| Iron chromium redox flow battery(ICRFB)has a broad application prospect in the energy storage market due to be a cost-effective energy storage system among many redox systems.Increasing the power density of ICRFB can reduce the consumption of materials which further expands the cost advantage of ICRFB.However,the increase of power density will increase the polarization loss in a ICRFB,resulting in lower energy efficiency.Therefore,to maintain the acceptable energy efficiency,the key of improving the power density of ICRFB is to minimize various polarizations.As one of the key components,carbon felt is the most commonly electrode used in ICRFBs.Carbonation and activation are the key processes for the propeerties of carbon felts which have an important effect on electrochemical,ohmic and concentration polarization.In order to minimize the polarizations above,the properties of’carbon felts would be mutually complementary and restricted.But the structure-performance relationship between carbon felt and polarization is not clear in ICRFBs,so that the development of ICRFB is greatly limited.Therefore,considering the faultiness of the existed theory for carbonization and activation of carbon felt,the following work has been done in this paper on the precursor materials,carbon content,oxygen functional groups and surface catalyst in order to identify the influence mechanism of polarization behavior of carbon felts.The study can provide the theoretical foundation for the development of ICRFB.The main points can be summarized as following.Firstly,the structure characteristics of rayon-and polyacrylonitrile(PAN)based carbon felts are characterized in this study.The electrochemical properties of the two felts are investigated by electrochemical analysis and ICRFB charge-discharge tests.It is pointed out that mass transport is the main control step to affect the polarization behavior of two carbon felts differed by precursor materials.The results show that the larger surface roughness of rayon-based carbon felt is more favorable for reactants transported from the pores to the surfaces of the electrode.However,as the precursor of PAN-based carbon felt is easier to be graphitized,its graphitization degree is higher resulting in a smaller loss of ohmic polarization.It is also found that PAN-based carbon felt shows better electrochemical activity and kinetic reversibility,especially faster charge exchange rate for Fe2+/Fe3+ redox couple.Therefore,PAN-based carbon felt with lower cost should be further investigated from the aspect of expanding mass transport and improving negative reaction activity for commercialized ICRFBs.Secondly,aiming at the issue between carbon content and polarizations for PAN based carbon felt in ICRFBs,several PAN based carbon felts including 90%-99.2%of carbon content are investigated for the structural evolution and electrochemical behavior.The results show that the electrochemical properties of carbon felts with different carbon content depend on electrochemical polarization and ohmic polarization.High purity and high crystallinity of carbon felts accelerate electron transport and electrode reaction of negative Cr3+/Cr2+ ion,which is beneficial to decrease ohmic and electrochemical polarization.The cell assembled by carbon felts with 99.2%of carbon content exhibits the best performance,which the value of energy efficiency(EE)is 82.88%at a current density of 60 mA/cm2.The EE value is 12.88%higher than that of conventional ICRFB.When the carbon content is 90%-99.2%,the variation range of pore-level transport and surface-level transport is not enough to affect concentration polarization.Microcrystalline structure of the carbon felts with higher carbon content is more conducive to decrease the ohmic polarization.For the positive reaction,the double layer effect of C=O functional group limits the bridging mechanism of-OH functional group in the structure,resulting in the best electrocatalytic activity for the carbon felt with 98%of carbon content.However,for the negative reaction,as the increasing of carbon content,the crystallinity in carbon basal plane increases,which decreases the electrochemical polarization of carbon felts.The electrochemical polarization for the negative reaction is reduced synergisticly with the double layer effect of C=O functional group in carbon basal plane,especially for the carbon felt with 96%of carbon content.On the basis of the above work,the effects of oxygen functional groups on the structure and polarization of PAN-based carbon felts are obtained.The structure-performance relationship between oxygen functional groups in the carbon basal or edge plane and polarization of carbon felts is proposed in this study.The results show that the electrochemical properties of carbon felts mainly depend on the electrochemical polarization and concentration polarization.The oxygen functional group in the carbon edge plane of carbon felts is beneficial to activate the positive reaction of ICRFBs.When the number of OH functional group on the surface of carbon felt is less than 1.6%,the bridging mechanism of-OH functional group and the double layer effect of C=O functional group are limited by each other,which the carbon felt with 1.25%of OH functional group exhibits the highest electrocatalytic activity.Then the further increasing of the number of OH functional groups in the carbon felt is helpful to counteract the double layer effect of C=O functional groups.The the negative reaction of ICRFBs is improved by the oxygen functional group in the carbon basal plane of carbon felts.Due to the coupling effect of the carbon basal structure and the oxygen functional group in the carbon basal plane,the carbon felt with 2%of C=O functional group exhibits the highest electrocatalytic activity.Cr3+/Cr2+ redox couple is easily absorbed by oxygen functional group in the carbon basal plane.The surface oxidation etching of carbon felt is beneficial to decrease the concentration polarization and increase the cycle stability.The loss in carbon basal plane is the main reason for the increase of ohmic polarization during the formation of oxygen functional groups in the carbon basal and edge plane.Finally,in order to further decrease the electrochemical polarization,bismuth particles are synchronously electrodeposited onto the surface of a carbon felt electrode during operation of ICRFB.The activation mechanism of Bi on the surface of carbon felts mainly depends on the formation reaction of BiHx,which is controlled by the mixture of charge transfer and diffusion.The cell assembled by Bi modified carbon felts can work at room temperature mainly owing to the increase of the electrocatalytic activity from the introduction of Bi particles,which the EE value is 63.15%at a current density of 60 mA/cm2.The current density of Bi deposition increases moderately with the catalyst electrodeposition increasing.A ICRFB with the carbon felts achieved Bi loading of 10 mg/cm2 at 50 mA/cm2 exhibits 5-15 percentage points of energy efficiency higher than other catalysts. |
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