Investigations into Electrochemical CO2 Reduction Catalysts via Gas Diffusion Electrodes

Abstract

The advancement of carbon dioxide (CO2) electroreduction technology is crucial for address ing climate change by transforming CO2 into valuable chemicals and fuels. eCO2RR offers a route to decarbonise energy requirements for industries where energy-dense molecules are typically used to provide energy, such as aviation and domestic heating. However, developing catalysts that afford selectivity towards multi-carbon products is still the focus of much re search. With longer reduction pathways requiring more electrons, multi-carbon products are still hard to produce at significant rates. Furthermore, degradation of the gas diffusion elec trode is an issue under the electrochemical conditions for CO2 electroreduction. Flooding of the gas diffusion electrode reduces the availability of CO2 at the ideal triple phase reaction centres and hence the selectivity towards carbonaceous products reduces and hydrogen evo lution (HER) increases. This thesis presents research on the optimization of gas diffusion elec trodes for CO2 electroreduction, focusing on the effects of bimetallic catalyst selectivity and hydrophobicity. The hypothesis of this work was the addition of gold with the Cu2O catalyst would have a higher faradaic efficiency for multi-carbon products. The catalytic performance of Au-Cu2O bimetallic catalysts in gas diffusion electrodes was evaluated and it showed enhanced selec tivity for CO2 reduction to ethanol and propanol when compared to the Cu2O catalyst. The research found that the presence of gold in the catalyst composition significantly influences the reaction pathway by facilitating CO insertion and dimerization, leading to a higher selec tivity for C2+ oxygenates over ethylene. Chapter 4 presents the work on Au decorated Cu2O microparticles via electrodeposition were observed to produce higher faradaic efficiency of longer chain alcohols compared to Cu2O microparticles, however the overall rate of production of carbonaceous products decreased. The rate of ethanol and propanol production increased by 2 and 4.5 nmol s-1 cm-2 respectively with the AuCu2O compared to Cu2O. Overall the inclusion of gold in the catalytic layer resulted in higher *CO concentration which favoured high length carbon products. Faradaic efficiency of hydrogen was marginally lower at 16% faradaic efficiency with AuCu2O with respect to Cu2O. In exploring the role of hydrophobicity, the impact of modifying gas diffusion layers with hy drophobic additives was assessed. Nanoporous carbon particles, functionalized with polyfluoroaromatic compounds, were employed aiming to enhance CO2 diffusion whilst main taining hydrophobicity. The hypothesis of this work was that the use of polyfluorophenyl car bons as an additive in the catalyst layer would prevent flooding in the gas diffusion electrode. Initial results indicate that these hydrophobic substrates improve the faradaic efficiency for carbon monoxide production with Ag catalysts. However, the durability of hydrophobicity un der electrochemical conditions poses a challenge, as the effectiveness of these additives de creases over time, affecting the overall stability and efficiency of the CO2 reduction process. The hydrophobicity of nanoporous carbon particles functionalised with fluorophenyl mole cules was measured with capacitance in different potential windows and compared to na noporous carbons absent of functionalisation. It was found that the capacitance per mass was 2.69 F/g of PhF5CIC with respect to a capacitance per mass of 10.92 F/g of CIC. When a more negative potential window was applied, the capacitance of PhF5CIC per mass increased to 6.8 F/g for the PhF5CIC, providing evidence for electrowetting of PhF5CIC. PhF5CIC was utilised as a hydrophobic additive in the catalyst layer with Ag particles and the ratio of CO/H2 was measured. At 5 min, the ratio of CO/H2 faradaic efficiency was 50, showing preference of CO2 electroreduction with respect to HER. However, under longer electrochem ical conditions, the CO/H2 faradaic efficiency ratio lowered to 3 at 22 min, indicating electro lyte intrusion. This is in agreement with the capacitance studies conducted to measure hydro phobicity, whereby hydrophobicity of PhF5CIC decreases significantly under electrochemical conditions.