Enabling High Capacity and Stable Sodium Capture in Simulated Saltwater by Highly Crystalline Prussian Blue Analogues Cathode

Capacitive deionization is proven to be a viable resource-efficient alternative to remove dissolved salts from water. Further improvements in electrode materials are, however, necessary to achieve a higher efficiency of water desalination. In this work, X-Fe (X = Mn, Co, Cu) Prussian blue analogues of homogeneous particle sizes were grown on electrically conducting activated carbon cloth electrodes by a simple and facile synthesis process. As two-dimensional/three-dimensional (2D/3D) nanostructured Prussian blue analogue materials present favorable electrochemical behavior and ion-exchange possibilities, they are promising for application in capacitive deionization. The modified carbon cloth electrodes exhibit higher salt adsorption capacity and better charge efficiency compared to those constructed with pristine carbon cloth. Cobalt hexacyanoferrate-modified electrodes show the most stable performance, reaching a salt adsorption capacity of 14.47 mg g–1 at 1.0 V, accompanied by the lowest energy consumption (0.389 kWh m–3). This simple strategy is expected to offer possibilities for designing and synthesizing advanced electrode materials with superior performances for application in capacitive deionization devices.

Recent progress in graphene-based and ion-intercalation electrode materials for capacitive deionization

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.

Copper hexacyanoferrate/carbon sheet combination with high selectivity and capacity for copper removal by pseudocapacitance

Mesopore-enhanced graphene electrodes with modified hydrophilicity for ultrahigh capacitive deionization

Prussian blue analogue based integrated membrane electrodes for desalination and selective removal of ammonium ions in a rocking-chair capacitive deionization

Capacitive deionization (CDI) technology is a challenge on an economical and effective electrosorption desalination method. The paper analyses the CDI current status and progress of carbon electrode materials, and describes the types of CDI and its performances of testing materials. The electrosorption capacities are summarized on the carbon electrode materials and the current hurdles. The reported numbers from the literature vary in a wide range between 0.25 and 26.42 mg/g of both electrodes CDI cell, we suggest that the CDI electrodes should have an adsorption of at least 9.0 mg/g NaCl when the applyed voltage is 2.0 V. The potential capacitive deionization technologies are proposed here.

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

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

A direct comparison of flow-by and flow-through capacitive deionization

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

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

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

Capacitive deionization (CDI) is a promising technology for desalination of seawater and recovery of mineral resources. By applying a voltage to a pair of electrodes within the potential of water electrolysis, ions such as Li+ and Na+ can be recovered by electrosorption. In order to increase the salt adsorption capacity (SAC) of the electrodes, various metal oxides have been proposed.1 For example, manganese oxide, titanium oxide and ruthenium oxide show higher SAC than activated carbon, owing to the contribution from both electric double-layer capacitance and pseudocapacitance. In particular, the SAC of ruthenium oxide electrodeposited on activated carbon was ~4 times higher than that of pristine activated carbon due to the additional pseudocapacitance.2 Since hydrous RuO2 shows higher pseudocapacitance compared with anhydrous RuO2,3 it is anticipated that hydrous RuO2 would show higher SAC compared to anhydrous RuO2. Here we evaluate and discuss the pseudocapacitance and SAC capability of hydrous and anhydrous RuO2-based electrode materials. RuO2·0.5H2O or RuO2 were prepared by heating of RuO2·xH2O at 200°C or 450°C, respectively.3 The slurry, prepared by mixing RuO2·0.5H2O or RuO2, polyvinylidene fluoride and N, N-dimethylformamide, was pasted on a Ti plate and dried at 120oC to prepare electrodes (10-11 mg cm-2) for batch-mode CDI experiments. Activated carbon (MSP-20) electrode was prepared as a control sample. The CDI experiment was carried out with a voltage of 1.2 or 0 V, by circulating 100 mL NaCl feed solution (initial concentration: 5 mM NaCl) at a flow rate of 9.8 mL min-1 per electrode. The SAC was determined by measuring the change in the conductivity on the negative electrode side, using a calibration curve of conductivity and concentration of NaCl solution.The specific capacitance of RuO2·0.5H2O and RuO2 were 172 F g-1 and 13 F g-1, while the SAC was 7.4 mg g-1 and 4.0 mg g-1, respectively. This means that RuO2·0.5H2O shows 13.2 times higher capacitance and twice higher SAC. The results suggest that the increase in the specific capacitance by applying RuO2·0.5H2O might be attributed to the increase in the SAC value. On the other hand, the specific capacitance of RuO2·0.5H2O was higher than that of activated carbon, while the SAC of RuO2·0.5H2O was lower than that of activated carbon. There seems to be a difference in the effect of pseudocapacitance and electric double layer capacitance on the SAC value.This work was supported in part by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Challenging Research (Exploratory) Grant Number 17K19174. References K. Singh, S. Porada, H. D. de Gier, P. M. Biesheuvel and L. C. P. M. de Smet, Desalination, 455, 115 (2019).X. Ma, Y. Chen, K. Zhou, P. Wu and C. Hou, Electrochim. Acta, 295, 769 (2019).W. Sugimoto, H. Iwata, K. Yokoshima, Y. Murakami and Y. Takasu, J. Phys. Chem. B, 109, 7330 (2005). Figure 1

Selective removal of ammonium ions with transition metal hexacyanoferrate (MHCF) electrodes