a b s t r a c tOscillatory fluctuations of a single polymer electrolyte fuel cell appear upon operation with a dry cathode air supply and a fully humidified anode stream. Periodic transitions between a low and a high current operation point of the oscillating state are observed. The transition time of 20-25 s for the change from the low to the high operation is fast and does not depend on the operating parameters. Contrasting with this behavior, the downward transition depends strongly on the operating conditions. Impedance measurements indicate a high ionic resistance with low water content for the low current operation and a low ionic resistance of the membrane with high water content for the high current operation. An insight into the transitions is obtained by current density distributions at distinct times indicating a propagating active area with defined boundaries. The observations are in agreement with assuming a liquid water reservoir at the anode with a downward transition period depending on the operation conditions. The high current operation possesses a high electro-osmotic drag and a high permeation rate (corresponding to liquid-vapor permeation) leading to a large water flux to the cathode. Subsequently, the liquid reservoir at the anode is consumed leading to an anode drying. The system establishes a new quasi-stable operation point associated with a low current, low electro-osmotic drag coefficient, and a low water permeation (corresponding to vapor-vapor permeation). When liquid water is formed at the anode interface after some time the fast transition to the high current operation occurs. This interpretation is supported by conductive atomic force microscopy current images of the membrane showing a strong dependence of the ionic conductivity on the activation procedures with or without liquid water and also showing oscillatory behavior after the membrane is activated. Specifically, activation with liquid water yields a high conductivity with currents larger by three orders of magnitude.

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SUMMARYThe properties of the components of a membrane electrode assembly in a polymer electrolyte fuel cell (PEFC) determine its efficiency and performance. This paper aims at demonstrating the importance of nanoscale properties of PEFC membranes and electrodes and discussing the information obtained by various experimental techniques. The nanostructure and conductivity of freshly prepared as well as artificially degraded Nafion membranes and Pt/C electrodes are investigated by contact atomic force microscopy (AFM), conductive AFM, pulsed force mode (PFM)-AFM, in situ scanning tunnelling microscopy (STM), and scanning electron microscopy. The different techniques can provide complementary information on structure and conductivity. With in situ STM on Pt catalyst covered graphite, a layer of very small Pt particles between the catalyst particles is imaged, which is probably not visible with TEM and can explain a systematic discrepancy between TEM and XRD in particle size distribution. Conductive AFM is used to investigate the conductivity of Nafion. The images show a quite inhomogeneous distribution of current at the surface. The percentage of conductive surface increases with humidity, but regions without any current still present up to 80% of relative humidity (RH). Comparison with PFM-AFM images, where differences in adhesion forces are measured, indicates that hydrophobic regions are present at the surface with comparable dimensions, which are attributed to non-conductive PTFE-like polymer backbone. The changes in hydrophilic and hydrophobic parts after artificial degradation by plasma etching in air plasma can be imaged by PFM. High-resolution current images of the membrane were used to directly compare the measured nanostructure of the single conductive channels with model predictions from the literature. Recent models in the literature propose the formation of water-filled inverted micelles, with a mean diameter of 2.4 nm, and their agglomeration into clusters agrees well with the current images.

In this contribution, the possibilities and limits of atomic force microscopy (AFM) for investigation of fuel cell component degradation are evaluated. In particular the adhesion force and dissipation energy of the surface measured by a material sensitive AFM technique -the HarmoniX-mode (Bruker Corp.) -have been used as a measure for the relative polytetrafluoroethylene (PTFE) content of surfaces and could be quantified by calibrating with sample of known composition. Differently operated samples with microporous layers (MPLs) of commercial gas diffusion layers (GDLs) were investigated before and after operation and were compared to artificially aged and reference samples. A larger degradation of the cathode material compared to the anode was always found. As an additional example for the potential of AFM and infrared absorption spectroscopy (FTIR-ATR) the local PTFE content of a cell with a segmented anode flow field has been investigated. The results of PTFE loss at MPL and electrode surfaces from AFM measurements and infrared spectroscopy delivered different results which were explained by the distinct information depth of both methods. The large relative differences of PTFE content of the different segments were correlated with the mechanical properties of the special design of the segmented cell.

In this contribution we report on the nanostructure and conductivity of freshly prepared as well as artificially degraded Nafion membranes investigated by contact atomic force microscopy (AFM), conductive AFM, and pulsed force-mode (PFM)-AFM. The different techniques can provide complementary information on structure and conductivity. Conductive AFM is used to investigate the conductivity of Nafion membrane surfaces. The images show a quite inhomogeneous distribution of current at the surface. The percentage of conductive surface increases with humi¬dity, but regions without any current are still present up to 80% relative humidity [1,2]. Comparison with PFM-AFM images, where differences in adhesion forces are measu¬red, indicates that hydrophobic regions, which are attribu¬ted to PTFE-like polymer back bone without conductivity, are present at the surface with comparable dimensions. Conductive AFM gives information about the internal structure of ionic clusters during current flow. High resolution current images of the membrane were used to directly compare the measured nanostructure of the single conductive channels with model predictions from the literature (Fig. 3). The influence of H2O2 treatment as a method for artificial degradation is investigated. The analysis of adhesion forces demonstrates a significant change of the surface properties with different membrane treatment. Topography and adhesion measurements clearly show materials changes with high resolution and correlate with changes in conductivity distributions. In particular, small protrusions in the topography correlating with strongly diminished conductivity in the current images are observed. These areas are interpreted as crystalline polymer backbone which higher stability against radical decomposition.