There are numerous reports of the effects of electrochemical pretreatment on the rates of electrochemical reactions on carbon electrodes. In the case of the VIV-VV redox couple, the kinetic rates at carbon felt electrodes were reported14 to decrease after anodic treatment. Likewise, electrodes consisting of graphene oxide on a glassy carbon substrate were reported15 to show enhanced kinetics after cathodic treatment. There are also reports16–19 of enhancement of VIV/VVkinetics after anodic treatment.
We have previously reported20-25 for a range of carbon materials that the electrode kinetics of the VIV-VVreaction is enhanced by cathodic treatment of the electrode and inhibited by anodic treatment, as shown in Fig. 1a. Furthermore, after several oxidation and reduction treatments, an electrode can be repeatedly and reproducibly toggled between cathodically activated and anodically deactivated states. However, an oxidized or a reduced electrode treated at the same potential may result in different activities.
In this investigation we show that there is even greater dependence on the initial state of the electrode. As shown in Fig. 1b, initial treatment of the electrode is not reproducible and it is only after several anodization and cathodization steps that the electrode can be reproducible toggled between activated and deactivated states; i.e.to attain this stable cathodization-anodization treatment regime the electrode must be first “aged”. In considering the effects of anodization on a carbon surface it must be borne in mind that carbon can corrode at anodic potentials and that this can sometimes lead to roughening of the surface with a consequent effect on electrode current. Therefore, it is not surprising that, as shown in Fig. 1b, anodization of an electrode initially results in activation. However, after repeated cathodization and anodization, the effect of anodization reverses so as to give the stable regime of anodization resulting in deactivation and cathodization resulting in activation.
Acknowledgements
M. Al Hajji Safi, M. Balandeh and A. Bourke would like to thank the Irish Research Council (IRC) for PhD scholarships and R.P. Lynch acknowledges funding from a IRC - Marie Skłodowska Curie Fellowship under grant no. INSPIRE PCOFUND-GA-2008-229520 to perform this research.
References
1. M.J. Leahy, et al., Wind Energy Storage Technologies, Wind Power Generation and Wind Turbine Design, ed. W. Tong, WIT Press, Southhampton, pp. 661 (2010).
2. D. Connolly, A Review of Energy Storage Technologies: For the integration of fluctuating renewable energy, ed. M. J. Leahy, Charles Parsons Initiative, University of Limerick, pp. 22 (2010).
3. M. Skyllas-Kazacos, et al., J. Electrochem. Soc., 158, R55 (2011).
4. H. Bindner, et al., Characterisation of Vanadium Flow Battery, Riso Report(2011).
5. C.P. de Leon, et al., J. Power Sources, 160, 716 (2006).
6. Z. Yang, et al., Chem. Rev., 111, 3577 (2011).
7. G. Kear, et al., Int. J. Energy Res., 36, 1105 (2012).
8. K. L. Huang, et al., Renew. Energy, 33, 186 (2008).
9. M. Rychcik, M. Skyllas-Kazacos, J. Power Sources, 22, 59 (1988).
10. A. Weber, et al., J. Appl. Electrochem., 41, 1137 (2011).
11. M.J. Watt-Smith, et al., Journal of Chemical Technology & Biotechnology, 88, 126 (2013).
12. A.H. Whitehead, M. Harrer, J. Power Sources, 230, 271 (2013).
13. X. Gao, et al., in Electrochem 2012: L. D. Burke Energy Symposium, p. 29, Dublin, Ireland (2012).
14. A. Di Blasi, et al., J. Power Sources, 227, 15–23 (2013).
15. W. Li, et al., Carbon, 55, 313–320 (2013).
16. W. Zhang, et al., Electrochim. Acta, 89, 429–435 (2013).
17. J. Xi, et al., Int. J. Electrochem. Sci., 8, 4700–4711 (2013).
18. Y. Men, T. Sun, Int. J. Electrochem. Sci., 7, 3482–3488 (2012).
19. X.G. Li, et al., Transactions of Nonferrous Metals Society of China (English Edition), 17, 195–199 (2007).
20. A. Bourke, et al., ECS Trans., 53, 59 (2013).
21. A. Bourke, et al., ECS Trans., 61, 15 (2014).
22. A. Bourke, et al., in Conference Papers The International Flow Battery Forum 2014, p. 16, Hamburg, Germany (2014).
23. A. Bourke, et al., ECS Trans., 64, 1 (2015).
24. A. Bourke, et al., J. Electrochem. Soc., 162, A1547 (2015).
25. A. Bourke, et al., J. Electrochem. Soc., 163, A5097 (2016).