At present, the urgent need to address the intermittency of renewable energy has significantly promoted the rapid development of large-scale energy storage systems. Among all electrochemical energy storage technologies, water-based redox flow batteries, especially vanadium flow batteries (VFBs), have received widespread attention due to their excellent characteristics of high power density, long cycle life, non flammability, and independent design of power and capacity. At present, facing the shortage of vanadium ore resources, liquid flow battery developers have always attached great importance to improving the working current density of fuel cells, which is of great significance for reducing the cost of the entire VFB system.
However, at high current densities, Ohmic polarization can also be significantly amplified, leading to a decrease in the voltage efficiency of the battery. At present, the effective design at high current density is to use thin carbon felt electrodes, which can greatly reduce the resistance between the electrolyte and the electrode, thereby reducing the Ohmic resistance and achieving an improvement in voltage efficiency at high operating current density. However, using thin electrodes has two prominent drawbacks. The use of thin carbon felt electrodes in large battery stacks may inevitably lead to an increase in pressure drop, thus the battery stack may face the risk of electrolyte leakage and pumping loss. Both of these drawbacks greatly hinder the further development of high-power density VFB stacks. In this context, advanced flow channel design can enhance the mass transfer process and improve uniformity in liquid flow batteries, resulting in lower overpotential. Therefore, it is crucial to explore the laws and strategies of carbon felt flow field design to overcome the limitations associated with the use of thin porous felt electrodes in large VFB stacks under high current density.
Research Highlights
Huanhuan Hao et al. from the Institute of Metals, Chinese Academy of Sciences achieved a significant improvement in the power density of vanadium flow batteries by adjusting the flow field design on carbon felt electrodes. The author revealed through finite element analysis the reduction of pressure drop, uniform distribution of reactants, and improvement of flow velocity in parallel and cross design flow fields on carbon felt. According to the measured local mass transfer coefficients, carbon felt with both cross and parallel flow fields showed a decrease in simulated concentration polarization at 200 mA cm-2, and the parallel flow field was superior to the cross flow field design. The experimental verification further confirms that carbon felt with parallel flow field has a voltage efficiency of up to 78% at 200 mA cm-2, and the discharge capacity of the battery has also been improved. Finally, under industrial scale conditions of 32 kW and current density of 200 mA cm-2, the dynamic modeling and simulation results of the battery stack using parallel flow carbon felt design achieved 70% system efficiency. This also demonstrates the enormous potential of adjusting the flow field on the carbon felt to improve the efficiency of actual liquid flow battery packs.
research contents
The following diagram is a typical structure diagram of a liquid flow battery, mainly including end plates, current collectors, bipolar plates, flow frames, porous electrodes, and exchange membranes. The flow field design is mainly aimed at carbon felt electrodes.
The author first revealed the flow characteristics of carbon felt under different flow fields through finite element analysis. Compared with carbon felt without flow field design, both cross flow and parallel flow fields show significant pressure drop reductions, with reductions of 36.8% and 37.2%, respectively, which is beneficial for saving pumping energy consumption, especially for large VFB stacks. In addition, it can be seen that the flow rate continuously increases along the cross shaped and parallel channels, significantly promoting the distribution of reactants. In the absence of channels, trivalent vanadium ions mainly stay near the inlet, which can cause hydrogen gas precipitation and damage the Coulombic efficiency. When using cross channel and parallel channel carbon felt, the distribution of reactants and the increase in flow rate also exhibit a more uniform distribution pattern.
The author then measured the limit current of carbon felt with three different channel designs as the flow rate increased, and calculated the corresponding local mass transfer coefficients. The carbon felt without flow channels exhibits an average concentration polarization of 36.55 mV at 150 mA cm-2, and the flow outlet region has much greater concentration polarization than the inlet region. When the flow field is introduced, the concentration polarization of the cross channel and parallel channel carbon felt is significantly reduced, and the carbon felt designed with parallel channels has the minimum concentration polarization (about 6.53 mV) at 150 mA cm-2. The parallel channel design demonstrates its outstanding ability in reducing concentration polarization. From the perspective of mass transfer coefficient, it has also been confirmed that the mass transfer coefficient of carbon felt in cross flow channels and parallel flow channels is higher, while parallel flow channels remain the most prominent.
