Die Normal-Wasserstoffelektrode als Bezugselektrode in der Direkt-Methanol-Brennstoffzelle

Stähler, Markus

Dissertation / PhD Thesis/Book

Die Normal-Wasserstoffelektrode als Bezugselektrode in der Direkt-Methanol-Brennstoffzelle

Stähler, M. (Corresponding author)FZJ*

2006
Forschungszentrum Jülich GmbH Zentralbibiothek, Verlag Jülich
ISBN: 3-89336-428-5

Jülich : Forschungszentrum Jülich GmbH Zentralbibiothek, Verlag, Schriften des Forschungszentrums Jülich. Reihe Energietechnik / Energy Technology 47, V, 96 S. (2006) = RWTH Aachen, Diss., 2005

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Please use a persistent id in citations: http://hdl.handle.net/2128/546

Abstract: Direct methanol fuel cells (DMFCs) are able to directly convert the chemical energy of methanol into electrical energy. The engineering effort required for handling methanol is not great and therefore the design of DMFC systems is much simpler than that of other fuel cell systems. A disadvantage of DMFCs in comparison to hydrogen-driven fuel cells is the low electrochemical efficiency as a result of several physicochemical processes. These processes, important for the conversion of methanol, are coupled and have to be adjusted in the operation of the DMFC. The aim of the present work is the further development of electrochemical measuring methods for investigating the physicochemical processes in the DMFC during operation. Interest is focused on the reference electrode and impedance measurements. The main part of a DMFC is the membrane electrode assembly (MEA). Developments have so far concentrated on analysing the entire MEA consisting of two substrate layers, two microlayers, two catalyst layers and one membrane. Due to the uncertainty about the contribution of each layer to the measured signal, it is very difficult to physically interpret the measured information. Therefore, the first part of this work concentrates on a new procedure for the production of catalyst-coated membranes (CCMs). The CCMs are used in the test cells without substrates or microlayers and therefore the measured signal contains only the contributions of the catalyst layers and the membrane. For electrode-selective measurements it is necessary to split this cell signal into two single electrode signals. For this purpose a special test cell with a third electrode inside the cell, a so-called reference electrode, has been developed. The technique of using reference electrodes in fuel cells is well known. Different simulations have shown that this method involves several sources of errors. Because of these errors it is not clear whether the measured signals are useful or not, and no experiments systematically investigating these errors are known. For this reason, the influence of different errors on the application of reference electrodes and the effect on the measured voltage signal is investigated in the present work. It can be shown that impedance measurements are indispensable for validating the single electrode signals measured with the reference electrode. A precondition for the application of impedance spectroscopy is a mathematical model for the investigated system. An impedance model for the whole frequency range of the DMFC anode catalyst layer was not available and therefore had to be developed. With this model, the reference electrode measurements can be combined with the impedance measurements and this combination enables the measured signals to be validated. The combination of the methods presented here considerably reduces the error potential of reference electrode measurements. Single electrode measurements for the DMFC anode, measured with a hydrogen reference electrode, are demonstrated for small current densities. The problems with these kinds of measurements have not been completely solved, but with the methods presented it is possible to detect the influence of measuring errors and to test the reliability of the measured signals.

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