Fabrication and properties of hybrid membranes based on salts of heteropolyacid,zirconium phosphate and polyvinyl alcohol

The fabrication and properties of a hybrid membrane based on cesium salt of heteropoly acid, zirconium phosphate and polyvinyl alcohol are described. The fabricated membranes were characterized for their intra molecular interaction, thermal stability, surface morphology, water content and surface-charge properties using Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), water uptake and ion-exchange capacity measurements. These membranes showed reduced methanol crossover (for possible application in DMFC) relative to that of Nafion^® 115. At 50% of relative humidity, the protonic conductivity of the hybrid membranes was in the range of 10⁻³ to 10⁻² S cm⁻¹. The feasibility of these hybrid membranes as proton conducting electrolyte in direct methanol fuel cell (DMFC) was investigated and preliminary results are compared with that of Nafion^® 115. A maximum power density of 6 mW cm⁻² with PVA–ZrP–Cs₂STA hybrid membrane was obtained with the cell operated in passive mode at 373 K and atmospheric pressure. Open circuit voltage of the cell operated with hybrid membranes are high compared to that of Nafion^® 115 indicating reduced methanol crossover.     

Nano-TiO2-coated polymer electrolyte membranes for direct methanol fuel cells

Composite polymer electrolyte membranes with nano-TiO₂ films are fabricated by deposition of titania nanoparticles from a sol solution. Measurements of ion conductivity, methanol permeability and single-cell performance of the modified Nafion membranes are conducted. The TiO₂ films adhere well and are crack-free. The protonic conductivity of the composite membranes decreases with increasing titania content, but methanol permeability is reduced. Preliminary tests on a single-cell of a direct methanol fuel cell (DMFC) indicate that a titania-coated membrane with 0.009 mg cm⁻² content gives the highest cell voltage and maximum power density.     

Preparation and characterization of sulfonated PEEK-WC membranes for fuel cell applications: A comparison between polymeric and composite membranes

Sulfonated poly(etheretherketone) with a cardo group (SPEEK-WC) exhibiting a wide range of degree of sulfonation (DS) was used to prepare polymeric membranes and composite membranes obtained by incorporation of an amorphous zirconium phosphate sulfophenylenphosphonate (Zr(HPO₄)(O₃PC₆H₄SO₃H), hereafter Zr(SPP)) in a SPEEK-WC matrix. The nominal composition of the composite membranes was fixed at 20 wt% of Zr(SPP). Both types of membrane were characterized for their proton conductivity, methanol permeability, water and/or methanol uptake, morphology by SEM and mechanical properties. For comparison, a commercial Nafion 117 membrane was characterized under the same operative conditions. The composite membranes exhibited a reduced water uptake in comparison with the polymeric membranes especially at high DS values and temperature higher than 50 °C. As a result, the water uptake into composite membranes remained about constant in the range 20–70 °C. The methanol permeability (P) of both polymeric and composite membranes was always lower than that of a commercial Nafion 117 membrane. At 22 °C and 100% relative humidity (RH), the proton conductivities (σ) of the polymeric membranes increased from 6 × 10⁻⁴ to 1 × 10⁻² S cm⁻¹ with the increase of DS from 0.1 to 1.04. The higher conductivity value was comparable with that of Nafion 117 membrane (3 × 10⁻² S cm⁻¹) measured under the same operative conditions. The conductivities of the composite membranes are close to that of the corresponding polymeric membranes, but they are affected to a lesser extent by the polymer DS. The maximum value of the σ/P ratio (about 7 × 10⁴ at 25 °C) was found for the composite membrane with DS = 0.2 and was 2.5 times higher than the corresponding value of the Nafion membrane.     

