TEMPO addition into pre-irradiated fluoropolymers and living-radical graft polymerization of styrene for preparation of polymer electrolyte membranes

We prepared proton exchange membranes (PEMs) by 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO)-mediated living-radical graft polymerization (LRGP) of styrene into fluoropolymer films and subsequent sulfonation. Poly(vinylidene fluoride) (PVDF) and poly(ethylene-co-tetrafluoroethylene) (ETFE) films were first irradiated and then treated with TEMPO solutions in various solvents. TEMPO addition was confirmed by the test of styrene grafting into TEMPO-treated films at 60 °C, at which the LRGP never proceeds. This test enabled us to differentiate the LRGP from the conventional graft polymerization. In order to gain a deep insight about TEMPO-addition reaction, the TEMPO-penetration behavior into the base polymer films was examined by a permeation experiment and computer simulation. Xylene and dioxane were appropriate solvents for the complete introduction of TEMPO into PVDF and ETFE films, respectively. Then, the LRGP of styrene was performed based on the fully TEMPO-capped films at 125 °C with various solvents. By using an alcoholic solvent, the degree of grafting was enhanced and it reached a maximum of 38%. This grafted film was sulfonated to prepare a PEM showing an ion exchange capacity of 2.2 meq/g and proton conductivity of 1.6×10^?1 S/cm.     

Radiation-induced grafting of styrene into poly(vinylidene fluoride) film by simultaneous method with two different solvents

Radiation-induced grafting of styrene into poly(vinylidene fluoride) (PVDF) films with 0.125 mm thickness at doses of 1 and 2.5 kGy in the presence of a styrene/N,N-dimethylformamide (DMF) solution (1:1, v/v) and at doses of 20, 40 and 80 kGy in presence of a styrene/toluene solution (1:1, v/v) at dose rate of 5 kGy h^?1 was carried out by the simultaneous method under nitrogen atmosphere and room temperature, using gamma rays from a Co-60. The films were characterized before and after modification by calculated grafting yield (GY %), infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and thermogravimetry (TG/DTG). GY results shows that grafting increases with dose, and the grafting of styrene was confirm by FT-IR due to the new characteristic peaks and by the TG and DSC attributed to changes in thermal behavior of the grafted material. Results showed that the system allows the controlled grafting of styrene into PVDF using gamma rays at doses as low as 1 kGy in DMF.     

Novel ETFE based radiation grafted poly(styrene sulfonic acid-co-methacrylonitrile) proton conducting membranes with increased stability

Styrene radiation grafted ETFE based proton conducting membranes are subject to degradation under fuel cell operating conditions and show a poor stability. Lifetimes exceeding 250 h can only be achieved with crosslinked membranes. In this study, a novel approach based on the increase of the intrinsic oxidative stability of uncrosslinked membranes is reported. Hence, the co-grafting of styrene with methacrylonitrile (MAN), which possesses a protected α-position and strong dipolar pendant nitrile group, onto 25 μm ETFE base film was investigated. Styrene/MAN co-grafted membranes were compared to a styrene based membrane in durability tests in single H₂/O₂ fuel cells. It is shown that the incorporation of MAN considerably improves the chemical stability, yielding fuel cell lifetimes exceeding 1000 h. The membrane preparation based on the co-grafting of styrene and MAN offers the prospect of tuning the MAN content and introduction of a crosslinker to enhance the oxidative stability of the resulting fuel cell membranes.     

Sulfonated polyethersulfone Cardo membranes for direct methanol fuel cell

Novel proton conducting membranes, sulfonated polyethersulfone Cardo (SPES-C), were prepared with concentrated sulfonic acid at room temperature. The degree of sulfonation was controlled by reaction time. Their proton conductivity and methanol permeability as a function of temperature were investigated. The SPES-C membranes with 70% DS were still not water soluble and had low degree of swelling. With the level of 70% sulfonation, proton conductivity was 0.011 S/cm at 80 °C, 0.0338 S/cm at 110 °C, which approached that of Nafion^® 115 membrane at the same conditions. Methanol permeability of SPES-C membranes was considerably smaller than that of Nafion^® 115 membrane over the temperature 25–80 °C.     

