Search

A significantly challenging issue in polymer electrolyte fuel cells (PEFCs) is the insufficient durability of the membrane electrode assembly (MEA) electrodes caused by crack formation and growth. A decrease in interconnections of the electrical pathways in the electrodes leads to poor performance and durability of the PEFCs. Therefore, fundamental understanding of the nature of the crack formation and growth in electrodes is critical for improving fuel cell durability. In this study, an electromechanical diagnostic method is proposed to monitor the cracks in the PEFC electrodes. The electrical resistance of electrode is measured through the in situ four-wire electrical resistance measurements under tension. The crack areal density is proposed and measured as a quantitative parameter to define the presence of cracks in the electrodes using an optical microscope. It is found that the change of electrical resistances under tension increases with the electrode thickness (i.e., Pt loading), which results from the crack growth in the electrodes. This electromechanical diagnostic method is useful for understanding the crack mechanics consisting of initiation, propagation, and widening stages, and expected to facilitate the design of robust electrodes for highly durable fuel cells.

Ethylene-propylene-diene-termonomer (EPDM) rubber based fuel cell gasket compounds have been designed and explored the effects of various vulcanization systems on different properties. Three types of sulphur-accelerated vulcanization systems such as conventional vulcanization (con), semi-efficient vulcanization (sev) and efficient vulcanization (ev) and also a peroxide vulcanization system were employed in this study. The curing characteristics, tensile, hardness and compression set properties of the cured compounds were evaluated. The crosslink density was assessed by equilibrium swelling method in dodecane. The chemical stability of the cured EPDM compounds was also evaluated through an accelerated durability test (ADT) using a solution (1 M H2SO4 + 10 ppm HF) very close to the fuel cell atmosphere. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM) were employed to investigate the chemical and physical changes of the cured EPDM compounds before and after exposure to the ADT solution over time. The results indicate that the EPDM compounds cured with peroxide exhibit the highest crosslink density with lowest compression set value at both room temperature and at elevated temperature. The FTIR and the corresponding SEM results show no significant chemical degradation of the peroxide cured EPDM compounds due to ADT ageing.

Fundamental understanding of the mechanical behavior of polymer electrolyte fuel cell electrodes as free-standing materials is essential to develop mechanically robust fuel cells. However, this has been a significant challenge due to critical difficulties, such as separating the pristine electrode from the substrate without damage and precisely measuring the mechanical properties of the very fragile and thin electrodes. We report the mechanical behavior of free-standing fuel cell electrodes on the water surface through adopting an innovative ice-assisted separation method to separate the electrode from decal transfer film. It is found that doubling the ionomer content in electrodes increases not only the tensile stress at the break and the Young's modulus (E) of the electrodes by approximately 2.1-3.5 and 1.7-2.4 times, respectively, but also the elongation at the break by approximately 1.5-1.7 times, which indicates that stronger, stiffer, and tougher electrodes are attained with increasing ionomer content, which have been of significant interest in materials research fields. The scaling law relationship between Young's modulus and density (ρ) has been unveiled as E ∼ ρ(1.6), and it is compared with other materials. These findings can be used to develop mechanically robust electrodes for fuel cell applications.

Here, we present physical–chemical properties of Linde type A (LTA) zeolite crystals synthesized via conventional hydrothermal and microwave heating methods. Both heating methods produced LTA crystals that were sub-micron in size, highly negatively charged, super-hydrophilic, and stable when dispersed in water. However, microwave heating produced relatively narrow crystal size distributions, required much shorter heating times, and did not significantly change composition, crystallinity, or surface chemistry. Moreover, microwave heating allowed systematic variation of crystal size by varying heating temperature and time during the crystallization reaction, thus producing a continuous gradient of crystal sizes ranging from about 90 to 300 nm. In ion-exchange studies, colloidal zeolites exhibited excellent sorption kinetics and capacity for divalent metal ions, suggesting their potential for use in water softening, scale inhibition, and scavenging of toxic metal ions from water.

