Novel multilayer Nafion/SPI/Nafion composite membrane for PEMFCs

A novel multilayer membrane for the proton exchange membrane fuel cell (PEMFC) was developed. Nafion was dispersed uniformly onto both sides of the sulfonated polyimide (SPI) membrane. The Nafion/SPI/Nafion composite membrane was prepared by immersing the SPI into the Nafion-containing casting solution. Through immersing both membranes into the Fenton solution at 80 °C for 0.5 h for an accelerated ex situ test, chromatographic analysis of the water evacuated from the cathode and the anode of the cells and a durability test of a single proton exchange membrane fuel cells, it was proved that the stability of the composite membrane has been greatly improved by adding the Nafion layer compared with the SPI membrane. The fuel cell performance with the SPI and Nafion/SPI/Nafion membranes was similar to the performance with the commercial product Nafion^® NRE-212 membrane at 80 °C.     

High performance composite membranes with enhanced dimensional stability for use in PEMFC

Sulfonated poly(sulfide sulfone) (SPSSF)/porous polytetrafluoroethylene (PTFE) reinforced composite membranes were prepared from a mixed solvent containing n-butanol and DMSO. To improve their dimensional stability, SPSSF/PTFE membranes were further oxidized to obtain sulfonated poly(phenylene sulfone) (SPSO2)/PTFE composite membranes under an optimized H₂O₂ oxidation procedure in acidic medium. Thin composite membranes with good mechanical stability can be fabricated due to the PTFE reinforcement. SEM and FTIR indicated the sulfonated polymers were fully impregnated into the expanded PTFE. SPSO2/PTFE membranes show better thermal and dimensional stability than SPSSF/PTFE membranes. Both composite membranes exhibited very excellent single cell performance. A maximum power density of 1.34 W cm⁻² for the SPSO2/PTFE membrane was obtained at 80 °C and 100 RH%.     

Fabrication and characterization of a PTFE-reinforced integral composite membrane for self-humidifying PEMFC

A novel PTFE-reinforced self-humidifying membrane based on low-cost sulfonated poly (ether ether ketone) (SPEEK) resin was fabricated. In the membrane a base layer and a thin protective layer were bonded by porous polytetrafluoroethylene (PTFE) film. The base layer, which is composed of silicon oxide supported platinum catalyst (abbreviated as Pt-SiO₂) dispersed in SPEEK resin, can suppress reactant crossover and achieve good membrane hydration due to the imbedded hygroscopic Pt-SiO₂ catalysts. The thin protective layer, which constitutes of H₂O₂ decomposition catalyst Pt-SiO₂ and high H₂O₂-tolerant Nafion resin, aims to prevent the SPEEK resin degradation by H₂O₂ produced at the cathode side by incomplete reduction of oxygen. The porous PTFE film tightly bonds with the SPEEK and the Nafion resins to form an integral membrane and accordingly to avoid delamination of the two different resins. The self-humidifying membrane was characterized by TEM, SEM and EDS, etc. The self-humidifying membrane exhibits higher open circuit voltage (OCV) of 0.98 V and maximum power density value of 0.8 W cm⁻² than 0.94 V, 0.33 W cm⁻² of SPEEK/PTFE membrane under dry condition, respectively. The primary 250 h fuel cell durability experiment was conducted and suggested that this low-cost self-humidifying membrane was durable both on fuel cell performance and the membrane structure under fuel cell operation condition with dry H₂/O₂.     

