Electrochemical CO₂ reduction (CO₂RR) offers a promising route for converting greenhouse gas emissions into valuable chemicals. Among the catalysts investigated for CO₂RR, noble metals such exhibit excellent selectivity toward CO, but their high cost and limited availability hinder large-scale use. Zinc oxide (ZnO) has emerged as an attractive earth-abundant alternative due to its low cost and environmental friendliness; however, its relatively low surface area limits its CO₂ adsorption capacity. ¹. Metal-organic frameworks (MOFs) are promising materials for overcoming this limitation due to their high porosity and tunable surface chemistry. In particular, ZIF-8 exhibits high CO₂ adsorption capacity and has shown promising selectivity toward CO formation. ²

Mechanochemical synthesis, in contras with conventional solvothermal approaches,  provides a sustainable route for different MOF production by drastically reducing solvent consumption and enabling the direct use of poorly soluble metal precursors. ³ In the case of ZIF-8, mechanochemistry allow for the use of ZnO as the zinc source and its direct conversion into the ZIF-8 phase. ^4–6 Unlike conventional ZnO/ZIF-8-derived catalysts, where ZnO is typically generated by calcination of a pre-synthesized ZIF-8 sacrificial template, the mechanochemical approach enables a simple and controllable one-step conversion of ZnO into ZIF-8, providing direct control over the ZnO/ZIF-8 ratio and, consequently, the catalytic properties of the resulting materials.

Here, ZnO/ZIF-8 composites were synthesized via mechanochemistry using different grinding times (1.5 to 12 h) and optimized liquid-assisted grinding conditions, requiring only 1 μL of methanol per mg of reagent. The mechanochemical route enabled precise control over the extent of ZnO-to-ZIF-8 conversion, yielding materials containing between 58.1% (1.5 h) and 99.4% (12 h) ZIF-8, as determined by thermogravimetric analysis. The fully converted sample showed no signs of amorphization and presented a specific surface area of 1704 m² g⁻¹ (BET) and a CO₂ uptake of 1.44 mmol g⁻¹ at 273 K and 100 kPa. The resulting materials were evaluated as gas-diffusion electrodes in a membrane electrode assembly (MEA) electrolyser operating at technologically relevant current densities. While higher ZIF-8 contents enhanced CO₂ adsorption, the best CO₂RR performance was obtained for partially converted materials, demonstrating that an optimal balance between the CO₂-concentrating capability of ZIF-8 and the electrical conductivity provided by residual ZnO is required to maximize catalytic activity and CO selectivity. The optimized catalyst containing 58% ZIF-8 achieved 69% FE_CO at 250 mA cm⁻², outperforming both pristine ZnO and nearly fully converted ZIF-8-rich materials.