Preparation Of Manganese Oxide/Carbon Nanofiber For Capacitive Deionization

Capacitive deionization（CDI）, as a new desalination technology, has attracted much attention due to its low energy consumption, easy operation, no secondary pollution. The performance of electrode materials is the key to CDI. Ideal electrode materials should have high specific surface area, good electrical conductivity, good wettability, reasonable pore distribution and easy preparation. Electrospun carbon nanofibers（ECNFs） are suitable as self-supporting electrode for desalination due to high mechanical strength, flexibility and free of binder. However, low wettability and specific surface area of ECNF leads to low specific capacitance, which limiting its application in desalination.Transition metal oxide ECNF possesses many merits such as high specific capacitance, wide voltage window, environmentally friendly, wealth sources and so on. In this paper, MnO₂ film was modified on the ECNF surface by simply being immersed in KMn O₄ solution for a certain amount of time. Based on the redox reaction principle of KMn O₄ and carbon, MnO₂ was deposited on the ECNF surface. And the electrochemical performance of the MnO₂/ECNF composite has been much improved after further annealing treatment. The morphology and electrocapacitive desalination properties of the resulting the MnO₂/ECNF composite was investigated by adjusting the reaction time. The main results are as follows:Under the conditions of various concentrations of KMn O₄ solution and reaction time, the optimal deposition amount was obtained. The scanning electron microscope（SEM） and transmission electron microscopy（TEM） have been applied to characterize the morphology and structure of the composite fibers. It was found that the MnO₂ film is dense, uniform, and the film with the thickness of around 30 nm was obtained after immersion 4 h. X-ray diffraction（XRD） and TEM demonstrate that K+ doped birnessite type MnO₂ exists on the ECNF surface. The dense film has been transferred into the brush-like porous structure after annealing treatment, characterized by SEM and TEM. Thermal gravimetric analysis（TGA） and X-ray photoelectron spectroscopy（XPS） characterization indicates that annealing treatment process promotes the reaction between MnO₂ and carbon, accompanying with the partial formation of III-valent Mn. The mixed-valent composition will be helpful to the ionic transportation and adsorption in the electrochemical process.The as-prepared MnO₂/ECNF composite fiber was used as the self-supporting electrode for the electrochemical and capacitive deionization study in the asymmetric capacitor. Cyclic voltammetry（CV） and electrochemical impedance spectroscopy（EIS） analysis reveal that the annealed MnO₂/ECNF sample with 1 h immersion in KMn O₄ solution presents a optimal electrochemical properties, and the specific capacitance arrives at 46.14 F/g, far higher than ECNF（8.54 F/g）. From EIS test, it has been found that the diffusion resistance decreases, and there is no obvious increase of the electron transfer resistance. Desalination test indicates a stable desalination amount of 8.14 mg/g and high current efficiency of 68.54% were obtained on the annealed composite electrode, which is better than those of composite electrode without any annealing treatment ECNF（desalination amount of 4.46 mg/g; current efficiency of 40.42%）, much higher than those of ECNF（desalination amount of 2.01 mg/g; current efficiency of 18.36%）.Therefore, the annealed MnO₂/ECNF composite nanofiber has better electrochemical and desalination performance, which is much promising for capacitive deionization application.