Sulfonic-functionalized heteropolyacid–silica nanoparticles for high temperature operation of a direct methanol fuel cell

Sulfonic-functionalized heteropolyacid–SiO₂ nanoparticles were synthesized by grafting and oxidizing of a thiol-silane compound onto the heteropolyacid–SiO₂ nanoparticle surface. The surface functionalization was confirmed by solid-state NMR spectroscopy. The composite membrane containing the sulfonic-functionalized heteropolyacid–SiO₂ nanoparticles was prepared by blending with Nafion^® ionomer. TG–DTA analysis showed that the composite membrane was thermally stable up to 290 °C. The DMFC performance of the composite membrane increased the operating temperature from 80 to 200 °C. The function of the sulfonic-functionalized heteropolyacid–SiO₂ nanoparticles was to provide a proton carrier and act as a water reservoir in the composite membrane at elevated temperature. The power density was 33 mW cm⁻² at 80 °C, 39 mW cm⁻² at 160 °C and 44 mW cm⁻² at 200 °C, respectively.     

Sulfonated poly(ether sulfone) for universal polymer electrolyte fuel cell operations

The performances of the proton exchange membrane fuel cell (PEMFC), direct formic acid fuel cell (DFAFC) and direct methanol fuel cell (DMFC) with sulfonated poly(ether sulfone) membrane are reported. Pt/C was coated on the membrane directly to fabricate a MEA for PEMFC operation. A single cell test was carried out using H₂/air as the fuel and oxidant. A current density of 730 mA cm⁻² at 0.60 V was obtained at 70 °C. Pt–Ru (anode) and Pt (cathode) were coated on the membrane for DMFC operations. It produced 83 mW cm⁻² maximum power density. The sulfonated poly(ether sulfone) membrane was also used for DFAFC operation under several different conditions. It showed good cell performances for several different kinds of polymer electrolyte fuel cell applications.     

Surface modified Nafion® membrane by ion beam bombardment for fuel cell applications

The interfacial structure between an electrolyte membrane and an electrode catalyst layer plays an important role in determining performance of proton exchange membrane fuel cell (PEMFC) since the electrochemical reactions produce electricity occur on the interfaces that are in contact with hydrogen or oxygen gas, so-called three phase boundaries. To improve performance of the PEMFC by enlarging effective area of the interfaces, surface of Nafion^® 115 membrane was roughened by Ar⁺ ion beam bombardment before being coated with a catalyst ink to form the electrode layer. With increasing ion dose density from 0 to 1 × 10¹⁷ ions cm⁻², roughness and hydrophobicity of the membrane surface increased, which could be favored for a high-performance PEMFC. In fuel cell tests, the single cell using Nafion^® membrane bombarded at an ion dose density of 10¹⁶ ions cm⁻² exhibited maximum power density of 0.62 W cm⁻², which was two times higher than that of the single cell employing untreated Nafion^® 115 membrane, i.e. 0.30 W cm⁻².     

Development of materials for mini DMFC working at room temperature for portable applications

Methanol permeability measurements and direct methanol fuel cell tests were performed at room temperature with different commercially available or recast Nafion^® membranes and sulfonated polyimide (SPI) membranes. Power densities as high as 20 mW cm⁻² could be obtained with Nafion^® 115. However, in order to meet the technological requirements for portable applications, thinner membranes have to be considered. As the MeOH crossover increases greatly (from (7 to 20) × 10⁻⁸ mol s⁻¹ cm⁻²) while Nafion^® membranes thickness decreases, non-perfluorinated polymers having high IEC are promising candidates for DMFC working at room temperature. The development catalysts tolerant to methanol is also relevant for this application. In spite of the low permeability to MeOH of SPI membranes, the obtained electrical performance with E-TEK electrodes based MEAs was lower than that obtained with Nafion^® membranes. No significant increase of performances was neither evidenced by using homemade PtCr(7:3)/C and PtRu(4:1)/C catalysts instead of E-TEK electrodes with recast Nafion^® based MEAs. However, MEAs composed with thin SPI membranes (50 μm) and homemade PtCr/C catalysts gave very promising results (18 mW cm⁻²). Based on experimental observations, a speculative explanation of this result is given.     