Hybrid zeolite-polyamide thin film nanocomposite (TFN) reverse osmosis membranes were synthesized by incorporating Linde type A (LTA)-type zeolite molecular sieve nanocrystals in the interfacial polymerization reaction used to form polyamide thin films. Nanocrystals were prepared with two different mobile cations (Na+ and Ag+) exchanged within the LTA crystal matrix. Incorporation of molecular sieve nanocrystals into polyamide thin films during interfacial polymerization was verified by infrared spectroscopy. Both TFN membranes exhibited higher water permeability, while maintaining similar salt rejection to pure polyamide thin film composite membranes. Nanocomposite thin films containing LTA nanocrystals in the silver form (AgA) produced a greater increase in water permeability than those in the sodium form (NaA). Furthermore, AgA-TFN membranes exhibited more hydrophilic and smooth interfaces, which appeared to inhibit adhesion of bacteria cells onto the membranes. The AgA nanocrystals exhibited significant bactericidal activity; however, when encapsulated within polyamide thin films the antimicrobial activity was significantly reduced.

Here, we model the effects of fouling on the performance of a reverse osmosis (RO) system treating micro-filtered secondary effluent. The model considers system properties, water quality, and transport phenomena. The model is fitted to operating data from the Orange County Water District's Groundwater Replenishment System RO pilot plant, which experienced severe fouling over a period of 3–4 months. Modeling results suggest the following: (a) permeate production, mass transfer, and solute rejection decrease from inlet to outlet, (b) osmotic and hydraulic pressure losses increase from inlet to outlet, (c) surface fouling by cake formation mirrors the permeate production rate (highest in lead elements, tapering off from inlet to outlet), (d) the higher fouling rate near the inlet shifts permeate production downstream, leveling the flux profile from inlet to outlet, (e) fouling-induced flux leveling leads to higher flux and rejection in tail elements, and thus, enhanced concentration polarization, (f) fouling-enhanced concentration polarization may induce premature scale formation even at fixed recovery, and (g) long-term, irreversible fouling results from combined effects of incomplete membrane cleaning and physical membrane compaction. This new model appears to be a powerful tool for understanding the behavior of full-scale RO processes.

Here we report on the impacts of organic solvent properties, reaction conditions, and curing conditions on polyamide composite reverse osmosis membrane separation performance, film structure, and interfacial properties. We provide direct experimental evidence that: (1) MPD diffusivity in the organic phase governs MPD–TMC thin film water permeability, (2) MPD diffusivity and solubility influence MPD–TMC thin film crosslinking in competing ways, (3) water permeability correlates most strongly with MPD–TMC film structure (i.e., crosslinking), and (4) salt rejection correlates most strongly with MPD–TMC film thickness and morphology. Overall, higher flux membranes with good salt rejection appear to comprise thinner, more heavily crosslinked film structures. Such high performance RO membranes are obtained by (1) selecting high surface tension, low viscosity solvents, (2) controlling protonation of MPD and hydrolysis of TMC during interfacial polymerization, and (3) optimizing curing temperature and time based on organic solvent volatility. Finally, although more research is necessary, our results suggest the rugose morphology and relative hydrophobicity of high performance MPD–TMC membranes might enhance concentration polarization and exacerbate surface fouling.

Here, we report on a new concept for formation of mixed matrix reverse osmosis membranes by interfacial polymerization of nanocomposite thin films in situ on porous polysulfone supports. Nanocomposite films created for this study comprise NaA zeolite nanoparticles dispersed within 50–200 nm thick polyamide films. Hand-cast pure polyamide membranes exhibit surface morphologies characteristic of commercial polyamide RO membranes, whereas nanocomposite membranes have measurably smoother and more hydrophilic, negatively charged surfaces. At the highest nanoparticle loadings tested, hand-cast nanocomposite film morphology is visibly different and pure water permeability is nearly double that of hand-cast polyamide membranes with equivalent solute rejections. Comparison of membranes formed using pore-filled and pore-opened zeolites suggest nanoparticle pores play an active role in water permeation and solute rejection. The best performing nanocomposite membranes exhibit permeability and rejection characteristics comparable to commercial RO membranes. As a concept, thin film nanocomposite membrane technology may offer new degrees of freedom in tailoring RO membrane separation performance and material properties. © 2007 Elsevier B.V. All rights reserved.