Cross-linked miscible blend membranes of sulfonated poly(arylene ether sulfone) and sulfonated polyimide for polymer electrolyte fuel cell applications

Cross-linked miscible blend (CMB) membranes were prepared from sulfonated poly(arylene ether sulfone) (SPAES) and sulfonated polynaphthalimide (SPI). They were transparent and insoluble in solvents. They showed the intermediate properties between SPAES and SPI concerning mechanical strength, water uptake, membrane swelling and proton conductivity. As for membrane swelling and proton conductivity, SPAES was almost isotropic, whereas SPI was highly anisotropic. CMB membranes were moderately anisotropic and had the advantages of the smaller in-plane membrane swelling and the larger through-plane conductivity compared to SPAES and SPI, respectively. Polymer electrolyte fuel cell performance of CMB2 membrane with an equal weight ratio of SPAES/SPI and an ion exchange capacity (IEC) of 1.74 meq g⁻¹ was investigated, compared to SPI membrane (R1) with a slightly higher IEC of 1.86 meq g⁻¹. At 90 °C, 0.1 MPa and relatively high humidification of 82/68% RH or 0.2 MPa and low humidification of 50-30% RH, CMB2 showed the reasonably high cell performances. At 110 °C and 50-33% RH, the cell performance was fairly high only at a high pressure of 0.3 MPa, but low at 0.2-0.15 MPa. At these conditions, the cell performance was better for CMB2 than for R1 due to the more effective back-diffusion of water formed at cathode into membrane. CMB2 showed the fairly high PEFC durability at 110 °C.     

Syntheses of sulfonated star-hyperbranched polyimides and their proton exchange membrane properties

We synthesized novel sulfonated star-hyperbranched polyimides composed of a hydrophobic hyperbranched polymer for polymer stability and a hydrophilic sulfonated polyimide as the proton-transport site in the core-shell structure. The proton conductivities of the star-hyperbranched polyimide membranes were measured as functions of the relative humidity and temperature using electrochemical impedance spectroscopy. Although the water uptake and IEC value for the sulfonated star-hyperbranched polyimide membranes were almost constant, the proton conductivity of the membrane strongly depended on the molecular weight of the hydrophilic sulfonated polyimide as the shell. Especially, the conductivity of the high molecular weight star-hyperbranched polyimide membranes was significantly superior to that determined in Nafion^® at all temperatures and was 0.51 S cm⁻¹ at 80 °C and 98% RH, which may suggest that a good proton-transport pathway in the core-shell structure is formed. Consequently, this material proved to be promising as a proton exchange membrane and may have potential applications for use in fuel cells.     

Electrospun nanofibrous blend membranes for fuel cell electrolytes

We have synthesized the novel blend membranes composed of sulfonated polyimide nanofibers and sulfonated polyimide for proton exchange membrane fuel cell. The proton conductivities of the blend membrane containing nanofibers were measured as functions of the relative humidity and temperature using electrochemical impedance spectroscopy. The proton conductivity of the blend membrane indicated a higher value when compared to that determined for the blend membrane without nanofibers prepared with conventional solvent-casting method. In addition, the membrane stability, such as oxidative and hydrolytic stabilities, of the blend membrane containing nanofibers strongly depended on the amount of nanofiber and was significantly improved with an increase in nanofiber. Oxygen permeability of the membrane was also investigated under dry condition at 35 °C and 760 mm Hg. Oxygen permeability coefficient of the blend membrane slightly decreased when compared to that determined in the blend membrane without nanofibers. Consequently, nanofibers proved to be promising materials as a proton exchange membrane and the blend membrane containing nanofibers may have potential application for use in fuel cells.     

Tuning transport properties by manipulating the phase segregation of tetramethyldisiloxane segments in modified polyimide electrolytes

