Activated Carbon Electrodes In Vanadium Redox Flow Battery

In this paper, preparation and modification of carbon material electrodes for vanadium redox battery (VRB) was mainly studied. Multi-walled carbon nanotubes-graphite powder (MWNT-GP) composite electrodes were prepared by mixture of multi-walled carbon nanotubes and graphite power with various ratios, and their electrochemical properties and stability were studied. Polyacrylonitrile (PAN)-based felt electrodes were graphitized at high temperature with or without boric acid as catalyst and mechanism of improving the electrochemical activity of boron-doped felt electrode was also investigated. The main points can be summarized as following.The surface morphologies of the MWNT-GP composite electrodes were characterized by scanning electron microscope (SEM), and electrochemical behaviors were investigated by cyclic voltammograms (CV), impedance spectroscopy (EIS) and charge-discharge technique. The best composition for the positive electrode of VRB with different content of MWNT is 15wt.%. The current efficiency of VRB using 15wt.% MWNT-GP composite electrode has values above 93% and the voltage efficiency decrease with current densities increasing under current densities of 20 mA·cm-2～80 mA·cm-2. The best composition for the negative electrode of VRB with different content of MWNT is 5wt.%. The improvement in the electrochemical activity of 15wt.% MWNT-GP composite electrode is ascribed to the decrease in the total resistivity of vanadium ions adsorption and desorption from MWNT-GP composite electrode and the charge transfer resistivity of MWNT-GP electrode at the electrolyte/electrode interface.PAN-based felt electrodes were graphitized at different high heat temperature of 2000℃,2300℃,2500℃,2700℃,with different treated time of 7min,10min,20min,30min. Some PAN-based carbon felt sheets were immersed into 1 wt.%,5 wt.%,7 wt.%,10 wt.% boric acid solution before graphitization. The electrochemical behaviors were investigated by CV and charge-discharge technique. The results indicate that electrochemical activity increase with temperature and the concentration of boric acid solution, and the appropriate time is 20min.Mechanism of improving the electrochemical activity of boron-doped felt electrode was investigated by XRD,SEM,CV,EIS. Graphite crystallite grains on boron-doped PAN-based felt are smaller than on non-boron-doped PAN-based felt. Resistivities measurement shows that the resistivity of PAN-based felts after graphitization decrease apparently and PAN-based felt containing boron has lower resistivity than the boron-free controls. The improvement in the electrochemical activity of boron-doped PAN-based felt electrode is ascribed to the increase in the electrical conductivity and the increase in fringe carbon atoms resulting from small size of graphite crystallite grains.