Radiation-induced grafting of styrene onto ultra-high molecular weight polyethylene powder and subsequent film fabrication for application as polymer electrolyte membranes: I. Influence of grafting conditions

Ultra-high molecular weight polyethylene (UHMWPE) powder was irradiated by gamma rays using a ⁶⁰Co source. Simultaneous and pre-irradiation grafting was performed in air and in inert atmosphere at room temperature. The monomer selected for grafting was styrene, since the styrene-grafted UHMWPE could be readily post-sulfonated to afford proton exchange membranes (PEMs). The effect of absorbed radiation dose and monomer concentration in methanol on the degree of grafting (DG) is discussed. It was found that the DG increases linearly with increase in the absorbed dose, grafting time and monomer concentration, reaching a maximum at a certain level. The order of rate dependence of grafting on monomer concentration was found to be 2.32. Furthermore, the apparent activation energy, calculated by plotting the Arrhenius curve, was 11.5 kJ/mole. Lower activation energy and high rate dependence on monomer concentration shows the facilitation of grafting onto powder substrate compared with film. The particle size of UHMWPE powder was measured before and after grafting and found to increase linearly with increase in level of grafting. FTIR-ATR analysis confirmed the styrene grafting. The grafted UHMWPE powder was then fabricated into film and post-sulfonated using chlorosulfonic acid for the purposes of evaluating the products as inexpensive PEM materials for fuel cells. The relationship of DG with degree of substitution (DS) of styrene per UHMWPE repeat unit and ion exchange capacity (IEC) is also presented.     

Chemically stable hybrid polymer electrolyte membranes prepared by radiation grafting,sulfonation, and silane‐crosslinking techniques

To prepare a crosslinked hybrid polymer electrolyte membrane (PEM) with high chemical stability, a silane monomer, namely p‐styryltrimethoxysilane (StSi), was first grafted to poly(ethylene‐co‐tetrafluoroethylene) (ETFE) film by γ‐ray preirradiation. Hydrolysis‐condensation and sulfonation were then performed on the StSi‐grafted ETFE (StSi‐g‐ETFE) films to give them crosslinks and proton conductibility, respectively. Thus, a crosslinked proton‐conducting hybrid PEM was obtained. The crosslinks introduced by the silane‐condensation have an inorganic ? Si? O? Si? structure, which enhance the chemical and thermal stabilities of the PEM. The effect of the timing of the hydrolysis‐condensation (before or after sulfonation) and the sulfonation method (by chlorosulfonic acid or H₂SO₄) on the properties of the resulting hybrid PEMs such as ion‐exchange capacity, proton conductivity, water uptake, chemical stability, and methanol permeability were investigated to confirm their applicability in fuel cells. We conclude that the properties of the new crosslinked hybrid StSi‐grafted PEMs are superior to divinylbenzene (DVB)‐crosslinked styrene‐grafted membranes. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5559–5567, 2008     

Electro-electrodialysis of HI–I2–H2O mixture using radiation-grafted polymer electrolyte membranes

In the thermochemical water-splitting iodine–sulfur process for hydrogen production, new polymer electrolyte membranes were applied in an electro-membrane process (electro-electrodialysis, EED) to increase the HI molality of HIx solution (HI + I₂ + H₂O mixture) to be over quasi-azeotropic. Radiation grafting of a styrene monomer into a poly(ethylene-co-tetrafluoroethylene) base film and subsequent sulfonation provided electrolyte membranes that had ion exchange capacities (IECs) of 1.1–1.6 mmol/g. With the EED of the HIx solutions using [HI] = [I₂] = 10 mol/kg at 40 °C the transport number of protons, ratio of permeated quantities of water to the protons, and current efficiency all appeared to depend on the IEC of the resulting membranes. When compared to Nafion, the self-made membranes exhibited lower electric cell resistance, and thereby decreasing up to 32% of the overall energy required in the concentration operation.     

