Modeling & Simulation of VRFB with Interdigitated Flow Field for Optimizing Electrode Architecture

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Journal of The Electrochemical Society, 2020 167 020553

Modeling and Simulation of Vanadium Redox Flow Battery with Interdigitated Flow Field for Optimizing Electrode Architecture Shohji Tsushima*,z and Takahiro Suzuki* Department of Mechanical Engineering, Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan The fibrous electrodes used in redox flow batteries are a key component of the batteries and have a determining effect on their performance. In this work, a two-dimensional numerical model of redox flow batteries was developed and used to optimize the architecture of the electrodes employed in vanadium redox flow batteries with interdigitated flow fields. The developed model was validated and subsequently used to determine the optimized electrode architecture. During the optimization process, we considered the fiber diameter, porosity, and thickness of the fibrous electrode as well as the geometrical properties of the channel. Numerical simulations revealed that the cell performance can be improved significantly by employing electrodes consisting of finer fibers. We also show that multiple-parameter optimization that considers the electrode properties and channel geometry is essential for improving the design of redox flow batteries. © 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BYNC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. [DOI: 10.1149/1945-7111/ab6dd0] Manuscript submitted September 25, 2019; revised manuscript received December 23, 2019. Published February 3, 2020.

Electrical energy storage systems that can be used for the efficient utilization of renewable energy are attracting increasing attention, owing to their swift response, which allows them to compensate for the intermittent availability of solar and wind power. Redox flow batteries (RFBs) are among the devices being explored for this purpose, as they allow for the storage of large amounts of energy, which can then be used during periods of peak shaving and load leveling.1–5 In the RFB system, the energy and power are decoupled, since the energy is stored in chemical form in solutions held in external tanks, and is only converted into electrical energy during use. Hence, these subsystems can be designed independently, allowing for the development of versatile electrical energy storage systems for a low-carbon society. Further improvements in the energy and power densities of RFBs are necessary for their commercialization.6–8 Recently, redox pairs based on both inorganic and organic chemistries9–13 have been proposed and demonstrated for use in high-energy-density and lowcost RFB systems. In RFBs, aqueous and non-aqueous solutions are circulated from the external tanks to the cells during operation. A high energy density can be achieved by concentrating the active species in the solution or by ensuring that the solution contains the active materials in solidified form.14–16 However, this inherently increases the viscosity of the solution and causes the cell performance to deteriorate, owing to mass transport and pumping losses. While high-power-density operations remain a challenge and depend on the cell architecture, there is a lot of freedom with respect to the architecture, especially when designing the electrodes and flow channels.17 In the last decade, the designs and materials developed for polymer electrolyte membrane fuel cells have also been evaluated for use in RFBs. Serpentine and interdigitated flow fields18 result in superior performance and reduce the overpotential of RFBs when used in combination with thin, fibrous carbon electrodes.19–21 They are thus replacing the flow-through channel geometry used in conventional RFBs. Different types of flow fields have been investigated through numerical and experimental studies22–33 in order to further improve the performance of RFBs. Some of the approaches under exploration for energy loss reduction in RFBs include modifying the conventionally used electrodes and developing new ones; this is because the cell performance is highly affected by the electrode architecture34–42 and catalytic activity.43–45 To enhance the electrochemical reaction and reduce the activation overpotential, the surface area of the *Electrochemical Society Member. z E-mail: tsushima@mech.eng.osaka-u.ac.jp

electrode must be large. Thus, electrospun fibrous electrodes are being tested, as the diameter of the fibers that make up these electrodes can be controlled with ease. The aim is to reduce the diameter of the electrode fibers, which are synthesized by electrospinning techniques,46–49 to be significantly lower than 10 μm, which is the diameter of the fibers of conventional carbon papers and cloths. On the other hand, electrodes consisting of finer fibers can potentially lead to poorer cell performance because of their lower permeability. The large flow resistance of such electrodes means that the flow velocity of the electrolyte within them is low. This results in an insufficient supply of the active species to the electrode surface, resulting in an overpotential during the electrode reaction.50 Therefore, the fiber diameter is an important parameter that must be optimized, as it can potentially have both positive and negative effects on the cell performance. The porosity and thickness of the electrode as well as the geometrical properties of the channel are also important parameters to be optimized, because these parameters also have a determining effect on the cell performance. The aforementioned approaches are promising for improving the cell performance. However, optimizing the electrodes and flow field architecture of RFBs remains an open question. In particular, optimizing the electrode architecture is of great importance with respect to the development of fibrous electrodes suitable for use in RFBs. Therefore, in this work, a two-dimensional numerical model of RFBs in which the electrode properties and the channel geometry are taken into account, was developed for optimizing the electrode architecture. Numerical simulations were performed using different sets of parameters to computationally determine the optimal parameter values. In addition, experimental tests were performed to validate the numerical model. Next, the different parameters were optimized individually, in order to highlight those that affect the electrode surface area and electrolyte flow velocity. Finally, multiple-parameter optimization was performed to determine the set of parameters corresponding to the optimal electrode architecture. Mathematical Model of RFBs Focusing on the optimization of the electrode architecture, a twodimensional RFB system was modeled in this study. An interdigitated flow flied was assumed for the simulations, as it results in superior cell performance. Figure 1 shows a schematic of a single cell of an RFB with an interdigitated flow field and the calculation domain to be modeled. A proton exchange membrane is sandwiched between the two fibrous carbon electrodes. Negative and positive electrolytes are supplied from the inlet channels, and they flow to the outlet channels via the electrodes.


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