Covalent Organic Frameworks as Porous Solid Electrolytes for Electrochemical CO₂ Reduction

The electrochemical reduction of carbon dioxide (CO₂RR) to high-value chemicals offers a promising approach to storing renewable energy while helping to close the anthropogenic carbon loop. The use of porous solid electrolytes (PSE) makes CO₂RR more sustainable because they are recyclable and can produce pure liquid products free of electrolyte contaminants. However, current PSEs have several limitations including insufficient ion conduction and poor scalability. Here, sulfonated NUS-10 covalent organic framework (COF) foams are demonstrated as PSEs for efficient CO₂RR capable of producing valuable products including formic acid, carbon monoxide, and C₂₊ compounds. A straightforward, scalable microwave-assisted synthesis method is outlined for producing NUS-10 COF nanosheets, which can be freeze-dried to create NUS-10 proton exchange foams. These foams exhibit high proton conductivity (σ = 0.052 S cm⁻¹ at 30 °C) and can be easily recycled without mass loss across multiple device operations. When applied in PSE CO₂RR electrolyzers, NUS-10 COF exhibites exceptional electrochemical efficiency, requiring less than 4 V of cell voltage at 200 mA cm⁻² current density to produce high-purity formic acid. This study advances the development of COF PSE for CO₂RR, offering a path toward more sustainable and practical applications of CO₂RR.