Identification of Possible Mechanistic Pathways during the Electrochemical Reduction of CO2: Results from Constant Potential DFT Calculations

Identification of Possible Mechanistic Pathways during the Electrochemical Reduction of CO₂: Results from Constant Potential DFT calculations

Jason D. Goodpaster,^a,*  Martin Head-Gordon,^b Alexis T. Bell^c

^aDepartment of Chemistry, University of Minnesota Twin Cities, Minneapolis, Minnesota 55455, USA^bDepartment of Chemistry, University of California at Berkeley, Berkeley, California 94720, USA

^cDepartment of Chemical and Biomolecular Engineering, University of California at Berkeley, Berkeley, California 94720

*Corresponding author: jgoodpas@umn.edu

We investigated the mechanistic pathways by which CO₂ on copper surfaces is electrochemically reduced using periodic Kohn-Sham Density Functional Theory that includes the effects of the electrochemical potential, solvent, and electrolyte. Our analysis revealed that at low-applied potential, C-C bond formation occurs through a CO dimer; however, at high-applied potentials, C-C bond formation occurs through reaction of adsorbed CHO and CO. Additionally, our model allowed us to study proton transfer from water via the Heyrovsky mechanism, which we show to be an important pathway for CO₂ reduction.¹

The electrode surface and all adsorbates were treated using DFT with the revised Perdew-Burke-Ernzerhof (RPBE) functional. The solvent was treated as a continuum dielectric and the electrolyte was described by a linearized Poisson-Boltzmann model.^2,3 The electrode potential was calculated from the Fermi energy. The Fermi energy can be varied by changing the number of electrons in the simulation cell; therefore, the electrochemical potential was set by calculating the number of electrons the simulation cell required for a specific Fermi energy. Using this model, the free energy profile for the reaction could be calculated as a function of the applied potential. The free energy of activation for each elementary step was then used to determine the rate coefficient for that step. The potential-dependent rate coefficients were then used in a microkinetic model to determine the rate of product formation.

References:

1. J. D. Goodpaster, A. T. Bell, and M. Head-Gordon J. Phys. Chem. Lett. 7, 1471 (2016).
2. K. Letchworth-Weaver and T. A. Arias Phys. Rev. B 86, 075140 (2012).
3. K. Mathew, R. Sundararaman, K. Letchworth-Weaver, T. A. Arias, and R. G. Hennig.
   J. Chem. Phys. 140, 084106 (2014).