Electrochemical impedance spectroscopy and cyclic voltammetry were used to investigate the electrode kinetics of VII-VIII and VIV-VV in H2SO4 on glassy carbon, carbon paper, carbon xerogel, and carbon fibers. It was shown that, for all carbon materials investigated, the kinetics of VII-VIII is enhanced by anodic, and inhibited by cathodic, treatment of the electrode; in contrast, the kinetics of VIV-VV is inhibited by anodic, and enhanced by cathodic, treatment. The potential region for each of these effects varied only slightly with carbon material. Rate constants were always greater for VIV-VV than for VII-VIII except when anodized electrodes were compared, which may explain discrepancies in the literature. The observed effects are attributed to oxygen-containing functional-groups on the electrode surface. The considerable differences between the potentials at which enhancement of VII-VIII and inhibition of VIV-VV occur indicates that they do not correspond to a common oxidized state of the electrode. Likewise inhibition of VII-VIII and enhancement of VIV-VV do not correspond to a common reduced state of the electrode. It is possible that enhancement of both VII-VIII and VIV-VV is due to the same (active) state of the electrode.
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The authors acknowledge support from the Irish Research Council(IRC), Marie-Skłodowska-Curie Actions (INSPIRE PCOFUND-GA-2008-229520), and the U.S. National Science Foundation, Sustainable Energy Pathways Program (NSF-1230236); and thank Graphite Engineering Supplies, Shannon, Co. Clare, Ireland for donating the glassy carbon.
Bourke, A.; Miller, M. A.; Lynch, R. P.; Gao, Xin; Landon, James; Wainright, J. S.; Savinell, R. F.; and Buckley, D. N., "Electrode Kinetics of Vanadium Flow Batteries: Contrasting Responses of VII-VIII and VIV-VV to Electrochemical Pretreatment of Carbon" (2015). Center for Applied Energy Research Faculty and Staff Publications. 22.