Structure Design And Regulation Of Thin Skinned Asymmetric Membrane For Redox Flow Batteries

Redox flow batteries（RFB）have broad application prospects in electrochemical energy storage because of their flexible design,high safety,high efficiency and long cycle life.Membrane or separator is one of the most important components in RFB and responsible for transferring balancing ions and preventing active species.An ideal RFB membrane should have both high ionic conductivity and high selective.Generally,the conductivity and selectivity are mutually restricted for the present homogeneous and porous membranes.In this dissertation,a series of thin skinned asymmetric ion conductive membranes are designed for RFB,which reduce the thickness of the separation layer and ensure the high selectivity of the membrane.The membrane structure is further regulated and different methods are studied in depth to build ion conductive channels in the skin layer.The prepared membranes are applied to RJFB and the cell performance are studied in detail.Using uncharged polymer polybenzimidazole（PBI）as membrane material,a series of integrally thin skinned asymmetric PBI membranes have been fabricated by solvent evaporation-induced phase separation method.The effective thickness of the separation layer is reduced by the asymmetric morphology and the defect-free skin layer prevents vanadium ions effectively.The membrane morphology is readily regulated by non-solvent content.As the non-solvent content increases,the pore connectivity and porosity of the membrane increase,and the thickness of the skin layer decreases（as thin as 3-4 μm）.As a result,the area resistance（AR）of the membrane is reduced while the vanadium blocking ability is rarely affected and vanadium permeability is in the range of 1.6-3.6×10^-9 cm² min^-1.The asymmetric PBI membrane is applied to vanadium flow battery（VFB）and the energy efficiency(EE=78.4%,100 mA cm^-2)is increased by 28.7%compared to the dense PBI membrane（EE=61.0%）and is also superior to the EE of the commercial Nafion membrane（EE=71.8%）.The proton transfer resistance of asymmetric membranes is mainly concentrated in the dense skin layer.In order to improve the proton conductivity of the skin layer,a low-volatile,non-oxidizing concentrated-H₃PO₄ preswelling strategy has been proposed.The high concentration of the acid solution could increase the acid-base interaction between PBI and acid,so that the hydrogen bonds among PBI molecules could be broken during the preswelling process.As a result,the acid doping level（ADL）of PBI membrane in 3 M H₂SO₄ has been improved and a continuous proton transfer network is formed in the membrane,thus improving the proton conductivity.The decent vanadium-blocking ability(vanadium permeability is 3.0×10^-8 cm² min^-1)and mechanical strength（tensile strength is 63.8 MPa）are maintained at the same time.When applying the preswelled PBI membrane to VFB,the EE(80.9%,80 mA cm^-2)is increased by 21.1%compared to the cell performance of the non-preswelled PBI membrane（EE=66.8%）.Applying the preswelling strategy to the asymmetric PBI membrane to enhance the proton conductivity of the skin layer,EE(87.2%,100 mA cm^-2)of the VFB could be further improved by 11.2%compared to the performance of the original asymmetric PBI membrane（EE=78.4%）.To improve the proton conductivity of the skin layer,PBI grafted with non-ionic hydrophilic side chains（GPBI）have been designed and synthesized for VFBs.Free from ion exchange groups,GPBI membranes maintain the good chemical stability of the pristine PBI membrane.The molecular modification induces nanophase separation and formation of hydrophilic clusters（diameter of 5-8 nm）in the membrane,which act as effective proton transfer pathways and reduce the AR by one order of magnitude.The vanadium permeability is inappreciable due to the Donnan exclusion of protonated GPBI.The GPBI-based（substitution degree=188%）VFB has achieved an excellent performance under 20-200 mA cm^-2.Specially,EE is as high as 91.8%under 60 mA cm^-2 and it is up to 80.0%even under high current density of 160 mA cm^-2,the highest EE achieved by homogeneous PBI membrane for VFB so far.Fabricating GPBI into asymmetric morphology to enhance the proton conductivity of the skin layer.To obtain defect-free skin layer,the substitution degree is reduced to 50%.EE of the VFB is improved from 78.4%of the original asymmetric PBI membrane to 80.6%at 100 mA cm^-2.Then,the thin skinned asymmetric membrane structure design is extended to the charged membrane in VFB,cation exchange membrane（CEM）.A novel integrally thin skinned asymmetric CEM（ITSA-CEM）has been successfully fabricated for VFB,disentangling the typical conductivity-selectivity dilemma in CEM.Using sulfonated poly ether ether ketone（SPEEK）as the membrane material,the effect of sulfonation degree（DS）on the membrane morphology has been revealed.When the DS is low,the resulting skin layer is dense and defect-free.As DS increasing,pores present in the skin layer.Molecular dynamics simulation results show that there are hydrogen bonds between the sulfonic acid group of SPEEK and the ester group of DBP,which may be an important factor affecting the membrane morphology.The symmetric morphology reduces the effective thickness of separation layer and the AR is reduced by 60.4%.The low DS together with the defect-free skin layer leads to extremely low vanadium permeability.VFB performance is greatly improved and EE is 86.4%at 40 mA cm^-2,which is 39.5%higher than that of the dense SPEEK membrane（EE=61.9%）.Finally,the thin skinned asymmetric membrane structure design is further extended to the membrane for non-aqueous redox flow battery（NARFB）.It is proposed that utilizing metal organic frameworks（MOFs）with 3D porous structure to construct continuous balancing ions transport channels in the skin layer,using sieving effect of MOFs to reduce cross-contamination of active materials and maintain high permeation rate of balancing ions at the same time.Besides,the size uniformity and controllability of the ion conductive channels in the skin layer could be further improved.A gradient-distributed MOF/Celgard asymmetric membrane is fabricated for NARFB by a new method of "seeding by solvent evaporation-second growth".The asymmetric structure and continuous balancing ion transfer channels in the skin layer are obtained at the same time.Li/ferrocene NARFB with the membrane delivers an excellent high-rate capability and an enhanced cycling stability.The discharge capacity reaches as high as 94.0%of the theoretical value at a current density of 4 mA cm^-2,and achieves 76.1%even at 12 mA cm^-2.Moreover,a much slower capacity decay rate is achieved（0.09%per cycle over 300 cycles）by using the asymmetric membrane compared with the pristine Celgard membrane（0.24%per cycle）.