Graphene–carbon 2D heterostructures with hierarchically-porous P,N-doped layered architecture for capacitive deionization†

Capacitive deionization characteristics of nanostructured carbon aerogel electrodes synthesized via ambient drying

Integrating cost-effective microbial fuel cells and energy-efficient capacitive deionization for advanced domestic wastewater treatment

Treatment of industrial brine using capacitive deionization (CDI) towards zero liquid discharge – challenges and optimization

2D-2D nanohybrids of Ni–Cr-layered double hydroxide and graphene oxide nanosheets: Electrode for hybrid asymmetric supercapacitors

Defect-rich molybdenum disulfide as electrode for enhanced capacitive deionization from water

Nitrogen-doped nanostructured carbons: A new material horizon for water desalination by capacitive deionization

Improvement of water softening efficiency in capacitive deionization by ultra purification process of reduced graphene oxide

Capacitive deionization (CDI) has shown great promise as a water treatment and conditioning technique. However, a large majority of studies using this technology have focused solely on NaCl removal, while other ions commonly found in natural waters might pose unaddressed challenges for CDI, such as specific adsorption, diffusional limitations and scaling precipitation. For instance, precipitation of inorganic salts such as calcium carbonate is an issue associated with brackish waters and RO plants. Similarly, high concentrations of sulfate and silicate salts in groundwater may lead to membrane scaling due to their low solubility [1]. Therefore, there is still a need to identify the effect of both individual ions and mixtures on CDI performance. In this work, calcium and sulfate sorption was studied using a CDI reactor outfitted with low-cost, high surface area carbons coated with two different metal oxides (SiO2 on the cathode and Al2O3 on the anode). The metal oxide coatings were chosen because of previous efforts in the literature showing improved electrode performance in the CDI process with the addition of these coatings [2-4]. Two calcium and two sodium salts (CaSO4, CaCl2, NaCl and Na2SO4) were tested to examine and compare the sorption characteristics of different anions and cations. Previous testing of this CDI system in a “Single Pass” (passing the water through the device only once) operational mode using a CaSO4 solution showed reductions in both Ca2+ and SO4 2- concentrations [5]. However, issues related with ion desorption and faradaic reactions (due to high cell potentials) were also observed. In this work, cycles of consecutive adsorption/desorption tests were performed to evaluate the long-term performance of this CDI system. Ion sorption and electrode stability were analyzed in terms of variables such as influent concentration, flow rate, and regeneration potential. These analyses also included performance metrics related with charge efficiency and energy consumption. For comparison purposes, CDI cells with uncoated carbon electrodes were evaluated following similar protocols. The results showed that increasing the concentration of CaSO4 in the feed solution (from 1.5 to 4.5 mM) increased the ion removal to values up to 3.29 ± 0.25 mg·g-1 and reduced energy consumption from 0.16 down to 0.09 kWh·mol-1. It was also observed that electric charge efficiencies increased by 15-30 % when asymmetrically coated configurations were employed. Nevertheless, the presence of SO4 2- significantly hindered salt desorption; conversely, recovery values close to 95 % were achieved using NaCl electrolyte. Finally, the evaluation of the long-term CDI performance revealed a slight but progressive loss in the ion electrosorption capability, probably due to the oxidation of the positive electrodes.

Prussian blue analogues (PBAs) are considered as promising cathode materials for capacitive deionization (CDI) technology due to their 3D open‐frame structure and tunable redox active sites. However, the inevitably high content of [Fe(CN)6] vacancies in the crystal structure results in a low salt adsorption capacity (SAC) and poor recycling performance. Herein, a high‐salt nano‐reaction system is established by mechanochemical ball milling, enabling the preparation of a variety of highly crystallized PBAs (metal hexacyanoferrate, MHCF‐B‐170, M = Ni, Co, or Cu) with low vacancies (0.05–0.06 per formula unit). The reduction of vacancies in the PBAs lattice not only strengthens the conductivity and promotes the rapid transfer of electrons, but also reduces the migration energy barrier and accelerates the fast and reversible diffusion of Na+ ions. The structural characterization method and theoretical simulation demonstrates the excellent reversibility and crystal structure stability of MHCF‐B‐170 during the CDI process. Impressively, the NiHCF‐B‐170 exhibits excellent CDI performance, characterized by an exceptionally high SAC of up to 101.4 mg g−1 at 1.2 V, and demonstrates remarkable cycle stability with no significant degradation observed even after 100 cycles. This PBAs with low Fe(CN)6 vacancies are expected to be a competitive candidate material for CDI electrodes.

2D hollow leaf shaped mesoporous carbon derived from ZIF-L for efficient capacitive deionization

Enhanced capacitive deionization of a low-concentration brackish water with protonated carbon nitride-decorated graphene oxide electrode

Capacitive deionization has been developed as a promising desalination alternative for removing ions from aqueous solutions. In this study, the evaluation of capacitive performance was carried out by galvanostatic charge/discharge and cyclic voltammetry experiments. The good capacitive and electrosorption behaviors suggest carbon aerogel not only treated as an electrical double layer capacitor, but also as a potential electrode in capacitive deionization processes. Also, the capacitive deionization characteristics indicate that electrosorption/regeneration can be controlled by polarization and depolarization of each electrode. It implies that sodium and chloride ions are electrostatically held to form electrical double layer on the surface of charged electrodes. The electrosorption performance at different applied voltages and solution concentrations was investigated. It is found that the removal of sodium chloride increases with increasing applied voltage and solution concentration, resulting from stronger electrostatic interactions, higher concentration gradient, and less double layer overlapping effect. Based on Langmuir isotherm, the equilibrium electrosorption capacity at 1.2 V is determined as 270.59 μmol/g. Under this condition, due to the presence of micropores associated with the double layer overlapping, the effective surface area for electrosorption of ions at 1.2 V is estimated in the range of 12.18–14.25 % of the Brunauer–Emmett–Teller surface area. The results provide a fundamental understanding of electrosorption of ions and help promoting capacitive deionization technology for water purification and desalination.

We employ slow cyclic voltammetry to study the quasi‐steady‐state capacitive deionization (CDI) of aerated 10 mM NaCl from Vcell=−0.2 to 2 V. The method allows for the deconvolution of capacitive and faradaic processes, which are significant even at low voltage, in conjunction with pH, conductivity, dissolved oxygen (DO), and electrode potential measurements. The pH at three locations and effluent DO data identify asymmetric electrosorption in the cell configured with pristine activated carbon electrodes, where charge is irreversibly consumed to produce H+/OH− instead of reversibly adsorbing Cl−/Na+. Implementing a reference electrode resolves ion adsorption as a function of the individual potential at each electrode. Near‐electrode pH probes reveal pH environments that diverge by up to 8 units during cell polarization and allow more accurate calculation of faradaic redox potentials in a flow‐by CDI cell. In the aerated NaCl solution that will be relevant to industrial‐scale water treatment, we find that DO reduction occurs to a greater degree than Na+ adsorption at the cathode until DO removal becomes mass transfer limited at <−0.2 V vs. SHE. Two cell architectures – flow‐by and flow‐through – corroborate our findings. Finally, we reconcile all measurements to generate a map displaying how the potential is partitioned amongst the capacitive and faradaic processes occurring during CDI operation over a 2.2 V window.

The effect of the flow-regime, reversal of polarization, and oxygen on the long term stability in capacitive de-ionization processes

Three‐dimensional graphene/MWCNT-MnO2 nanocomposites for high‐performance capacitive deionization (CDI) application