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We report on the synthesis of carbon nanotubes (CNTs) by direct thermal decomposition of ferrocene (Fe(C5H5)2). Our studies indicate that presence of a small amount of sulfur along with the ferrocene in the decomposition process strongly affects the quality of the CNTs produced and is crucial for obtaining thin diameter nanotubes. Raman spectroscopic investigations suggest that the atomic ratio of sulfur to the total iron plus sulfur content of approximately 0.09 yields CNTs with highly crystalline structure having diameters ranging from 0.85 nm to 1.75 nm. Electrochemical double layer capacitor electrodes fabricated from these CNTs show impressive energy storage properties, capable of delivering a maximum power density of approximately 27 KW/kg and energy density of approximately 2.12 Wh/Kg.

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Owing to the lack of systematic kinetic theory about the redox reaction of V(III)/V(II), the poor electrochemical performance of the negative process in vanadium flow batteries limits the overall battery performance to a great extent. As the key factors that influence electrode/electrolyte interfacial reactivity, the physicochemical properties of the V(III) acidic electrolyte play an important role in the redox reaction of V(III)/V(II), hence a systematic investigation of the physical and electrochemical characteristics of V(III) acidic electrolytes with different concentrations and related diffusion kinetics was conducted in this work. It was found that the surface tension and viscosity of the electrolyte increase with increasing V(III) concentration, while the corresponding conductivity shows an opposite trend. Both the surface tension and viscosity change slightly with increasing concentration of H 2 SO 4 , but the conductivity increases significantly, indicating that a lower V(III) concentration and a higher H 2 SO 4 concentration are conducive to the ion transfer process. The electrochemical measurements further show that a higher V(III) concentration will facilitate the redox reaction of V(III)/V(II), while the increase in H 2 SO 4 concentration only improves the ion transmission and has little effect on the electron transfer process. Furthermore, the diffusion kinetics of V(III) have been further studied with cyclic voltammetry and chronopotentiometry. The results show that an elevated temperature facilitates the V(III)/V(II) redox reaction and gives rise to an increased electrode reaction rate constant ( k s ) and diffusion coefficient [ D V(III) ]. On this basis, the diffusion activation energy (13.7 kJ·mol −1 ) and the diffusion equation of V(III) are provided to integrate kinetic theory in the redox reaction of V(III)/V(II).

Carbon nanofibers with multi-scale pores have been easily constructed by synchronous water etching during the carbonization process of PAN nanofibers, reducing the additional consumption of energy and time. After etching by high-temperature water vapor, the fiber surface becomes more coarse, and large amounts of etched pits are formed, effectively increasing the electrode’s specific surface area and hydrophilicity. Oxygen content is also significantly increased, which may effectively increase the electrocatalytic active sites of the electrode. Electrochemical tests verified the improved electrocatalytic activity and increased effective surface area. As a result, the VRFB single cell with water vapor etched carbon nanofibers as its electrode shows higher battery efficiencies than that with pristine carbon nanofibers; the energy efficiency improves by nearly 9.4% at 200 mA·cm-2. After 100 charge/discharge cycles, the battery efficiency has no obvious attenuation, and the capacity attenuation rate of single cycle is nearly 0.26%，suggesting a satisfactory cycling stability. This green and simple method for constructing multi-scale porous carbon nanofibers electrode is expected to achieve large-scale production of high-performance electrode materials, and can be applied in various electrochemical energy storage systems.