Poly (Phthalazinone Ether Ketone) Amphoteric Ion Exchange Membranes For Vanadium Redox Flow Battery Applications

Vanadium redox flow battery(VRB)is suited to be used as a large-scale energy storage device.The separator in VRB functions to isolate electrolytes in two half-cells and allow the transport of current carriers.Cation exchange membranes(CEMs)for VRB applications maintain increased ion conductivity while suffer from poor resistance to vanadium ion cross-over.Anion exchange membranes(AEMs)possess low vanadium ion permeability due to Donnan exclusive effect while suffer from poor ion conductivity.Amphoteric ion exchange membranes(AIEMs)with both cationic and anionic groups have the promising to address the above problem.Aromatic-polyether-type polymers are extensively employed for the preparation of AIEMs.Poly(phthalazione ether ketone)firstly explored by our group exhibits good chemical stability and mechanical properties.A series of AEMs based on poly(phthalazione ether ketone)were prepared through chloromethylation-amination or bromination-amination process.They exhibited decreased vanadium ion permeability while the chemical stability and ion conductivity needed further improvement.In this work,AIEMs based on poly(phthalazione ether ketone)for VRB were prepared.The bromination of poly(phthalazinone ether ketone)with methyl groups was conducted to obtain brominated poly(phthalazione ether ketone)(BPPEK).BPPEK and poly(phthalazione ether ketone)(PPEK)were mixed and dissolved in N-methyl-2-pyrrolidone.Blend base membranes were prepared with solution casting method,which were soaked in trimethylamine aqueous solutions to obtain AEMs(Q/P).Compared with quaternized poly(phthalazinone ether ketone)membrane(QBPPEK),Q/P membranes exhibited lower vanadium ion permeability and ion conductivity.The ex-situ test in VO2+ solution presented that Q/P membranes showed lower weight loss(4.0-6.3%)and VO2+content(0.92-1.1 Omol%)than QBPPEK,exhibiting the increased chemical stability.AIEMs based on poly(phthalazinone ether ketone)(Q/S-x)with sulfonic acid and quaternary ammonium groups were prepared from BPPEK with the degree of substitution(DS)of 88%and sulfonated poly(phthalazinone ether ketone)(SPPEK)with ion exchange capacity(IEC)of 1.37 mmol/g through blending membrane formation and amination process,in which x presented the mass content of SPPEK.With the increasing of SPPEK content,water content and vanadium ion permeability in Q/S-x membranes decreased while area resistance firstly reduced and then increased.The increased content also led to the improvement of water transport volume to positive half-cell and chemical stability in VO2+.Q/S-20%membrane showed the area resistance of 0.57? cm2,which was lower than the values of other Q/S-x membranes.V3+and V02+permeability of Q/S-x membranes were 1.08-1.61%and 0.29-5.08%of the values for Nafion115.When SPPEK content increased from 10wt.%to 50wt.%,the ratio of VO2+permeability to V3+permeability decreased from 2.97 to 0.26.The static water transport volume of Q/S-30%membrane was 96%less than that of Nafion115(0.8 mL).The energy efficiency(EE)of VRB with Q/S-30%membrane reached around 90%,higher than that with Nafion115.Throughout cycling test lasting for more than 500 h,VRB with Q/S-30%membrane exhibited increased EE and capacity retention.And the cycling water transport volume of Q/S-30%membrane was 87.7%less than that of Nafion115.The effect of amounts of sulfonic acid and quaternary ammonium groups on properties of AIEMs was detected,respectively.BPPEK and SPPEK with IEC of 0.70 and 1.37 mmol/g were employed to prepare Q/S-y,where y represented the IEC of SPPEK.Water content,vanadium ion permeability and water transport volume to positive half-cell of Q/S-y membranes increased while area resistance reduced with the increased IEC.Q/S-1.37 membrane exhibited much lower vanadium ion permeability related to Nafion115,and the area resistance was 0.58 ? cm2.The voltage efficiency(VE)of VRB with Q/S-y membranes increased from 89.3%to 91.8%with the increasing of IEC,and VRB with Q/S-1.37 membrane exhibited higher EE than that with QBPPEK and Nafion115 membranes.Q/S-y membranes showed better chemical stability in VO2+solution than QBPPEK.And increased content of sulfonic acid groups negatively affected the chemical stability.BPPEK with the DS of 60-95%and SPPEK were applied to prepare Q-z/S,and z represented the DS.With the increasing of DS,for Q-z/S membranes vanadium ion permeability and water transport volume to negative half-cell increased while area resistance and chemical stability in VO2+decreased.Q-95/S membrane showed the area resistance of 0.54 ? cm2.V3+ and VO2+permeability of Q-z/S membranes were 98.4-99.8%and 96.3-99.7%less than those of Nafionl 15.The VE of VRB with Q-z/S membranes enhanced with the increasing of DS.The EE of VRB with Q-95/S membrane reached 90.6%,higher than that of Nafionl 15.Aiming to address the problem of decreased ion conductivity induced by the dense microstructure of membranes after SPPEK incorporation and further improve chemical stability,Q/S-M membranes were prepared from BPPEK and SPPEK substituted by counter-ions(SPPEK-M)through blending membrane formation and amination process,and M represented the counter-ion(Li+,Na+,K+and Mg2+).Compared with Q/S membrane prepared from BPPEK and SPPEK in acid form,Q/S-M membranes showed higher water content and ion conductivity,which enhanced as the increasing of ion radius of counter-ions.Q/S-K membrane showed the area resistance of 0.67 ? cm2,which was half the value of Q/S membrane.The VE and EE of VRB with Q/S-M membranes enhanced with the increasing of ion radius.The EE of VRB with Q/S-K membrane was 89.4%,higher than those of Nafion115 and Q/S membranes(86.5%and 83.2%).The chemical stability in VO2+for Q/S-M membranes was poorer than that for Q/S membrane,which improved with the decreasing of ion radius.Q/S-M membranes exhibited better chemical stability in VO2+ than Q/S-20%and Q/S-30%membranes.The efficiencies of VRB with Q/S-K membrane were stable in 100-cycles test.SPPEK-Mg1 and SPPEK-Mg2 were obtained from the substitution of sulfonic acid groups in SPPEK by magnesium ions with the molar ratio between Mg2+ and sulfonic acid groups of 1/1 and 1/2.Q/S-Mg1 and Q/S-Mg2 membranes were prepared from BPPEK,and SPPEK-Mgl and SPPEK-Mg2 through blending membrane formation and amination process.Q/S-Mgl membrane showed uniform microstructure while obvious phase separation was observed in Q/S-Mg2 membrane.Q/S-Mg1 and Q/S-Mg2 membranes exhibited lower area resistance(0.85 ? cm2 and 0.87 ? cm2)than Q/S membrane.In order to further improve chemical stability of membranes,poly(phthalazinone ether ketone)with pyridinium groups(PyPPEK)and SPPEK were blended to prepare amphoteric ion exchange membranes(Py/S),which possessed low IEC.The optimization of preparation condition was conducted to obtain Py/S membranes with improved properties.When Py/S membrane prepared from the casting solution with the concentration of 13wt.%was evaporated at 60? for 1.60 h and the NMP content in coagulation bath reached 8vol.%,VRB with the membrane exhibited coulombic efficiency(CE)of 95.7%,VE of 90.1%and EE of 86.2%,which were close to those of Nafion115.Py/S membranes exhibited similar chemical stability in VO2+ solution compared with Nafion115.