Hossein Ostadi

An x-ray tomography based lattice Boltzmann simulation study on gas diffusion layers of polymer electrolyte fuel cells

This work reports a feasibility study into the combined full morphological reconstruction of fuel cell structures using X-ray computed micro- and nanotomography and lattice Boltzmann modeling to simulate fluid flow at pore scale in porous materials. This work provides a description of how the two techniques have been adapted to simulate gas movement through a carbon paper gas diffusion layer (GDL). The validation work demonstrates that the difference between the simulated and measured absolute permeability of air is 3%. The current study elucidates the potential to enable improvements in GDL design, material composition, and cell design to be realized through a greater understanding of the nano- and microscale transport processes occurring within the polymer electrolyte fuel cell.

Calculating the anisotropic permeability of porous media using the lattice Boltzmann method and X-ray computed tomography

A lattice Boltzmann (LB) method is developed in this article in a combination with X-ray computed tomography to simulate fluid flow at pore scale in order to calculate the anisotropic permeability of porous media. The binary 3D structures of porous materials were acquired by X-ray computed tomography at a resolution of a few microns, and the reconstructed 3D porous structures were then combined with the LB model to calculate their permeability tensor based on the simulated velocity field at pore scale. The flow is driven by pressure gradients imposed in different directions. Two porous media, one gas diffusion porous layer used in fuel cells industry and glass beads, were simulated. For both media, we investigated the relationship between their anisotropic permeability and porosity. The results indicate that the LB model is efficient to simulate pore-scale flow in porous media, and capable of giving a good estimate of the anisotropic permeability for both media. The calculated permeability is in good agreement with the measured date; the relationship between the permeability and porosity for the two media is well described by the Kozeny–Carman equation. For the gas diffusion layer, the simulated results showed that its permeability in one direction could be one order of magnitude higher than those in other two directions. The simulation was based on the single-relaxation time LB model, and we showed that by properly choosing the relaxation time, it could give similar results to those obtained using the multiple-relaxation time (MRT) LB method, but with only one third of the computational costs of MRTLB model.

Lattice Boltzmann simulation of water and gas flow in porous gas diffusion layers in fuel cells reconstructed from micro-tomography

The porous gas diffusion layers (GDLs) are key components in hydrogen fuel cells. During their operation the cells produce water at the cathode, and to avoid flooding, the water has to be removed out of the cells. How to manage the water is therefore an important issue in fuel cell design. In this paper we investigated water flow in the GDLs using a combination of the lattice Boltzmann method and X-ray computed tomography at the micron scale. Water flow in the GDL depends on water-air surface tension and hydrophobicity. To correctly represent the water-gas surface tension, the formations of water droplets in air were simulated, and the water-gas surface tension was obtained by fitting the simulated results to the Young-Laplace formula. The hydrophobicity is represented by the water-gas-fabric contact angle. For a given water-gas surface tension the value of the contact angle was determined by simulating the formations of water droplets on a solid surface with different hydrophobicity. We then applied the model to simulate water intrusion into initially dry GDLs driven by a pressure gradient in attempts to understand the impact of hydrophobicity on water distribution in the GDLs. The structures of the GDL were acquired by X-ray micro-tomography at a resolution of 1.7 microns. The simulated results revealed that with an increase in hydrophobicity, water transport in GDLs changes from piston-flow to channelled flow.

Three-dimensional surface reconstruction of diatomaceous frustules.

Single-celled diatoms have attracted much attention due to their delicate nanostructures and wide applications in photonics, biology, nanofluidics, drug delivery, and so on. Three-dimensional (3D) surface reconstruction of diatoms has profound significance both in theory and in applications. The scanning electron microscope (SEM) stereo imaging technique was used to reconstruct the surface features of the diatomaceous frustules to quantitatively evaluate specimens. Geometrical parameters such as volume and area were given based on the reconstructed 3D image. This approach provides a simple and efficient way to measure the morphological parameters of diatoms at nanometer scale in three dimensions.

Multiscale Modeling of Single-Phase Multicomponent Transport in the Cathode Gas Diffusion Layer of a Polymer Electrolyte Fuel Cell

This research reports a feasibility study into multi-scale polymer electrolyte fuel cell (PEFC) modelling through the simulation of macroscopic flow in the multi-layered cell via 1D electrochemical modelling, and the simulation of microscopic flow in the cathode gas diffusion layer (GDL) via 3D single-phase multi-component lattice Boltzmann (SPMC-LB) modelling. The heterogeneous porous geometry of the carbon-paper GDL is digitally reconstructed for the SPMC-LB model using X-ray computer micro-tomography. Boundary conditions at the channel and catalyst layer interfaces for the SPMC-LB simulations such as specie partial pressures and through-plane flow rates are determined using the validated 1D electrochemical model, which is based on the general transport equation (GTE) and volume-averaged structural properties of the GDL. The calculated pressure profiles from the two models are cross-validated to verify the SPMC-LB technique. The simulations reveal a maximum difference of 2.4% between the thickness-averaged pressures calculated by the two techniques, which is attributable to the actual heterogeneity of the porous GDL structure. 1 Department of Aeronautical and Automotive Engineering, Loughborough University, Leicestershire LE11 3TU, United Kingdom 2 Corresponding author: p.rama@lboro.ac.uk 3 Department of Mechanical Engineering, University of Birmingham, Birmingham B15 2TT, United Kingdom 4 Department of Engineering, University of Liverpool, Liverpool L69 3GQ, United Kingdom 5 Technical Fibre Products Ltd., Kendal LA9 6PZ, United Kingdom 2

