Tuning Ionic Screening To Accelerate Electrochemical CO₂ Reduction in Ionic Liquid Electrolytes

Abstract

Electric double layer formation often governs the rate and selectivity of CO2 electrochemical reduction. Ionic correlations critically define double layer properties that are essential to electrocatalytic performance, including capacitance and localization of potential gradients. However, the influence of ionic correlations on CO2 electroreduction remains unexplored. Here, we use electrochemical conversion of CO2 to CO in ionic liquid-based electrolytes to investigate how the emergence of ionic correlations with increasing ion concentration influences reaction rates and selectivity. Remarkably, we find substantial acceleration of potential-dependent CO2 reduction rates and enhancement of faradaic efficiency to CO at intermediate concentrations of 0.9 M ionic liquid in acetonitrile, a concentration regime that has not been studied previously. We find that onset potentials for CO2 reduction remain relatively unchanged at −2.01 V vs Ag/Ag+ from 0.025 M up to 1.1 M and increase to −2.04 V vs Ag/Ag+ in the limit of neat ionic liquids. Hence, the acceleration of CO2 reduction we observe originates from the amplification of potential-dependent driving forces, as opposed to changes in onset potential. Importantly, our findings are general across cocatalytic and noncatalytic ions. We propose that concentrations of maximum reactivity correspond to conditions where electric double layers exhibit the strongest screening, which would localize electric fields to stabilize polar intermediates. Our study demonstrates that tuning bulk electrostatic screening lengths via modulation of ionic clustering provides a general approach to accelerating both inner-sphere and outer-sphere electrochemical reactions.

Alternative Energy Carriers: Unique Interfaces for Electrochemical Hydrogenic Transformations

“…This is consistent with the fact that ionic liquids exhibit only modest double layer capacitances [38] and often need to be blended with higher permittivity solvents, like acetonitrile or water, to facilitate catalysis (Figure 4). [49][50][51] Figure 3. A) Classical electric double layers consist of a bound ion layer in series with a diffuse layer.…”

“…Together, these approaches can be leveraged to enhance selective CO 2 reduction to CO by fivefold, compared to electrolytes with a more typical ion concentration of 0.1 m. Reproduced with permission. [51] Copyright 2022, American Chemical Society.…”

“…This is consistent with the fact that ionic liquids exhibit only modest double layer capacitances [ 38 ] and often need to be blended with higher permittivity solvents, like acetonitrile or water, to facilitate catalysis ( Figure 4 ). [ 49–51 ]…”

“…Further control over product selectivity is also obtained by sourcing protons from imidazolium cations, instead of water molecules. Together, these approaches can be leveraged to enhance selective CO 2 reduction to CO by fivefold, compared to electrolytes with a more typical ion concentration of 0.1 m. Reproduced with permission [51]. Copyright 2022, American Chemical Society.…”

Efficient electroreduction of CO2to syngas over ZIF-8 derived oxygen vacancy-rich ZnO nanomaterials

“…The [Bmim]PF 6 -containing solutions have been widely applied as electrolytes for developing the electrocatalytic performance of CO 2 reduction. [36][37][38] In this work, the effect of [Bmim]PF 6 concentration in the electrolyte was measured on ZnO-n at À1.9 V vs. Ag/ Ag + . In Table S2 (ESI †), we observed the 30 wt% [Bmim]PF 6 in the electrolyte was the most efficient for CO 2 reduction.…”