Influence of Process Parameters on Electrochemical CO2 Reduction to C2+ Products

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With an increasing concentration of carbon dioxide (CO2) in the atmosphere, greenhouse gas emissions must be mitigated to prevent further global warming. A promising technology to achieve this goal is the electrochemical reduction of CO2. Thereby, CO2 is converted to value-added chemicals using renewable electrical energy. Ethylene or ethanol are valuable C2+ products, which may be synthesized with copper based catalysts. However, many studies lack control of temperature and CO2 excess factors as well as the application of industrially relevant current densities above 200 mA cm-2. Additionally, recent literature proposes benefits of pulsed operation as opposed to static operation. Pulsed operation employing gas diffusion electrodes at elevated current densities must be studied in more detail to assess the full potential of dynamic operation. This work demonstrates the continuous electrochemical reduction of CO2 to C2+ products at industrially relevant conditions. Current densities from 100 mA cm-2 to 400 mA cm-2 are applied to an electrochemical flow cell with a geometrical electrode area of 25 cm2. CuO nanoparticles were deposited on gas diffusion layers as catalyst via automated ultrasonic spray-coating. Throughout electrolysis experiments, constant electrolyte temperatures of 20 °C or 40 °C and constant CO2 excess factors of 2 or 5 are maintained. In contrast to statically applied current, pulsed current experiments focus on enhanced CO2 mass transfer to the catalyst layer. Throughout static electrolysis, an increase in Faradaic efficiency from 30 % to 60 % for C2+ products is observed when the current density is increased from 100 mA cm-2 to 400 mA cm-2. This is attributed to a more alkaline pH in the catalyst layer caused by high current densities which is favorable to produce C2+ products. Furthermore, it was found that an electrolyte bulk temperature of 20 °C increases the Faradaic efficiency for C2+ products compared to experiments conducted at 40 °C. We hypothesize that this is due to less intermediate desorption from the catalyst. Furthermore, the reduction of CO2 excess leads to an increase in Faradaic efficiency for C2+ products. The highest Faradaic efficiency to C2+ products was found at 400 mA cm-2 with an electrolyte bulk temperature of 20 °C at a CO2 excess factor of 2. Additionally, the influence of pulsed current on the yield of C2+ products was investigated and different pulsing procedures were tested. The results show that pulsed current operation reduces the parasitic hydrogen evolution. However, the C2+ formation was not increased compared to static operation in an electrochemical flow cell at high current densities. In summary, this work shows how different process parameters influence the product distribution in electrochemical CO2 reduction on a CuO catalyst to improve the selectivity towards C2+ products. This is a further step towards a sustainable process to produce platform chemicals.