Heterogeneous Porosity Distribution Research Articles

Deep learning assisted anode porous transport layer inverse design for proton exchange membrane water electrolysis

Proton exchange membrane water electrolysis cell (PEMEC) offers a clean and promising way for hydrogen production. The spatial internal current density and temperature distribution uniformity significantly determines its reliability and durability, as well as the hydrogen production performance. Here, a non-isothermal 3D computational fluid dynamics (CFD) model for PEMEC with parallel flow fields is employed to investigate the impacts of the heterogeneous porosity distribution within the anode porous transport layer (APTL) on the internal current density and temperature distribution uniformity and energy conversion performance. 800 heterogeneous APTL porosity distributions are applied for CFD calculation, providing dataset for deep learning. The deep operator network (DeepONet) is employed to mapping the heterogeneous APTL porosity distribution to the real physical fields such as temperature, oxygen molar fraction and current density distribution fields. Full-connected neural network is employed to construct the relationship between the heterogeneous APTL porosity distribution to the performance metrics. The gradient descent algorithm is applied to obtain APTL porosity distributions corresponding to the optimal internal current density and temperature distribution uniformity, respectively. Compared with the uniform porosity distribution, the current density and temperature uniformity are improved by 45.544 % and 26.680 %, respectively, at an average APTL porosity of 0.5.

The combined effects of a heterogeneous porosity distribution and stress gradient on the high cycle fatigue behavior of high pressure die cast AlSi9Cu3

In order to expand the number of possible applications of High Pressure Die Cast (HPDC) components using the AlSi9Cu3 alloy, the automotive industry is giving greater attention to the fatigue design of components produce by this process-material combination. The process is well known for introducing a heterogeneous distribution of casting defects. This study explores the impact of the heterogeneous porosity distribution and stress gradients on the high cycle fatigue behavior of HPDC AlSi9Cu3 components. Fatigue tests under fully-reversed tension–compression and fully-reversed bending loads are used to investigate the influence of porosity and stress gradients on the fatigue behavior, by machining different specimen thicknesses. The results demonstrate that stress gradients and the heterogeneous porosity distribution determine the material’s fatigue response, including both the fatigue strength and crack initiation mechanisms. The Murakami parameter and local stress level at the critical defect make it possible to interpret the gradient effects. The nature of the entrapped gas in internal defects may influence the in-bulk crack propagation behavior. The comparison of fracture mechanics models (El Haddad, Murakami, Kitagawa-Takahashi) make it possible to highlight the link between the El Haddad’s parameter to parameters that are more microstructurally based.

Towards Prediction of Blood Flow in Coiled Aneurysms Before Treatment: A Porous Media Approach.

Modeling blood flow in aneurysms treated with coils could be used to understand the complete embolization of the aneurysm, through thrombus formation that fills the entire sac. Modeling of the endovascular coil mass as a porous medium is a technique that allows for study of aneurysm hemodynamics, efficiently for patient-specific treatment outcome predictions. Models in the literature use mean porosity of coils in the aneurysmal volume, proving inadequate for outcome prediction. However, models that consider heterogeneous porosity distribution have shown more accurate hemodynamics. We recently published the porous crown model, considering the heterogeneous coil mass distribution, validated on two patients. This study aims (i) to validate the porous crown model for a larger cohort (eight patients), and (ii) to propose a porous medium model translatable to clinical practice in treatment planning. We analyzed the porosity distribution of the endovascular coils deployed inside the cerebral aneurysm phantoms of eight patients using 3D x-ray synchrotron images. The permeability and inertial factor of the porous crown model are calculated using previously published methodology. We propose a new "bilinear" porous model, that uses the same hypothesis, but the permeability and inertial factor can be defined from just basic information available in the neuro-suite, i.e., the aneurysmal sac volume and the coil volume fraction targeted by the neurosurgeon. These two models are compared to the coil-resolved simulations, considered as the gold standard. The results show that both the porous crown model and the bilinear model produce similarly accurate hemodynamics in the aneurysm. The error in the standard (mean porosity) porous model is 66%, whereas the error of the bilinear model is 26%, compared to the coil-resolved. The bilinear model is promising as a means of treatment outcome prediction at time of intervention.

