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decreases continuously. When the pH is up to 12, multi-oxidation peaks appear on the voltammograms without the corresponding reduction peaks. This indicates that the oxidation products of tiron are difficult to be reduced in strongly alkaline solutions of pH up to 12. It is manifested by the above cycle voltammograms that in acidic solutions of pH lower than 4, the electrode reaction of tiron is simple with a relatively high potential, which is favorable for the application of tiron in RFBs. The proposed charge-discharge processes of the tiron redox reaction can be described as follows: (7.3) Hence, following the pH study described above, we decided to use acidic electrolyte solutions in order to facilitate the quinones redox reaction. Moreover, selecting an appropriate acidic solution for benzoquinones derivates (p-hydroxybenzene, o- hydroxybenzene or disodium 4,5-dihydroxy-1,3-benzenedisulfonate) as catholyte and anthraquinone derivates as anolyte (anthraquinone 2,7-disulfonate) (Figure 7.6a). Initially, it is done a cyclic voltammetry for different solvents (Figure 7.6b) at low scan speed, 1mV/s, therefore study the reversibility parameters in several conditions for p-hydroxybenzene, which are shown in Table 7.1. Moreover, PEIS measurements (Figure 7.6c) show a lower charge transfer and also minor electrical resistance when using acid solvents (3 M H2SO4 and 3 M MeSO3H) not mixed with ethanol. 1500 1000 500 0 -500 -1000 -1500 b) 1 mV s-1 20 15 10 5 0 c) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 E vs. NHE / V 0 5 a) E0 = 0.70 V vs. NHE + 2H+ + 2e- 3 M H SO 24 3 M MeSO H 3 EtOH:H SO 3M (1:1) 24 EtOH:MeSO3H 3M (1:1) Equivalent circuit 10 15 20 Re(Z) / Ohm 0.05 M Hydroquinone i / mA/g Im(Z) / Ohm Figure 7.6. - a) p-benzoquinone redox reaction showing the standard potential. b) Cyclic voltammetry done for 0.05 M p-benzoquinone reaction showed in a) using different solvents (3 M H2SO4, 3 M MeSO3H, EtOH:3 M H2SO4 (1:1) and EtOH:3 M MeSO3H (1:1)) at 1mV/s. c) PEIS at 0.7 V vs. NHE in the same conditions as b) for carbon felt as working electrode. It also shows the equivalent circuit for the system. 157PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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