In-Plane Liquid Electrolyte Permeability of Porous Electrode in Vanadium Redox Flow Battery
Authors: Ertugrul Tugrul Yavuz , Daugherty Michael Cyrus , Aaron Douglas , Ekici Kivanc , Mench Matthew M 
Published: July 2021

Vanadium redox flow batteries (VRFBs) have received broad recognition due to their attractive characteristics, i.e., flexibility and scalability, high coulombic efficiency, and long cycle life. However, rapid market penetration of this technology is still restrained by high system capital cost and relatively low energy and power density compared to other energy storage technologies. At the cell level, mass transport contributes significantly to performance losses, limiting VRFB performance, especially for cells operating at high power densities [1]. Improving mass transport in the VRFB electrodes can lower system cost by enabling higher power densities and larger depth of discharge windows. In these systems, convective mass transport in the electrode is directly proportional to the electrochemical performance of the cell [2]. Permeability and diffusivity of the vanadium active species are also significant mass transport parameters affecting electrochemical performance [3]. In this study, the in-plane electrolyte permeability of the porous electrode is investigated computationally and experimentally as a function of electrode compression.

Lattice Boltzmann models (LBMs) are a class of numerical methods which can be used to simulate fluid flows, mass transfer, heat transfer and many relevant physical phenomena which occur in these fields. The implementation of boundary conditions combined with the ease of parallelization make LBMs attractive for modeling mass transfer in geometrically complex domains such as porous media. A three-dimensional single-relaxation-time (SRT) LBM is employed (utilizing the Palabos library written in C++) to simulate the liquid electrolyte in the porous electrode [4]. Pressure-driven flow in porous media is achieved by imposing a constant pressure at the inlet and a constant lower pressure at the outlet. A computational domain (100 x 100 x 100 lattice unit) created within Python (Porespy module) [5] consists of randomly-generated fibers, having uniform diameter to simulate the carbon paper electrode pore structure as seen in Figure 1. Porosity of the unit structure is achieved by controlling the number of fibers in the domain. Permeability parameters are calculated for generated pore structures as functions of the electrode porosity.

The in-plane electrolyte permeability of porous electrodes was measured to compare with computational predictions. The sample electrode was placed between two plates as shown in Figure 2. Various electrode thicknesses were tested using an incompressible PTFE gasket. The end plates were secured by eight bolts to a torque of 10 N m each to ensure uniform compression and tight sealing. The electrode thickness during compression was converted to porosity to compare experimental measurements with mathematical model predictions [6]. The inlet pressure was measured via pressure transducer (Omega Engineering Inc, 0-50 psi, 0.25% accuracy, Norwalk, CT, USA) for a range of flow rates from 10 mL min-1 up to 50 mL min-1 at each electrode thickness. The permeability was then calculated using Darcy’s law for incompressible fluid flow.

Good agreement was achieved between experimental measurements and computational predictions. The result of this study will provide useful data for three-dimensional multi-physics VRFB models.

References

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Figure 1