Xingtao Xu

Review on carbon-based composite materials for capacitive deionization

The last five decades have witnessed the rapid development of capacitive deionization (CDI) as a novel, low-cost and environment-friendly desalination technology. During the CDI process, salt ions are sequestered by the porous electrodes once exposed to an electric field. These electrodes, acting as an ion storage container, play a vital role during desalination. In this review, various carbon-based composite electrode materials, including carbon–carbon composites, carbon–metal oxide composites, carbon–polymer composites and carbon–polymer–metal oxide composites, are systematically presented. Applications of these carbon-based composite materials for the removal of the salt ions from solution are demonstrated and they exhibit improved CDI performances compared with pristine carbon electrodes.

Facile synthesis of novel graphene sponge for high performance capacitive deionization.

Capacitive deionization (CDI) is an effective desalination technique offering an appropriate route to obtain clean water. In order to obtain excellent CDI performance, a rationally designed structure of electrode materials has been an urgent need for CDI application. In this work, a novel graphene sponge (GS) was proposed as CDI electrode for the first time. The GS was fabricated via directly freeze-drying graphene oxide solution followed by annealing in nitrogen atmosphere. The morphology, structure and electrochemical performance of GS were characterized by scanning electron microscopy, Raman spectroscopy, nitrogen adsorption-desorption, X-ray photoelectron spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The electrosorption performance of GS in NaCl solution was studied and compared with pristine graphene (PG). The results show that due to the unique 3D interconnected porous structure, large accessible surface area and low charge transfer resistance, GS electrode exhibits an ultrahigh electrosorption capacity of 14.9 mg g−1 when the initial NaCl concentration is ~500 mg L−1, which is about 3.2 times of that of PG (4.64 mg g−1), and to our knowledge, it should be the highest value reported for graphene electrodes in similar experimental conditions by now. These results indicate that GS should be a promising candidate for CDI electrode.

Novel nitrogen doped graphene sponge with ultrahigh capacitive deionization performance

As water shortage has become a serious global problem, capacitive deionization (CDI) with high energy efficiency and low cost, is considered as a promising desalination technique to solve this problem. To date, CDI electrodes are mainly made up of porous carbon materials. However, the electrosorption performance obtained by now still cannot meet the demand of practical application. Therefore, a rationally designed structure of electrode materials has been an urgent need for CDI application. Here, a novel nitrogen-doped graphene sponge (NGS), with high specific surface area and rationally designed structure was fabricated, and used as CDI electrodes for the first time. The results show that NGS exhibits an ultrahigh electrosorption capacity of 21.0 mg g−1 in ∼500 mg L−1 NaCl solution, and to our knowledge, it is the highest value reported for carbon electrodes in similar experimental conditions by now. NGS in this work is expected to be a promising candidate as CDI electrode material.

Enhanced capacitive deionization performance of graphene by nitrogen doping

Nitrogen-doped graphene (NG) was fabricated via a simple thermal treatment of graphene oxide in an ammonia atmosphere. The morphology, structure and electrochemical performance of NG were characterized by scanning electron microscopy, Raman spectroscopy, nitrogen adsorption-desorption, X-ray photoelectron spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The electrosorption performance of NG in NaCl solution was studied and compared with pristine graphene (PG). The results show that due to its high specific surface area, increased specific capacitance and low charge transfer resistance, NG exhibits high electrosorption capacity of 4.81 mg g(-1) when the initial solution conductivity is 100 μS cm(-1), which are much higher than those of PG (3.85 mg g(-1)).

Metal–organic framework-engaged formation of a hierarchical hybrid with carbon nanotube inserted porous carbon polyhedra for highly efficient capacitive deionization

Capacitive deionization (CDI) is an emerging desalination technique to offer clean water. In order to obtain high CDI performance, a rationally designed structure of electrode materials has been an urgent need for CDI application. Here for the first time a hierarchical porous carbon nanotube (CNT)/porous carbon polyhedra (PCP) (hCNT/PCP) hybrid was fabricated via in situ insertion of CNTs into ZIF-8 and a subsequent pyrolysis process. The potential of the hCNT/PCP hybrid for CDI application was demonstrated, and the results indicate that the hCNT/PCP hybrid exhibits a high electrosorption capacity of 20.5 mg g−1 with stable cycling stability due to its novel CNT-inserted-PCP porous structure, high specific surface area and good electrical conductivity. It should be expected that hCNTs/PCP should be a promising candidate for highly efficient CDI electrode materials.