The chemical behavior and degradation mitigation effect of cerium oxide nanoparticles in perfluorosulfonic acid polymer electrolyte membranes

Perfluorosulfonic acid membranes are susceptible to degradation during hydrogen fuel cell operation due to radical attack on the polymer chains. Mitigation of this attack by cerium-based radical scavengers is an approach that has shown promise. In this work, two formulations of crystalline cerium oxide nanoparticles, with an order of magnitude difference in particle size, are incorporated into said membranes and subjected to proton conductivity measurements and ex-situ durability tests. We found that ceria is reduced to Ce(III) ions in the acidic environment of a heated, humidified membrane which negatively impacts proton conductivity. In liquid and gas Fenton testing, fluoride emission is reduced by an order of magnitude, drastically increasing membrane longevity. Sideproduct analysis demonstrated that in the liquid Fenton test, the main point of attack is weak polymer end groups, while in the gas Fenton test, there is additional side-chain attack. Both mechanisms are mitigated by the addition of the ceria nanoparticles, whereby the extent of the concentration-dependent durability improvement is found to be independent of particle size.     

Effect of polyaniline (PANI) on Poly(vinylidene fluoride-co-hexaflouro propylene) (PVDF-co-HFP) polymer electrolyte membrane prepared by breath figure method

Poly(vinylidene fluoride-co-hexaflouro propylene) is a well-known material for polymer electrolyte membranes (PEMs) due to its low cost, high mechanical integrity and excellent chemical resistance; however, its pure form has limited characteristics that require further modification to achieve optimum results. Therefore, the different dosages of polyaniline (PANI) (10 wt%, 20 wt%, 30 wt%, 40 wt% and 50 wt%) were incorporated into PVDF-HFP blend to fabricate PVDF-HFP/PANI polymer electrolyte membrane by using breath-figure method. The FTIR peaks of PVDF-HFP and PVDF-HFP/PANI membrane confirms the successful incorporation of PANI into PVDF-HFP blend, while TGA, DSC and XRD analysis shows the PANI effect on stability and ionic conductivity of PVDF-HFP membrane. The PVDF-HFP/PANI membrane with 30 wt% PANI found superior with the highest porosity of 83%, electrolyte uptake of 270% and ionic conductivity of 1.96 mS cm⁻¹; however, the other concentrations of PANI were also effective and enhanced the performance of PVDF-HFP membrane. This shows the improved performances of PVDF-HFP membrane were attributed to successful incorporation of PANI and the proposed membrane can be a suitable alternative PEM or a separator for energy devices.     

The interaction of water vapors with H3PO4 imbibed electrolyte based on PBI/polysulfone copolymer blends

The interaction of steam with phosphoric acid imbibed electrolyte composed of PBI/PPy(50)coPSF 50/50 polymer blend and its effect on fuel cell performance was studied regarding its permeability through and its chemical interaction with the membrane. It was found that steam is the only gas that permeates the membrane with a permeability coefficient 1.1 × 10⁻¹⁴ mol cm cm⁻² s⁻¹ Pa⁻¹ at 150 °C. This is attributed either to the high solubility of water in phosphoric acid or to the chemical interaction with pyrophosphoric acid_. The latter was demonstrated by carrying out TGA experiments under various water vapor partial pressures. Water reacts with pyrophosphoric acid in order to maintain the equilibrium concentration of phosphoric acid at high level, thus improving proton conductivity and fuel cell performance. In addition it is shown that excess water dissolves in the membrane thus maintaining the “membrane/acid” system at high hydration level. This depends both on temperature and steam partial pressure. Although in the present study it is shown that steam plays a significant role in the performance of the high temperature Polymer electrolyte membrane (PEM) fuel cell, nevertheless its feed with humidified gases is not necessary, due to the back transport of the water produced at the cathode.     

