(639f) Understanding Ionic Liquid Assisted Selective CO2 Reduction over Bi Metal Catalyst

The electrocatalytic reduction of carbon dioxide (CO₂) to useful fuel and chemical precursors such as carbon monoxide (CO) or formic acid (HCOOH) is a potential way to mitigate increasing CO₂ levels in the atmosphere and simultaneously provide storage for electricity from renewable energy sources. While significant progress has been made in the design of electrocatalysts for CO₂ reduction, the rates at viable potentials are still rather low. Recent experimental results^1–4 show that ionic liquids (IL) can function as co-catalysts in conjunction with inexpensive metal catalysts to offer a lower energy path for CO₂ reduction by effectively stabilizing the rate determining intermediate. In particular, imidazolium and amidine based ILs on Bismuth (Bi) were found to reduce CO₂ to CO and HCOOH, respectively, with a performance on par with noble metal catalysts.^5,6 Herein we use potential dependent ab-initio molecular dynamics (AIMD) and density functional theory (DFT) methods to simulate the reactivity at the electrolyte/metal interface, elucidate the nature of the IL-intermediate-catalyst surface interactions, and explain the low overpotentials and high selectivity to the high energy density products. The simulations carried out use explicit solvent molecules at conditions that imitate the experimental concentrations to examine the structure of double layer that forms at sufficiently negative potentials needed to carry out electrocatalytic reduction of CO₂. Subsequent calculations carried out with CO₂ under reaction conditions show the formation of IL-CO₂-IL catalytic pockets at the metal surface that stabilize the *CO₂(-) radical anion, helping in lowering the overpotential for both the classes of ILs. The subsequent proton and electron transfer steps in the 2H⁺/2e^- reduction path are explored with different modes of proton transfers from the IL to *CO₂(-) as a function of reduction potential. The nature of deprotonated ILs, their structure, charge distribution and stability are found to control the selectivity to specific products. Their interaction with the negatively charged electrode surface and the subsequent free energy calculations of intermediates involved in the CO and HCOOH formation pathways at experimentally relevant potentials suggest that the imidazolium type ILs predominantly generate CO and H₂O as the major products with the proton transfer from vicinal imidazolium cations while amidine cations predominantly form HCOOH as the major product. The product distribution is studied as a function of operating potential and compared to experimental results for validation.

References:

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