Influence of Pretreatment on Kinetics at Carbon Electrodes and Consequences for Flow Battery Performance
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Influence of Pretreatment on Kinetics at Carbon Electrodes and Consequences for Flow Battery Performance D. N. Buckley1,2, A. Bourke3, R. P. Lynch1, N. Quill1, M. A. Miller2, J. S. Wainright2 and R. F. Savinell2 1
Department of Physics and Energy, Bernal Institute, University of Limerick, Ireland Department of Chemical Engineering, Case Western Reserve University, Cleveland OH, USA 3 School of Engineering, Waterford Institute of Technology, Ireland 2
ABSTRACT Using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), it was shown for four different types of carbon that electrode treatments at negative potentials enhance the kinetics of VIV-VV and inhibit the kinetics of VII-VIII while electrode treatments at positive potentials inhibit the kinetics of VIV-VV and enhance the kinetics of VII-VIII. These observations may explain conflicting reports in the literature. The potentials required for activation and deactivation of electrodes were examined in detail. The results suggest that interchanging the positive and negative electrodes in a vanadium flow battery (VFB) would reduce the overpotential at the negative electrode and so improve the performance. This is supported by flow-cell experiments. Thus, periodic catholyte-anolyte interchange, or equivalent alternatives such as battery overdischarge, show promise of improving the voltage efficiency of VFBs. INTRODUCTION Vanadium flow batteries (VFBs), also known as vanadium redox flow batteries (VRFBs or VRBs), are attracting significant attention as a promising technology for grid-scale storage of electricity [1-15]. Active areas of current research include cell design and modeling [1-3], performance and state-of-charge monitoring [8,9], coulombic and energy efficiencies [4, 15], electrolytes [1,11-14], membranes [1-3], and electrodes [1,5-7]. Cells typically have porous carbon electrodes and electrode performance can depend strongly on electrode treatment. Various electrochemical, chemical, and thermal treatments have been reported and it is clear that vanadium redox reactions are very sensitive to the chemistry of the carbon surface. The kinetics of both VII-VIII and VIV-VV redox couples have been studied for a range of different carbon materials using a variety of techniques, and it is clear that the kinetic rates depend strongly both on the type of carbon used and on the preparation of the electrode surface [5-7,16-21]. Generally, the kinetics are reported [5-7, 16-18] to be faster for VIV-VV than for VII-VIII; however there are also reports [19-21] that VIV-VV has slower kinetics than VII-VIII. Even on the same electrode material, e.g. glassy carbon, and for the same electrolyte species, e.g. VII-VIII, the kinetic rate metrics reported vary by over an order of magnitude from author to author. Reported values [5-7,16-21] of the ratio of the kinetic rate constant ko for VIV-VV to that for VII-VIII varies from 0.24 to 150. Thus, not only are there discrepancies as to the magnitude of the relative activities but there are also discrepancies as t
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