The energy efficiency of batteries is dependent on their voltage efficiency and coulombic efficiency. Voltage inefficiencies arise due to sluggish electrode kinetics and, as a result, large overpotentials at the electrode. Furthermore, the higher the overpotential the greater the number of side reactions such as hydrogen formation^1-7 at the negative electrode or degradation of the carbon at the positive electrode. Side reactions at either electrode reduce coulombic efficiency and in turn can cause the battery to become unbalanced (e.g. more V^V on the positive side than V^II on the negative side, in the case of H₂formation at the negative electrode of a vanadium flow battery). Therefore, in order to better understand and improve the performance of batteries, it is important to investigate the electrode kinetics.

We have previously shown^8-13 that electrochemical treatment of carbon electrodes significantly affects the electrode kinetics of V^IV-V^V oxidation-reduction reactions. In this study, we investigate the effect of electrochemical treatment of glassy carbon on the electrode kinetics of the oxidation-reduction reactions of Fe^II-Fe^III and hydroquinone-quinone and compare the behaviour of these systems to that of the V^IV-V^V system. Similar to the V^IV-V^V system, the electrode kinetics of both Fe^II-Fe^III and hydroquinone-quinone are enhanced by cathodic treatment of the electrode and inhibited by anodic treatment (as shown in Fig. 1 (i) and (ii), respectively).  Furthermore, the electrode can be toggled between these activated (reduced) and deactivated (oxidised) states.  The effects of treatment potential on both Fe^II-Fe^III and hydroquinone-quinone kinetics were investigated in detail for both anodic treatment of a reduced electrode and cathodic treatment of an oxidised electrode. As can be seen in Fig. 2, although there are significant differences between the behaviour of the electrode in the two systems there are also many similarities. We attribute the observed activation and deactivation effects to meta-stable oxygen-containing functional-groups on the electrode surface.

Acknowledgements

M. Balandeh, M. Al Hajji Safi 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.  C. Fabjan, J. Garche, B. Harrer, L. Jörissen, C. Kolbeck, F. Philippi, G. Tomazic and F. Wagner, Electrochim. Acta, 47, 825 (2001).

2.  M. H. Chakrabarti, R. A. W. Dryfe and E. P. L. Roberts, Electrochim. Acta, 52, 2189 (2007).

3.  X. Gao, R. P. Lynch, M. J. Leahy and D. N. Buckley, in Electrochem 2012: L. D. Burke Energy Symposium, p. 29, Dublin, Ireland (2012).

4. A. H. Whitehead and M. Harrer, J. Power Sources, 230, 271 (2013).

5.  D. Pletcher, ECS Trans., 28, 1 (2010).

6. A. A. Shah, H. Al-Fetlawi and F. C. Walsh, Electrochim. Acta, 55, 1125 (2010).

7. F. Chen, J. Liu, H. Chen and C. Yan, Int. J. Electrochem. Sci., 7, 3750 (2012).

8. A. Bourke, R. P. Lynch and D. N. Buckley, ECS Trans., 53, 59 (2013).

9. A. Bourke, N. Quill, R. P. Lynch and D. N. Buckley, ECS Trans., 61, 15 (2014).

10. A. Bourke, N. Quill, R. P. Lynch and D. N. Buckley, in Conference Papers The International Flow Battery Forum 2014, p. 16, Hamburg, Germany (2014).

11. A. Bourke, R. P. Lynch and D. N. Buckley, ECS Trans., 64, 1 (2015).

12. A. Bourke, M. A. Miller, R. P. Lynch, J. S. Wainright, R. F. Savinell and D. N. Buckley, J. Electrochem. Soc., 162, A1547 (2015).

13. A. Bourke, M. A. Miller, R. P. Lynch, X. Gao, J. Landon, J. S. Wainright, R. F. Savinell, and D. N. Buckley, J. Electrochem. Soc., 163, A5097 (2016).