Na₃V₂(PO₄)₃@C as Faradaic Electrodes in Capacitive Deionization for High-Performance Desalination

Abstract

Among various desalination technologies, capacitive deionization (CDI) has rapidly developed because of its low energy consumption and environmental compatibility, among other factors. Traditional CDI stores ions within the electric double layers (EDLs) in the nanopores of the carbon electrode, but carbon anode oxidation, the co-ion expulsion effect, and a low salt adsorption capacity (SAC) block its further application. Herein, the Faradaic-based electrode is proposed to overcome the above limitations, offering an ultrahigh adsorption capacity and a rapid removal rate. In this paper, the open framework structure Na3V2(PO4)3@C is applied for the first time as a novel Faradaic electrode in the hybrid capacitive deionization (HCDI) system. During the adsorption and desorption process, sodium ions are intercalated/deintercalated through the crystal structure of Na3V2(PO4)3@C while chloride ions are physically trapped or released by the AC electrode. Different concentrations of feedwater are investigated, and a high SAC of 137.20 mg NaCl g–1 NVP@C and low energy consumption of 2.157 kg-NaCl kWh–1 are observed at a constant voltage of 1.0 V, a concentration of 100 mM, and a flow rate of 15 mL min–1. The outstanding performance of the Na3V2(PO4)3@C Faradaic electrode demonstrates that it is a promising material for desalination and that HCDI offers great future potential.

Faradaic Electrodes Open a New Era for Capacitive Deionization

“…Unlike carbon, Faradaic materials capture ions by Faradaic reactions involving the charge transfer between electrodes and the ions in solution. Such charge transfer can be accomplished by ion insertion within crystal structures, [40,41] conversion reaction with forming new compounds, [42,43] or ion-redox active moiety interaction. [44,45] This endows Faradaic materials with three advantages over carbon as CDI electrode.…”

“…In addition, redox-active polymers are also promising for water desalination, as they can show either strong interactions with Na + (such as redox-active polyimide) [56] or Cl − (such as polymers with [Fe(CN) 6 ] 4− ) [44] depending on the tunable redox active moieties. In addition to the aforementioned novel ion-selective Na + /Cl − capture electrode materials [47,57,58] and various cell designs, [43,46,49] some recent promising developments of Faradaic electrodes have been achieved, such as research on the influences of operational parameters on CDI performance metrics, [41,43,59] and some typical scientific and practical application. [40,[60][61][62][63] However, it appears that Faradaic electrodes used in CDI cells are still not mature enough to meet the requirement for practical implementation and commercialization, which can be attributed to major challenges such as not fully understanding the ion capture mechanisms and behaviors of materials, matching issues between the Na + capture cathode and Cl − capture anode, and the need to establish standardized test conditions, etc.…”

“…First, a higher influent salt concentration reduces the ionic resistance of the electrolyte, thus enhancing the electrochemical activity of the electrodes. [41,95] Second, a higher influent salt concentration (i.e., higher ionic strength) leads to a reduced effective size of the hydrated ion (i.e., lower number of water molecules in the hydration shell), facilitating a more facile and effective ion insertion. [97][98][99] Finally, a higher influent salt concentration enhances the concentration gradient between the flow channel and the electrode, as well as within the electrode, thus improving the diffusion and convection effect.…”

“…Similar factors mentioned in salt removal capacity will also affect the value of salt removal rate. Increasing the influent salt concentration, [41,111] current density in CC mode [8,94,102] or the applied voltage in CV mode [107,111] can contribute positively to the salt removal rate of the desalination cell up to a certain extent. It should be noted that although an increase in current density increases salt removal rate, the improvement of salt removal rate with Faradaic electrodes normally comes at the expense of a sharp loss of salt removal capacity.…”

Hazardous wastes treatment technologies

“…Additionally, core-shell nanotube architecture increased the specific surface area and lowered the resistivity, which effectively increased ion penetration and ion removal rate. Cao, Wang, Wang, Yu, and Ma (2019) applied Na 3 V 2 (PO 4 ) 3 [NVP] as the open framework structure for C@NVP electrode in a hybrid CDI system. The crystal structure of C@NVP facilitated the interspersion and deinterspersion of Na + while chloride ions were trapped or released by the activated carbon in the hybrid electrodes.…”