Silicate and zirconium phosphate modified Nafion/PTFE composite membranes for high temperature PEMFC

The PEMFC performance of MEAs prepared from Nafion-212 (thickness 50 μm, Du Pont Co), porous poly(tetrafluoro ethylene) (PTFE, thickness 15 ~ 18 μm) film reinforced Nafion (NF, thickness 20 ± 2 μm), silicate hybridized NF (NF-Si, thickness 21 ± 2 μm), and zirconium phosphate hybridized NF (NF-Zr, thickness 21 ± 2 μm) membranes were investigated at 110 °C/ 51.7% RH, 120 °C/ 38.2% RH, and 130 °C/ 28.6% RH. We show PEMFC performances of these MEAs decrease in the sequence of: NF-Zr> NF-Si> NF> Nafion-212. The NF, NF-Si, and NF-Zr membranes have lower membrane thickness and lower Nafion content and require less water for proton transport than Nafion-212 at temperatures above 110 °C, and thus have higher conductivity and better PEMFC performance than Nafion-212. Incorporating silicate and zirconium phosphate into NF membranes enhances water retention of membranes at temperatures above 110 °C and improves PEMFC performances. Besides enhancing water retention, incorporating zirconium phosphate into membranes also provides more routes for proton transport via H⁺ exchange between H₃ ⁺O and HPO₄-Zr- and between H₂ ⁺PO₄-Zr- and HPO₄-Zr-. Thus NF-Zr has a higher conductivity and better PEMFC performance than NF and NF-Si.     

Modification of Nafion membranes with ternary composite materials for direct methanol fuel cells

Composite membranes for direct methanol fuel cells (DMFCs) were prepared by using Nafion115 membrane modification with polyvinyl alcohol (PVA), polyimide (PI) and 8-trimethoxysilylpropyl glycerin ether-1,3,6-pyrenetrisulfonic acid (TSPS). The performance of the composite membranes was evaluated in terms of water sorption, dimensional stability, thermal stability, proton conductivity, methanol permeability and cell performance. The proton conductivity was slightly decreased by 1-3% compared with Nafion115, which still kept the high proton conduction of Nafion115. The methanol permeability of Nafion/PI-PVA-TSPS composite membranes was remarkably reduced by 35-55% compared with Nafion115. The power density of DMFCs with Nafion/PI-PVA-TSPS composite membranes reached to 100 mW/cm², exceeding that with Nafion115 (68m W/cm²).     

A polymer electrolyte membrane for high temperature fuel cells to fit vehicle applications

Poly(tetrafluoroethylene) PTFE/PBI composite membranes doped with H₃PO₄ were fabricated to improve the performance of high temperature polymer electrolyte membrane fuel cells (HT-PEMFC). The composite membranes were fabricated by immobilising polybenzimidazole (PBI) solution into a hydrophobic porous PTFE membrane. The mechanical strength of the membrane was good exhibiting a maximum load of 35.19 MPa. After doping with the phosphoric acid, the composite membrane had a larger proton conductivity than that of PBI doped with phosphoric acid. The PTFE/PBI membrane conductivity was greater than 0.3 S cm⁻¹ at a relative humidity 8.4% and temperature of 180 °C with a 300% H₃PO₄ doping level. Use of the membrane in a fuel cell with oxygen, at 1 bar overpressure gave a peak power density of 1.2 W cm⁻² at cell voltages >0.4 V and current densities of 3.0 A cm⁻². The PTFE/PBI/H₃PO₄ composite membrane did not exhibit significant degradation after 50 h of intermittent operation at 150 °C. These results indicate that the composite membrane is a promising material for vehicles driven by high temperature PEMFCs.     

Optimisation of polypyrrole/Nafion composite membranes for direct methanol fuel cells

Acidic and neutral Nafion^® 115 perfluorosulphonate membranes have been modified by in situ polymerization of pyrrole using Fe(III) and H₂O₂ as oxidizing agents, in order to decrease methanol crossover in direct methanol fuel cells. Improved selectivities for proton over methanol transport and improved fuel cell performances were only obtained with membranes that were modified while in the acid form. Use of Fe(III) as the oxidizing agent can produce a large decrease in methanol crossover, but causes polypyrrole deposition on the surface of the membrane. This increases the resistance of the membrane, and leads to poor fuel cell performances due to poor bonding with the electrodes. Surface polypyrrole deposition can be minimized, and surface polypyrrole can be removed, by using H₂O₂. The use of Nafion in its tetrabutylammonium form leads to very low methanol permeabilities, and appears to offer potential for manipulating the location of polypyrrole within the Nafion structure.     

