Macro‐ and Nano‐Porous Ag Electrodes Enable Selective and Stable Aqueous CO ₂ Reduction

Abstract Electrochemical carbon dioxide (CO ₂ ) reduction from aqueous solutions offers a promising strategy to overcome flooding and salt precipitation in gas diffusion electrodes used in gas‐phase CO ₂ electrolysis. However, liquid‐phase CO ₂ electrolysis often exhibits low CO ₂ reduction rates because of limited CO ₂ availability. Here, a macroporous Ag mesh is employed and activated to achieve selective CO ₂ 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 CO ₂ ‐to‐CO conversion. Notably, it is found that a combination of dissolved CO ₂ in bicarbonate solution with CO ₂ generated in situ from bicarbonate ions enables increased CO ₂ availability and a CO ₂ ‐to‐CO conversion rate over 100 mA cm ⁻² . By optimizing the oxidation/reduction cycles to fine‐tune the structure of Ag surface, CO ₂ ‐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.