Novel Phosphoric Acid Doped Cross-linked Polybenzimidazole High-temperature Proton Exchange Membranes: Synthesis And Property Studies

Fuel cell is an electrochemical energy device, which can convert chemical energydirectly into electricity with limited energy loss. Fuel cells have attracted increasingattention because of their high efficiency, environmentally benign nature. Clean andhighly effective energy resources are needed for a sustainable future due to thedepletion of the fossil fuel resources and environmental problems associated with theburning of fossil fuels. It is inevitable to use fuel cell electric vehicles to take place ofthe gasoline automobiles. Because the only product of chemical reactions within thefuel cells is water, no other hazardous side products. Moreover, the energy conversionefficiency of fuel cells is2to3times higher than the internal combustion engine.Among several types of fuel cells, proton exchange membrane fuel cells(PEMFCs) have attracted major research activities in the past decade due to theirexcellent comprehensive performance for transportation, residential appliance andportable power applications. The key component of PEMFCs is proton exchangemembrane (PEM), which should have high proton conductivity and offer an attractivecombination of chemical, physical and mechanical properties. Nafion is the mostcommercially available PEM due to its high proton conductivity, reliability anddurability. However, Nafion still has some significant limitations: firstly, theperfluorosulfonated ionomer synthesis condition is harsh, and the raw material costsare high ($600m-2); secondly, the optimum operation temperature of Nafion is around 80oC, so low humidity or high temperature will lead to low conductivity; thirdly, highfuel permeability, which has limited its practical large-scale applications.Due to the stringent requirements of the automotive market, researchers arepushed to investigate the membranes, which are able to work at high temperature(120-200oC) and under anhydrous conditions. High temperature proton exchangemembrane fuel cell (HT-PEMFC) could not only improve the reaction kinetics at bothelectrodes, but also enhance the CO-tolerance and the overall performance.Polybenzimidazole (PBI) membranes as electrolyte were first proposed by Wainrightet al. Henceforth, PBI membranes have been widely investigated due to their excellentthermal stability and outstanding mechanical properties. Acid-doped PBI membranescould be obtained by adding different kinds of acids such as sulfuric acid, phosphoricacid, perchloric acid, hydrochloric acid or nitric acid. Among these acids, phosphoricacid is the most widely used by researchers. Phosphoric acid-dopedpolybenzimidazole (PA/PBI) membranes exhibit high proton conductivity at hightemperature ranging from100to200oC under anhydrous conditions, as well as goodmechanical properties. Moreover, benzimidazole rings act as both donor and acceptorof protons, and absorb the PA through hydrogen bonding. PA doping level is one ofthe three main factors affecting the conductivity of PBI membranes; the other two aredoping temperature and operational relative humidity (RH). Obviously, higher PAdoping level leads to higher proton conductivity. However, the chemical stability andmechanical strength deteriorate significantly at high PA doping levels. Thus, manyefforts have been focused on obtaining a kind of material with both high PA dopinglevels and excellent mechanical properties.Cross-linking is an efficient method to achieve high doping levels and maintainenough mechanical strength. Regarding the chemical structures and properties ofcross-linkers, we choose different kinds of cross-linkers to prepare series ofcross-linked PBI membranes. In chapter3, PBI is synthesized by amelt-polymerization method. The chemical structure is confirmed by the FT-IRmeasurement and1H NMR spectrum. An approach has been proposed to prepare thereinforced PA doped cross-linked PBI membranes HT-PEMFCs, using 1,3-bis(2,3-epoxypropoxy)-2,2-dimethylpropane (NGDE) as the cross-linker and thecross-linking process is completed by the program-temperature control. Generally,imidazoles are common curing agent for epoxy groups which are reported by manyresearch groups. Therefore, the reaction between the imidazole rings of PBI and theepoxy groups of NGDE could be easily completed by a facile and gentle heatingmethod. FT-IR measurement and solubility test showed the successful completion ofthe crosslinking reaction. The resulting cross-linked membranes PBI-NGDE-X%(X isthe content of epoxy resin in the cross-linked membranes) exhibited improved volume,chemical and oxidative stability. Moreover, the mechanical strength is affected by theconcentration of the PA solution, which decreases with the increasing PAconcentration. PA doping level is also influenced by the PA concentration, but thecross-linking structure makes it possible to obtain higher phosphoric acid dopinglevels and therefore relatively high proton conductivity. In addition, the thermalstability of PBI-NGDE-X%is sufficient enough within the operation temperature ofPBI-based fuel cells, and the initial decomposition temperature of which is about200°C. The PBI-NGDE-X%membranes displayed relatively high proton conductivityunder anhydrous conditions. For instance, PBI-NGDE-5%membrane with aciduptake of193%exhibited a proton conductivity of17mS cm-1at200°C. All theresults indicate that it may be a suitable candidate for applications in HT-PEMFCs.PBI membranes