In the electrocatalytic carbon dioxide reduction reaction (CO2RR), supercritical carbon dioxide (scCO2) as an alternative solvent can produce various carbon-based products on the surfaces of suitable catalysts. The scCO2 solvent has been found capable of suppressing the most significant competing side reaction, hydrogen evolution reaction (HER), that impacts the selectivity and Faraday efficiency of desired carbon-based products. To decipher the role of the scCO2 solvent and other environmental conditions like the temperature and the pressure, we performed quantum chemical calculations of CO2RR and HER that produce formic acid (HCOOH) and molecular hydrogen (H2) as the leading products. We used a simplified model with a Cu4 cluster as the electrocatalyst and the electron donor, explicit CO2 molecules as the starting material, H2O molecules as the proton donors, and implicit scCO2 molecules as the solvent. We also implemented the temperature and the pressure based on an NPT ensemble. Using a combination of quantum mechanics and statistical mechanics, we evaluated the profiles of Gibbs free energies for important molecular configurations along environmentally dependent reaction paths from CO2 and H2O to HCOOH and H2, including reactants, products, intermediates, and transition states. Based on our models, we managed to predict dominant products from electrocatalytic CO2RR and the environmental dependence of their efficiency and selectivity.

Quantum mechanical assessment of supercritical carbon dioxide as alternative solvent for electrocatalytic carbon dioxide reduction reactions

ACS Spring 2022

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