Improved Proton Conductivity Research Articles

Enhanced proton conductivity of polymer electrolyte membrane doped with titanate nanotubes

Nafion-titanate nanotubes composite membranes were prepared through a casting process. With the addition of 5 wt.%, the nanotubes were homogenously distributed in Nafion solution. The formed composite membrane showed a comparable mechanical strength to Nafion membrane. The proton conductivity of the composite membrane without external humidification is higher than that of the Nafion membrane, reaching 0.034 Scm−1 and 0.01 Scm−1 at 100 °C and 120 °C, respectively. The improved proton conductivity was attributed to the great water retention ability of the doped nanotubes.

Optimizing end-group cross-linkable polymer electrolytes for fuel cell applications

This paper demonstrates the optimization of proton conductivity and water uptake for cross-linkable polymer electrolytes through synthesis and characterization of end-group cross-linkable sulfonated poly(arylene ether) copolymers. The extent of reaction of cross-linking was controlled by reaction time resulting in a series of polymers with two, independent tunable parameters, degree of sulfonation (DS) and degree of cross-linking (DC). For the polymers presented, cross-linking improved proton conductivity while reducing water uptake, an uncommon trend in polymer electrolytes where water is critical for proton conduction. Other trends relating to changes are reported and the results yield insight into the role of DS and DC and how to optimize electrochemical properties and performance of polymer electrolytes through these tunable parameters. Select polymer electrolytes were tested in fuel cells where performance and durability with accelerated relative humidity cycling were compared with Nafion ®.

Preparation of high-performance polymer electrolyte nanocomposites through nanoscale silica particle dispersion

Nano-level dispersion with a minimum amount of non-porous and surface-functionalized nanoparticles is a key to tune physically a common polymer material with poor durability to a powerful material with excellent stability even under harsh fuel cell conditions. Surfactants composed of hydrophobic cores and hydrophilic outer shells are used to assist a homogenous distribution of surface-treated (hydrophilic and hydrophobic) silica nanoparticles. In particular, their effect on nanoparticle dispersion is conspicuous in polymer electrolyte nanocomposites containing hydrophilic surface-treated silica. The hydrophilic silica acts as an additional proton conductor in the acid electrolyte medium, leading to improved proton conductivity without any negative side-effects on the mechanical and chemical durability of the membrane material. The well-distributed hydrophilic silica nanoparticles are beneficial in preventing methanol permeation via compact polymer packing and in strengthening the membrane stability under hot aqueous conditions. Finally, the efficacy of the nano-level dispersion is electrochemically verified in terms of high single-cell performance and further extended life time as a result of a synergistic effect of improved proton conductivity, reduced methanol permeability and excellent hydrolytic durability.

Surface fluorinated poly(vinyl alcohol)/poly(styrene sulfonic acid- co-maleic acid) membrane for polymer electrolyte membrane fuel cells

Surface fluorination effectively reduced the water absorption of crosslinked polyvinyl alcohol (PVA)/poly(styrene sulfonic acid- co-maleic acid) (PSSA_MA) membranes. The crosslinked PVA membranes were prepared using PSSA_MA as a crosslinking agent as well as a donor of the hydrophilic group (–SO 3H and/or –COOH). Surface treatment by gaseous fluorine treatment disrupted the C–OH bonds and generated C–F and C–F 2 groups at the membrane surface where atomic percent of fluorine increased up to 5.4%. The membranes with highly hydrophobic fluorinated surface exhibited improved proton conductivity and methanol permeability at a relatively low water uptake.