Preparation of poly(ether ether ketone)-based polymer electrolytes for fuel cell membranes using grafting technique

Poly(ether ether ketone) (PEEK)-based polymer electrolyte membranes (PEMs) was successfully prepared by radiation grafting of a styrene monomer into PEEK films and the consequent selective sulfonation of the grafting chains in the film state. Using milder sulfonation, the sulfonation reactions proceeded at the grafted chains in preference to the phenylene rings of PEEK main chains; as a result, the grafted films could successfully transform to a PEM with conductivity of more than 0.1 S/cm. The ion exchange capacity (IEC) and conductivity of the grafted PEEK electrolyte membranes were controlled to the ranges of 1.2–2.9 mmol/g and 0.03–0.18 S/cm by changing the grafting degree. It should be noted that this is the first example of directly transforming super-engineering plastic films into a PEM using radiation grafting.     

Performance of plasma sputtered fuel cell electrodes with ultra-low Pt loadings

Ultra-low Pt content PEMFC electrodes have been manufactured using magnetron co-sputtering of carbon and platinum on a commercial E-Tek^® uncatalyzed gas diffusion layer in plasma fuel cell deposition devices. Pt loadings of 0.16 and 0.01 mg cm^?2 have been realized. The Pt catalyst is dispersed as small clusters with size less than 2 nm over a depth of 500 nm. PEMFC test with symmetric electrodes loaded with 10 μg cm^?2 led to maximum reproducible power densities as high as 0.4 and 0.17 W cm^?2 with Nafion^®212 and Nafion^®115 membranes, respectively.     

Oxygen permeability and the state of water in Nafion® membranes with alkali metal and amino sugar counterions

The mobility of hydration water and the dissolved oxygen permeability through different cation forms of the Nafion^® membranes were determined. Two alkali metals (Na and K) and two amino sugars (an equivalent molar mixture of d-glucopyranosyl-α(1′ → 6)-2-amino-2-deoxy-d-mannitol and its sorbitol (GPA)and d-glucosamine (GLU)) were used as counterions. Based on the two-state model, the content and mobility of hydration water were determined using DSC and ¹⁷O NMR. The dissolved oxygen permeability through the Nafion^® membrane containing GPA was the lowest value in this study because, for the membrane, the fraction of hydration water was the greatest and the mobility of hydration water was the lowest. The amount and location of the hydrophilic group contained in substances as well as the kind of hydrophilic group affected the fraction and mobility of hydration water and dissolved oxygen permeability through the membranes.     

Viscoelastic phase diagram of fluorinated and grafted polymer films and proton‐exchange membranes for fuel cell applications

The influence of temperature and moisture activity on the viscoelastic behavior of fluorinated membranes for fuel cell applications was investigated. Uncrosslinked and crosslinked ethylene tetrafluoroethylene (ETFE)‐based proton‐conducting membranes were prepared by radiation grafting and subsequent sulfonation and their behavior was compared with ETFE base film and commercial Nafion^® NR212 membrane. Uniaxial tensile tests and stress relaxation tests at controlled temperature and relative humidity (RH) were carried out at 30 and 50 °C for 10% < RH < 90%. Grafted films were stiffer and exhibited stronger strain hardening when compared with ETFE. Similarly, both uncrosslinked and crosslinked membranes were stiffer and stronger than Nafion^®. Yield stress was found to decrease and moisture sensitivity to increase on sulfonation. The viscoelastic relaxation of the grafted films was found to obey a power‐law behavior with exponent equal to ?0.04 ± 0.01, a factor of almost 2 lower than ETFE, weakly influenced by moisture and temperature. Moreover, the grafted films presented a higher hygrothermal stability when compared with their membranes counterparts. In the case of membranes, a power‐law behavior at RH < 60% was also observed. However, a markedly different behavior was evident at RH > 60%, with an almost single relaxation time exponential. An exponential decrease of relaxation time with RH from 60 s to 10 s was obtained at RH ≥ 70% and 30 °C. The general behavior of grafted films observed at 30 °C was also obtained at 50 °C. However, an anomalous result was noticed for the membranes, with a higher modulus at 50 °C when compared with 30 °C. This behavior was explained by solvation of the sulfonic acid groups by water absorption creating hydrogen bonding within the clusters. A viscoelastic phase diagram was elaborated to map critical conditions (temperature and RH) for transitions in time‐dependent behavior, from power‐law scaling to exponential scaling. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1139–1148     

Development of MEMS-based direct methanol fuel cell with high power density using nanoimprint technology

Performance of MEMS-based DMFC is low, because graphite-based porous electrodes show poor compatibility with MEMS technology. Nanoimprint technology was adopted in this paper to prepare fine pattern on proton exchange membrane (PEM) in MEMS-based DMFC as a promising alternative to the graphite-based porous electrodes. Micro-convex with the diameter of about 600 nm and the height of 50–70 nm was prepared on Nafion^® 117 membrane by the nanoimprint at 130 °C using silicon mold. Thick Pt film (20 nm) was deposited as catalyst directly on the nanoimprinted Nafion^® 117 membrane. Then the Pt-coated PEM was sandwiched with micro-channeled silicon plates to form a micro-DMFC. With passively feeding of 1 M methanol solution and air at room temperature, the as-prepared cell had the open circuit voltage (OCV) of 0.74 V and the maximum power density of 0.20 mW/cm². The measured OCV was higher than those (0.1–0.3 V) of the state-of-the-art MEMS-based DMFC with planar electrode and pure Pt catalyst.     

