Influence of the MPL on PEM fuel cell performance

Kurzfassung

The gas diffusion layer (GDL) plays a crucial role for PEM fuel cell performance. The main requirements of a GDL are the provision of a gas and water transport as well a significant electrical and thermal conductivity. For the development of PEMFC diffusion media the still unknown influence of the micro porous layer (MPL) on fuel cell performance is a major obstacle. Therefore a self-supporting MPL was developed because it allows basically to measure the properties of the MPL separately from the substrate. This MPL consists of a thin nonwoven of synthetics coated on one side with a mixture of carbon and PTFE produced by 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 the global intrinsic properties of the MPL, like through-plane permeability, electrical conductivity or hydrophobicity, to fuel cell performance, Ucell(i)-curves up to limiting current densities and electrochemical impedance spectra are measured in a 5 cm² fuel cell setup. Additionally the function of the MPL structure on water distribution is investigated. Synchrotron radiography studies proved the importance of liquid water pathways through the porous structure for the water management. These pathways can be formed by natural cracks in the MPL and the texture of the carbon fibre substrate or by artificial pore paths through the GDL. With artificial paths in a carbon fiber GDL produced by laser perforation an overall performance gain has been obtained. In the presented work, we performed further experiments to investigate the influence of artificial pores in the MPL on the water management. Therefore the liquid water transport of nonperforated GDL/MPLs is 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.