Prediction of methanol and water fluxes through a direct methanol fuel cell polymer electrolyte membrane

Development of a direct methanol fuel cell (DMFC) mass flux model, using conventional transport theory, is presented and used to predict the fluid phase superficial velocity, methanol and water molar fluxes, and the chemical species (methanol and water) dimensionless concentration profiles in the polymer electrolyte membrane, Nafion^® 117, of a DMFC. Implementation of these equations is illustrated to generate the numerical data as functions of the variables such as the pressure difference across the membrane, methanol concentration at the cell anode, temperature, and position in the membrane.     

Influence of ligands of palladium complexes on palladium/Nafion composite membranes for direct methanol fuel cells by supercritical CO2 impregnation method

Palladium/Nafion composite membranes were synthesized by supercritical impregnation method to reduce methanol crossover in direct methanol fuel cells. The palladium complexes used in this study were palladium(II) acetylacetonate, palladium(II) hexafluoroacetylacetonate, and palladium (II) bis(2,2,6,6-tetramethyl-3,5-heptane-dionato). The palladium complexes with various loading amounts from 0.010 to 0.050 g in a high-pressure vessel were dissolved in supercritical CO₂, and impregnated into Nafion membranes.The SEM images indicated that the palladium complexes were successfully deposited into Nafion membrane, and there were no problems such as cracking and pinhole. The EDX analysis showed that the palladium particles were distributed both at the membrane surface and also extended deeper into the membrane. The TEM images indicated that thin dense band of agglomerated Pd particles can be observed near the membrane surface, and a significant number of isolated Pd particles can be seen deeper into the membrane, when Pd(II) acetylacetonate was used as palladium complex. When palladium(II) hexafluoroacetylacetonate and palladium (II) bis(2,2,6,6-tetramethyl-3,5-heptane-dionato) were used, dense band of agglomerated Pd particles cannot be observed near the membrane surface, and small Pd particles were observed inside the membranes.The XRD analysis indicated that the crystalline peak of Nafion membrane at 2θ = 17° increased with the supercritical CO₂ treatment. It means that the degree of crystallinity for Nafion membrane increased by supercritical CO₂. The metal Pd peak at 2θ = 40° was observed for the Pd/Nafion membranes.The methanol crossover was reduced and the DMFC performance was improved for the Pd/Nafion membranes compared with Nafion membrane at 40 °C. The successful preparation of Pd/Nafion membranes by supercritical CO₂ demonstrated an effective alternative way for modifying membranes and for depositing electrode catalytic nanoparticles onto electrolyte.     

Fabrication and characterization of PFSI/ePTFE composite proton exchange membranes of polymer electrolyte fuel cells

PFSI/ePTFE composite proton exchange membranes were fabricated by impregnating perfluorosulfonic acid resin (PFSI resin, Nafion) into chemically modified expanded PTFE (ePTFE) matrix. Chemical modification of sodium-naphthalene treatment and N-methylol acrylamide (NMA) grafting decreased the contact angle of the as-received ePTFE from 125 ± 0.5° to 67 ± 0.5°, effectively converting the as-received hydrophobic ePTFE to a hydrophilic ePTFE matrix. The composite membrane fabricated with the hydrophilic ePTFE have higher impregnated PFSI loading, much lower porosity and better PTFE/PFSI interface contact, as compared to the composite membranes with the as-received ePTFE. This leads to much lower gas permeability and significantly improves the durability under an accelerated dry/wet cycle test. The fuel cell made from the PFSI/ePTFE composite membranes with hydrophilic ePTFE showed superior performance as compared to that with the composite membrane made from the as-received ePTFE and Nafion 211 membrane.     

