Synthesis of silica immobilized phosphotungstic acid (Si-PWA)-poly(vinyl alcohol) (PVA) composite ion-exchange membrane for direct methanol fuel cell

A 80 μm thick composite ion-exchange membrane was synthesized by uniformly dispersing sub-micron to nano sized silica immobilized phosphotungstic acid (Si-PWA) inorganic ion exchanger into cross-linked poly(vinyl alcohol) (PVA) matrix. ATR-IR spectrum confirmed the PVA cross-linking and presence of Si-PWA in membrane. Amorphous behavior of the membrane indicated uniform blending of crystalline Si-PWA particles with cross-linked PVA. Membrane's tensile strength (93 MPa) was much higher than Nafion 117 (34 MPa). Ion exchange capacity of the membrane (0.90 meqg⁻¹) was higher than the values reported for the other PVA based membranes. Na⁺ transport number was 0.92, indicating good ion-selectivity of the membrane. Membrane showed a high water uptake of 35% while its methanol uptake was low (8.4%) and thereby reduced methanol permeability (1.6 × 10⁻⁷ cm² s⁻¹) compared to Nafion-117 was observed, a highly desirable property for DMFC application. Proton conductivity increased from 7.04 mS cm⁻¹ to 10.5 mS cm⁻¹ with increase in temperature from 30 °C to 50 °C. At 35 °C, the single cell DMFC with membrane showed higher OCV (0.8 V) and comparable peak power density to Nafion-117.     

NMR investigation of water and methanol transport in sulfonated polyareylenethioethersulfones for fuel cell applications

We report an investigation of water and methanol transport in polymer electrolyte membranes based on highly sulfonated polyarelenethioethersulfones (SPTES) for direct methanol fuel cell (DMFC) applications. Measurements of both water and methanol self-diffusion coefficients of SPTES polymer as well as in a reference sample of Nafion-117 equilibrated in 2 M methanol solution have been carried out, using the pulsed gradient spin echo technique, over a temperature range of 20–140 °C. The selectivity of the membrane, defined as (D_OH/D_CH3), decreased from 6 to 2.4 as temperature increased from 20 to 140 °C in SPTES sample while in Nafion, the value decreased from 3.2 to 1.4 as temperature increased from 20 to 100 °C. These results indicate significantly lower fuel molecular permeability in SPTES compared to that of Nafion. All results suggest high-temperature stability in these materials, offering the possibility of fuel cell operation at temperatures >120 °C. High pressure NMR diffusion measurements were also carried out for three different water contents (between 20 and 55 wt.%) in a static field gradient in order to get supplemental information regarding water transport in SPTES materials. The calculated activation volume increased from 1.54 to 8.40 cm³/mol as the water content decreased from 55 to 20%. This behavior is qualitatively similar to previously reported results for Nafion-117.     

A high-performance chitosan-based double layer proton exchange membrane with reduced methanol crossover

A novel double layer proton exchange membrane (PEM) comprising a layer of structurally modified chitosan, as a methanol barrier layer, coated on Nafion^®112 was prepared and assessed for direct methanol fuel cell (DMFC) applications. Scanning electron microscope (SEM) micrographs of the designed membrane revealed a tight adherence between layers, which indicate the high affinity of opposite charged polyelectrolyte layers. Proton conductivity and methanol permeability measurements showed improved transport properties of the designed membrane compared to Nafion^®117. Moreover, DMFC performance tests revealed a higher open circuit voltage and power density, as well as overall fuel cell efficiency for the double layer membrane in comparison with Nafion^®117, especially at elevated methanol solution feed. The obtained results indicate the designed double layer membrane as a promising PEM for high-performance DMFC applications.     

Low methanol crossover and high performance of DMFCs achieved with a pore-filling polymer electrolyte membrane

We compared the performance of the membrane electrode assembly for direct methanol fuel cells (DMFCs) composed of a pore-filling polymer electrolyte membrane (PF membrane) with that composed of a commercial Nafion-117 membrane. In DMFC tests, the methanol crossover flux was 23% lower in the PF membrane than in the Nafion-117 membrane even though the thickness of the PF membrane was 43% that of Nafion-117. This led to a higher DMFC performance and the lower overpotential of the cathode of the PF membrane. Feeding an aqueous 10 M methanol solution at 50 °C produced a low cathode overpotential, as low as 0.40 V at 0.2 A in the PF membrane, whereas the potential was 0.65 V at 0.2 A in the Nafion-117 membrane. In contrast, the ohmic loss and anode overpotential were almost the same in the two membranes. We confirmed that a reduction in methanol crossover using the PF membrane results in lower cathode overpotential and higher DMFC performance. In addition, the electro-osmotic coefficient was estimated as 1.3 in the PF membrane and 2.6 in Nafion-117, based on a water mass-balance model and values showing that the PF membrane prevents the flooding of the cathode at a low gas flow rate using. A highly concentrated methanol solution can be applied as a fuel without decreasing DMFC performance using PF membranes.     