Increasing the operation temperature of polymer electrolyte membranes for fuel cells: From nanocomposites to hybrids

Among the possible systems investigated for energy production with low environmental impact, polymeric electrolyte membrane fuel cells (PEMFCs) are very promising as electrochemical power sources for application in portable technology and electric vehicles. For practical applications, operating FCs at temperatures above 100 °C is desired, both for hydrogen and methanol fuelled cells. When hydrogen is used as fuel, an increase of the cell temperature produces enhanced CO tolerance, faster reaction kinetics, easier water management and reduced heat exchanger requirement. The use of methanol instead of hydrogen as a fuel for vehicles has several practical benefits such as easy transport and storage, but the slow oxidation kinetics of methanol needs operating direct methanol fuel cells (DMFCs) at intermediate temperatures. For this reason, new membranes are required. Our strategy to achieve the goal of operating at temperatures above 120 °C is to develop organic/inorganic hybrid membranes. The first approach was the use of nanocomposite class I hybrids where nanocrystalline ceramic oxides were added to Nafion. Nanocomposite membranes showed enhanced characteristics, hence allowing their operation up to 130 °C when the cell was fuelled with hydrogen and up to 145 °C in DMFCs, reaching power densities of 350 mW cm⁻². The second approach was to prepare Class II hybrids via the formation of covalent bonds between totally aromatic polymers and inorganic clusters. The properties of such covalent hybrids can be modulated by modifying the ratio between organic and inorganic groups and the nature of the chemical components allowing to reach high and stable conductivity values up to 6.4 × 10⁻² S cm⁻¹ at 120 °C.     

PVDF-g-PSSA and Al2O3 composite proton exchange membranes

Poly(vinylidene fluoride) grafted polystyrene sulfonated acid (PVDF-g-PSSA) membranes doped with different amount of Al₂O₃ (PVDF/Al₂O₃-g-PSSA) were prepared based on the solution-grafting technique. The microstructure of the membranes was characterized by IR-spectra and scanning electron microscope (SEM). The thermal stability was measured by thermal gravity analysis (TGA). The degree of grafting, water-uptake, proton conductivity and methanol permeability were measured. The results show that the PVDF-g-PSSA membrane doped with 10% Al₂O₃ has a lower methanol permeability of 6.6 × 10⁻⁸ cm² s⁻¹, which is almost one-fortieth of that of Nafion-117, and this membrane has moderate proton conductivity of 4.5 × 10⁻² S cm⁻¹. Tests on cells show that a DMFC with the PVDF/10%Al₂O₃-g-PSSA has a better performance than Nafion-117. Although Al₂O₃ has some influence on the stability of the membrane, it can still be used in direct methanol fuel cells in the moderate temperature.     

A new direct methanol fuel cell with a zigzag-folded membrane electrode assembly

We propose a new direct methanol fuel cell with a zigzag-folded membrane electrode assembly. This fuel cell is formed by a membrane, which is made up of anode and cathode electrodes on a zigzag-folded sheet, separated by insulation film and current collectors. Individual anodes, cathodes and membranes form a unit cell, which is connected to the adjacent unit cell. The fuel cell can achieve high output voltage through easy in-series connection. Since it is not necessary to connect electrodes, as in the manner of conventional bipolar plates, there is no increase in fabrication cost and no degradation in reliability. The fuel feeds for the anode and cathode are achieved through methanol and air feeds on each electrode, which do not require electricity to run a pump or blower. The experimental cells were formed with an active area of 16 cm × 2 cm on membrane-folded cells. Filter papers with slits were inserted between anodes to improve their methanol supply. A power density of 3 mW cm⁻² was obtained at a methanol concentration of 2 M at ambient temperature. The cell power was affected by the slit area on cathode.     