CFD-DEM Simulation of Fast Fluidization of Fine Particles in a Micro Riser

In recent years, the discrete element method (DEM) has gradually been applied to the traditional fluidization simulation of fine particles in a micro fluidized bed (MFB). The application of DEM in the simulating fast fluidization of fine particles in MFB has not yet received attention. This article presents a drag model that relies on the surrounding environment of particles, namely the particle circumstance-dependent drag model or PCDD model. Fast fluidization in an MFB of fine particles is simulated using DEM based on the PCDD model. Simulations indicate that the local structure in an MFB exhibits particle aggregation, which is a natural property of fast fluidization, forming a structure where a continuous dilute phase and dispersed concentrated phase coexist. There exists a strong effect of solid back-mixing in an MFB, leading to relatively low outlet solid flux. The gas back-mixing effect is, however, not so distinct. The axial porosity shows a monotonically increasing distribution with the bed height but does not strictly follow the single exponential distribution. The solid volume fraction at the bottom of the bed is significantly lower than the correlated value in CFB. The axial heterogeneous distribution of the cross-sectional average porosity in the lower half of the bed is also weakened. The radial porosity shows a higher distribution pattern in the central region and a lower one in the sidewall region.

The evolving representative elementary volume size in crystalline and granular rocks under triaxial compression approaching macroscopic failure

SUMMARY Determining the size of the representative elementary volume (REV) for properties of fracture networks, such as porosity and permeability, is critical to robust upscaling of properties measured in the laboratory to crustal systems. Although fractured and damaged rock may have higher porosity and permeability than more intact rock, and thus exert a dominant influence on fluid flow, mechanical stability and seismic properties, many of the analyses that have constrained the REV size in geological materials have used intact rock. The REV size is expected to evolve as fracture networks propagate and coalesce, particularly when fracture development becomes correlated and the growth of one fracture influences the growth of another fracture. As fractures propagate and open with increasing differential stress, the REV size may increase to accommodate the larger fractures. The REV size may also increase as a consequence of the increasing heterogeneity of the fracture network, as many smaller fractures coalesce into fewer and longer fractures, and some smaller fractures stop growing. To quantify the evolving heterogeneity of fracture networks, we track the REV size of the porosity throughout eleven triaxial compression experiments under confining stresses of 5–35 MPa. Acquiring X-ray tomography scans after each increase of differential stress provides the evolving 3-D fracture network in four rock types: Carrara marble, Westerly granite, quartz monzonite and Fontainebleau sandstone. In contrast to expectations, the REV size does not systematically increase toward macroscopic failure in all of the experiments. Only one experiment on sandstone experiences a systematic increase in REV size because this rock contains significant porosity preceding loading, and it subsequently develops a localized fracture network that spans the core. The REV size may not systematically increase in most of the experiments because the highly heterogeneous porosity distributions cause the REV to become larger than the core. Consistent with this idea, when the rock does not have a REV, the fractures tend to be longer, thicker, more volumetric, and closer together than when the rock hosts a REV. Our estimates of the REV for the porosity of the sandstone are similar to previous work: about two to four times the mean grain diameter, or 0.5–1 mm. This agreement with previous work and the <15 per cent change in the REV size in two of the sandstone experiments suggests that when a system composed of sandstone does not host a localized, system-spanning fracture network, estimates of the REV derived from intact sandstone may be similar to estimates derived from damaged sandstone. Using the existing REV estimates derived from intact sandstone to simulations with more damaged crust, such as the damage zone adjacent to large crustal faults, will allow numerical models to robustly simulate increasingly complex crustal systems.

Performance evaluation of the Habanero enhanced geothermal system, Australia: Optimization based on tracer and induced micro-seismicity data

Understanding of hydraulic heterogeneity of the reservoir is the basis for Enhanced geothermal systems (EGS) optimization. This, however, is challenging, as there are limited well tests available for reservoir characterization. To overcome this challenge, this study developed a methodology for determining hydraulic parameters by integrating the induced micro-seismic data collected during the hydraulic stimulation, and tracer test data in the subsequent trial production period for the first time. The spatio-temporal distribution of induced micro-seismicities is indicative of the hydraulic diffusivity distribution, and is subsequently converted into the heterogeneous distribution of permeability and porosity, by quantitative calibrating models outputs with tracer test observations. This approach was verified and applied to the Habanero EGS, Australia, where the accuracy in calibrating the tracer test responses was improved by over 50%, attributed to the constraints of micro-seismic data. The well placement was then optimized based on new insights of hydraulic parameters in the reservoir. As a result, the electrical power efficiency was increased by 5.59 times in 30 years. The wide existence of tracer and induced micro-seismic data promotes the generality of this methodology to improve the reservoir characterization and well placement optimization for the sustainable development of EGS.

