Impacts of electrode potentials and solvents on the electroreduction of CO₂: a comparison of theoretical approaches

Abstract

Since CO2 is a readily available feedstock throughout the world, the utilization of CO2 as a C1 building block for the synthesis of valuable chemicals is a highly attractive concept. However, due to its very nature of energy depleted "carbon sink", CO2 has a very low reactivity. Electrocatalysis offers the most attractive means to activate CO2 through reduction: the electron is the "cleanest" reducing agent whose energy can be tuned to the thermodynamic optimum. Under protic conditions, the reduction of CO2 over many metal electrodes results in formic acid. Thus, to open the road to its utilization as a C1 building block, the presence of water should be avoided to allow a more diverse chemistry, in particular for C-C bond formation with alkenes. Under those conditions, the intrinsic reactivity of CO2 can generate carbonates and oxalates by C-O and C-C bond formation, respectively. On Ni(111), almost exclusively carbonates and carbon monoxide are evidenced experimentally. Despite recent progress in modelling electrocatalytic reactions, determining the actual mechanism and selectivities between competing reaction pathways is still not straight forward. As a simple but important example of the intrinsic reactivity of CO2 under aprotic conditions, we highlight the shortcomings of the popular linear free energy relationship for electrode potentials (LFER-EP). Going beyond this zeroth order approximation by charging the surface and thus explicitly including the electrochemical potential into the electronic structure computations allows us to access more detailed insights, shedding light on coverage effects and on the influence of counterions.

Practical Considerations for Continuum Models Applied to Surface Electrochemistry

“… is printed in VASPsol as the negative of the output flag FERMI‐ SHIFT and decreases as a function of cell height. Figure also shows that the electrolyte potential is already flat at typical cell sizes, and so the slow convergence of energies with respect to cell height arises solely from variations in rather than a slow decay of countercharge density in the continuum electrolyte …”

“…F vac is printed in VASPsol as the negative of the output flag FERMI-SHIFT and decreases as a function of cell height. Figure 1 also shows that the electrolyte potential is already flat at typical cell sizes, and so the slow convergence of energies with respect to cell height [23][24][25][26][27] arises solely from variations in F vac rather than a slow decay of countercharge density in the continuum electrolyte. [26] In a charged LPB system, the absolute value of the potential is fixed even in the presence of periodic boundary conditions due to the additional ionic screening term in the Poisson equation.…”

Solvation free energies for periodic surfaces: comparison of implicit and explicit solvation models

“…16,31 As clearly seen, PCM τ=0 overestimates the solvation free energy rather dramatically (mean absolute deviation, MAD of 0.14 eV (> 3 kcal/mol)). § The cavitation energy is, by definition, positive and its inclusion reduces the solvent affinity.…”

“…Since the cavitation energy usually only gives a small contribution (in the order of 0.05 eV), it is often neglected. 16,31 The model contains three empirical parameters (ρ c , σ , τ) that have been fitted to reproduce the reference data, 32 comparing the computed…”

Recent Progress in the Theoretical Investigation of Electrocatalytic Reduction of CO2

“…Steinmann et al modeled CO 2 electroreduction in aprotic conditions with this approach. [57] They proposed a surface charging method to introduce the potential effect on the charge variation of adsorbate. To really achieve a fixed potential, Goodpaster et al developed a fixed potential selfconsistent algorithm combining with implicit solvation model.…”