In-situ x-ray absorption experiments with a PEM fuel cell in hydrogen and methanol operation mode

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In-situ x-ray absorption experiments with a PEM fuel cell in hydrogen and methanol operation mode

Th. Buhrmester, C. Roth, N. Martz, H. Fuess

University of Technology Darmstadt, Department of Materials Science, Structure Research, Petersenstrasse 23, D-64287 Darmstadt, Germany Fax: +49 6151 16-60 23; E-mail: [email protected] ABSTRACT In-situ XAS studies have been carried out on PEM fuel cells in methanol and hydrogen operation. This has become possible due to a recently developed PEM fuel cell (single cell) equipped with inherent carbon fibre windows to allow x-ray studies in transmission geometry. The set-up chosen allows the in-situ monitoring of the structural changes of the Pt-Ru catalyst, utilised as electrode active material, during cell operation. The analysis of the white line intensities, edge shifts of the Pt-LIII edge and the XAFS signal, which was modelled for the first co-ordination shell around Platinum, exhibited no significant changes during operation in terms of the first neighbouring shell. Nevertheless, the white line intensity decreased comparing the ex-situ to in-situ measurements. From the latter, can be concluded that the catalyst is reduced during operation and further, the redox behaviour of the catalyst does not change the local environment of the Pt-centres in the first co-ordination shell to a measurable extend. INTRODUCTION Pt-Ru systems on carbon support are currently regarded as the state of the art anode catalysts in polymer electrolyte membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC). The performance, the life time and especially the CO tolerance of such cells can somehow be controlled by additional parameters (i.e. particle size distribution, degree of alloying) during manufacturing. Accordingly, this system was under manifold ex-situ [1,2] and in-situ [3,4,5,6,7] investigation. Catalysts have been characterised by different groups and different methods before and after cell operation but still these so called quenched situations can only be assumed to exist during operation and are not representative for a real in-situ measurement. This view is also supported by the work of O’Grady et al. [6], and Russell et al. [7]. To closely relate the behaviour of a PEMFC (E/i-curves) to the catalyst performance, the structural features of the material have to be monitored in-situ accordingly. So far, only a few investigations under fuel cell relevant conditions have been published [6,7], and to our knowledge even only one in-situ study in reformate-air operation by Viswanathan et al [8,9]. This work is in line with the results of our in-situ XAFS experiments in hydrogen and methanol operation, which will be presented in this paper. EXPERIMENTAL DETAILS The membrane electrode assemblies (MEA) were produced by utilising a conventional spraying method [10]. Three (MEAs) have been prepared at the ZSW in Ulm by applying a metal loading of approx. 1.2 mg cm-2 metal per electrode on Nafion©117 used as proton conducting membrane. This comparatively high metal load