Understanding and Improving Electrochemical CO2 Capture by Supercapacitive Swing Adsorption

Abstract

Arguably the biggest problem faced today on a global scale is climate change. The UK government set the target of reaching net zero CO2 emissions by 2050 under the Climate Change Act 2008 to mitigate the impact of climate change. While there have been important strides in decarbonisation this will not be enough to reach net zero by 2050 making CO2 capture an essential technology. CO2 capture is the selective removal of CO2 from a mixture of gases. The captured CO2 is then stored underground or used (for example as a chemical feedstock) to prevent its release into the atmosphere. Current CO2 capture technology uses amines that react with CO2 and form carbamates. This is an efficient capture process but has high energy costs to release the CO2 and the amines used are both toxic and corrosive. Electrochemical carbon capture has been proposed as a safer and more sustainable alternative. One specific example is Supercapacitive Swing Adsorption (SSA). This avoids the traditional temperature swing used in amine scrubbing and instead relies on a change in the voltage applied to the supercapacitor to reversibly capture CO2. The most significant advantage of using SSA for carbon capture is the sustainable materials used. The electrodes are activated carbon made from coconut shells, and an aqueous sodium sulphate electrolyte is used, both of which are environmentally inert. The main limitation with SSA is the lower adsorption capacity (~ 60 mmol kg−1 of the gas exposed electrode) compared to amine scrubbing (~ 800 mmol kg−1 of amine) and a lack of understanding of the mechanism which is impeding the design of improved systems. To increase the CO2 adsorption capacity for SSA, the effect of the charging protocols on performance was investigated. By increasing the applied voltage window, the adsorption capacity could be more than doubled. This investigation also revealed that the relative polarity of the gas-exposed electrode controls the capture and release of CO2 and not solely the charge and discharge of the supercapacitor. In other words, when the gas exposed electrode is made more positive, CO2 release occurs but when the gas exposed electrode is made more negative, CO2 is captured regardless of the charging protocol applied. This highlighted the need for an investigation into the mechanism. Computational models that solved finite-element calculations were used to track the movement of ions through the supercapacitor. This showed that the accumulation (or depletion) of bicarbonate ions controlled the release (or capture) of CO2 as the cell voltage of the supercapacitor was changed, through disturbances to the equilibrium between CO2 and bicarbonate in the presence of water. Analysis of the system kinetics revealed the capture of CO2 was an inherently kinetic effect and becomes a reversible process when the voltage was held for longer durations. This reversibility showed the system performance would be dependent on the charging rate and investigation revealed the charging rate that maximised the CO2 adsorption capacity for the current set-up was 30 mA g−1. If the supercapacitor was charged slower than this, the reversibility dominated the behaviour, thus decreasing the CO2 adsorption capacity. Alternatively, when the supercapacitor was charged faster than 30 mA g−1 a decrease was seen due to mass transport limitations of both CO2 and bicarbonate ions. The potential mechanism uncovered suggested the current flowing through the gas exposed electrode controls the behaviour of the device. Therefore, to investigate this further, the gas cell architecture was changed. By allowing gas to contact both electrodes, instead of just a single electrode in the normal set-up, a decrease in CO2 adsorption capacity was seen. This was caused by increasing the rate of the reversible reaction. To exaggerate this process, a fully symmetric cell was designed. This revealed both a further decrease in the CO2 adsorption capacity and a change of mechanism such that the CO2 capture and release is no longer dependent on the direction of charge. Finally, as the objective of CO2 capture is to capture more CO2 than it produces, the real-world applications of SSA were investigated. Results demonstrated that SSA performance was robust against changes in temperature, composition of gases and over prolonged cycling. To further understand how SSA fits with current technologies, a Life Cycle Assessment (LCA) was performed to give quantitative comparison of the environmental impacts for SSA, electrochemical CO2 capture with quinones and the traditional amine scrubbing. Overall, the work in this thesis helps to increase our understanding of the mechanism of SSA and the conditions required to maximise the performance. This will help to inform future rational design of the system to make SSA a commercially viable option for CO2 capture in the future.