Experimental Characterization of the Compressive Behaviour of Gas Diffusion Layers in PEM Fuel Cells
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Successful commercialization of polymer electrolyte membrane (PEM) fuel cells is highly dependant on performance of the membrane electrode assembly (MEA). The performance of the gas diffusion layer (GDL) of the MEA is highly influenced by applied compressive load. Compression of the GDL changes its porosity and microstructure, which will conse- quently change the physical properties of the GDL. Hence to improve the properties of the GDL, understanding its compressive behaviour is crucial. In the present study, the compressive behaviour of the GDL is characterized by per- forming a series of compression tests using a compression apparatus. The compressive test is first performed on an untreated carbon paper by measuring its thickness as a function of compressive load at room temperature. The obtained results are analyzed and presented in a compressive stress-strain curve. The compressive stress-strain curve of the GDL is divided into 3 distinct regions. The first region represents the stress-strain behaviour at very low stress values. In this region, the compressive strain of the GDL increases rapidly and linearly with the applied stress. In the second region, the compressive strain increases less rapidly and non-linearly with applied stress. In the last region, the compressive stress- strain behaviour becomes linear again, but the strain increases at a lower rate. Analysis of the SEM images for both compressed and uncompressed GDL samples suggest that the stress-strain behaviour of the GDL in the first two regions is induced by the surface roughness and local thickness variations. The compressive behaviour of the GDL is further analyzed by investigating the effect of temperature, relative humidity, and hydrophobic treatment. The compressive stress-strain curves obtained at temperatures of 25 ◦ C, 45 ◦ C, 65 ◦ C, and 85 ◦ C are almost identical within the experimental margin of error, which suggests that fuel cell’s operating temperature has no appreciable effect on the compressibility of the GDL. The compressive stress-strain curve obtained from compressive test at humidified environment shows a higher change in strain in the second region of the curve. The study on the effect of Polytetrafluo- roethylene (PTFE) coating on compressibility and pore distribution shows that increase in PTFE content decreases the overall pore volume of the GDL and hence decreases the compressibility. At last, the effect of cyclic loading is studied by measuring the thickness of GDL samples during 2500 loading/unloading cycles. The strain of the GDL samples increases by 50% after the samples undergo 2500 cycles of compression. The effect of unloading was further investigated by measuring the thickness of the sample as a function of applied compressive load while loading the sample by compression pressures of up to 3.5MPa and then unloading the sample to zero MPa for seven cycles. It is found that the structure of the GDL degrades significantly under compression cycles. This study concludes that the compressive behaviour of the GDL under cyclic compression is a major drawback and without further improvements, the required durability will not be achieved.