Preparation and characterization of polymer electrolyte membrane from chloroacetate chitosan/chitosan blended with epoxidized natural rubber

Chitosan-based membranes are among the most effective and efficient PEMs for fuel cells, however their low proton conductivity needs to be improved. In this study, chitosan, chloroacetate chitosan (CCS), chitosan blend with epoxidized natural rubber (ENR), and CCS with ENR blend based membranes were prepared by solution casting, crosslinked with NaOH and H₂SO₄, and investigated for physical, chemical, electrical and ionic properties. The functional groups were identified by ATR-FTIR spectroscopy and the peaks matched improved membrane properties. The surface roughness of the membranes was determined by AFM, and it increased with the amount of ENR. The electrical properties measured with an LCR meter showed that the CCS, CS and CS-B had the highest conductance, conductivity, capacitance and dielectric constant, while the CCS10/ENR8, CS10/ENR8 and CS15/ENR3 showed the highest resistance and resistivity. Furthermore, the CCS gave the lowest dissipation factor, which indicates its suitability for use in a PEM. In addition, the contact angle was relatively high for CS-B, CS and CCS.     

Pre-irradiation induced grafting of styrene into crosslinked and non-crosslinked polytetrafluoroethylene films for polymer electrolyte fuel cell applications. I: Influence of styrene grafting conditions

Crosslinked and non-crosslinked polytetrafluoroethylene (PTFE) films [RX-PTFE and V-PTFE films, respectively], were irradiated in air at room temperature using γ-rays from a ⁶⁰Co source. The irradiated films were grafted with styrene in liquid phase. The grafting of styrene into PTFE films was proved by FT-IR spectroscopy. The influence of the reaction temperature and pre-irradiation doses on the resulted degree of grafting was discussed. The grafting speed and the degree of grafting were determined by the reaction temperature and pre-irradiation doses. The apparent activation energies were calculated as 39.7 kJ/mol for RX-PTFE films and 59.5 kJ/mol for V-PTFE films. The dependence index on absorbed doses at pre-irradiation for RX-PTFE films is 0.66, and for V-PTFE films it is 1.57. The geometric size changes of the grafted films were measured and discussed. Interestingly, the thickness of the grafted films was strongly influenced by the reaction temperature. The tensile strength and the elongation at break of the non-grafted and grafted RX-PTFE and V-PTFE films were measured. The grafted films then are sulfonated by chlorosulfonic acid for polymer electrolyte fuel cell (PEFC) applications and the highest IEC value gained is over 3. The analysis of the sulfonated films are now in progress.     

Effect of γ irradiation on optical properties of Ce-doped phosphate and silicate scintillating glasses

A set of Ce³⁺ activated silicate- and phosphate-based scintillating glasses were submitted to γ irradiation in the ⁶⁰Co radioisotope source “Calliope” (ENEA-Casaccia in Rome, Italy) in the dose range between 1 and 250 Gy (3.7 Gy/h). The effect of ionising radiation was probed by transmission measurements performed before and after each irradiation on all analysed samples. From these data, the radiation-induced absorption coefficient was calculated, proportional to the density of colour centres induced by irradiation in the solid matrix. Results are discussed by taking into account the possible dependence of radiation hardness on the composition of glass matrix.     

γ-Radiation effect on ionic liquid [bmim][BF4]

The effect of γ radiation on 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]) was investigated in detail. It was found that radiation induced an increase of UV absorption of the ionic liquid but no change in its glass transition point. Raman spectroscopy analysis indicates that the radiolysis of [bmim][BF₄] induces detectable destruction of alkyl-chain and scission of H-atoms of ring of imidizolium cation, and the relatively small changes of BF₄⁻ anion. Viscosities of irradiated [bmim][BF₄] samples were measured at different shear rates and temperatures, and compared with irradiated [bmim][PF₆] samples. The plots of shear stress against shear rate indicates that [bmim][BF₄] is a Newtonian fluid, no matter with irradiation. The viscosity of irradiated [bmim][BF₄] decreased with increasing of temperature and followed an Arrhenius equation. However, radiation caused no change in the viscosity of [bmim][BF₄], but a decrease in the viscosity of [bmim][PF₆] by up to 10%. We conclude that the radiation stability of [bmim][BF₄] is higher than [bmim][PF₆].     

