Activated Electrospun Carbon Nanofibers For Capacitive Desalination

Fresh water is a precious resource, and is also the basic substance for human survival and development. Currently, the shortage of freshwater resources has become an important factor restricting the social and economy development. Fresh water can be obtained through the desalination of brackish water, sea water and wastewater reclamation and reuse, etc. Therefore, the development of efficient desalination technology is of great importance. There also exist some problems such as high energy consumption, high cost, hardly regeneration in traditional desalination methods such as electrodialysis and reverse osmosis (RO), which limit its practical application. Capacitive deionization technology is a new kind of desalting technology developed recently. It is based on the electrochemical electric double layer charge and discharge theory to realize desalting and electrode regeneration with DC as driving force. It has some advantages such good regeneration reversibility, low operation pressure, low energy consumption, high water recovery, no redox reaction and no secondary pollution etc. Compared with the existing RO and electrodialysis, the capacitive deionization technology has a promising application prospect.In this paper, with commercial activated carbon fiber (ACF) and activated electrospun carbon nanofiber (A-ECNF) as self-supporting electrode materials, the capacitors were assembled for desalination study. The effects of target voltage, charging current density and flow velocity on the desalination properties of the capacitors have been investigated. The A-ECNF electrode was characterized by cyclic voltammetry, electrochemical impedance and scanning electron microscopy etc. The main achievements were summarized as follows:1. The commercial ACF as self supporting electrode was assembled in the capacitor for desalination study. The effects of target voltage and charging current density on the desalination amount and current efficiency were studied. The results showed that with the increasing of the target voltage, the desalination amount became higher, and the current efficiency increased at first and then decreased. And too high target voltage results in the pH value variation of the electrode surface. Under the conditions of1.2V target voltage and the current density of38mA/g, the desalination amount is4.92mg/g with the current efficiency of32.3%. The electrochemical characterization indicated the hindered ion diffusion on the electrode surface, which leads to a low desalination capacity.2. The electrospun carbon nanofibers (ECNF) were prepared by electrospinning with the polyacrylonitrile (PAN) as the precursor, followed by preoxidation and carbonization. The effects of spinning parameters on the morphology of ECNF, such as the concentration of PAN solution, spinning voltage and receiving distance were studied. The results showed that the ECNF possessed a uniform morphology with the fiber diameter of about220nm under the optimized electrospining conditions with the PAN concentration of9%, the applied voltage of15kV and the receiving distance of20cm,. The ECNF was further treated with the ZnCl2as the activating reagent to obtain the activated electrospun carbon nanofibers(A-ECNF). As a result, the A-ECNF surface still kept its original morphology and good flexibility, indicating that the A-ECNF can be used as a self supporting electrode for the capacitor assembly. The contact angle measurement showed that the water wettability of A-ECNF was improved significantly compared with the pristine ECNF. Besides, the effects of the activation dosage (ZnCl2/ECNF ratio) had been investigated by N2-adsorption/desorption test and electrochemical characterization. It was found that the fiber specific surface area increased gradually with increasing the ZnCl2/ECNF ratio, and the A-ECNF specific surface area reached as high as430m2/g with the ratio of2:1, much higher than that of the pristine ECNF of12.4m2/g. Cycling voltammetry and electrochemical impedance measurement demonstrated that with the increase of ZnCl2/ECNF, the capacitance current of A-ECNF electrode enhanced, and the charge transfer resistance decreased accompanying with the elevation of the ion migration rate on the electrode surface.3. The A-ECNF was used as a self supporting electrode to assembly capacitor for desalination study with constant current charge-discharge mode. The desalination performance of A-ECNF electrode of different ZnCl2/ECNF ratios had been compared. It was found that the desalination performance of A-ECNF electrode had a significant improvement after activation of ZnCl2. And the desalination amount of the A-ECNF electrode increased with the increase of ZnC^/ECNF ratio. At the ZnCl2/ECNF ratio of2:1, the desalination rate of A-ECNF electrode was the fastest, and the desalination amount was highest. So the A-ECNF with the ZnCl2/ECNF ratio of2:1was chosen as electrode material for capacitive desalination. The multi-cycling test by constant current charge-discharge mode demonstrated the stability of the A-ECNF electrodes. And the effects of charging current density, solution flow rate and target voltage on the desalination amount and current efficiency of the capacitor were investigated. It was concluded that under the optimized conditions(i.e. charge current density of36mA/g, solution flow rate of10ml/min; the target voltage of1.2V), the desalination amount of A-ECNF electrode was up to10.2mg/g, with the57.1%current efficiency obtained, which was much higher than that of the pristine ECNF electrode. In addition, the desalination performance of ZnCl2activated ECNF was compared to literature results, and it is comparable to the state-of-art results of capacitive desalination. The good desalination performance and the cycling stability demonstrated that the A-ECNF is promising for the electrochemical capacitive desalination application.