The high specific capacity of graphite as a negative electrode material for redox flow batteries, coupled with its immunity to surface contamination and very good corrosion resistance makes it one of the most attractive electrode materials for this technology. The open porosity in graphite, which is a good conductor of electricity and has a very low resistance to electrical current, limits the effective surface area. To increase the surface area of electrodes, many pretreatments, such as electrolytic or abrasive treatments, are necessary. However, the exact relationship between the increased BET surface area of a carbon sample and its electrochemical performance remains unclear.
Raman spectroscopy was used to investigate the surface properties after abrasive and etching treatments of graphite. FIG. shows Raman typical spectra from the examined graphite. 8 (a-e). Graphite is characterized by the G-band.
As was expected, the surface area of the abrasive graphite is higher than that of the etching treatment samples. This surface area is approximately three to five times larger than the theoretical BET surface area of untreated graphite. It is believed that the increased surface area will result in an increase in electrode capacity, which can be used to measure the rate-limiting factor of the electrochemical reaction. The abrasive treated samples also show better reversibility and capacity retention after a cycle.
This study also showed that oxygen production rates were significantly higher on graphite electrodes that had been etched compared with those treated by abrasion. The oxygen in this mixture is made up of the oxygen generated at the GE surfaces as oxidized groups on graphite (OxSFG) or adsorbed, trapped oxygen. The oxidation of the different SFG commences with the easily oxidizable ones, E-OxSFG, in the potential range AI and continues though the more anodic potentials till oxidizing the most difficult ones, D-OxSFG. PII and PIII plateaus will be seen during the following discharge/charge cycles due to the reduction in adsorbed or trapped oxygen.
The adsorption and dissolution of oxygen during the charge/discharge reactions of the abrasive-treated and etching-treated graphite was further studied by performing SEI layer characterization via SEM and TEM images. SEM shows that the SEI of the etching treatment graphite was thicker and had more pores than the one from the abrasive treatment. Furthermore, the etching-treated graphite has a more pronounced crystalline structure than the abrasive-treated graphite.
To investigate how roughness affects the electrochemical properties of an electrode, various impedance measurement were carried out on the treated electrodes. The values of Rct, j0 and the surface area calculated by geometrical surface (implicitly assumed in many reports) was compared to those obtained by assuming an electrode with a smooth surface. This trend was observed in their dependence on treatment.
English
Write a Message