Capacitive deionization (CDI) has emerged as a promising technique for brackish water desalination. Here, composites of polypyrrole grafted activated carbon (Ppy/AC) were prepared via in situ chemical oxidative polymerization of pyrrole on AC particles. The Ppy/AC cathode was then coupled with a MnO 2 anode for desalination in a membrane-free CDI cell. Both the Ppy/AC and MnO 2 electrodes exhibited pseudocapacitive behaviors, which can selectively and reversibly intercalate Cl − (Ppy/AC) and Na + (MnO 2 ) ions. Compared to AC electrodes, the specific capacitances of Ppy/AC electrodes increased concurrently with the pyrrole ratios from 0 to 10%, while the charge transfer and ionic diffusion resistances decreased. As a result, the 10%Ppy/AC-MnO 2 cell showed a maximum salt removal capacity of 52.93 mg g −1 (total mass of active materials) and 34.15 mg g −1 (total mass of electrodes), which was higher than those of conventional, membrane, and hybrid CDI cells. More notably, the salt removal rate of the 10%Ppy/AC-MnO 2 cell (max 0.46 mg g −1 s −1 to the total mass of active materials and 0.30 mg g −1 s −1 to the total mass of electrodes) was nearly 1 order of magnitude higher than those in most previous CDI studies, and this fast and efficient desalination performance was stabilized over 50 cycles.

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Salinity gradient (SG) is a natural and renewable energy source existing in estuaries, and can also be produced during various desalination and industrial processes. Here, a new method is proposed to efficiently recover SG energy based on chloride-ion (Cl − ) extraction and insertion with metal chloride electrodes and the Donnan potential over a cation-exchange membrane in a concentration flow cell. Three different metal chloride electrodes (BiCl 3 , CoCl 2 , and VCl 3 ) were investigated in the cell, and their properties after discharging in 30 g L −1 (seawater) and 1 g L −1 (river water) NaCl solutions were studied by cyclic voltammetry, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy. The cell with BiCl 3 electrodes yielded the largest power density (max. = 3.17 W m −2 ) compared to that of CoCl 2 and VCl 3 electrodes, which was higher than those of most previous technologies for SG energy recovery. Fast Cl − extraction and insertion processes were observed on BiCl 3 electrodes due to small charge transfer resistance and Cl − diffusion resistance. BiCl 3 was reduced to metal Bi as Cl − released from the electrode to river water, while metal Bi was oxidized to BiCl 3 as Cl − inserted into the electrode from seawater.

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