This paper presents a 2-D transient, isothermal model of a vanadium redox flow battery that can predict the species crossover and related capacity loss during operation. The model incorporates the species transport across the membrane due to convection, diffusion, and migration, and accounts for the transfer of water between the half-cells to capture the change in electrolyte volume. The model also accounts for the side reactions and associated changes in species concentration in each half-cell due to vanadium crossover. A set of boundary conditions based on the conservations of flux and current are incorporated at the electrolyte|membrane interfaces to account for the steep gradients in concentration and potential at these interfaces. In addition, the present model further improves upon the accuracy of existing models by incorporating a more complete version of the Nernst equation, which enables accurate prediction of the cell potential without the use of a fitting voltage. A direct comparison of the model predictions with experimental data shows that the model accurately predicts the measured voltage of a single charge/discharge cycle with an average error of 1.83%, and estimates the capacity loss of a 45 cycle experiment with an average error of 4.2%.

Availability of grid-scale electric energy storage systems with response rates on the order of seconds plays a key role in wide implementation of renewable energy sources. Here, a new concept called the electrochemical fl ow capacitor (EFC) is presented. This new concept shares the major advantages of both supercapacitors and fl ow batteries, providing rapid charging/discharging while enabling the decoupling of the power and energy ratings. Like in supercapacitors, energy is stored in the electric double layer of charged carbon particles. A fl owable carbon-electrolyte mixture is employed as the active material for capacitive energy storage, and is handled in a similar fashion to fl ow or semi-solid batteries (i.e., for charging/discharging, it is pumped into an electrochemical cell, and for storage, it is pumped into reservoirs). This study presents the proof-of-concept of this technology and reports initial EFC performance data obtained under static and intermittent fl ow operations. Adv. Energy Mater. 2012, 2, 895-902 896 www.MaterialsViews.com www.advenergymat.de

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