Surface Modification of Carbon Materials for Electrocatalysis: From Catalyst Development to the Hierarchical Build-up of Electrode Structures

Michael Bron 

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

Abstract

Despite the huge progress in the field of polymer electrolyte membrane fuel cells (PEM-FCs) during the last two decades, there are still some drawbacks, which have to be overcome in the course of fuel cell commercialisation. One of these is related to electrocatalysis: Typical electrocatalysts for PEM-FCs consist of nanoparticles of Pt or its alloys, which are deposited onto a conducting carbon black. While Pt is still the most active metal for the cathode process in PEM fuel cells, namely the oxygen reduction reaction (orr) in acid environment, it is rather expensive, and still exhibits large overvoltages for the orr. Alternative approaches are therefore pursued in the development of active and stable electrocatalysts. One of these alternatives, which will be discussed in this presentation, uses iron-nitrogen centres incorporated into the surface of a carbon black. Such surface-modified carbons can prepared from a variety of precursors, like metalloporphyrins and -phthalocyanines, metal salts together with acetonitrile or metal complexes of e.g. phenanthroline. These precursors are deposited onto the carbon and submitted to an appropriate heat treatment step at 500 to 900 °C, in which the active electrocatalyst forms. The properties of the carbons used in the preparation, like surface area and nitrogen content, strongly influence the activity of the derived electrocatalysts. Recently, we have successfully modified carbon nanotubes with catalytically active centres. We were able to show, that the electrocatalytic activity of these nanotubes depends on their structural parameters (length and diameter). However, catalyst development for fuel cells can not just focus on catalyst activity. Further important points are the investigation of catalyst selectivity (i.e., the ratio of water to H2O2 formation) as well as the overall electrode structure of the so called gas diffusion electrodes, which is rather complex and includes at least two layers (catalyst layer as well as gas diffusion layer) and additives. In order to optimise electrode structure, a hierarchical approach would be desired, in which an appropriate substrate is modified step by step to build up a gas diffusion electrode, e.g. by the growth of carbon fibers and tubes and subsequent deposition of electroactive species like Pt nanoparticles or electropolymerized metalloporphyrines. First steps in this direction are currently carried out in our laboratories. The methods we use for investigating the catalytic active materials include RDE (rotating disc electrode), impedance and SECM (scanning electrochemical microscope) measurements.

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Presentation: Keynote lecture at SMCBS'2007 International Workshop, by Michael Bron
See On-line Journal of SMCBS'2007 International Workshop

Submitted: 2007-07-17 18:15
Revised:   2009-06-07 00:44
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