Preparation And Characterization Of Poly (Phenylene Oxide) Based Proton Exchange Membranes

Fuel cells are known as alternative energy conversion systems with high efficiency,renewable fuel(hydrogen,methanol,etc.),and environmental benignity. Hydrogen Proton exchange membrane fuel cells(PEMFCs) were studied the most extensively due to their high effiency,relatively easy operation,and broad range of operating temperature.As the key component in PEMFCs,proton exchange membranes serve as a barrier for fuel and oxidant with the function of transporting protons and blocking electrons.Accordingly,they should possess(1) high proton conductivity and low electronic conductivity,(2) good thermal and chemical stability, (3) good mechanical properties,(4) low permeability,(5) dimensional stability,(6) capability for facbrication into membrane electrode assemblies,and(7) low cost. Perfluorosulfonic acid polymers like Nation^R have some properties good for PEMFC applications and have been the standard membrane for industry and academia during the last decade,but they also have a number of drawbacks,such as high cost and large methanol permeability.These factors have limited its commercial applications;hence, there is a urgent need to explore new proton exchange membranes that can achieve high performance at a low cost.Poly(2,6-dimethyl-1,4-phenylene oxide)(PPO) is a versatile,thermally stable engineering plastic with high mechanical strength and excellent hydrolytic stability. The structure of PPO is simple as compared to other aromatic polymers and it allows many modifications in both aryl and benzyl positions.Especially,modified PPO could possess high ion exchange capacity and good membrane forming properties.Although PPO was considered a promising material for proton exchange membranes,current studies on PPO has not reported good fuel cell performances.The main problems are low proton conductivity and difficulty in controlling membrane swelling.Therefore,preparation of proton exchange membranes by introducing hydrophilic/hydrophobic balance and crosslinking structure to highly sulfonated PPO-based polymers become the focus of this research.The preparation routes and results are as follows.A series of low-cost proton exchange membranes with covalent crosslinking structure were prepared from sulfonated PPO(SPPO) only through a heat treatment. The crosslinking degree could be controlled by adjusting the time or temperature for heat treatment.With a proton conductivity of 0.128 S cm^-1 and tensile strength of 52.8 MPa,as well as proper water uptake and low crossover,the optimum membrane showed a maxim power density of 0.893 W cm^-2 while that of Nafion^R 112 is 0.602 W cm^-2 in a single cell test at 60℃.In view of the competitive properties with Nafion^R series,the crosslinked membranes are qualified for a potential application in fuel cells.New composite proton exchange membranes were prepared by mixing SPPO with bromomethylated PPO(BPPO) for hydrophilic-hydrophobic balance.By properly compromising the conductivity and methanol permeability,membranes with 60-80 wt%SPPO content exhibited comparable proton conductivity to that of Nafion^R 117,and only half the methanol permeability of Nafion^R 117,thereby demonstrating higher single cell performance.The composite was then treated with different amines,such as ammonia,methyl amine,dimethyl amine,and trimethyl amine.These amines have different pKb values and induce distinctive acid-base interactions between aminated BPPO and SPPO.The change in proton conductivity depends on amine type and the SPPO content.Dimethyl amine is recommendable because it can form tertiary ammonium groups to facilitate the proton transfer without appreciable increase in methanol permeability and can form effective acid-base polymer complex with SPPO to increase the thermal stability.A series of hybrid acid-base polymer membranes were prepared by blending SPPO with(3-aminopropyl) triethoxysilane through a sol-gel process.As indicated by scanning electron microscopy,energy-dispersive X-ray analysis,and thermogravimetric analysis,the acid-base interaction improves not only the membrane homogeneity and thermal stability but also the mechanical strength and flexibility.Apart from the low cost,the developed membranes exhibit high proton conductivity and low methanol permeability as compared to Nafion^R 117.Further,the optimal membrane showed better performance than Nafion^R 117 in a single cell test. All these properties make the hybrid membranes suitable for application in fuel cells.For direct methanol fuel cell applications,a series of proton exchange membranes were developed via sulfonation of BPPO base membranes.Besides the low manufacture cost,the membranes exhibited an excellent control on methanol crossover and swelling,and a sound balance with high proton conductivities.These can be attributed to the inherent properties of membrane structures:(â…°) benzyl-substitution with bromine,which imparts the membrane stronger hydrophobicity,(â…±) cross-linking between BPPO chains,which enhances the dimensional stability and renders the membrane a dense texture,(â…²) proper content of sulfonic acid groups,which guarantees high proton conductivities.The optimal membrane exhibited a methanol permeability of 2.64×10^-8 cm² s^-1 and characteristic factorΦof thirty times higher than that of Nafion^R 117.A series of organic-inorganic hybrid membrane were prepared by introducing mercapto silanes and amino silanes to sulfonated BPPO membranes through in-situ sol-gel.Mercapto groups can be oxidated to sulfonic acid groups to accelerate proton conduction,while amino silanes react with benzyl bromine to connect organic and inorganic components by chemical bones.The hybrid membrane with mercapto silane content of 75 wt%showed the best performance;i.e.,it had a proton conductivity of 0.06 S cm^-2,water uptake of 8.71%,and methanol permeability of 1.55×10^-7 cm² s^-1. During oxidation,the deterioration of performance is caused by the decomposition of the sulfonic goup and bromine at the pendant of PPO backbones.The introduction of silanes improves the oxidative stabilility of hybrid membranes in strong or mild oxidative conditions.Its proton conductivity was kept for 1 h even in Fenton agent at 80℃.In summary,it seems attractive and effective to prepare proton exchange membranes by introducing hydrophilic/hydrophobic balance and crosslinking structure to highly sulfonated PPO-based polymers.The resulting low-cost membranes show a great potential in fuel cell applications.