Mechanistic Understanding of Capacitive Deionisation (MU-CDI)

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The capture and management of ions in water systems are of widespread importance to society. One of the most prominent applications is water desalination, which is becoming an increasingly important technology due to population growth and climate change putting pressure on freshwater resources. In recent years, capacitive de-ionisation (CDI) has gained increasing attention as a potentially low-energy alternative to more common desalination methods such as reverse osmosis. CDI works by passing a saline solution through an electrochemical cell where the positive and negative salt ions are immobilized on the surfaces of oppositely-charged porous carbon electrodes. One of the advantages of CDI over other desalination methods is that following the initial ion capture step, the electrode can be regenerated by discharging into a separate effluent stock. In this step, some of the energy used for the ion capture is recovered, and furthermore, the efficient regeneration of the electrode reduces fouling. Despite the promise of CDI, its efficiency reduces at high salt concentrations. In this respect, it does not compete with other methods such as reverse osmosis for treatment of seawater. In recent years there have been considerable research efforts to extend the concentration range in which CDI is effective. Most development has focused on optimisation of materials and cell designs with considerable success, yet, surprisingly little consideration has been given to details of the the ion behaviour or the elementary processes taking place at each electrode. One of the primary considerations is to ensure that ionic charge is stored by ions being captured by the electrode, rather than being exchanged with those in the feed electrolyte (which does not reduce the salt concentration). This proposal seeks to develop a mechanistic understanding of CDI and apply this knowledge to control the ion storage mechanism to optimize the salt removal efficiency. This will be done through the use of detailed electrochemical analysis and the use of nuclear magnetic resonance (NMR), which allows us to "see" and count ions that are captured in the electrode, and correlate this with the electrochemical response and salt removal efficiency. We will investigate how the electrode pore size and electrolyte properties, such as concentration and the nature of the ions present, affect how they are captured. This information will then be used to inform and optimise the cell design and operational conditions (e.g., flow rate and cell voltage). Our proposed work is necessarily fundamental in nature with the key aim of improving the understanding of the underlying science of CDI, rather than fabrication of prototype CDI stacks. However, through our collaborations with academic and industrial partners, we aim to work with, and identify, scalable and commercially-relevant electrode materials.