Enhanced high-temperature polymer electrolyte membrane for fuel cells based on polybenzimidazole and ionic liquids

Polybenzimidazole (PBI)/ionic liquid (IL) composite membranes were prepared from an organosoluble, fluorine-containing PBI with ionic liquid, 1-hexyl-3-methylimidazolium tri?uoromethanesulfonate (HMI-Tf). PBI/HMI-Tf composite membranes with different HMI-Tf concentrations have been prepared. The ionic conductivity of the PBI/HMI-Tf composite membranes increased with both the temperature and the HMI-Tf content. The composite membranes achieve high ionic conductivity (1.6 × 10⁻² S/cm) at 250 °C under anhydrous conditions. Although the addition of HMI-Tf resulted in a slight decrease in the methanol barrier ability and mechanical properties of the PBI membranes, the PBI/HMI-Tf composite membranes have demonstrated high thermal stability up to 300 °C, which is attractive for high-temperature (>200 °C) polymer electrolyte membrane fuel cells.     

Characteristics of membrane humidifiers for polymer electrolyte membrane fuel cells

Water management plays an important role in obtaining high performance from a polymer electrolyte membrane fuel cell (PEMFC). To reduce the volume and energy consumption of widely-used bubble humidifiers, membrane humidifiers were fabricated by using an ultrafiltration (UF) membrane and Nafion membranes. The performance of the membrane humidifiers was examined as a function of gas flow rate and operating temperature. A single cell was operated using the UF membrane humidifiers exhibiting almost the same performance with that employing bubble humidifiers.     

Computational fluid dynamics simulation of polymer electrolyte membrane fuel cells operating on reformate

Carbon monoxide (CO) can extremely diminish the polymer electrolyte membrane fuel cell (PEMFC) performance since it is preferentially absorbed on the platinum catalyst layer blocking and reducing the number of catalyst sites available for the hydrogen oxidation reaction. To gain a good insight of CO poisoning characteristics so as to provide a remedial solution for CO-poisoned PEMFCs, a two-dimensional, isothermal, and single phase CO poisoning numerical model taking into account the transport phenomena, electrochemical reactions and multi-component gas mixture transport is developed for such purpose. Linear and bridged-bonded adsorbed CO modes were considered to occur in parallel on the highly dispersed nano-crystalline Pt/C and PtRu/C catalysts. By performing computational fluid dynamics numerical simulations, this study clearly demonstrates the CO poisoning mechanisms and characteristics of PEMFCs. The numerical results obtained are in reasonably good agreement with the experimental data showing the predictive capability of the model.     

Silicate-based polymer-nanocomposite membranes for polymer electrolyte membrane fuel cells

Proton-exchange membrane fuel cells have emerged as a promising emission free technology to fulfill the existing power requirements of the 21st century. Nafion^® is the most widely accepted and commercialized membrane to date and possesses excellent electrochemical properties below 80 °C, under highly humidified conditions. However, a decrease in the proton conductivity of Nafion^® above 80 °C and lower humidity along with high membrane cost has prompted the development of new membranes and techniques. Addition of inorganic fillers, especially silicate-based nanomaterials, to the polymer membrane was utilized to partially overcome the aforementioned limitations. This is because of the lower cost, easy availability, high hydrophilicity and higher thermal stability of the inorganic silicates. Addition of silicates to the polymer membrane has also improved the mechanical, thermal and barrier properties, along with water uptake of the composite membranes, resulting in superior performance at higher temperature compared to that of the virgin membrane. However, the degrees of dispersion and interaction between the organic polymer and inorganic silicates play vital roles in improving the key properties of the membranes. Hence, different techniques and solvent media were used to improve the degrees of nanofiller dispersion and the physico-chemical properties of the membranes. This review focuses mainly on the techniques of silicate-based nanocomposite fabrication and the resulting impact on the membrane properties.     

Proton-conducting electrolyte membranes based on hyperbranched polymer with a sulfonic acid group for high-temperature fuel cells

The hyperbranched polymers (HBP-SA-Acs) with both a sulfonic acid group as a functional group and an acryloyl group as a cross-linker at terminals in different ratios of sulfonic acid group/acryloyl group (SO₃H/Ac) were successfully synthesized as a new thermally stable proton-conducting electrolyte. The cross-linked hyperbranched polymer electrolyte membranes (CL-HBP-SAs) were prepared by thermal polymerizations of the HBP-SA-Acs using benzoyl peroxide, and their ionic conductivities under dry condition and thermal properties were investigated. The ionic conductivities of the CL-HBP-SAs were found to be in the range of 2.2 × 10⁻⁴ to 3.3 × 10⁻⁶ S/cm, depending upon the SO₃H unit contents, at 150 °C under dry condition, and showed the Vogel-Tamman-Fulcher (VTF) type temperature dependence, indicating that proton transfer is cooperated by local polymer chain motion. All CL-HBP-SAs were thermally stable up to 260 °C, and they had suitable thermal stability as electrolyte membranes for the high-temperature fuel cells under dry condition. Fuel cell measurement using a single membrane electrode assembly cell with a cross-linked electrolyte membrane was successfully performed under non-humidified condition. It was demonstrated that applying the concept of dry polymer system to proton conduction is one possible approach toward high-temperature fuel cells.     

