The development of PTFE/Nafion/TEOS membranes for application in moderate and high temperature proton exchange membrane fuel cells

PTFE/Nafion (PN) and PTFE/Nafion/TEOS (PNS) membranes were fabricated for the application of moderate and high temperature proton exchange membrane fuel cells (PEMFCs), respectively. Membrane electrode assemblies (MEAs) were fabricated by PTFE/Nafion (and PTFE/Nafion/TEOS) membranes with commercially available low and high temperature gas diffusion electrodes (GDEs). The effects of relative humidity, operation temperature, and back pressure on the performance and durability test of the as-prepared MEAs were investigated. Incorporating TEOS into a PNS membrane and adding another layer of carbon onto a GDE would result in low membrane conductivity and low fuel cell performance respectively. However, in this work it is shown that HT-PNS MEAs demonstrate a higher performance than LT-PN MEAs in severe conditions - high temperature (118 °C) and low humidity (25% RH). The TEOS and additional carbon layer function as water retaining agents which are especially important for high temperature and low humidity conditions. The HT-PNS MEA showed good stability in a 50 h fuel cell test at high temperature, moderate relative humidity (50% RH) and back pressure of 14.7 psi.     

Polymer electrolyte membranes containing titanate nanotubes for elevated temperature fuel cells under low relative humidity

Nafion-titanate nanotubes composite membranes prepared through casting process have been investigated as electrolytes for polymer electrolyte membrane fuel cell applications under low relative humidity. The glass transition temperature and the decomposition temperature of composite membrane at dry state are higher than those of pristine Nafion membrane. Cracks have been observed in the membrane at the concentration of nanotubes above 5 wt.%. The maximum proton conductivity at 100 °C and 50% relative humidity is observed with the concentration of doped titanate nanotubes of 5 wt.%. Solid nuclear magnetic resonance spectrum is applied to qualitatively characterize the status of water inside the membrane at different temperatures. The power densities at 0.8 V for cell assembled from composite membrane containing 5 wt.% of titanate nanotubes are about 13% and 35% higher than that for plain Nafion cells under 50% relative humidity at 65 °C and 90 °C, respectively.     

Stabilizing phosphotungstic acid in Nafion membrane via targeted silica fixation for high-temperature fuel cell application

With PWA as proton transfer and silica as water retainer, stable phosphotungstic acid/silica/Nafion (PWA/Si–N) composite membrane is non-destructively fabricated and exhibits excellent stability and high temperature proton conductivity. Compared with pristine Nafion, high temperature proton conductivity is significantly enhanced due to the collaboration between –SO₃H ionic clusters and the in-situ filled silica embedded PWA nanoparticles. PWA is stabilized in the ionic clusters via in-situ catalyzing the hydrolysis silica precursor targeted filled into the –SO₃H ionic clusters. Stable proton conductivity of the PWA/Si–N membrane at 110 °C and 60% RH is high to 0.058 S/cm, which is 2.4 folds of that of Nafion. At the same time, the composite membrane still maintains good mechanical and thermal stability. As a result, high temperature fuel cell performance of the composite membrane is improved by 41% compared with the pristine Nafion membrane. The in-situ coating method proved to be an effective method to solve the stability of PWA in Nafion membrane, especially the inorganic oxide with good hygroscopicity as the modifier.     

Fuel cell catalyst layers containing short-side-chain perfluorosulfonic acid ionomers

Porous catalyst layers (CLs) containing short-side-chain (SSC) perfluorosulfonic acid (PFSA) ionomers of different ion exchange capacity (IEC: 1.3, 1.4 and 1.5 meq g⁻¹) were deposited onto Nafion 211 to form catalyst-coated membranes. The porosity of SSC-PFSA-based CLs is larger than Nafion-CL analogues. CLs incorporating SSC ionomer extend the current density of fuel cell polarization curves at elevated temperature and lower relative humidity compared to those based on long-side chain PFSA (e.g., Nafion)-based CLs. Fuel cell polarization performance was greatly improved at 110 °C and 30% relative humidity (RH) when SSC PFSI was incorporated into the catalyst layer.     

