The use of fossil based energy sources over the last two centuries has played a huge role in the upliftment of human society. Per-capita energy consumption and population have both been sharply increasing, leading to an ever increasing demand for energy. The current reliance on fossil-based energy has come at the expense of our environment as is seen by the rising CO2 greenhouse gas levels, which pose a threat to society as we know it. Curbing CO2 emissions by closing the anthropogenic carbon cycle and electrifying processes that release CO2 and other greenhouse gases is going to be essential going forward. Recent technological and engineering advancements have led to alternative non-fossil based renewable energy sources that are sustainable, cyclical and do not create an imbalance in the existing natural carbon cycles. Using these renewable energy sources (electrifying) to power the transportation and industrial sectors besides just being a primary energy source is also crucial to remove dependence on fossil-based energy.
Electrochemical CO2 conversion to produce hydrocarbons is a CO2 negative approach that provides alternative pathways to synthesize chemicals and energy dense fuels, essential for both industry and society. As this approach relies only on electrical power, it is an important field of research that could be a cornerstone of a fully electric grid of the future. The ability to access/synthesize chemicals and fuels with the primary energy input being renewable electricity could help decarbonize the chemical sector. While a promising technology, this approach still has to overcome some technological hurdles to reach industrial applicability. Increasing the scale, stability and selectivity whilst overcoming energy losses is the main scientific challenge. We conducted a study to benchmark three different carbon conversion approaches across various performance and operational parameters. We compared plasma based approaches and two electrochemical approaches, high-temperature (HT) electrolysis and low-temperature (LT) electrolysis. Our bench-marking study indicated to us that whilst HT-electrolysis was the most power efficient approach to synthesize CO from CO2 its inability to access larger hydrocarbons made LT-electrolysis a more viable approach for such compounds.
While initial research in the LT-electrolysis field focused on catalyst development recent studies have highlighted the crucial role of the catalytic microenvironment on the performance of electrochemical cells. In this thesis we mainly investigate the role of different microenvironment constituents on the performance of electrolysers operated with commercial Cu catalysts. These micro-environmental parameters were investigated for electrochemical approaches to convert both CO2 and CO to value added C2+ products. The initial part of the work focused on understanding the impact of electrolyte ions on the performance (stability, activity) of CO2 electrolysers. Here we were able to highlight the role of unintended cation crossover from the anode to the cathode. Lowering electrolyte ion concentration (anolyte) led to lower cation concentration and a change in the selectivity of zero-gap gas diffusion electrode (GDE) CO2 electrolysers. It was observed that for lower electrolyte concentrations the primary product formed was carbon monoxide (CO) as opposed to ethylene C2H4, which was predominantly formed at higher electrolyte concentrations. These electrolyte concentration studies were then replicated for two other electrolyser configurations (catholyte-flow GDEs and H-cells), where it was found that the different electrolysers were impacted differently on changing the electrolyte concentration. The catholyte-flow electrolysers showed a lesser impact of electrolyte concentration on product selectivity.
Next, we evaluated the impact of ionomers on the performance of CO2 electrolysers by systematically varying the ionomer amount in the catalyst layer. Ionomers are typically used for their role as binders and also ion conducting species. Through our experiment in the catholyte-flow GDE electrolysers it was observed that incorporation of an ionomer (Nafion) led to drastic shift in selectivity from C2H4 to CO. On the incorporation of Nafion we were able to selectively reduce CO2 to CO on copper (Cu) based catholyte-flow GDEs with faradaic efficiencies (FE) reaching upto 90\%. These experiments were also repeated for two other electrolyser configurations and here as well a dependence on the electrolyser configuration was found.
To assess if CO2 and CO behaved similarly, we conducted experiments with replicate Cu GDEs and it was observed that to facilitate meaningful CO reduction currents, an ionomer was crucial. We hypothesize that this is due to the vastly lower solubility of CO in aqueous solutions in comparison to CO2.This result highlighted the necessity of ionomers for CO reduction and was a crucial milestone for further CO reduction studies.
Besides altering the catalytic microenvironment, we also wanted assess the impact of a second element (Sn/In) on the behaviour of Cu catalysts. A composition dependent selectivity trend was observed where for Cu rich catalysts (low Sn or In amounts) predominantly CO was formed and on increasing the amount of Sn/In the selectivity switched towards formate indicating synergistic effects in bimetallic catalyst systems.
Over the course of this work we highlighted crucial interfacial microenvironment parameters that affected the behaviour of Cu based electrolysers for both electrochemical CO2and CO conversion. Having gained a better insight into the interfacial parameters affecting the electrolyser performance the future challenges are to incorporate non Cu based catalysts that are more selective towards a given product.
