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A membrane-free hybrid system employing the battery-type copper hexacyano-ferrate (CuHCF) and the pseudocapacitive polypyrrole (PPy) as the positive and negative electrodes are designed for faradaic desalination. Both materials exhibit a memory effect, a unique characteristic of the faradaic reaction, with the dual function of ion removing and concentrating. The appropriate mass ratio of two materials is adjusted according to the positive/negative charge balance to ensure that both electrodes work in their respectively suitable potential windows. Through the little inverted voltage (−0.2 V) during the discharge process in order to keep the two electrodes in the desirable potential windows in comparison with the case discharged at 0 V, the salt removal capacity (SRC) of this hybrid system is significantly increased from 21.17 mg g−1 to 26.62 mg g−1 in 8 mM NaCl. Under this operation condition, the cell keeps >90 % of the original SRC in a 100-cycle stability test. The SRC obtained in 8 mM NaCl solution can increase to 38.64 mg g−1 after employing the acid-treated CNT/CuHCF composite. The system also exhibits superior performance on removing other cations including K+, Mg2+, and Ca2+. All results reveal the promising application potential of this system to the faradaic desalination and metal-ion concentrating and recovering. • A hybrid system with high SRC is constructed with CuHCF and polypyrrole via charge balance. • SRC in 8 mM NaCl is increased from 21.17 to 26.62 mg g−1 by small inverted cell voltages. • The SRC retention of this system is >90 % in a 50-cycle test, applicable to other mono−/di-valence cations. • SRC can be further increased from 26.62 to 38.64 mg g−1 by adding CNT in CuHCF. [ABSTRACT FROM AUTHOR]

A highly efficient pseudocapacitive desalination system is constructed with positive MnO 2 and negative polypyrrole electrodes. Both materials deposited by electrochemical methods exhibit faradaic reactions with the "memory effect" and permit the unique multi-function application with salt-removing and concentrating abilities. The quasi-steady state electrode potentials of both electrodes under charged/discharged states and the cell salt removal capacity (SRC) highly depends on the mass loading of active materials. To ensure the quasi-steady state electrode potentials of materials in their working potential windows, the mass loadings of active materials are carefully controlled by the deposition charge density. After the mass loading adjustment, SRC reaches 78 mg g−1 in a 60-min salt-removing process. The salt removal rate (SRR) reaches the highest value of ca. 5 mg g−1 min−1 during the initial 5 min and continuously decreases to near 0 with prolonging the desalination time. In the stability test, this MnO 2 //PPy cell remains over 90% of its original SRC in a 100-cycle test. Shortening the time of a single operation segment is an efficient way to increase the average SRR which reaches 212.7 mg g−1 in this MnO 2 //PPy cell transferring NaCl from one solution to another in a 120-min test using the 4/4-min model. Unlabelled Image • A simple method of fabricating pseudocapacitive materials for capacitive deionization system is provided. • SAC highly depends on discharging time and reach 78 mg g−1 for 60-min discharging process. • MnO 2 //PPy exhibits high salt adsorption rate in the front end of whole desalination process. • MnO 2 //PPy shows excellent stability through a 100-cycle test. • An innovative system that achieves both desalination and salt-concentration is invented. [ABSTRACT FROM AUTHOR]

[Display omitted] • Na+ and Cl− ions can be captured by PPy electrodes modified with different dopants. • The designed PPy//PPy system ensures a low cell voltage to reduce energy consumption. • The designed system displays high SAC and low EC of 74.3 mg/g and 0.19 kWh/kg-NaCl. • The designed system shows great stability with SAC retention of 88% after 250 cycles. • A first-ever 3-D ECDI Ragone plot is proposed as a powerful tool for ECDI systems. Electrochemical deionization (ECDI) systems are attractive desalination devices that achieve relatively high salt adsorption capacity (SAC) with acceptable energy consumption, which can replace or be a promising complement to other desalination techniques. However, most ECDI systems are only suitable for brackish water desalination, and the energy consumption is significantly increased with the salt concentration of brine water without considering their short cycle life. Here a new concept that an ECDI cell constructed with both electrodes of the same conducting polymer (polypyrrole, PPy) shows the high SAC of Faradaic desalination in the brackish water of a wide concentration range and low cell voltages of ion-capturing/ion-releasing, leading to low energy consumption and excellent cycle life, is demonstrated. Anionic dopants of different sizes make PPy films exhibit distinct ion exchange abilities. Trapping large dopants, 4-methylbenzene-sulfonic acid (p-TS), enables the cation-capturing characteristics of the PPy-p-TS electrode, while the free movement of small dopants, perchlorate (ClO 4), facilitates the anion exchange capability of the PPy-ClO 4 electrode. Moreover, the operation parameters, including charging/discharging time ratio, mass loading ratio of PPy-p-TS/PPy-ClO 4 , and overall mass loading on two electrodes, are optimized to achieve excellent desalination performance, long cycle life, and the dual function of salt-removal and salt-concentrating. Finally, a three-dimensional ECDI Ragone plot is proposed to judge the performance of all ECDI systems. This work shows the design guidelines for constructing ECDI systems with high desalination capacity, fast deionization rate, low energy consumption, and long cycle life. [ABSTRACT FROM AUTHOR]

A highly efficient water purification device is designed by a combination of electrochemical deionization and microfluidic techniques. The device is constructed with seven layers, in which MnO 2 and polypyrrole are employed as positive and negative electrodes, respectively. The advantages of high sensitivity, large surface-to-volume ratio, and short ion diffusion path provided by such a microfluidic device dramatically increase the salt removal capacity (SRC) and salt removal rate (SRR) to 132 mg g−1 and 30 mg g−1 min−1 in the desalination test using a 600 mM NaCl solution. The salt removal percentage in a single-pass test of this microfluidic device using an 8 mM NaCl solution can reach 88 %, 33 times higher than 2.6 % obtained from a macro-scale experiment setup. The SRC remains over 75 % after a 180-cycle stability test. The "memory effect" provides both deionization and concentration ability without using a membrane. Over 160 mg g−1 of NaCl can be transferred from one solution to another under a 120-min deionization/concentration test. This innovative device in a semi-automatic system has been tested in real salinized underground water and seawater with the SRC of 199.4 mg g−1 and 456.6 mg g−1 for removing several ions such as Na+, K+, Mg2+, and Ca2+. [Display omitted] • A novel microfluidic electrochemical desalination (MFECDI) device was invented. • SRC and SRR of MFECDI highly depend on the NaCl concentration, reaching 130 mg g-1 and 30 mg g-1 min-1 in 600 mM NaCl. • The salt removal percentage increases to 88% in the MFECDI compared to 2.6% in a traditional macro-system. • MFECDI shows excellent stability with over 90% and 75% of SRC in the 100-cycle and 180-cycle operation. • A semi-automatic MFECDI shows 199 mg g-1 and 615 mg g-1 of SRC for salinized underground water and seawater. [ABSTRACT FROM AUTHOR]