Rational design and fabrication of graphene/carbon nanotubes hybrid sponge for high-performance capacitive deionization

Capacitive deionization (CDI) is an emerging technology offering a green and efficient route to obtain clean water. Up to now, the key of CDI technology has been focused on the exploration of electrode materials with a rationally designed structure and excellent performance, because the electrosorption performance of the carbon-based electrodes reported to date cannot meet the demands of practical applications of CDI. Herein, novel graphene/carbon nanotubes (CNTs) hybrid sponge (GNS) structures were designed and fabricated via directly freeze-drying graphene oxide/CNTs mixed solution followed by annealing in nitrogen atmosphere. The morphology, structure and electrochemical performance of GNS were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, nitrogen adsorption–desorption, cyclic voltammetry and electrochemical impedance spectroscopy. The results show that GNS with 20 wt% CNTs has a maximum specific surface area of 498.2 m2 g−1 and a highest specific capacitance of 203.48 F g−1 among all the samples. When used as CDI electrode, it exhibits an ultrahigh electrosorption capacity of 18.7 mg g−1, and, to our knowledge, this value is superior to those of other carbon electrodes reported recently. GNS should be a promising electrode material for high-performance CDI.

Carbon aerogels electrode with reduced graphene oxide additive for capacitive deionization with enhanced performance

Carbon aerogels (CAs) electrodes with reduced graphene oxide (RGO) additive were fabricated and used as electrosorption electrodes. The capacitive deionization (CDI) performance of the CAs electrodes with different proportions of RGO was investigated. The results show that the CAs electrodes with RGO additive exhibit better electrosorption performance compared with pure CAs electrodes and an electrode with acetylene black additive, indicating that RGO can serve as a flexible bridge to form a “plane-to-point” (RGO-to-CAs) conducting network, which can improve the electron transfer within the CAs electrode. The CAs electrode with 15 wt% RGO was further used in membrane capacitive deionization which integrates ion-exchange membranes with CDI and an extremely high desalination efficiency of 98% was obtained.

Nitrogen-doped carbon nanorods with excellent capacitive deionization ability

Nitrogen-doped carbon nanorods (NCNRs) were prepared from naturally based nanocrystalline cellulose through simple freeze drying and subsequent thermal treatment under an ammonia atmosphere at different temperatures. The morphology, structure and electrochemical performance of the NCNRs were characterized using scanning electron microscopy, transmission electron microscopy, nitrogen adsorption–desorption, X-ray photoelectron spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy, and their electrosorption performance in NaCl solution was studied. The results show that NCNRs treated at 1000 °C exhibit an extremely high electrosorption capacity of 17.62 mg g−1 when the initial NaCl concentration is 500 mg l−1, which shows great improvement compared with their undoped counterparts. The nitrogen doping proved to be a very effective method for improving the electrosorption performance, and the NCNRs should be very promising candidates as electrode materials for CDI applications.

Nitrogen-doped electrospun reduced graphene oxide–carbon nanofiber composite for capacitive deionization

A nitrogen-doped electrospun reduced graphene oxide–carbon nanofiber composite (NG–CNF) was fabricated via electrospinning by adding graphite oxide into a precursor solution and subsequent thermal treatment under an ammonia atmosphere. The morphology, structure and electrochemical performance of the composite were characterized by scanning electron microscopy, nitrogen adsorption–desorption, cyclic voltammetry and electrochemical impedance spectroscopy, and their capacitive and electrosorption performances in NaCl solution were studied. The NG–CNF composite electrode shows excellent specific capacitance (337.85 F g−1) and electrosorption capacity (3.91 mg g−1), much higher than those of pure carbon nanofibers (171.28 F g−1 and 3.13 mg g−1) and the reduced graphene oxide–carbon nanofiber composite (264.32 F g−1 and 3.60 mg g−1). The enhanced performance of the NG–CNF is ascribed to the nitrogen doping and the formation of an effective “plane-to-line” conducting network in the composite, which facilitates the electron transfer and ion transport as well as increases the specific surface area.

Carbon nanorods derived from natural based nanocrystalline cellulose for highly efficient capacitive deionization

Carbon nanorods (CNRs) were fabricated from natural based nanocrystalline cellulose through a simple thermal treatment at 800, 1000 and 1200 °C. The morphology, structure and electrochemical performance of CNRs were characterized by atomic force microscopy, Raman spectroscopy, nitrogen adsorption–desorption, cyclic voltammetry and electrochemical impedance spectroscopy. Their electrosorption performance in NaCl solution was studied. The results show that CNRs treated at 1200 °C exhibit the highest specific capacitance of 264.19 F g−1 and electrosorption capacity of 15.12 mg g−1 with the initial NaCl concentration of 500 mg l−1, due to their high specific surface area and low charge transfer resistance.