Fundamental Aspects of Polymer Electrolyte Fuel Cells: Nanoscale Conductivity of Polymer Membranes Factors Influencing Membrane Degradation

Kurzfassung

Polymer electrolyte fuel cells (PEFC) remain the main fuel cell technology for electric power trains in automobiles. Therefore this technology has benefited from a huge research effort over the past decades. However, still some fundamental properties and interactions of PEFC electrode/electrolyte interface remain elusive due to the experimental difficulty of analysis. One of the major challenges in the development PEFC is to exploit the whole capacity that inheres a given membrane electrode assembly (MEA) as well as to ascertain superior reliability. In practice, the water management and the corresponding local mass transport effects have to be optimized. Suboptimal operation leads to heterogeneous current distributions, which reduce the efficiency of a MEA and hence that of a PEFC. In order to investigate factors limiting the performance, the DLR has developed several measurement and visualization techniques to determine the local current density distribution in fuel cells without interfering with the cell operation. This method is applied to investigate oscillatory fluctuations of a single proton exchange membrane fuel cell which appear if pronounced humidity differences exist between anode (wet) to cathode (dry) compartments. An insight into the transitions between high and low current operation points 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 and changing water fluxes to the cathode due to distinct water content dependent electro-osmotic drag rates and permeation rates (corresponding to liquid-vapour permeation). The results are discussed with regards to water management of fuel cells. Our research also comprises the characterization of fundamental processes like the conductivity of fuel cell membranes on the nanometer scale by conductive atomic force microscopy. We have investigated the ionic conductivity of different solid electrolyte membranes - mostly perfluorinated sulfonic acid membranes - for fuel cell application by contact mode as well as with different tapping mode atomic force microscopy techniques for several years. This methodological approach yields high resolution images of the conductive surface structure and – by investigating cross-sections – gives insight into the bulk structure and the conducting network. The results demonstrate that the dc current as well as the activation procedure has a prominent influence on the conductivity surface distribution. In addition to conductivity other mechanical properties like i.e. adhesion forces, energy dissipation, and stiffness can be measured simultaneously with the current.