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.
In a vanadium redox flow battery, the traditional polyacrylonitrile based graphite felt (GF) electrode suffers the problems of low electrochemical catalytic activity and low specific surface area. To improve the performance of the GF electrode, we prepared phosphorus and sulphur co-doped reduced graphene oxide (PS-rGO) as catalyst with the simple treatment of reduced graphene oxide (rGO) in the mixture of phytic acid and sulfuric acid. The GF electrode modified with PS-rGO (PS-rGO-GF) was characterized by scanning electron microscope, specific surface area, X-ray photoelectron spectroscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and charge-discharge tests. The PS-rGO-GF shows enhanced performance toward VO2+/VO2+ redox reaction. The battery with the PS-rGO decorated GF presents an excellent battery performance with the energy efficiency of 81.37 % at the current density of 80 mA cm-2 and the corresponding discharge capacity of 772 mAh due to the high catalytic activity of PS-rGO.
Redox flow batteries are considered a promising technology for grid energy storage. However, capacity decay caused by crossover of active materials is a universal challenge for many flow battery systems, which are based on various chemistries. In this paper, using the vanadium redox flow battery as an example, we demonstrate a new gel polymer interface (GPI) consisting of crosslinked polyethyleneimine with a large amount of amino and carboxylic acid groups introduced between the positive electrode and the membrane. The GPI functions as a key component to prevent vanadium ions from crossing the membrane, thus supporting stable long-term cycling. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted to investigate the effect of GPI on the electrochemical properties of graphitic carbon electrodes (GCFs) and redox reaction of catholyte. X-ray photoelectron spectroscopy (XPS) and 1H nuclear magnetic resonance (NMR) spectra demonstrated that the crosslinked GPI is chemically stable for 100 cycles without dissolution of polymers and swelling in the strong acidic electrolytes. Results from inductively coupled plasma mass spectrometry (ICP-MS), Fourier-transform infrared (FTIR) spectroscopy, and energy-dispersive X-ray (EDX) spectroscopy proved that the GPI is effective in maintaining the concentration of vanadium species in their respective half-cells, resulting in improved cycling stability because of it prevents active species from crossing the membrane and stabilizes the oxidation states of active species.