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 (hV2O5)‐decorated multi‐walled carbon nanotube (MWCNT) electrode and one activated carbon electrode enables sodium ions to be removed by pseudocapacitive intercalation with the MWCNT–hV2O5 electrode and chloride ion to be removed by non‐faradaic electrosorption of the porous carbon electrode. The MWCNT–hV2O5 electrode was synthesized by electrochemical deposition of hydrated vanadium pentoxide on the MWCNT paper. The stable electrochemical operating window for the MWCNT–hV2O5 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 mm NaCl (brackish water) and 600 mm 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 mm 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–hV2O5 electrode. Additionally, we propose a normalization method for the electrode material with faradaic reactions based on sodium uptake capacities. The life of brine: A hybrid cell consisting of a hydrated vanadium pentoxide (hV2O5)‐decorated multi‐walled carbon nanotube (MWCNT) electrode and one activated carbon electrode is used to remove sodium ions by pseudocapacitive intercalation with the MWCNT–hV2O5 electrode and remove chloride ions by non‐faradaic electrosorption of the porous carbon electrode.