Electrodeposition of metalloporphyrines for the preparation of electrocatalytically active surfaces

Thorsten Schilling ,  Justus Masa ,  Ayodele O. Okunola ,  Wolfgang Schuhmann ,  Michael Bron 

Ruhr-Universität Bochum, Anal. Chem. - Elektroanalytik & Sensorik (ELAN), Universitätsstr. 150, Bochum 44780, Germany

Abstract
PEM fuel cells (PEM: proton exchange membrane) are since long discussed as alternative energy-supplying system for e.g. household or mobile application. The commercialisation of PEM fuel cells, however, is strongly impeded by the high costs of the electrocatalysts, which mainly consist of carbon supported platinum and its alloys. Thus, cheaper alternatives are under investigation, one of them is carbon materials modified with metal-nitrogen centres, where the metal usually is iron or cobalt. These materials have initially been derived from metalloporphyrines or -phthalocyanines, which exhibit high activity for the oxygen reduction reaction (which constitutes the cathode reaction in fuel cells), albeit with low stability. The stability of these materials can be enhanced, when they are deposited onto carbon black and subjected to a heat treatment step.

Another important issue in current fuel cell research is the utilization of the catalysts within the gas diffusion electrodes needed to obtain high current densities during application. To prepare these, the catalysts are processed in form of a slurry containing additives, which is then sprayed onto a gas diffusion layer (e.g. carbon cloth) or the electrolyte membrane. An alternative to this approach would be, if firstly the gas diffusion electrode is prepared without the active material, and the latter would then be deposited onto this gas diffusion electrode. The advantage here is that the active material will only be deposited at sites which are electrochemically connected and will later participate in the respective electrocatalytic reaction, thus saving costly material.

Having this in mind, we are currently investigating techniques to electrodeposit metalloporphyrines onto carbon-based electrode materials. The employed porphyrines contain Fe, Co or Mn as central metal ions and are deposited via a pulsed potential technique. Substrates include flat glassy carbon surfaces but also layers of CNTs. CNTs have been chosen since they chemically much more stable then the usually employed carbon blacks, which tend to corrode under the conditions of a working fuel cell. Activity for the oxygen reduction reaction is determined by cyclic voltammetry and rotating disc electrode measurements; further characterisation includes e.g. SEM and XPS. It will be shown that electrodeposition of metalloprophyrines onto CNTs enhances their catalytic activity for the oxygen reduction reaction. Furthermore, activity is strongly influenced by the substituents at the macrocyclic ring as well as the nature of the central metal ion, with Mn porphyrines exhibiting activity similar to that of the more frequently used iron porphyrines. Heat treatment of metalloporphyrines when deposited onto CNTs enhances their activity with the optimum heat treatment temperature being in the range of 650 - 850 °C.

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Presentation: Short communication at SMCBS'2009 International Workshop, by Michael Bron
See On-line Journal of SMCBS'2009 International Workshop

Submitted: 2009-09-02 19:36
Revised:   2009-09-28 22:44
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