The detrimental consequences of global warming and climate change continue to accumulate and manifest through increased frequency of extreme weather events, increased mean temperatures, and rising sea levels. Global warming can be predominantly attributed to the evergrowing concentration of greenhouse gases in our atmosphere, which is a result of the combustion of fossil fuels. To decrease the atmospheric greenhouse gas concentration, energy use must not only be switched to renewable means such as wind, hydro, solar, and geothermal, but CO2 must also be captured and stored. The electrochemical CO2 reduction reaction (eCO2RR) has the potential to remove carbon from the atmosphere by utilising renewable energy, while producing value-added products for the chemical processing industry, such as carbon monoxide, formate, ethanol, acetate, and ethylene.

While the eCO2RR has been studied with interest since the mid-1980s, the process tends to lack the efficiency, selectivity, activity, stability or a combination of these (depending on the catalyst of interest) for it to be commercially relevant. The work presented in this thesis focuses in particular on the eCO2RR using copper-based electrodes. Copper is unique as it is the only monometallic catalyst that is capable of reducing CO2 to products requiring more than two electrons, such as ethylene and ethanol, with reasonable faradaic efficiencies. Copper also has the benefit of being relatively active for the eCO2RR, however, it has poor stability and selectivity for a specific product, and tends to require high overpotentials, decreasing its energy efficiency. The literature review also uncovered some more specific areas of interest that are addressed in this thesis.

Firstly, the literature review highlighted significant discrepancies in the selectivity of the eCO2RR on copper electrodes. It was proposed that these discrepancies could be due to several factors, such as the incorrect, or lack of use of iR compensation, variations in surface pretreatments of the copper, and variations in the metallurgical properties of the copper, such as grain size and grain orientation. By examining 72 random eCO2RR publications, it was found that 63% used no form of iR compensation (or at least they did not report that they used it). Only 22% publications used full iR compensation through either current interrupt or positive feedback correction coupled with post-experiment corrections. It was then shown that by using only partial, or no iR compensation, the actual electrode potential can be hundreds of millivolts from the desired electrode potential, resulting in significant current and selectivity variations. A novel piece of software was also written here to allow 100% iR compensation while using 80% PF correction, which regularly checks the actual electrode potential to update the applied potential until the actual electrode potential is the desired potential, once iR losses have been accounted for. Additionally, it was found that the method of surface pretreatment of the polycrystalline copper surface also had an impact on the activity and selectivity. While the electropolished copper had the lowest activity, it had significantly higher selectivity for CO compared to the mechanically roughened and polished electrodes, achieving up to 36.2% CO faradaic efficiency. As for the mechanically roughened sample, it was able to achieve up to 15% faradaic efficiency for ethylene, while the polished and electropolished samples achieved less than 2% over a wide potential range. This was suspected to be due to the variation in the specific surface area, distribution of exposed crystal facets, grain boundaries, defects, and undercoordinated sites.

The effect of increasing interfacial pH favouring C2+ formation is a commonly discussed phenomenon in the literature, particularly in H-cell configurations. However, the effect of interfacial pH in a flowcell is something that has had little attention. It was found that by adjusting the electrolyte flowrate as well as the electrode length, the selectivity and activity for the eCO2RR could be optimised, and different selectivity regimes could be achieved. By maximising the electrode length, the mean interfacial pH increases due to the formation of OH− ions as a by-product of the eCO2RR. It could be thought that low electrolyte flowrates would be beneficial as this would increase the interfacial pH, however, this also resulted in decreased CO2 concentration at the electrode surface due to mass transfer limitations. It was found that electrolyte flowrate should be optimised to provide sufficient CO2 transport to the electrode surface, while allowing the interfacial pH to increase sufficiently to increase C2+ formation.

It is often thought that subsurface oxygen in copper electrodes can enhance C2+ selectivity, however, some attribute the enhanced performance to increased surface roughness, which can increase the number of grain boundaries, edge sites and defective sites. The effect of surface roughness and subsurface oxygen tends to be difficult to decouple as inducing subsurface oxygen also causes surface roughness. To address this, ion implantation of neon and oxygen ions was used as it was hoped that the oxygen implantation would implant oxygen and create surface roughness, while the neon would only create surface roughness. Interestingly, it was found through XPS analysis that both the neon and oxygen ion implanted samples saw a decrease in the amount of oxides present and the number of oxygen vacancies, which was correlated to higher surface roughness, but worse electrochemical performance than the as-made electrode. This suggests that subsurface oxygen plays a far more crucial role in promoting eCO2RR performance, compared to surface roughness effects.

Ionomers such as Nafion are commonly used as binders in catalyst layers to improve electrical connectivity and adherence to the support, however, they can also significantly impact the interfacial properties and the reaction environment of the catalyst. Here, it was found that by tuning the Nafion and the catalyst loading, the performance of a CuO nanoparticle catalyst could give selectivity similar to that of a gold electrode, with current efficiencies of close to 80% for CO at a current density of 197 mA cm−2 in a gas diffusion electrode setup. Through in-situ and ex-situ XAS experiments, it was also found that the Nafion ionomer can inhibit the reduction kinetics of the CuO during electrolysis.