Heavy is the Crown: Crown Ether Modulation of Cobalt Porphyrin CO2 Electroreduction in Zero-Gap Electrolyzers

Since decades, metalloporphyrins have been studied to catalyze the electrochemical CO2 reduction (eCO2R) with the most recent studies focusing on immobilized complexes aiming for heterogeneous, scalable catalysis. However, reports for the application in industrially relevant zero-gap type electrolyzer cells (ZGEs) are especially rare. Herein we present the synthesis of four novel crown ether (CE) substituted cobalt porphyrins to benefit from an increased local cation concentration. Following their electrochemical characterization all catalysts have been tested in ZGEs. Experiments under laboratory-scale conditions (≤100 mA/cm2) revealed that the positioning of the CE influences the catalytic performance in terms of Faradaic Efficiency for CO (FECO) as well as cell voltage. A maximum selectivity for CO of 96% at 100 mA/cm2 is reached, ranking the ortho substituted complex among the best state of the art systems. Post-mortem analysis of the prepared electrodes proved that the introduction of CEs enhances the complex stability significantly. At higher current densities (≤500 mA/cm2) the positioning of the CEs is less impactful. Instead, the type and concentration of cations in the reactor play a dominant role determining reaction performance, achieving up to 43% FECO at 300 mA/cm2 with a high potassium concentration.