Analysis of local stress/strain fields in an HPDC AM60 plate containing pores with various characteristics

Casting pores are regarded as the main factor causing stress/strain concentrations, facilitating the initiation of microcracks and shortening the plasticity of a cast component. The influence of pore characteristics (i.e., size, morphology, and position) on local stress concentration in a High Pressure Die Cast (HPDC) AM60 sample is studied utilizing X-ray tomography (XRT) and image-based finite element (FE) techniques. Heterogeneous porosity distribution is identified in terms of volume fraction and projected area ratio per segment. Statistically results demonstrate that small pores (volume < 0.01 mm3) have higher number frequency and sphericity. In contrast, large pores (volume > 0.01 mm3) account for a very low number frequency (0.3%), and complex shape. The present study addresses the local stress/strain behavior in a cast-pore containing HPDC AM60 sample during tensile loading investigated using 3D image-based models which consider the actual size, morphology, and spatial distribution of pores based on XRT images and consider the actual material properties of the AM60 alloy. Simulation results show that the local stress/strain around a pore evolves progressively with deformation. The strain at a certain pore location may be amplified or reduced in the presence of other pores, depending on the specific location of the concerning pores. Also, the ratio between the projected area of a pore and its shortest distance to the free surface (PA/SD) could give a good indication of the max local stress concentration around the pore surface. The weakening of Ktpl values of pores found in defect bands are principally due to the pore orientation concerning the load direction. These findings may be extended to provide additional insight into the damage mechanisms in material containing casting pores and to enhance understanding of the ductility variation in terms of local deformation fields around pores and their interactions.

Characterization and upscaling of hydrodynamic transport in heterogeneous dual porosity media

We study the upscaling of pore-scale transport of passive solute in a carbonate rock sample. It is characterized by microporous regions displaying heterogeneous porosity distribution that are accessible due to diffusion only, and a strongly heterogeneous mobile pore space, characterized by a broad distribution of flow velocities. We observe breakthrough curves that are characterized by strong tailing, which can be attributed to velocity variability in the flowing medium portion, and solute retention in the microporous space. Using accurate numerical flow and transport simulations, we separate these two mechanisms by analyzing the statistics of residence times in the mobile phase, and the trapping and residence time statistics in the mmobile phase. We employ a continuous time random walk framework in order to upscale transport using a particle based implementation of mobile-immobile mass transfer, and heterogeneous advection. This approach is based on the statistics of the characteristic mobile and immobile residence times, and mass transfer rates between the two continua. While classical mobile-immobile approaches model mass transfer as a constant rate process, we find that the trapping rate increases with increasing mobile residence times until it reaches a constant asymptotic value. Based on these findings and the statistical characteristics of travel and retention times, we derive an upscaled Lagrangian transport model that separates the processes of heterogeneous advection and diffusion in the immobile microporous space, and provides accurate descriptions of the observed non-Fickian breakthrough curves. These results shed light on transport upscaling in highly complex dual-porosity rocks for which mobile-immobile mass transfer are controlled by a dual multirate process controlled by the heterogeneity of both the flow field in the connected porosity and the diffusion in the no-flow regions.

Characteristics of flow through randomly packed impermeable and permeable particles using pore resolved simulations

Pore resolved simulations are performed to study the mean flow characteristics in porous media formed by random distribution of impermeable and permeable particles. Permeability of the medium and reactive surface areas are analyzed for a range of solid fractions, packing patterns and flow rates. The results are compared with models presented in the literature and with simulations of flow through ordered packings. Permeability of a randomly packed porous medium is found to depend on the heterogeneous distribution of porosity which results from particle agglomeration and is typically ignored by simpler models. The reactive surface area is dependent on the flow field at pore scale in addition to geometry. Moreover, the wake formed downstream of the particles at moderately high flow rates also affects the reactive surface area. Similar analysis is also performed by assuming particles to be permeable to account for dual scale porosity relevant for various engineering applications.

