Recent advances in catalyst materials¹ and reactor configurations² for the electrochemical reduction of carbon dioxide (CO₂) have led to significant increases in current efficiency and product throughput, but further improvements are necessary for process viability. In particular, understanding and controlling reaction mechanisms and kinetic rates are critical for balancing voltage efficiency and current density. To this end, the ubiquitous Butler-Volmer (BV) kinetic model, from which Tafel analysis is derived, has been used extensively to evaluate current-potential relationships for CO₂ reduction reactions to various products on a range of catalytic surfaces. Though relatively easy to implement, the BV model has a phenomenological basis that is not as strongly rooted in physics as alternative electrokinetic models, which, in turn, can limit its ability to describe complex reactions that exhibit nonlinear Tafel behavior.

Here we investigate the applicability of several different electrokinetic models for describing the electroreduction of CO₂ to carbon monoxide on gold (Au) surfaces. Specifically, we compare the BV model to the Marcus-Hush-Chidsey (MHC) model, which has been successfully demonstrated in LiFePO₄-based Li-ion batteries³, and a BV model that incorporates a series resistance term (BV+R). These models are applied to kinetic data collected from gas diffusion electrodes coated with Au nanoparticles⁴ and integrated into an instrumented flow electrolyzer that enables in-situ single electrode analysis coupled with product quantification. The resulting current-potential response, shown in Figure 1a, displays sublinear behavior at high overpotentials and, based on a literature survey, appears characteristic of Au surfaces irrespective of catalyst morphology, reactor configuration, or reactant delivery mode. In this presentation, we seek to describe the underlying causes of this general behavior and to apply appropriate kinetic modeling that accurately represent this electrochemical response (Figure 1b).

References:

(1) Lu, Q.; Jiao, F. Electrochemical CO₂ Reduction: Electrocatalyst, Reaction Mechanism, and Process Engineering. Nano Energy 2016, 29, 439–456.

(2) Endrődi, B.; Bencsik, G.; Darvas, F.; Jones, R.; Rajeshwar, K.; Janáky, C. Continuous-Flow Electroreduction of Carbon Dioxide. Prog. Energy Combust. Sci. 2017, 62, 133–154.

(3) Bai, P.; Bazant, M. Z. Charge Transfer Kinetics at the Solid–solid Interface in Porous Electrodes. Nat. Commun. 2014, 5.

(4) Salih, T.; Brown, S.; Kim, C.; Carroll, K.; Brushett, F.; Bumajdad, A. Cost Effective and Scalable Synthesis of Supported Au Nanoparticles for the Electroreduction of CO₂ to CO. Sci. Adv. Mater. 2017, 9 (6), 888–895.

Figure 1: (a) Tafel analysis of two linear regions using a Butler-Volmer kinetic model. The charge transfer coefficients and exchange current densities are calculated for the high and low overpotential regions and exhibit significant differences. (b) Best fit of the data with BV (red), BV+R (green), and MHC (blue) models. Fit parameters for each model are provided.