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Electrocatalytic CO2 conversion in flow reactors Salvatore, Danielle

Abstract

Electrochemical reduction of CO₂ to value-added liquid fuels and chemical feedstocks is a sustainable approach to off-peak electricity utilization. The propagation of a CO₂ value chain through capture-conversion technology is bottlenecked by a lack of systems capable of catalytic CO₂ conversion with the efficiency, selectivity, robustness, and economic viability relevant to industry. The design of a CO₂ electrolyzer that can operate at current densities (J) > 200 mA/cm² , Faradaic efficiencies (FE) > 85%, voltages < 3 V is germane to this challenge. Improving the efficiencies, selectivities and current densities of CO₂ electrocatalytic systems are of scientific, environmental, and economic importance. In this thesis, I first demonstrate the development of a flow reactor that utilizes gas-phase CO₂ to achieve higher current densities (J = 200 mA/cm²) than is possible with conventional aqueous-fed systems. I use a silver catalyst dispersed on a gas diffusion layer to produce CO with high selectivity ( > 50%). I deploy a bipolar membrane which allows the use of non-corrosive conditions on either electrode, thereby prolonging cell lifetime. I then detail the design and use of an analytical device to compare the voltages for different CO₂ flow reactor architectures to identify which cell components should be optimized to most effectively lower the overall cell voltage. Analysis of the voltages across the components of three different flow reactor configurations highlights that the reactions at the anodes and cathodes are relatively efficient and that much of the voltage loss occurs at the membran. These results illuminate that a better understanding of membranes and the membrane-catalyst interface is needed. Finally, I demonstrate the incorporation of a molecular catalyst into a CO₂ flow reactor for efficient conversion of CO₂ to CO. Molecular catalysts are known to have high activity for the CO₂ reduction reaction but typically at low rates of conversion (i.e., < 30 mA/cm²). In this work, I show that a widely available molecular catalyst, cobalt phlalocyanine, can mediate CO₂ to CO formation in a flow reactor with high conversion rates commensurate with solid-state metal catalysts (ie., > 150 mA/cm² and FE > 85%).

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