Self-supporting porous carbon nanofibers with opposite surface charges for high-performance inverted capacitive deionization

Capacitive deionization (CDI) has attracted much attention as a novel desalination technology. However, the oxidation of carbon anode and co-ion repulsion during charging are key issues which limit the CDI development. Inverted capacitive deionization (i-CDI) can attenuate these effects. Here, a simple method was presented to get the self-supporting carbon nanofibers electrodes with opposite surface charges to improve the deionization performance of i-CDI device. The electrospun polyacrylonitrile/polymethyl methacrylate (PAN/PMMA)-derived flexible carbon nanofiber was found to be negatively charged (N-CNF). The positively charged porous carbon nanofiber (P-CNF) was obtained by further heat treatment of N-CNF in the presence of ZnCl2. The P-CNF possesses much low negative potential of zero charge (PZC = 0.68 V), opposite to N-CNF (+0.32 V), measured by differential capacitance tests. With P-CNF as cathode and N-CNF as anode, the asymmetric capacitor device realized high performance i-CDI, and delivered a super high deionization capacity of 30.4 mg/g and very fast deionization rate (1.0 mg/g/min) with low specific energy consumption. The adsorption-desorption process in iCDI was analyzed. The device exhibits superior cycling stability and regenerability over 70 cycles. These flexible carbon nanofibers with substantial surface charge difference render it favorable for i-CDI application.