Utilizing Mass Transport to Enhance CO₂ Conversion to Multi-Carbon Products

This thesis focusses on the use of mass transport to improve the selectivity and production rate of electrochemical CO₂ reduction to multi-carbon (C₂₊) products. In the first chapter, the effects of mass transport on CO₂ reduction are discussed. Briefly, mass transport has been used to improve the selectivity of CO₂ reduction by reducing the transport of proton sources to suppress the competing hydrogen evolution reaction and improve the selectivity towards C₂₊ products by keeping intermediates near the surface. In addition, gas diffusion electrodes (GDEs) have been designed to achieve high current densities by minimizing the diffusion length of CO₂ to the catalyst surface. In the second chapter, additive-derived films were studied as an approach to improve the selectivity towards CO₂ reduction when using oxygen containing feedstocks, such as flue gas. Oxygen is problematic as it introduces a competing side reaction, the oxygen reduction reaction. The films were able to increase the selectivity towards CO₂ reduction by suppressing the transport of oxygen and demonstrated high selectivity towards multi-carbon production when using a simulated flue-gas stream. In the third chapter, a methodology to prepare 3D-printed flow-through electrodes is developed. While GDE can achieve high current densities, they are susceptible to flooding and salt precipitation which can significantly hinder performance. As such, liquid flow-through electrodes are an attractive alternative electrode design to achieve high current densities. A 3D-printed flow-through electrode was fabricated using the methodology and achieved a higher current density compared to a flat plate electrode. The methodology developed in Chapter 3 opens the door to the fabrication of rationally designed flow-through electrodes. In the last chapter, two tandem systems were assembled to convert CO₂, water, and sunlight into a butene (C₄) and 2-methyl-2-pentenal (C₆). A limitation of electrochemical CO₂ reduction is that the selectivity towards products with 3-carbon chains (C₃₊) is relatively low. These devices used a two-step tandem process, where in the first step CO₂ is electrochemically reduced to the desired intermediate (ethylene, CO, and/or hydrogen) using a PV-driven electrochemical cell. The gas stream is fed to a photothermocatalytic reactor where the gaseous products are converted to butene (via ethylene dimerization) or 2-methyl-2-pentenal (via ethylene hydroformylation followed by aldol condensation). This chapter highlights the wide range of potential products that can be derived from CO₂ and the importance of co-designing of the thermochemical and electrochemical steps.