Cation-Coordinated Inner-Sphere CO2 Electroreduction at Au-Water Interfaces
Authors: Qin Xueping , Vegge Tejs , Hansen Heine Anton 
Published: September 2023

The electrocatalytic CO2 reduction reaction (CO2RR) is a promising technology to store renewable energy and mitigate the increase of CO2 concentration.1-2 Understanding the electrode-electrolyte interfacial reaction mechanism from the atomic level is instrumental for both catalyst design and catalytic system’s optimization. Electrolyte ions are critical for CO2RR and the competitive hydrogen evolution reaction (HER), however, the role of alkali metal cations is highly controversial. For example, a recent study by Koper and co-workers highlighted the role of alkali metal ion coordination,3 whereas another work by Hu et al. ascribed the cation effect to the modulation of local electric field.4 In order to explore the crucial cation role in electrocatalytic reactions, electrode-electrolyte interfacial models should be properly constructed by including explicit water solvents and alkali metal cations.
In our previous work, CO2 activation step at Au-water-2K interfaces has been studied by ab initio molecular dynamics (AIMD) simulations with the kinetic barrier of 0.61 eV.5 To determine the rate-determining step (RDS) during CO2RR on Au surfaces, the full reaction pathway involving the subsequent electron and proton transfer steps is further investigated, and the reaction kinetics is evaluated by the slow-growth sampling approach integrated with AIMD simulations (SG-AIMD). As shown in Figure 1, the complete free energy diagram of CO2RR under electrochemical conditions is constructed for the first time, where the transition state (TS) structures are shown on top. It is illustrated that such a cation-coordinated inner-sphere CO2RR at Au-water interfaces is facile, and the first electron transfer with the concomitant adsorption during CO2 activation is the RDS. Furthermore, HER pathway is also explored, which is highly suppressed by local alkali cations showing much higher kinetic barrier in the rate-limiting Volmer step.
Our systematic atomic-scale study via ab initio molecular dynamics simulations demonstrates that CO2RR shows superior catalytic activity under the cation promotion effect which is originated from the short-range coordination interaction between reaction intermediates and cations. This study motivates the development of periodic models mimicking the electrode-electrolyte interface under electrochemical conditions, which can be extended into studying various electrocatalytic reactions via AIMD simulations.

References

(1) Nitopi, S.; Bertheussen, E.; Scott, S. B.; Liu, X.; Engstfeld, A. K.; Horch, S.; Seger, B.; Stephens, I. E. L.; Chan, K.; Hahn, C.; Nørskov, J. K.; Jaramillo, T. F.; Chorkendorff, I., Chem. Rev. 2019, 119, 7610-7672.
(2) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F., Science 2017, 355, eaad4998.
(3) Monteiro, M. C., Dattila, F., Hagedoorn, B., García-Muelas, R., López, N. and Koper, M., Nat. Catal., 2021, 4, 654-662.
(4) Gu, J., Liu, S., Ni, W., Ren, W., Haussener, S. and Hu, X., Nat.
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(5) Qin, X.; Vegge, T.; Hansen, H. A., J. Chem. Phys. 2021, 155, 134703.

Figure 1. Complete free energy landscape of CO2RR at Au-water interfaces with two K cations. The structures of transition state (TS) are shown on top. The charges transferred to key intermediates (*CO2, *COOH, *CO) are shown in the insert figure, where the charge transfer in CHE model is indicated by dashed lines. Color code: Au, golden; K, purple; C, blue; O, red; H, white.

Figure 1