Properties and fuel cell performance of a Nafion-based,sulfated zirconia-added,composite membrane

The effect of an acidic inorganic additive, i.e. sulfated zirconia, on Nafion-based polymer electrolytes is evaluated by comparing the properties in terms of conductivity and fuel cell performance of a composite sulfated zirconia-added Nafion membrane with those of an additive-free Nafion membrane. The peculiar surface properties of the selected filler promote a higher hydration level and a higher conductivity for the composite membrane under unsaturated conditions, i.e. at 20% RH. Tests on H₂-air fully humidified cells, monitored at 70 °C and at atmospheric pressure, reveal small differences when passing from a plain Nafion to a composite Nafion/sulfated zirconia membrane as electrolyte. However, remarkably great improvements are observed for the composite membrane-based cell when the comparison tests are run at low relative humidity and high temperature, this outlining the beneficial role of the sulfated zirconia additive.     

Preparation and performance of nano silica/Nafion composite membrane for proton exchange membrane fuel cells

Composite membranes made from Nafion ionomer with nano phosphonic acid-functionalised silica and colloidal silica were prepared and evaluated for proton exchange membrane fuel cells (PEMFCs) operating at elevated temperature and low relative humidity (RH). The phosphonic acid-functionalised silica additive obtained from a sol–gel process was well incorporated into Nafion membrane. The particle size determined using transmission electron microscope (TEM) had a narrow distribution with an average value of approximately 11 nm and a standard deviation of ±4 nm. The phosphonic acid-functionalised silica additive enhanced proton conductivity and water retention by introducing both acidic groups and porous silica. The proton conductivity of the composite membrane with the acid-functionalised silica was 0.026 S cm⁻¹, 24% higher than that of the unmodified Nafion membrane at 85 °C and 50% RH. Compared with the Nafion membrane, the phosphonic acid-functionalised silica (10% loading level) composite membrane exhibited 60 mV higher fuel cell performance at 1 A cm⁻², 95 °C and 35% RH, and 80 mV higher at 0.8 A cm⁻², 120 °C and 35% RH. The fuel cell performance of composite membrane made with 6% colloidal silica without acidic group was also higher than unmodified Nafion membrane, however, its performance was lower than the acid-functionalised silica additive composite membrane.     

Poly(ethylene glycol) modified activated carbon for high performance proton exchange membrane fuel cells

A high water retention membrane is developed by co-assembling poly(ethylene glycol) (PEG) grafted activated carbon (AC-PEG) with Nafion. The AC-PEG is prepared via a sol–gel process. The use of PEG as a transporting medium in AC-PEG shows a largely improved water retention ability, a higher proton conductivity and a reduced swelling ratio, making it well suited for proton exchange membrane fuel cells (PEMFCs). Further, the composite membranes show improved mechanical properties at high temperature, thus ensuring the structural stability of membranes during the fuel cell operation. Compositional optimized AC-PEG/Nafion composite membrane (15 wt% compared to Nafion) demonstrates a better performance than the commercially available counterpart, Nafion 212, in fuel cell measurements. To identify the key factor of the improved performance, current interrupt technique is used to quantitatively verify the changes of resistance under different relative humidity environment.     

Nafion/polyaniline composite membranes specifically designed to allow proton exchange membrane fuel cells operation at low humidity

A Nafion and polyaniline composite membrane (designated Nafion/PANI) was fabricated using an in situ chemical polymerization method. The composite membrane showed a proton conductivity that was superior to that obtained with Nafion^® 112 at low humidity (e.g. RH = 60%). Water uptake measurements revealed similarities between the Nafion^® 112 and Nafion/PANI membranes at different humidities. The high conductivity of the Nafion/PANI membrane at low humidity is hypothesized to be due to the existence of the extended conjugated bonds in the polyaniline; proton transfer is facilitated via the conjugated bonds in lower humidity environments allowing retention of the relatively high conductivity. Correspondingly, the performance of a single cell fuel cell containing the Nafion/PANI composite membrane is improved compared to a Nafion^® 112-containing cell under low humidity conditions. This is important for portable fuel cells, which are required to operate without external humidification.     

