High-Index-Faceted Cu Nanoparticles in Ag-Cu Tandem Catalysts for Selective CO₂ Electroreduction

Facet engineering of Cu-based nanocatalysts is a promising strategy for regulating the selectivity of CO₂ and CO electroreduction because surface atomic arrangements strongly influence intermediate adsorption and reaction pathways. Compared with low-index facets, high-index facets contain abundant steps, kinks, and low-coordinated atoms, providing diverse active ensembles that can facilitate CO adsorption and subsequent C-C coupling. However, the catalytic roles of high-index-faceted Cu nanocatalysts remain largely unexplored. Herein, tetrahexahedral (THH) and hexoctahedral (HOH) high-index-faceted Cu nanoparticles were integrated with a porous Ag foam to construct Cu/Ag tandem catalysts for CO₂ electroreduction. The C₂ ⁺ selectivity follows the order HOH Cu/Ag > THH Cu/Ag > non-facet-controlled Cu/Ag, with HOH Cu/Ag achieving the highest ethylene Faradaic efficiency of 60.37%. In situ spectroscopic measurements using HOH Cu/Ag as a representative high-index-faceted catalyst indicate the formation of abundant adsorbed CO and possible CO-derived coupling intermediates within the potential range associated with pronounced ethylene production. Stability measurements further show that the morphology and catalytic selectivity are largely retained under moderate operating conditions, whereas more negative potentials induce surface roughening and gradual performance decay. This work provides a platform for investigating morphology-dependent selectivity in high-index-faceted Cu catalysts during CO₂ electroreduction.