Revisiting the Impact of Morphology and Oxidation State of Cu on CO₂ Reduction Using Electrochemical Flow Cell

Electroreduction of carbon dioxide (CO₂) in a flow electrolyzer represents a promising carbon-neutral technology with efficient production of valuable chemicals. In this work, the catalytic performance of polycrystalline copper (Cu), Cu₂O-derived copper (O(I)D-Cu), and CuO-derived copper (O(II)D-Cu) toward CO₂ reduction is unraveled in a custom-designed flow cell. A peak Faradaic efficiency of >70% and a production rate of ca. -250 mA cm^-2 toward C₂₊ products have been achieved on all the catalysts. In contrast to previous studies that reported a propensity for C₂₊ products on OD-Cu in conventional H-cells, the selectivity and activity of ethylene-dominated C₂₊ products are quite similar on the three types of catalysts at the same current density in our flow reactor. Our analysis also reveals current density to be a critical factor determining the C-C coupling in a flow cell, regardless of Cu catalyst's initial oxidation state and morphology.