Nafion-based composite electrolytes for proton exchange membrane fuel cells operating above 120 °C with titania nanoparticles and nanotubes as fillers

Nafion/titania-based fillers composites are prepared by casting and tested in proton exchange membrane fuel cells (PEMFCs) operating at elevated temperatures (130 °C). Three types of titania-based fillers are studied: nanoparticles with nearly spherical shape, mesoporous particles with high surface area, and hydrogen titanate nanotubes. Properties of composites related to PEMFC operation, such as water absorption/retention and proton conductivity, are determined and correlated with microstructural data obtained by small-angle X-ray scattering. The addition of titanate nanotubes changes more markedly the physical properties of the composite electrolytes as compared to titanium oxide nanoparticles with different surface area, a feature probably related to the intrinsic hydration and proton conductivity of the nanotubes. Polarization curves of H₂/O₂ PEMFCs using composite electrolytes indicate that composite electrolytes contribute to a significant boost of H₂/O₂ PEMFC performance at 130 °C.     

Balancing dimensional stability and performance of proton exchange membrane using hydrophilic nanofibers as the supports

In this work, we developed a novel composite membrane by anchoring perfluorosulfonic acid into the hydrophilic poly(lactic-co-glycolic acid) (PLGA) nanofibrous network which was synthesized by electrospinning method. It was clear that the PLGA/Nafion composite membranes possessed high Nafion loading, excellent dimensional stability and proton transport capacity. When the humidity of the membrane changed from soaking in water to 25 RH% at 90 °C, the PLGA fiber network effectively controlled the swelling of Nafion resin and reduced the humidity-generated shrinkage stress from 2.2 MPa (Nafion211 membranes) to 0.5 MPa (PLGA/Nafion composite membranes). The proportion of humidity-induced stress to the yield strength was also reduced to 4.4%, in comparison to 21.2% of that of Nafion211 membrane. The area proton conductivity of the PLGA/Nafion composite membrane achieved 48.2 S cm⁻², compared with 36.0 S cm⁻² of Nafion211 membranes in the same condition. The excellent proton transport capacity greatly improved the performance of fuel cell assembled with PLGA/Nafion composite membranes and effectively reduced the dynamic response time from 22 s (Nafion211 membranes) to 7 s (PLGA/Nafion composite membranes).     

Sulfonated mesoporous benzene-silica-embedded sulfonated poly(ether ether ketone) membranes for enhanced proton conduction and anti-dehydration

In polymer electrolyte fuel cell operation, a decrease in the proton conductivity of the membrane at reduced humidity is a main cause for poor cell performance at high temperature. To alleviate the dehydration of the membrane at high temperature, sulfonated mesoporous benzene-silica (sMBS) particles are embedded in sulfonated poly(ether ether ketone) (sPEEK) membranes. As the sMBS itself is highly sulfonated on both organic and inorganic moieties, the proton conductivity of composite membranes is much higher than that of the pristine sPEEK membrane, and it reaches that of Nafion 117 at a high relative humidity (RH) of 90%. The dehydration rate of the membrane is reduced significantly by the capillary condensation effect of sMBS particles with the nanometer-scale 2-D hexagonal cylindrical pores, and the proton conductivity of the composite membranes, 0.234 × 10⁻¹ S cm⁻¹, is much higher than that of pristine sPEEK membrane, 0.59 × 10⁻³ S cm⁻¹, at a relatively low humidity of 40% RH. This maintenance of high conductivity at low humidity is attributed to the high water-holding capacity of the sMBS proton conductors. The sMBS-embedded sPEEK composite membranes show a much lower methanol permeability of 2–5 × 10⁻⁷ cm² s⁻¹ compared to that of Nafion 117, which is 1.6 × 10⁻⁶ cm² s⁻¹ at room temperature.     

