Sep 24 2011
The heart of the Polymer Electrolyte Membrane Fuel Cell (PEMFC) is the Membrane Electrode Assembly (MEA), which consists of a proton exchange membrane (typically a perfluorosulfonated polymer) and two catalyst layers (i.e. anode and cathode). However, between the Bipolar Plate (BP) and the MEA it is common practice to insert a third component, the so-called Gas Diffusion Layer (GDL), which is now considered indispensable for the proper working of the FC. The GDL is a critical component in a PEM because it must carry reactant gases from the flow fields to the catalyst layer as well as electrons from the bipolar plate to the catalyst layer, while at same time has to remove reaction products (exhausted gases and water) from the catalyst layer and help take heat out of the MEA up to the cooling channels in the BP. Last but not least, it has to guarantee good contact between BP and MEA. So, among the many properties that are required for the GDL there are electronic and thermal conductivity, porosity, hydrophobicity to assist in water management, compressibility and elasticity.
At present, GDLs are constructed from porous carbon paper, or carbon cloth, with a thickness in the range of 100–300 µm. The gas diffusion layers are typically wet-proofed with a PTFE (Teflon) coating to ensure that pores do not become congested with liquid water; this positive contribution of the PTFE is usually counterbalanced by some drawbacks such as an increase in electrical resistance and thickness, together with an inhomogeneous coating distribution and the need of a thermal treatment to ensure the adhesion between PTFE and carbon substrate.
Both carbon paper and cloth are typically macroporous materials, but to improve water removal and gas permeability it is necessary to introduce micropores and this is accomplished by coating a thin Micro-Porous Layer (MPL) onto the GDL surface adjacent to the membrane; this coating also improves the smoothness of the GDL surface allowing a better contact with the catalytic layer. In addition to being microporous and to preserve high gas permeability, the MPL has to be quite hydrophobic to drive out water and electrically conductive. In this paper the PEM component obtained by coating the MPL onto the carbon cloth will be referred to as Gas Diffusion Medium (GDM), reserving the term GDL to the macroporous substrate made of woven carbon cloth.
The catalyst layer, which is in direct contact with the membrane and the GDM, is either applied to the membrane (Catalyst Coated Membrane, CCM) or to the MPL (the so-called Gas Diffusion Electrode, GDE). In either cases, the objective is to place the catalyst particles, typically platinum or platinum alloys, within close proximity of the membrane. In this work we will use CCMs only.
In commercialized products the MPL is almost universally made from dispersion of carbon black and PTFE particles in water in different concentrations. However, in the recent literature an increasing attention has been paid to nanocarbon-based materials to be used in the formulation of innovative PEM-FC components.
To improve thermal and electrical conductivity of GDL, Single walled carbon nanotubes free-standing membrane (CNTFSM) have been developed by the Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta” and tested into a LT-PEMFC.
Kannan et al. made GDLs using a new type of partially ordered graphitized nano-carbon black. They were fabricated with teflonized nonwoven carbon paper as a substrate coated with a MPL, whose hydrophobicity was enhanced by the use of the graphitized carbon black provided by a Teflon suspension. The robustness of the micro-porous layer was improved by combining the graphitized nano-carbon black with a fibrous nano-carbon. The fuel cell performance in terms of power density was excellent.
Soehn et al. fabricated two types of gas diffusion electrodes (GDE) with nanocarbon as structural component. In the first one carbon nanotubes were directly grown on the fiber surface of a traditional carbon cloth in a CVD process. The second GDL uses the buckypaper preparation technique which allows a flexible design of layer-type GDEs with tuneable properties (wetting behaviour, catalyst concentration) or gradient materials.
Minimizing catalyst load and maximizing catalyst utilization can be achieved using platinum supported on high surface area nanocarbon materials, such as carbon nanofibers, graphite nanofibers multi-wall carbon nanotubes, although the effective attachment of Pt nanoparticles onto CNTs, for instance, remains a challenging task.
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