Characterization And Basic Research Investigations At Pefc Electrodes And

M. Schulze, N. Wagner, G. Steinhilber, E. Gülzow, M. Wöhr, K. Bolwin

Institut für Technische Thermodynamik Deutsche Forschungsanstalt für Luft- und Raumfahrt (DLR) Pfaffenwaldring 38-40, D-70569 Stuttgart, Germany

Introduction

For the study of electrochemical and transport mechanisms in polymere electrolyte fuel cells (PEFC) electrodes and for a further development of PEFC electrodes it is important to characterize these electrodes. The characterization of the electrodes was performed by electrochemical analytical as well as physical methodes on both single electrodes and electrode-membrane assemblies (MEA). In addition to voltage-current characteristics the electrodes were electrochemically measured by cyclic voltammetry, electrochemical impedance spectroscopy and chronopotentiometry. To determine the pore systems nitrogen adsorption and mercury porosimetry were used. Chemical composition and microstructure of the electrodes were studied by surface science methodes like scanning electron microscopy or X-ray induced photoelectron spectroscopy. The results of characterization are the base for theoretical simulation of fuel cells and fuel cell stacks.

Surface Science Methods

The X-ray Photoelectron Spectroscopy yields quantitive informations on concentration of elements on surfaces and the binding states of these elements. Up to lOOnm depth profiles can be recorded by measuring XP-spectra and ion etching the surface alternatingly. We have studied gas diffusion electrodes which were produced by DLR [1, 2] and commercial E-TEK electrodes. The main components of the surface are carbon and fluorine from carbon black and PTFE. Additional platinum from the platinized carbon black, oxygen and sulfur are detected in the XP-spectra of the electrodes. In the electrode produced by DLR the platinum concentration is equal to the platinum concentration in the used carbon black and keeps constant during depth profiling. In depth profiles of DLR produced electrodes XP-spectra show only a decomposition of the polymers [3], which is induced by X-ray exposure and ion bombardment. E-TEK electrodes show different results in depth profiling. The surface of E-TEK is covered by a PTFE-film with a thickness of 3 nm; so the platinum concentration in these electrodes increases at the begining of depth profile measurements.

In all electrodes sulfur can be observed in a concentration of 0.2 - 0.4 %. This sulfur concentration is also measured in the carbon black; sulfur from Nafion in these electrodes gives only a minor portion of the total sulfur signal.

In polymer electrolyte fuel cells (PEFC), Nafion is used as the electrolyte. Its contact with the cell electrode also enlarges the three-phase boundary between the catalyst, gas and proton-conducting membrane. Nafion is inserted in electrodes to enlarge the three phase boundary, which is relevant for the reaction [4-6], The distribution of Nafion is critical to optimize the electrodes. Addition of Nafion powder to the electrode during preparation is an solvent free procedure [7], The electrodes are produced by a rolling process, which is described in literature [2, 8-12], These electrodes consist of platinated carbon black, polytetrafluorethylene (PTFE) as an organic binder and Nafion. The particle size of the used pulverized Nafion is some pm [13], The distribution of the different components of the electrodes is important for their operation behavior. The structure of electrodes was investigated by Scanning Electron Microscopy (SEM). The SEM images provide no information about the materials of the particles. The Energy Dispersive X-ray spectroscopy (EDX) allows to study the element distribution on the electrode surface. Since Nafion is chemically related to the polytetraflourethylene (PTFE) used as binder in the electrode, distinction between both is only possible through observation of sulfur peaks, obtained by analysis of the electrodes using surface science methods. By means of EDX analysis one is able to determine the local distribution of elements. Using these methods however, platinum in the electrodes give an X-ray signal, which is superposed to the sulfur signal. Therefore it is a problem to distinguish the two elements. Sulfur seems to be detected at those places where platinum is present. The second problem of Nafion detection by the sulfur signal is, that the sulfur concentration in Nafion is not significantly higher then in the carbon black. Figure 1 shows a micrograph of the cross section and the EDX mappings for carbon, sulfur, platinum and fluorine of a Nafion powder containing electrode. Light areas in the element maps indicate a high concentration of the elements, dark areas a low concentration. Thus sulfur can not be used to distinguish Nafion from PTFE. Further Scanning Auger Electron Spectroscopy (SAES) is not able to distinguish between Nafion and PTFE, since the same problem arises- the superposition of Auger electrons from sulfur and platinum.

In order to investigate the electrodes by these methods, the Nafion must be marked somehow. One possibility is to exchange the conducting protons in the Nafion with alkali ions. Through the choice of alkali ions, one is able to choose a specific area of the spectra which does not coincide with signals ftom other electrode components; sodium is a good tracer-candidate for the surface science methods. Having marked Nafion with alkaline ions the distribution of Nafion in a fuel

Fig. 1: SEM-image of a gas diffusion electrode for PEFC and EDX-mappings for the elements: C, S, Pt, F and Na after marking Nafion by Na

cell electrode can be measured. For this, the element distribution of fluorine and the alkaline metal must be determined.

Secondly the membrane was characterized by XPS to determine the element composition. Sulfur can be identified by XPS measurements, however the spatial element distribution on the surface can not be determined. After ion exchange sodium produces the Nals-signal at a binding energy of about Eb>'ai. = 1075 eV in the XP-spectrum. The sulfur signal is observed at a binding energy of Et,,s2p = 170 eV.

Figure 1 also shows the element distribution of sodium on the electrode after ion exchange additionally. In the SEM image a Nafion particle is labeled with a. At this position a concentration of fluorine and sodium is detected. The particle labeled with b consists of PTFE; a concentration of fluorine is found, but no concentration of sodium. By marking Nafion with alkaline ions, Nafion and PTFE in an electrode can be distinguished by the sodium concentration using EDX mapping. However sodium concentrations not only exist on Nafion particles. Sodium is also concentrated at platinum particles. For a reliable identification of Nafion the distribution of sodium and fluorine has to be measured.

Porosimetry

For the transport mechanism and the structure of the electrodes the pore systems are significant. Using nitrogen adsorption and mercury porosimetry [IS, 16] the pore systems of membrane electrode assemblies, single electrodes and of their components are measured. The electrodes consist of the catalyst supported carbon black and a carbon backing. Electrodes produced by a rolling process at DLR [1, 2, 7], wet chemically prepared electrodes and commercial electrodes from E-Tek and GDE have been measured. In addition the pore system of membrane-electrode-assemblies (MEA) was measured.

Two different pore systems have been found; the carbon backing has a porosity with a pore radius of about 10 to 30 pm, the catalyst supporting carbon black has a pore system in the range of 7 to 30 nm pore radius. The porosimetry measurement of the supporting carbon black shows two maxima in the pore size distribution. The maxima in the pore size distribution for the supporting carbon black are at pore radii of about 10 nm and 20 nm.

For all electrodes the pore systems of the carbon backing and the catalyst supporting carbon black can be observed. Preparing the electrodes wet chemically or by the rolling process the pore system of the carbon backing in not changed. The pore system of the carbon black is unchanged at wet chemical preparation, but the pore size of the carbon black decreases at the rolling process.

A rolled electrode has more narrow pores then the catalyst supporting carbon black. The two maxima in the pore size distribution of the carbon black shift 2-4 nm to lower pore radii. This compression of the pores is been observed also on commercial electrodes, which were rolled at DLR to study the influence of the rolling process.

BET measurements on MEA shows the pore systems of the applied electrodes. The protonic conducting polymere yields no additional porosity, so only the pore system of the electrode is determined by the porosimetric measurements. The porosimetry allows to measure the pore size distribution of electrodes in a v MEA structure as well as that of single electrodes.

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