By switching the electrolyte employed in electrochemical CO2 reduction, researchers have gone on to discover a potentially game-changing finding; one that may explain how product selectivity—the formation of 1 or very few products—can be influenced. There are a lot of other options for how we can customize the process to produce useful molecules from CO2 now.
The Versatile MOF Catalyst
To achieve this, scientists have created a new type of metal-organic framework (MOF) catalyst called FICN-8, which has a broad substrate that can be made to generate many specific products relating to CO2 reduction. FICN-8 was prepared in 3D porous structure based on Cu(porphyrin)-ligands and one-to-three direct link framework with Cu(pyrazolate) building units to exhibit highly accessible of the metalloporphyrin catalytic sites.
Moreover, FICN-8 displayed excellent electroactivity toward the electrochemical reduction of CO2 when operated as a type of heterogeneous electrocatalyst. What was more interesting was that the team realised they could manipulate this selectivity of products by just changing what other elements were in their water-based electrolyte. When applied for electrocatalytic CO2 reduction in a tetrabutylammonium hexafluorophosphate (TBAPF6)/acetonitrile (MeCN) electrolyte, FICN-8 exhibited excellent selectivity to formate CO with the highest Faradic efficiency of ≈95 % at the potential of −1.OFFSET V/S RHE. On the other hand, as a proton source in water or trifluoroethanol (TFE) was added, gradually the major CO2 reduction product turned from CO to formic acid with the highest Faradaic efficiency of formate production up to 48% at 2.65 mol/L water or 0.55 mol/L TFE.
Mechanistic Insights from Reaction Coordination
To understand the causal mechanisms driving this interestingly unpredictable selectivity switch, however, researchers conducted a set of designed experiments. Important clues were gleaned from kinetic isotope effect (KIE) measurements.
In the synthesis of CO, a near-unity KIE demonstrated that C–H bond cleavage was not involved along the reaction pathway. Formic acid production, on the other hand showed a much higher KIE value of 3.7±0.7 confirming that protons are involved in the reaction pathway directly.
The findings were supported theoretically by thermodynamic calculations, which pointed to reductive adsorption of either chloride or hydride (*H) being promoted and forming a dimu-hydrido complex on the N site of the central porphyrin unit as an essential step for formic acid formation. Given this, the production of CO follows an alternate pathway that is not related to the proton concentration.
These results indicate which only by considering the joint effects of catalysis and electrolyte composition, can rational, structure-property-based design of a selective CO2 reduction process truly be achieved. These findings allow to control useful product formation more efficiently and selectively in CO2 utilization in the future, by adjusting only the electrolyte.
Conclusion
Professor Chan’s research team has made a radical discovery that demonstrates the profound effect electrolyte composition can have on controlling reaction selectivity in CO2 reduction. The researchers managed to steer the CO2 reduction process toward selective generation of valuable chemicals (CO, formic acid) by using the versatile FICN-8 MOF catalyst and systematically tuning the electrolyte. Because of this discovery, new possibilities have been found in the work for designing improved catalyst-electrolyte systems for CO2 utilization, and one with more sustainability and economic feasibility.
