Comparison of Porous Gas Diffusion Electrodes Obtained By Different Support Morphologies Using FIB-SEM Tomography

Abstract

Porous gas diffusion electrodes (GDE) are an integral part of polymer electrolyte fuel cells (PEMFC) and of significant importance for their performance. State-of-the-art electrodes are manufactured using a mixture of carbon-supported platinum nanoparticles and conducting ionomer (e.g. NafionTM). To reach maximum performance with as low noble metal content as possible, a high amount of triple phase boundary is needed. Thus, an ideal GDE would combine an improved accessibility of the noble metal with a good electronic contact of the ion- and electron-conducting component. However, large-scale electrode manufacturing techniques, such as airbrush, decal or sieve-printing, are limited in the control of the final electrode structure. Electrode structuring has so far either been performed by changing the amount of contents (e.g. NafionTM, Pt-loading) (Litster & McLean, 2004) or by use of different fabrication techniques (Zils et al., 2010).Another strategy to modify the electrode structure is to use support materials with different morphologies. However, one has to ensure that the morphology does not affect the properties, as for instance the fraction of graphitic carbon in the material or the functional groups. Different carbon materials like glassy carbon, graphite or carbon nanotubes also have an impact on platinum deposition, which influences the performance more than the electrode structure does (Sevjidsuren et al., 2010). In this case, a systematic comparison is not possible.In the present approach, polyaniline with different morphologies was synthesized. We produced long fibers, short fibers and a granular material. FT-IR measurements verified the chemical identity of the material. These materials were decorated with platinum nanoparticles having nearly the same size distribution. A subsequent heat treatment up to 750°C converted the PANI into a nitrogen-containing carbon support, with homogeneously distributed platinum nanoparticles having a mean diameter of roughly 3 nm for all three morphologies.Before using these novel catalysts as cathode materials in PEMFC, XRD, TEM, SEM and BET analysis were performed. The final carbon support materials only differed in their morphology. Thus, we were able to attribute the observed differences in fuel cell performance to the shape of the support material. For further insights, FIB-SEM tomography was performed on these electrodes revealing significant differences in the structure of the porous electrode. The pore structure generated by the long fibers provides the highest performance, while short fibers still provide better performance than granular material (see Figure 1). A first reconstruction of a volume of roughly 4x4x4 µm³from the cathode manufactured of the long fibers is depicted in Fig. 2.Litster, S., & McLean, G. (2004). PEM fuel cell electrodes. Journal of Power Sources, 130(1-2), 61–76. doi:10.1016/j.jpowsour.2003.12.055Sevjidsuren, G., Zils, S., Kaserer, S., Wolz, a., Ettingshausen, F., Dixon, D., … Ganzorig, C. (2010). Effect of Different Support Morphologies and Pt Particle Sizes in Electrocatalysts for Fuel Cell Applications. Journal of Nanomaterials, 2010, 1–9. doi:10.1155/2010/852786Zils, S., Timpel, M., Arlt, T., Wolz, a., Manke, I., & Roth, C. (2010). 3D Visualisation of PEMFC Electrode Structures Using FIB Nanotomography. Fuel Cells, 10(6), 966–972. doi:10.1002/fuce.201000133