Electropolymerized Cobalt Porphyrin Films with Fine-Tuned Molecular Hydrophobicity for Boosted Electrocatalytic CO2 Reduction

Catalyst hydrophobicity plays critical roles in electrocatalytic CO2 reduction reaction (CO2RR). Regulating catalyst intrinsic hydrophobicity at the molecular level is required but challenging. Herein, we report on electropolymerized films with distinct hydrophobicity for electrocatalytic CO2RR. By altering the linkage position of four meso-9-phenylcarbazole groups, we synthesized two Co porphyrin isomers Co-1 and Co-2. Theoretical studies showed that both isomers have nearly identical electronic structure, but Co-1 is more hydrophobic than Co-2. Electropolymerized films EPCo-1 and EPCo-2 are both active for CO2RR, but EPCo-1 reveals higher efficiency and selectivity than EPCo-2 for the CO2-to-CO conversion. In flow cells, the mass activity for the CO formation with EPCo-1 (iCO = 2.31 × 105 A/gCo, FECO = 99%) is two times larger than that with EPCo-2 (iCO = 1.20 × 105 A/gCo, FECO = 65%) at −1.20 V versus RHE. Significantly, a zero-gap membrane electrode assembly (MEA) electrolyzer using EPCo-1 operated stably at 250 mA (50 mA/cm2) for 75 h with 96% CO selectivity, representing an example of robust MEA electrolyzers using electropolymerized films. Mechanistic studies revealed that compared to EPCo-2, more hydrophobic EPCo-1 preserved more stable hydrogen-bonded water networks at the interface under large cathodic potentials, which effectively suppresses competing hydrogen evolution.