Preparation Of Conductive Additives/Activated Carbon Composite Electrode For Asymmetric Capacitive Desalination

In recent years, capacitive deionization (CDI) has attracted the scientists’interest as a new and promising technology for desalination of brackish or sea water due to its high energy efficiency, high water recovery, simple operation process with no secondary pollution. The low current efficiency and adsorption amount, however, are still the most important causes limiting the development of CDI. This paper focuses on the studies about preparation and properties of the composite active carbon electrodes by combining with conductive additives, such as the chemically modified graphenes and ZnO nanoparticles. According to the difference of the Zeta potential of the conductive additives, asymmetric capacitors were designed and assembled. With the addition of conductive additives in the active carbon electrodes, the salt removal amount and current efficiency can be enhanced by adjusting the internal charge state of electrodes, thus weakening the effect of co-ions or conter-ions transportation. And the electrical resistance of the electrodes can be decreased due to excellent conductivity of the conductive additives. The major results are as follows:1. Sulfonated graphene was prepared by grafting of sulfonated diazoniun salt on graphene oxide followed by reduction with NaBH4. The sulfonated graphene was characterized by Fourier transform infrared spectroscope (FT-IR) and scanning electron microscope (SEM), respectively. It indicates that the sulfonic groups have been successfully modified on grphene. The sulfonated graphene/activated carbon composite electrode (GP-SO3H/AC) was prepared by mixing GP-SO3H as an additive. Comparing with AC electrode, this composite electrode exhibits an ideal double layer capacitive behavior and high conductivity, confirmed by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The asymmetric capacitor (GP-SO3H/AC I AC) was assembled by using the resultant GP-SO3H/AC as negative electrode and AC as positive electrode for capacitive deionization. Under the constant current charging-discharging condition, the salt removal amount of10.87mg/g in single cycle was obtained, about2.4times higher than that of the normal AC丨AC capacitor (4.5mg/g). And the current efficiency was improved dramatically to88.2%owing to the facile adsorption of sulfonic groups to cations, and the shielding effect of sulfonic groups on the counter-ions.2. Aminated graphene was fabricated by directly grafting GO with3-aminopropyltriethoxysilane (AMPTS). The covalent functionalization of amine functional groups on graphene oxide was confirmed by FT-IR, XRD, EDX and Raman spectroscopy. The aminated graphene/activated carbon composite electrode (GP-NH2/AC) was prepared by mixing GP-NH2with AC. The electrochemical properties of GP-NH2/AC composite electrode were tested by CV and EIS, indicating the improvement of specific capacity and conductivity. The asymmetric capacitor (GP-NH2/AC I AC) was assembled by using the resultant GP-N2H/AC as positive electrode and AC as negative electrode for capacitor deionization. The experimental results show that the salt removal amout of GP-NH2/AC丨AC of7.63mg/g is achieved, about1.7times higher than that of normal AC I AC capacitor, and the current efficiency is increased to77.63%.The asymmetric capacitor (GP-SO3H/AC I GP-NH2/AC) capacitor was assembled by using the resultant GP-N2H/AC as positive electrode and GP-SO3H/AC as negative electrode for capacitor deionization. The salt removal of10.58mg/g was obtained, comparable to that on GP-SO3H/AC I AC. It is worthy to note that electrosorption rate is significantly improved with the charge-discharge time30%faster than that of GP-SO3H/AC丨AC, and the current efficiency increased to92.79%. The stability was tested by continuous charge-discharge cycling with only15%decrease in adsorption capacity. The re-desalination amount levels off in3-4days without further reduction by time.3. Zinc oxide/activated carbon composite electrode was prepared by mixing ZnO nanoparticles as additive. SEM spectroscopy confirms the uniform dispersion of ZnO in AC. Two kinds of ZnO/AC electrodes were prepared with two grain sizes of50nm (ZnO-50) and200nm (ZnO-200). By comparison, ZnO/AC has more even surface and better adsorption performance. Using AC electrode as the counter one, the asymmetric capacitor with ZnO/AC as positive electrode (+ZnO/AC丨AC) and capacitor with ZnO/AC as negative electrode (-ZnO/AC I AC) were studied, respectively, for the first time. It is found that-ZnO/AC丨AC has good salt removal ability and excellent cycling stability, comparing with+ZnO/AC丨AC.Salt removal amount of9.35mg/g in single cycle and current efficiency of80.53%was achieved, two times higher than that of normal AC丨AC capacitor. While+ZnO/AC丨AC has poor desalination performance. The causes that ZnO in the+ZnO/AC丨AC capacitor is noneffective were further investigated by studying the Zeta potential of ZnO, polarization of electrode in charging process and ZnO release amount during long term cycling. The feasibility of-ZnO/AC丨AC capacitor with ZnO additives in positive electrode is demonstrated. Meanwhile, these studies provide important criteria and theoretical foundation for selection of electrode materials and their application in capacitive desalination.