TY - JOUR AB - A novel measurement principle for measuring the current distribution in polymer electrolyte fuel cells (PEFCs) is introduced. It allows, in contrast to all other known techniques, for the first time for a resolution smaller than the channel/rib scale of the flow field in PEFCs. The current density is obtained by considering the electron conductors in the cell as a two-dimensional resistor with the voltage drop caused by the current. The method was applied to a cell operated on oxygen up to high current densities. The results show that the ohmic resistances govern the current distribution in the low current regime, whereas mass transport limitations hamper the current production under the land at high loads. AU - Freunberger, Stefan Alexander AU - Reum, Mathias AU - Wokaun, Alexander AU - Büchi, Felix N. ID - 7329 IS - 9 JF - Electrochemistry Communications SN - 1388-2481 TI - Expanding current distribution measurement in PEFCs to sub-millimeter resolution VL - 8 ER - TY - JOUR AB - Polymer electrolyte fuel cells (PE fuel cells) working with air at low stoichiometries (<2.0) and standard electrochemical components show a high degree of inhomogeneity in the current density distribution over the active area. An inhomogeneous current density distribution leads to a non-uniform utilization of the active area, which could negatively affect the time of life of the cells. Furthermore, it is also believed to lower cell performance. In this work, the homogenization of the current density, realized by means of tailored cathodes with along-the-air-channel redistributed catalyst loadings, is investigated. The air stoichiometry range for which a homogenization of the current density is achieved depends upon the gradient with which the catalyst is redistributed along the air channel. A gentle increasing catalyst loading profile homogenizes the current density at relatively higher air stoichiometries, while a steeper profile is suited better for lower air stoichiometries. The results show that a homogenization of the current density by means of redistributed catalyst loading has negative effects on cell performance. Model calculations corroborate the experimental findings on homogenization of the current density and deliver an explanation for the decrease in cell performance. AU - Santis, M. AU - Freunberger, Stefan Alexander AU - Reiner, A. AU - Büchi, F.N. ID - 7330 IS - 25 JF - Electrochimica Acta SN - 0013-4686 TI - Homogenization of the current density in polymer electrolyte fuel cells by in-plane cathode catalyst gradients VL - 51 ER - TY - JOUR AB - A quasi-two-dimensional, along-the-channel mass and heat-transfer model for a proton exchange membrane fuel cell (PEFC) is described and validated against experimental current distribution data. The model is formulated in a dimensional manner, i.e., local transport phenomena are treated one-dimensional in through-plane direction and coupled in-plane by convective transport in the gas and coolant channels. Thus, a two-dimensional slice running through the repetitive unit of a cell from the anode channel via membrane-electrode assembly (MEA) and cathode channel to the coolant channel and from inlet to outlet is modeled. The aim of the work is to elucidate the influence of operating conditions such as feed gas humidities and stoichiometric ratios on the along-the-channel current density distribution and to identify the distinct underlying voltage loss mechanisms. Furthermore, a complicated technical flow field is modeled by a combination of co- and counterflow subdomains and compared with experimental current densities. AU - Freunberger, Stefan Alexander AU - Santis, Marco AU - Schneider, Ingo A. AU - Wokaun, Alexander AU - Büchi, Felix N. ID - 7332 IS - 2 JF - Journal of The Electrochemical Society SN - 0013-4651 TI - In-plane effects in large-scale PEMFCs VL - 153 ER - TY - JOUR AB - A previously developed mathematical model for water management and current density distribution in a polymer electrolyte fuel cell (PEFCs) is employed to investigate the effects of cooling strategies on cell performance. The model describes a two-dimensional slice through the cell along the channels and through the entire cell sandwich including the coolant channels and the bipolar plate. Arbitrary flow arrangements of fuel, oxidant, and coolant stream directions can be described. Due to the serious impact of temperature on all processes in the PEFC, both the relative direction of the coolant stream to the gas streams and its mass flow turns out to significantly affect the cell performance. Besides influencing the electrochemical reaction and all kinds of mass transfer temperature, variations predominantly alter the local membrane hydration distribution and subseqently its conductivity. AU - Freunberger, Stefan Alexander AU - Wokaun, Alexander AU - Büchi, Felix N. ID - 7331 IS - 5 JF - Journal of The Electrochemical Society SN - 0013-4651 TI - In-plane effects in large-scale PEFCs: II. The influence of cooling strategy on cell performance VL - 153 ER - TY - JOUR AB - Phylogenetic relationships between the extinct woolly mammoth (Mammuthus primigenius), and the Asian (Elephas maximus) and African savanna (Loxodonta africana) elephants remain unresolved. Here, we report the sequence of the complete mitochondrial genome (16,842 base pairs) of a woolly mammoth extracted from permafrost-preserved remains from the Pleistocene epoch - the oldest mitochondrial genome sequence determined to date. We demonstrate that well-preserved mitochondrial genome fragments, as long as ∼1,600-1700 base pairs, can be retrieved from pre-Holocene remains of an extinct species. Phylogenetic reconstruction of the Elephantinae clade suggests that M. primigenius and E. maximus are sister species that diverged soon after their common ancestor split from the L. africana lineage. Low nucleotide diversity found between independently determined mitochondrial genomic sequences of woolly mammoths separated geographically and in time suggests that north-eastern Siberia was occupied by a relatively homogeneous population of M. primigenius throughout the late Pleistocene. AU - Rogaev, Evgeny I AU - Moliaka, Yuri K AU - Malyarchuk, Boris A AU - Fyodor Kondrashov AU - Derenko, Miroslava V AU - Chumakov, Ilya M AU - Grigorenko, Anastasia P ID - 854 IS - 3 JF - PLoS Biology TI - Complete mitochondrial genome and phylogeny of pleistocene mammoth Mammuthus primigenius VL - 4 ER -