Inorganic-organic hybrid polymer electrolyte based on polysiloxane/poly(maleic imide-co-styrene) network

Covalently cross-linked nonfluorinated hydrocarbon ionomers are synthesized by introducing sulfonate groups and a siloxane cross-linker through thermally and chemically stable imide bonding on poly(styrene-co-maleic anhydride). The three-dimensional polysiloxane framework, which does not only act as a robust scaffold but also provide sites for the hydrogen bonding with water, contribute to the increase in bound water degree, higher proton conductivity at lower ion exchange capacity, and greatly decreased methanol permeability. The spherical-shaped ionic clusters produce a comparable proton conductivity (10⁻¹ S cm⁻¹ above 60 °C) to Nafion-117. The conductivity of the hybrid ionomer does not decrease to gain its selectivity, but instead increased. Methanol permeability is ∼70% lower than that of Nafion-117, but has a higher water uptake and IEC. The membrane with IEC values of 1.1 mequiv. g⁻¹ exhibits a constant conductivity for 200 h in hydrolytic stability test, and produce a power density 20% higher than Nafion-117 in single DMFC operation.     

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.     

Development of polyvinyl alcohol (PVA) supported zirconium tungstate (ZrW/PVA) composite ion-exchange membrane

A polyvinyl alcohol supported zirconium tungstate (ZrW/PVA) composite ion exchange membrane has been synthesized by the sol-gel technique. The membrane was prepared by blending zirconium tungstate sol into polyvinyl alcohol gel which was cross-linked with glutaraldehyde (GA). The prepared membrane was analyzed by FT-IR, optical microscopy, SEM, XRD & EDS. The membrane was found to be devoid of any cracks and defects. Electrochemical properties which include transport number, ion exchange capacity (IEC), and proton conductivity were also measured. An IEC count of 0.92 meq.g⁻¹ and transport number of 0.88, thus obtained, were indicators of good electrochemical properties that shows its potential to be used in the direct methanol fuel cell. Moreover, water uptake, methanol uptake and chemical resistivity in the acidic and basic medium of the prepared membrane have also been assessed. Synthesized membrane showed high water holding capacity as compared to Nafion-117. The water uptake of ZrW/PVA composite membrane was found to be 114% which is much higher than that of Nafion-117 (29.2%) [18]. The final weight loss of the ZrW/PVA composite membrane in acidic medium was 10%. Oxidative stability of the membrane was evaluated by using Fenton's reagent. The weight loss in the reagent after 24 h was 17% without disruption.     

Sulfonic acid functionalized graphene oxide paper sandwiched in sulfonated poly(ether ether ketone): A proton exchange membrane with high performance for semi-passive direct methanol fuel cells

Sulfonated poly(ether ether ketone) (SPEEK) membranes have been deposited on the both sides of a sulfonic acid functionalized graphene oxide (SGO) paper to form a proton exchange membrane (PEM) with a sandwiched structure. The obtained SPEEK/SGO/SPEEK membrane could exhibit proton conductivity close to Nafion^® 112 and lower methanol permeability. The use of this SPEEK/SGO/SPEEK membrane greatly improves the performance of the semi-passive direct methanol fuel cell (DMFC). The semi-passive DMFC with the SPEEK/SGO/SPEEK membrane is found to be capable of delivering the peak power density 60% higher than that with the commercial Nafion^® 112. This, along with its comparable durability to Nafion^® 112, strongly suggests the great promise of using the SPEEK/SGO/SPEEK membrane as the PEM.     

