Electrochemical reduction of CO2 has been long viewed as a promising approach to utilize CO2. Selective generation of a certain product is an important topic, but most effort on controlling the product selectivity has been devoted to catalyst design and modification. The impact of electrolyte composition is less studied.

In a study published in Angewandte Chemie Internation Edition, Prof. CAO Rong, Prof. ZHANG Teng, and colleagues from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences, reported a metal-organic framework (MOF) electrocatalyst that exhibits CO2 reduction product selectivity depending on the composition of electrolytes, providing a possible approach to control and tune reaction selectivity towards value-added CO2 reduction products.

Researchers constructed the MOF catalyst, namely FICN-8, from Cu(porphyrin)-derived ligands and Cu(pyrazolate) building units. FICN-8 has a three dimensional porous structure that ensures substrate-accessibility of the metalloporphyrin catalytic sites. It is used as a heterogenous electrocatalyst which allows systematic solvent/electrolyte composition tuning in a wide range.

Electrochemical tests revealed that FICN-8 has a high activity for electrochemical CO2 reduction. In tetrabutylammonium hexafluorophosphate (TBAPF6)/acetonitrile (MeCN) electrolyte, FICN-8 catalyzed the reduction of CO2 to CO with a high selectivity up to 95%. When water or trifluoroethanol (TFE) was added to the electrolyte as a proton source, the major CO2 reduction product gradually switched from CO to formic acid. The highest observed Faradaic efficiency for formic acid production reached 48% with 2.65 mol/L water or 0.55 mol/L TFE addition.

Besides, researchers designed a series of experiments to find out the reaction mechanism that accounts for this selectivity switching. Kinetic isotope effect (KIE) measurements gave a near-identity kH/kD KIE value for CO production and a large KIE value of 3.7±0.7 for formic acid production, suggesting direct involvement of proton in the reaction path of HCOOH formation. Theoretical calculations identified reductive adsorption of hydride(*H) on the porphyrin N site as the crucial step for HCOOH formation, while production of CO follows another route that is independent on proton concentration.

This study highlights the importance of electrolyte composition on product selectivity of electrochemical CO2 reduction, and opens up the possibility of constructing novel catalyst-electrolyte systems for value-added CO2 reduction products.