The author subsequently conducted electrochemical tests, and the CV results in the figure below compared the reversibility of V4+/V5+on the original carbon felt and the carbon felt with flow channels. It can be seen that the ratio of peak potential to peak current is very close, indicating that both have excellent redox reversibility. The electrochemical impedance spectra of the original carbon felt and the carbon felt with flow channels show similar high-frequency resistance (about 0.26 Ω) and charge transfer resistance (Rct=7.31 Ω cm 2). This means that electrode dynamics will not be affected by the introduction of flow channel design on the carbon felt. The results of linear sweep voltammetry further indicate that the three carbon felt materials have the same hydrogen and oxygen evolution potentials (i.e. -0.56 V and 1.75 V vs. SCE) in sulfuric acid, and also exhibit strong corrosion resistance. In addition, all three types of carbon felt have similar slopes corresponding to diffusion control processes. All the above electrochemical characteristics demonstrate that carbon felt with flow field retains excellent electrochemical performance compared to the original carbon felt. In addition to electrochemical properties, the author also evaluated the physical properties of the carbon felt. By measuring the contact resistivity between the carbon felt and the bipolar plate, it was found that the flow field has little effect on the key physical properties of the carbon felt, and the contact resistivity remains almost unchanged.
Subsequently, the author conducted a full battery test to confirm the superiority of carbon felt with flow field in reducing concentration polarization. The table below shows the cycling efficiency under high current density in the range of 150~200 mA cm-2. It can be observed that both cross channel and parallel channel carbon felt have higher voltage efficiency than the original carbon felt. The charge discharge curve also clearly indicates that the carbon felt with parallel flow field design generates a low voltage platform during charging and a high platform during discharge. In addition, the cyclic test results also indicate that carbon felt with flow field design can generate higher and more stable voltage efficiency, and it can be inferred that the improvement of voltage efficiency under high current density is caused by the effective reduction of concentration polarization. In addition, the cross channel and parallel channel designs of carbon felt achieve greater discharge capacity at 150-200 mA cm-2, while the parallel channel design provides the most ideal capacity for each situation. Of particular note is that the flow field design allows for a longer discharge voltage plateau, which means that concentration polarization is significantly reduced at the end of discharge, resulting in high voltage efficiency of the battery. Therefore, the cycle efficiency of parallel flow channel design is superior to that of cross flow channel design and no flow channel design at different operating current densities of 150~250 mA cm-2, which is in good agreement with simulation results, thus proving the effectiveness of introducing flow field on carbon felt to improve the efficiency of high-power liquid flow batteries.
Finally, in order to further evaluate the impact of carbon felt with flow channels on industrial large-scale stacks, the author established a 32 kW VFB stack dynamic model. The simulation numerical and experimental results clearly indicate that the flow field design on the carbon felt is very beneficial for reducing pressure drop and concentration polarization in vanadium flow batteries. Based on Darcy's law and pressure simulation, the permeability of cross channel and parallel channel carbon felt is one order of magnitude higher than that of the original carbon felt, which will result in a significantly reduced pressure drop at high flow rates in large VFB stacks. The prediction of the dynamic model further confirms this expectation, as compared to the original carbon felt, the carbon felt with a flow channel design requires significantly less pumping energy for the stack to operate at 200 mA cm-2. Through dynamic simulation, it was found that cross channel and parallel channel carbon felt stacks have higher voltage efficiency for concentration polarization and its impact on stack voltage efficiency. By comparing energy and system efficiency, it was found that variable flow rate is very effective in saving pumping losses. Meanwhile, the cross channel and parallel channel carbon felt still outperform the original felt in terms of voltage efficiency, indicating that concentration polarization is effectively reduced in this variable flow mode, thereby achieving higher system efficiency. For a 32 kW stack, the system efficiency through conversion to a 5-hour battery is 70%, and in variable flow mode, parallel channel carbon felt is superior to cross channel and original felt.
In summary, the author found through experiments and simulations that carbon felt electrodes with flow field design can achieve high-power vanadium flow batteries that simultaneously reduce pressure drop and concentration polarization. Compared with the original carbon felt, both cross shaped and parallel carbon felt can significantly reduce pressure drop, make the concentration distribution of reactants uniform, increase the flow rate of the entire porous electrode, and predict an ideal system efficiency of 70% in the established dynamic model of a 32 kW stack.From an application perspective, this is fundamentally different from the flow field design on conventional fuel cell bipolar plates. It can avoid the high cost caused by referring to the flow field design in thick graphite bipolar plates in fuel cells (which may account for more than 50% of the cost of VFB stacks), and also avoid electrolyte leakage caused by mechanical damage to thin composite bipolar plates. Therefore, the design of carbon felt with flow channels provides a new approach for the design of high-power VFB stacks.
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