Pseudocapacitive Desalination of Brackish Water and Seawater with Vanadium-Pentoxide-Decorated Multiwalled Carbon Nanotubes

A hybrid membrane pseudocapacitive deionization (MPDI) system consisting of a hydrated vanadium pentoxide (hV(2)O(5))-decorated multi-walled carbon nanotube (MWCNT) electrode and one activated carbon electrode enables sodium ions to be removed by pseudocapacitive intercalation with the MWCNT-hV(2)O(5) electrode and chloride ion to be removed by non-faradaic electrosorption of the porous carbon electrode. The MWCNT-hV(2)O(5) electrode was synthesized by electrochemical deposition of hydrated vanadium pentoxide on the MWCNT paper. The stable electrochemical operating window for the MWCNT-hV(2)O(5) electrode was between -0.5V and +0.4V versus Ag/AgCl, which provided a specific capacity of 44mAhg(-1) (corresponding with 244Fg(-1)) in aqueous 1m NaCl. The desalination performance of the MPDI system was investigated in aqueous 200mm NaCl (brackish water) and 600mm NaCl (seawater) solutions. With the aid of an anion and a cation exchange membrane, the MPDI hybrid cell was operated from -0.4 to +0.8V cell voltage without crossing the reduction and oxidation potential limit of both electrodes. For the 600mm NaCl solution, the NaCl salt adsorption capacity of the cell was 23.6 +/- 2.2mgg(-1), which is equivalent to 35.7 +/- 3.3mgg(-1) normalized to the mass of the MWCNT-hV(2)O(5) electrode. Additionally, we propose a normalization method for the electrode material with faradaic reactions based on sodium uptake capacities.