Study On Influencing Factors Of Mass Transfer Coefficient In Vanadium Redox Flow Batteries

Large-scale and efficient energy storage technologies play a key role in the massive integration of renewable energy.Among a variety of large-scale energy storage technologies,a vanadium flow battery features in its decoupled power and energy configuration for adding extra flexibilities with respect to the battery system configuration,modular-based design for ease in massive production and maintenance,and same kinds of ions in both negative and positive electrolytes for alleviating the electrolyte cross contamination.There are three main voltage losses in the vanadium flow battery,namely ohmic loss,charge transfer loss,and mass transfer loss.With the rapid development of novel electrode materials,the mass transfer loss gradually becomes the limiting factor to further improve the power density of the flow cell.Considering that the active ions participate in the electrode reaction only through the diffusion layer on the electrode surface,it is important for characterizing this process which is typically described by using the parameter of the mass transfer coefficient.In addition,the mass transfer coefficient is one of the essential parameters in the numerical studies on flow batteries.Although previous studies have comprehensively investigated a variety of influencing factors of the mass transfer coefficient in aqueous flow batteries,the conventional measurement for the mass transfer coefficient is limited by certain polarization curves and few of previous studies have addressed the effects of the surface morphology,functional groups,and the direction of the flow in the cell with the serpentine flow field on the mass transfer coefficient.Therefore,the present study first establishes the fitting model for the mass transfer coefficient based on the Butler-Volmer equation.Second,the electrode materials are pre-treated through different processes,and the pretreated material with changes in the surface morphology or functional groups are obtained,respectively.Based on the experimental and fitting results,the effects of the surface morphology and functional groups on the mass transfer coefficient are studied.Finally,three serpentine flow fields leading to electrolyte flow directions within the electrode perpendicular to,45° relative to,and parallel to the fiber alignment direction are devised.Based on the results of the experiments,fitting,and simulation,the effects of the flow channel’s relative direction with respect to the fiber alignment on the mass transfer coefficient are studied.(1)In the first part of this thesis,the fitting model for obtaining the mass transfer coefficient by using the electrochemical-mass transfer dominated polarization zone is established.Based on the developed model,the mass transfer coefficient of the vanadium flow battery at relatively high flow rates and high reactant concentrations can be fitted.(2)In the second part of this thesis,the experimental and fitting results show that both surface morphology and functional groups significantly affect the mass transfer coefficient.The larger the surface morphology changes,the larger the mass transfer coefficient per unit volume.In addition,the effect of the surface morphology on the mass transfer coefficient depends on the pore size.Specifically,large pores significantly increase the mass transfer coefficient,while nano-scale pores show minor effects.Regarding the effects of the amount of the functional groups,the mass transfer coefficient per unit volume and mass transfer coefficient increase with increasing amount of the functional groups.(3)In the third part of this thesis,the flow direction of the electrolyte within the electrode is maintained different from that of alignment direction of the carbon fibers with the developed serpentine flow fields.The experimental,fitting and simulation results show that the mass transfer coefficient per unit volume and mass transfer coefficient increase with increasing relative flow direction angle.Overall,the mass transfer coefficient per unit volume under the flow vertical to the fibers is 8.36% higher than that under the flow parallel to the fibers.To summarize,the influencing factors of the mass transfer coefficient in the vanadium redox flow battery are systematically studied in the present thesis,which could provide guidance for the further improvement of the electrode structure and flow field designs.