Investigating The Effects Of Electrode Thickness And Electrolyte Supply On The Performance And Efficiency Of Redox Flow Batteries

Owing to the exclusive advantage of decoupling the processes of energy conversion and storage,the redox flow battery（RFB）has been considered as a critical candidate for large-scale electrical energy storage（EES）.Improving the energy efficiency of a RFB system is always an attractive research topic.The energy conversion efficiency of a RFB is closely related to the voltage loss resulting from the reaction activation,ohmic resistance and mass transfer resistance.As one critical component in the RFB,the electrode provides electrochemical reaction sites for active ions.Consequently,the electrode thickness has an important influence on the voltage loss due to the dependence of activation loss on the reaction area and ohmic loss on the electrical conductivity.In addition,the pumping loss caused by forcing electrolyte to flow through porous electrode is also affected by the electrode thickness,which is another determinant of the energy efficiency of a RFB system.To avoid excessive pumping loss,the electrolyte flow rate is hoped to be low,but which is opposite to the requirement of minimizing mass transfer loss.In this study,numerical analyses and experimental measurements were conducted to systemly investigate effcts of electrode thickness on the activaton loss,ohmic loss and mass transfer loss for different reactions.Based on the optimal electrode thickness,electrolyte feeding schemes were explored to achieve the balance between the mass transfer loss and the pumping loss.The detailed research contents are described as follows:1.Voltage losses of the high reversible flow battery at high current densities.Due to the fast reaction kinetics,high reversible RFBs are considered to have inherent advantages in realizing efficient and fast energy conversion.Taking the high reversible H₂/Br₂ flow battery（HBFB）for example,its voltage losses at high current densities were numerically investigated to provide guidance for optimizing the battery performance.With a numerical model,the voltage loss was resolved into activation,ohmic and mass transfer parts,which provided quantitative assessment of the effect of electrode thickness on each part of the voltage loss.With a change in electrode thickness,the dominant part of voltage loss alternates between activation loss and ohmic loss.Consequently,the optimal electrode thickness is mainly determined by the balance between these two factors.Since hydrogen bromide solution serves as both reactant/product and supporting electrolyte,the ionic conductivity varies during charge/discharge,which in turn leads to a difference of optimal electrode thickness at different state-of-charge.For reactant transfer,decreasing electrode thickenss leads to a higher demand on the mass transfer rate in pore scale,while thickening electrode increases the reactant diffusion distance from concentration area to shortage area.2.The effect of electrolyte velocity on mass transfer in porous carbon felt electrode.In addition to the electrode thickness,the electrolyte supply also has an important influence on the battery performance.In this study,the dependence of reactant transfer on the electrolyte velocity was investigated.A lab-scale flow battery,fed with identical electrolyte solutions containing Fe²⁺/Fe³⁺as active substances in both the anode and the cathode,was used to realize stable tests free from side reactions in a broad range of current densities.By measuring limiting currents at a typical electrolyte velocity range from 2.5 to 15 mm s^-1,a correlation between the mass transfer coefficient and the velocity in dimensionless form was obtained as?=1.680)^0.9.Meanwhile,voltage losses of the battery fed with adequate reactants at different velocities were both experimentally measured and numerically simulated.The agreement between simulated results and experimental data verifies the applicability of this correlation under normal operating conditions below limiting currents.3.Kinetics-introduced difference in the effect of electrode thickness on the voltage loss.In consideration of the influence of electrolyte velocity on reactant transfer,the difference in effects of electrode thickness on each part of voltage loss（i.e.activation,ohmic and mass transfer loss）was investigated between different reactions.For the reaction Fe²⁺/Fe³⁺with extremely fast kinetics,even electrode thickness as thin as 2mm provides sufficient reaction area and few activation loss is presented.Thickening electrode gives rise to an obvious rise of voltage loss,which is attributed to the increased ohmic resistance.In the condition of feeding adequate reactants,the mass transfer loss is still induced owing to the existent mass transfer resistance.Moreover,at a fixed flow rate,the mass transfer loss varies slightly with the increase of electrode thickness,because the detriment to mass transfer resulted from the reduced velocity is counterbalanced by the decreased demand for local mass transfer rate.For reactions with relatively sluggish kinetics,due to the balance between activation loss and ohmic loss,the optimal electrode thickness varies with the current density.Since the slower kinetics leads to a more uniform reaction distribution in the electrode,the ohmic loss is larger and mass transfer loss is a little smaller than those of reaction Fe²⁺/Fe³⁺respectively.Optimizing electrode thickness based on the specific requirement of differnet reactions brings a great benefit to the battery performance,which is indicated by comparing energy efficiencies of Fe/Cr flow batteries employing differently thick electrodes at positive and negative.4.Electrolyte supply for the large-scale high-performance RFB.On the premise of no significant activation loss,employing thinner electrodes has been proved to be effective in improving battery performance.When these high-performance thinner electrodes are scaled up to larger sizes required for kW-scale stacks,adding interdigitated flow fields is a simple solution in maintaining low pressure drops.To optimize the construction of flow fields,a 3-D model of a half-battery with an active area of 900 cm² was developed in this study.Optimizing the number and size of channels is essentially striking a balance between the pressure drop and the electrolyte velocity in the electrode,which have important effects on the pumping loss and mass transport loss respectively.In addition to the magnitude of the average velocity,the uniformity of velocity distribution should also be paid attention to in optimizing flow fields,which is determined by the ratio of flow resistance in the electrode to that in the channels.Acceptably thicker channels are recommended to improve uniformity of velocity distribution.This thesis not only clarifies the effect of electrode thickness on batter performance by decomposing the voltage loss into activation,ohmic and mass transfer parts,but also investigates the dependence of mass transfer and pumping loss on the electrolyte supply,which provides an important theoretical basis for improving the energy efficiency of a RFB system.