Effect of kappa-carrageenan on the properties of poly(N-vinyl pyrrolidone)/kappa-carrageenan blend hydrogel synthesized by γ-radiation technology

A series of hydrogels in the form of rods were prepared from kappa-carrageenan (KC) and poly (N-vinyl pyrrolidone) (PVP) by gamma radiation with ⁶⁰Co γ source at room temperature. The properties of the prepared hydrogels, such as the gel strength, gel fraction and swelling behavior were investigated. Incorporation of KC into the PVP/water system increased obviously the gel strength and equilibrium degree of swelling (EDS) of PVP hydrogel. The experimental analyses showed that the crosslinking reaction of PVP was quicker than the degradation of KC at a low dose (less than 30 kGy), and the degradation of KC was inhibited in the PVP/KC mixture system. So an interpenetrating polymer network (IPN) hydrogel composed of PVP (a chemical crosslinking network) and KC (a physical crosslinking network) was proposed here. The existence of different classes of water in this IPN system was shown by water melting curves using DCS.     

Ionic transport studies on (PEO)6:NaPO3 polymer electrolyte plasticized with PEG400

Conduction characteristics of the poly(ethylene oxide) based new polymer electrolyte (PEO)₆:NaPO₃, plasticized with poly(ethylene glycol) are investigated. Free standing flexible electrolyte films of composition (PEO)₆:NaPO₃ + x wt.% PEG₄₀₀ (30 ? x ? 70) are prepared by solution casting method. A combination of X-ray diffraction (XRD), optical microscopy and differential scanning calorimetry (DSC) studies have indicated enhancement in the amorphous phase of polymer due to the addition of plasticizer. Further, a reduction in the glass transition temperature observed from the DSC result has inferred increase in the flexibility of the polymer chains. The cationic transport number (t_Na⁺) of 0.42 determined through combined ac-dc technique has confirmed ionic nature of conducting species. Ionic conductivity studies are carried out as a function of composition and temperature using complex impedance spectroscopy. The electrolyte with maximum PEG₄₀₀ content has exhibited an enhancement in the conductivity of about two orders of magnitude compared to the host polymer electrolyte. The complex impedance data is analyzed in conductivity, permittivity and electric modulus formalism in order to throw light on transport mechanism. A solid state electrochemical cell based on the above polymer electrolyte with a configuration Na|SPE|(I₂ + acetylene black + PEO) has exhibited an open circuit voltage of 2.94 V. The discharge characteristics are found to be satisfactory as a laboratory cell.     

Preparation and hydrogen permeation of SrCe0.95Y0.05O3−δ asymmetrical membranes

Asymmetrical thin membranes of SrCe_0.95Y_0.05O_3−δ (SCY) were prepared by a conventional and cost-effective dry pressing method. The substrate consisted of SCY, NiO and soluble starch (SS), and the top layer was the SCY. NiO was used as a pore former and soluble starch was used to control the shrinkage of the substrate to match that of the top layer. Crack-free asymmetrical thin membranes with thicknesses of about 50 μm and grain sizes of 5–10 μm were successfully pressed on to the substrates. Hydrogen permeation fluxes (J_H2) of these thin membranes were measured under different operating conditions. At 950 °C, J_H2 of the 50 μm SCY asymmetrical membrane towards a mixture of 80% H₂/He was as high as 7.6 × 10⁻⁸ mol/cm² s, which was about 7 times higher than that of the symmetrical membranes with a thickness of about 620 μm. The hydrogen permeation properties of SCY asymmetrical membranes were investigated and activation energies for hydrogen permeation fluxes were calculated. The slope of the relationship between the hydrogen permeation fluxes and the thickness of the membranes was −0.72, indicating that permeation in SCY asymmetric membranes was controlled by both bulk diffusion and surface reaction in the range investigated.     

Influence of substituted groups on the binding ability of benzoyl-modified β-cyclodextrins

A series of substituted benzoyl modified β-cyclodextrins, including mono-6-O-(p-methylbenzoyl)-β-CD (1), mono-6-O-(m-methylbenzoyl)-β-CD (2), mono-6-O-(o-methylbenzoyl)-β-CD (3), mono-6-O-(p-methoxylbenzoyl)-β-CD (4), mono-6-O-(m-methoxylbenzoyl)-β-CD (5), mono-6-O-(o-methoxylbenzoyl)-β-CD (6), mono-6-O-(m, p-dimethoxylbenzoyl)]-β-CD (7), mono-6-O-(o,m-dimethoxylbenzoyl)-β-CD (8), and mono-(6-O-benzoyl)-β-CD (9) were synthesized and their inclusion properties were studied by using fluorescence spectroscopy. The binding constants (K_a) of the modified β-CD derivatives with 2-p-toluidinylnaphthalene-6-sulfonate (TNS) were determined on the basis of the fluorescence spectroscopy. The effect of types and location of substituted groups of the benzene ring of the modified β-cyclodextrins on the binding property was discussed. Results indicated that the substituents had significant influences on the binding abilities of modified β-cyclodextrins.     

Conductivity and discharge characteristics of (PVC + NaClO4) polymer electrolyte systems

A sodium ion conducting polymer electrolyte based on polyvinyl chloride (PVC) complexed with NaClO₄ was prepared using a solution-cast technique. Investigations were conducted using XRD, FTIR and SEM. Frequency dependent conductivity (σ_ac) values were obtained from complex impedance plots (Cole-Cole plots). Activation energy values were determined from the conductivity data are decreased with increasing ionic conductivity. The transference number measurement was performed to characterize the polymer electrolyte for battery applications. Transference number values show that the charge transport in this electrolyte is predominantly due to the ions (t_ion = 0.98). Using the electrolyte, cells with a configuration Na/(PVC + NaClO₄)/(I₂ + C + electrolyte) were fabricated and their discharge profiles studied.     

Mechanical stability and transport properties of the Sn-promoted SrCo0.8Fe0.2O3−δ ceramic membrane

In this article, the phase compositions, thermal, mechanical and transport properties of both the SrCo_0.8Fe_0.2O_3−δ (SCF) and the SrCo_0.8Fe_0.1Sn_0.1O_3−δ (SSCF) ceramic membranes were investigated systematically. As compared with the SCF membrane, the SSCF one had a more promoted thermal shock resistance, which related to its small thermal expansion coefficient between them and an enhanced composite structure for it. For the SCF membrane, a permeation rate of 1.9 × 10⁻⁶ mol cm⁻² s⁻¹ was obtained at 1000 °C and under the oxygen partial pressure gradient of P_O2 (h)/P_O2 (l) = 0.209 atm/0.012 atm; however, the permeation rate was 2.5 × 10⁻⁶ mol cm⁻² s⁻¹ for the SSCF one in the same measuring condition. In addition, both peak values of total electrical conductivity (σ_e) for SSCF sample appeared with increasing temperature. The second peak value of σ_e for SSCF one was regarded as the contribution from its minor phase, which appeared with the mixed conducting behavior resulting from partly Co-dissolving into its lattice.     

Barrier properties of polymer/alumina nanocomposite membranes fabricated by atomic layer deposition

Novel polymer/ceramic nanocomposite membranes were fabricated, characterized and tested for their barrier performance. Atomic layer deposition (ALD) was used to deposit alumina films on primary, micron-sized (16 and 60 μm) high-density polyethylene (HDPE) particles at a rate of 0.5 nm/cycle at 77 °C. Well-dispersed polymer/ceramic nanocomposites were obtained by extruding alumina coated HDPE particles. The dispersion of alumina flakes can be controlled by varying the number of ALD coating cycles and substrate polymer particle size. The diffusion coefficient of fabricated nanocomposite membranes can be reduced to half with the inclusion of 7.29 vol.% alumina flakes. However, a corresponding increase in permeability was also observed due to the voids formed at or near the interface of the polymer and alumina flakes during the extrusion process, as evidenced by electron microscopy. The low surface wettability of the alumina outerlayers was believed to be one of the main reasons of void formation. Particle surface wettability was improved using 3-aminopropyltriethoxysilane (APS) to coat the particle ALD surface modified polymer particles prior to extrusion. The diffusion coefficient and permeability of the membrane using surfactant-modified particles decreased by 20%, relative to the non-modified case.     

The effect of ferric ions on the conductivity of various types of polymer cation exchange membranes

Recently, rejuvenated interest to fuel cells has posed a number of problems regarding the polymer electrolyte membrane properties and their behaviour in different electrolyte solutions. This work was dedicated to study the conductivity of H⁺-, Fe³⁺- and mixed H⁺/Fe³⁺-forms of cation exchange membranes Neosepta CMS, Nafion 112, 115 and 117 and Selemion HSF under conditions similar to these in the Fe³⁺/Fe²⁺–H₂/H⁺ fuel cell in the range of current densities 0–90 mA/cm². It was found that the conductivities of these membranes in 1.09 M H₂SO₄ solution decrease in the following order: Selemion HSF › Nafion 117 ≈ Nafion 115 ≈ Neosepta CMS › Nafion 112. Conductivities of perfluorinated membranes were discussed in terms of Hsu and Gierke percolation theory [20]. The Fe³⁺-forms of Nafion membranes studied displayed a monotonous decline in the resistance when current increased, which is a manifestation of gradual conversion of the Fe³⁺-form into H⁺-form of these membranes. Unlike the Nafion membranes, the Fe³⁺-forms of Neosepta CMS and Selemion HSF membranes exhibited a sharp jump of resistance at relatively high current densities (more than 70 mA/cm²) that is most probably a result of concentration polarization.     

Ionic transport in P(VDF-HFP)-PMMA-LiCF3SO3-(PC + DEC)-SiO2 composite gel polymer electrolyte

Composite gel polymer electrolytes composed of poly(vinylidene fluoride-co-hexafluoropropylene) P(VDF-HFP) and polymethylmethacrylate PMMA polymers, PC + DEC as plasticizer and LiCF₃SO₃ as salt and fumed silica as filler have been synthesized by solvent casting technique with varying plasticizer-filler ratio systematically. Films of thickness in the range of 40-70 μm were characterized by a.c. impedance measurements in the temperature range 303 K to 373 K. Addition of filler to the polymer electrolyte was found to result in an enhancement of the ionic conductivity. A maximum electrical conductivity of ∼1 × 10⁻³ S/cm at 303 K and ∼2.1 × 10⁻³ S/cm at 373 K has been achieved with the dispersion of the SiO₂. FTIR spectral studies confirmed the polymer-salt interaction. XRD patterns exhibit the increased amorphicity in the blended composite gel polymer electrolytes. Scanning electron micrograph shows the dispersion of SiO₂ particle in the polymer electrolyte.     

Al2O3-CeO2复合电解质的制备及其半导体离子燃料电池性能

通过共沉淀法制备了Al₂O₃-CeO₂复合材料,并将其作为电解质应用于半导体离子燃料电池（SIFC）。探究了Al₂O₃、CeO₂物质的量之比不同的Al₂O₃-CeO₂复合电解质对SIFC电化学性能的影响。采用X射线衍射（XRD）、X射线光电子能谱（XPS）、扫描电子显微镜（SEM）和透射电子显微镜（TEM）对材料进行了表征。其中,Al₂O₃、CeO₂物质的量之比为1：0.5的Al₂O₃-CeO₂（1：0.5）电池获得了最佳性能,在550℃下,开路电压为1.099 V时最大功率密度为1 142 mW·cm^-2。得益于复合电解质在测试气氛下两相间的界面效应,Al₂O₃-CeO₂（1：0.5）电池在较低测试温度下取得了优异的混合离子传导和功率输出性能。