Investigation of the hydro-mechanical behaviour of GMZ bentonite pellet mixtures

Bentonite pellet mixtures are considered as one of the candidate sealing materials for deep geological disposals of radioactive waste. One of the particularities of this material is the initial heterogeneous distribution of pellets and porosity within the mixture, leading to complex hydro-mechanical behaviour. In this paper, the hydro-mechanical properties of GMZ bentonite pellet mixtures were investigated in the laboratory by carrying out water retention tests on pellet mixtures under constant-volume condition and single pellets under free swelling condition, as well as a infiltration test on a column specimen of pellet mixture. In the infiltration test, the relative humidity and radial swelling pressure were monitored at five heights, the axial swelling pressure was also recorded. The instantaneous profile method was applied to determine the unsaturated hydraulic conductivities. Results show that, in high suction range (> 10 MPa) the water retention curve of pellet mixture under constant-volume condition was comparable to that of a single pellet under free swelling condition, while in low suction range (< 10 MPa) the latter exhibits a much higher water retention capacity. Due to clogging of large pores, the unsaturated hydraulic conductivity decreases as suction decreases to around 25 MPa. However, with further suction decrease, the hydraulic conductivity increases continuously until the value at saturated state, as in the case of most unsaturated soils. The radial swelling pressure at different heights develops with local sudden increase and decrease, which was attributed to local rearrangement of pellets upon wetting. By contrast, as the axial swelling pressure was measured on the global surface of the specimen, it develops in a more regular fashion.

A modified pressure-gradient-based (PGB) sand failure criterion for dynamically and preferentially characterizing wormhole growth and propagation during CHOPS processes

In this paper, a recently developed pressure-gradient-based (PGB) sand failure criterion has been modified to improve its applicability for accurately quantifying the dynamic and preferential wormhole growth and propagation during cold heavy oil production with sand (CHOPS) processes. More specifically, the resistance of sand mobilization is analyzed and mechanical balance at pore-scale is established. The heterogeneous porosity distribution is newly integrated into the PGB sand failure criterion to identify the preferential directions of wormhole propagation. The sand failure is quantitatively examined by comparing the pressure gradients along with the potential directions at a threshold pressure gradient. Such an improved PGB sand failure criterion has been incorporated with a commercial reservoir simulator, in which three sets of relative permeability are utilized to represent the three physically distinct stages of the CHOPS processes. Subsequently, the newly modified PGB sand failure criterion is validated by laboratory sand production experiments and then extended to a field application. Good agreements between the simulated and measured data (i.e., production profiles and pressure) have been achieved from the laboratory experiments. As the preferential directions of the wormhole propagation can be dynamically determined through pressure gradient and porosity distribution, the simulated wormholes are much closer to the experimental observations than the previous work (Fan et al., 2019). The accuracy of the simulated production profiles has also been significantly improved by using three sets of permeability to separately represent the dominant impacts (i.e., sand failure, slurry flow, and boundary effect) on different stages of the CHOPS processes. In addition, good agreements between the simulated and measured data have been obtained from the field case by use of the improved PGB sand failure criterion, demonstrating its robustness and versatile applicability of predicting the dynamic and preferential wormhole network under various laboratory and field conditions.

Porosity quantification for ductility prediction in high pressure die casting AM60 alloy using 3D X-ray tomography

Porosity plays an important role in determining the ductility in magnesium alloy high pressure die castings. In this work, X-ray Tomography (XRT), fractographic analysis and Digital Image Correlation(DIC) have been implemented to understand the inter-relation between various pore characteristics, specifically porosity spatial distribution and morphology, and the ductility in thin-wall AM60 alloy high pressure die casting. The results show that a great amount of porosity was detected in the 20 studied samples. The pore size follows the three-parameter lognormal distribution. Among the detected porosity, small porosity (equivalent diameter ≤90 μm) is numerically dominant and the proportion in number reaches up to ~80%, which constitutes ~50% of the volume of porosity. In comparison, the large porosity (equivalent diameter >425 μm) accounts for a very low percent of the porosity population, namely ~1%, and only occupies ~5% of the volume of porosity. For through-thickness distribution of porosity, two defect bands were observed consisting of concentrated porosity. A great amount of pores randomly distributed pores in the skin and core, and some large pores (equivalent diameter >425 μm) are presented in the core. In addition, along tensile axis, heterogeneous porosity distribution is displayed in terms of local porosity and pore size. Tensile test coupled with DIC measurement has been conducted for the twenty samples with XRT data and the influences of various parameters of pore, including local porosity, size and position of pore, on the ductility have been analyzed. It can be seen that the local porosity in the fracture segment plays the primary role in determining the ductility, although the location and size of pore have effect on deformation heterogeneity and thereby affect the ductility. Therefore, the ductility can be considered as a function of the local porosity in the fracture segment. Moreover, though it's difficult to obtain the local porosity in the fracture segment prior to tensile test and the ductility can't be predicted based on it, there is a clear correlation between the elongation and the highest local porosity in the gauge, which can be got before tensile test, and thus ductility can be estimated accordingly.