Experimental investigation of current distributions in a direct methanol fuel cell with various gas diffusion layers and Nafion membranes

The influences of the gas diffusion layer (GDL) properties on the current distributions of a direct methanol fuel cell are investigated. Cathode GDLs with different hydrophobicity/hydrophilicity, air permeability, microporous layer (MPL), thickness, and texture properties are examined. Among the GDLs examined, a thin hydrophobic GDL with an MPL has the most homogeneous current distribution, which is primarily ascribed to the better water management capabilities of the cathode GDL properties. The differences in the current distribution among the different GDLs are more apparent when the air flow rate and loaded current are lower. The effect of the membrane thickness on the current distributions is also investigated. Among the membranes examined, Nafion^® 112 has different current distributions from the others, whereas there is no noticeable difference between the current distributions with Nafion^® 115 and Nafion^® 117. The current distribution with Nafion^® 112 is most affected by the enhanced methanol crossover and the high mixed potential.     

Nanocomposite membrane electrolyte of polyaminobenzene sulfonic acid grafted single walled carbon nanotubes with sulfonated polyether ether ketone for direct methanol fuel cell

Nanocomposite membranes of sulfonated polyether ether ketone (sPEEK) are prepared with polyaminobenzene sulfonic acid grafted single-walled carbon nanotubes copolymer (PABS-SWCNT) and its zwitterion interactions are studied. The nanocomposite membranes are prepared through solution cast technique using PABS-SWCNT as additive in different weight % (0.1, 0.15, and 0.2) ratio. The additive and nanocomposite membranes are characterized for its surface morphology, composition, thermal and physico-chemical properties. The nanocomposite membrane comprising optimized content of PABS-SWCNT (0.15 wt %) shows improved proton conductivity and reduced methanol crossover resulting in enhanced DMFC peak power density of 150 mW cm⁻² in comparison to 110 mW cm⁻² for sPEEK and 80 mW cm⁻² for Nafion® 117 respectively. The improved durability till 100 h for sPEEK/PABS-SWCNT (0.15 wt %) compared to sPEEK and Nafion-117 confirms its viability in DMFC application.     

Sulfonated poly(ether ether ketone) membranes with sulfonated graphene oxide fillers for direct methanol fuel cells

Sulfonated organosilane functionalized graphene oxides (SSi-GO) synthesized through the grafting of graphene oxide (GO) with 3-mercaptopropyl trimethoxysilane and subsequent oxidation have been used as a filler in sulfonated poly(ether ether ketone) (SPEEK) membranes. The incorporation of SSi-GOs greatly increases the ion-exchange capacity (IEC), water uptake, and proton conductivity of the membrane. With well-controlled contents of SSi-GOs, the composite membranes exhibit higher proton conductivity and lower methanol permeability than Nafion^® 112 and Nafion^® 115, making them particularly attractive as proton exchange membranes (PEMs) for direct methanol fuel cells (DMFC). The composite membrane with optimal SSi-GOs content exhibit over 38 and 17% higher power densities, respectively, than Nafion^® 112 and Nafion^® 115 membranes in DMFCs, offering the possibilities to reduce the DMFC membrane cost significantly while keeping high-performance.     

Current distribution measurements with a free-breathing direct methanol fuel cell using PVDF-g-PSSA and Nafion 117 membranes

The methanol crossover and other mass transfer phenomena have been investigated in a free-breathing direct methanol fuel cell (DMFC). The current distribution profile along the MeOH flow channel was measured and information of local concentrations of the reacting species was obtained. The DMFC with a segmented cathode was found to be very useful for a detailed analysis of the interrelated parameters, which cause the local variations of the cell current. The connections between different operating parameters were clarified in detail for two different membranes. The measurements were done for both an experimental poly(vinylidene fluoride)-graft-poly(styrene sulfonic acid) (PVDF-g-PSSA) membrane and the commercial Nafion^® 117 membrane, which have different methanol permeabilities. The MeOH concentration and the flow rate were varied in a wide range in order to determine their optimum values. The deviations from an even current density distribution were observed to increase as a function of MeOH concentration and decrease as a function of temperature. The power production of a free-breathing DMFC was observed to be proportional to the local oxygen concentration at the cathode side and inadequate air convection together with the MeOH crossover phenomenon was observed to decrease the cell performance locally.     

