Microscale imaging of novel porous media used in polymer electrolyte fuel cells

The polymer electrolyte fuel cell (PEFC) is a promising clean power conversion technology that directly converts chemical energy into electrical energy at relatively low operation temperatures. A wet-proofed, porous layer located between the catalyst layer and flow-field plate in the PEFC is termed as the gas diffusion layer (GDL). The commercially available GDLs are mostly made of electrically conductive carbon fibre-based materials. However, such materials are likely to be subject to different types of degradation. Hence, many studies have been conducted to investigate alternative materials for the GDLs.
Metal foams have attracted a good deal of attention to be used as GDL materials and this is due to their appealing features: high mechanical strength, high and controllable porosity, high electrical and thermal conductivity, and high specific surface area. Nickel (Ni) foam is particularly of great interest to PEFC developers, and this is owing to, in addition to the above-mentioned features, its relatively high corrosion resistance. To this end, the main aim of this work is to further explore the viability of the use of nickel foams for PEFC GDLs.
The first phase of this thesis focuses on developing a computationally economical X-ray computed tomography (X-ray CT) framework to precisely estimate the crucial structural and transport properties of nickel foam-based GDL. The findings indicate that the nickel foam-based GDL shows a greater level of uniformity and isotropy in comparison to the traditional carbon-based GDLs, along with enhanced structural and transport properties, thereby reinforcing its potential as a suitable GDL material for PEFC applications.
The second phase of the thesis is to investigate the effects of compression on the key structural and transport characteristics using a custom-built compression device and X-ray computed tomography-based models. A key discovery from this research is that, in contrast to porosity and ligament thickness, the average pore size considerably decreases with compression. Moreover, the study revealed that the gas permeability, as opposed to effective diffusivity, exhibits a significant degree of anisotropy under compression. This observation is critical for PEFC modelling, as the GDL properties are commonly assumed to be isotropic.
The last phase of the thesis presents a three-dimensional model that was developed to study the impact of the compression imposed by the ribs of the bipolar plates on the fuel cell performance operating with carbon fibre-based or nickel foam-based GDLs. The findings show that the nickel foam-based GDLs display a significantly more uniform oxygen and water distribution than the conventional carbon-based counterpart. Notably, while reduced channel sizes tend to improve cell performance, they also demand increased pumping power because of substantial pressure drop in the flow channels. For the carbon-based PEFCs, the highest net power density observed was 0.473 W/cm2 with 1 mm channel height and 0.75 mm channel width. Conversely, the nickel foam-based PEFCs reached the highest net power density at 0.945 W/cm2 with a channel height of 0.25 mm and a width of 1 mm.