A series of copolymer electrolytes containing 4,4′-oxydianiline (ODA)-based sulfonated polyimide and siloxane segments, in various ratios, are prepared and characterized for direct methanol fuel cell applications. The chemical Structure of the sulfonated copolymers is confirmed by FT-IR and NMR. The prepared composite membranes are found to be flexible and show good thermal stability as well as good proton conductivity. A maximum proton conductivity of 5.78 × 10⁻² S cm⁻¹ (cf. Nafion117 = 8.31 × 10⁻² S cm⁻¹) is obtained for the sulfonated polyimide blended with sulfonated polyimide with a grafted tetramethyldisiloxane segment (cf. SPI_DSX75 membrane) at 90 °C. The membranes showed low methanol crossover below 10⁻⁷ cm² s⁻¹ (cf. Nafion117 = 10⁻⁶ cm² s⁻¹). The transport properties of the membranes are found to be strongly influenced by water uptake and by the number and nature of the ionic clusters in the hydrophilic domains. When the number of siloxane segments is increased, the relationship between the methanol self-diffusion coefficient (D_M) and water molecules per sulfonic acid group (λ) indicate that the water molecules are interacting with channels inside the membrane. In addition, the segregated nanophase also affects the ion transport and sometimes enhances the corresponding ionic conductivity. TEM and SAXS analyses shows evidence for phase segregation in the membranes and reveal the influence of flexible siloxane segments in ionic clustering.     

Fabrication of protic ionic liquid/sulfonated polyimide composite membranes for non-humidified fuel cells

We have demonstrated that a protic ionic liquid, diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]) functions as a proton conductor and is suitable for use as an electrolyte in H₂/O₂ fuel cells, which can be operated at temperatures higher than 100 °C under non-humidified conditions. In this study, in order to fabricate a polymer electrolyte fuel cell, matrix polymers for [dema][TfO] are explored and sulfonated polyimides (SPI), in which the sulfonic acid groups are in diethylmethylammonium form, are found to be highly compatible with [dema][TfO]. Polymer electrolyte membranes for non-humidified fuel cells are prepared by the solvent casting method using SPI and [dema][TfO]. The SPI, with an ion exchange capacity of 2.27 meq g⁻¹, can retain four times its own weight of [dema][TfO] and produces uniform, tough, and transparent composite membranes. The composite membranes have good thermal stability (>300 °C) and ionic conductivity (>10⁻² S cm⁻¹ at 120 °C when the [dema][TfO] content is higher than 67 wt%) under anhydrous conditions. In the H₂/O₂ fuel cell operation using a composite membrane without humidification, a current density higher than 240 mA cm⁻² is achieved with a maximum power density of 100 mW cm⁻² at 80 °C.     

Sulfonated polyimide hybrid membranes for polymer electrolyte fuel cell applications

Sulfonated Si-MCM-41 (SMCM) with an ion exchange capacity (IEC) of 2.3 mequiv. g⁻¹ was used as a hydrophilic and proton-conductive inorganic component. Sulfonated polyimide (SPI) based on 1,4,5,8-naphthalene tetracarboxylic dianhydride and 2,2′-bis(3-sulfophenoxy) benzidine was used as a host membrane component. The SMCM/SPI hybrid membrane (H1) with 20 wt% loading of SMCM and an IEC of 1.90 mequiv. g⁻¹ showed the high mechanical tensile strength and the slightly higher water vapor sorption than the host SPI membrane (M1) with an IEC of 1.86 mequiv. g⁻¹. H1 and M1 showed anisotropic membrane swelling with about 10 times larger swelling in thickness direction than in plane one. The proton conductivity at 60 °C of H1 was lower in water than that of M1, but comparable at 30% RH. At 90 °C, H1 showed the rather lower performance of polymer electrolyte fuel cell (PEFC) at 82% RH than M1 and fairly better performance at 30% RH. On the other hand, at 110 °C and low humidity less than 50% RH, H1 showed the much better PEFC performance than M1 and Nafion 112. This was due to the promoted back diffusion of produced water by the superior water-holding capacity of SMCM. The SMCM/SPI hybrid membranes have high potential for PEFCs at higher temperatures and lower humidities.     

