Gas-diffusion measurements

ACKNOWLEDGMENTS Many thanks to H. D. Lehmann for his gas-diffusion measurements. We thank C. Burger for help during X-ray analysis, H. Kamuse-witz for contact angle measurements, and S. Forster for the synthesis ofthe PS-b-PBd block copolymers. S. Oestreich wants to thank the Fonds der Chemischen Industrie for a Kekule fellowship. Financial support by the Max-Planck Society is gratefully acknowledged. 

The diffusion coefficient of S02 through the product layer has also been measured, the value of which is about 10 12 m2/s. Figure 9 shows comparison of intraparticle gas diffusivity between the sulfurated sorbent and its calcined sample, indicating calcination can enhance gas diffusivity. The inert matter in the sorbent, however, is beneficial for improving gas diffusion. Measurements (Zhang, 1992) indicated that pores greater than 700 A in diameter for different limestones possess the same distribution function /i(r), as can be expressed by the following correlation ... 

Hoo YL, Jin W, Ho HL, Ju L, Wang DN (2005) Gas diffusion measurement using hollow-core photonic bandgap fiber. Sensors Actuators B Chem 105 183-186... 

Kobayashi, Y, et al. (1994). Evaluation of Polymer Free Volume by PositronAnnihilation and Gas Diffusivity Measurements. Polymer, 35(5), 925-928. 

Surface Area Determination The surface-to-volume ratio is an important powder property since it governs the rate at which a powder interacts with its surroundings. Surface area may be determined from size-distribution data or measured directly by flow through a powder bed or the adsorption of gas molecules on the powder surface. Other methods such as gas diffusion, dye adsorption from solution, and heats of adsorption have also been used. It is emphasized that a powder does not have a unique surface, unless the surface is considered to be absolutely smooth, and the magnitude of the measured surface depends upon the level of scrutiny (e.g., the smaller the gas molecules used for gas adsorption measurement the larger the measured surface). 

Figure 3.5 The apparent vapour pressure of gold in gas transportation measurements as a function of the gas flow rate. Low flow rates, which were used earlier to assure equilibrium, are now known to be too high as a result of thermal diffusion in the gas mixture which is saturated with gold vapour...

L. M. Schwartz, R. L. Walswofhry 2001, (Tortuosity measurement and the effects of finite pulse widths on xenon gas diffusion NMR studies of porous media), Mag. Reson. Imag. 19, 345. 

Various carbon-based catalysts for the electrochemical oxygen reduction have been tested in the air gas-diffusion electrodes [7]. The polarization curves of the air electrodes were measured when operating against an inert electrode in 2 N NaCl-solution. The potential of the air electrodes was measured versus saturated calomel electrode (SCE). 

In Figure 3, we have presented the experimentally obtained reciprocal values of (Di )t.ff of oxygen in a sample of the nano-porous hydrophobic material as a function of the total pressure P of gas mixture (02-N2) when the oxygen concentration in the mixture is 21%. From the intercept of the straight line with the ordinate the value of the Knudsen diffusion coefficient can be also determined. It must be underlined that the value of Knudsen diffusion coefficient obtained by these diffusion measurements (2,86.10"2 cm2/s) is in very good coincidence with the value obtained by the gas permeability measurements. 

It must be noted that the effective diffusion coefficient (Di)eff is obtained by electrochemical measurements of air gas-diffusion electrodes with sufficiently thick gas layer so that the limiting process is the gas... 

In Figure 4 we have presented the experimental Tafel plots of air electrodes with catalysts from pure active carbon and from active carbon promoted with different amounts of silver. The obtained curves are straight lines with identical slopes. It must be underlined that the investigated electrodes possess identical gas layers and catalytic layers, which differ in the type of catalyst used only. Therefore, the differences in the observed Tafel plots can be attributed to differences in the activity of the catalysts used. The current density a at potential zero (versus Hg/HgO), obtained from the Tafel plots of the air electrodes is accepted as a measure of the activity of the air gas-diffusion electrodes the higher value of a corresponds to higher activity of the air electrode. 

An experimental method is proposed for estimation of the transport hindrances in air gas-diffusion electrodes. As a measure of the transport hindrances in the air gas-diffusion electrodes is introduced the difference AE between the potentials of one and the same electrode when operating with air and with pure oxygen at one and the same current density. The difference AE can be theoretically described by the equation ... 

J. D. Fairweather, P. Cheung, J. St-Pierre, and D. T. Schwartz. A microfluidic approach for measuring capillary pressure in PEMFC gas diffusion layers. Electrochemistry Communications 9 (2007) 2340-2345. 

K. Ito, K. Ashikaga, H. Masuda, et al. Estimation of flooding in PEMFC gas diffusion layer by differential pressure measurement. Journal of Power Sources 175 (2008) 732-738. 

H. Yamada, T. Hatanaka, H. Murata, and Y. Morimoto. Measurement of flooding in gas diffusion layers of polymer electrolyte fuel cells with conventional flow field. Journal of the Electrochemical Society 153 (2006) A1748-A1754. 

Lim, C. Wang, C. Y. Measurement of contact angles of liquid water in PEM fuel cell gas diffusion layer (GDL) by sessile drop and capillary rise methods. Penn State University Electrochemical Engine Center (ECEC) Technical Report no. 2001 03, Perm State University State College, PA, 2001. 

However, as mentioned previously, gas-diffusion electrodes usually deviate substantially from traditional electrochemical—kinetic behavior, often being limited by multiple rate-determining factors and/or changes in those factors with overpotential or other conditions. In attempting to analyze this type of electrode, one of the most influential experimental techniques to take hold in the solid-state electrochemical literature in the last 35 years is electrochemical impedance spectroscopy (EIS)—also know as a.c. impedance. As illustrated in Figure 6, by measuring the sinusoidal i— response as a function... 

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