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

Authors: 
Singh, M., Chemical Sciences Division
Goodpaster, J., University of California Berkeley
Weber, A., Lawrence Berkeley National Laboratory
Head-Gordon, M., University of California - Berkeley
Bell, A. T., University of California, Berkeley
Electrochemical reduction of CO2 over Ag is affected by various physical processes such as transport of species (H+,OH-, CO2, HCO3-, CO32-, 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.