The electrode material is one of the key materials for vanadium redox flow battery. Due to the large surface area and good stability, the graphite felt is dominant. However, the graphite felt material has poor hydrophilicity, and its electrochemical activity is not good enough. This paper on the graphite felt electrode respectively chromic acid oxidation method, mixed acid electrochemical oxidation method and hydrothermal ammoniation method for processing vanadium redox flow battery to improve its activity, the main conclusions are as follows: after chromic acid oxidation, hydrophilicity and electrochemical activity of the Shi Mozhan electrode increased, the optimum processing time is 5 h. In the 10 to 70mA/cm2, the battery with the graphite felt after the treatment with the chromate has a good charge discharge performance.
The efficiency of the battery voltage is about 90.9% and the energy efficiency is up to 79.4% at mA/cm2. FT-IR and SEM show that the carbon fiber processing after the oxygen functional groups increased and the surface gully increased, the former increased the number of active sites on surface of carbon fiber in VO2+/VO2+ electrode reaction, which increases the contact area of vanadium ion and electrode. The hydrophilicity and electrochemical activity of the graphite felt electrode were obviously enhanced after the electrochemical anodization of sulfuric acid and phosphoric acid. The optimum treatment conditions were as follows: the current density was 50 mA/cm2 and the treatment time was 4min. The processing of graphite felt is loaded into the cell, showing good charge and discharge performance in 10 to 50 in mA/cm2, 20 in mA/cm2, the battery voltage efficiency of 88.2%, energy efficiency reached 79.6%. FT-IR and SEM tests showed that the oxygen functional groups on the surface of carbon fiber increased, and the surface roughness increased. The results of XRD show that the crystallite size and the space between the carbon fibers decrease, which indicates that the electrochemical oxidation etching the graphite crystallites in the outer layer, which is the reason for the increase of the surface roughness of the carbon fiber. For the first time put forward 180 degrees in the hydrothermal ammoniation of graphite felt electrode, hydrophilicity and electrochemical activity of treated graphite felt electrode increased, the optimum processing time is 15 H.
The processing of graphite felt is loaded into the battery, charge and discharge can be stabilized in 20 to 60 in the range of mA/cm2. In 20mA/cm2, the voltage efficiency can reach 83.7% and the energy efficiency is up to 85.3%. The reason lies in the improvement of electrode active carbon fiber surface increased nitrogen content of functional groups and nitrogen functional groups to enhance the positive charge density of the alkaline carbon fiber and its adjacent carbon atoms, to speed up the transfer speed of vanadium ions and electrons in the electrode / solution interface, improve the vanadium ion diffusion process, reduces the electron transfer resistance.