Properties and morphology of Nafion/polytetrafluoroethylene composite membrane fabricated by a solution-spray process

The Nafion/polytetrafluoroethylene (Nafion/PTFE) composite membrane is fabricated by a solution-spray process. The performance and morphology of the composite membrane are studied in terms of the mechanical properties, conductivity, and permeability. The results of TEM and X-ray studies show that the morphologies of crystalline and ion cluster of the perfluorosulfonated acid (PFSA) in composite membrane are apparently similar to that of Nafion^® NR211 membrane. The composite membrane has higher stiffness and strength and lower swelling than that of Nafion^® NR211. The conductivity at 85 °C of 0.375 S cm⁻¹ is relatively high in comparison to that of 0.300 S cm⁻¹ for Nafion^® NR211. The 20 kW stack with the composite membranes is evaluated. The mean single cell voltage is 0.67 V @1000 mA cm⁻². The stack has behaved performance uniformity and steadily operated under low humidifying condition. In consideration of the integration of complex structure and perfect morphology, the solution-spray process is feasible for composite proton exchange membrane manufacture.     

Preparation and characterization of a modified montmorillonite/sulfonated polyphenylether sulfone/PTFE composite membrane

Organically modified montmorillonites are valuable materials that have been used to improve the permeability, water retention, and proton conductivity of proton exchange membrane for fuel cells. A sulfonated montmorillonite/sulfonated poly (biphenyl ether sulfone)/Polytetrafluoroethylene (SMMT/SPSU-BP/PTFE) composite membrane was prepared for fuel cells. The thermal stability of the SMMT was tested by the thermogravimetry-mass spectrometry (TGA-MS) and its structure in the composite membrane was characterized by X-ray diffraction (XRD). It was found that SMMT was stable up to 205 °C and the interlayer distance of the nanoclay expanded from 1.43 nm to 1.76 nm after the organic sulfonic modification. The SMMT was completely exfoliated in the composite membranes. The properties of ion-exchange capacity, water uptake, swelling ratio, proton conductivity, and mechanical strength of the composite membranes were investigated as well. The good water retention of SMMT made the SMMT/SPSU-BP and SMMT/SPSU-BP/PTFE composite membranes have about 20% more bound water than the SPSU-BP membrane. Due to the reinforce effect of the PTFE porous film, the SMMT/SPSU-BP/PTFE composite membrane presented low swelling even at elevated temperature and high stress strength. All of the properties indicate that the SMMT/SPSU-BP/PTFE composite membrane is very promising as the PEM for medium temperature PEMFCs.     

Functionalized titania nanotube composite membranes for high temperature proton exchange membrane fuel cells

In this study, functionalized titania nanotubes (F-TiO₂-NT) were synthesized by using 3-mercaptopropyl-tri-methoxysilane (MPTMS) as a sulfonic acid functionalization agent. These F-TiO₂-NT were investigated for potential application in high temperature hydrogen polymer electrolyte membrane fuel cells (PEMFCs), specifically as an additive to the proton exchange membrane. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) results confirmed that the sulfonic acid groups were successfully grafted onto the titania nanotubes (TiO₂-NT). F-TiO₂-NT showed a much higher conductivity than non-functionalized titania nanotubes. At 80 °C, the conductivity of F-TiO₂-NT was 0.08 S/cm, superior to that of 0.0011 S/cm for the non-functionalized TiO₂-NT. The F-TiO₂-NT/Nafion composite membrane shows good proton conductivity at high temperature and low humidity, where at 120 °C and 30% relative humidity, the proton conductivity of the composite membrane is 0.067 S/cm, a great improvement over 0.012 S/cm for a recast Nafion membrane. Based on the results of this study, F-TiO₂-NT has great potential for membrane applications in high temperature PEMFCs.     

