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Electrochemical investigations

Electrodes were characterized electrochemically using Electrochemical Impedance Spectroscopy (EIS), Cyclic Voltammetiy (CV) and measurements on a rotating disc electrode [17], These measurements were performed in a half cell configuration with a 1 mol H2S04 electrolyte. Also the MEA was measured using EIS.

Cyclic voltametric measurements on polished platium yields two hydrogen adsorption peaks and the potential ranges of hydrogen and oxygen evolution. Measuring the commercial ElectroChem electrodes the hydrogen adsorption peaks are not observed. At a potential more negative than 700 mV the current density shifts to more negative values if one changes from an inert gas saturated electrolyte to a oxygen saturated electrolyte. In this potential range below 700 mV a current caused by the oxygen reduction und limited by diffusion processes, leads to this shift in the current density.

At porous electrodes the electrochemically active surface is higher than the geometric surface. The geometric factor as the ratio of active and geometric surface was determined to be about 40 by EIS measurement using the half cell configuration with sulfur acid

The kinetics of the oxygen reduction and the hydrogen oxidation was studied. For the oxygen reduction on an ElectroChem electrode the kinetic parameters are determined: the exchange current density is about 2 pA/cm2 and the Tafel slope is 92 mV/decade. The kinetic inhibition of the hydrogen oxidation is 3 decades lower, so the anode in a MEA can be used as reference and counter electrode.

For the interpretation of the EIS measurement of the porous electrodes a model is used, which takes cylindric pores into account [18,19]. At current densities lower than 100 mA/cm2 the current-voltage characteristics is dominated by kinetic inhibitions, above 100 mA/cm2 the resistance of the electrolyte leads to a linear increase of the characteristics in addition to a constant amount from the kinetic inhibition.

Conclusions

The platinum concentration in gas diffusion electrodes produced by DLR is constant in the examined depth range and equal to the platinum concentration in the catalyst supporting carbon black. In contrast commercial E-TEK electrodes are covered by a 3 nm PTFE-film. The catalyst supporting carbon blacks contains sulfur, which can be detected by XPS. Laterally resolved surface science methods allow to determine the distribution of the different components of electrodes. The sulfur signal from Nation is superposed by a platinum signal, so the sulfur concentration does not allow a distinction of Nation and PTFE in an electrode. Nafion can be marked by ion exchange with alkaline ions. In order to identify Nafion and PTFE unambigously, the concentration of fluorine and the tracer alkaline must be measured. Thus the distribution of Nafion and PTFE in an electrode can be determined by EDX. Porosimetty measurements yield two pore systems in the electrode, one from the carbon backing and one from the catalyst supporting carbon black. At a rolling process the pore size of the carbon black decreases and the pore size distribution shifts to lower pore radii. The porosimetty allows to determine the pore size distribution of electrodes in a MEA structure as well as that of single electtodes. The protonic conducting polymere yields no additional porosity, so only the pore system of the electrode is determined by the porosimetric measurements. This allows to investigate degradation processes of the electrodes under electrochemical operation. Electrochemical investigations yield insight into informations on the electrochemical kinetics, the active surface and the degree of utilization of the electrodes and the electrochemically relevant pore structure. Additionally it is possible to separate infuences of different parameters on the overvoltage in fuel cells.

Acknowlcdgcmcnts

The authors gratefully acknowledge the financial support of Deutsche Forschungsgemeinschaft (DFG). The surface science studies in this work were performed as part of the Sonderforschungsbereich 270. Further the authors acknowledge the financial support of the State of Baden Württemberg (Wirtschaftsministerium, Fuel Cell Program of Baden Württemberg). The authors thank their collegues for experimental support, mainly D. Bevers for the preparation of the electrodes and Dr. M. v. Bradke for the EDX measurements.

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