Electrode Kinetics in All-Vanadium Flow Batteries: Effects of Electrochemical Treatment

The practical importance of vanadium redox flow batteries (VRFBs) has made the kinetics of the redox reactions at carbon electrodes of great interest.  The kinetics of the redox reactions at the positive and negative electrodes have been investigated in flow batteries and three-electrode cells for a range of carbon materials.  The reports reveal that the kinetics of these reactions are not fully understood^1,2 and discrepancies exist around which half-cell has slower kinetics^1–7.

Carbon surfaces are, however, very sensitive to changes in their environment which can then alter kinetics at their surfaces.^8,9  A variety of investigations have shown either enhancement or inhibition of V^II/V^III or V^IV/V^V kinetic rates by various pretreatments either electrochemical, chemical or thermal. There are, however, apparent disagreements amongst these reports.

Pretreatment of electrodes by polarization at either positive or negative potentials has a very significant effect on the kinetic rates of both the V^II/V^III or V^IV/V^V redox reactions. We have found^3–5 that pretreatment of various carbon electrodes at positive potentials leads to inhibition of the rates of the V^IV/V^V reactions but enhancement of the rates of the V^II/V^III reactions.  Conversely, pretreatment at negative potentials leads to enhancement of the V^IV/V^V reactions but inhibition of the V^II/V^III reactions. 

We will present results on the effect of electrochemical pretreatment of single carbon fibers extracted from carbon felt.  The kinetic rates for both the V^II/V^III and V^IV/V^V couples on these electrodes were measured and compared using linear sweep voltammetry, electrochemical impedance spectroscopy, and current measurements at constant potential.  The surface of the carbon fibers after electrochemical treatments was characterized using x-ray photoelectron spectroscopy.  These results are directly applicable to the performance of VRFBs which are typically operated using carbon felt electrodes.

 Acknowledgements

The authors wish to acknowledge support from the Irish Research Council (IRC), the National Science Foundation, Sustainable Energy Pathways Program (NSF-1230236), and the  IRC-Marie Curie International Mobility Fellowship in Science, Engineering and Technology (grant no. INSPIRE PCOFUND-GA-2008-229520).

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