Construction of low-cost and long-time proton exchange membranes by using slow-release radical scavenger

The bottlenecks of commercial application of proton exchange membranes (PEM) fuel cell are cost and oxidation stability of PEM. Hence, we encapsulate Resveratrol (Res, a kind of reductant) in hydroxypropyl-β-cyclodextrins (CDs) to prepare the inclusion complexes of Res and CDs (Res@CDs) under the guidance of theoretical arithmetic. Then the Res@CDs are evenly dispersed in Nafion emulsion, which is subsequently combined with porous polytetrafluoroethylene (PTFE) substrate by emulsion pouring method to form the antioxidative composite membrane (Res@CDs-Nafion/PTFE). The as-prepared Res@CDs-Nafion/PTFE shows the similar performance on proton conductivity (103.9 mS cm⁻¹) and hydrogen-air fuel cell (317.84 mW cm⁻²) compared to the Nafion/PTFE composite membrane. The content of Nafion in the Res@CDs-Nafion/PTFE is less than 30%, which dramatically reduces the production cost compared to pure Nafion membrane. The weight loss of Res@CDs-Nafion/PTFE and Nafion/PTFE immersed in Fenton's reagent after 36 h is 4.97% and 16.49%, respectively, which demonstrate that Res@CDs can enhance oxidation stability of composite membrane. The Res@CDs-Nafion/PTFE offer huge merits of low cost and enhanced oxidation stability, which greatly promotes the application process of long-lifetime PEM fuel cell.     

Temperature-dependent performance of the polymer electrolyte membrane fuel cell using short-side-chain perfluorosulfonic acid ionomer

We report on polymer electrolyte membrane fuel cells (PEMFCs) that function at high temperature and low humidity conditions based on short-side-chain perfluorosulfonic acid ionomer (SSC-PFSA). The PEMFCs fabricated with both SSC-PFSA membrane and ionomer exhibit higher performances than those with long-side-chain (LSC) PFSA at temperatures higher than 100 °C. The SSC-PFSA cell delivers 2.43 times higher current density (0.524 A cm⁻¹) at a potential of 0.6 V than LSC-PFSA cell at 140 °C and 20% relative humidity (RH). Such a higher performance at the elevated temperature is confirmed from the better membrane properties that are effective for an operation of high temperature fuel cell. From the characterization technique of TGA, XRD, FT-IR, water uptake and tensile test, we found that the SSC-PFSA membrane shows thermal stability by higher crystallinity, and chemical/mechanical stability than the LSC-PFSA membrane at high temperature. These fine properties are found to be the factor for applying Aquivion™ E87-05S membrane rather than Nafion^® 212 membrane for a high temperature fuel cell.     

Cross-linked membranes based on sulfonated poly (ether ether ketone) (SPEEK)/Nafion for direct methanol fuel cells (DMFCs)

A series of cross-linked membranes based on SPEEK/Nafion have been prepared to improve methanol resistance and dimension stability of SPEEK membrane for the usage in the direct methanol fuel cells (DMFCs). Sulfonated diamine monomer is synthesized and used as cross-linker to improve the dispersion of Nafion in the composite membranes and decrease the negative effect of cross-linking on proton conductivity of membranes. FT-IR analysis shows that the cross-linking reaction is performed successfully. The effects of different contents of Nafion on the properties of cross-linked membranes are investigated in detail. All the cross-linked membranes show lower methanol permeability and better dimensional stability compared with the pristine SPEEK membrane. SPEEK-N30 with the 30 wt % Nafion shows a methanol permeability of 0.73 × 10⁻⁶ cm² s⁻¹ and a water uptake of 24.4% at 25 °C, which are lower than those of the pristine membrane. Meanwhile, the proton conductivity of SPEEK-N30 still remains at 0.041 S cm⁻¹ at 25 °C, which is comparable to that of the pristine SPEEK membrane. All the results indicate that these cross-linked membranes based on SPEEK/Nafion show good prospect for the use as proton exchange membranes.     

