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

Abstract 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 −0.5 V and +0.4 V versus Ag/AgCl, which provided a specific capacity of 44 mAh g −1 (corresponding with 244 F g −1 ) in aqueous 1 m NaCl. The desalination performance of the MPDI system was investigated in aqueous 200 m m NaCl (brackish water) and 600 m m NaCl (seawater) solutions. With the aid of an anion and a cation exchange membrane, the MPDI hybrid cell was operated from −0.4 to +0.8 V cell voltage without crossing the reduction and oxidation potential limit of both electrodes. For the 600 m m NaCl solution, the NaCl salt adsorption capacity of the cell was 23.6±2.2 mg g −1 , which is equivalent to 35.7±3.3 mg g −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.