Conductive area ratio of multiblock copolymer electrolyte membranes evaluated by e-AFM and its impact on fuel cell performance

Abstract The correlation between membrane surface morphology and fuel cell performance was investigated using a series of hydrophilic–hydrophobic multiblock copolymers based on poly(arylene ether sulfone) with different block lengths. The proton conductive regions on the membrane surface were successfully observed by using electrochemical atomic force microscopy (e-AFM). The results revealed a strong dependence of the hydrophilic/hydrophobic microphase-separated structure on the block length. The conductive area ratio (CAR) estimated from the proton conduction image decreased as the block length increased, and it was found to be closely connected with cell resistance that determines fuel cell performance. The well-defined phase-separated structure of multiblock copolymers can improve proton conductivity without any undesirable increments in water uptake or swelling, but in some instances, it affects the interfacial connection with the catalyst layer, resulting in lower fuel cell performance. The results of this study suggest the necessity for further improvement of the membrane morphology by optimizing both the casting conditions and the molecular design of the block sequences.

Implantation of Nafion® ionomer into polyvinyl alcohol/chitosan composites to form novel proton-conducting membranes for direct methanol fuel cells

Abstract A series of cost-effective, proton-conducting composite membranes, comprising of Nafion® ionomer, chitosan (CS), and polyvinyl alcohol (PVA), is successfully prepared. By taking advantage of the strong electrostatic interactions between Nafion® ionomer and CS component, Nafion ionomer is effectively implanted into the PVA/CS composite membranes, and improves proton conductivity of the PVA/CS composite membranes. Furthermore, this effect dramatically depends on the composition ratio of PVA/CS, and the optimum conductivity is obtained at the PVA/CS ratio of 1:1. The developed composite membranes exhibit much lower methanol permeability compared with the widely used Nafion® membrane, indicating that these novel membranes have great potential for direct methanol fuel cells (DMFCs).

Surface-Fluorinated Proton-Exchange Membrane with High Electrochemical Durability for Direct Methanol Fuel Cells

Random disulfonated poly(arylene ether sulfone)-silica nanocomposite (FSPAES-SiO2) membranes were physicochemically tuned via surface fluorination. Surface fluorination for 30 min converted about 20% of the C-H bonds on the membrane surface into C-F bonds showing hydrophobicity and electronegativity at the same time. The membranes with hydrophobic surface properties showed high dimensional stability and low methanol permeability when hydrated for direct methanol fuel cell applications. In particular, the surface enrichment of fluorine atoms led to anisotropic swelling behavior, associated with a stable electrode interface formation. Interestingly, in spite of the use of a random copolymer as a polymer matrix, the low surface free energy of the C-F bonds induced a well-defined continuous ionic channel structure, similar to those of multiblock copolymers. In addition to the morphological transition, fluorine atoms with high electron-withdrawing capability promoted the dissociation of sulfonic acid (-SO3H) groups. Consequently, FSPAES-SiO2 membranes exhibited improved proton conductivity. Thus, FSPAES-SiO2 membranes exhibited significantly improved single-cell performances (about 200%) at a constant voltage of 0.4 V in comparison with those of Nafion 117 and nonfluorinated membranes. Surprisingly, their good electrochemical performances were maintained with very low nonrecovery loss over the time period of 1400 h and interfacial resistances 380% times lower than those of conventional membrane-electrode assemblies comprising the control hydrocarbon membrane and a Nafion binder for the electrodes.

Investigation of the role of benzimidazole-based model compounds on thermal stability and anhydrous proton conductivity of sulfonated poly(ether ether ketone)

The present work shows a strategy to apply hydrogen bond connected benzimidazole compounds for improvement of proton conductivity and thermal stability of sulfonated poly(ether ether ketone) (SPEEK). A series of benzimidazole derivatives, i.e., mono-, di- and trifunctional benzimidazole, are applied to study the role of their packing structures related to the specific properties for SPEEK in being a proton exchange membrane. The benzimidazole-based compounds significantly increase the thermal stability of the SPEEK and, at the same time, improve proton conductivity of SPEEK under anhydrous condition at temperatures higher than 80 °C. The investigation of the structural effect of the benzimidazole based model compounds implies that the trifunctional benzimidazole molecule is the most effective compound to improve the thermal stability and the proton conductivity of SPEEK as compared to mono- and difunctional ones.

