Size-dependent selectivity of Cu2O nanocube catalysts for CO2 reduction at industrial current densities

• Cu 2 O catalysts exhibit size-dependent selectivity in CO 2 electroreduction • Cubes <100 nm achieve the highest ethylene selectivity in CO 2 electroreduction • 45–75 nm cubes achieve up to 50% Faradaic efficiency for C₂ products. • Trade-off between stability and ethylene selectivity in zero-gap cell. • Electrolyte composition strongly influences CO₂RR selectivity in zero-gap cell. The electrochemical reduction of CO₂ (CO₂RR) to value-added chemicals offers a promising route for carbon recycling and renewable energy storage. Cu-based catalysts are uniquely capable of producing multi-carbon products such as ethylene, but their selectivity is highly sensitive to their morphology. In this work, we systematically investigate the impact of Cu₂O nanocube size (45–600 nm) on CO₂RR performance in both alkaline flow cell and zero-gap electrolyzer, both operating at industrially-relevant current densities. The catalysts were synthesized with well-controlled geometries and edge lengths. In the flow-cell, smaller nanocubes (45–75 nm) exhibited superior selectivity toward ethylene and liquid C 2 products, achieving Faradaic efficiencies toward C 2 products (FE C2 ) of up to 50%, attributed to an optimal balance between edge and facet sites. In contrast, in the zero-gap cell, although 45 nm cubes were the most ethylene-selective, overall FE C2 was reduced and strongly influenced by operational parameters, such as anolyte composition. Long-term tests revealed a trade-off between catalyst durability and ethylene selectivity. These findings demonstrate the critical interplay between nanostructure, testing configuration, and electrolyte, and emphasize the need to assess catalyst performance under industrially-relevant conditions.