Effect of Repeated Oxidation and Reduction of Glassy Carbon on VIV-VV Electrode Kinetics
Authors: Al Hajji Safi Maria , Balandeh Mehrdad , Murphy Joseph A , Quill Nathan , Lynch Robert Patrick , Bourke Andrea , Buckley D. Noel 
Published: February 2020

There is considerable interest in flow batteries for storing energy from non-dispatchable power sources such as solar and wind energy.1-3 Vanadium flow batteries (VFB),3-12 also known as vanadium redox flow batteries (VRFB or VRB), are particularly attractive because, in addition to having long cycle life, they are essentially immune to cross-contamination problems due to mass transfer across the membrane.2-10

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.

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Figure 1