Electrosorption of ions from aqueous solutions by mesoporous carbon materials
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Electrosorption involves the application of an electrical potential between carbon electrode pairs submerged in brackish water, effectively “trapping” the ions in an electrical double layer at the solid-liquid interface. Electrosorption has significant applications in environmental engineering, including desalination of water by capacitive deionization (CDI), and in energy storage by supercapacitors. This work combines experimental and modeling studies to investigate the transport and sorption mechanisms of ions in the pores of mesoporous carbon materials that were synthesized at the Oak Ridge National Laboratory (ORNL). The main contribution of this research is examining the effects of operational parameters such as applied potential, solution temperature, ionic concentration, and valence of ions, on the electrosorption behavior of mesoporous carbon materials with the aim to improve the desalination efficiency in the CDI process. It was found that the rates of sorption by mesoporous carbon electrodes and their regeneration increased at a higher temperature and on application of a high-frequency, low-amplitude AC signal. Neutron imaging has been employed as a tool to visually observe and quantify the transport and distribution of ions within the carbon electrodes. The neutron images revealed interesting ion transport phenomena that can aid in the optimization of the CDI process. From the ion concentration profiles inside the electrodes, the effective diffusivities of gadolinium and lithium ions were obtained under various conditions of applied potential. Information on the diffusivity of ions can aid in theoretical modeling of the CDI process as well as guide strategies for the design of advanced electrode materials. In the final part of the study, the extraction of salinity gradient energy or ‘blue energy’ by mixing fluids of different salinities was assessed based on the principle of capacitive double layer expansion. Neutron imaging of blue energy recovery cycles was conducted to observe the ion transport behavior during the various steps of the cycle. The results obtained provide insights into the fundamental ion transport mechanisms during electrosorption by mesoporous carbon materials. This research has important implications for developing advanced system designs for desalination of saline water and energy storage devices.