Optimization And Regulation Of Relative Selectivity Of Proton Exchange Membranes For Direct Methanol Fuel Cells

Energy is the base of a country's development.Under the pressure of energy crisis and environmental pollution,lots of countries have begun to develop new energy sources.Proton exchange membrane fuel cell(PEMFC)is one of the new energy sources,which possess low pollution,high energy conversion efficiency and energy density.Direct methanol fuel cell(DMFC)is a type of PEMFC which uses methanol as a fuel.As one of the core components of DMFC,proton exchange membrane(PEM)mainly plays a role of blocking fuel and transferring protons.At present,the most widely used PEM is Nafion,which are made by Dupont.Perfluorosulfonic acid proton exchange membranes represented by Nafion have good chemical stability and excellent medium-low temperature proton transport capability.However,perfluorosulfonic acid proton exchange membranes have high cost,terrible methanol permeability coefficient and proton conductivity under the high temperature.Therefore,it is necessary to develop new membranes to replace Nafion.In this paper,we chose aromatic and aliphatic polymers as matrix materials to prepare new PEMs.N-heterocyclic modification,acid-base interaction and cross-linking were used to give new PEMs better relative selectivity.The relative selectivity is meaning of that the ratio of proton conductivity and methanol permeability coefficient.Firstly,we chose the sulfonated poly(arylene ether ketone sulfone)(SPAEKS)as the matrix material.SPAEKS possessed good mechanical property and thermal stability,which was seen as a good material to alternate Nafion.However,the proton transmission in SPAEKS membranes was depended on the sulfonic acid group.In order to get high proton conductivity,the SPAEKS membranes had to possess high degree of sulfonation(DS).But the membranes which had too high DS will swell unduly,and that will lead to the decrease of mechanical property and alcohol blocking property.To overcome the shortcomings of SPAEKS membranes,we used N-heterocyclic modification,acid-base interaction and cross-linking to improve the relative selectivity of SPAEKS membranes.Carboxyl was leaded into the pendant of SPAEKS macromolecular to obtain the SPAEKS-COOH-m copolymers.2-Aminobenzimidazole(BM)groups were grafted onto the side-chain of SPAEKS macromolecular to prepare the SPAEKS-BM-x copolymers.As the results indicated,the introduction of BM made the SPAEKS-BM-x membranes had the low water uptake(WU),swelling ratio(SR)and methanol permeability coefficient than those of the pure SPAEKS membranes.At 60 oC,the methanol permeability coefficient of SPAEKS-BM-40% was 9.97×10-7 cm2 s-1,which was half of that of pure SPAEKS membrane.Due to the steric effect and high p Ka(acid dissociation constant)of BM,the proton conductivities of SPAEKS-BM-x membranes were much lower than that of pure SPAEKS membrane.The introduction of BM groups made the SPAEKS-BM-x membranes had lower methanol permeability coefficient,but also had lower proton conductivity at same time.In addition,the relative selectivity of SPAEKS-BM-x membranes was almost same as pure SPAEKS membrane at 20 oC,which was not what we expected.Therefore,a new N-heterocyclic 4-(2-aminoethyl)-imidazole was used to modify the SPAEKS matrix material.4-(2-aminoethyl)-imidazole(IM)groups were grafted onto the side-chain of SPAEKS macromolecular to prepare the SPAEKS-IM-x copolymers.It was found that the WU and SR of SPAEKS-IM-x membranes were significantly decreased as the content of IM increased.IM groups had weaker steric effect,higher chain movement ability and lower p Ka than those of BM groups,which leaded to the SPAEKS-IM-x membranes possessed higher proton conductivity and methanol permeability coefficient than those of SPAEKS-BM-x membranes.However,high proton conductivity made the SPAEKS-IM-x membranes have better relative selectivity than that of pure SPAEKS membrane.At 20 oC,the relative selectivity of SPAEKS-IM-40% was 6.52×104 S scm-3,which was twice than that of pure SPAEKS membrane.The introduction of IM groups achieved the improvement of proton conductivity of the pure SPAEKS membrane.But the decrease of methanol permeability coefficient had not met our expectation.Thanks to the SPAEKS-IM-40% membrane had the highest proton conductivity in SPAEKS-IM-x membranes,we chose the SPAEKS-IM-40% copolymers as the matrix material to carry on further modification.Different content of SPVA were added into SPAEKS-IM-40% copolymers to obtain the Semi-interpenetrating network composite membranes(SPAEKS-IM/SPVA-x).The results showed that the WU,SR and methanol permeability coefficient of SPAEKS-IM/SPVA-x membranes were obviously decreased as the content of SPVA increased.As the density of cross-linking increased,the intermolecular gap of SPAEKS-IM/SPVA-x membranes was reduced gradually.At 60 oC,the methanol permeability coefficient of SPAEKS-IM/SPVA-40% membrane was 2.37×10-7 cm2 s-1,which was an order of magnitude lower than that of pure SPAEKS membrane.Furthermore,the introduction of SPVA made the SPAEKS-IM/SPVA-x membranes possess much better relative selectivity than that of SPAEKS-IM membrane.At 20 oC,the relative selectivity of SPAEKS-IM/SPVA-40% was 33.70×104 S scm-3,which was twelve times than that of pure SPAEKS membrane.N-heterocyclic modification and cross-linking had improved the relative selectivity of SPAEKS membranes effectively.However,the preparation and modification of SPAEKS were more cumbrous than aliphatic polymer.Therefore,the last part of this paper,we chose poly(2-Acrylamido-2-methyl-1-propanesulfonic acid)(PAMPS)as proton transport carrier material and SPVA as the membrane-forming support material.According to the free radical polymerization and esterification to achieve the interpenetrating network cross-linking membranes(SPVA/PAMPS).As the result revealed,the density of cross-linking increased with the content of PAMPS increased,which leaded to the WU and SR of SPVA/PAMPS membranes decrease gradually.The unique interpenetrating cross-linking network caused SPVA/PAMPS membranes had low methanol permeability coefficient.In addition,the introduction of PAMPS made SPVA/PAMPS membranes possessed numerous sulfonic acid groups,which leaded to the proton conductivity of SPVA/PAMPS membranes was much higher than that of pure SPVA membrane.At 20 oC,the relative selectivity of SPVA/PAMPS-40% was 7.50×104 S scm-3,which was triple than that of pure SPAEKS membrane.Compared to the SPAEKS membrane,the preparation of SPVA/PAMPS membranes is much easier and the reaction condition of SPVA/PAMPS membranes is mild.