Tuning Asymmetric Membranes With The Narrow Ionic Channels For Vanadium Flow Batteries

All vanadium flow batteries（VFBs）hold a promising prospect in electrochemical energy storage,due to decoupling of power and capacity and facile remediation of capacity decay.Ion-conducting membrane is one of the key components of VFB,allowing the transport of charge carriers and separating the positive and negative redox species.An ideal ion-conducting membrane should possess both high ion-selectivity and low area resistance（AR）simultaneously,which could not be achieved for the most commonly applied homogeneous membrane.In this thesis,asymmetric membranes with dense separation layer and porous supporting sublayer were proposed for VFBs.Compared with a homogeneous membrane,the porous supporting sublayer has little resistance to ion transport,and the mass transfer resistance of asymmetric membranes is mainly contributed from the dense separation layer.By tuning the size of the ion transport channels and reducing the thickness of the dense separation layer,high proton/vanadium ion selectivity and low AR can be achieved.A series of asymmetric membranes with narrow ion channels were prepared to improve the electrochemical performance of VFB by decreasing ion exchange capacity（IEC）and incorporating cross-linked graphene oxide membranes with angstrom-scale channels.The effect of narrow ion channels on vanadium ion transfer was also studied.To enhance ion selectivity,an integrally thin skinned asymmetric quaternized polysulfone anion exchange membrane with a low IEC(～0.72 meq.g^-1)was successfully prepared via a n-pentanol/water double gel bath phase inversion method,using chloromethylated polysulfone with chloromethylation degree of 40%as the precursor.The defect-free skin layer with～2 nm ionic clusters can effectively block vanadium ions.The permeability coefficient of VO²⁺ was less than 0.295×10^-7 cm² min^-1.Meanwhile,the membrane structure was readily adjusted by changing the immersion time in n-pentanol.With the immersion time shortened to 20 s,the pore connectivity of the porous supporting sublayer was improved,and the thickness of the dense separation layer was reduced to～5μm.Area resistance significantly declined from 36.22 Ω cm² for the homogeneous membrane to 6.72 Ωcm².As this asymmetric membrane was applied to VFB,energy efficiency(EE=81.0%,60 mA cm^-2)increased by 23.5%compared to the homogeneous membrane（EE=65.6%）,which was also superior to Nafion212（EE=79.1%）.It is difficult to reach the angstrom level uniformity for the ionic channels of polymeric ion-conducting membranes and the sizes of vanadium ions are around 8.0 （?）.To further improve the ion selectivity,it was proposed to employ graphene oxide（GO）nanosheets to construct a dense separation layer with uniform ion transport channels with angstrom-level size.In order to ensure the repeatability of the experiment,Nafion212 was selected as the base membrane to prepare the GO/Nafion212 composite membrane.Amino groups were introduced on the surface of Nafion212 using 3-aminopropyltriethoxysilane sol-gel method.Subsequently,GO nanosheets were deposited on the surface of the amino-functionalized Nafion212 using spin-coating approach.At the same time,m-xylylenediamine（mXDA）was selected as a cross-linker to bond adjacent GO nanosheets,forming a cross-linked GO（CLGO）layer to increase the mechanical stability.Eventually,a CLGO/Nafion212 composite membrane with a CLGO layer thickness of about 100 nm was obtained.The size of the ion transport channel in the CLGO layer was estimated to be around 7.4 A,which substantially reduced the permeation of vanadium ions.Compared with Nafion212,the composite membrane exhibited much lower V02+permeability coefficient(0.30×10^-6 cm² min^-1 vs.1.88×10^-6 cm² min^-1).Within the range of 20-100 mA cm^-2,it displayed higher coulombic efficiency（CEs）（93.0-97.2%vs.68.0-91.6%）and EEs（88.9-85.2%vs.65.7-82.2%）.At 80 mA cm^-2,the discharge capacity decayed more slowly（0.25%/cycle vs.0.47%/cycle）,An epoxy-functionalized polysulfone porous membrane with stronger anti-swelling ability and better mechanical properties was chosen as a supporting membrane,and the reaction between amino and