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 (see the Methods). 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; the same potential scale is used in the following discussion) and C₂ species (ethanol, ethylene, and ethane) at more negative potentials. In this work, we 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. We aimed to examine the interplay between the mass transfer and local pH effects in the electrocatalysis for CO₂ reduction and elucidate their roles in the structure-performance relationships of the high-surface-area nanowire catalysts. The ultimate goal is to explore the optimal electrochemical conditions for CO₂ reduction to produce valuable C₂ products.