Innovative Elektrokatalyse

6. Dezember 2009 by j.jensen

Platinfreie Kathodenkatalysatoren für Brennstoffzellen

Das elektrochemische Prinzip der Polymer-Elektrolyt-Membran Brennstoffzelle (PEM-FC) stellt zurzeit die effektivste Methode dar,
um die im Wasserstoff gespeicherte Energie in Elektrizität zu wandeln.
Daher wird der Brennstoffzelle ein bedeutender ökologischer und ökonomischer Stellenwert in der stetig wachsenden regenerativen Energiewirtschaft vorhergesagt. Der breiten Markteinführung der PEM-FC stehen unter anderem allerdings noch die hohen Kosten der darin benötigten Edelmetall-Katalysatoren entgegen.
Im vorliegenden Buch wird die Entwicklung einer neuartigen Klasse preisgünstiger Katalysatoren für die Sauerstoffreduktion an der Kathode behandelt, die auf in Kohlenstoff eingebundenen molekularen Kobalt- bzw. Eisen-Zentren beruht. Sowohl die innovativen Präparationsverfahren als auch die physikalisch-chemische Strukturanalyse dieser Elektrokatalysatoren werden vorgestellt und im Detail diskutiert.

Das Buch weist neue Wege für die Elektrokatalyse in der Sauerstoffreduktion auf. Es wendet sich an Forschungsgruppen und Studierende auf dem Gebiet der Natur- und Ingenieurwissenschaften.

  • Verlag: Vdm Verlag Dr. Müller (April 2008)
  • Broschiert: 248 Seiten
  • ISBN: 978-3836476041
  • von Iris Herrmann (Autor)
Innovative Elektrokatalyse

Innovative Elektrokatalyse

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Abstract

Pyrolysed cobalt-tetramethoxyphenylporphyrin-based electrocatalysts (CoTMPP) for the electro-chemical oxygen reduction in 0.5 M H2SO4 have been investigated in the present work. In the process of pyrolysis, a conducting carbon matrix, in which highly active catalytic centres (presumable CoN4) are chemically and electronically integrated, is formed from the precursor. In the present thesis, the formed microscopic and molecular structure of the carbon matrix is characterised by Raman spectroscopy, XRD, XPS, REM, gassorption and thermogravimetric measurements and is correlated with the electrochemical activity. The latter one has been determined by RDE, RRDE and CV measurements.
The formation of carbon matrix takes place in the temperature range from 400 to 550 °C. Graphen layers with an expansion of at least 4 nm, in which graphite nano-particles are embedded, have been found. The electrochemical activity of the samples increases with decreasing size of their graphite nano-crystallites (XRD). Obviously, the noncrystalline areas are more efficient for the oxygen reduction reaction. In the temperature range from 600 to 750 °C, reorganisation of the carbon matrix to smaller graphen layers and larger graphite crystallites is detected, which is related with meaningful decreasing of electrochemical activity. Beside the molecular structure, the pore structure is identified as important factor for the electrochemical activity. The variation of pore structure has been enabled by the application of a new preparation method in presence of structure forming agents (metal oxalates) and shows an unexpected super proportional increasing of activity versus the electrochemical accessible surface. Possible molecular and structural reasons for this effect are discussed in this thesis. The pore analysis reveals that the oxygen reduction is mainly affected by the mesopores (5 – 10 nm). Mesopore surface could be doubled by the addition of sulphur to the precursor mixture. It has been shown that the sulphur modifies the structure forming agent as well as the molecular structure of the carbon matrix. Therefore, the expansion of graphen layers has been increased and the size of graphite crystallites has been reduced. Due to this optimised catalyst design, high activities have been obtained for the first time, which are similar to that of commercial platinum catalyst (10 % Pt/C by Etek). However, the size of the as prepared, highly porous particles (10 to 30 μm) is not suitable for the application in gas diffusion electrodes (GDE) of fuel cells. Hence, the reduction of size is essential in order to obtain high performances.
Therefore, low temperature plasma treatment has been successfully applied to the carbonisation of CoTMPP for the first time and intermediates have been characterised by IR, Raman and UV-Vis spectroscopy. The predominant impact of electronic energy leads to a distinct reduction of particle size. Furthermore sputter technology has been proven its feasibility in order to open up the way to an automatised production of platinum-free GDE. The method has been applied for patent.