1,720,963 research outputs found
Electrical conductivity and chemical equilibria of the phosphoric acid – water system at HT-PEM fuel cells relevant condition
No full textHigh temperature polymer electrolyte membrane (HT-PEM) fuel cells typically work at 120-200°C and are mainly based on phosphoric acid (PA) swollen basic polymer membranes like phosphoric acid doped polybenzimidazole. An overview can be found in [1-7]. Although PA is a widely used material even outside the field of electrochemical transformers, only little is known about the thermodynamical but also physical properties at temperatures above 100°C. Especially the correlation between different parameters provided from different sources in literature is often demanding due to different experimental approaches.
In this work, an alternative approach is used simulating directly the conditions inside an operational HT-PEM. The obtained physical and thermodynamic data are critically compared to results from other laboratories
Investigations on the H3PO4-Uptake of Polybenzimidazole type Polymers using RAMAN Spectroscopy — Correlations between Adsorption Process and Electrolyte - Polymer Molecular Interactions
No full textAn important application of phosphoric acid doped polybenzimidazol (PBI) is the use as proton conducting electrolyte membrane in high temperature polymer electrolyte fuel cells (HT-PEFC). At typical operation temperatures of 120 to 200 °C and very low humidity ionic conductivities of 10-1 to 10-2 S cm-1 can be measured.
The H3PO4-doping process of different PBI-type materials were still investigated in se¬ve¬ral experimental studies, e.g. [1,2]. Unfortunately, no general applicable model for the kinetic of the adsorption process available which is able to describe the whole acces¬sible range of doping degrees has been published yet. The correlation between the interactions between H3PO4 molecules and polymer chains, the ad-sorp¬tion iso¬therm as well as the polycondensation equilibria of H3PO4 and the corresponding implications on the pro¬ton conductivity are also not finally illuminated.
We have investigated the ad¬sorption process of H3PO4 on a com¬mer¬cial cross-linked PBI deri¬vative (Fuma¬pem AM-55). A num¬ber of mem¬bra¬nes has been pre¬pared at dif¬fe¬rent do¬ping levels and analysed to elucidate the ad¬sorp¬tion process of H3PO4 as func¬tion of temperature and con¬cen¬tra¬tion. Karl-Fischer-, pH-titration and RAMAN spec¬tro¬scopy are used to cha¬rac¬terise the mem¬branes [3,4]. The ad¬sorp¬tion equi¬li¬bria of the up¬take pro¬cess have been ana¬ly¬sed with dif¬fe¬rent ki¬ne¬tic mo¬dels for own and for lite¬ra¬ture data on non-cross¬lin¬ked m-PBI and AB-PBI. The be¬ha¬viour of all PBI-type poly¬mers can be de¬scri¬bed sa¬tis¬fac¬torily with a BET-like ad¬sorp¬tion iso¬therm. Using the RAMAN data, re-gions in the isotherm can be cor¬re¬la¬ted to the pro¬tonation of the poly¬mer chains, for¬ma¬tion of H-bonds di¬rec¬tly to the chains and to still adsorbed H3PO4 mole¬cules.
[1] Q. Li et al., Solid State Ionics, 2004, 168, 177-185 [2] J. A. Asensio et al., Chem. Soc. Rev., 2010, 39, 3210-3239 [3] F. Conti et al., submitted to Fuel Cells, Jan. 2014 [4] C. Korte et al., to be submitted 201
Adsorption process of phosphoric acid on polybenzimidazole membranes: a crucial step inside operational high temperature PEM fuel cells
No full textHigh temperature polymer electrolyte fuel cells (HT-PEFC) typically work at 120-200°C and are mainly based on phosphoric acid (PA) swollen basic polymer membranes like poly¬benzimidazole (PBI). Although PA doped PBI membranes were investigated in several ex¬perimental studies, the kinetic of the adsorption process, the mo¬lecular interactions bet¬ween the PA molecules and the polymer chains, poly¬con¬densation equilibria of the PA molecules as well as the implications on the proton conductivity is not finally illuminated.
In this work, we have investigated the adsorption process of PA on m-PBI and a commercial AB-PBI derivative (Fumapem AM-55). A number of membranes (cross and uncross-linked) has been prepared at different doping level and analysed to elucidate the adsorption process of the PA as function of temperature, acid concentration and chemical equilibria between PA derivatives. Impedance measurements, Karl Fischer titration method and RAMAN spectra have been used to characterise the membranes. The thermodynamic aspects related to the adsorption process have been analysed with different kinetic models. Considering own and literature data on non-crosslinked m-PBI, a BET-like adsorption kinetics explains the behaviour of this polymer type satisfactorily. Using the RAMAN data, different regions in the BET-like isotherm can be correlated to the protonation of the polymer chain, formation of H-bonds directly to the chain and indirectly to still adsorbed PA molecules
Liquid – gas phase equilibria of phosphoric acid at high temperature electrolyte polymer fuel cell condition
No full textPhosphoric acid is used in most high temperature polymer electrolyte membrane fuel cells (HT-PEFC) especially because it exhibits high proton conductivity together with high availability. The nature of the proton conductivity in phosphoric acid and the interaction between the polymer matrix and phosphoric acid were investigated recently by MD simulations [1]. Experimental conductivity data are available extensively for temperatures up to 100°C and different water contents, while it gets scares for higher temperatures [2, 3]. The reason for this is the complex nature of the phosphoric acid, tending to polymerize from its monomeric form at low temperature and surplus of water towards chain or even ring structures at temperatures above 100°C with depletion of water [4]. However, the different species have different chemical but also physical properties. This circumstance is either neglected or not considered for most examinations.
In contrast to most published methods starting with P2O5 and defined water content, we start from aqueous orthophosphoric acid (85w%) and impose a fixed water vapour pressure using a climate chamber. The development of the equilibrium state with the gas phase is investigated using time sequenced impedance measurements. Temporal snapshot samples are taken and examined using Karl-Fischer titration method to determine the content of unbound water and Raman spectroscopy to determine changes in molecular interactions. The transitions between the different species and adducts of phosphoric acid can be precisely determined
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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