Developing and Investigating the Scope of Quinone-Functionalised Carbons for Electrochemical Carbon Dioxide Capture

Abstract

The amount of CO2 emissions is rising exponentially, contributing to a global temperature increase beyond 1.5°C. To limit emission levels, the demand for cost-effective carbon capture technology is rapidly increasing. Current commercial carbon capture processes use amine molecules to capture CO2; however, these systems face high regeneration energy, high operational costs, and degradation issues. An electrochemical approach can avoid large energy losses via. heat and selectively uptake CO2 by utilising redox-active organic molecules. To compete with conventional chemical scrubbing, the electrochemical cell needs high CO2 uptake, long cycle stability and low energy consumption. Redox-active molecules such as quinone-based molecules have been utilised in this area but suffer from low cycling stability due to organic molecules leaking into the electrolyte. Anthraquinone is a common organic molecule for this process because it is highly stable under different conditions and exhibits electrochemically reversible redox behaviour. However, anthraquinones in solution can react with oxygen, suffer from electrolyte loss through evaporation and have low solubility in aqueous conditions. Therefore, immobilising anthraquinone onto conductive materials can combine the charge capacity of the conductive material and the redox activity of anthraquinone. Quinone-polymer electrodes have shown high efficiency for CO2 capture however are plagued by low active-material mass and quinone degradation, therefore a new alternative is needed. In this thesis, anthraquinone was synthetically grafted onto various carbon materials: CMK-3, BP-1300 and YP-80F. The carbon materials were extensively characterised, and a standardised workflow of techniques was developed, involving gas sorption analysis, solid-state NMR, and electrochemical methods. The characterisation techniques evidenced successful anthraquinone loading, and quantitative loadings were estimated from electrochemistry and solid-state NMR spectroscopy. The materials were tested for electrochemical CO2 capture and showed capture of CO2 during anthraquinone reduction and release of CO2 during anthraquinone oxidation. This was then tested in long cycling experiments, showing adsorption capacities of 0.1 – 0.3 mmol g–1 up to 200 cycles, comparable to reported electrochemical capture technologies. A stark difference in stability was observed under CO2 compared to N2, particularly for functionalised carbon materials with predominantly mesoporous environments, such as CMK-3 and BP-1300. In contrast, the predominantly microporous carbon YP-80F showed enhanced stability, with charge storage capacity retention of 80.0% after 1000 charge/discharge cycles under CO2. The reactivity of anthraquinone in the presence of oxygen was then investigated. To avoid the oxygen reduction reaction in solution, which occurs at negative potentials, electron-withdrawing groups were added to the anthraquinone molecular structure. However, this modification decreased the binding constant for CO2, creating an inversely proportional relationship. The CO2 capture performance of anthraquinone-functionalised carbons was then tested in the presence of oxygen. CO2 capture was observed at approximately 0.1 – 0.2 mmol g–1, similar to values seen under pure CO2, but the Faradaic efficiency lowered by 10%. The anthraquinone-mediated pH swing was also tested using the functionalised carbon materials, resulting in apparent CO2 capture at low energy consumptions. Overall, this thesis details the ease of synthesis and characterisation of anthraquinone-functionalised carbon materials. All materials demonstrate the ability to electrochemically capture CO2; however, functionalised carbon materials with microporous environments showed better stability and lower energy consumption. These materials were then tested for CO2 capture in the presence of oxygen and in a pH swing system. In both cases, promising results indicated that these materials are highly tuneable for CO2 capture, allowing for future development and implementation.