Open Collections

Mowbray, Benjamin

University of British Columbia

Electrochemical CO₂ reduction is a means of using captured CO₂, water, and electricity to produce carbon-based chemicals and fuels. The CO₂ reduction reaction (CO2RR) provides access to a range of products including carbon monoxide (CO), which can be further upgraded into methanol and diesel. The CO2RR therefore provides a path to generating liquid fuels from captured CO₂, provided that electrochemical reactors (electrolysers) capable of efficiently reducing CO₂ into CO at high rates can be developed. In this thesis, I demonstrate how to design materials for electrolysers that efficiently reduce CO₂ into CO. This thesis first describes how to prepare anion-exchange membranes that attenuate water transport in CO₂(g)-fed electrolysers. Prior to this work, CO₂(g)-fed electrolysers suffered from poor durability due to excess water accumulating in the cathode. Here, I demonstrate that water transport to the cathode can be attenuated by increasing the water sorption or decreasing the thickness of membranes. These modifications enabled a 37% increase in CO formation and 450 mV decrease in cell voltage at 200 mA cmˉ² relative to a reference electrolyser. I then show how cathode fabrication methods affect the efficiency of CO₂(g)-fed electrolysers. CO₂(g)-fed electrolyser cathodes are commonly prepared by depositing dispersions of catalysts and polymeric binders (ionomers) onto porous substrates. This work resolves how the dispersion solvent modulates the surface area, hydrophobicity, and wettability of catalyst layers, which modulate CO2RR efficiency in electrolysers. My results show that the dispersion solvent identity can cause > 50% deviations in the faradaic efficiency for CO at 200 mA cmˉ² between compositionally identical electrodes. I then investigate how to increase the CO2RR activity of Ag cathodes for KHCO₃(aq)-fed electrolysers. The use of KHCO₃(aq) feedstocks bypasses energy intensive CO₂(g) separation steps, but also increases H₂O reduction activity. Here, I show that CO2RR activity can be selectively increased by functionalizing the Ag cathode surface with methylimidazolium. This functionalization increased CO₂RR activity by 10-fold in an electrochemical cell, and increased CO formation by 30% in a KHCO₃(aq)-fed electrolyser. I used operando Raman spectroscopy to show that methylimidazolium adsorbs to the cathode during electrolysis in an orientation which may stabilize intermediates.

Text

2023-01-03

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/83549

Chemistry

University of British Columbia

UBCV

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