Silver electrodes are highly selective for CO in CO$_2$ electroreduction due to interplay between voltage dependent kinetics and thermodynamics

Michele Re Fiorentin; Francesca Risplendi; Clara Salvini; Juqin Zeng; Giancarlo Cicero; Hannes Jónsson

Silver electrodes are highly selective for CO in CO$_2$ electroreduction due to interplay between voltage dependent kinetics and thermodynamics

Michele Re Fiorentin, Francesca Risplendi, Clara Salvini, Juqin Zeng, Giancarlo Cicero, Hannes Jónsson

Abstract

Electrochemical reduction is a promising way to make use of CO$_2$ as feedstock for generating renewable fuel and valuable chemicals. Several metals can be used in the electrocatalyst to generate CO and formic acid but hydrogen formation is an unwanted side reaction that can even be dominant. The lack of selectivity is in general a significant problem, but silver-based electrocatalysts have been shown to be highly selective for CO with over over 90% faradaic efficiency when the applied voltage is below -1 V vs. RHE. Hydrogen formation is then insignificant and little formate is formed even though it is thermodynamically favored. We present calculations of the activation free energy for the various elementary steps as a function of applied voltage at the three low index facets, Ag(111), Ag(100) and Ag(110), as well as experimental measurements on polycrystalline electrodes, to identify the reason for this high selectivity. The formation of formic acid is suppressed because of the low coverage of adsorbed hydrogen and kinetic hindrance to the formation of the HCOO* intermediate, while *COOH, a key intermediate in CO formation, is thermodynamically unstable until the applied voltage reaches -1 V vs. RHE, at which point the kinetics for its formation are more favorable than for hydrogen. The calculated results are consistent with experimental measurements carried out for acidic conditions and provide an atomic scale insight into the high CO selectivity of silver-based electrocatalysts.

Silver electrodes are highly selective for CO in CO$_2$ electroreduction due to interplay between voltage dependent kinetics and thermodynamics

Abstract

Electrochemical reduction is a promising way to make use of CO

as feedstock for generating renewable fuel and valuable chemicals. Several metals can be used in the electrocatalyst to generate CO and formic acid but hydrogen formation is an unwanted side reaction that can even be dominant. The lack of selectivity is in general a significant problem, but silver-based electrocatalysts have been shown to be highly selective for CO with over over 90% faradaic efficiency when the applied voltage is below -1 V vs. RHE. Hydrogen formation is then insignificant and little formate is formed even though it is thermodynamically favored. We present calculations of the activation free energy for the various elementary steps as a function of applied voltage at the three low index facets, Ag(111), Ag(100) and Ag(110), as well as experimental measurements on polycrystalline electrodes, to identify the reason for this high selectivity. The formation of formic acid is suppressed because of the low coverage of adsorbed hydrogen and kinetic hindrance to the formation of the HCOO* intermediate, while *COOH, a key intermediate in CO formation, is thermodynamically unstable until the applied voltage reaches -1 V vs. RHE, at which point the kinetics for its formation are more favorable than for hydrogen. The calculated results are consistent with experimental measurements carried out for acidic conditions and provide an atomic scale insight into the high CO selectivity of silver-based electrocatalysts.

Paper Structure (6 equations, 5 figures)

This paper contains 6 equations, 5 figures.

Figures (5)

Figure 1: Gibbs free energies of formation of *COOH (red bars), HCOO* (purple bars) and *H (blue bars) computed with the TCM. From left to right: Ag(110), Ag(100) and Ag(111) surfaces.
Figure 2: Diagram of the studied reaction pathways, TSs and intermediate states for CO2RR and HER. Carbon, oxygen and hydrogen atoms are represented by black, red and white circles respectively. Grey rectangles represent the silver slab.
Figure 3: Geometries of three representative TSs on Ag(111) at $U=-0.8$ V vs. RHE. Left panel: HER step in Eq. (\ref{['eq:H+e->*H']}). Central panel: CO2RR step in Eq. (\ref{['eq:CO2_H+e->*COOH']}). Right panel: CO2RR step in Eq. (\ref{['eq:CO2_H+e->HCOO*']}). Carbon, oxygen, hydrogen and silver atoms are represented by black, red, white and gray spheres, respectively. Lighter colors are used to mark spectator water molecules.
Figure 4: Grand-canonical activation energies $\Delta\Omega^\ddagger$ of the studied steps of HER and CO2RR. Top to bottom: Ag(110), Ag(100) and Ag(111) surfaces. Each group represents a reaction step and consists of three bars, corresponding to the three studied potentials, arranged from left to right (darker to lighter shades): $U=-1.1,\,-0.8,\,-0.4$ V vs. RHE.
Figure 5: (a) and (b): Grand-canonical free energy variations $\Delta G$ on Ag(111) along CO2RR and HER at $U=-0.4$ V vs. RHE (a) and $U=-1.1$ V vs. RHE (b). The inset in panel (b) shows a zoom on the activation grand-canonical free energies of *H production in HER (blue line) and *COOH formation in CO2RR. (c) Comparison of reaction (upper panel) and activation (lower panel) grand canonical free energies for reaction steps in Eqs. (\ref{['eq:CO2_H+e->*COOH']}) (red bars) and (\ref{['eq:H+e->*H']}) (blue bars). (d) Experimental FEs of CO2RR to CO (red bars) or to HCOOH (purple bars) and HER (blue bars) of a synthesized polycristalline Ag electrocatalyst in a pH 2 electrolyte.