A novel Gd3+ and Yb3+ co-doped ceria-sulphate composite electrolyte for intermediate-temperature fuel cells

In this study, Yb³⁺ and Gd³⁺ co-doped CeO₂ and the corresponding (Li/K)₂SO₄ composite electrolyte were prepared. The structures and morphologies of Ce_0.8Yb_0.1Gd_0.1O_2-α and Ce_0.8Yb_0.1Gd_0.1O_2-α-Li₂SO₄–K₂SO₄ were investigated using X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM). The conductivity of Ce_0.8Yb_0.1Gd_0.1O_2-α (1550 °C) as a function of time during humidification in a nitrogen atmosphere at 700 °C was investigated. The log(σT) vs. 1000T⁻¹ plots, logσ vs. log(pO₂) curves, and fuel cell performances of Ce_0.8Yb_0.1Gd_0.1O_2-α (1550 °C) and Ce_0.8Yb_0.1Gd_0.1O_2-α-Li₂SO₄–K₂SO₄ (1550 °C) were investigated. At 700 °C, Ce_0.8Yb_0.1Gd_0.1O_2-α-Li₂SO₄–K₂SO₄ (1550 °C) showed a power density of 197 mW cm⁻², which is five times higher than that of Ce_0.8Yb_0.1Gd_0.1O_2-α (1550 °C).     

Preparation and electrochemical characterization of Er3+-doped ceria-chloride composite electrolyte for intermediate-temperature solid oxide fuel cells

In this study, Ce_0.8Er_0.2O_2-α was prepared via a microemulsion method. Then, Ce_0.8Er_0.2O_2-α powder was mixed with melted NaCl-KCl (1:1 mol ratio) at the weight ratio of 80%: 20% to obtain Ce_0.8Er_0.2O_2-α-KCl-NaCl at 750 °C. Ce_0.8Er_0.2O_2-α and Ce_0.8Er_0.2O_2-α-KCl-NaCl were characterized by thermogravimetry analysis and differential scanning calorimetry (TGA-DSC), Raman spectrometer, X-ray diffraction (XRD) and scanning electron microscope (SEM). The log (σT) ~ 1000 T^-1 plots and fuel cell performances of Ce_0.8Er_0.2O_2-α and Ce_0.8Er_0.2O_2-α-KCl-NaCl were tested at 400–700 °C. The maximum output power density of Ce_0.8Er_0.2O_2-α-KCl-NaCl was 187 mW cm^-2 at 700 °C which is six times greater than that of Ce_0.8Er_0.2O_2-α.     

Impact of acid containing montmorillonite on the properties of Nafion membranes

The counter-ions of montmorillonite have been exchanged for ammonium cations containing either a sulfonic acid or a carboxylic acid in order to improve the performances of sulfonated membranes in direct methanol fuel cell. These layered silicates have been dispersed within Nafion^® by solution mixing. Comparison with conventional organo-modified montmorillonite (Cloisite 30B) shows that the incorporation of carboxylic acid in the clay galleries improves the filler dispersion and, consequently, the methanol barrier properties. Moreover, the negative impact of Cloisite 30B on the ionic conductivity is restricted.     

Silica-sol-templated mesoporous carbon as catalyst support for polymer electrolyte membrane fuel cell applications

Mesoporous carbon (MC) samples having especially high specific surface area, pore size, and pore volume (e.g. pore volume in excess of 4 cm³ g⁻¹) were prepared and their suitability as Pt catalyst supports in polymer electrolyte membrane fuel cells was examined. Pt particles on the MC support were slightly larger than those on commercial samples of Pt on carbon black, and they showed a greater tendency to agglomerate on the MC support than on carbon black. Ex situ cyclic voltammetry gave values for electrochemically active surface area that were about half that for a commercial Pt-on-carbon black sample. Preliminary attempts to prepare thin-film electrodes from Pt/MC samples with a Nafion binder using conventional ink formulations failed, probably because much of the Nafion electrolyte was taken up inside support pores and was not available to bind the support particles together. An alternate approach involving painting of catalyst inks directly onto gas diffusion layers was used to prepare membrane electrode assemblies (MEAs) from Pt/MC samples, which were tested using single-cell test hardware. Performance of these Pt/MC sample MEAs was compared with that prepared by decal transfer method with commercially obtained Vulcan XC-72R supported Pt catalyst. The reasons for the lower performance of Pt/MC were discussed.     