Proton-conducting methacrylate–silica sol–gel membranes containing tungstophosphoric acid

The increase of the operation temperature in proton-exchange membrane fuel cell (PEMFC) above 100 °C is a great concern for the application of this type of cells in electric vehicles. Hybrid organic–inorganic membranes with nano-sized interfaces can combine the main properties to meet this objective. Methacrylate–silica covalent hybrid membranes have been synthesised by polymerization of 2-hydroxyethyl methacrylate and 3-methacryloxypropyl trimethoxysilane, and hydrolysis–polycondensation of alkoxide radicals. Tungstophosphoric acid hydrate was incorporated to endow the membranes with proton conductivity. The composition and synthesis conditions to promote organic polymerisation and sol–gel condensation avoiding phase separation have been optimised. The structural analysis shows homogeneous membranes without phase separation. The water retention properties provided by SiO₂ and tungstophosphoric acid leads to high proton conductivity (maximum values around 3 × 10⁻² S cm⁻¹) at 100–150 °C. A 0.5 M % of Tungstophosphoric acid (PWA) is enough to build well-connected paths for proton conduction.     

Improved photographic production of Pt nano-particles for fuel cell electrodes

An improved photographic Pt printing process has been developed, which is called the print-out process (POP). No developer is required in this process and the deposition efficiency was significantly improved by more than 6 times on carbon paper (CP) and 22 times on carbon-black-coated carbon paper (CB/CP) over the previously reported develop-out process (DOP) [1]. The Pt particle size can be easily controlled by varying the moisture content in the substrate and was reduced to 5 nm on blank CP by adding a stabilizing agent, ethylene glycol (EG), to the photo-emulsion. Due to the hydrophobic nature of CB/CP, both Nafion ionomer solution and ethylene glycol (EG) were mixed with the emulsion during the printing. SEM revealed that on this substrate Pt was distributed as ∼25 nm clusters consisting of 5 nm particles on the carbon-black. The mass specific catalytic activity for methanol oxidation of Pt printed on CB/CP by POP was increased five times compared to that of Pt printed by the previous DOP. The performance of the POP Pt in a H₂ PEM single fuel cell (5 cm²) was also evaluated. A peak power density of 288 mW cm⁻² was achieved with an anode POP Pt catalyst loading of 0.16 mg cm⁻² at 70 °C and 0.9 mg cm⁻² JM Pt at the cathode. Compared to the DOP Pt catalyst at about the same loading, peak power density was improved more than four times by using the POP Pt.     

Proton-conducting polymer membrane comprised of a copolymer of 2-acrylamido-2-methylpropanesulfonic acid and 2-hydroxyethyl methacrylate

In order to identify a proton-conducting polymer membrane suitable for replacing Nafion^® 117 in direct methanol fuel cells (DMFC), we prepared a cross-linked copolymer of hydrophilic 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and 2-hydroxyethyl methacrylate (HEMA). Fumed silicas were also added in an attempt to increase the amount of water adsorbed by the membrane and to enhance water retention. Hydrated copolymer membranes adsorbed significantly more water than Nafion^® 117, but were no better at retaining water during drying under ambient conditions. Films composed of 4% AMPS—96% HEMA had a room temperature proton conductivity of 0.029 S cm⁻¹, which increased to 0.06 S cm⁻¹ at 80 °C.     

Advanced air-breathing direct methanol fuel cells for portable applications

A novel MEA is fabricated to improve the performance of air-breathing direct methanol fuel cells. A diffusion barrier on the anode side is designed to control methanol transport to the anode catalyst layer and thus suppressing the methanol crossover. A catalyst coated membrane with a hydrophobic gas diffusion layer on the cathode side is employed to improve the oxygen mass transport. It is observed that the maximum power density of the advanced DMFC with 2 M methanol solution achieves 65 mW cm⁻² at 60 °C. The value is nearly two times more than that of a commercial MEA. At 40 °C, the power densities operating with 1 and 2 M methanol solutions are over 20 mW cm⁻² with a cell potential at 0.3 V.     