A polytetrafluoroethylene/quaternized polysulfone membrane for high temperature polymer electrolyte membrane fuel cells

A polytetrafluoroethylene (PTFE)/quaternized polysulfone (QNPSU) composite membrane has been fabricated for use in proton exchange membrane fuel cells (PEMFCs). The composite membrane is made by immobilizing a QNPSU solution into a hydrophobic porous PTFE membrane. The structure of the composite membrane is examined by SEM and EDX. The ionic conductivity of the PTFE/QNPSU membrane, at a relative humidity lower than 0.5% and a temperature of 180 °C, is greater than 0.3 S cm⁻¹, when loaded with 400% H₃PO₄. A hydrogen fuel cell with this membrane operating at 2.0 atmosphere absolute (atma) pressure and 175 °C gives voltages >0.4 V at current densities of 1.0 A cm⁻² using oxygen.     

Investigation of sulfonated polysulfone membranes as electrolyte in a passive-mode direct methanol fuel cell mini-stack

This paper reports on the development of polymer electrolyte membranes (PEMs) based on sulfonated polysulfone for application in a DMFC mini-stack operating at room temperature in passive mode. The sulfonated polysulfone (SPSf) with two degrees of sulfonation (57 and 66%) was synthesized by a well-known sulfonation process. SPSf membranes with different thicknesses were prepared and investigated. These membranes were characterized in terms of methanol/water uptake, proton conductivity, and fuel cell performance in a DMFC single cell and mini-stack operating at room temperature. The study addressed (a) control of the synthesis of sulfonated polysulfone, (b) optimization of the assembling procedure, (c) a short lifetime investigation and (d) a comparison of DMFC performance in active-mode operation vs. passive-mode operation.The best passive DMFC performance was 220 mW (average cell power density of about 19 mW cm⁻²), obtained with a thin SPSf membrane (70 μm) at room temperature, whereas the performance of the same membrane-based DMFC in active mode was 38 mW cm⁻². The conductivity of this membrane, SPSf (IEC = 1.34 mequiv. g⁻¹) was 2.8 × 10⁻² S cm⁻¹. A preliminary short-term test (200 min) showed good stability during chrono-amperometry measurements.     

Water channel morphology of non-perfluorinated hydrocarbon proton exchange membrane under a low humidifying condition

Water channel formation of non-perfluorinated proton exchange membranes (PEMs) under a low humidifying condition is a very important issue, due to weaker phase separation between hydrophilic and hydrophobic moieties than in the case of perfluorinated PEMs such as Nafion. In this study, we performed Molecular dynamics (MD) simulations of hydrated sulfonated polyimide (SPI) models, one of the representative non-perfluorinated PEMs, under different temperature and humidifying conditions by removing water molecules continuously, reflecting experimental conditions of actual low humidifying fuel cell. The water channel morphology of sulfonated polyimide (SPI) models had no apparent temperature dependence. The hydrated SPI models show weak water channel formation even in a fully hydrated condition (λ = 16.4), consistent with our previous study, and they do not display significant temperature dependence on the water molecule distribution. As the λ value decreases from 16.4 to 2 (i.e., low humidifying conditions), the water molecules in the hydrated SPI models are evenly reduced. In particular, when the λ value of the hydrated SPI model decreases from 8.5 to 6, the size of the water clusters is significantly narrowed and the clusters become segregated, and this is also confirmed by an X-ray scattering analysis. As a result, the proton conducting performance of hydrated SPI models shows similar behavior with the change in water channel morphologies, which will be very important to design a novel non-perfluorinated hydrocarbon PEM with high performance for practical fuel cell systems.     

