High selectivity of CO2 conversion to formate by porous copper hollow fiber: Microstructure and pressure effects

Highlights: • Copper hollow fiber with interconnected pore structure was fabricated via a controllable method. • A high selectivity for CO₂ reduction to formate with a maximum FE of 77.1% was achieved at −34.7 mA cm⁻². • The high pressure induced unique crystal facets attributes to the superior electrocatalytic activity and selectivity. -- Abstract: Electrochemical reduction of CO₂ by Cu hollow fibers to CO with high selectivity has previously been reported but selective conversion of CO₂ to formic acid at high current densities, although highly desirable, is still challenging. Herein, a Cu hollow fiber with an interconnected pore structure is fabricated via a facile method and used as a stand-alone cathode for highly efficient electrochemical reduction of CO₂ to formate. We obtain a high selectivity for CO₂ reduction to formate with a maximum FE of 77.1% at a high current density of 34.7 mA cm⁻², one of the highest FE on Cu-based materials. Our results suggest that delivering the CO₂ gas into the inner space of the hollow fiber leads to a higher CO₂ partial pressure in the pores due to the pressure drop across the wall of the Cu hollow fiber. As both the CO₂ and H⁺ ions (from the electrolyte) compete for adsorption on the Cu hollow fiber active sites, the higher CO₂ partial pressure makes CO₂ adsorption more favorable, thereby reducing the concentration of the H⁺ on the active sites. This effectively suppresses the major competing reaction, hydrogen evolution reaction (HER), from 46.9% Faradaic efficiency (FE) to 15.0%. Furthermore, our studies reveals the impotency of the co-existence of Cu(100) and Cu(110) active site facets towards excellent selectivity of formate formation under high CO₂ partial pressure. Additionally, the designed catalyst also exhibits out-standing long-term stability at high current density, demonstrating potential for large-scale practical applications.