Compressibility of SIGRACET® GDL
For the mechanical design of a fuel cell the thickness of the gas diffusion layer (GDL) under compression is an important parameter. SGL measures and specifies the GDL thickness at a fixed compression stress of 5 N/cm². But compression forces in the fuel cell stack are typically much higher. In this newsletter we provide more detailed information on the compression behavior of our portfolio materials to support engineers optimizing the stack design. The data underlying the figures presented is based on measurement results taken from a multitude of samples and production batches. Compressibility is determined predominantly by the SIGRACET® GDL substrate.
Figure 1 shows the compressive behavior of our standard portfolio SIGRACET® GDL materials with 5% PTFE in the web and 23% PTFE in the Micro Porous Layer (MPL) for wetproofing purpose. The data presented is representative for GDL with other hydrophobic treatments and/or MPL coatings. All GDL show a similar trend - a non-linear thickness reduction at lower compression forces and a nearly linear correlation at compression stress above ~50 N/cm². The denser GDL materials i.e. GDL 24 and GDL 34 have higher compression stiffness than the more open materials i.e. GDL 25 and GDL 35.
For our paper type GDL on the 200 ́m platform we offer a pre-compression treatment (HyAmp) that besides a reduction of the nominal thickness significantly reduces thickness tolerances and smoothens the MPL surface. Pre-compressed GDL materials have reduced bending stiffness and tensile strength as compared to standard GDL.
Our broad portfolio of SIGRACET® GDL materials covers the entire thickness range from 150 ́m to 400 ́m. With the data presented the engineer will be able to calculate the expected stack length under compression with the GDL of choice.
SGL recommends a contact pressure of 1 MPa (100 N/cm²).
SGL SIGRACET® GDL 100 AA developmental material Thin Gas Diffusion Layers for Portable Direct Methanol Fuel Cells
Methanol micro-fuel cells are promising candidates as energy source in consumer electronics such as camcorders, laptops but also in small-size medical devices. Within a public funded project called DDMFC (funded by BMBF), a project consortium (FWB Kunststofftechnik, Fraunhofer Institute of Solar Energy Systems ISE, Solvicore, Fumatech, Medset, Filtrona Fibertec, HIAT, and SGL Carbon) developed an innovative direct methanol micro fuel cell which operates with methanol vapor. For the completely passive operation (self-breathing), tailored ultrathin gas diffusion layers are required in order to minimize the fuel diffusion paths.
The fuel cells are manufactured based on a planar design using injection molded parts (see picture) which enables a lightweight system with a modular structure. A prototype with an electric power output of about 1mW has been manufactured and successfully implemented into a portable ECG device by Medset. SGL supplied a 150 ́m thin developmental GDL (SIGRACET® GDL 100 AA) for this application with optimized vapor transport properties facilitating operation in passive mode.
Picture of micro- fuel cell by courtesy of FWB / ISE