Self-supporting Microporous Layers (MPLs) for PEM fuel cells

Kurzfassung

The main requirements of a gas diffusion layer (GDL) of polymer electrolyte membrane fuel cells (PEMFC) are the provision of a gas and water transport as well as a significant electrical and thermal conductivity. The development of membrane electrode assemblies (MEA) for a wide temperature range (-20°C until +130°C) still favours membranes that require a certain level of humidification for sufficient proton conductivity. For the improvement of PEMFC diffusion media for these conditions, the still mostly unknown influence of the micro porous layer (MPL) on fuel cell performance is a major obstacle. An important function of the cathode side MPL at high temperature operation of PEMFCs is the prevention of membrane drying. However, concurrently a flooding of porous structures in the GDL or catalyst layers at lower temperatures has to be avoided. To get an insight that could lead to an improved GDL design for a broad range of operating conditions, a self-supporting MPL was developed, because this allows the manufacturing and the following treatments of the MPL independent from the GDL substrate. This MPL consists of a thin nonwoven synthetics coated on one side with a mixture of carbon and PTFE produced with the dry spraying technology. For in-situ experiments and some ex-situ measurements these layers are pressed with the non coated side on a commercial GDL without MPL (Sigracet® GDL25BA from SGL). To get a correlation of fuel cell performance to the global intrinsic properties of the MPL, like through-plane permeability, electrical conductivity or hydrophobicity, U(i)-curves up to limiting current densities and electrochemical impedance spectra are measured in a 5 cm² fuel cell setup. To obtain information about the influence of MPL structure on water distribution, synchrotron X-ray radiography studies were performed additionally. In such studies the importance of liquid water pathways through the porous structure for the water management is proven. With artificial paths in a carbon fibre GDL produced by laser perforation an overall performance gain has been obtained. With selfsupporting MPLs it was feasible to investigate the liquid water transport of nonperforated GDL/MPLs compared to the perforation of both layers as well as to the exclusive perforation of MPL and the GDL, by means of in-situ synchrotron imaging.