Key factors affecting the internal flow of flow batteries -Ⅲ

3.Compression ratio of electrode

The compression ratio of porous electrodes is an effective assembly factor for improving flow battery performance. On the one hand, the reduction in electrode thickness reduces the overall resistance of the battery. On the other hand, the compressed porous electrode also reduces the concentration loss in the thickness direction, further improving the performance of the flow battery.

Wang proposed an accurate model to study the effect of non-uniformly compressed porous electrodes on the performance of all-vanadium redox flow batteries at different compression ratios. The porous electrode is usually compressed to a certain extent to reduce the contact resistance between the bipolar plate and the carbon felt. However, the flow field engraved in the bipolar plate causes uneven pressure load distribution in the porous electrode under the sub-rib area, the channel area, and the intrusion area inside the channel, and the uneven deformation causes the electrode to intrude into the channel, thereby significantly affecting the physical properties of the electrode includes porosity, permeability, thickness and depth of invasion. As shown in Figure 2, an experimental device was designed to detect the morphological characteristics of the compressed felt under different compression ratios, such as intrusion rate and local porosity. The study found that the presence of the invading area leads to the deterioration of the mass transfer process of the electrolyte in the electrode, resulting in a high overpotential.

Figure 2 Comparison of experimental and simulation results

Yue coupled the polarization model with a previously developed three-dimensional model to obtain a mechanistic understanding of the relationship between electrode compression ratio and battery polarization. The effects of different compression ratios on flow rate, pressure drop, local current density, overpotential, and velocity distribution were studied, and the all-vanadium flow battery performance predicted by the numerical model was matched with experimental data. It was found that the pressure and flow velocity in the flow channel increased with the increase of the compression ratio due to the decrease in the cross-sectional area of the flow channel.

Furthermore, increased compression ratio limits electrolyte penetration due to reduced permeability, porosity, and electrode volume. The porous electrode with an optimized compression ratio of 28% showed maximum electrolyte permeability. Taking the intrusion area into account, proper electrode compression can significantly improve the transport of reactants and the reaction area. The all-vanadium flow battery with an optimal felt compression ratio of 55.7% shows optimal electrolyte uniformity, low current density and overpotential.

Figure 3 Diagram of electrode compression device

Latha reported a study on the hydrodynamics of serpentine flow fields in all-vanadium flow batteries. Two different sizes of serpentine flow fields, 25 mm × 25 mm and 80 mm × 51 mm, were selected, and their effects on parameters such as pressure drop and electrolyte permeability were studied. Compressed carbon felt reduces the cross-sectional area of the channel, which increases flow rate, and reduces the hydraulic diameter, which increases pressure drop.

Therefore, the authors estimated the permeability at different compression ratios, which ranged from 5–8 × 10−11 m². Furthermore, the pressure drops for both channel geometries were measured over a wide range of Reynolds numbers. It was found that in this case the measured pressure drop was in good agreement with the predicted pressure drop, while the other case showed a large difference.