Cation effects on CO2 reduction catalyzed by single-crystal and polycrystalline gold under well-defined mass transport conditions

Abstract

The presence of alkali metal cations in the electrolyte substantially affects the reactivity and selectivity of electrochemical carbon dioxide (CO₂) reduction (CO₂R). This study examines the role of cations in CO₂R on single-crystal and polycrystalline Au under controlled mass-transport conditions. It establishes that CO₂ adsorption is the rate-determining step regardless of cation type or surface structure. Density functional theory calculations show that electron transfer occurs to a solvated CO₂-cation complex. A more positive potential of zero charge enhances CO₂R activity only on Au with similar surface coordination. The symmetry factor (β) of the rate-determining step varies with surface structure and cation identity, with density functional theory calculations indicating β's sensitivity to surface and double-layer structures. These findings emphasize the importance of both surface and double-layer structures in understanding cation effects on CO₂R.

Fig. 1.. Cyclic voltammetry characterization of different Au electrodes.

CVs of Au(100), Au(110), Au(111) single-crystal pcAu, EP-pcAu, and FA-pcAu recorded at 50 mV s⁻¹ in Ar-saturated 0.1 M H₂SO₄ with a rotation rate of 1600 rpm.

Fig. 2.. Partial current densities of CO measured by RRDE voltammetry on different Au electrodes in 0.1 M alkali metal bicarbonate electrolytes.

Partial current densities of CO measured on (A) Au(100), (B) Au(110), (C) Au(111), (D) EP-pcAu, and (E) FA-pcAu during CO₂R at 15 mV s⁻¹ and 1600 rpm in CO₂-saturated 0.1 M bicarbonate electrolytes (pH = 6.8, T = 25 ± 1°C). The current densities represent the average values, and the error bars (SDs) are calculated on the basis of three independent experiments.

Fig. 3.. Partial current densities of CO plotted as a function of PZC at −0.5 V_RHE on the same Au electrode.

Partial current densities of CO measured on (A) Au(100), (B) Au(110), (C) Au(111), (D) EP-pcAu, and (E) FA-pcAu in CO₂-saturated 0.1 M bicarbonate electrolytes (pH = 6.8, T = 25 ± 1°C). Error bars (SDs) are calculated on the basis of three independent experiments.

Fig. 4.. Partial current densities of CO plotted as a function of PZC at −0.5 V_RHE in the same electrolyte.

Partial current densities of CO measured in CO₂-saturated 0.1 M (A) LiHCO₃, (B) NaHCO₃, (C) KHCO₃, (D) RbHCO₃, and (E) CsHCO₃ electrolytes (pH = 6.8, T = 25 ± 1°C). The Au electrodes with comparable surface CN are linked with dashed lines for a better comparison. Error bars (SDs) are calculated on the basis of three independent experiments.

Fig. 5.. Values of symmetry factor (β) derived from Tafel plots of CO₂R to CO as a function of cation identity.

Values of β obtained in 0.1 M (A) H₂O-based and (B) D₂O-based bicarbonate electrolytes (pH = 6.8, T = 25 ± 1°C). Error bars (SDs) are calculated on the basis of three independent experiments.

Fig. 6.. DFT modeling of formation of *CO₂⁻ with explicit cations and H₂O.

(A) Profile of β_DFT with reference to EDL width from different transition state models of *CO₂⁻ formation with explicit K* and H₂O. The dash horizontal black line represents β_exp = 0.44 for the 0.1 M KHCO₃ electrolyte on the Au(111) surface. The blue solid line is the profile of β_DFT for the simple *CO₂⁻ model as seen in Fig. 7. The red solid and dashed lines represent the profiles of β_DFT for the coadsorbed cation models, respectively, for *CO₂⁻ + *K(H₂O) and *CO₂⁻ + *K(H₂O)₅. The purple solid and dashed lines represent the profiles of β_DFT for the coordinated cation model, respectively, for *CO₂-K and *CO₂-K-H₂O. A dielectric constant of 2 was chosen when calculating β_DFT. (B) Optimized geometries of *CO₂⁻ with explicit H₂O and K* are shown. The structure of *CO₂⁻ is derived from constrained geometries on the basis of COOH*. Atom colors are as follows: yellow, Au; blue, K; gray, C; red, O. Each activated complex label is color coded according to the lines in fig. S6A.

Fig. 7.. Comparison between DFT-predicted symmetry factor and experimental symmetry factor.

(A) DFT-predicted symmetry factors across different alkali metal cations and surface facets of Au. The EDL width is assumed constant across alkali metal cations on the Au(111) surface. A Helmholtz EDL width of 5.9 Å was used, fitted to give β_DFT = β_exp for K⁺ on Au(111). A cation-coordinated model represents the transition state for *CO₂⁻. (B) Experimentally derived symmetry factors across different alkali metal cations and surface facets of Au derived from Fig. 5.