The role of ambient conditions on the performance of a planar,air-breathing hydrogen PEM fuel cell

This paper presents experimental data on the effects of varying ambient temperature (10–40 °C) and relative humidity (20–80%) on the operation of a free-breathing fuel cell operated on dry-hydrogen in dead ended mode. We visualize the natural convection plume around the cathode using shadowgraphy, measure the gas diffusion layer (GDL) surface temperature and accumulation of water at the cathode, as well as obtain polarization curves and impedance spectra. The average free-convection air speed was 9.1 cm s⁻¹ and 11.2 cm s⁻¹ in horizontal and vertical cell orientations, respectively. We identified three regions of operation characterized by increasing current density: partial membrane hydration, full membrane hydration with GDL flooding, and membrane dry-out. The membrane transitions from the fully hydrated state to a dry out regime at a GDL temperature of approximately 60 °C, irrespective of the ambient temperature and humidity conditions. The cell exhibits strong hysteresis and the dry membrane regime cannot be captured by a sweeping polarization scan without complete removal of accumulated water after each measurement point. Maximum power density of 356 mW cm⁻² was measured at an ambient temperature of 20 °C and relative humidity of 40%.     

Effects of platinum loading on performance of proton-exchange membrane fuel cells using surface-modified Nafion® membranes

The interface between the electrolyte and electrode catalyst plays an important role in determining the performance of proton-exchange membrane fuel cells (PEMFCs) since the electrochemical reactions take place at the interface in contact with the reactant gases. To enhance catalyst activity by enlarging the interfacial area, the surface of a Nafion^® membrane is roughened by Ar⁺ ion beam bombardment that does not change the chemical structure of the membrane, as confirmed by FT-IR spectra. Among the membranes treated with ion dose densities of 0, 10¹⁵, 10¹⁶, 5 × 10¹⁶ and 10¹⁷ ions cm⁻² at ion energy of 1 keV, the membrane treated at ion dose density of 5 × 10¹⁶ ions cm⁻² exhibits the highest performance. Using the untreated and the treated membrane with 5 × 10¹⁶ ions cm⁻², the effects of platinum loading on cell performance are examined with Pt loadings of 01, 0.2, 0.3, 0.4 and 0.55 mg cm⁻². Except for a Pt loading of 0.55 mg cm⁻² where mass transport limits the cell performance, the single cell using a treated membrane gives a higher performance than that using an untreated membrane. This implies that the cell performance can be improved and the Pt loading can be reduced by ion beam bombardment.     

Influence of composition and acid treatment on proton conduction of composite polybenzimidazole membranes

Inorganic/organic composite membranes formed by polybenzimidazole, silicotungstic acid and silica with different ratio between them have been prepared and characterized before and after treatment in phosphoric acid in order to evaluate the influence of composition and acid treatment on some main characteristics of the membranes. In particular the proton conductivity, the mechanical stability and the structural characteristics of the membranes were evaluated. Silica behaved as a support on which the heteropolyacid remained blocked in finely dispersed state and as an adsorbent for water, thus determining a beneficial effect on proton conduction. The membrane with 50 wt.% of SiWA–SiO₂/PBI, mechanically stable, gave proton conductivity of 1.2×10⁻³ S cm⁻¹ at 160°C and 100% relative humidity. After treatment with phosphoric acid the proton conductivity of membranes increased to 2.23×10⁻³ S cm⁻¹ under the same test conditions. All the materials prepared had amorphous structure.     

