Carbon-based electric double layer capacitors for water desalination

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 68-72).In capacitive deionization (CDI), salt water is passed through two polarized electrodes, whereby salt is adsorbed onto the electrode surface and removed from the water stream. This approach has received renewed interest for water desalination due to the development of new high-surface area, carbon-based nanomaterials. However, there is limited understanding as to how electrode geometry, surface properties, and capacitance affect ion capture. In this work, we experimentally investigated various standard carbon-based electrode materials, including activated carbon and carbon cloths, as well as microfabricated silicon structures for CDI. Electrochemical characterization through cyclic voltammetry was used to determine the electrochemical properties of each material. The capacitance values of the carbon materials tested were 40 F/g for 2000 m2 /g carbon cloth, 32 F/g for 1000 m2 /g carbon cloth, and 25 F/g for activated carbon. In addition, we constructed two iterations of flow test channels to perform parametric studies on ion capture. The first flow cell utilized a commercial conductivity probe to measure salt concentration after charging the electrodes without flow. We showed that the ion capture on both the carbon cloth and activated carbon electrodes were proportional to the applied voltage, however two orders of magnitude smaller than what is expected from the electrode charge. We addressed a significant experimental limitation in the second flow cell by integrating conductivity sensors into the flow channel to measure effluent salt concentration during electrode charging. We found that the salt adsorption increased from 33.1 pmol/g in the first flow cell to 63.5 pmol/g in the redesigned flow for an applied potential of 1.2 V. Future directions will focus on controlling electrode geometry and chemistry to help elucidate transport mechanisms and provide insight into the design of optimal materials for capacitive deionization.by Batya A. Fellman.S.M