The graphene oxide (GO) was prepared by sonication-induced exfoliation from graphite oxide, which was produced by oxidation from graphite flakes with a modified Hummer's method. The GO was then treated by hydrazine to obtain reduced graphene oxide (rGO). On the basis of the characterization results, the GO was successfully reduced to rGO. Acrylonitrile–butadiene rubber (NBR)–GO and NBR–rGO composites were prepared via a solution-mixing method, and their various physical properties were investigated. The NBR–rGO nanocomposite demonstrated a higher curing efficiency and a change in torque compared to the gum and NBR–GO compounds. This agreed well with the crosslinking density measured by swelling. The results manifested in the high hardness (Shore A) and high tensile modulus of the NBR–rGO compounds. For instance, the tensile modulus at a 0.1-phr rGO loading greatly increased above 83, 114, and 116% at strain levels of 50, 100, and 200%, respectively, compared to the 0.1-phr GO loaded sample. The observed enhancement was highly attributed to a homogeneous dispersion of rGO within the NBR matrix; this was confirmed by scanning electron microscopy and transmission electron microscopy analysis. However, in view of the high ultimate tensile strength, the NBR–GO compounds exhibited an advantage; this was presumably due to strong hydrogen bonding or polar–polar interactions between the NBR and GO sheets. This interfacial interaction between GO and NBR was supported by the marginal increase in the glass-transition temperatures of the NBR compounds containing fillers. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42457.

Yttria stabilized zirconia (YSZ) powders comprised of Y/Zr in molar ratios of 0.01, 0.04, and 0.08, were prepared by a sol–gel method and their crystal structures and pore size distributions were determined. The 8‐mol% Y‐doped zirconia (8YSZ) powder restrained the crystallization and had the smallest pores. Porous 8YSZ membranes, prepared on a porous α‐alumina support tube by a sol–gel method, showed a low permselectivity of 5.1 for H2 to CO2. The membranes were modified by impregnation with Pt or Ni to improve the H2 permselectivity. The permselectivities of H2 to CO2 at 500°C through the Pt–YSZ and Ni–YSZ membranes were increased to 22 and 29, respectively.

A simple mathematical model assuming isothermal operation and a plug flow pattern was developed to evaluate the performance of an FAU-type zeolite membrane reactor for use in the catalytic dehydrogenation of cyclohexane. The membrane reactor consisted of a catalyst bed and membrane, containing an impermeable region at the reactor inlet, followed by a permeable region. The impermeable region was required in order to achieve an equilibrium conversion before entering the permeable region, and the selective permeation of benzene and hydrogen was sufficient to shift the equilibrium. The results of the simulation for the membrane reactor were in good agreement with the experimental results. On the basis of the simulation, the zeolite membrane reactor was superior to the Knudsen membrane reactor. The effect of co-feeding hydrogen with cyclohexane, to restrain coke formation on the catalyst due to the high hydrogen concentration in the feed side was clearly demonstrated. The effect of permeance and the separation factor on the conversion was evaluated, and as a result, the permeance was more important in terms of increasing the conversion than the separation factor.

Porous oxide membranes of γ-alumina, zirconia and silica were prepared on porous α-alumina tubes by sol-gel processes. γ-Alumina and zirconia membranes impregnated with platinum were also prepared. The permeation properties of these membranes were investigated by using unary and binary feeds of H2 and CO2 at 423 K. After permeation for 5 h with humidification at a concentration of 3 mol%, no large changes were found for the zirconia and γ-alumina membranes, but the permeances to H2 and CO2 for the silica membrane were decreased by 10–20%. A 70-h exposure to humidified feeds showed that the zircomaand γ-alumina-based membranes were more resistant than the silica membrane. The decrease in H2 permeance was only 5% for the zirconia-based membranes and 17.4% for the silica membrane. The Pt-loaded gg-alumina membrane remained defect-free after one-month of exposure to the humidified feeds at 423 K.

