Introduction of proton‐exchange membranes as solid electrolytes has permitted the development of fuel cells that utilize hydrogen, reformate gas and methanol as reactants. Modern fuel cells have extended service life and reliability and increased power and energy density. The need for fuel cell operating in excess of 5000 h for transportation and 30 000 h for stationary applications has led to extensive investigations of the failure modes in an effort to understand the primary mechanical, chemical and electrochemical mechanisms. This chapter reviews the progress in understanding the limitations of fuel cell operation with historical and current state‐of‐the‐art membranes and suggests directions for achieving further improvement.
A study was conducted to understand the physical and chemical changes in fuel cell membranes that result from Freeze/Thaw (F/T) cycling which might occur in electric vehicles. Nafion™ membranes and membrane electrode assemblies (MEA) were subjected to 385 temperature cycles between +80 °C and –40 °C over a period of three months to examine the effects on key properties. These studies were done on both compressed and uncompressed materials in the un‐humidified state. Although no catastrophic failures were seen, the analytical results shed some light on the relationship of temperature cycling to membrane structure, water management, ionic conductivity, gas permeability and mechanical strength. Changes in water swelling behavior and dry densities were noted and the effect on ionic conductivity and cell performance was examined. The impact on catalyst activity and structural integrity of MEAs was evaluated electrochemically.
Using small-angle X-ray scattering from solutions of yeast hexokinase, we have measured the radii of gyration of the monomeric B isozyme and its complexes with sugar substrates. We find that the radius of gyration decreases by 0.95 +/- 0.24 A upon binding glucose and 1.25 +/- 0.28 A upon binding glucose 6-phosphate. This observed reduction in radius of gyration in the presence of glucose is the same as that calculated from the coordinates of the high-resolution crystal structures of native hexokinase B and a glucose complex with hexokinase A. Thus, these measurements suggest that the dramatic closing of the slit between the two lobes of hexokinase observed in the crystal structures (Bennett, W.S., & Steitz, T.A. (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 4848--4852) occurs in solution when either glucose or glucose 6-phosphate is bound.
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