Methanol crossover through PtRu/Nafion composite membrane for a direct methanol fuel cell

This study examined methanol crossover through PtRu/Nafion composite membranes for the direct methanol fuel cell. For this purpose, 0.03, 0.05 and 0.10 wt% PtRu/Nafion composite membranes were fabricated using a solution impregnation method. The composite membrane was characterized by inductively coupled plasma-mass spectroscopy and thermo-gravimetric analysis. The methanol permeability and proton conductivity of the composite membranes were measured by gas chromatography and impedance spectroscopy, respectively. In addition, the composite membrane performance was evaluated using a single cell test. The proton conductivity of the composite membrane decreased with increasing number of PtRu particles embedded in the pure Nafion membrane, while the level of methanol permeation was retarded. From the results of the single cell test, the maximum performance of the composite membrane was approximately 27% and 31% higher than that of the pure Nafion membrane at an operating temperature of 30 and 45 °C, respectively. The optimum loading of PtRu was determined to be 0.05 wt% PtRu/Nafion composite membrane.The PtRu particles embedded in the Nafion membrane act as a barrier against methanol crossover by the chemical oxidation of methanol on embedded PtRu particles and by reducing the proton conduction pathway.     

Preparation of polysiloxane modified perfluorosulfonic acid composite membranes assisted by supercritical carbon dioxide for direct methanol fuel cell

Polysiloxane modified perfluorosulfonic acid (PFSA) composite membranes are prepared by using (3-mercaptopropyl) methyldimethoxysilane (MPMDMS) as a precursor of silicon alkoxide in supercritical carbon dioxide (Sc-CO₂) system. In the Sc-CO₂ system with the presence of water, Sc-CO₂ is not only used as a solvent and swelling agent, but also functioned as an acid catalyst for the condensation polymerization of MPMDMS. Characteristics of the modified composite membranes are investigated by using attenuated total reflection-infrared spectra, scanning electron microscopy and transmission electron microscopy. The modified membrane with 13.9 wt.% poly(MPMDMS) is the best one among all the modified membranes, whose methanol permeability is extremely lower and selectivity (ratio of proton conductivity to methanol permeability) is about 5.49 times higher than that of pristine membrane and 5.88 times than that of Nafion^® 117, respectively. This modified PFSA membrane still can maintain its higher selectivity value than that of Nafion^® 117 in the temperature range of 25-65 °C. Therefore, the modified membranes prepared in Sc-CO₂ system may be the suitable candidate electrolytes for direct methanol fuel cell applications.     

Fabrication and evaluation of Nafion nanocomposite membrane based on ZrO2–TiO2 binary nanoparticles as fuel cell MEA

Synthesis and characterization of nanocomposite membranes for proton exchange membrane fuel cell (PEMFC) operating at different temperatures and humidity were investigated in this study. Recast Nafion composite membrane with ZrO₂ and TiO₂ nanoparticles with 75 nm in mean size diameter, prepared for PEM fuel cells. Nafion/TiO₂ composite membranes have been also fabricated by in-situ sol–gel method. However, fine particles of the ZrO₂ were synthesized and Nafion/ZrO₂ composite membrane were produced by blending a 5% (w/w) Nafion-water dispersion with the inorganic compound. All nanocomposite membranes demonstrated higher water retention in comparison with unmodified membranes. Proton conductivity increased with increasing ZrO₂ content while TiO₂ additive (with mean size of 25 nm) enhanced water retention. Subsequently, structures of the membranes were investigated by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) as well as X-Ray Diffraction (XRD). In addition, water uptake and proton conductivity of the modified membranes were also measured. The nanocomposite membrane was tested in a 25 cm² commercial single cell at the temperature range of 80–110 °C and in humidified H₂/O₂ under different relative humidity (RH) conditions. The membrane electrode assembly (MEA) prepared from Nafion/TiO₂, ZrO₂ presented highest PEM fuel cell performance in respect of I–V polarization under condition of 110 °C, 0.6 V and 30% RH and 1 atm.