(66e) Interplay of Mass Transfer and Local pH Effects in CO2 Reduction Electrocatalysis on Cu Nanowires

Besides the catalyst structures, the electrocatalysis for CO₂ reduction is also dependent on the electrolyte and CO₂ partial pressure, which can affect the catalytic activity and selectivity via mass transfer and/or local pH effects. While protons are readily available in aqueous solutions, the supply of reactants to the electrode surface is limited by the concentration and diffusivity of dissolved CO₂ molecules, which places a hurdle on the current density and efficiency of CO₂ reduction. The reduction of CO₂, as well as the evolution of H₂, produces hydroxide anions (OH^–) and causes rise of the local pH on the electrode surface.

Here we report on the study of mass transfer and local pH effects on CO₂ reduction by using Cu nanowires as the electrocatalysts. Highly dense Cu nanowires were synthesized by first thermal oxidation of Cu mesh in air, and then reduction by applying a cathodic electrochemical potential or thermal annealing in a reducing atmosphere. Our previous report has shown that the electrochemically reduced (ECR) Cu nanowires are highly active for CO₂ reduction, with favorable production of CO and formate at potentials more positive than -0.5 V (versus reversible hydrogen electrode) and C₂ species (ethanol, ethylene, and ethane) at more negative potentials. We have further performed more systematic studies in the high-overpotential region, i.e., potentials more negative than -0.5 V, where the mass transport of CO₂ and high local pH near the electrode surface are believed to play a significant role in determining the electrocatalytic performance. Simulations have been developed to understand the interplay between the mass transfer and local pH effects and elucidate their roles in the structure-performance relationships of the high-surface-area nanowire catalysts. Based on these results, discussions are provided for optimizing the electrochemical conditions for CO₂ reduction to produce valuable C₂ products.