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Lees, Eric William

University of British Columbia

The CO₂ reduction reaction (CO2RR) is a means of producing chemicals and fuels using waste CO₂ instead of fossil fuels. Carbon monoxide (CO) is an appealing target product because it can be converted to carbon-neutral diesel and methanol using established industrial chemistry. However, a source of purified gaseous CO₂ is often a prerequisite to forming CO at high rates in a low-temperature electrochemical reactor (“electrolyser”). This feedstock of CO₂ for the electrolyser is typically produced using >100 kJ mol⁻¹ thermal energy from fossil fuel combustion. In this thesis, I show that liquid bicarbonate solutions produced by reactive carbon capture can be converted into CO without the need to thermally isolate gaseous CO₂ upstream of the electrolyser. In this thesis, I first investigate the conversion of aqueous bicarbonate (HCO₃–(aq)) solutions into CO in an electrolyser composed of nickel anode, bipolar membrane, and silver cathode. Before this work, it was not known whether liquid bicarbonate solutions could be converted into CO at product formation rates (current densities) greater than 100 mA cm⁻². Here, I demonstrate an electrolyser design that reacts protons with bicarbonate to form CO₂ in situ at the membrane|cathode interface in the electrolyser. This CO₂ intermediate is reduced to the CO product at the silver cathode surface. I then demonstrate silver electrodes that enable electrolysis of bicarbonate solutions into CO at product formation rates that rival gas-fed CO₂ electrolysers. While hydrophobic cathodes yield high CO formation rates in gas-fed CO₂ electrolysers, my experiments show that hydrophobic electrodes are not suitable for bicarbonate electrolysers, which use liquid feedstocks. I demonstrate that hydrophilic silver cathodes enable faradaic efficiencies for CO formation of 82% at 100 mA cm⁻², which is higher than the 37% achieved with the hydrophobic cathodes. I then developed a physical model that simulates the acid-base and electrochemical reactions in the cathode of the bicarbonate electrolyser. The model is validated with experimental results and is used to elucidate the rate limiting step in the bicarbonate electrolyser. This model confirmed my experimental results which showed that the rate of in-situ CO₂ formation governs the rate of CO formation.

Text

2022-08-24

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/82510

Chemical and Biological Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/