Electrochemical reduction of carbon dioxide using gold cathodes
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Electrochemical reduction of carbon dioxide (CO2) is a promising means of storing energy from intermittent electrical resources (like solar and wind) in chemical forms. To be able to make this process feasible, selective catalysts with low overpotential are needed. The present study is concerned with the electrochemical CO2 reduction at ambient conditions in an aqueous electrolyte.
The majority of this study focuses on electrochemical CO2 reduction on gold cathodes (polycrystalline gold, gold on carbon paper and gold clusters). The thesis begins with the electrochemical CO2 conversion to CO using a custom electrolysis H-cell, which is coupled to a Gas Chromatography (GC) for the gaseous product detection. The gaseous products analysis confirmed that H2 and CO were the major products of the reaction. Then, the general aspects of electrochemical CO2 reduction experiment were investigated. The next step was designing the electrochemical flow cell that significantly increases Faradaic efficiency of the reaction toward CO (15-45%). During these attempts, we aimed to study different polycrystalline gold surfaces as catalyst for CO2 electroreduction. This enabled us to investigate the effect of surface morphology on the gold catalytic activity and selectivity. Our results suggest that the rougher gold surface favours CO production over unwanted hydrogen evolution reaction.
The next step was performing prolong galvanostatic CO2 reduction experiment with stable activity and selectivity by using periodic cyclic voltammetry, open circuit potential or short anodic pulses . In contrast with the previous literature, our results suggest that no gold oxide would form by these cleaning steps and a constant catalytic activity and selectivity can be maintained over long-term electrolysis by removing the impurities like Fe, Zn and Cu sources from the electrolyte. The results obtained in this part, cover an important gap in the literature and provide useful information for many researchers who are trying to understand the mechanism of electrochemical CO2 reduction during deactivation.
The sensitivity of CO2 reduction reaction on cell hydrodynamics at various electrolyte solutions has been investigated using a flow cell set-up. Our observations show that the Faradaic efficiency for CO production is around 90% regardless of the electrolyte concentration whereas in the custom H-cell, the Faradaic efficiency for CO decreases from 75% to 35% when the electrolyte concentration is increased from 0.1 mol L-1 to 0.5 mol L-1.
Lastly, the electrocatalytic CO2 reduction was performed on different gold clusters (Au6, Au9, Au13 and Au101) adsorbed onto carbon paper substrates. These clusters showed very high Faradaic efficiency toward CO (around 90%) at -1.5 V vs Ag|AgCl using relatively low gold loadings (90 μg cm-2). Also, Au13 was found to show the highest selectivity for CO production amongst other clusters. The understanding of the gold clusters behaviour studied experimentally in this thesis, could help to augment the activity and selectivity of the electrochemical CO2 reduction.