New polymer bipolar plates for polymer electrolyte membrane fuel cells: Synthesis and characterization

Carbon black (CB) and polyvinylidene fluoride (PVDF) composites were obtained and subsequently characterized, both microstructurally (DSC and DMA) and electrically. In addition, the electrochemical performance of these materials was tested in the form of bipolar plates, expressly manufactured for this purpose and incorporated in a conventional fuel cell. The results obtained allow for the conclusion that CB incorporation into PVDF yields polymer composite materials with electrical conductivity of about 2.4 S/cm, which may be thermically processed and given any convenient shape with the means conventionally applied in the field of polymer technologies. It was found that CB concentration slightly affects the microstructural parameters of the composites (melting temperature, glass‐transition temperature, Avrami kinetic parameters, etc.). © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2817–2822, 2002; DOI 10.1002/app.10257     

Plasma modification on a Nafion membrane for direct methanol fuel cell applications

This research focuses on Nafion modification using plasma techniques for direct methanol fuel cell applications. The results indicated the both argon (Ar) and carbon tetrafluoride (CF₄) plasma treatments modified the Nafion surface substantially without altering the bulk properties. The Nafion surface exposed to CF₄ plasma resulted in a more hydrophobic layer and an even lower MeOH permeability than the Ar-treated membrane. The plasma operating conditions using CF₄ were optimized by utilizing an experimental design. The minimum MeOH permeability was reduced by 74%. The conductivity was 1–2×10^-3 S/cm throughout the entire experimental range. Suppressed MeOH permeability can be achieved while maintaining the proton conductivity at a satisfactory level by adjusting the plasma operating conditions.     

New solid polymer electrolyte membranes for alkaline fuel cells

New solid polymer electrolyte composite membranes have been prepared using chitosan as matrices and incorporating potassium hydroxide as the functional ionic source. These membranes were featured as a three‐layer structure having a porous intermediate layer while the two crosslinked surface layers are dense. Results from impedance spectroscopy analysis showed that the conductivity of some hydrated composite membranes, after hydration for 1 h at room temperature, reached about 10⁻² S cm⁻¹. Several composite membranes were then tested in alkaline fuel cells, using hydrogen as fuel, air as oxidant and platinum as the electrode catalyst. A current density of 35 mA cm⁻² has been achieved at 60 °C with a flow rate of hydrogen at 50 ml min⁻¹ and air at 200 ml min⁻¹. Copyright © 2004 Society of Chemical Industry     

A novel Sn0.9Ni0.1P2O7/KPO3 composite electrolyte with improved electrical properties for intermediate temperature fuel cells

In this study, a novel Sn_0.9Ni_0.1P₂O₇/KPO₃ composite electrolyte was prepared by a mechanical mixing method. The Raman spectrum and X-ray diffraction of Sn_0.9Ni_0.1P₂O₇/KPO₃ indicated that the main microstructure is pyrophosphate phase and KPO₃ mainly exists in amorphous state in the composite. The intermediate temperature (400–700 °C) electrical properties of Sn_0.9Ni_0.1P₂O₇/KPO₃ were studied by means of electrochemical impedance spectroscopy. The maximum conductivity of Sn_0.9Ni_0.1P₂O₇/KPO₃ was 6.1 × 10⁻² S cm⁻¹ in a dry nitrogen atmosphere at 700 °C. The log σ ～ log (pO₂) results showed that Sn_0.9Ni_0.1P₂O₇/KPO₃ is a pure ionic conductor in an oxidizing atmosphere and a hybrid conductor of proton and electron hole under a reductive atmosphere. The maximum power density of Sn_0.9Ni_0.1P₂O₇/KPO₃ (thickness: 1.2 mm) was 434 mW cm⁻² at 700 °C.