Here, we detail how the catalytic behavior of immobilized molecular electrocatalysts for the CO 2 reduction reaction (CO 2 RR) can be impacted by catalyst aggregation. Operando Raman spectroscopy was used to study the CO 2 RR mediated by a layer of cobalt phthalocyanine (CoPc) immobilized on the cathode of an electrochemical flow reactor. We demonstrate that during electrolysis, the oxidation state of CoPc in the catalyst layer is dependent upon the degree of catalyst aggregation. Our data indicate that immobilized molecular catalysts must be dispersed on conductive supports to mitigate the formation of aggregates and produce meaningful performance data. We leveraged insights from this mechanistic study to engineer an improved CO-forming immobilized molecular catalyst�cobalt octaethoxyphthalocyanine (EtO 8 −CoPc)�that exhibited high selectivity (FE CO ≥ 95%), high partial current density (J CO ≥ 300 mA/cm 2 ), and high durability (ΔFE CO < 0.1%/h at 150 mA/cm 2 ) in a flow cell. This work demonstrates how to accurately identify CO 2 RR active species of molecular catalysts using operando Raman spectroscopy and how to use this information to implement improved molecular electrocatalysts into flow cells. This work also shows that the active site of CoPc during CO 2 RR catalysis in a flow cell is the metal center.
Electrolyzers that electrochemically convert aqueous (bi)carbonate solutions (solutions containing captured CO2, or “reactive carbon solutions”) into commodity chemicals couple CO2 capture with CO2 conversion. Industrial exhaust streams contain nitrogen oxides (NO x ) and sulfur oxides (SO x ) that form redox-active anions in reactive carbon solutions that can interfere with downstream CO2 reduction. We therefore designed experiments to test how impurities produced from the dissolution of NO x (NO2 – and NO3 –) and SO x (SO3 2– and SO4 2–) impact the electrochemical conversion of (bi)carbonate to CO. We found that CO production was unaffected by SO x compounds in a 3.0 M KHCO3 feedstock, but 2000 ppm of NO x impurities decreased CO selectivity from ∼60% to <5%. This decrease was caused by the preferential reduction of NO2 – and NO3 – over CO2. Our study establishes tolerance limits for common flue gas impurities in reactive carbon solutions and provides strategies to mitigate toxification effects.
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