Water desalination and other processes involving separation and recovery of ionic species are energy-intensive, cost-intensive and generate waste. This issue poses a particular challenge from the perspective of future fresh water supply: 98% of water available in our planet is sea or brackish water, and it is estimated that two thirds of the world will be dealing with water scarcity by 2025. Electrosorption of ions onto nano-structured electrodes is a process that could potentially enable deionization technologies with lower energy and cost requirements and less waste than traditional deionization technologies.
Electrosorption is a process where ions of opposite charge are immobilized within a region known as the electrical double layer (EDL), which forms in the vicinity of the solid/liquid interface of a charged electrode.
We studied the combined effects of pore diameter, applied potential and ion size during electrosorption of three monovalent ions onto titanium dioxide nanotubes with three tailored pore sizes in the mesoporous range. The pore sizes examined were 36, 41 and 45 nm. The results demonstrate that electrosorption capacity is not strictly proportional to surface area and applied potential for the nanostructured electrodes used in this work in contradiction to classical electrosorption theory. The results also suggest that there are minima in terms of electrosorption capacity at the intermediate pore size examined, where competitive energy and steric hindrance effects may take place. Only two combinations of applied potential and pore diameter did not show differences in electrosorption capacity with different hydrated ion radii. A key implication of the results to be presented is that nano-porous materials with tailored pore sizes may be used to improve the absolute electrosorption capacity of an electrode, and when coupled with voltage, it is possible to tailor ion selectivity to ion type.