241 research outputs found
PEMFC MEA and System Design Considerations
Proton exchange membrane fuel cells (PEMFCs) are being developed and sold commercially for multiple near term markets. Ballard Power Systems is focused on the near term markets of backup power, distributed generation, materials handling, and buses. Significant advances have been made in cost and durability of fuel cell products. Improved tolerance to a wide range of system operation and environmental noises will enable increased viability across a broad range of applications. In order to apply the most effective membrane electrode assembly (MEA) design for each market, the system requirements and associated MEA failures must be well understood. The failure modes associated with the electrodes and membrane degradation are discussed with respect to associated system operation and mitigating approaches. A few key system considerations that influence MEA design include expected fuel quality, balance-of-plant materials, time under idle or open circuit operation, and start-up and shut-down conditions
Investigating the effects of proton exchange membrane fuel cell conditions on carbon supported platinum electrocatalyst composition and performance
Changes that carbon-supported platinum electrocatalysts undergo in a proton exchange membrane fuel cell environment were simulated by ex situ heat treatment of catalyst powder samples at 150 #2;C and 100% relative humidity. In order to study modifications that are introduced to chemistry, morphology, and performance of electrocatalysts, XPS, HREELS and three-electrode rotating disk electrode experiments were performed. Before heat treatment, graphitic content varied by 20% among samples with different types of carbon supports, with distinct differences between bulk and surface compositions within each sample. Following the aging protocol, the bulk and surface chemistry of the samples were similar, with graphite content increasing or remaining constant and Pt-carbide decreasing for all samples. From the correlation of changes in chemical composition and losses in performance of the electrocatalysts, we conclude that relative distribution of Pt particles on graphitic and amorphous carbon is as important for electrocatalytic activity as the absolute amount of graphitic carbon presen
X-ray absorption spectroscopy study of diluted magnetic semiconductors: Zn1-xMxSe (M = Mn, Fe, Co) and Zn1-xMnxY (Y = Se, Te)
We have investigated 3d electronic states of doped transition metals in II-VI
diluted magnetic semiconductors, Zn1-xMxSe (M = Mn, Fe, Co) and Zn1-xMnxY (Y =
Se, Te), using the transition-metal L2,3-edge X-ray absorption spectroscopy
(XAS) measurements. In order to explain the XAS spectra, we employed a
tetragonal cluster model calculation, which includes not only the full ionic
multiplet structure but also configuration interaction (CI). The results show
that CI is essential to describe the experimental spectra adequately,
indicating the strong hybridization between the transition metal 3d and the
ligand p orbitals. In the study of Zn1-xMnxY (Y = Se, Te), we also found
considerable spectral change in the Mn L2,3-edge XAS spectra for different
ligands, confirming the importance of the hybridization effects in these
materials.Comment: This paper consists of 22 pages including 4 figures. This paper is
submitted to Physical Review
Li 4 Ti 5 O 12 Nanocrystals Synthesized by Carbon Templating from Solution Precursors Yield High Performance Thin Film LiāIon Battery Electrodes
Nanocrystals of Li 4 Ti 5 O 12 (LTO) have been prepared by processing an ethanolātoluene solution of LiOEt and Ti(OiPr) 4 using a carbon black template. The mechanism of crystal growth has been tracked by SEM and TEM microscopies. The resulting nanocrystals grown using the carbon template (CāLTO) show less aggregation than materials prepared from solution without the template (SāLTO), which is reflected in higher surface area (27 m 2 /g) and concomitantly smaller particle size (58 nm) for CāLTO compared to 20 m 2 /g and 201 nm for SāLTO. Electrochemically, thināfilm electrodes composed of CāLTO demonstrate reversible cycling, storing ā¼160 mAh/g at both 1 C (175 mA/g) and 10 C current. Important is that resistance to charge transfer between the CāLTO nanocrystals and added conducting carbon is 3 times smaller than that for SāLTO. Accordingly, CāLTO shows excellent rate capability, maintaining an energyāstorage capacity >150 mAh/g even at 100 C current. These characteristics solidify CāLTO a suitable replacement for carbon as a Liāion battery anode. High power Li 4 Ti 5 O 12 (LTO) nanocrystals can be synthesized by a carbonātemplating method for Liāion battery electrodes . These electrodes demonstrate reversible cycling of 160 mAh/g at both 1 C and 10 C current, and remains above 150 mAh/g even at 100 C.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98313/1/aenm_201200964_sm_suppl.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/98313/2/753_ftp.pd
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