Acid Doping Ability Research Articles

Spectroscopic Study of Reinforced Cross-Linked Polymeric Membranes for Fuel Cell Application.

Acid-doped reinforced polymer electrolyte membranes for high-temperature polymer electrolyte membrane fuel cell applications (HT PEMFCs) are presented and spectroscopically studied. Fully aromatic polyethers are employed bearing main chain pyridine units as the proton accepting sites, which have two different substitution patterns of the pyridine units, namely, 2,5- or 2,6-pyridine. This fact enables control of the solubility and of the acid doping ability of the polymeric membranes. Reinforcement is accomplished via incorporation of a PTFE woven fabric during the casting procedure for fabrication of the membranes. High acid uptake of the reinforced membranes was maintained for the 2,6-pyridine-based copolymers with high pyridine unit content. Studies of the swelling behavior of these reinforced membranes revealed that they expand mainly along the z-axis, which helps to avoid extensive damage in case of humidity or temperature changes during the fuel cell operation. Additionally, spectroscopic techniques are employed, namely, X-ray photoelectron spectroscopy, X-ray photoelectron spectroscopy with depth profile, near-edge X-ray absorption fine structure, and reflection electron energy loss spectroscopy, for the in-depth study of the two copolymer membranes doped with phosphoric acid. Through these spectroscopic evaluations, modifications in the membranes’ chemical structure, orientation, composition, and electronic structure after the reinforcement and doping processes were elaborated and unveiled.

Enhancement on both phosphoric acid retention and proton conduction of polybenzimidazole membranes by plasma treatment

In this study, plasma treatment is designed to solve phosphoric acid (PA) leaching problem as well as enhance the proton conduction and tensile strength of PBI membranes at high temperature. The hydrophily of polybenzimidazole (PBI) membrane surface is remarkably improved during plasma process according to the water contact angle decreasing from 84.0° to 9.9°. The effect of plasma generating power, gas pressure, Ar/O2 gas proportion and exposure time on water contact angle was investigated. The optimized plasma treating condition was 350 W, 5 Pa, 50 % O2 and 7 min. Fourier transform infrared spectroscopy (FT-IR) and x-ray photoelectron spectroscopy (XPS) were performed to investigate the effects of plasma treatment on the chemical composition of PBI membranes. The results of FT-IR and XPS reveal the formation of oxygen groups and a progressive increase of oxygen content on the PBI membrane surface due to the plasma modification. Furthermore, the pristine-PBI and plasma-PBI membranes were doped into phosphoric acid (PA) for high temperature proton exchange membrane use. Compared with PA doped pristine-PBI, the PA doped plasma-PBI membranes were found to present higher PA doping levels, higher proton conductivity and better PA retention ability. This indicates plasma treatment could promote acid doping ability, maintain good mechanical strength and reduce PA leaching of PBI membranes.

(Invited) Crosslinked High Temperature Polymer Electrolyte Membranes

High temperature polymer electrolyte membranes containing basic units that are able to interact with strong protic acids providing ionically conductive composite membrane have shown respectable performance and long operational stability. Further increase of the operation temperature and even an increase on the obtained conductivity values would facilitate the construction of efficient systems providing both electricity and heat, increasing thus the total efficiency. Initial attempts to stabilize the HTPEM electrolyte membranes through covalent crosslinking have proven that the operation temperature can be shifted up to 220oC 1,2. Different crosslinking methodologies have been tested to stabilize either PBI or aromatic polyethers bearing pyridine units3,4. In the present work we focused on the optimization of the crosslinking methodology using side double bonds and we also develop new crosslinking methodologies leading to wholly aromatic crosslinked structures. Different type of crosslinkers have been used, controlling thus the acid doping ability of the final membranes while the use of dopable crosslinkers that enable the high acid uptake and retention. Moreover, depending on the chemical structure of the crosslinked membranes, a significant increase of the conductivity values of the final acid doped composite membranes was obtained. High temperature operation as the one achieved here enables the use of liquid feed like methanol in a compact methanol reformer-high temperature PEM fuel Cell setup that is under construction and testing5.

End-group cross-linked polybenzimidazole blend membranes for high temperature proton exchange membrane

A cross-linked network is obtained by an end-group cross-linkable PBI (E-PBI) and 4,4′-diglycidyl (3,3′,5,5′-tetramethylbiphenyl) epoxy resin (TMBP). The formation of the network is proved by FT-IR and gel fraction test. Then, high temperature proton exchange membranes are constructed by incorporating the cross-linked network into poly(2,2′-(1,3-phenylene)-5,5′-bibenzimidazole) (m-PBI). The influences of the end-group cross-linked network on the properties of membranes are studied, such as, phosphonate acid doping ability, thermal stability, mechanical property, oxidative stability and proton conductivity. The E-PBI/TMBP in the membranes contributes to the improvement of acid doping ability, proton conductivity, mechanical properties and oxidative stabilities. The blend membranes with 20–90wt% E-PBI/TMBP show higher proton conductivity than pristine PBI at 120°C–180°C. These results indicate that the end-group cross-linked blend membranes are promising materials as high temperature proton exchange membranes.