Electric field processing to control the structure of poly(vinylidene fluoride) composite proton conducting membranes

A novel method is reported for controlling the structure of poly(vinylidene fluoride) (PVdF) composite proton conducting membranes. When proton conducting Nafion or zirconium phosphate sulfophenylenphosphonate (ZrPSPP) particles are dispersed in a mixed colloidal suspension with PVdF particles, the proton conducting particles selectively respond to an applied electric field. Under appropriate conditions, the proton conducting particles are induced to assemble into chains that rapidly grow to span the gap between electrodes as the electric field is applied. By removing the solvent and melting the PVdF phase while applying the electric field, composite membranes were formed that have field-induced structure. In comparison to randomly structured composites, the electric field-processed Nafion/PVdF or ZrPSPP/PVdF composite membranes showed improved proton conductivity, water sorption, selectivity for protons over methanol, and controlled surface area changes upon swelling with water. The transport and mechanical properties of the electric field-processed composite membranes suggest the potential for improved performance in direct methanol fuel cells.

The interaction of water vapors with H 3PO 4 imbibed electrolyte based on PBI/polysulfone copolymer blends

The interaction of steam with phosphoric acid imbibed electrolyte composed of PBI/PPy(50)coPSF 50/50 polymer blend and its effect on fuel cell performance was studied regarding its permeability through and its chemical interaction with the membrane. It was found that steam is the only gas that permeates the membrane with a permeability coefficient 1.1 × 10 −14 mol cm cm −2 s −1 Pa −1 at 150 °C. This is attributed either to the high solubility of water in phosphoric acid or to the chemical interaction with pyrophosphoric acid . The latter was demonstrated by carrying out TGA experiments under various water vapor partial pressures. Water reacts with pyrophosphoric acid in order to maintain the equilibrium concentration of phosphoric acid at high level, thus improving proton conductivity and fuel cell performance. In addition it is shown that excess water dissolves in the membrane thus maintaining the “membrane/acid” system at high hydration level. This depends both on temperature and steam partial pressure. Although in the present study it is shown that steam plays a significant role in the performance of the high temperature Polymer electrolyte membrane (PEM) fuel cell, nevertheless its feed with humidified gases is not necessary, due to the back transport of the water produced at the cathode.

Synthesis and characterization of sulfonated poly(arylene ether)s with sulfoalkyl pendant groups for proton exchange membranes

Side-chain-type sulfonated poly(arylene ether)s bearing sulfoalkyl pendant groups (PSA-SPAEs) as proton exchange membranes for fuel cells were prepared via nucleophilic substitution polycondensation reactions of sodium 3-(4-(2,6-difluorobenzoyl)phenyl)propane-1-sulfonate (SDFPPS), 4,4′-dihydroxyldiphenylether (DHDPE) and 4,4′-dichlorodiphenyl sulfone (DCDPS). The sulfonic acid group content (SC), as the quantity per repeat unit of polymer, ranging from 0.5 to 0.8, was readily controlled by changing the feed ratio of SDFPPS to DCDPS. Good thermal properties of PSA-SPAE copolymers were indicated by observing glass transition temperatures ( T gs) ranging from 210 to 230 °C in sodium salt form and decomposition temperatures ( T ds) over 220 °C in acid-form and over 400 °C in sodium-form in air. All PSA-SPAE membranes exhibited reasonable flexibility and tensile strength in the range of 41–78 MPa. PSA-SPAE copolymers bearing sulfonic acid groups on flexible side chains showed considerably reduced swelling ratio and improved proton conductivities. Proton conductivity curves parallel to those of Nafion 117 were obtained with proton conductivity of 10 −1 S/cm at equivalent ion exchange capacities (IEC) of 1.47 and 1.64, comparable to Nafion 117. PSA-SPAE copolymer with SC 0.8 had the best data including PEM mechanical strength, water swelling and proton conductivity.

