Preparation And Characterization Of Bipolar Membranes For Fuel Cell Applications

Bipolar membrane fuel cell(BPMFC)is a new kind of fuel cell with bipolar membrane(BPM)as its electrolyte.It contains some advantages.First,cheap non-platinum catalysts and/or lower catalyst loading can be used in alkaline cathode.And this is helpful to lower the cost.Second,in H2/O2 BPMFC,protons produced in anode and hydroxides produced in cathode would react in the intermediate layer of bipolar membrane which provides the possibility to devise passive self-humidification over the entire cell.At the same time,the self-humidifying characteristic offers a chance to simplify cell system complexity.Third,the water production reaction facilitates the consumption of H+ and OH-which accelerates oxygen reduction reaction and hydrogen oxidation reaction and helps to get a better fuel cell performance.Bipolar membrane is used to transfer protons and hydroxides simultaneously and separate the anode and cathode.It is very vital to the fuel cell performance.Herein,a simple casting method was used to prepare a novel bipolar membrane and following is the main research aspects:(1)The imidazolium salt 1-(4-vinylbenzyl)-3-methyl-imidazolium chloride was synthetized by 4-vinylbenzyl chloride and 1-methylimidazole;then imidazolium salt and purified styrene were used to prepare imidazolium cation copolymer through free radical copolymerization.ATR-FTIR spectrum verifies the successful preparation of copolymer,and there is 4.394 wt.%of nitrogen element in copolymer due to the imidazole groups.Anion exchange membranes(AEM)were prepared by 5 wt.%copolymer solution through a solution-casting and evaporation method.In 30 ?,the ion exchange capacity,water uptake and ionic conductivity of AEM are 1.030 mmol/g,29.0%and 29.7 mS/cm,respectively.All these data show that the imidazolium cation copolymer can be used in BPM preparation.(2)Polytetrafluoroethylene(PTFE)porous membrane was chosen as base film of BPM,and Nafion solution was used to prepare cation exchange layer of BPM.The best preparation condition of Nafion/PTFE cation exchange layer(NPCEL)is placing 4 h at room temperature,drying at 80 ? in a vacuum oven and followed by annealing at 120 ?.Tensile strength test and swelling ratio data show that NPCEL has a better mechanical strength and dimensional stability than Nafion membrane by introducing PTFE.At the same time,NPCEL could have very low Nafion resin loading,which is helpful to decrease cost and prepare thinner bipolar membrane.(3)The BPMs with different loadings of Nafion resin and imidazolium cation copolymer were prepared.Properties of BPMs were investigated by ATR-FTIR,TGA,SEM-EDS and so on.At the same time,ionic conductivity and water uptake(WU)were also studied.SEM pictures show that both anion exchange layer(AEL)and cation exchange layer(CEL)of the bipolar membrane are smooth and dense.Besides,not only Nafion resin but also imidazolium cation copolymer is well filled in PTFE micro pores.ATR-FTIR analyses and EDS data show that there are single AEL and CEL with different structures,and there is an intermediate district between CEL and AEL.The BPMs have good thermal stability under 205.7 ?.At room temperature,Water uptakes of BPMs are 10.3%?28.3%.BPM-2c has the max ionic conductivity of 35.0?76.5 mS/cm in 30-80 ? and its Nafion resin loading is 4.00 mg/cm2 and copolymer loading is 0.40 mg/cm2.(4)The best hot-pressing condition of membrane electrode assembly is 2.0 MPa,50 ? for 300 s.BPM-2c was utilized to H2/O2 fuel cell tests since it has the higher ionic conductivity among all the samples.It has a peak power density of 40.3 mW/cm2 at the current density of 126.7 mA/cm2 at 30? under anhydrous operation conditions.