanol on copper using a bicarbonate system.

Abstract: The electrochemical CO₂ reduction reaction (CO₂RR) has been proposed as a sustainable way of closing the carbon cycle while synthesizing useful commodity chemicals. One of the possible routes to scale up the process is the elevated pressure CO₂RR, as this increases the concentration of the poorly soluble CO₂ in aqueous systems. Yet, there are not many studies that focus on this route owing to the inherent challenges with high pressure systems. In this study, a novel high pressure flow cell setup has been designed and validated. The modular design uses a clamp system, which facilitates simple stacking of multiple cell parts while being capable of handling pressures up to 50 bar. The effects of CO₂ pressure on the reaction were investigated on a gold (Au) foil cathode in a 0.1 M KHCO₃ electrolyte. We successfully measured gaseous products produced during high pressure operation using an inline gas chromatograph. We find that the selectivity toward CO₂ reduction products is enhanced while that of H₂ evolution is suppressed as the pressure is increased from 2 to 30 bar. The reported setup provides a robust means to conduct high pressure electrolysis experiments in an easy and safe manner, making this technology more accessible to the electrochemical CO₂RR community. Graphical abstract: [Figure not available: see fulltext.].