Proton Conducting Membranes Based on Poly(2,2′‐imidazole‐5,5′‐bibenzimidazole)

AbstractPoly(2,2′‐imidazole‐5,5′‐bibenzimidazole) (PBI‐imi) was synthesized via the polycondensation between 3,3′,4,4′‐tetraaminobiphenyl and 4,5‐imidazole‐dicarboxylic acid. Effects of the reaction conditions on the intrinsic viscosity of the synthesized polymers were studied. The results show that the molecular weight of the polymers increases with increasing monomer concentration and reaction time, and then levels off. With higher reaction temperature, the molecular weight of the polymer is higher. With the additional imidazole group in the backbone, PBI‐imi shows improved phosphoric acid doping ability, as well as a little higher proton conductivity when compared with widely used poly[2,2′‐(m‐phenylene)‐5,5′‐bibenzimidazole] (PBI‐ph).Whereas, PBI‐imi and PBI‐ph have the similar chemical oxidation stability. PBI‐imi/3.0 H3PO4 composite membranes exhibit a proton conductivity as high as 10–4 S cm–1 at 150 °C under anhydrous condition. The temperature dependence of proton conductivity of acid doped PBI‐imi can be modeled by an Arrhenius equation.

Polymer blends based on copolymers bearing both side and main chain pyridine units as proton exchange membranes for high temperature fuel cells

New blend systems consisting of copolymers based on aromatic polyethers bearing both main and side chain pyridine units have been prepared in order to be evaluated as proton exchange membranes. The blend membranes showed high glass transition temperatures up to 265°C and high thermal (Td's up to 450°C) as well as oxidative stability. The acid doping ability was controlled based on the blend constituents and composition while the maximum doping levels were up to 480wt%. Comparison of different blend membranes with the same doping level showed that different conductivity values were obtained (up to 1.1×10−1Scm−1), denoting the important role of the polymer matrix itself on proton conduction. Morphological investigation by means of SEM revealed nanophase separated structures with spherical clusters of average sizes between 50 and 70nm. MEAs based on the newly developed blends were prepared and preliminary fuel cell tests at temperatures up to 180°C using dry gases were conducted. The performance obtained was satisfactory (723mV at 0.2Acm−2 at 180°C using H2/O2 gases) and close to the state of the art PBI MEAs.

The effect of structural variations on aromatic polyethers for high‐temperature PEM fuel cells

AbstractThree series of new aromatic polyether sulfones bearing phenyl, p‐tolyl or carboxyl side groups, respectively, and polar pyridine main chain groups were developed. Most of the polymeric materials presented high molecular weights and excellent solubility in common organic solvents. More importantly, they formed stable, self‐standing membranes that were thoroughly characterized in respect to their thermal, mechanical and oxidative stability, their phosphoric acid doping ability and ionic conductivity. Particularly, the copolymers bearing side p‐tolyl or carboxyl groups fulfill all necessary requirements for application as proton electrolyte membranes in high temperature fuel cells, which are glass transition temperatures higher than 220 °C, thermal stability up to 400 °C, oxidative stability, high doping levels (DLs) and proton conductivities of about 0.02 S/cm. Initial single fuel cell results at high temperatures, 160 °C or 180 °C, using a copolymer bearing p‐tolyl side groups with a relatively low DLs around 200 wt % and dry H2/Air feed gases, revealed efficient power generation with a current density of 0.5 A/cm2 at 500 mV. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Synthesis and properties of poly[2,2′-(4,4′-(2,6-bis(phenoxy) benzonitrile))-5,5′-bibenzimidazole] for proton conducting membranes in fuel cells

Poly[2,2′-(4,4′-(2,6-bis(phenoxy) benzonitrile))-5,5′-bibenzimidazole] (PBI-CN) was synthesized via the polycondensation of 3,3′,4,4′-tetraaminobiphenyl and 2,6-bis(4-carboxyphenoxy) benzonitrile. The introduction of the nitrile group to the main chain of PBI-CN can provide both strong polarity and a reactive site for further reaction. The effects of different reaction conditions on the intrinsic viscosity of the synthesized polymer are discussed, which is consistent with the regular pattern. After appropriate purification of starting materials and optimization of polymerization conditions, the synthesized polymer can exhibit higher intrinsic viscosity, which means higher molecular weight. The solubility, chemical oxidation stability, thermal degradation behavior, acid doping ability, and proton conductivity of the synthesized polymers were studied. Compared with poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (PBI-ph), the introduction of –C≡N and –O– group in PBI-CN provides the polymer with better acid doping ability and chemical oxidation stability, while the solubility and thermal stability decrease a little. The proton conductivity of PBI-CN/3.5H3PO4 composite membranes can reach ∼10−3 S cm−1 at 170 °C under anhydrous condition. The temperature dependence of proton conductivity of the acid doped membranes can be modeled by an Arrhenius relation.