Sulfonated polystyrene/polyvinyl chloride composite membranes for PEMFC applications

The sulfonated polystyrene (SPSt)/polyvinyl chloride (PVC) composite electrolyte membranes were prepared via styrene-impregnated polymerization within PVC matrix and subsequent sulfonation. The propagated polystyrene chains were interpenetrated with those of the PVC film, and hence the two components were completely compatible. The effects of some preparation parameters, including monomer solution composition, polymerization and sulfonation conditions, on membrane performance were investigated. It was found that crosslinking with DVB improved oxidative stability, while excess crosslinking might lead to decrease in the proton conductivity. Polymerization should be carried out at 80 °C for more than 8 h; otherwise the produced polystyrene had relatively low molecular weight and might leach out of the membrane. The higher concentration of chlorosulfonic acid or the higher sulfonation temperature caused side reactions, which led to decrease in proton conductivity as well as slight improvement in oxidative stability. Furthermore, repetitious polymerization under hot-pressing plates further improved proton conductivity with slight increase in membrane thickness. Preliminary data in single cell test showed that the prepared membrane had comparable performance with commercial Nafion. This study demonstrates that an inexpensive polymer electrolyte membrane suitable for low-temperature PEMFC applications can be prepared easily by styrene/DVB-impregnated polymerization within PVC substrate with subsequent sulfonation in chlorosulfonic acid.

Structural and transport effects of doping perfluorosulfonic acid polymers with the heteropoly acids, H 3PW 12O 40 or H 4SiW 12O 40

A perfluorosulfonic acid (PFSA) polymer with pendant side chain –O(CF 2) 4SO 3H was doped with the heteropoly acids (HPAs), H 3PW 12O 40 and H 4SiW 12O 40. Infrared spectroscopy revealed a strong interaction between the HPA and the PFSA ionomer. Modes associated with the peripheral bonds of the HPA were shifted to lower wave numbers when doped into PFSA membranes. Small-angle X-ray scattering (SAXS) measurements showed the presence of large crystallites of HPA in the membrane with d spacings of ca. 10 Å, close to the lattice spacing observed in bulk HPA crystals. Under wet conditions the HPA was more dispersed and constrained the size of the sulfonic acid clusters to 20 Å at a 5 wt% HPA doping level, the same as in the vacuum treated ionomer samples. Under conditions of minimum hydration the HPA decreased the E a for the self-diffusion of water from 27 to 15 kJ mol −1. The reverse trend was seen under 100% RH conditions. Proton conductivity measurements showed improved proton conductivity of the HPA doped PFSAs at a constant dew point of 80 °C for all temperatures up to 120 °C and at all relative hummidities up to 80%. The activation energy for proton conduction generally was lower than for the undoped materials at RH ≤80%. Significantly the E a was 1/2 that of the undoped material at RHs of 40 and 60%. A practical proton conductivity of 113 mS cm −1 was observed at 100 °C and 80% RH.

Transport Properties of Proton Exchange Membranes

Disulfonated poly(arylene ether sulfone) random copolymers have shown promise as potential candidates for proton exchange membranes. The chemical structure of these copolymers has served as a basis for the molecular design of new random and multiblock copolymers. In random copolymers the sulfonic acid is distributed statistically, while in the multiblock copolymers the chemical structure is designed to have co continuous hydrophilic and hydrophobic domains. The effect of these variables on the transport properties of the membranes was investigated. For the random copolymers, high proton conductivity was observed at higher IEC along with high methanol permeability. But for the multiblock copolymers, it was possible to combine high proton conductivity with low methanol permeability. Improved proton conductivity was observed under partially hydrated conditions for samples with higher block lengths. Self diffusion coefficient of water was also measured by NMR which provided better understanding of the transport process.