Polybenzimidazole/zwitterion-coated polyamidoamine dendrimer composite membranes for direct methanol fuel cell applications

The zwitterion-coated polyamidoamine (ZC-PAMAM) dendrimer with ammonium and sulfonic acid groups has been synthesized and used as filler for the preparation of PBI-based composite membranes for direct methanol fuel cells. Polybenzimidazole (PBI)/ZC-PAMAM dendrimer composite membranes were prepared by casting a solution of PBI and ZC-PAMAM dendrimer, and then evaporating the solvent. The presence of ZC-PAMAM dendrimer was confirmed by FT-IR and energy-dispersive X-ray spectroscopy (EDS) mapping of sulfur and oxygen elements. The water uptake, swelling degree, proton conductivity, and methanol permeability of the membranes increased with the ZC-PAMAM dendrimer content. For the PBI/ZC-PAMAM-20 membrane with 20 wt% of ZC-PAMAM, it shows a proton conductivity of 1.83 × 10⁻² S/cm at 80 °C and a methanol permeability of 5.23 × 10⁻⁸ cm² s⁻¹. Consequently, the PBI/ZC-PAMAM-20 demonstrates a maximum power density of 26.64 mW cm⁻² in a single cell test, which was about 2-fold higher than Nafion-117 membrane under the same conditions.     

Self-assembly of highly charged polyelectrolyte complexes with superior proton conductivity and methanol barrier properties for fuel cells

The paper is concerned with the formation of Layer-by-Layer (LbL) self-assembly of highly charged polyvinyl sulfate potassium salt (PVS) and polyallylamine hydrochloride (PAH) on Nafion membrane to obtain the multilayered composite membranes with both high proton conductivity and methanol blocking properties. Also, the influences of the salt addition to the polyelectrolyte solutions on membrane selectivity (proton conductivity/methanol permeability) are discussed in terms of controlled layer thickness and charge density.The deposition of the self-assembly of PAH/PVS is confirmed by SEM analysis and it is observed that the polyelectrolyte layers growth on each side of Nafion membrane regularly. (PAH/PVS)₁₀-Na⁺ and (PAH/PVS)₁₀-H⁺ with 1.0 M NaCl provide 55.1 and 43.0% reduction in lower methanol permittivity in comparison to pristine Nafion, respectively, while the proton conductivities are 12.4 and 78.3 mS cm⁻¹. Promisingly, it is found that the membrane selectivity values (Φ) of all multilayered composite membranes in H⁺ form are much higher than those of Na⁺ form and perfluorosulfonated ionomers reported in the literature. These encouraging results indicate that composite membranes having both superior proton conductivity and improved methanol barrier properties can be prepared from highly charged polyelectrolytes including salt for fuel cell applications.     

Self-humidifying nanocomposite membranes based on sulfonated poly(ether ether ketone) and heteropolyacid supported Pt catalyst for fuel cells

In the present study, the self-humidifying nanocomposite membranes based on sPEEK and Cs_2.5H_0.5PW₁₂O₄₀ supported Pt catalyst (Pt-Cs_2.5H_0.5PW₁₂O₄₀ catalyst or Pt-Cs_2.5) and their performance in proton exchange membrane fuel cells with dry reactants has been investigated. The XRD, FTIR, SEM-EDXA and TEM analysis were conducted to characterize the catalyst and membrane structure. The ion exchange capacity (IEC), water uptake and proton conductivity measurements indicated that the sPEEK/Pt-Cs_2.5 self-humidifying nanocomposite membranes have higher water absorption, acid and proton-conductive properties compared to the plain sPEEK membrane and Nafion-117 membrane due to the highly hygroscopic and acidy properties of Pt-Cs_2.5 catalyst. The single cells employing the sPEEK/Pt-Cs_2.5 self-humidifying nanocomposite membranes exhibited higher cell OCV values and cell performances than those of plain sPEEK membrane and Nafion-117 membrane under dry or wet conditions. Furthermore, the sPEEK/Pt-Cs_2.5 self-humidifying nanocomposite membranes showed good water stability in aqueous medium. After investigation of several membranes such as sPEEK and sPEEK/Pt-Cs_2.5 membranes, the self-humidifying nanocomposite membrane with sulfonation degree of 65.12% for its sPEEK and 15 wt.% of catalyst with 1.25 wt.% Pt within catalyst was found to be the best proton exchange membrane for fuel cell applications. This self-humidifying nanocomposite membrane has a higher single cell performance than the Nafion-117 which was frequently used as a proton exchange membrane for fuel cell applications.     

