Radiation grafted membranes

The development of proton-exchange membranes for fuel cells has generated global interest in order to have a potential source of power for stationary and portable applications. The membrane is the heart of a fuel cell and the performance of a fuel cell depends largely on the physico-chemical nature of the membrane and its stability in the hostile environment of hydrogen and oxygen at elevated temperatures. Efforts are being made to develop membranes that are similar to commercial Nafion membranes in performance and are available at an affordable price. The radiation grafting of styrene and its derivatives onto existing polymer films and subsequent sulfonation of the grafted films has been an attractive route for developing these membranes with requiredchemistry and properties. The process of radiation grafting offers enormous possibilities for design of the polymer architecture by careful variation of the irradiation and the grafting conditions. A wide range of crosslinkers are available, which introduce stability to the membrane during its operation in fuel cells. Crosslinking of the base polymer prior to grafting has also been an attractive means of obtaining membranes with better performance. A systematic presentation is made of the grafting process into different polymers,the physical properties of the resultant membranes, and the fuel cell application of these membranes

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

Cross-linker effect in ETFE-based radiation-grafted proton-conducting membranes II. Extended fuel cell operation and degradation analysis

In this study the effect of crosslinker (divinylbenzene (DVB)) content on the chemical stability of poly(ethylene-alt-tetrafluoroethylene) (ETFE) based membranes using an H2O2 solution was carried out. Furthermore, the first long term-testing of single H2/O2 cell over 2180h of an MEA assembled using an optimized ETFE-based membrane prepared by radiation-induced grafting of styrene / DVB and subsequent sulfonation with a graft level of 25 % was carried out. The in situ MEA properties were characterized over the testing period using auxiliary current-pulse resistance, electrochemical impedance spectroscopy, polarization and H2 permeation. It is shown that the crosslinking dramatically improves the ex situ chemical stability, while no significant trend with the crosslinker content was observed. The performance of the tested MEA exhibits a decay rate of 13 μV.h-1 in voltage over the testing time at 500 mA.cm-2 at 80°C, while the hydrogen permeation shows a steady increase over time. This indicates clearly that to some extent changes in the membrane morphology occur over the operating time. The local post mortem analysis of the tested membrane reveals that high degradation was observed in areas adjacent to the O2 inlet and in other areas nearb

Fuel-cell performance of multiply-crosslinked polymer electrolyte membranes prepared by two-step radiation technique

A multiply-crosslinked polymer electrolyte membrane was prepared by the radiation-induced co-grafting of styrene and a bis(vinyl phenyl)ethane (BVPE) crosslinker into a radiation-crosslinked polytetrafluoroethylene (cPTFE) film. We then investigated its H2/O2 fuel-cell performance at 60 and 80ºC in terms of the effect of radiation and chemical crosslinking. At 60ºC, all the membranes initially exhibited similar performance, but only the cPTFE-based membranes were durable at 80ºC, indicating the necessity of radiation crosslinking in the PTFE main chains. Importantly, cell performance of the multiply-crosslinked membrane was found high enough to reach that of a Nafion112 membrane. This is probably because the BVPE crosslinks in the graft component improved the membrane-electrode interface in addition to membrane durability. After severe OCV hold tests at 80 and 95ºC, the performance deteriorated, while no significant change was observed in ohmic resistivity. Accordingly, our membranes seemed so chemically stable that an influence on overall performance loss could be negligible

Stable schedule matching under revealed preference

Baiou and Balinski (Math. Oper. Res., 27 (2002) 485) studied schedule matching where one determines the partnerships that form and how much time they spend together, under the assumption that each agent has a ranking on all potential partners. Here we study schedule matching under more general preferences that extend the substitutable preferences in Roth (Econometrica 52 (1984) 47) by an extension of the revealed preference approach in Alkan (Econom. Theory 19 (2002) 737). We give a generalization of the GaleShapley algorithm and show that some familiar properties of ordinary stable matchings continue to hold. Our main result is that, when preferences satisfy an additional property called size monotonicity, stable matchings are a lattice under the joint preferences of all agents on each side and have other interesting structural properties

Deep Spin-Glass Hysteresis Area Collapse and Scaling in the d=3d=3 ±J\pm J Ising Model

We investigate the dissipative loss in the $\pm J$ Ising spin glass in three dimensions through the scaling of the hysteresis area, for a maximum magnetic field that is equal to the saturation field. We perform a systematic analysis for the whole range of the bond randomness as a function of the sweep rate, by means of frustration-preserving hard-spin mean field theory. Data collapse within the entirety of the spin-glass phase driven adiabatically (i.e., infinitely-slow field variation) is found, revealing a power-law scaling of the hysteresis area as a function of the antiferromagnetic bond fraction and the temperature. Two dynamic regimes separated by a threshold frequency $\omega_c$ characterize the dependence on the sweep rate of the oscillating field. For $\omega < \omega_c$, the hysteresis area is equal to its value in the adiabatic limit $\omega = 0$, while for $\omega > \omega_c$ it increases with the frequency through another randomness-dependent power law.Comment: 6 pages, 6 figure

Synthesis of hierarchically structured Y2O3:Eu3*@ Ag nanocomposites with plasmon enhanced luminesencence via ultrasonic spray pyrolysis

Y2O3:Eu3+@Ag nanocomposites have been successfully synthesized by ultrasonic spray pyrolysis (USP) and examined to reveal effects of surface plasmon resonance, associated to silver nanoparticles, to the luminescence efficiency of Y2O3: Eu3+ red-emitting phosphors. Various Ag concentrations (1, 2.5 and 5 wt. %) and heat-treatment regimes (as prepared, 2h, 12h) were applied to understand how size and distribution of the Ag nanoparticles affect the luminescence efficiency. Samples were characterized by TEM, XRPD and STEM to evaluate crystal structure and distribution of Eu3+ in Y2O3 matrix. In terms of Y, O and Eu ions, uniform distribution was observed in the particles interior, while the Ag is present at the particles surface showing that USP is feasible for synthesis of hierarchically organized Y2O3:Eu3+@Ag. In the case of higher Ag concentration, a deviation from uniform and finely distributed Ag nanoparticles on Y2O3:Eu3+ surface was detected having detrimental effect to the plasmon enhanced luminescence. Regardless from silver concentrations, all heat treated samples exhibited superior luminescence with respect to asprepared ones, while decrease of luminescence efficiency was detected with the increase of Ag concentration. The most intense red luminescence at 612 nm which is due Eu3+ 5D0→7F2 transition was observed in Y2O3:Eu3+@Ag system for sample with 1wt% Ag, annealed for 12 hours