Water-stable crosslinked sulfonated polyimide–silica nanocomposite containing interpenetrating polymer network

Sulfonated polyimide (SPI) interpenetrating polymer network (IPN) (IXSPI)–silica (SiO 2) nanocomposite membranes were fabricated as proton conducting solid electrolytes for fuel cells. Urethane acrylate non-ionomers (UANs) were used as dispersants to homogeneously distribute nanosized SiO 2 and, simultaneously, as crosslinkers to induce IPN structure formation. IXSPI–SiO 2 nanocomposite membranes showed high proton conductivity and hydrolytic stability, and low methanol permeability as compared with those of pristine SPI. Interestingly, the casting solvent for membrane fabrication influenced membrane performances, especially proton conductivity. In particular, dimethyl sulfoxide exhibited a strong interaction with sulfonic acid groups in the polymer matrix, which hindered them from spontaneously releasing protons and reduced the proton conductivity and electrochemical performances of the resulting membranes. Crosslinkers with long polyethylene oxide chains also contributed to improved proton conductivity and increased single cell performances.

Influence of silica content in sulfonated poly(arylene ether ether ketone ketone) (SPAEEKK) hybrid membranes on properties for fuel cell application

Sulfonated poly(arylene ether ether ketone ketone) (SPAEEKK) copolymer containing pendant sulfonic acid group (sulfonic acid content (SC) = 0.67) was synthesized from commercially available monomers such as sodium 6,7-dihydroxy-2-naphthalenesulfonate (DHNS), 1,3-bis(4-fluorobenzoyl)-benzene (BFBB), and hexafluorobisphenol A (6F-BPA). SPAEEKK/silica hybrid membranes were prepared using the sol–gel process under acidic conditions. The SPAEEKK/silica hybrid membranes were fabricated with different silica contents and the membranes were modified to achieve improved proton conductivity incorporating P–OH groups (H 3PO 4 treatment). The silica particles within the membranes were used for the purpose of blocking excessive methanol cross-over and for forming a pathway for proton transport due to water absorption onto the hydrophilic SiOH surface. The proton conductivities of H 3PO 4-doped membranes were somewhat higher than the un-doped (H 3PO 4-free) membranes due to increasing hydrophilicity of the membranes. The presence of silica particles within the organic polymer matrix, which decreases the ratio of free water to bound water due to the SiOH on the surface of silica derived from sol–gel reaction, results in hybrid membranes with reduced methanol permeability and improved proton conductivity.

Influence of chemical composition and sequence length on the transport properties of proton exchange membranes

AbstractOne of the integral parts of the fuel cell is the proton exchange membrane. Our research group has been engaged in the past few years in the synthesis of several sulfonated poly(arylene ether) random copolymers. The copolymers were varied in both the bisphenol structure as well as in the functional groups in the backbone such as sulfone and ketones. To compare the effect of sequence length, multiblock copolymers based on poly(arylene ether sulfone)s were synthesized. This paper aims to describe our investigation of the effect of chemical composition, morphology, and ion exchange capacity (IEC) on the transport properties of proton conducting membranes. The key properties examined were proton conductivity, methanol permeability, and water self diffusion coefficient in the membranes. It was observed that under fully hydrated conditions, proton conductivity for both random and block copolymers was a function of IEC and water uptake. However, under partially hydrated conditions, the block copolymers showed improved proton conductivity over the random copolymers. The proton conductivity for the block copolymer series was found to increase with increasing block lengths under partially hydrated conditions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2226–2239, 2006

Proton Conductivity of Mesoporous Silica Incorporated with Phosphorus under High Water Vapor Pressures up to 150.DEG.C.