MWCNTs reinforced Nafion membrane prepared by a novel solution-cast method for PEMFC

An improved solution-cast method is presented to prepare multi-wall carbon nanotubes (MWCNTs)/Nafion^® reinforced membrane with different MWCNTs content (from 1 to 4 wt.%). MWCNTs were oxidized by H₂O₂ and sodium hydroxide (NaOH) was added into the MWCNTs/Nafion^®/N,N-dimethylacetamide (DMAC) solution. The long-term stability of the resulting dispersions was much better than the unmodified dispersions. The as-cast membrane was observed by scanning electron microscope (SEM). The MWCNTs were uniformly dispersed in the Nafion^® resin. The tensile strength and the elongation at break were greatly improved for the reinforced membranes compared to the recast Nafion^® membranes (54 and 27%, respectively). The fuel cell performance of the reinforced membranes with different MWCNTs contents was also tested at 80 °C under fully humidified conditions. By comparing the mechanical properties, proton conductivity and fuel cell performance of the reinforced membranes, we concluded that the content of MWCNTs in the reinforced membranes should not exceed 3 wt.%. The MWCNTs/Nafion^® reinforced membrane with 3 wt.% MWCNTs content showed the best mechanical characteristics and excellent fuel cell performance.     

Sulfonated polyimides: Synthesis,proton conductivity and water stability

Six-membered ring sulfonated polyimides (SPIs) are promising membrane materials for low temperature (<100 °C) fuel cell application. This paper describes our recent work on the synthesis of various sulfonated diamine monomers and the related sulfonated polyimides. The relationship between the chemical structure and the proton conductivity and water stability of various sulfonated polyimide membranes is discussed in detail.     

Multilayered hydrocarbon ionomer/PTFE composite electrolytes with enhanced performance for energy conversion devices

A new multilayered composite membrane was prepared by impregnating a sulfonated poly(arylene ether sulfone) (SPAES) ionomer into a porous polytetrafluoroethylene (PTFE) substrate for application in proton exchange membrane fuel cells (PEMFCs) and water electrolyzers (PEMWEs). The PTFE substrate was treated with n-propyl alcohol to mediate the interfacial interactions between the SPAES solution and PTFE. Using the 10- and 5-μm-thick PTFE substrates, three-layered and five-layered composite membranes were prepared, respectively, to investigate the effect of PTFE thickness on impregnation of the SPAES ionomer. When 5-μm-thick PTFE was applied, the SPAES ionomer was effectively impregnated without noticeable defects, indicating a strong interlocking structure between the two incompatible components. Therefore, the five-layered composite membrane showed enhanced dimensional stability and mechanical properties compared to the SPAES membrane, and the effect of the PTFE on proton conductivity was minimized. Consequently, the cell performances of the five-layered composite membrane were reflected by current densities of 1.71 A/cm² at 0.5 V and 9.76 A/cm² at 1.9 V for PEMFC and PEMWE, respectively, corresponding to 44% and 32% increases compared to those of Nafion 212, owing to its smaller membrane resistance. Moreover, the prepared composite membrane presented excellent durability, which resulted in stable wet-dry cyclability and low degradation rates.     

Properties and fuel cell performance of proton exchange membranes prepared from disulfonated poly(sulfide sulfone)

A series of disulfonated poly(sulfide sulfone)s (SPSSF)s copolymers are synthesized via direct aromatic nucleophilic substitution polycondensation of 4,4′-dichlorodiphenylsulfone (DCDPS), 3,3′-disulfonate-4,4′-dichlorodiphenylsulfone (SDCDPS) and 4,4′-thiobisbenzenethiol at various molar ratios. Tough and flexible membranes with 30 mol% (SPSSF30) to 50 mol% (SPSSF50) SDCDPS monomers are obtained by casting from DMAc solution. Their physicochemical properties including thermal properties, mechanical properties, water uptake, swelling ratio and oxidative stability are fully investigated. And the fuel cell performance of SPSSF membranes at different temperature and relative humidity is evaluated comprehensively for the first time. It is found that the SPSSF40 membrane exhibited low dimensional change in the temperature range of 20–100 °C, good mechanical properties, high oxidative stability and comparable fuel cell performance to Nafion 212 membrane. Besides, the H₂ crossover density of the SPSSF40 membrane is only 50% of that of Nafion 212 membrane. Consequently, SPSSF40 membranes prove to be promising candidates as new polymeric electrolyte materials for proton exchange membrane (PEM) fuel cells operated at medium temperatures.     

