Macro- and Nano-Porous Ag Electrodes Enable Selective and Stable Aqueous CO₂ Reduction

Electrochemical carbon dioxide (CO₂) reduction from aqueous solutions offers a promising strategy to overcome flooding and salt precipitation in gas diffusion electrodes used in gas-phase CO₂ electrolysis. However, liquid-phase CO₂ electrolysis often exhibits low CO₂ reduction rates because of limited CO₂ availability. Here, a macroporous Ag mesh is employed and activated to achieve selective CO₂ conversion to CO with high rates from an aqueous bicarbonate solution. It is found that activation of Ag surface using oxidation/reduction cycles produces nanoporous surfaces that favor CO₂-to-CO conversion. Notably, it is found that a combination of dissolved CO₂ in bicarbonate solution with CO₂ generated in situ from bicarbonate ions enables increased CO₂ availability and a CO₂-to-CO conversion rate over 100 mA cm^-2. By optimizing the oxidation/reduction cycles to fine-tune the structure of Ag surface, CO₂-to-CO conversion is reported from a bicarbonate solution with CO Faradaic efficiency of over 85% at current density of 100 mA cm^-2, high concentration of 24.7% at outlet gas stream and stability of over 100 h with maintaining CO FE over 85% during whole reaction time.