Preparation of sulfonated poly(phthalazinone ether sulfone ketone) composite nanofiltration membrane

Thin film composite (TFC) membranes were prepared from sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK) as a top layer coated onto poly(phthalazinone ether sulfone ketone) (PPESK) ultrafiltration (UF) support membranes. The effects of different preparation conditions such as the SPPESK concentration, organic additives, solvent, degree of substitution (DS) of SPPEK and curing treatment temperature and time on the membrane performance were studied. The SPPESK concentration in the coating solution was the dominant factor for the rejection and permeation flux. The TFC membranes prepared from glycerol as an organic additive show better performance then those prepared from other additives. The rejection increased and the flux decreased with increasing curing treatment temperatures. The salt rejections of the TFC nanofiltration (NF) membranes increased in the order MgCl₂ < MgSO₄ < NaCl < Na₂SO₄. TFC membranes showed high water flux at low pressure. SPPESK composite membranes rejections for a 1000 mg L⁻¹ Na₂SO₄ feed solution was 82%, and solution flux was 68 L m⁻² h⁻¹ at 0.25 MPa pressure.     

Effect of gamma radiation on the mechanical and barrier properties of HEMA grafted chitosan-based films

Chitosan films were prepared by dissolving 1% (w/v) chitosan powder in 2% (w/v) aqueous acetic acid solution. Chitosan films were prepared by solution casting. The values of puncture strength (PS), viscoelasticity coefficient and water vapor permeability (WVP) of the films were found to be 565 N/mm, 35%, and 3.30 g mm/m² day kPa, respectively. Chitosan solution was exposed to gamma irradiation (0.1–5 kGy) and it was revealed that PS values were reduced significantly (p≤0.05) after 1 kGy dose and it was not possible to form films after 5 kGy. Monomer, 2-hydroxyethyl methacrylate (HEMA) solution (0.1–1%, w/v) was incorporated into the chitosan solution and the formulation was exposed to gamma irradiation (0.3 kGy). A 0.1% (w/v) HEMA concentration at 0.3 kGy dose was found optimal-based on PS values for chitosan grafting. Then radiation dose (0.1–5 kGy) was optimized for HEMA grafting. The highest PS values (672 N/mm) were found at 0.7 kGy. The WVP of the grafted films improved significantly (p≤0.05) with the rise of radiation dose.     

Enhancement of the performances of a single concentric glucose/O2 biofuel cell by combination of bilirubin oxidase/Nafion cathode and Au–Pt anode

This work deals with a novel preparation method of bilirubin oxidase/2,2′-azinobis-3-ethylbenzothiazoline-6-sulfonic acid electrode. The enzyme and its mediator were adsorbed on carbon Vulcan XC-72R before their immobilization into a Nafion^® matrix. Promising results were obtained when this biocathode was associated with Au₇₀Pt₃₀ nanoparticles as anode in a single concentric glucose/O₂ biofuel cell (BFC). The latter BFC delivered at 37 °C a power density of 90 μW cm^?2 for a cell voltage of 0.4 V in phosphate buffer (pH 7.4) containing 0.01 M glucose. Moreover, the electrical performances were increased with the concentration of glucose by generating up to 190 μW cm^?2 for a cell voltage of 0.52 V when the concentration of the renewable fuel reached 0.7 M.     

Hydrogen/oxygen polymer electrolyte membrane fuel cells (PEMFCs) based on alkaline-doped polybenzimidazole (PBI)

When complexed with alkaline such as potassium hydroxide, sodium hydroxide or lithium hydroxide, films (40 μm thick) of polybenzimidazole (PBI) show conductivity in the 5 × 10⁻⁵–10⁻¹ S/cm⁻¹ range, depending on the type of alkali, the time of immersion in the corresponding base bath and the temperature of immersion. It has been shown that PBI has a remarkable capacity to concentrate KOH, even in an alkaline bath of concentration 3 M. The highest conductivity of KOH-doped PBI (9×10⁻² S cm⁻¹) at 25°C obtained in this work is higher than the we had obtained previously as optimum values for H₂SO₄-doped PBI (5 × 10⁻² S cm⁻¹ at 25°C) and H₃PO₄-doped PBI ( 2 × 10⁻³ S cm⁻¹ at 25°C). PEMFCs based on an alkali-doped PBI membrane were demonstrated, and their characteristics exhibited the same performance as those of PEMFCs based on Nafion® 117. Their development is currently under active investigation.     

