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

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 increasing 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.

Automated summary

This study investigates the influence of ionic correlations on CO2 electrochemical reduction. The findings show that increasing ion concentration leads to an acceleration of CO2 reduction rates and an enhancement of the efficiency to produce CO. Importantly, the acceleration of CO2 reduction is due to the amplification of potential-dependent driving forces. This phenomenon is observed across different types of ions.