In the present work, the kinetic behavior of vanadiumion reactions on novel single carbon-fiber electrodes is investigated. The theory of reaction orders and charge-transfer coefficients is reviewed and typical sources of error due to incorrectly determined electrochemically active surface area, inhomogeneous current density distributions, mass transfer resistances, and aging are highlighted. The measured rate constants are in a range of 2.5 • 10 À 7-1.1 • 10 À 5 s À 1 for the positive and 7.0 • 10 À 8-2.6 • 10 À 6 s À 1 for the negative electrolyte. Despite these different activities of individual fibers, the reaction orders of V 2 + , V 3 + , VO 2 + and VO 2 + species are precisely determined as a function of the concentration and the state of charge. Moreover, charge-transfer coefficients are calculated with two different approaches based on Tafel slopes and through adjustment of the Butler-Volmer equation.

Vanadium redox-flow batteries are a promising energy storage technology due to their safety, long-term stability, and independent adjustability of power and capacity. However, the vanadium crossover through the membrane causes a self-discharge, which results in a capacity shift towards one half cell. This leads to a gradual decrease in its efficiency over time. Capacity balancing methods for compensation of this effect require a reliable online state of charge (SoC) monitoring. Most common methods cannot provide exact values of the individual concentration of each species in both electrolytes. In particular, the state of the positive electrolyte cannot yet be precisely determined. In this work, an amperometric SoC monitoring is proposed as a new approach. First, the suitability of the principle is investigated with a rotating disc electrode (RDE). Then, a sensor based on a gas diffusion layer (GDL) is developed and tested in the positive electrolyte. The dependencies between oxidative current and V(IV)-concentration are examined as well as those between reduction current and V(V)-concentration. Using both relationships, a reliable measurement of all relevant concentrations is possible.

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