with porous structure will definitely enhance the ability ofabsorbing PA and possess high proton conductivity. However, the porous PBImembranes display a degradation of mechanical properties. Therefore, in chapter4,we proposed a new approach to obtain high-temperature proton exchange membranesbased on novel cross-linked porous PBI. The cross-linked porous PBI membranes(CpPBI) were prepared by leaching out a low-molecular-weight compound dibutylphthalate (DBP) and mixing4,4’-diglycidyl (3,3’,5,5’-tetramethylbiphenyl)(TMBP)epoxy resin as a cross-linker. To some extent, the cross-linking structure makes up thedegradation properties due to the porous structure. According to the surfacemorphology and mechanical properties of the porous membranes, we confirm anoptimum content of porogen of50%. Compared with pPBI membranes, the mechanical properties and the chemical stability of CpPBI membranes weresignificantly improved. The mechanical properties of PA/CpPBI-X membranesmaintain a proper value, which can be used as PEM. In general, the increasedtemperature results in a deterioration of mechanical strength of the membranes, butthe value of elongation at break increase due to the plasticizing effect. Both theuncross-linked and cross-linked membranes were doped with phosphoric acid，thedoping levels of all the pPBI membranes were higher than that of pristine PBI, thusleading to higher proton conductivity. Especially the CpPBI-10(with a cross-linkercontent of10wt%and a porogen content of50wt%) had a relative high protonconductivity of46mS cm-1at200oC, which is higher than some of the known porousPBI membranes measured under similar conditions.Introduction of nanoscale additives such as SiO2into the polymer matrix is apromising way to simultaneously elevate proton conductivities and mechanicalproperties. Especially, incorporating the silane-functionalized SiO2to the PBI systemcan significantly increase the PA doping level and thus elevate the proton conductivity.In chapter5, silane-cross-linked PBI membranes with high proton conductivity andexcellent mechanical properties were successfully prepared by using a silanemonomer, γ-(2,3-epoxypropoxy) propyltrimethoxysilane (KH560), as a cross-linker.The cross-linking process was divided into two steps:(i) through a facile and generalheating method to complete the reaction between the imidazole rings of PBI and theepoxy groups of KH560;(ii) after the hydrolysis process, KH560in the matrix formedthe silane-cross-linked structure, which can absorb PA through electric interaction,thus the cross-linked SCPBI-X (X is the weight percent of KH560) membranes wereobtained. The FT-IR and solubility test were used to characterize and confirm thecross-linked structure in the membranes. The silane-cross-linked membranesdisplayed excellent chemical stability and improved mechanical strength. Especiallyat high temperature (130oC), the tensile strength value was in the range of68.6to99.3MPa, and that of the pristine PBI was61.7MPa. Moreover, the protonconductivity was significantly enhanced because the silane-cross-linked structure inthe membranes could absorb more phosphoric acid. The Td5%of PA/SCPBI-X is in the range of213.5-234.1oC. Considering the tradeoff of mechanical properties and protonconductivity,3%KH560in weight was demonstrated to be an optimum content in themembranes, for instance, the SCPBI-3/7.95PA (the cross-linker content was3wt%and the PA doping level was7.95) had a proton conductivity of81mS cm-1and thatof the SCPBI-3/9.07PA was114mS cm-1at200oC.The structures of cross-linkers greatly affect the properties of cross-linkedmembranes. For instance, in our previous work, we have prepared two kinds ofepoxy-based cross-linked PBI, one cross-linker was4,4’-diglycidyl(3,3’,5,5’-tetramethylbiphenyl) epoxy resin (TMBP), the other was1,3-bis(2,3-epoxypropoxy)-2,2-dimethylpropane (NGDE). The most obviousdifference between these two cross-linkers was the chemical structure, the former wasof rigid aromatic ring structure and the latter was totally flexible alkyl chain structure.The results indicated that cross-linked PBI membranes using rigid structurecross-linker displayed better combination properties of mechanical strength, oxidativestabilities, high doping levels and proton conductivity. What’s more, severalresearchers have prepared cross-linked polymer membranes using macromolecularcross-linker, which resulted in high cross-linking density and excellent mechanicalproperties and chemical stabilities. In chapter6, a series of macromolecular-cross-linked PBI membranes have been successfully prepared for the HT-PEMFCapplications. Bromomethylated poly (aryl ether ketone)(BrPAEK) is synthesized andused as a macromolecular cross-linker, and the cross-linking reaction can beaccomplished at160oC using an easy facial heating treatment. The resultingcross-linked membranes CBrPBI-X (X is the weight fraction of the cross-linker)display excellent chemical and oxidative stability. The mechanical strength is alsoimproved due to the cross-linking networks. The thermal stability of PA/CBrPBI-Xmembranes is good enough, and the Td5%of which is around200oC. After PA doping,the proton conductivity is also improved. Considering the tradeoff of the mechanicalstrength and proton conductivity,10wt%BrPAEK is demonstrated to be an optimumcontent in the matrix. For instance, the proton conductivity of CBrPBI-10is38mScm-1at200oC, which is higher than that of pristine PBI with the proton conductivity of29mS cm-1at the same temperature.