Theoretical insight into effect of cation–anion pairs on CO₂ reduction on bismuth electrocatalysts

<p dir="ltr">This study presents theoretical insight into the mechanism of the CO₂ reduction reaction (CO₂RR) to formic acid (HCOOH) on Bi (0 1 2) surfaces in the presence of alkali metal cations (M⁺: Cs⁺, K⁺, and Li⁺) and/or halide anions (X⁻: Cl⁻, Br⁻, and I⁻) using density functional theory (DFT). The adsorption energy (Eads) and work function (Wf) of the anions increases with decreasing anion size (i.e., Cl⁻ > Br⁻ > I⁻). On the other hand, the larger the cation size is, the higher is the Eads (i.e., Li⁺< K⁺ < Cs⁺) but the lower is the Wf (i.e., Cs⁺ < K⁺ < Li⁺). In the presence of the cation–anion pairs (M⁺/X⁻), Eads of the pairs on hydrated Bi (Bi-2H) becomes more negative than that in the cases of anions or cations alone, particularly when the ionic radius of the paired cation and anion do not differ significantly. Such a synergistic effect of the mixed ions is also observed for the work function values. In the case of anions alone, CO₂ molecules prefer to coordinate directly with hydrated Bi atoms via the oxygen bidentate mode; in the case of cations alone, CO₂ molecules directly bind to the cations via the oxygen monodentate mode, rather than the hydrated Bi atoms. Between two possible CO2RR pathways involving *OCHO and *COOH intermediates on Bi-2H pre-adsorbed with M⁺/X⁻, the former pathway requires less energy for all M⁺/X⁻ pairs. In addition, cascaded reaction profiles from CO₂* to HCOOH are obtained with Cs⁺/Cl⁻ and K⁺/Cl⁻ pairs in the former. This indicates that once CO₂ is adsorbed, the following reactions proceed spontaneously on Bi-2H with Cs⁺/Cl⁻ or K⁺/Cl⁻pairs. This study thus shed light on the positive effects of supporting electrolytes (e.g., CsCl and KCl) on catalytic CO₂RR.</p><h2>Other Information</h2><p dir="ltr">Published in: Applied Surface Science<br>License: <a href="http://creativecommons.org/licenses/by/4.0/" target="_blank">http://creativecommons.org/licenses/by/4.0/</a><br>See article on publisher's website: <a href="https://dx.doi.org/10.1016/j.apsusc.2020.147459" target="_blank">https://dx.doi.org/10.1016/j.apsusc.2020.147459</a></p>