Carbon dioxide has been in the spotlight in the last decades due to its high concentration in the Earth's atmosphere. Fossil fuels and natural gas combustion companies are in the public eye, but humanity is still dependable of them and for that reason a solution needs to be found to reduce their emissions. Therefore, new methodologies to capture and convert CO₂ to produce value chemicals are required.

Electrochemical reduction of carbon dioxide (CO₂ER) can combine the processes of capturing CO₂ while also converting it into a value-added product like carbon monoxide, a commonly used petrochemical combustible for electricity generation together with hydrogen molecules. Commercialization of this technology relies on developing highly selective and efficient catalysts operating at a high current density. Operational voltage is also an important performance factor because it defines the electrolyser’s efficiency. At industrial scale it is preferable to operate below 3 V.

In this research, high Faradaic Efficiencies (FE_CO) and current densities were reached applying a voltage of 3 V. In order to accomplish this research, several factors were under study as a way of reaching the most efficient set-up such as the electrolytes, cell type, membranes, substrate loading, gases diffusion layer, etc. By applying a membrane electrode assembly (MEA) as the electrolyzer, wherein a porous metal organic framework (MOF) serves as CO₂ concentrator and as a catalyst which electrochemically converts CO₂ into syngas products altogether with K₂CO₃ as electrolyte. The MOF, which is supported in a carbon fibre (CF) sheet that acts as a platform to conduct the current, facilitates the CO₂ capture and its electrochemical reduction from wet stream.

The capture material is a porous Zn-based MOF called CALF20, which is used to capture CO₂ from industrial combustion flue gases in a cement-making company Svante (Vancouver) capturing 1t-CO₂/day. CALF20 has shown a high capacity and selectivity towards CO₂ gas adsorption with the presence of other gases even at ambient temperature and under humid conditions (Figure 1a); being the first MOF to be scaled up (made in ton quantities) and used in the field. The significant capacity of CALF20 can be represented by its 528 m²/g Langmuir surface area.

Spray coating technique was used to spray CALF20 onto the CF sheets. The spray coating process took place after mixing CALF20 particles in Methanol/Nafion solution to maintain a certain level of rigidness and stability on the substrate surface.

The hydrophobicity of the material was tested employing wet CO₂ flows where a high relative humidity (RH) was applied resulting in the increment of H₂ production due to the competitivity of H₂O with CO₂ molecules on CALF20 pores. Many MOF structures get saturated with H₂O with low humidity values however, CALF20 simulations showed that adsorption kinetics are unaffected by water even at 40% RH which illustrates the exceptional nature of this material. This degree of water resistance in the CO₂ adsorption of CALF20 has currently not been noticed in other MOF. CALF20 rigid structure and pore size match CO₂ dimensions better than any other gases, especially at the CO₂RR conditions, room temperature and about atmospheric pressure.

The initial results showed high conversion efficiency with the highest CO partial current density of 100 mA/cm² for CALF20. CALF20 showed the highest reported FE for a Zn-based MOFs with a 60% of CO production at 3 V in a MEA-type electrolyser with 1 M K₂CO₃ electrolytes passing through the cell.

This CALF20/CF catalyst produced CO as a main product with a FE_CO of 60% together with H₂ generation making an entire syngas production at a high partial current density of 100 mA/cm².

SEM images in Figure 1(b-e) show the carbon fibre sheet sides before and after the electrochemical reduction of CO₂ in the MEA cell, displaying the nanostructured material on the CF sheets.

Therefore, the beforementioned device uses carbon fibre sheets as a platform to conduct current and support an integrated CO₂ capture and electrochemical reduction reaction. It uses a scale-up synthesis MOF-based catalyst that acts to concentrate and permeate CO₂ through that layer, while catalyse the mentioned reaction into CO gas with high current and stability. More optimizations are being performed in order to enhance the aforementioned features of this technology.

In conclusion, CALF20 is presented as the first MOF in being scaled up for CO₂ capture and also acting as an electrocatalyst for the reduction of this gas. CALF20 is a promising material with an inexpensive and easy synthesis that acts as concentrator and also catalyst for CO₂ conversion.