Nafion nanocomposite membranes: Effect of fluorosurfactants on hydrophobic silica nanoparticle dispersion and direct methanol fuel cell performance

Nafion^®–silica nanocomposite membranes are successfully prepared by adding hydrophobic silica nanoparticles to a Nafion^® solution. To distribute these nanoparticles evenly in the Nafion^® matrix, various fluorosurfactants of different ionic character are employed. Fluorosurfactants with acid groups such as phosphonic acid and sulfonic acid play an important role in simultaneously increasing the homogeneous dispersion of silica nanoparticles, enhancing proton conductivity, and reducing the methanol permeability of the nanocomposite membranes. Therefore, the dispersion properties of inorganic fillers such as silica can significantly affect nanocomposite performance in direct methanol fuel cell (DMFC) applications, whereas surfactants, if used properly, can improve the nanocomposite membrane properties. In particular, a commercial fluorosurfactant containing a sulfonic acid group (Zonyl^® TBS) at the end of the surfactant chain exhibits better miscibility with the Nafion^® ionomer. This feature results in a reduction in the dimensional change of the nanocomposite membrane due to relatively lower water swelling and significantly reduced methanol permeability through the membrane. A membrane–electrode assembly (MEA) prepared from a Nafion^®–silica nanocomposite membrane with TBS shows the highest DMFC performance in terms of voltage vs. current density (V–I) and power density vs. current density (P–I). The current densities at 0.4 V and 90 °C are 342, 508, and 538 mA cm⁻² with 1, 3 and 5 M methanol being fed at the anode side, respectively.     

Effects of recombination parameters in pn indium phosphide solar cells

This work, using a numerical code PC-1D, describes the effects of surface and bulk recombination on the performance of p⁺n indium phosphide solar cells. It is shown that surface recombination velocity and minority carrier diffusion lengths play a dominant role in controlling the efficiency of p⁺n cells. In order to have an acceptable series resistance, a p⁺n cell must have an emitter that is thicker than a n⁺p cell emitter. Consequently the performance of a p⁺n cell is more sensitive to the front surface recombination velocity. Improved surface and bulk recombination parameters can lead to cell efficiencies in excess of 24% AMO at 25°C.     

Synthesis and characterization of partially fluorinated sulfonated poly(arylene biphenylsulfone ketone) block copolymers containing 6F-BPA and perfluorobiphenylene units

A new series of partially fluorinated and sulfonated poly(biphenylsulfone ketone) block copolymers as proton exchange membrane materials were prepared from hydrophilic and hydrophobic oligomers. The copolymers were characterized by proton NMR, FT-IR, GPC, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) thermograms. The membranes showed excellent thermal and oxidative stability; e.g., TGA and DSC demonstrated that all sulfonated block copolymers exhibited good thermal stability with an initial weight loss at temperatures above 200 °C. Block-30 membranes showed low water uptake and acceptable ionic exchange capacity (IEC). The proton conductivity of the block-30 was 75 mS cm⁻¹ at 90 °C and 100% relative humidity (RH), while Nafion-117 had a value of 98 mS cm⁻¹ under the same conditions. AFM analysis of the Block-30 clearly showed that the morphology of the membranes separated the hydrophilic and hydrophobic domains which provided an effective proton-transport pathway.     

Direct methanol fuel cell performance of sulfonated polyimide membranes

Sulfonated polyimides (SPIs) derived from 1,4,5,8-naphthalene tetracarboxylic dianhydride, 4,4′-bis(4-aminophenoxy) biphenyl-3,3′-disulfonic acid and hydrophobic aromatic diamines showed the much lower methanol permeability and the lower proton conductivity than Nafion 112. The performance and the water and methanol crossover for direct methanol fuel cells (DMFCs) with the SPI membranes were investigated in comparison with Nafion membranes. The methanol and water fluxes increased significantly with increasing load current density for Nafion membranes but not for the SPI membranes, indicating that they were controlled by both the electro-osmotic drag and the molecular diffusion for the former but by only the molecular diffusion for the latter. These resulted in the much better DMFC performance for the SPIs than Nafion membranes especially at high methanol feed concentrations. The Faraday's efficiency and overall DMFC efficiency at 60 °C and 200 mA cm⁻² for SPI membrane with IEC of 1.51 meq g⁻¹ were 75% and 21%, respectively, at 5 wt.% methanol feed concentration, and 36% and 9.5%, respectively, at 20 wt.% methanol concentration. They were about two times and three times higher at 5 wt.% and 20 wt.% methanol concentrations, respectively, than those for Nafion 112. The short-term durability test for 300 h at 60 °C revealed no deterioration in the DMFC performance. The SPI membranes have high potential for DMFC applications at mediate temperatures (40–80 °C).