Preparation and Characterisation of Proton Exchange Membranes Based on Crosslinked Polybenzimidazole and Phosphoric Acid

AbstractCrosslinked polybenzimidazole (PBI) was synthesised via free radical polymerisation between N‐vinylimidazole and vinylbenzyl substituted PBI. The degree of crosslinking increases with increasing content of the crosslinker. The phosphoric acid doping behaviour, mechanical properties, proton conductivity and acid migration stability of crosslinked PBI and linear PBI are discussed. The results show that the acid doping ability decreases with increasing degree of crosslinking of PBI. The introduction of N‐vinylimidazole in PBI is beneficial to its oxidation stability. The mechanical stability of crosslinked PBI/H3PO4 membrane is better than that of linear PBI/H3PO4 membrane. The proton conductivity of the acid doped membranes can reach ∼10–4 S cm–1 for crosslinked PBI/H3PO4 composite membranes at 150 °C. The temperature dependence of proton conductivity of the acid doped membranes can be modelled by an Arrhenius relation. The proton conductivity of crosslinked PBI/H3PO4 composite membranes is a little lower than that of linear PBI/H3PO4 membranes with the same acid content. However, the migration stability of H3PO4 in crosslinked PBI/H3PO4 membranes is improved compared with that of linear PBI/H3PO4 membranes.

New proton conducting polymer blends and their fuel cell performance

Novel polymer blends based on aromatic polyethers with pyridine units have been prepared for their use as electrolytes after being doped with phosphoric acid for high temperature PEM fuel cells. They exhibit very good film-forming properties, mechanical integrity, high modulus up to 230 °C, high glass transition temperatures (up to 260 °C) and high thermal stability up to 400 °C. In addition to the above required properties, these novel materials show high oxidative stability and acid doping ability, enabling proton conductivity in the range of 10 −2 S cm −1 at 130 °C. The preparation and fuel cell testing of membrane electrode assemblies, demonstrated very promising performance, and an initial study has shown the positive effect of humidity on the measured conductivity.

Blends of Aromatic Polyethers Bearing Polar Pyridine Units and Their Evaluation as High Temperature Polymer Electrolytes

AbstractAromatic polyethers containing polar pyridine units in the main chain have been synthesized using different difluoride monomers. Copolymers of 2,5‐(4′,4″dihydroxy biphenyl)‐pyridine and 3,3′,5,5′‐tetramethyl‐[1,1′‐biphenyl]‐4,4′‐diol with bis(4‐fluorophenyl) sulfone or phenyl phosphine oxide difluoride or decafluorobiphenyl (PTMPySF, PTMPyPO, PTMPyDF) were synthesized. These polymeric structures despite their common structural characteristics, showed totally different behavior in terms of solubility and acid doping ability. Blends of these copolymers have been prepared in order to be evaluated in terms of fuel cell relevant parameters like acid doping ability and conductivity. In most cases flexible membranes were obtained by solution casting. The acid doping ability was controlled based on the blend constituents and composition. The doped membranes exhibited high conductivity values, in the range of 10−3 S/cm at room temperature which is increased at 2.5 × 10−2 S/cm at temperatures up to 180 °C.

Novel Polymer Electrolyte Membrane, Based on Pyridine Containing Poly(ether sulfone), for Application in High‐Temperature Fuel Cells

AbstractSummary: Novel poly(aryl ether sulfone) copolymers containing 2,5‐biphenylpyridine and tetramethyl biphenyl moieties were synthesized by polycondensation of 4‐fluorophenyl sulfone with 2,5‐(4′,4″ dihydroxy biphenyl)pyridine and tetramethyl biphenyl diol. Copolymers with different molecular weights and different monomer compositions were obtained. These copolymers exhibit excellent film‐forming properties, mechanical integrity, and high modulus up to 250 °C, high glass transition temperatures (above 280 °C) as well as high thermal stability up to 400 °C. In addition to the above properties required for PEMFC application, this novel material shows high oxidative stability and acid doping ability, enabling proton conductivity in the range of 10−2 S · cm−1 above 130 °C.Synthesis of copolymers with high acid uptake and ionic conductivity.magnified imageSynthesis of copolymers with high acid uptake and ionic conductivity.