Preparation Of 3D Graphene-based Asymmetric Electrodes For Capacitive Deionization

The water crisis has become one of the biggest problems due to the development of the industry,the lack of fresh water resources and the increase of human demand for water.The deionization of seawater or brackish water is an effective approach to solve the crisis.Capacitive deionization?CDI?is a new technology on the basis of the principle of electric double layer capacitors,with low cost,energy efficient and without secondary pollution.According to theoretical basis of CDI,the electrode materials are the critical factor for the deionization performance.The ideal electrode materials should possess good conductivity,high specific surface area,excellent physical and chemical stability and reasonable structure.Graphene is a promising candidate for the capacitive deionization due to its high theoretical specific surface area and good electrical conductivity and outstanding chemical stability.However,in the traditional capacitive deionization,the co-ion effects lead to a lower charge efficiency and unsatisfactory deionization performance.Based on the above research hotspots and problems,we designed and fabricated 3D graphene-based asymmetric electrodes by grafting functional groups on the 3D graphene.The morphologies and surface physicochemical properties of the functional3D graphene were analyzed.The deionization performance of 3D graphene-based asymmetric electrodes was investigated.The main contents are listed as follows:?1?In order to overcome the co-ion effects in the symmetric capacitive deionization?CDI?and improve the desalination performance,a novel ion-selective3D graphene electrode was rationally designed and originally prepared.These grafted groups can act as ion-selective functional coatings which can minimize the co-ion effects and increase the charge efficiency.Besides,they can increase the hydrophilicity and wettability of the electrode surface,which is beneficial for the ion transmission from the solution to the electrode.The CDI performance of functionalized 3DGR were evaluated at various operating conditions by four flow-through capacitors assembled with different combination of the sulfonated,aminated and original 3DGR.The CDI performances of the flow-through capacitor based on the ion-selective 3DGR were improved in both salt adsorption capacity and charge efficiency.The results show that the 3DNGR-3DSGR asymmetric electrodes exhibit a salt adsorption capacity of 13.72 mg g^-1 in a 500 mg L^-1 NaCl aqueous solution,which is higher than that of 3DGR-3DGR symmetric electrodes(9.48 mg g^-1).The charge efficiency reaches to 0.85 and higher than that of 3DGR-3DGR symmetric electrodes?0.40?.The improved desalination performance was attributed to the reduced co-ions effects by ion-selective groups on the 3DGR.?2?In order to separate and recover the heavy metal ions and salty ions from wastewater,a novel concept for wastewater treatment by 3D graphene-based asymmetric electrodes via capacitive deionization is presented for the first time.Instead of the traditional practice to adsorb heavy metal by stirring method,we rationally designed functional 3D graphene by grafting ethylenediamine triacetic acid?EDTA?and 3-aminopropyltriethoxysilane on the 3D graphene surfaces,respectively,and take advantage of capacitive deionization for wastewater treatment.In this process,the Pb²⁺was adsorbed by EDTA through chelation reaction and the Na⁺was adsorbed into the 3D graphene pores by electrosorption.Meanwhile the aminated 3D graphene with 3-aminopropyltriethoxysilane was used as an anode to minimize the co-ion effects and improve the removal efficiency.This research investigated the adsorption and desorption behaviors of Pb²⁺and Na⁺,and investigates the influences of operation conditions,such as pH,voltage,concentration and time on the Pb²⁺and Na⁺removal.The removal efficiency was99.9%for Pb²⁺and 98.7%for Na⁺at pH 6.0 with a flowrate of 40 mL min^-1 at 1.4 V.It is worth noting that the Pb²⁺and Na⁺can be separated and recovered in the desorption process by two steps depending on the different adsorption mechanism between Pb²⁺and Na⁺.The desorption rates were⁹9.6%for Pb²⁺and⁹7.2%for Na⁺,respectively,and remained at⁹4.3%and⁸8.2%without further degradation after the 8 cycles.Overall,the CDI with 3D graphene-based asymmetric electrodes is a promising route for the separation and recovery of heavy metals and salty ions from wastewater.