Determination of the anisotropic permeability of a carbon cloth gas diffusion layer through X-ray computer micro-tomography and single-phase lattice Boltzmann simulation

An investigation of the anisotropic permeability of a carbon cloth gas diffusion layer (GDL) based on the integration of X-ray micro-tomography and lattice Boltzmann (LB) simulation is presented. The method involves the generation of a 3D digital model of a carbon cloth GDL as manufactured using X-ray shadow images acquired through X-ray micro-tomography at a resolution of 1.74 µm. The resulting 3D model is then split into 21 volumes and integrated with a LB single-phase numerical solver in order to predict three orthogonal permeability tensors when a pressure difference is prescribed in the through-plane direction. The 21 regions exhibit porosity values in the range of 0.910–0.955, while the average fibre diameter is 4 µm. The results demonstrate that the simulated through-plane permeability is about four times higher than the in-plane permeability for the sample imaged and that the corresponding degrees of anisotropy for the two orthogonal off-principal directions are 0.22 and 0.27. The results reveal that flow channelling can play an important role in gas transport through the GDL structure due to the non-homogeneous porosity distribution through the material. The simulated results are also applied to generate a parametric coefficient for the Kozeny–Carman (KC) method of determining permeability. The current research reveals that by applying the X-ray tomography and LB techniques in a complementary manner, there is a strong potential to gain a deeper understanding of the microscopic fluidic phenomenon in representative models of porous fuel cell structures and how this can influence macroscopic transport characteristics which govern fuel cell performance

Effective Contact Potential of Thin Film Metal-Insulator Nanostructures and Its Role in Self-Powered Nanofilm X‑ray Sensors

We studied the effective contact potential difference (ECPD) of thin film nanostructures and its role in self-powered X-ray sensors, which use the high-energy current detection scheme. We compared the response to kilovoltage X-rays of several nanostructures made of disparate combinations of conductors (Al, Cu, Ta, ITO) and oxides (SiO₂, Ta₂O₅, Al₂O₃). We measured current–voltage curves in parallel-plate configuration separated by an air gap and determined three characteristic parameters: current at zero voltage bias <i>I</i>₀, the voltage offset for zero current ECPD, and saturation current <i>I</i>_sat. We found that the metals’ ECPD values measured with our technique were higher than the CPD values measured with photoelectron spectroscopy <i>in situ</i>, i.e., no air contact. These differences are related to natural oxidization and to the presence of photo-/Auger-electron current leaking from the high-<i>Z</i> toward the low-<i>Z</i> electrode, as suggested by additional experiments carried out in vacuum. Further, the deposition of the 40–500 nm oxide layer on the surface of metallic substrates strongly affects their contact potential. This technique exploits ionization and charge carrier transport in both solid insulators and in air, and it opens the possibility of measuring the ECPD between metals separated by a solid insulator in a metal–insulator–metal (MIM) configuration. Additionally, we demonstrated that certain configurations of MIM structures are suitable for X-ray detection in self-powered mode

A numerical study of structural change and anisotropic permeability in compressed carbon cloth polymer electrolyte fuel cell gas diffusion layers

The effect of compression on the actual structure and transport properties of the carbon cloth gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC) are studied here. Structural features of GDL samples compressed in the 0.0 – 100.0 MPa range are encapsulated using polydimethylsiloxane (PDMS) and by employing X-ray micro-tomography to reconstruct direct digital 3D models. Pore size distribution (PSD) and porosity data are acquired directly from these models while permeability, degree of anisotropy and tortuosity are determined through lattice Boltzmann (LB) numerical modelling. The structural models reveal that structural change proceeds through a three-step process, while PSD data suggests a characteristic peak in the pore diameter of 10-14 microns and a decrease in the mean pore diameter from 33 to 12 microns over the range of tested pressures. A mathematical relationship between compression pressure and permeability is determined based on the Kozeny-Carman equation, revealing a one order of magnitude reduction in through-plane permeability for a two order of magnitude increase in pressure. The results also reveal that the degree of anisotropy peaks in the 0.3 – 10.0 MPa range, suggesting that in-plane permeability can be maximised relative to through-plane permeability within a material-specific range of compression pressures