Performance Investigation Of Proton Exchange Membranes For Vanadium Redox Flow Batteries Based On Phosphotungstic Acid Anchoring

The seperator is the key component of vanadium redox flow battery（VRB）,which is used to separate the positive and negative electrolytes and conduct the internal charge fluids.At present,perfluorosulfonic acid proton exchange membrane（PEM）is widely used.However,its sulfonic acid clusters are easy to form micelle channels with large pore size,which leads to the rapid penetration of vanadium ions(Vⁿ⁺).Unfortunately,increasing the vanadium resistance of PEM is usually at the expense of its proton conductivity.In view of this,phosphotungstic acid（PWA）with super Br?nsted acidity was introduced as a high-efficiency proton donor to fully maintain or increase the proton conductivity of PEM while inhibiting Vⁿ⁺crossover.Since PWA is easily soluble in water and leaks from the membrane,this thesis takes the anchoring of PWA as the starting point,and carries out the systematic research work around the core purpose of improving the H⁺/Vⁿ⁺selectivity of PEM.The common inorganic anchoring agent is difficult to disperse well in the matrix owing to the great difference of the solubility parameters between polymer and inorganic anchoring agent.In view of this,PWA was anchored on Nano Kevlar Fibers（NKFs）.The hybrid membranes of PBI-（NKFs@PWA）and Nafion-（NKFs@PWA）were prepared using polybenzimidazole（PBI）and perfluorosulfonic acid resin（Nafion）as the matrix.High loading level of PWA endowed PBI with appropriate proton conductivity.The polar channels formed by sulfonic acid（-SO₃H）clusters were blocked through the cross-linking network formed by NKFs and Nafion chains,and then the barrier effect of NKFs@PWA for Vⁿ⁺was fully exerted.The VRB single cell assembled with PBI-（NKFs@PWA）-22.5 wt%exhibited similar energy efficiency to recast Nafion as the current density was below 80 mA·cm^-2,and the maximum coulombic efficiency could reach 97.6%.The lowest VO²⁺permeability was 2.46×10^-7 cm²·min^-1,which was about 12%of recast Nafion.In addition,the composite membranes exhibited similar proton conductivities to recast Nafion due to the compensation of ion exchange capacity（IEC）.The self-discharge time of VRB assembled with Nafion-（NKFs@PWA）-30 wt%was 76 h,which was about 1.95 times that of recast Nafion.A nanocomposite carrier containing NKFs and silica nanoparticles（SN）was designed using SN as the hygroscopic material.The PWA was anchored on the carrier and incorporated into the Nafion matrix,which improved the water uptake（WU）value and proton conductivity of the composite membrane.Influenced by the incompatibility between SN and the polymer matrix,Nafion-（SN@NKFs/PWA）had a slight decrease in Vⁿ⁺suppression compared with Nafion-（NKFs@PWA）.The VRB based on Nafion-（SN@NKFs/PWA）-15 wt%showed a lower charging voltage and a slightly higher discharge voltage than that with recast Nafion.Inspired by the Donnan effect generated by Vⁿ⁺and the positive charge in the anion exchange membrane,3-aminopropyl-triethoxysilane（APTES）was grafted onto the SN to introduce the-NH₂ group,and then PWA was coupled with SN through the electrostatic interaction with-NH₂.The Donnan-rejection effect of Vⁿ⁺and-NH₃⁺formed by the protonation of-NH₂ effectively inhibited the crossover of Vⁿ⁺,and the proton-conducting ability of the composite membrane was greatly improved due to the promoted dissociation of the PWA proton donor by the-NH₂ proton receptor.The proton/vanadium selectivity of Nafion-（SN@APTES-PWA）-4.5 wt%was 2.92×10⁵S·min·cm^-3,which was about 8.5 times that of recast Nafion.The voltage efficiency of the VRB with the composite membrane could reach 95.3%and the discharge capacity retention rate was 16.4%higher than that of recast Nafion at 80 mA·cm^-2.Mesoporous silica SBA-15 was synthesized by using P123 as a template,and APTES was grafted on SBA-15 to introduce-NH₂ groups.PWA was then coupled with-NH₂ as an acid-base pair by electrostatic interaction to reduce its leakage.The Si-OH of SBA-15 provided water molecule adsorption sites,and the porous structure provided water molecule storage space,which promoted the overall carrier transport of protons in the form of H_2n+1O_n⁺and hopping transfer between adjacent proton groups.The proton conductivity of the prepared Nafion-（SBA-15@APTES-PWA）-4.5 wt%composite membrane was up to 0.089 S·cm^-1.The initial discharge capacity of the composite membrane was about 110 mAh higher than that of recast Nafion.The metal-organic framework compounds（MOFs）named as UiO-66-NH₂ with bulk-NH₂ groups were synthesized by using 2-aminoterephthalic acid and ZrCl₄.The microporous structure of UiO-66-NH₂ had good storage capacity for water molecules and protons could be transported rapidly through the coordination structure of MOFs,which accelerated the transfer process of protons inside the membrane.Additionally,compared with the introduction of-NH₂ groups via the grafting of APTES,the in-situ introduction of-NH₂ groups in the formation of MOFs had better homogeneity at micro-scale,which can fully form-NH₃⁺species pervasive in the Nafion matrix and create electrostatic repulsion with Vⁿ⁺to achieve higher Vⁿ⁺shielding ability.The Nafion-（UiO-66-NH₂@PWA）-3 wt%membrane presented a proton conductivity of0.096 S·cm^-1,VO²⁺permeability of 2.74×10^-7 cm²·min^-1.The proton/vanadium selectivity was as high as 3.51×10⁵ S·min·cm^-3,which was about 10.2 times that of recast Nafion.Furthermore,the single cell with Nafion-（UiO-66-NH₂@PWA）-3 wt%exhibited an overall improved VRB performance at all testing current densities.