Proton conductive thin films prepared by plasma polymerization

Proton conductive membranes were prepared as thin films of about 10 μm thickness by an ion beam assisted plasma polymerization process. Argon ions were generated in a high frequency plasma and accelerated towards a PTFE target where CF fragments were released as a consequence of the ion impact. Various sulfur components (SO₂, CF₃SO₃H or ClSO₃H) were added to achieve proton conductivity by the formation of sulfonic acid groups. The CF fragments combined with the sulfur components to form a coherent thin film on a substrate. Mass spectrometric investigations revealed, however, that sulfur oxygen compounds were extremely delicate towards reduction to sulfur carbon compounds like CS₂ or SCF₂. The best membrane conductivities (>10⁻⁴ S/cm) and highest ion exchange capacities (0.15 mmol/g) were achieved with chlorosulfonic acid involved in the plasma polymerization process. Ultra-thin layers of these of these plasma polymers (ca. 300 nm) were subsequently deposited onto Nafion^® membranes in order to suppress methanol permeation for a potential application in a direct methanol fuel cell (DMFC). The ratio of proton conductivity and methanol diffusion coefficient was employed for an assessment of the transport characteristics of the coated membrane. Diffusion coefficients were determined in a flow cell coupled to a mass spectrometer. The plasma polymer coating decreased both the methanol permeation and the proton conductivity. With a proton conductive plasma polymer coating the decrease of methanol diffusion could outweigh the loss of proton conductivity. Plasma coating offers a way to suppress methanol crossover in DMFCs and to maintaining the proton conductivity.     

Preparation of palladium membranes from the reaction of Pd(C3H3)(C5H5) with H2: wet-impregnated deposition

A new technique to prepare a palladium membrane for high-temperature hydrogen permeation was developed: Pd(C₃H₃)(C₅H₅) an organometallic precursor reacted with hydrogen at room temperature to decompose into Pd crystallites. This reaction together with sintering treatment under hydrogen and nitrogen in sequence resulted in the formation of dense films of pure palladium on the surface of the mesoporous stainless steel (SUS) support. Under H₂ atmosphere the palladium membrane could be sintered at 823 K to form a skin layer inside the support pores. The hydrogen permeance was 5.16×10⁻² cm³ cm⁻² cm Hg⁻¹ s⁻¹ at 723 K. H₂/N₂ selectivity was 1600 at 723 K.     

Proton-conducting membranes based on poly(2,5-benzimidazole) (ABPBI) and phosphoric acid prepared by direct acid casting

We report the preparation of phosphoric acid doped poly(2,5-benzimidazole) (ABPBI) membranes for PEMFC by simultaneously doping and casting from a poly(2,5-benzimidazole)/phosphoric acid/methanesulfonic acid (MSA) solution. The evaporation of MSA yields a very homogeneous membrane having a better controlled composition, avoiding the use of solvent-intensive procedures. Membranes have been prepared with contents of up to 3.0H₃PO₄ molecules per ABPBI repeating unit. These membranes achieve a maximum conductivity of 1.5 × 10⁻² S cm⁻¹ at temperatures as high as 180 °C in dry conditions. These ABPBI membranes are more conveniently prepared than those conventionally formed and doped in separate steps while featuring comparable conductivities (ABPBI × 2.7H₃PO₄ prepared by the soaking method showed a conductivity of 2.5 × 10⁻² S cm⁻¹ at 180 °C in dry conditions).     