Influence of pore structure on electric double-layer capacitance of template mesoporous carbons

The behavior of two types of mesoporous carbons with different pore structures (i.e. unimodal and bimodal) as electrode material in an electrochemical double-layer capacitor has been analyzed. The carbon samples were prepared using mesostructured silica materials (MSM) as templating agents. The unimodal mesoporous carbon has a BET surface area of 1550 m² g⁻¹, and a pore volume of 1.03 cm³ g⁻¹; the porosity is mainly made up of structural mesopores of ca. 3 nm that exhibit a narrow pore size distribution (PSD). The bimodal carbon shows larger surface area (1730 m² g⁻¹) and larger pore volume (1.50 cm³ g⁻¹); the porosity is composed of two types of mesopores: structural (size around 3 nm) and complementary (size around 16 nm) mesopores. Both carbons show a disordered 3-D pore structure. Heat treatments at high temperatures (1000 °C) for long times (11 h) do not significantly change the pore structure with respect to the two synthesised carbons (800 °C). From the synthesized and heat-treated carbons, electrodes were processed as composites in which the carbons, polivinilidene fluoride (PVDF) and carbon black (CB) were the components. The effect of the heat treatment and relative CB content on specific capacitance, energy density and power density were studied. We found a specific capacitance of 200 F g⁻¹ for low current density (1 mA cm⁻²) and 110 F g⁻¹ for high current density (150 mA cm⁻²). Moreover, the curve of the specific capacitance versus current density shows three regimes, which are related to the three types of pore: micropores, structural mesopores and complementary mesopores. An energy density of 3 Wh kg⁻¹ at a power density of 300 W kg⁻¹ was obtained in some particular cases.     

Optimization of water and air management systems for a passive direct methanol fuel cell

A multi-phase, multi-component, thermal and transient model is applied to simulate the operation of a passive direct methanol fuel cell and optimize the design. The model takes into consideration the thermal effects and the variation of methanol concentration at the feeding reservoir above the fuel cell. Polarization and constant current cases are numerically simulated and compared with experiments for liquid feed concentration, membrane thickness, water management and air management systems. Parameters considered when determining an optimal design include power density, fuel utilization and energy efficiencies and water balance coefficients. An optimal liquid feed concentration is determined to be 2.0 mol kg^?1, which achieved a maximum power density of 21 mW cm^?2 and a fuel utilization efficiency of 63.0%. An optimal design of a cell uses a thick membrane (Nafion 117) to reduce methanol crossover and two additional cathode GDLs to improve the water balance coefficient and efficiency of the cell. This combination results in a power density of 23.8 mW cm^?2 and a water balance coefficient of ?1.71. An air filter may also be added to improve the efficiency and water balance coefficient of the cell, however, a small loss in power density will also occur. Using an Oil Sorbents air filter the water balance coefficient is increased to ?0.85, the fuel utilization efficiency is improved by 27.35% and the maximum power density decreased to 21.6 mW cm^?2.     

Progress in the development of a high-power,direct ethylene glycol fuel cell (DEGFC)

We recently reported on a high-power nanoporous proton-conducting membrane (NP-PCM)-based direct methanol fuel cell (DMFC) operated with triflic acid. However, accompanying the advantages of methanol as a fuel, such as low cost and ease of handling and storage, are several pronounced disadvantages: toxicity, high flammability, low boiling point (65 °C) and the strong tendency to pass through the polymer-exchange membrane (high crossover). The focus of this work is the development of a high-power direct ethylene glycol fuel cell (DEGFC) based on the NP-PCM. Ethylene glycol (EG) has a theoretical capacity 17% higher than that of methanol in terms of Ah ml⁻¹ (4.8 and 4, respectively); this is especially important for portable electronic devices. It is also a safer (bp 198 °C) fuel for direct-oxidation fuel cell (DOFC) applications. Maximum power densities of 320 mW cm⁻² (at 0.32 V) at 130 °C have been achieved in the DEGFC fed with 0.72 M ethylene glycol in 1.7 M triflic acid at 3 atm at the anode and with dry air at 3.7 atm at the cathode. The cell platinum loading was 4 mg Pt cm⁻² on each electrode. The overpotentials at the cathodes and at the anodes of the DEGFC and DMFC were measured, compared and discussed.