1,721,150 research outputs found
Uptake of protic electrolytes by polybenzimidazole-type polymers – Model for the absorption isotherm and electrolyte/polymer interactions
No full textPhosphoric acid doped polybenzimidazol (PBI) membranes are commonly used as proton
conducting membrane in high temperature polymer electrolyte fuel cells (HT-PEFC). In this
study we want to present a general (thermodynamic) model for the absorption process of
protic electrolytes by PBI-type polymers, which is able to describe the whole accessible
doping range. We have analysed literature and own data on the uptake of phosphoric, sulphuric
and perchloric acid by non-cross-linked m-PBI [1, 2] and AB-PBI [3] and by a commercial
cross-linked PBI derivative (Fumapem AM-55©). The uptake of protic electrolytes
by PBI-type polymers can be described satisfactorily by a BET-like adsorption isotherm
(GAB model), assuming a multilayer-like adsorption process.
In addition to the thermodynamic data spectroscopic data from Raman studies are taken
into account [4]. It is possible to correlate domains in the adsorption isotherms to the protonation
of the polymer chains, the formation of H-bonds directly to the chains and to still absorbed
electrolyte molecules
Investigations on the H3PO4-Uptake of Polybenzimidazole type Polymers using RAMAN Spectroscopy
No full textElectrical conductivity and water effects in phosphoric acid solutions for doping of membranes in polymer electrolyte fuel cells
Full text linkFuel cells (FCs) are among the more efficient solutions to limit the emission of greenhouse gases. Based on the conversion of the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electrical energy, a typical FC produces a voltage of 0.7 V under load. The potential is highly increased by placing the cells in series to obtain a stacked cell. Among the types of FCs, the polymer electrolyte membrane FCs (PEMFCs) are developed mainly for transport applications, because of their low impact on the environment, high power density and light weight compared with other types of FCs. Phosphoric acid (H3PO4) doped polybenzimidazole (PBI) membranes are widely used as efficient electrolytes. The performance of a (high temperature, 130–200 °C) HT-PEMFC depends mainly on the amount of H3PO4 in the solid polymer membrane. The strong autoprotolysis of H3PO4 is res-ponsible for the high proton conductivity also in the anhydrous state. In this study, the H2O-H3PO4 system is investigated in the temperature range 60–150 °C with varying water vapour activity at constant atmospheric pressure. Main purpose is to gain more insights into the kinetics of the equilibria in the H2O-H3PO4 system, which influence the fuel cell performance. Density, water content, electrical conductivity and activation energy are determined by exposing H3PO4 solutions for sufficiently long periods to controlled gas atmosphere in order to reach near-equilibrium conditions. The coexistence of ortho- and pyrophosphoric acid is analysed and higher condensed species are also considered. A new setup fully made in quartz is designed and developed to mix the phosphoric acid solutions in a climate chamber. The experimental results are compared to literature data to validate the developed setup and the methodology
The water vapor equilibrium in the phosphoric acid – water system
No full textFor high temperature polymer electrolyte fuel cells (HT-PEFCs), phosphoric acid (PA, H3PO4) is still the state-of-the-art electrolyte material, since it shows excellent proton conductivity [1] in the temperature range between 373 K and 473 K. Another advantage is the chemical compatibility with polymers which is exceptional considering the acidity of the acid (pH < 0 unless highly diluted). However, care has to be taken that strongly dehydrating conditions have to be avoided when using PA in HT-PEFCs, since enhanced degradation of the catalyst [2] appears for these conditions. This can be explained with the formation of PA condensates [3], which are formed at low water contents and high temperatures, typically above 390 K. It is therefore crucial to avoid the formation of PA condensates by adjusting the operating conditions of a HT-PEFC.
In this work, PA is exposed at a constant water vapor pressure of 10 kPa until equilibration is reached at different temperatures in the range between 333 K and 433 K. The equilibration time and the evolution of the electrical parameters during the equilibration process are discussed with respect to the water content at the end of the tests. The results not only show the formation of PA condensates for temperatures above 413 K, but also indicate the existence of a local PA hemihydrate structure occurring in the liquid before condensation occurs
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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
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