On the structural and magnetoelectrical characterization of epitaxial SrRuO3 thin films grown on (001) SrTiO3 substrates

High-quality epitaxial thin films of the ferromagnetic metallic oxide SrRuO₃ (SRO) were fabricated by dc-sputtering at high oxygen pressure and their structural and magnetoelectrical properties were carefully studied. The films featured a Curie temperature T_C ～ 160 K and a magnetic moment of ～0.7 μ_B per Ru ion. The temperature dependent magnetization could be well described by the scaling relation M(T) ∝ (T_C ? T)^β with a critical exponent β = 0.53 over the entire ferromagnetic temperature range. A negative magnetoresistance, MR, on the order of a few percent was found up to room temperature. MR showed a maximum of ～4% right at T_C where a kink structure of the resistivity, ρ, at zero field was flattened out on magnetic field application. This ρ contribution could be related to scattering due to orientational disorder of the Ru magnetic moments which become aligned by an external magnetic field. In addition, an equally strong MR effect, related to localization phenomena, could be observed at lower temperature. Particularly, the second MR peak at ～35 K might be related to a Fermi-liquid to non-Fermi-liquid crossover. A scaling behavior dρ/dT ∝ |T ? T_C|^α was observed only above T_C. Here, values for the exponent α ≈ ?0.4 and α ≈ ?1.4 were obtained in zero field and in a field of 9 T, respectively. The commonly observed ρ minimum, appearing at low temperatures (～3 K in the present case), is correlated with the structural disorder of the SRO films and is believed to have its origin in quantum corrections to the conductivity (QCC).     

Electrochemical and thermodynamic properties of hexacyanoferrate(II)/(III) redox system on multi-walled carbon nanotubes

Novel films consist of multi-walled carbon nanotubes (MWCNT) were fabricated by means of catalytic chemical vapor deposition (CVD) technique with decomposition of either acetonitrile (ACN) or benzene (BZ) using ferrocene (FeCp₂) as catalyst. The electrochemical and thermodynamic behavior of the ferrocyanide/ferricyanide, [Fe(CN)₆]^3−/4− redox couple on synthesized MWCNT-based films was investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques at T = (278.15, 283.15, 293.15, and 303.15) K. The redox couple [Fe(CN)₆]^3−/4− behaves quasi-reversibly on fabricated MWCNT-based films and its reversibility is enhanced upon increasing temperature. Namely, the findings establish that with the rise in temperature the barrier for interfacial electron transfer decreases, leading, consequently, to an enhancement of the kinetics of the charge transfer process. According to thermodynamics the equilibrium of the redox process is shifted towards the formation of [Fe(CN)₆]³⁻ at elevated temperatures.     

Towards a combined use of Mn(Salen) and quaternary ammonium salts as catalysts for the direct conversion of styrene to styrene carbonate in the presence of dioxygen and carbon dioxide

The use of oxygen in combination with carbon dioxide to afford the direct conversion of alkenes into cyclic carbonates could help to promote the greenhouse gas while minimizing the impact of the oxidation reaction on the environment. In this work, we focused, for the first time, on the association of two catalytic systems individually efficient for the epoxidation of styrene (Mn(salen)/O₂ bubbling/isobutyraldehyde at 80 °C) and the cyclocarbonatation of styrene oxide (choline chloride/CO₂ at 15 bar and 120 °C). First, the feasibility of the cyclocarbonatation reaction, starting from the non-isolated epoxide, has been proven as styrene carbonate was formed with a 24% yield. The objective was, then, to determine the best conditions allowing the overall transformation in a common solvent. Taking into account the differences in optimal temperatures and kinetics of the two individual steps, it was decided to vary the temperature during the reaction [first 80 °C (3 h) and 120 °C (23 h)]. Under these conditions, styrene was converted into the epoxide but, unfortunately, styrene carbonate formation could not be demonstrated. Blank experiments have clearly shown that isobutyraldehyde, which is essential to the first step, must be completely consumed before the temperature rise. Otherwise, autoxidation of the aldehyde in the presence of styrene oxide at 120 °C leads to other products than styrene carbonate.     

Thermodynamic study of the sublimation of crystalline tris(1,1,1-trifluoro-2,4-pentanedionate)ruthenium(III) and tris(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate)ruthenium(III)

The Knudsen mass-loss effusion technique was used to measure the vapour pressures at different temperatures of two crystalline ruthenium complexes: tris(1,1,1-trifluoro-2,4-pentanedionate)ruthenium(III) {Ru(tfacac)₃}, between T =  350.20 K and T =  369.17 K and tris(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate)ruthenium(III) {Ru(hfacac)₃} between T =  299.15 K and T =  313.14 K. From the temperature dependence of the vapour pressure of the crystalline compounds, the standard molar enthalpies of sublimation were derived by the Clausius–Clapeyron equation and the molar entropies of sublimation at equilibrium pressures were calculated. By using an estimated value for the heat capacity differences between the gas and the crystal phases the standard, p^o =  10⁵Pa, molar enthalpies, entropies, and Gibbs energies of sublimation at T =  298.15 K, were derived: