Investigation and Assessment of basic Flow Field Designs for the cathodic Gas Supply in low Temperature Fuel Cells

One of the major challenges in the development of polymer electrolyte fuel cells (PEFCs) is to exploit the whole potential that inheres a given membrane electrode assembly. For this purpose, the fuel cell performance can be maximised by optimising the flow field design with regard to the intended operating point. As a prerequisite for such an optimisation, the electrochemical performance of PEFCs with various cathodic flow fields (Parallel flow, serpentine, spiral and interdigitated) has been investigated. In addition, the influence of the rib width was studied by utilising each structure with different rib widths, whereby the width and the height of the channels remain constant. The characterisation of the gas distributor structures was carried out by measuring overall polarization curves and local current density distributions by means of a segmented laboratory cell with an active area of 25 cm2. The optimum gas distributor structure strongly depends on the particular operating point. Consequently, each basic structure has to be adapted for the intended operating conditions. For example, a serpentine flow field generates the highest current densities for high air fluxes whereas an interdigitated structure is preferred for small flow rates. In the same way it can be shown that a spiral structure produces the most uniform current density distributions, whereas the current density distribution of a flow field with parallel channels is extremely inhomogeneous and becomes unsteady for higher water generation rates. The variation of the rib width has shown that all areas with a high catalytic activity are situated directly over the gas channels. In contrast, the covered areas represent only a small fraction of the global performance, whereby the general tendency is independent of the basic structure of the flow field

The acquisition of intensifiers versus reflexive pronouns

The ultimate goal of the study is to examine the acquisition of intensifiers in English and German. In this paper an overview of the first results regarding four L1 English-speaking children will be given. Contrary to previous claims in the literature (e.g. Thomas 1990), it will be argued that intensifiers are used by children in early phases of language acquisition. Intensifiers play an important role in early phases of language acquisition since they can be used to express the wish either to be included or excluded in a certain action and thus contribute to structuring a central aspect of the child's discourse

Early development of verb-constructions in an english-speaking child

In this paper the first results concerning the development of early verb morphology in an L1-English speaking child are presented. Adopting the framework of morphological development of Dressler (Dressler, this volume) the data of a girl from the CHILDES database, Nina of the Suppes corpus, is analysed with regard to the emergence of early verbal categories (e.g. number and person) and their appearance in a first mini-paradigm. In the sessions analysed so far the child Nina has reached an age of 2;2 when the first mini-paradigm emerges

Zerstörungsfreie Bestimmung elastischer Eigenschaften quadratischer 3-schichtiger Brettsperrholzplatten mit symmetrischem Aufbau

Zusammenfassung: Die Steifigkeiten von Holzwerkstoffen, wie z.B. von Brettsperrholzplatten, zählen bei statischen Berechnungen zu den wichtigsten Materialeigenschaften. Bei Platten sind es insbesondere die zwei Elastizitätsmoduln (Eii) in Plattenebene und die drei Schubmoduln (Gij). Um diese Parameter zerstörungsfrei und mit wenig Arbeitsaufwand bestimmen zu können, wurde ein Verfahren entwickelt, mit dem auf Grundlage von Schwingungsanalysen alle fünf Steifigkeiten in nur einem Versuch bestimmt werden können. Das Verfahren wurde auf 24 aus zwei verschiedenen Herstellerwerken stammenden, quadratischen Brettsperrholzplatten mit Seitenlänge 2.5m und einer Dicke von 70mm mit zwei unterschiedlichen, symmetrischen Aufbauten angewandt. Die so experimentell ermittelten Steifigkeiten wurden mit Biegeversuchen verifiziert und mit auf Grundlage der Verbundtheorie berechneten Steifigkeiten verglichen. Für alle Platten konnten unabhängig von deren Produktionsmethode und Aufbau vier Steifigkeiten sicher bestimmt und die Richtigkeit der Werte verifiziert werden. Die Bestimmbarkeit der Steifigkeit G23 ist von der Geometrie der Platte abhängig. Bemerkenswert ist die Tatsache, dass in der Praxis oft angewendete Steifigkeitsberechnungen, die unter der Annahme durchgeführt wurden, dass das Rohmaterial der einzelnen Schichten der Festigkeitsklasse C24 entspricht, zu optimistische Werte liefern könne

Influence of wood moisture content on bending and shear stiffness of cross laminated timber panels

The bending and shear stiffness of cross-laminated timber (CLT) made from softwood depend on the moisture content of the panel. In principle, the stiffness properties drop with increasing moisture content within the hygroscopic range. However, swelling provokes closing of open gaps of the middle layers and leads to an apparent increase of stiffness due to internal friction. This increase in stiffness is only relevant for low deflections and can provoke misinterpretation of measuring results when deriving stiffness properties of CLT by means of dynamic methods such as modal analysis. Drying, on the other hand causes cracking within CLT, which manifests itself in a distinct reduction of the bending stiffness perpendicular to the grain direction of the face layers. Calculating bending stiffness of CLT perpendicular to the grain direction of the face layers by means of the compound theory not taking into account the face layers therefore is a common and correct procedur

Metaecosystem Dynamics of Marine Phytoplankton Alters Resource Use Efficiency along Stoichiometric Gradients

Metaecosystem theory addresses the link between local (within habitats) and regional (between habitats) dynamics by simultaneously analyzing spatial community ecology and abiotic matter flow. Here we experimentally address how spatial resource gradients and connectivity affect resource use efficiency (RUE) and stoichiometry in marine phytoplankton as well as the community composition at local and regional scales. We created gradostat metaecosystems consisting of five linearly interconnected patches, which were arranged either in countercurrent gradients of nitrogen (N) and phosphorus (P) supply or with a uniform spatial distribution of nutrients and which had either low or high connectivity. Gradient metaecosystems were characterized by higher remaining N and P concentrations (and N∶P ratios) than uniform ones, a difference reduced by higher connectivity. The position of the patch in the gradient strongly constrained elemental stoichiometry, local biovolume production, and RUE. As expected, algal carbon (C)∶N, biovolume, and N-specific RUE decreased toward the N-rich end of the gradient metaecosystem, whereas the opposite was observed for most of the gradient for C∶P, N∶P, and P-specific RUE. However, at highest N∶P supply, unexpectedly low C∶P, N∶P, and P-specific RUE values were found, indicating that the low availability of P inhibited efficient use of N and biovolume production. Consequently, gradient metaecosystems had lower overall biovolume at the regional scale. Whereas treatment effects on local richness were weak, gradients were characterized by higher dissimilarity in species composition. Thus, the stoichiometry of resource supply and spatial connectivity between patches appeared as decisive elements constraining phytoplankton composition and functioning in metaecosystems

O−O Bond Formation and Liberation of Dioxygen Mediated by N5‐Coordinate Non‐Heme Iron(IV) Complexes

Formation of the O−O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High‐valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O−O bond in solution, from non‐heme, N5‐coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta‐chloroperbenzoic acid is administered in excess. Oxygen‐isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)‐initiated free‐radical pathways of the peroxides, which are typical of catalase‐like reactivity, and iron‐borne O−O coupling, which is unprecedented for non‐heme/peroxide systems. Interpretation in terms of [FeIV(O)] and [FeV(O)] being the resting and active principles of the O−O coupling, respectively, concurs with fundamental mechanistic ideas of (electro‐) chemical O−O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.DFG, 12489635, SFB 658: Elementarprozesse in molekularen Schaltern auf OberflächenTU Berlin, Open-Access-Mittel - 201