Elucidating reaction pathways of the CO₂ electroreduction via tailorable tortuosities and oxidation states of Cu nanostructures

Copper-based 3D fractal nanostructures are integrated on the electrodes using a scalable and ink-free flame aerosol synthesis technique for electrochemical CO₂ reduction. The effects of tortuosity and oxidation state of copper are respectively investigated by isolating each effect from the others. By balancing the intermediate confinement and local availability of CO₂, CuO-derived Cu with optimal tortuosity exhibits a Faradaic efficiency of 65% toward C₂₊ products at an applied potential of −1.04 V versus reversible hydrogen electrode. A subsequent study of the effects of the oxidation state, which is free from the influence of tortuosity, reveals that Cu²⁺-derived Cu demonstrates suppressed hydrogen evolution reaction and a higher C₂₊/CH₄ ratio than metallic Cu. The preference for the formation of both ethanol and n-propanol versus ethylene, is found to follow the trend from metallic Cu > Cu²⁺-derived Cu > Cu⁺-derived Cu toward alcohols’ formation. These findings elucidate the underlying causes for the effects of tortuosity of porous Cu electrodes on selectivity and provide insights into the specific effects of the initial oxidation state on various reaction pathways during electrochemical CO₂ reduction.