An Improved MRT Lattice Boltzmann Model for Calculating Anisotropic Permeability of Compressed and Uncompressed Carbon Cloth Gas Diffusion Layers Based on X-Ray Computed Micro-Tomography

The gas diffusion layers (GDLs) in polymer proton exchange membrane fuel cells are under compression in operation. Understanding and then being able to quantify the reduced ability of GDLs to conduct gases due to the compression is hence important in fuel cell design. In this paper, we investigated the change of anisotropic permeability of GDLs under different compressions using the improved multiple-relaxation time (MRT) lattice Boltzmann model and X-ray computed micro-tomography. The binary 3D X-ray images of GDLs under different compressions were obtained using the technologies we developed previously, and the permeability of the GDLs in both through-plane and inplane directions was calculated by simulating gas flow at micron scale through the 3D images. The results indicated that, in comparison with the single-relaxation time (SRT) lattice Boltzmann model commonly used in the literature, the MRT model is robust and flexible in choosing model parameters. The SRT model can give accurate results only when using a specific relaxation parameter whose value varies with porosity. The simulated results using the MRT model reveal that compression could lead to a significant decrease in permeability in both through-plane and in-plane directions, and that the relationship between the decreased permeability and porosity can be well described by both Kozeny-Carman relation and the equation derived by Tomadakis and Sotirchos (1993, “Ordinary and Transition Rdgime Diffusion in Random Fiber Structure,” AIChE J., 39, pp. 397–412) for porosity in the range from 50% to 85%. Since GDLs compression takes place mainly in the through-plane direction, the results presented in this work could provide an easy way to estimate permeability reduction in both through-plane and in-plane directions when the compressive pressure is known.

Threshold Fine-Tuning and 3D Characterisation of Porous Media Using X-ray Nanotomography

A common challenge in the X-ray nanotomography of porous media, such as fuel cell gas diffusion layers (GDLs), is to binarize nanotomography greyscale images in order to differentiate between solids and voids for structural characterisation and numerical flow analysis. In the process threshold determination is critical. This paper presents a study on determination of and fine-tuning threshold value based on comparison of material porosity and average fibre diameter obtained from nanotomography images with porosity data from density experiments and average fibre diameter achieved from scanning electron microscopy images respectively. The more accurate 3D reconstructed model is then used to calculate pore size distribution and average pore size, while the gas permeability of the representative 3D binary images are calculated using a single phase Lattice Boltzmann (LB) model in the D3Q19 regime.

3D visualization and characterization of nano structured materials

This paper reports a study on 3D topography and tomography visualization of materials with micro and nano features. SEM stereo imaging technique is employed to characterize surfaces by reconstruction. With this technique, diatomaceous frustules and sputtering yield of metals were analyzed. X-ray nanotomography (nCT) is used for characterization of a polymer electrolyte fuel cell (PEFC) gas diffusion layer (GDL) with 680 nm resolution. Focused ion beam/SEM (FIB/SEM) nanotomography is developed to visualize materials with resolution from 8 to 60 nm. The technique was used to reconstruct glass micropipette tips and PEFC microporous layers (MPL). In addition, porosity of PEFC layers and roundness of micropipette tips can be extracted from the reconstructed models. As an example of the applications of the FIB/SEM technique, tomographic images of PEFC MPL is combined with the lattice Boltzmann (LB) numerical modeling to anticipate the permeability which helps understand the flow inside the porous layers of the fuel cell and provide the opportunity to improve the performance.

Morphological characterization of sub-micron PDMS bowl structures

In this study, both SEM stereoscopic technique and FIB milling method are used to characterize the morphology of PDMS bowl-shaped structure. The presented structure is created via a low-cost and high-throughput modified colloidal lithographic method. 3D reconstruction through SEM stereoscopic images is used to perform volume analysis while FIB milling is used to perform cross-sectional analysis. The results show that the average value of maximum peak to valley distance is 516 nm and the average height of the pyramids is around 325 nm. This approach provides a simple and efficient way to measure the morphological parameters in nanometer-sized structures.

Characterisation of nanoporous materials using Focused Ion Beam milling method

Focused Ion Beam (FIB) is generally used for machining of solid and bulk materials. However, new applications such as FIB nanotomography of nanoporous surfaces require sputtering yield characterisation. This paper presents the study of the FIB sputtering yield of Ga+ on nanoporous catalyst layers (CL) of a polymer electrolyte fuel cell (PEFC) based on analytical calculations and SEM stereo imaging experiments. It is shown that a porosity of around 50% has a significant effect (approximately 400%) on the sputtering yield of materials.