Structure and conductivity of fuel cell membranes and catalytic layers investigated by AFM
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Structure and conductivity of fuel cell membranes and catalytic layers investigated by AFM Renate Hiesgen1*, Tobias Morawietz1, Michael Handl1, K. Andreas Friedrich2 1 University of Applied Sciences Esslingen, Kanalstrasse 33, 73728 Esslingen, Germany 2 German Aerospace Center, Institute of Engineering Thermodynamics, Pfaffenwaldring 38-40, 70569 Stuttgart, Germany * Author to whom correspondence should be addressed: [email protected] ABSTRACT In this work, the structure and conductive structure of perfluorinated sulfonated ionomers were investigated by tapping mode, material sensitive atomic force microscopy (AFM). At cross section of membranes, large ordered lamellar-like areas were found. From adhesion force mappings, approximately 50 nm large water-rich areas could be identified by their low adhesion. These areas were interpreted as ionically conductive phase. They appeared circular and isolated before any forced current flow through the sample (activation). After activation, branched, long and flat ionically conductive phase structures in direction of applied voltage were found. They were interpreted as the formation of a continuous ionically conducting network formed by the current flow. In a second part, the material sensitive imaging was used to analyze the distribution of ionomer and platinum covered carbon particles in fuel cell electrodes. The analysis was based on the high adhesion of ionomers compared to the carbon supported catalyst particles. INTRODUCTION In fuel cells, performance and life time rely on a large extend on the nanoscale properties of the materials used as ionomer for the membrane or in electrodes. As membranes, mostly perfluorinated sulfonic acid (PFSA) membranes, e.g. Nafion® and Aquivion®, are used. Recently, novel materials such as sulfonated multi-block copolymers are increasingly considered for application due to their promising conductivity. The nanostructure of PFSA has been investigated since many decades, however, mostly on pristine samples equilibrated at different humidity. Extensive studies by SAXS and TEM combined with modelling of the data provided insight into the hydrophobic/hydrophilic phase separation. Typical separation sizes obtained are in the range of 3-7 nm, depending on the water uptake. Open questions concerning the conductive structure during dynamic fuel cell operation still remain. Aspects of the membranes that have not been considered sufficiently are the changes in the nanostructure induced by current flow and the structure of interfaces. In particular, a hydrophobic surface layer at pristine membranes exists that prevents fast water exchange through the interface, and which persists even after storage in water. Recently, atomic force microscopy (AFM) investigations have also shown that a major difference in conductive nanostructure of the ionomer occurs upon current flow [1]. In a fuel cell, typically an initial rise of current and performance for up to 20 h is observed until the performance becomes steady-state. This macroscopic effect can be
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