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Capacitive deionization (CDI) is an emerging technology that supplies deionized water to resolve the fresh water shortage. CDI electrodes are mainly made up of carbon materials, of which the deionization performance is closely related to their physical properties and structures. Hence, a rational design of electrode material structure is essentially significant. Functionalized graphene (fG) in particular has recently been regarded as characteristic CDI electrode material. However, preparation of fG based on graphene oxide usually results in serious secondary pollution due to usage of highly poisonous chemicals, and thus still cannot meet the demand of practical application. It is feasible that environmentally-friendly activated carbon (AC) and small amounts of fGs can be combined rationally, and used as CDI electrodes. Here, sono-assembled AC/m-phenylenediamine (mPEA) or p-phenylenediaminefG inter-connected network architecture has been constructed for the first time successfully. The specific capacitances of the AC/fG composites were found to be significantly higher than that of the AC electrode owing to mesoporous generation. Also, among all the samples, the AC composite with 5 wt % mPEA-fG exhibited an ultrahigh electrosorption capacity of 12.58 mg/g (or 0.22 mmol/g) in NaCl solution. These observations indicate that fG can serve as an efficient conductive bridge to decrease the aggregation of AC particles, and improve the electron transfer with the composite electrode. This work provides an effective strategy for the environmental and economical electrode architectures for general applications in CDI and energy storage. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) which removes ionic species from solution by applying electric energy to carbon based electrodes is one of excellent convergence technologies combined with energy storage technology and environmental systems. In recent decades, the technology for synthesizing new carbon materials has advanced, the electrical double layer capacitance in porous structures has become better understood, and novel deionization systems have been developed. Nevertheless, achieving a higher deionization performance is required for CDI to compete with reverse osmosis for deionization. The recently introduced Hybrid CDI (HCDI), which utilizes sodium manganese oxide (Na 4 Mn 9 O 18 ) and carbon material, successfully demonstrated its superior deionization performance over conventional CDI systems. Despite the great promise of the HCDI system, the limited availability of aqueous based intercalation materials and the lack of information regarding the characteristics of the HCDI system operations are obstacles to the advancement of HCDI. Thus, we report a new HCDI system with sodium iron pyrophosphate (Na 2 FeP 2 O 7 ), which is a promising material for sodium ion batteries due to its high capacity, low cost and environmentally benign nature. The major results of the HCDI system with Na 2 FeP 2 O 7 showed a superior maximum deionization rate performance (0.081 mg g −1 s −1 ) with a comparable deionization capacity (30.2 mg g −1 ) compared to the previous HCDI system with Na 4 Mn 9 O 18 . Furthermore, the analysis of the CDI Ragone plot revealed the hybrid behavior characteristics of this HCDI system and that the high deionization capacity originated from the high capacity of Na 2 FeP 2 O 7 at a low current density, whereas the fast deionization rate originated from the supercapacitor system at a high current density. Consequently, this study on a new HCDI system with Na 2 FeP 2 O 7 contributes to expanding the understanding of the kinetic properties of the HCDI system with respect to its diverse operations. [ABSTRACT FROM AUTHOR]