Membrane-Free Hybrid Capacitive Deionization System Based on Redox Reaction for High-Efficiency NaCl Removal

Abstract

Capacitive deionization (CDI) is a promising technology for desalination due to its advantages of low driven energy and environmental friendliness. However, the ion removal capacity (IRC) of CDI is insufficient for practical application because such a capacity is limited by the available surface area of the carbon electrode for ion absorption. Thus, the development of a novel desalination technology with high IRC and low cost is vital. Here, a membrane-free hybrid capacitive deionization system (HCDI) with hollow carbon@MnO 2 (HC@MnO 2 ) to capture sodium via redox reaction and hollow carbon sphere with net positive surface charges (PHC) for chloride adsorption is introduced. The as-obtained HC@MnO 2 with unique structure and high conductivity can improve the utilization of MnO 2 pseudocapacitive electrodes. Meanwhile, the PHC can selectively adsorb Cl − and prevent the adsorption of Na + due to electrostatic repulsion. As expected, the membrane-free HCDI system demonstrates excellent desalination performance. The system's IRC and maximum removal rate are 30.7 mg g −1 and 7.8 mg g −1 min −1 , respectively. Moreover, the proposed system has a low cost because of the absence of expensive ion exchange membranes (IEM), which is suitable for practical application. The excellent performance of this HCDI makes it a promising desalination technology for future use.

Tailorable Phase and Structural MnO2 as Electrode for Highly Efficient Hybrid Capacitive Desalination (HCDI)

“…Note that SAC of poorly crystalline MnO2-1h electrode is approximately proportional to the salt concentrations of the feed solutions ( Figure 6B), which was also observed for MnO2 electrodes by other investigators. 23,28 The SAC of MnO2-1h electrode in 100, 200, and 500 mg L −1 NaCl solutions are 13.84, 16.14, and 21.32 mg g -1 , respectively, suggesting that our poorly crystalline MnO2-1h is among the top two best MnO2 electrodes as compared to other MnO2-based electrodes ( Figure 7 and Table 2).…”

“…8,20 Of various faradaic electrode materials, MnO2 has been demonstrated as one of the most attractive electrode materials for water desalination due to its tailorable crystal structure, low cost, low aquatic toxicity, high stability in aqueous environment, high electrochemical activity, and high theoretical specific capacity (1370 F g −1 ). [7][8][20][21][22][23][24][25][26][27][28] Several attempts have been reported to evaluate the saline desalination performance with MnO2-based electrode materials. Zou et al reported for the first time the application of MnO2/nanoporous carbon (NC) composite as electrodes in CDI, and found that a SAC of 0.987 mg g −1 was achieved for desalination of 50 mL NaCl (35 mg g −1 ).…”

“…By using hollow carbon@MnO2 as the cathode and positive-charged hollow carbon sphere (PHC) as the anode in a membrane-free HCDI system, Qiu et al found that the SAC as high as 19.6 mg g −1 was obtained in 100 mg L −1 NaCl. 28 In addition to these hybridization approaches, other strategies such as electrode reversing, crystal structure tailoring, and morphologic altering of MnO2 materials were also reported. [7][8]23 By using MnO2 as the anode rather than the cathode in an HCDI cell, Qiu et al reported an interesting finding that a SAC up to 14.9 mg g −1 was achieved in 500 mg L −1 NaCl and that the cell was highly stable with a SAC retention ratio of 90.5% over 350 cycles.…”

A Polyoxometalate‐Based Binder‐Free Capacitive Deionization Electrode for Highly Efficient Sea Water Desalination

“…[40][41][42][43] To improvedesalination performance, integrationo fp seudocapacitance and electric double-layer capacitance in am ixed capacitive-deionization (MCDI) electrode is af easible method. [44][45][46][47][48][49][50] In MCDI, sea water desalination is realizedb yb oth Faradaic reaction and electrostatic attraction. [51][52][53] Recently,s everal MCDI electrodes have been explored and exhibited clearly higher SAC and SAR than neat CDI electrodes.…”

“…However, in CDI, carbon‐based materials can only provide electric double‐layer capacitance, which limits further enhancement of the salt adsorption capacity (SAC) and salt adsorption rate (SAR) . To improve desalination performance, integration of pseudocapacitance and electric double‐layer capacitance in a mixed capacitive‐deionization (MCDI) electrode is a feasible method . In MCDI, sea water desalination is realized by both Faradaic reaction and electrostatic attraction .…”