Development of self-supporting MPLs for investigations of water transport in PEM fuel cells

Kurzfassung

The performance of a polymer electrolyte membrane (PEM) fuel cell has a strong dependence of its water management. The membrane needs humidity to have sufficient ion conductivity. But at high humidity, especially at high current densities, flooding of the electrodes can occur and consequently the available active area begins to decrease. The primary purpose of a micro porous layer (MPL) on a gas diffusion layer (GDL) is the effective wicking of liquid water from the catalyst layer into the diffusion media as well as reducing electrical contact resistance with the adjacent layers. In synchrotron radiography studies the importance of liquid water pathways through the porous structure for the water management is proven. 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 fibre GDL produced by laser perforation an overall performance gain has been obtained. To get additional information about the function of the MPL as an interconnection between the reaction layer and the macro porous carbon fibre substrate a self-supporting MPL was developed. This allows the manufacturing and the following treatments of the MPL independent from the GDL 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. It is possible to perforate this layer alone and press it with the non-coated side on a commercial GDL without MPL (Sigracet® GDL25BA from SGL). Thus it was feasible to perform experiments for investigation of the influence of artificial pores in the MPL on the water management. As a consequence, the liquid water transport of non-perforated 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. Further measurements, in particular Ucell(i)-curves up to limiting current densities and electrochemical impedance spectra were done in a 5 cm² fuel cell setup, to obtain a correlation of the global intrinsic properties of the MPL, like through-plane permeability, electrical conductivity or hydrophobicity, with fuel cell performance.