Research On Flow Channel And Electrode Improvements For Mass Transport In Vanadium Redox Flow Batteries

Renewable energy sources enrich the forms of power generation;however,they suffer from drawbacks such as intermittency and significant fluctuations due to weather conditions.These shortcomings can be mitigated by incorporating energy storage systems into renewable energy setups,thereby enhancing their utilization efficiency.The Vanadium Redox Flow Battery(VRFB),known for its rapid response time,large storage capacity,and extended lifespan,has found widespread commercial use in the field of energy storage.To enhance the overall performance of VRFB,improving the mass transport within the system is a highly effective approach.The structure of flow channels and electrodes within VRFB plays a crucial role in the distribution of electrolyte flow velocity and active substance concentration within the electrodes,subsequently impacting the electrochemical performance and system efficiency of flow batteries.On a macroscopic scale,the research on mass transport within VRFB focuses on flow channels and electrodes,with the primary emphasis on how flow channel structures,electrode material properties,and operating conditions influence mass transport within the VRFB.At the pore scale,the study of mass transport pertains to porous electrodes,primarily examining how the microscopic structure of the electrode impacts mass transport.However,this field still offers opportunities for more in-depth investigations:To ensure a more uniform distribution of active substances within the VRFB,enhance mass transport rates of active substances,and accommodate the increasingly complex design of novel flow channels,achieving a balance between hydraulic pressure drop and charge-discharge performance while keeping the flow channel structure simple is a key challenge in improving the overall performance of VRFB.Graphite felt electrodes are not specifically designed for VRFB applications.While their high porosity benefits mass transport,their low specific surface area hinders electrochemical reactions.Addressing this issue and achieving a balance between electrode concentration polarization and electrochemical polarization is a prerequisite for the stable operation of VRFB at higher current densities.For a deeper understanding of mass transport mechanisms within porous electrodes,it is necessary to reduce the study scale to the pore level.Investigating the impact of pore-scale electrode structures on mass transport,and improving electrode structures based on the mass transport characteristics within the pores,offers the potential to enhance both the mass transport and electrochemical performance of the electrodes.Therefore,this study selects appropriate research scales,establishes models for numerical investigations,and collaborates with a constructed VRFB experimental platform to delve into the effects of flow channels and electrodes on mass transport processes within the battery.The ultimate goal is to reduce flow losses and overpotentials.By modifying flow channels and electrodes at various research scales,the comprehensive performance of VRFB is strengthened.The research findings and conclusions of this paper are as follows:I established a VRFB single cell experimental platform capable of conducting charge-discharge and hydraulic pressure drop tests.Subsequently,a three-dimensional multi-field coupled model was developed for the VRFB single cell used in experiments,simulating charge-discharge processes for different flow channel configurations.By altering the flow channel computational domain,the model facilitated charge-discharge simulations for various flow channel designs.Using this model,charge-discharge simulations were performed for SFF and IFF VRFBs,and the results were compared with experimental data to validate the model’s effectiveness.The results demonstrated that,in terms of hydraulic performance,the model closely matched experimental results,with maximum prediction errors of 9.21% for SFF and 9.34% for IFF within the chosen flow rates.Regarding charge-discharge performance,the simulated voltage profiles closely resembled experimental data,with maximum prediction errors of 2.75% for SFF-VRFB and 3.84% for IFF-VRFB.Employing a three-dimensional multi-field coupled model,I simulated the chargedischarge processes of SFF-VRFB to investigate the influence of serpentine flow channels on internal mass transport within the battery.I addressed the shortcomings in traditional SFF-VRFB performance by aiming to reduce pressure drop and overpotential.I introduced split serpentine flow fields(SFF(SX)and SFF(SY))and double spiral serpentine flow fields(DSFF)as alternatives to traditional SFF,studying their effects on mass transport.The results showed that both types of split serpentine flow channels effectively reduced pressure drop,but their charge-discharge performance was suboptimal.In contrast,the two types of double spiral serpentine flow fields,DSFF(OC)and DSFF(IC),exhibited similar performance and reduced hydraulic pressure drop by approximately 70% compared to SFF under the same flow rate.With the same pump power,they improved energy efficiency by about 5%.Using a three-dimensional multi-field coupled model to examine mass transport,I explained the shortcomings in traditional IFF performance.I proposed the concept of a parallel-interdigitated flow field(PIFF)by connecting multiple IFF basic units in parallel.By adjusting the arrangement of IFF units and inlet/outlet positions,various PIFF configurations were obtained.These configurations were analyzed to enhance the uniform distribution of active substances within the electrode.The optimal PIFF was identified,and the impact of the number of parallel IFF units on mass transport was studied.The results showed that all PIFF(2)configurations outperformed traditional IFF,improving charge-discharge performance and reducing hydraulic pressure drop.Among them,PIFF(2T-2)and PIFF(2T-3)exhibited the greatest enhancement in overall performance.PIFF(3T),compared to PIFF(2T-2)and PIFF(2T-3),did not significantly improve battery performance and complicated the flow field.For VRFBs with a relatively large aspect ratio,IFF worsened mass transport in the central region.Although PIFF enhanced internal mass transport,it required a higher number of parallel IFF units and had a more complex inlet/outlet arrangement.Using a two-dimensional multi-field coupled model,we simulated the discharge of different electrode types in VRFBs.I investigated the influence of graphite felt electrode fabrication parameters on the transport of active substances within the electrode and identified the optimal electrode parameter combination.This information serves as a reference for the production process of graphite felt electrodes specifically designed for VRFBs.Additionally,using the two-dimensional model,I compared the effects of different compression ratios on the performance of single-layer and doublelayer electrodes in VRFBs.The results indicated that when producing graphite felt electrodes,controlling electrode thickness,porosity,and using materials with differentdiameter carbon fibers can achieve a balance between high surface area and preventing mass transport degradation.Double-layer electrodes exhibited higher performance with an optimal compression ratio compared to single-layer compressed electrodes.I reduced the study scale to the pore level and used the TRT-LB(Two Relaxation Time Lattice Boltzmann)model to establish relationships between permeability and effective diffusion coefficients and the carbon fiber structures within different electrodes.This information was used to enhance the predictive accuracy of macroscopic models for VRFB performance.Subsequently,I proposed a threedimensional hole-punching process for graphite felt electrodes to alter their microstructure and enhance mass transport within the pores.Using a mesoscale model,I analyzed the impact of hole diameter on electrode material properties and internal flow transport.Finally,I combined a two-dimensional macroscopic model to evaluate the discharge performance of electrodes subjected to different hole-punching treatments and provided recommendations for suitable hole sizes based on the initial electrode conditions.The results showed that after the correction of the macroscopic model using the relationships obtained from the TRT-LB model,the model’s predictions for hydraulic pressure drop and discharge voltage were more accurate.The micro-holepunched graphite felt electrode’s material properties and internal flow transport were influenced by hole diameter,with smaller hole diameters increasing electrode surface area and larger hole diameters enhancing flow transport.For electrodes with initially low porosity,larger hole sizes were more suitable to enhance flow transport,whereas electrodes with initially high porosity benefited from smaller hole sizes to increase surface area.