1,721,023 research outputs found
Spectroscopic Investigation of Acid Doped Polybenzimidazole as Electrolyte Membrane for Fuel Cells
No full textOne fundamental component of a fuel cell (FC) is the electrolyte, which separates the electrocatalytic active sites of the two electrode configurations. Acid-doped poly(2,2’-(m-phenylene)-5,5’-bibenzimidazole) (PBI) polymers have been studied as membrane materials for the use in High Temperature Polymer Electrolyte FCs. [1]
In the present study, we report on a FT-Raman investigation of poly(2,5-benzimidazole) (AB-PBI) polymer membranes doped with various concentrations of ortho-phosphoric acid. Characteristic Raman spectra with three diagnostic regions and specific peaks have been studied [2]. Moreover, we present very recent NMR results on the PBI material [3]. The information is of fundamental importance in order to elucidate the role of H3PO4 in conductivity mechanisms in polybenzimidazoles.
[1] J. N. Asensio, E. M. Sánchez and P. Gómez- Romero, Chem. Soc. Rev., 2010, 39, 3210-3239.
[2] F. Conti, A. Majerus, V. Di Noto, C. Korte, W. Lehnert, D. Stolten, Phys. Chem. Chem. Phys., 2012, 14, 10022-10026.
[3] F. Conti, S. Willbold, C. Korte, W. Lehnert, S. Mammi, D. Stolten Polymer, 2012, under review
Physicochemical investigation of phosphoric acid doped poly(2,5-benzimidazole) as electrolyte membrane for fuel cells
No full textPhosphoric acid doped polybenzimidazole membranes are a promising candidate for use as electrolyte membrane in high temperature polymer electrolyte membrane fuel cells (HT-PEFC). They present high thermal stability and excellent proton conductivity at low humidity [1]. Although polybenzimidazole membranes are routinely doped with phosphoric acid, few studies on the exact nature of the acid inside the membrane have been published. It is not known whether the phosphoric acid is present as ortho- or pyrophosphoric acid at all operating conditions and how it interacts with the membrane at a high operating temperature of 160 °C.
In this study, we investigated cross-linked poly(2,5-benzimidazole) (ABPBI) membranes obtained from FuMA-Tech GmbH which were doped in 85 % phosphoric acid. Conductivity measurements of the doped membrane under dry conditions show a maximum conductivity at 160 °C. At higher temperatures, the conductivity decreases as the dehydration of the orthophosphoric acid to the less conductive pyrophosphoric acid becomes relevant. Thermogravimetric analyses of pure phosphoric acid and doped membranes help to assess the amount of dehydrated phosphoric acid as well as the content of water and phosphoric acid inside the doped membrane. The thermal signals show two distinct weight losses between 30 °C and 200 °C, whose onset varies with the doping level of the membrane. An explanation for this effect, besides limited heat transfer, could be the bonds formed between the phosphoric acid molecules and the imidazole rings of the polymer. While free phosphoric acid molecules, found in pure phosphoric acid or in highly doped membranes, are free to condensate to pyrophosphoric acid molecules as soon as the activation energy is reached, bonded molecules need more energy to first break the bond and then dehydrate.
To further consolidate these results, the structure of the as-received and phosphoric acid doped membranes were analyzed and compared by means of Raman spectroscopy [2]. They confirm the presence of hydrogen bonds between the phosphoric acid and the imidazole group of the membrane, thus supporting the results obtained by thermogravimetric analysis.
[1] Q. Li, J. O. Jensen, R. F. Savinell, and N. J. Bjerrum, Progress in Polymer Science, 34, 449 (2009)
[2] F. Conti, A. Majerus, V. Di Noto, C. Korte, W. Lehnert, and D. Stolten, Phys. Chem. Chem. Phys., 14, 10022 (2012)
SUMMARY
We present a physicochemical investigation of phosphoric acid doped polybenzimidazole membranes, including conductivity measurements, thermogravimetric analysis and Raman spectroscopy
Diagnostic Raman signals of the interaction polybenzimidazole – phosphoric acid in membrane for Fuel Cells
No full textOne fundamental component of a fuel cell is the electrolyte, which separates the electrocatalytic active sites of the two electrode configurations. Phosphoric acid-doped poly(2,2’-(m-phenylene)-5,5’-bibenzimidazole) (PBI) polymers have been studied as membrane materials for the use in High Temperature Polymer Electrolyte Fuel Cells (HT-PEFC), since they can be used at temperatures as high as 200 °C without humidification. Among the many possible PBI derivatives, a very promising material is poly(2,5-benzimidazole) (AB-PBI). [1-4]
In the present study, we report on a FT-Raman investigation of AB-PBI polymer membranes doped with various concentrations of ortho-phosphoric acid. Characteristic Raman spectra with three diagnostic regions and specific peaks have been studied. The information is of fundamental importance in order to elucidate the role of phosphoric acid in the conductivity mechanism of AB-PBI. [5]
Reference:
[1] J. N. Asensio, E. M. Sánchez and P. Gómez- Romero, Chem. Soc. Rev., 2010, 39, 3210-3239.
