Tuning C-C Coupling and Selectivity in CO₂ Electrochemical Reduction Reaction via Pyramidal Dilute Sn-Cu Alloy

The electrochemical conversion of carbon dioxide (CO₂) into value-added fuels is emerging as a promising strategy to combat climate change and support carbon neutrality. Despite recent advances, the selective generation of higher-order hydrocarbons (C₂₊ products) remains a significant challenge due to kinetic and thermodynamic limitations. In this study, we report the synthesis of electrocatalysts comprised of a pyramidal dilute Sn-Cu alloy, fabricated via electrodeposition onto titanium substrates. The pure Cu sample showed the lowest surface roughness with smooth, spherical particles, while the addition of trace Sn was crucial in transforming the morphology to faceted pyramidal structures. Incorporating 1 at % Sn into Cu nanopyramids significantly enhances catalytic activity and selectivity toward ethylene (C₂H₄) production. Electrochemical tests reveal that the Cu₉₉Sn₁ catalyst achieves a Faradaic efficiency of 37% for ethylene at -0.8 V versus RHE, alongside operational stability over 12 h of continuous electrolysis. The improved performance of the Cu₉₉Sn₁ nanostructures is attributed to multiple synergistic effects. First, alloying with Sn modulates the electronic structure of Cu, stabilizing key *CO intermediates that are critical for C-C coupling while concurrently suppressing the hydrogen evolution reaction (HER) by limiting H⁺ adsorption. Second, the unique pyramid-shaped morphology introduces high-index facets, abundant edge sites, and a high density of surface defects. These characteristics contribute to an enhanced active surface area, which is known to promote favorable adsorption configurations and accelerate reaction kinetics. Complementary density functional theory (DFT) calculations further support the experimental findings, showing that the pyramidal geometry modulates the local electronic environment and optimizes adsorption energies to facilitate C-C bond formation while inhibiting HER. This work highlights the powerful interplay between atomic-level alloying and nanostructural engineering in tailoring catalyst functionality for CO₂ electroreduction. The findings offer a promising route toward efficient, selective, and sustainable carbon utilization technologies.