Poly (Arylene Ether Ketone) Containing Alkaline Groups For Polymer Electrolyte Membranes:Preparation And Performance Studies

Energy shortage and environmental polution are serious issues associated with human's life all over the world.The development and utilization of new types of clean,green,renewable energy sources and technologies to get rid of the dependence on traditional fossil fuels is probably a viable solution to these problems.A fuel cell is a kind of clean,efficient energy conversion device,which could directly convert the chemical energy of a fuel directly into electrical energy.Due to its high energy conversion efficiency,low pollution and wide supplement of fuel,fuel cell has been drawn much attention.Polymer electrolyte membrane fuel cell,as a member of fuel cell family,became a hot spot of research for its advantages of compact structure,high power density,short start-up time and wide application field.Polymer electrolyte membrane is one of the core components of fuel cell.According to the type of membrane,polymer electrolyte membrane fuel cell could be divided into two types: proton exchange membrane fuel cell and anion exchange membrane fuel cell.Anion exchange membrane fuel cell has the advantages of non-precious metal catalyst,low fuel permeability and high electrode reaction efficiency,but its performance is still far from the level of commercialization for now.The main reason is that the commercial-available anion exchange membrane in hand does not possess excellent comprehensive performance,which could not meet the requirements of the anion exchange membrane fuel cell.At present,the main drawbacks of anion exchange membrane are low ionic conductivity,poor dimensional stability and poor alkaline stability.Therefore,the development of anion exchange membrane with high ionic conductivity,great alkaline stability and good dimensional stability is important for the study in this field.In this paper,we aimed to obtain anion exchange membrane with enchanced performance,thus we prepared a series of poly?arylene ether ketone?containing alkaline groups and investigated the influence of crosslinking structure on the membrane properties.The distribution density of ionic groups on side chains and the flexibility of side chains were changed to improve the performance.Furthermore,alkaline poly?arylene ether ketone?could be used in high temperature fuel cell after being doped with phosphoric aicd.The experimental process and results are described as following:?1?A series of bromomethylated poly?arylene ether ketone??Br PAEK?with different degree of bromine substitution were prepared.The bromomethyl groups in Br PAEK then reacted with quaternary ammonium reagent,1-vinylimidazilium to form vinlimidazolium functionalized poly?arylene ether ketone??VIm PAEK?.Dimethoxybenzoin was added as a photo-initiator and activated by UV light to induce the self-crosslinking reaction of VIm PAEK.The degree of crosslinking in VIm PAEK membranes were controlled through the exposing time under UV light.Gel fraction test showed that the longer the exposing time,the higher degree of cross-linking in polymer film.The DMA test showed that the Tg values of VIm PAEK membranes were all higher than 245 oC and increased with the increasing degree of crosslinking,while 5% weight loss temperatures?Td5%?of VIm PAEK membranes were all higher than 260 oC according to TGA test,indicating the good thermal stability of VIm PAEK membranes.The mechanical properties of crosslinked membranes were significantly improved,the best one was VIm PAEK-15 min,with a young's modulus of 1837 MPa and a tensile strength of 69 MPa.The test results of water absorption and swelling ratio proved that the crosslinking structure effectively controled the swelling behaviour of membrane material in water.The test of alkaline stability indicated that VIm PAEK and crosslinked ones were unstable in strong alkali,with the degradation of imidazole rings,but the membranes were very stable in the weak Na HCO3 solution.In conclusion,VIm PAEK membranes have the potential to be used in less alkaline electrochemical systems.?2?We have successfully prepared mono-quaternized poly?arylene ether ketone??MQPAEK?using triethylene diamine as the quaternary ammonium reagent,which molecular contains one quaternary ammonium group on every side chain.Then di-quatermized poly?arylene ether ketone??DQPAEK?,which contains two quaternary ammonium groups on each side chain,was also successfully synthesized,using CH₃ I as the methylation reagent.TGA test demonstrated that the 5% weight loss temperature of all membranes were above 270 oC,and mechanical property test showed that the tensile strength of all membranes were higher than 73 MPa,which proved that MQPAEK and DQPAEK had good thermal stability and mechanical properties.It could be found that DQPAEK membrane could absorb more water and maintain lower swelling ratio when DQPAEK membrane and MQPAEK membrane had similar IEC values.And with the similar IEC values,DQPAEK had higher ionic conductivity and better performance in the alkaline stability test.Both DQPAEK and MQPAEK had good resistance to methanol permeability.The results mentioned above showed that the ionic conductivity,dimensional stability and alkaline stability of quaternized polymer could be enhanced as the content of ionic groups on the side chain increased,providing a new strategy for designing alkaline polymer electrolyte membranes with better performance.?3?On basis of the study?2?,we synthesized a type of small molecule with a quaternary ammonium group and an alkyl bromide.After tethering these molecules onto MQPAEK,tri-quaternized poly?arylene ether ketone??TQPAEK?was successfully prepared with flexible side chains containing three quaternary ammonium groups.The tensile test results indicated that the introduction of flexible side chains significantly improved the elongation at the break of polymer membranes.Among TQPAEK membranes the elongation at the break of TQPAEK-1.50 reached the highest value of 103.2%.The DMA test showed that the existence of the flexible side chain decreased the Tg value of the polymer.With the grafting ratio of flexible side chains increasing,the lower Tg value of the polymer would be realized.Comparing the IEC values and ionic conductivities of TQPAEK and DQPAEK,TQPAEK showed much higher ionic conductivity than that of DQPAEK at similar IEC values.For instance,TQPAEK-0.81?IEC=1.75 mmol g-1?exhaibited higher ionic conductivity of 58.85 m S cm-1 than that of DQPAEK-1.25?IEC=1.85 mmol g-1?,which was 54.25 m S cm-1.For the alkaline stability test,the ionic conductivity loss of TQPAEK in the strong base solution was less than that of DQPAEK and MQPAEK,because the flexible side chains are tangled around ionic groups to protect them.The results mentioned above showed that the introduction of the flexible side chain improved the ionic conductivity and alkaline stability of the anion exchange membrane.?4?Through the substitution reaction of mercapto groups and bromomethyl groups,the triazole groups were successfully introduced into the poly?arylene ether ketone?to prepare the triazole funcionalized PAEK?MTZPAEK?.Phosphoric acid doping MTZPAEK?PA-MTZPAEK?was prepared by absorption of PA through triazole groups.The phosphoric acid doping level was positively correlated with the content of triazole groups.The DMA test showed that the Tg of MTZPAEK is higher than 250 oC,and TGA test proved that Td5% of PA-MTZPAEK were higher than 235 oC,while the Td5% of MTZPAEK were higher than 270 oC.The MTZPAEK was obviously plasticized after phosphoric acid doped,of which the elongation at break increased dramatically,reaching 112.9%.The proton conductivity of the membranes was improved with the increasing PA doping level,and the maximum one was 51 m S cm-1.MTZPAEK and phosphoric acid doped ones showed excellent oxidative stability.The reason was attributed to thioether groups,which were derived from mercapto groups in the substitution reaction,could quenche free radicals to enchance the oxidative stability.The conversion process of sulfide quenching radicals to sulfoxide groups and sulfone groups was confirmed by FT-IR.The maximum power density of PA-MTZPAEK?2.15?at 160 oC reached 61 m W cm-2 in a single cell test.It indicated that PA-MTZPAEK has potential as a high temperature proton exchange membrane.Moreover,the reaction between mercapto groups and bromomethyl groups could not only introduce target groups into polymer molecule,but also bring thioether groups to enchance the oxdative stability,which is a very promising method to functionalize polymer.