An FAU-type zeolite membrane was formed on a porous α-Al2O3 support tube, for use in the selective separation of benzene and hydrogen from cyclohexane. The membrane was used for the catalytic dehydrogenation of cyclohexane in a membrane reactor packed with a Pt/Al2O3 catalyst. The reaction was carried out in the temperature range of 423–523 K over a 1.0 wt.% Pt/Al2O3 catalyst prepared by means of an impregnation method. The activity of the catalyst was evaluated in a conventional packed bed and no significant deactivation was found after at least 3 days of use. The conversion of cyclohexane in the membrane reactor increased beyond the thermodynamic equilibrium value due to the simultaneous removal of hydrogen and benzene from the reaction site. The conversion of cyclohexane increased with increasing sweep flow rate, which can be attributed to the fact that the rapid removal of products leads to a high turnover rate. In addition, a decrease in the feed rate of cyclohexane led to a higher cyclohexane conversion. When the sweep flow rate was maintained at 100 cm3 (STP) min−1 and the cyclohexane feed rate was maintained at 1.1 mol h−1, the conversion of cyclohexane in the FAU-type zeolite membrane reactor reached 72.1% at 473 K, compared to the calculated equilibrium value, 32.2%.

The permeation and separation properties through an FAU-type zeolite membrane for mixtures of benzene and saturated C 4 –C 7 hydrocarbons were investigated at a permeation temperature of 373 K. The data related to permeation were cited from previous papers, and the new data on adsorption were obtained for such binary mixtures with and without benzene. The high benzene/cyclohexane separation factor can be attributed to the high adsorption selectivity in favor of benzene and the blockage against cyclohexane. The proposed model is as follows: For the feed of single-component benzene or cyclohexane, the adsorption of benzene is higher than that of cyclohexane, while the diffusivity of benzene is lower than that of cyclohexane. As a result, permeance to cyclohexane is higher than that to benzene. For the case of the feed consisting of an equimolar mixture of benzene and cyclohexane, the adsorption of benzene is smaller than that for the case of a single-component feed. Meanwhile, cyclohexane molecules are expelled from the adsorption sites by benzene, and the benzene/cyclohexane selectivity for adsorption reaches a value of approximately 19. The benzene molecules which are moving toward the permeate side frequently block the windows, and the diffusivity of cyclohexane through the windows is greatly retarded. The small separation factors for mixtures of saturated C 6 hydrocarbons can be explained based on a single-file diffusion mechanism.

An NaY-type zeolite membrane was formed on the outer surface of a 1.7-mm I.D. porous α-Al2O3 support tube using a hydrothermal synthesis. NaY-type zeolite powder was also prepared by the same procedure. The permeation and separation characteristics of single-component benzene (B) and cyclohexane (C), as well as an approximately equimolar mixture thereof, were investigated using the membrane at permeation temperatures of 358–413K. For the single-component systems, C permeated through the membrane faster than B, and the permeances to B and C increased with increase in permeation temperature in the ranges (0.6–3.0)×10−7 and (2.3–6.4)×10−7 mol m−2 sec−1 Pa−1, respectively. When the mixture was fed, however, B permeated faster than C, and the separation factor was dependent on the partial pressures on both the feed and permeate sides. The B/C separation factor reached 107 at a permeation temperature of 373K. The amounts of B and C, which were adsorbed to the NaY-type zeolite powder, were also determined at thi...

FAU-type zeolite membranes with different Si/Al ratios were hydrothermally synthesized on the outer surface of a porous α-Al2O3 support tube. The permeances of the membranes to CO2, CH4 and N2 were then measured at 308 K for single-component and equimolar binary systems. The separation properties were dependent on both the Si/Al ratio and the ion-exchange treatment. For single-component systems, a lower Si/Al ratio resulted in the incorporation of a larger number of Na+ ions. For a CO2-CH4 mixture, both CO2 permeances and CO2/CH4 selectivities were approximately half the values obtained for a binary CO2-N2 mixture. The highest selectivities, obtained using the NaX(1) zeolite membrane, were 28 for CO2/CH4 and 78 for CO2/N2. The RbY, RbX(1) and RbX(2) zeolite membranes showed larger CO2 permeances, compared with those of the original Na-type membranes. Ion-exchange with K+ ions was the most effective for the NaY zeolite membrane in that both the CO2 permeance and the CO2/CH4 selectivity were increased.

