(435b) Mechanistic Insights into the Electrochemical Reduction of CO2 over Ag Using an Integrated Transport-DFT-Microkinetic Model

Electrochemical reduction of CO₂ over Ag is affected by various physical processes such as transport of species (H⁺,OH^-, CO₂, HCO₃^-, CO₃^2-, M⁺, CO and H₂) in the electrolyte, adsorption/desorption of reactants (CO₂ and H₂O) and products (OH^-, CO and H₂), and energetics of electron/hydrogen transfer reactions involved in the formation of CO and H₂. Near cathode surface, the pH and CO₂ concentration gradients increase and the electric field decreases with decreasing potential at the cathode, which causes the coverage of adsorbed CO₂ to decrease and H to increase. Consequently, the rate of CO formation decreases and H₂ increases at higher negative applied potentials. Identification of optimal reaction conditions to obtain higher selectivity to CO requires a detailed microkinetic model based on quantum mechanical calculations coupled with the transport effects. We have used a periodic Kohn-Sham density functional theory (DFT) and a linearized Poisson-Boltzmann model to calculate coverage-dependent free energies of adsorption and energy barriers for a set of elementary reactions.¹ The rates of reactions in the microkinetic model were described using the transition-state theory. The transport of species involves diffusion, migration, convection and acid-base reactions, which was modeled according to a previously reported procedure.² In this talk, we will show the predictions of a fully integrated model for surface coverages and reaction rates as a function of applied potential, electrolyte properties (pH and buffer capacity) and operating conditions (temperature, mixing, CO₂ pressure and flowrate).

References:

(1) Goodpaster, J. D.; Bell, A. T.; Head-Gordon, M., Identification of Possible Pathways for Câ??C Bond Formation during Electrochemical Reduction of CO2: New Theoretical Insights from an Improved Electrochemical Model. The Journal of Physical Chemistry Letters 2016, 7, (8), 1471-1477.

(2) Singh, M. R.; Clark, E. L.; Bell, A. T., Effects of electrolyte, catalyst, and membrane composition and operating conditions on the performance of solar-driven electrochemical reduction of carbon dioxide. Physical Chemistry Chemical Physics 2015, 17, (29), 18924-18936.