(257ag) Modeling of Hydrogen-Bromine Flow Battery System: Thermodynamics Perspective

Large scale energy storage technologies are crucial to various applications on power grids including peak shaving, frequency regulation, etc. It is estimated that, by 2017, the demand of grid scale energy would reach 185 GWh, which has a potential market value of 110 billion US dollars [1]. The ballooning demand is, however, still far from being satisfied by current capacity, and pumped water as well as compressed air is conventionally employed to store energy [2]. Therefore, reliable large scale energy storage solutions with high energy conversion efficiency and low cost are highly desirable.

A flow battery is a rechargeable fuel battery with two separate tanks containing liquid oxidant and reductant. External tanks of reactants make the design of flow batteries scalable and flexible. Typical scheme of flow batteries consists metal or graphite electrodes in oxidant and reductant solutions and ion-selective membrane separating the solutions. Chemical systems such as bromine/polysulphide, vanadium, zinc/bromine, and so on, have been applied in design of flow batteries [3].

Much effort has been put on improvements of flow battery systems. More chemical systems are tested as reactants, and meanwhile, researches on geometry, flow pattern as well as separators of flow batteries are conducted to achieve high electrochemical performance and low cost [4,5]. The cost of ion-selective membranes in conventional flow battery designs can be up to 44% of the total cost of electrochemical stacks while the limitation of membrane lifetime is also a major concern [6]. Progress has been made in membrane-less flow batteries although the performance of these systems is still not competitive to systems with membranes and the same chemicals [7]. Based on a porous cathode design by Jayashree et al. [1], Braff built a membrane-less hydrogen bromine laminar flow battery [7]. In this configuration, HBr solution and pure Br₂ flow between anode and cathode as two liquid phases and gaseous H₂is supplied outside porous anode, providing protons to complete a closed circuit. Compared to other flow battery systems, hydrogen bromine system has advantages in fast reaction kinetics, high electrochemical efficiency and most importantly, low material and operational cost [8,9].

There have been efforts in modeling of flow battery systems including membrane-less ones, mostly based on correlation of experimental results [10] or fluid dynamics [7,11,12]. Non-ideality of components in the solutions is neglected in the calculations and unity is used for activity coefficients of strongly non-ideal component like HBr [7]. However, with molarity of HBr changing from 0.5 to 5, our calculation shows the activity coefficient of HBr varies from 0.77 to 0.55 in Br₂saturated aqueous solution.

Our recent work established a comprehensive thermodynamic model for the Br₂-HBr-H₂O ternary system [13]. This model provides a thermodynamically consistent basis to calculate all thermodynamic properties of the system including Gibbs free energy, which is related to equilibrium chemical potential via Nernst equation [11]. In this work, rigorous approach to the calculation of equilibrium cell potential is established with the comprehensive thermodynamic model. Rigorous thermodynamic modeling underlies a complete model for hydrogen bromine flow battery systems can then be developed in the future to support optimization of the battery system.

References

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12. Yarlagadda, Venkata, and Trung Van Nguyen. "A 1D Mathematical Model of a H2/Br2 Fuel Cell." Journal of Electrochemical Society 160.6 (2013): F535-F547.
13. Yue, Y., and Chen, Chau-Chyun. “Thermodynamic modelling of Br2-HBr-H2O ternary system with ENRTL model.” AIChE Spring National Meeting, Austin, TX, April 27-30, 2015, Poster No. 106