Novel single-layer gas diffusion layer based on PTFE/carbon black composite for proton exchange membrane fuel cell

A series of poly(tetrafluoroethylene)/carbon black composite-based single-layer gas diffusion layers (PTFE/CB-GDLs) for proton exchange membrane fuel cell (PEMFC) was successfully prepared from carbon black and un-sintered PTFE, which included powder resin and colloidal dispersion, by a simple inexpensive method. The scanning electron micrographs of PTFE/CB-GDLs indicated that the PTFE resins were homogeneously dispersed in the carbon black matrix and showed a microporous layer (MPL)-like structure. The as-prepared PTFE/CB-GDLs exhibited good mechanical property, high gas permeability, and sufficient water repellency. The best current density obtained from the PEMFC with the single-layer PTFE/CB-GDL was 1.27 and 0.42 A cm⁻² for H₂/O₂ and H₂/air system, respectively.     

Enhancement of the fuel cell performance of a high temperature proton exchange membrane fuel cell running with titanium composite polybenzimidazole-based membranes

The fuel cell performance of a composite PBI-based membrane with TiO₂ has been studied. The behaviour of the membrane has been evaluated by comparison with the fuel cell performance of other PBI-based membranes, all of which were cast from the same polymer with the same molecular weight. The PBI composite membrane incorporating TiO₂ showed the best performance and reached 1000 mW cm⁻² at 175 °C. Moreover, this new titanium composite PBI-based membrane also showed the best stability during the preliminary long-term test under our operation conditions. Thus, the slope of the increase in the ohmic resistance of the composite membrane was 0.041 mΩ cm² h⁻¹ and this is five times lower than that of the standard PBI membrane. The increased stability was due to the high phosphoric acid retention capacity - as confirmed during leaching tests, in which the Ti-based composite PBI membrane retained 5 mol of H₃PO₄/PBI r.u. whereas the PBI standard membrane only retained 1 mol H₃PO₄/PBI r.u. Taking into account the results obtained in this study, the TiO₂-PBI based membranes are good candidates as electrolytes for high temperature PEMFCs.     

An effective strategy to enhance the performance of the proton exchange membranes based on sulfonated poly(ether ether ketone)s

We report an effective and facile approach to enhance the dimensional and chemical stability of sulfonated poly(ether ether ketone) (SPEEK) type proton exchange membranes through simple polymer blending for fuel cell applications, using commercial available materials. The polymeric blends with sulfonated poly(aryl ether sulfone)s (SPAES) were simply fabricated by a three-component system, which contained SPEEK (10–50 wt%, 1.83 mmol/g), and SPAES-40 (1.72 mmol/g)/SPAES-50 (2.04 mmol/g) at 1:1 in weight. The SPAES-40 was selected for mechanical and dimensional stability reinforcing, while SPAES-50 for the good polymer compatibility. The obtained SPEEK/SPAES blend membranes showed depressed water uptake, better dimensional and oxidative stability, together with higher proton conductivity beyond 70 °C than the pristine SPEEK membrane. The apparent improvements in membrane properties were associated with the homogeneous dispersion of SPEEK and both SPAES copolymers inside the membranes as well as the rearrangements of the polymeric chains. The SPEEK content should be properly controlled in the range of 10–40% (B10 to B40). In a H₂/O₂ fuel cell test, B30 showed a maximum power density of 700 mW/cm², which was 1.6 times as high as that of B40 at 80 °C under 100% RH. The further cross-linking treatment produced more ductile and enduring blend membranes, indicating an appreciable prospective for fuel cell applications.     

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

Sulfonated polyimide (SPI) interpenetrating polymer network (IPN) (IXSPI)–silica (SiO₂) 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 SiO₂ and, simultaneously, as crosslinkers to induce IPN structure formation. IXSPI–SiO₂ 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.