Silica mesoporous structures incorporated with phosphorus were prepared and their proton conductivity was evaluated at temperatures of up to 150°C under saturated or controlled water vapor pressure. The fixing of phosphoric acid through heat treatment after forming the mesostructure had a limited effect on the improvement in proton conductivity only in a relatively low-temperature region. The incorporation of phosphorus during mesoporous silica formation decreased the surface area but was effective in improving proton conductivity. A sample with a low P/Si atomic ratio of 0.07 showed the highest proton conductivity above 10 -2 Scm -1 at 100 to 120°C under saturated water vapor pressure. A sample with a high P/Si atomic ratio of 0.25 showed relatively high conductivities being maintained even at low relative humidity at 150°C. An adequate phosphorus concentration and a large surface area were found necessary for high and humidity-independent proton conductivity in mesoporous silica.

Low-swelling proton-conducting copoly(aryl ether nitrile)s containing naphthalene structure with sulfonic acid groups meta to the ether linkage

Wholly aromatic poly(aryl ether ether nitrile)s containing naphthalene structure with sulfonic acid groups meta to ether linkage ( m-SPAEEN), intended for fuel cells applications as proton conducting membrane materials, were prepared via nucleophilic substitution polycondensation reactions. The incorporation of rigid naphthalene structure with meta-sulfonic acid groups was with the intent of improving the aggregation of hydrophilic and hydrophobic domains and to increase the acidity and conductivities. m-SPAEEN copolymers were readily synthesized by potassium carbonate mediated nucleophilic polycondensation reactions of commercially available monomers: 2,6-difluorobenzonitrile (2,6-DFBN), 2,8-dihydroxynaphthalene-6-sulfonate sodium salt (2,8-DHNS-6), and 4,4′-biphenol (4,4′-BP) in dimethylsulfoxide (DMSO) at 160–170 °C. The sulfonic acid group content (SC), expressed as a number per repeat unit of polymer, ranged from 0 to 0.6 and was readily controlled by changing the feed ratio of 2,8-DHNS-6 to 2,6-DFBN. High thermal stability of m-SPAEEN copolymers was indicated by observed glass transition temperatures ( T gs) ranging from 223 to 335 °C in sodium salt form and from 230 to 260 °C in acid form ( m-SPAEENH) and decomposition temperatures ( T d)s over 250 °C in acid form and over 350 °C in sodium form in both nitrogen and air. All m-SPAEENH copolymers exhibited reasonable flexibility and tensile strength in the range of 39–78 MPa, indicating they were mechanically stronger than Nafion ®117, which had an approximate value of 10 MPa under the same test conditions. As expected, m-SPAEENH copolymers showed considerably reduced moisture absorption compared to previously prepared sulfonated hydroquinone based poly(aryl ether nitrile). m-SPAEENH copolymers also showed improved proton conductivities. Proton conductivity curves parallel to that of Nafion 117 were obtained with proton conductivity of 10 −1 S/cm at equivalent ion exchange capacities (IEC) of 1.6 and 1.9, comparable to Nafion ®117. The best compromise combining PEM mechanical strength, water swelling and proton conductivity, was achieved at SC of 0.5 and 0.6.

Preparation and characterization of sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK)/silica hybrid membranes for direct methanol fuel cell applications

Sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK) membranes and sol-gel derived SPPESK/silica hybrid membranes have been investigated as potential polymer electrolyte membranes for direct methanol fuel cell (DMFC) applications. In comparison with the SPPESK membrane, the SPPESK/silica membranes exhibited higher water content, improved proton conductivity, and lower methanol permeability. Notably, the silica embedded in the membrane acted as a material for reducing the fraction of free water and as a barrier for methanol transport through the membrane. From the results of proton conductivity and methanol permeability studies, we suggest that the fractions of bound and free water should be optimized to obtain desirable proton conductivities and methanol permeabilities. The highly sulfonated PPESK hybrid membrane (HSP-Si) displayed higher proton conductivity (3.42 × 102 S/cm) and lower methanol permeability (4.15 × 107 cm2/s) than those of Nafion 117 (2.54 × 102 S/cm; 2.36 × 106 cm2/s, respectively) at 30°C. This characteristic of the SPPESK/silica membranes is desirable for future applications related to DMFCs.