The quantification procedure of oxygen-transport resistances for different fuel-cell layers and phenomena is described. The total transport resistance is obtained from limiting-current measurements under conditions where oxygen diffusion is dominant (i.e., high flow rates, small cell size, humidified but subsaturated feeds, and low feed oxygen partial pressure). By systematically varying the experimental conditions, the contributions of molecular and Knudsen diffusion and permeation through the ionomer film covering the catalyst-layer agglomerates are determined. It is found that the ionomer-film resistance is dominant, especially at lower temperatures and lower Pt loadings. The calculated film properties through the ionomer hint that it is much more resistive than the bulk membrane for state-of-the-art cells.

The accurate evaluation of the activation overpotential of a membrane electrode assembly (MEA) is essential for fuel cell design. We have developed a new method of precisely determining the activity of cathode catalyst layer in MEA by maintaining the effect of platinum oxides at a constant level and have subsequently studied activity changes resulting from various operational histories. Cyclic voltammetry was used to quantitatively correlate activity changes in the cathode catalyst layer with various operational conditions and demonstrated that activity decreased after low humidity operation and recovered after high humidity operation in a N2 atmosphere at potentials below 0.2 V. The activity changes were found to be very slow and a span of more than 8 hours was required for the activity to reach the steady state. This phenomenon is presumably caused by the adsorption/desorption of ionomer sulfonate groups on the Pt surface, based on observations that the activity changes are consistent with sulfonate group adsorption and dependent on the ionomer structure. Oxygen transport resistance in the catalyst layer also varied in conjunction with the activity changes. Two possible Pt/ionomer interface models are proposed in an attempt to explain the above observations.

It is well known that the proton-exchange membrane is perhaps the most critical component of a polymer-electrolyte fuel cell. Typical membranes, such as Nafion®, require hydration to conduct efficiently and are instrumental in cell water management. Recently, evidence has been shown that these membranes might have different interfacial morphology and transport properties than in bulk. In this paper, experimental data combined with theoretical simulations that explore the existence and impact of interfacial resistance on water transport for Nafion®21x membranes will be presented. A mass-transfer coefficient for the interfacial resistance is calculated from experimental data using different permeation cells. This coefficient is shown to depend exponentially on relative humidity or water activity. The interfacial resistance does not seem to exist for liquid/membrane or membrane/membrane interfaces. The effect of the interfacial resistance is to flatten the water content profiles within the membrane during operation. Under typical operating conditions, the resistance is on par with the water transport resistance of the bulk membrane. Thus, the interfacial resistance can be dominant especially in thin, dry membranes and can affect overall fuel cell performance.

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Plants take up inorganic nitrogen and store it unchanged or convert it to organic forms. The nitrogen in such organic compounds is stoichiometrically recoverable by the Kjeldahl method. The sum of inorganic nitrogen and Kjeldahl nitrogen has long been known to equal the total nitrogen in plants. However, in our attempt to study the mechanism of nitrogen dioxide (NO(2)) metabolism, we unexpectedly discovered that about one-third of the total nitrogen derived from (15)N-labeled NO(2) taken up by Arabidopsis thaliana (L.) Heynh. plants was converted to neither inorganic nor Kjeldahl nitrogen, but instead to an as yet unknown nitrogen compound(s). We here refer to this nitrogen as unidentified nitrogen ( UN). The generality of the formation of UN across species, nitrogen sources and cultivation environments for plants has been shown as follows. Firstly, all of the other 11 plant species studied were found to form the UN in response to fumigation with (15)NO(2). Secondly, tobacco ( Nicotiana tabacum L.) plants fed with (15)N-nitrate appeared to form the UN. And lastly, the leaves of naturally fed vegetables, grass and roadside trees were found to possess the UN. In addition, the UN appeared to comprise a substantial proportion of total nitrogen in these plant species. Collectively, all of our present findings imply that there is a novel nitrogen mechanism for the formation of UN in plants. Based on the analyses of the exhaust gas and residue fractions of the Kjeldahl digestion of a plant sample containing the UN, probable candidates for compounds that bear the UN were deduced to be those containing the heat-labile nitrogen-oxygen functions and those recalcitrant to Kjeldahl digestion, including organic nitro and nitroso compounds. We propose UN-bearing compounds may provide a chemical basis for the mechanism of the reactive nitrogen species (RNS), and thus that cross-talk may occur between UN and RNS metabolisms in plants. A mechanism for the formation of UN-bearing compounds, in which RNS are involved as intermediates, is proposed. The important broad impact of this novel nitrogen metabolism, not only on the general physiology of plants, but also on plant substances as human and animal food, and on plants as an integral part of the global environment, is discussed.