[2] C. Wannek, B. Kohnen, H.-F. Oetjen, H. Lippert and Mergel J., Fuel Cell, 2008, 8, 87-95.
[3] C. Wannek, W. Lehnert and J. Mergel, J. Power Sources, 2009, 192, 258-266.
[4] K. Wippermann, C. Wannek, H.-F. Oetjen, J. Mergel and W. Lehnert, J. Power Sources, 2010, 195, 2806-2809.
[5] F. Conti, A. Majerus, V. Di Noto, C. Korte, W. Lehnert, D. Stolten, Phys. Chem. Chem. Phys., 2012, 14, 10022-10026
Raman and NMR study of the interaction Polybenzimidazole – Phosphoric acid in membranes for fuel cells
No full textOne fundamental component of a fuel cell is the electrolyte, which separates the electrocatalytic active sites of the two electrode configurations. Phosphoric acid-doped poly(2,2’-(m-phenylene)-5,5’-bibenzimidazole) (PBI) polymers have been studied as membrane materials for the use in High Temperature Polymer Electrolyte Fuel Cells (HT-PEFC), since they can be used at temperatures as high as 200 °C without humidification. Among the many possible PBI derivatives, a very promising material is poly(2,5-benzimidazole) (AB-PBI) [1]
In the present study, we report on FT-Raman and NMR investigations of AB-PBI and PBI polymer membranes doped with various concentrations of ortho-phosphoric acid. Characteristic Raman and 1D- and 2D-NMR spectra with diagnostic signals have been studied. [2, 3] Ab-initio calculations were carried out using density functional theory methods to analyse the Raman data. The information is of fundamental importance in order to elucidate the role of phosphoric acid in the conductivity mechanism of membranes based on polybenzimidazole.
[1] J.N. Asensio, E.M. Sánchez and P.Gómez- Romero, Chem. Soc. Rev.39 (2010) 3210.
[2] F. Conti, A. Majerus, V. Di Noto, C. Korte, W. Lehnert, D. Stolten, Phys. Chem. Chem. Phys. 14 (28) (2012) 10022.
[3] F. Conti, S. Willbold, C. Korte, W. Lehnert, S. Mammi, D. Stolten, Polymer (2012) submitte
NMR investigations of acid doped polybenzimidazole, an electrolyte membrane for fuel cells
No full textFuel cells (FCs) are devices that convert the chemical energy of their reagents into electrical energy without resorting to a thermal Carnot cycle. Polymer electrolyte membrane FCs (PEMFCs) have raised considerable interest for their high energy conversion efficiency, high power density, ease of assembly, silent operation, and good environmental compatibility. Fuel cells consist mainly of an anode, a cathode and a membrane acting as electrolyte and which allows charges to selectively move between anode and cathode. Acid doped polybenzimidazole (PBI) polymers have become a very promising system for use as membranes in high temperature PEMFCs.
The conductivity in FCs is well known to be directly related to the mobility of the ions through the membrane, which might involve a multi-step proton hopping process (Grotthuss mechanism) or the bulk mobility of a proton carrier.
We report investigations of poly(2,5-benzimidazole) (AB-PBI) doped with phosphoric acid (PA) with different methods of NMR [1]. We have characterized the molecular interactions between the polymer matrix and the acid dopant. Heteronuclear correlation experiments were used to study hydrogen bonding characteristics. We have further studied the dynamics and order in the system at different doping levels and temperatures using 2H-NMR and 1H-Double-quantum spectroscopy. This information is of fundamental importance in order to elucidate the role of phosphoric acid in the conductivity mechanism of polybenzimidazole [2, 3].
[1] F. Conti, S. Willbold, C. Korte, W. Lehnert, D. Stolten, S. Mammi, Chem. Commun. 2012, submitted.
[2] C. E. Hughes, S. Haufe, B. Angerstein, R. Kalim, U. Mähr, A. Reiche, M. Baldus, J. Phys. Chem. B., 2004, 108, 13626-13631.
[3] S. Suarez, S. Greenbaum, The Chemical Record, 2010, 10, 377-393
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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