Numerical study of the structure and temperature effects in vanadium redox flow batteries

Abstract

The Vanadium Redox Flow Batteries (VRFBs) play an important role as energy storage devices on the roadmap to carbon neutrality. Many studies demonstrated good performance of VRFBs. However, simulation models are still limited, especially the structure and temperature effects in VRFBs. Comprehensive understandings of the velocity distributions, ion distributions and transfer current density distributions inside VRFB are unclear with different structure designs and temperatures. Therefore, a numerical model is built to study the structure and temperature effects on the performance of VRFB. First, a three-dimensional numerical model of VRFB is developed to understand effects of electrode parameters on battery performance with different flow patterns. The model fully considers the electrochemical reactions, fluid flow and mass transfer in the battery. The model is validated by experiment. The results show that increasing the electrode porosity improves the performance of battery with serpentine flow channels, especially when the inlet flow rate is low. The high porosity not only reduces the pressure drop when the electrolyte flows through the porous electrode, but also improves the uniformity of reactive ion concentration distribution in both in-plane and through-plane direction. Increasing the specific surface area of electrode also improves the battery performance rapidly within a certain range, but the effect is not obvious when increasing further. With the same specific surface area but larger fibre diameter, the pressure drop in interdigitated flow channel case is reduced, while the battery performance is improved by the increased average velocity in electrode with serpentine flow channels. Considering that flow batteries are expected to operate over a wide temperature range, the three-dimensional model is subsequently updated to investigate the effects of operation and electrode parameters on battery performance and pressure drop at different temperatures. It is found that with the increase of temperature, the battery performance increases due to increased reaction rate, electrolyte conductivity, ions diffusion coefficient. It is also found that with the increase of temperature, the pressure drop of battery decreases due to the decreased electrolyte viscosity. Increasing the inlet flow rate and state of charge (SOC), decreasing the electrode porosity and fibre diameter can improve the battery performance with interdigitated flow channels, and the improvement is more obvious at a high temperature. Due to the high electrolyte viscosity, the pressure drop increases sharply with increasing flow rate and decreasing porosity or fibre diameter of electrode at low temperature. Thin electrode performs better when the operating voltage is below 0.9 V at low temperature and performs better when the working voltage is above 0.7 V at high temperatures. Excellent VRFB performance was achieved by modified fibrous electrodes with 0.5 mm thickness and 0.9 porosity. The electrode with modified fibre of large diameter 20 μm shows improved battery performance with a low pressure drop. The electrode with gradient porosity, which rationally aims to achieve a balance between concentration overpotential and activation overpotential, has been proved as an efficient design in the application of fuel cells. In this thesis, electrodes with increased porosity in different directions are applied in VRFB with serpentine flow channels. However, it is found that such VRFBs cannot perform as good as the VRFB with 0.95 porosity electrode. Additionally, the gradient porosity electrode is also applied in VRFB with interdigitated flow channels. The electrochemical performance of VRFB with electrode (porosity increases from 0.8 at channel side to 0.93 at membrane side) performs similarly with the VRFB with 0.8 porosity electrode. However, compared to 0.8 porosity electrode, the pressure drop in the gradient electrode is reduced by 40%. Thus, a VRFB with gradient electrode is a preferable choice when the interdigitated flow pattern is applied. The aligned carbon nanofibre electrodes have been reported to improve battery performance in the previous experiments. In this thesis, aligned electrodes are applied in VRFB with interdigitated flow channels. The numerical results show that VRFB with θ=7° aligned fibre electrode reaches the highest limiting current density, but it has an evidently high pressure drop. At 323.15 K, the VRFB with θ=45° aligned fibre electrode showed 21% higher limiting current density and 36% lower pressure drop than VRFB with xy-plane isotropic electrode. Noteworthy, after the fibre surface modification, the sufficient specific surface area can be ensured, which leads to the insignificant effects of aligned electrode design. The results of VRFB model provides an insightful understanding of the operating conditions and electrode characteristics effects on the performance and pressure drop of VRFBs. The built three-dimension numerical toolkit can be further applied to conduct operation, flow pattern and electrodes optimization with wide temperature range in large-scale VRFB system.