Q2. What is the importance of electrodes in a battery?
the selection of suitable electrode materials, with a high electrical conductivity, a high surface area, high mechanical strength, a good electrolyte stability and electrochemical activity [5], is of fundamental importance for obtaining a battery with an excellent performance.
Q3. What is the effect of the residual C-OH groups present in TRGO-1?
In addition, the residual C-OH groups present in TRGO-1 may act as active reaction sites and contribute to enhance the electrochemical response.
Q4. What is the role of graphene in the development of more active positive electrodes?
As the electrochemical kinetic limitation of VRFBs is in the positive side [22], an understanding of how the different structural and physico-chemical properties of the graphene materials influence their electrochemical performance is key to the development of more active positive electrodes and, consequently, more efficient batteries.
Q5. What is the effect of the improved electrochemical performance of TRGO-2?
Its improved electrochemical performance could be attributed mainly to the less defective structure of the restored graphitic lattice (higher C-sp2 bond fraction).
Q6. What is the main cause of self-discharge in other redox flow batteries?
the use of the same metal in both halfcells ([VO2+]/ [VO2+] in the positive electrolyte and V3+/V2+ in the negative one), eliminates the problem of cross-contamination, the main cause of self-discharge in other redox flow batteries [4].
Q7. What are the drawbacks of graphite felts?
The poor kinetics and reversibility of commonly used graphite felts [6], carbon cloths [7] and carbon fibers [8] restrict their use as active electrodes.
Q8. What is the positive electrode in the VRFB?
TRGO-1 is shown to be the more suitable positive electrode in the VRFB, as it exhibits a markedly enhanced electrochemical activity (higher peak current densities and lower redox reactions overpotentials) and a better kinetic reversibility towards these oxidation/reduction vanadium processes than TRGO-2.
Q9. What is the effect of the thermal exfoliation/reduction on the graphite oxide?
The graphene materials TRGO-1 and TRGO-2, obtained by the thermal exfoliation/reduction at 1000ºC of graphite oxides with distinct characteristics, present different structural and physicochemical properties which result in different electrochemical performances towards the VO2+/VO2+ redox reactions.
Q10. What was the atomic oxygen content on the surface?
The atomic oxygen content on the surface was determined by XPS analysis in a VG-Microtech Multilab 3000 spectrometer (SPECS, Germany) equipped with a hemispherical electron analyser and a MgKα (hυ = 1253.6 eV) X-ray source.
Q11. What is the effect of the positive electrode on the coulombic efficiency of the VRB?
As a consequence of the better reversibility of the [VO2+]/[VO2+] redox processes on the TRGO-1 electrode and the lower electrochemical polarization of this electrode (see CVs, Figure 2), the coulombic efficiency of VRB-1 is clearly improved [41].
Q12. What are the reasons for the poorer electrochemical performance of TRGO-2?
All these factors confirm the poorer electrochemical activity of TRGO-2 [34], probably due to its greater number of structural defects and lower electrical conductivity (Table 1).
Q13. What is the ohmic resistance of the TRGO-1 electrode?
the Z´ value at Z´´ = 0 Ω, including the ohmic resistance of the electrolyte, the working electrode and the contact resistance is also smaller for TRGO-1 (3 Ω vs ~ 4 Ω for TRGO-2 electrode).
Q14. What is the effect of the thermal exfoliation on graphene materials?
In a previous paper [21] the authors investigated the suitability of graphene materials, prepared by the direct thermal exfoliation/reduction of a synthetic graphite-based GO at different temperatures, as positive electrodes in a VRFB and observed an excellent behavior in that obtained at 1000 °C.