The current global average CO2 concentration in the air is 427 ppm, rising at a rate of 2 ppm per year, faster than ever before. This underscores the urgency to capture and either utilise or sequester the carbon. The energy transition is driving a shift from fossil-based generation towards renewable energy sources, with wind and solar energy generation at the forefront. The intermittency of supply from renewable energy generation presents opportunities during peak electricity generation to reduce CO2 electrochemically.
The electrochemical utilisation of captured CO2 offers a sustainable pathway to close the carbon loop by converting waste carbon into value-added chemicals. This thesis explores the electrochemical dicarboxylation of 1,3-butadiene with CO2 to produce 3-hexene-1,6-dioic acid (3HDA), a direct precursor to adipic acid. Adipic acid is a key raw material in Nylon-66 production, and its conventional synthesis from cyclohexane emits significant quantities of N2O, a greenhouse gas with a global warming potential of 298 times that of CO2. In contrast, the pathway studied here aims to decarbonise the process using renewable electricity.
The electrochemical synthesis of 3HDA from 1,3-butadiene and CO2 is employed in an electrolyser cell. Using a sealed, undivided electrochemical cell with acetonitrile and tetraethylammonium chloride as the electrolyte solution, the study systematically investigates the influence of various electrocatalyst morphologies and materials, and process parameters on faradaic efficiency, product selectivity and yield.
Nickel wire was identified as the most effective working electrode, outperforming copper wire and other nickel-based morphologies, employed alongside aluminium coil as a sacrificial anode. Electrochemical characterisation using cyclic voltammetry and chronoamperometry revealed –2.6 V vs Ag/AgCl as the optimal reduction potential for 3HDA formation, with a faradaic efficiency achieved of 14% for 3HDA and 100% selectivity among the carboxylated products. Competing side reactions, such as 3-pentenoic acid (3PA), formate and oxalic acid production, were observed at other reduction potentials. Reduced moisture content in the electrolyte was found to poorly influence the faradaic efficiency. The dry acetonitrile has a similar 3HDA faradaic efficiency when the water content is reduced from 350 ppm to 150 ppm in the electrolyte. Temperature studies indicated that higher temperatures enhance the reaction rate but increase product solubility, reducing solid product formation. Solubility measurements confirmed this behaviour. On calculating the faradaic efficiency of the dissolved 3HDA as well, the efficiency stands at 6.58% for at 40◦C and 3.59% at 60◦C. Further, experimentation with alternative supporting electrolytes like TBABF4 and TBAPF6 was inconclusive due to practical limitations.
Overall, the thesis demonstrates a promising proof-of-concept for the sustainable electrochemical conversion of CO2 and 1,3-butadiene to value-added chemicals of 3HDA. The findings contribute to the broader goals of electrifying chemical synthesis and valorising captured CO2, inspiring future work recommended in improving the process conditions and refining electrolyte and electrode design.