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Modeling Membrane Degradation in Proton-Exchange-Membrane Fuel Cells
Get PDFDuring operation, proton-exchange-membrane fuel cells (PEMFCs) are subjected to mechanical and chemical stressors that contribute to membrane degradation, performance loss, and eventual failure. Together, synergistic effects between mechanical and chemical degradation mechanisms lead to accelerated degradation. A physics-based model is developed to understand the synergistic effects of chemical and mechanical degradation and the coupled nature of performance and durability in PEMFCs. The model includes pinhole existence and growth in the membrane, which increases crossover of reactant gases as well as subsequent formation of chemical-degradation agents that impact both transport and mechanical properties of the membrane. The underlying performance model accounts for the multi-component gas diffusion, reaction kinetics, and transport across the membrane. The membrane mechanical model assumes that a circular pinhole is present in the membrane and calculates the swelling strains and the elastic or plastic stresses on the pinhole. Additionally, an empirical model for the chemical degradation of the membrane via hydrogen peroxide and subsequent hydrogen fluoride generation is used to modify the mechanical properties as a result of chemical degradation. The fuel-cell model is fully coupled with a mechanical model to determine the stresses on the membrane and subsequent growth of pinholes during transient operation. Simulation results show pinhole growth under humidity cycling conditions and an increase in gas-crossover fluxes and decrease in performance. Sensitivity studies show how the membrane mechanical properties impact both the performance and degradation behavior of the membrane.Multiphase effects are incorporated into the model to account for the effects of flooding on membrane degradation. Liquid condensation in the fuel cell can cause defects such as pinholes to close. Modeling results analyze the conditions under which water condensation will occur in pinholes, which is determined by calculating the critical radius. As the surface of Nafion can change from hydrophobic to hydrophilic, a sensitivity analysis on the critical angle is carried out. In addition, liquid water also reduces the amount of catalyst surface area available and therefore slows down the formation of hydrogen peroxide that drives chemical degradation. The decrease in chemical degradation at high RH values is demonstrated.Cerium ions are added to the membrane to extend its lifetime by scavenging radicals produced by crossover of reactant gases during PEMFC operation. The cerium ions also lead to a decrease in performance due to changes in the PEM transport properties and possible site blockage in the catalyst layers. The PEMFC performance and durability model is extended to include micro-kinetic framework that accounts for gas-crossover-induced degradation and concentrated-solution theory describes transport in the PEM. The transport model takes into account the coupled nature of the electrochemical driving forces that cause transport of cerium ions, protons, and water. The cell model predicts the migration of cerium out of the membrane and into the catalyst layers and its impact on performance. A comparison of dilute-solution-theory and concentrated-solution-theory approaches illustrates the interactions between water and cerium transport in the cell. Transient simulations show that low concentrations of cerium in the membrane (less than 1% of membrane sulfonic acid sites occupied by cerium) are required to optimize these design tradeoffs.Combining the mechanical and chemical degradation models, the mitigation effects of cerium on the coupled degradation methods can be shown. The model results show how the presence of a pinhole in the membrane shifts the distribution of cerium in the cell from the cathode into the anode and membrane. As the presence of cerium slows down the chemical degradation rate of the membrane, the rate of change of the mechanical properties of the membrane decreases. The model also shows how cerium modifies the mechanical and chemical degradation rates of the membrane under humidity- and voltage-cycling conditions.Finally, three approaches for modeling the electrochemical impedance response of a PEMFC are compared using two case studies: a porous electrode with linear kinetics and a fuel cell cathode with Tafel kinetics. These approaches may be applied to the development of a physics-based electrochemical impedance model for a full fuel cell model. The first approach uses a transient-model approach, which is much slower and more prone to errors. However, this approach requires no additional modification of the time domain equations describing the system. The second and third approaches transform the transient model into frequency space and linearizing around the steady-state conditions. This approach is quick and accurate, but is impractical for highly coupled, nonlinear systems of equations. This approach applied to the fuel cell cathode with Tafel kinetics allows for analysis of system properties that change over time as a result of membrane degradation
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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