epoxy groups was employed to fix GO nanosheets to the surface of the supporting membrane through covalent bonds.The hydration diameters of H30+,V（H₂O）₆²⁺,V（H₂O）₆³⁺,VO（H₂O）₅²⁺ and VO₂（H₂O）₄⁺,which are 2.80,8.24,8.14,8.18 and 8.34 A separately,were taken into account.The dimension of ion transport channels in the GO layer was further reduced to achieve the size sieving of H₃O⁺ and hydrated vanadium ions.The Epoxy-functionalized polysulfone porous membranes were prepared by the dry/wet phase inversion method combined with in-situ sol-gel method.mXDA was selected as the cross-linker and GO nanosheets as the building blocks.The CLGO layer was firmly attached to the porous epoxy-functionalized polysulfone membrane using the pressure difference driven self-assembly method,and covalent bonds formed during thermal treatment to strengthen the interface of the CLGO composite membrane.As the thickness of the CLGO layer was 89 nm,the composite membrane possessed sufficient mechanical strength.As the size of the ion transport channel was about 7.70 A,the composite membrane exhibited relatively low area resistance,which delivered the highest voltage efficiency（VE）.As the effective pore size between adjacent GO nanosheets by thermal reduction was shrunk from 7.70 A to 6.70 A,CEs increased significantly,but the area resistance also obviously increased.The VEs of GX-10.10 （?） decreased markedly（77.7-70.2%vs.84.8-76.8%GX-11.10 A）and the rate performance was distinctly deteriorated.To reduce the area resistance of the GO composite membrane,phenylhydrazine-4-sulfonic acid was embedded to the angstrom-scale two-dimensional GO channel to introduce sulfonic acid groups with fixed negative charges to decline the free energy barrier of proton transfer.Using the identical membrane fabrication method,thermal reduction combined with L-ascorbic acid chemical reduction,sulfonated CLGO composite membrane（GXP-7.98 （?））with an effective pore size of～4.6 （?） was prepared,achieving complete size sieving of H3O+ over VO（H₂O）₅²⁺.At 60-100 mA cm^-2,the CEs of the sulfonated CLGO composite membrane were in the range of 96.4-97.6%with the VEs within the range of 80.4-87.9%.The CEs remained high,and the VEs significantly increased simultaneously.However,the VFB cell with GXP-7.98 A exhibited significant capacity fading at 80 mA cm^-2.A combination of experimental exploration and molecular dynamics simulation was employed to investigate the capacity decay mechanism in detail.The experiment found that only VO²⁺ appeared to permeate among four kinds of vanadium ions.The VO²⁺ permeation rate was 1.38×10-3 mmol cm^-2 h^-1 at a concentration difference of 0.375 M.It was concluded that the capacity decay was mainly due to the migration of VO²⁺ to the negative electrolyte.The underlying mechanism of the preceding experimental findings was explored by molecular dynamics simulation.As the interlayer spacing d between GO nanosheets was 8 A,free energy barriers of vanadium ions with four valence states(V²⁺,V³⁺,VO²⁺,and VO²⁺)were calculated to be 22.92,41.60,21.15,and 10.51 kcal mol-1,which verified the experimental results.Three main mechanisms for VO²⁺ permeation,i.e.,osmotic pressure differential induced convection,concentration difference induced diffusion,and electric field induced migration,account for 52.3%,16.9%,and 30.8%,respectively.Clearly,VO²⁺convection induced by osmotic pressure difference between the positive and negative electrolytes dominated the total transfer of VO²⁺.Based on this mechanism,asymmetric electrolyte concentration layouts（positive 1.5 M,negative 0.75 M）were used to regulate the osmotic pressure difference between the positive and negative electrolytes to acquire a stable cyclical capacity.Over 20 charge-discharge cycles,the capacity retention was 95.3%（vs.67.2%at symmetrical concentrations）.High ion-selectivity and low area resistance were achieved through reasonable design of ion transport channels and construction of asymmetric membrane structure,resulting in the improvements in CEs and VEs and good electrochemical performances.The size-sieving membrane leaded to VO²⁺ convection induced by osmotic pressure difference,which in turn caused significant capacity decay.