The vapor permeation properties of an FAU-type zeolite membrane, which was formed on the outer surface of a porous α-Al2O3 support tube, were determined in the temperature range of 358–413 K. The measurements were carried out using single-component benzene (Bz) and n-alkanes (C4-C7), as well as mixtures of benzene and n-alkanes. The hydrocarbons were diluted with nitrogen, and the partial pressures of hydrocarbons on the feed side were determined to be 7–8 kPa for the single-component systems and 4–5 kPa for each component of the binary systems. The permeate side was swept with a flow of nitrogen. The permeances were dependent on the permeation temperature and, to a greater extent, on the presence of benzene. For the single-component systems, the permeances were dependent on the diffusivities of the permeants, and the diffusivity of benzene through the membrane was the smallest of all the hydrocarbons tested. For the binary systems, however, benzene permeated at faster rates than any of the n-alkanes, and the permeances to n-alkanes were not directly dependent on their chain lengths. This benzene-selective permeation was realized using the FAU-type zeolite membrane with pore openings larger than the molecular size of benzene. The separation factors were affected by the partial pressures of permeants on the permeate side. When the partial pressures on the permeate side were in the range of 100–140 Pa for benzene and 1–4 Pa for n-alkanes, the separation factors at 373 K were 57 for Bz/n-C4, 70 for Bz/n-C5, 63 for Bz/n-C6, and 27 for Bz/n-C7. The adsorption selectivities for the binary systems on the feed-side surface of the membrane were 24 for Bz/n-C4 and 15 for Bz/n-C7, and were much higher than those for the single-component systems. This suggests that the adsorption of the n-alkanes on the FAU-type zeolites was strongly retarded by the selective adsorption of benzene.

Mechanical robustness of polymer electrolyte fuel cell (PEFC) electrodes is essential to keep the performance and to improve the durability. To increase the mechanical robustness, mechanical behavior of PEFC electrodes should be investigated because the mechanical failures such as cracks, tears and punctures are critical to lifetime decrease. However, there are no attempts to investigate the mechanical behavior of a pristine electrode without any substrates because it has been a significant challenge due to its porous and brittle nature. Therefore, we present a novel method that can be used to investigate the mechanical behavior of free-standing PEFC electrodes on the water surface. The water surface enables measurement of the mechanical properties of electrodes due to its high surface tension and low viscosity. To separate the pristine electrode from the decal transfer film without damage, we adopted an innovative ice-assisted separation method. The tensile tests on water surface were conducted to understand of the effect of the ionomer content and Pt loading. Young’s modulus, elongation at break and tensile stress at break increased with increasing ionomer content in all Pt loading conditions. Also, the scaling law relationship of E~ρ1.6 between Young’s modulus and density was suggested. Our method can be used to design mechanically robust electrode for PEFC or evaluate the degradation degree of electrodes.

Rubber blends based on ethylene-propylene-diene-monomer (EPDM) rubber and fluoroelastomer (FKM) hav- ing various blend ratios were prepared. The phase morphology, cure characteristics, hardness, tensile and dynamic vis- coelastic properties of the blends were investigated. The cure rate of the blends increased, while the crosslink-density decreased as the FKM content increased. The hardness, tensile strength and elongation at break, and storage modulus (E') increased with increasing FKM loading. A typical incompatible blend behavior was found by observing two distinct tanδ peaks corresponding to EPDM and FKM. Based on the morphology investigation by scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) analyses, a typical 'sea-island' phase morphology was observed for the EPDM/FKM blends. As the FKM loading was increased to 50 phr, the EPDM was found to remain as the con- tinuous phase. At the 80 phr of FKM, a co-continuous phase was observed and the phase inversion was observed at 90 phr of FKM.