The Influence of Supercritical CO₂ on CO₂RR using Carbon Supported Copper Catalysts

Kai junge Puring, Fraunhofer UMSICHT, Oberhausen, Germany
Olga Evers, Fraunhofer UMSICHT, Oberhausen, Germany
Michael Prokein, Fraunhofer UMSICHT, Oberhausen, Germany
Stefan Kaluza, Fraunhofer UMSICHT, Oberhausen, Germany
Manfred Renner, Fraunhofer UMSICHT, Oberhausen, Germany
Marcus Petermann, Ruhr-University Bochum, Insititute for Particle Technology, Bochum, Germany
Eckhard Weidner, Fraunhofer UMSICHT; Ruhr-University Bochum, Insititute for Particle Technology, Oberhausen; Bochum, Germany
Ulf-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. [1] 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. [2] We recently showed that high process CO₂ pressure, more precisely supercritical CO₂ (scCO₂) conditions, can hugely influence the product outcome. [3] 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
[1] Jinli Qiao, Yuyu Liu, Feng Hong, Jiujun Zhang, Chem. Soc. Rev., 2014, 43, 631.
[2] Abdalaziz Aljabour, Halime Oskun, Dogukan Hazar Apaydin, Faruk Ozel, Achim Walter Hassel, Philipp Stadler, Niyazi Serdar Sariciftci, Mahmut Kusa, Appl. Catal. B: Environmental 2018, 229, 163.
[3] Olga Melchaeva, Patrick Voyame, Victor Costa Bassetto, Michael Prokein, Manfred Renner, Eckhard Weidner, Marcus Petermann, Alberto Battistel, Chem. Sus. Chem., 2017, 10, 18, 3660–3670.

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