The Influence of Supercritical CO2 on CO2RR using Carbon Supported Copper Catalysts
Kai junge Puring, Fraunhofer UMSICHT, Oberhausen, GermanyOlga Evers, Fraunhofer UMSICHT, Oberhausen, GermanyMichael Prokein, Fraunhofer UMSICHT, Oberhausen, GermanyStefan Kaluza, Fraunhofer UMSICHT, Oberhausen, GermanyManfred Renner, Fraunhofer UMSICHT, Oberhausen, GermanyMarcus Petermann, Ruhr-University Bochum, Insititute for Particle Technology, Bochum, GermanyEckhard Weidner, Fraunhofer UMSICHT; Ruhr-University Bochum, Insititute for Particle Technology, Oberhausen; Bochum, GermanyUlf-Peter Apfel, Fraunhofer UMSICHT; Ruhr-University Bochum, Inorganic Chemistry I, Oberhausen; Bochum, Germany
The electrochemical conversion of carbon dioxide into commodity chemicals or fuels is considered one of the most attractive reactions for a sustainable carbon dioxide utilization owing to its high flexibility and product diversity.  Due to low solubility of carbon dioxide in aqueous solution the mass transport is limited. Thus, the efficiency of the carbon dioxide reduction reaction (CO2RR) is limited and hydrogen evolution is commonly promoted. To increase the mass transport of CO2, organic solvents such as acetonitrile can be applied due to their higher solubility for CO2.  We recently showed that high process CO2 pressure, more precisely supercritical CO2 (scCO2) conditions, can hugely influence the product outcome.  Along this line, we herein present carbon supported copper catalysts and its activity in CO2RR in acetonitrile containing 30 mg ml-1 water as well as under scCO2 conditions. During electrolysis at similar potentials under ambient (20 °C, 1 atm) and scCO2 (70 °C, 150 bar) conditions, we observed up to five-fold increase in current density under scCO2 conditions. Furthermore, we observed a significant shift in the product distribution. While at ambient conditions, mainly hydrogen is formed along with minor quantities of carbon monoxide and formic acid, at scCO2 conditions the hydrogen evolution could be suppressed significantly with H2 faradaic efficiencies < 10 %. In fact, high selectivity towards formic acid production is observed with faradaic efficiencies of up to 80 %. Moreover, in long-term experiments under scCO2 conditions, acetic acid is produced as major side product enabling the direct synthesis of C-C coupling products. Concluding these results, the product selectivity of the CO2RR can be tailored by adjusting the proton concentration and CO2 mass transport during the reaction.References
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