CFD simulation of a packed-bed solid-state fermentation bioreactor

In recent years, Solid-State Fermentation (SSF) has shown much promise for the development of bioprocesses and products. SSF involves the growth of microorganisms within a bed of moist solid particles permeated by a continuous gas phase and a minimum of visible water. SSF offers potential advantages over submerged culture: since the concentrations of products are often higher, smaller bioreactors can be used, reducing operational costs. However, there is a major challenge in obtaining adequate heat and mass transfer when this fermentation method is used at large scales. Mathematical models and computer simulations are useful tools for designing strategies to overcome this challenge; use of these tools can reduce costs of experimental development programs at pilot-scale and production scale, by reducing the number of fermentation experiments required. In the current work, we used the commercial CFD software ANSYS FLUENT® 16.0 to develop a mathematical model for heat and mass transfer in a pilot-scale packed-bed bioreactor. The model was used to simulate two different experiments that had been carried out previously in the bioreactor: first, the cooling of a bed of soybeans and, second, the heating of a bed composed of a mixture of wheat bran and sugarcane bagasse. The simulations considered the dynamics of airflow in the porous substrate bed and the non-equilibrium transfer of heat and moisture between the solid and gas phases. The second simulation considered a heterogeneous distribution of porosity within the substrate bed. Even though the two experiments were quite different, the same mathematical model was able to represent the temperature profiles observed experimentally. In the second simulation, the average temperature difference between the experimental and predicted values was 0.07°C. A third simulation was done for the growth of the filamentous fungus Aspergillus niger in SSF, with the predictions being compared to the results of a traditional mathematical model based on differential equations. Our work provides the basis for the development of a suitable and reliable mathematical model for testing operating conditions and control strategies for large-scale cultivation of microorganisms in SSF bioreactors.

Porosity Prediction Using Neural Network Based on Seismic Inversion and Seismic Attributes

It has done a study of porosity prediction by using neural network. The study uses 2D seismic data post-stack time migration (PSTM) and 2 well data at field “T”. The objective is determining distribution of porosity. Porosity in carbonate reservoir is actually heterogeneous, complex and random. To face the complexity the neural network method has been implemented. The neural network algorithm uses probabilistic neural network based on best seismic attributes. It has been selected by using multi-attribute method with has high correlation. The best attributes which have been selected are amplitude envelope, average frequency, amplitude weighted phase, integrated absolute amplitude, acoustic impedance, and dominant frequency. The attribute is used as input to probabilistic neural network method process. The result porosity prediction based on probabilistic neural network use non-linear equation obtained high correlation coefficient 0.86 and approach actual log. The result has a better correlation than using multi-attribute method with correlation 0.58. The value of distribution porosity is 0.05–0.3 and it indicates the heterogeneous porosity distribution generally from the bottom to up are decreasing value.

Investigation of the hydro-mechanical behaviour of a pellet/powder MX80 bentonite mixture using an infiltration column

Non-compacted pellet/powder bentonite mixtures are considered as candidate sealing plug materials in geological disposals of radioactive waste. Due to its nature, this mixture is characterized by a heterogeneous porosity network, which is responsible for its complex hydro-mechanical (HM) behaviour. The French Institute of Radioprotection and Nuclear Safety (IRSN) has investigated this mixture within the SEALEX project. Both in situ large-scale and laboratory small-scale experiments were carried out. This paper presents the results of a small-scale mock-up test at 1/10th scale of the in situ experiments, in which the pellet/powder mixture was saturated from both sides (top and bottom of the specimen). Both swelling pressure and relative humidity were monitored at several positions of the specimen. Different responses from the sensors were found, depending on the local porosity as well as the evolution of the hydration front. The HM response of the mixture is strongly conditioned by the initial pellet/powder distribution, which depends on the protocol followed for the specimen preparation. After 800-day hydration, an anisotropy was found between the axial and the radial swelling pressures, due to the presence of larger void at the top of the sample and the friction at the cell wall. The sample was still heterogeneous after 800-day hydration mainly due to the initial heterogeneous porosity distribution, combined with the effect of friction and the non-saturation of the mixture. The evolution of injected water with time revealed that the sample was not full saturated after 800-day hydration.

Digital autoradiography on C-14-labelled PMMA impregnated rock samples using the BeaverTM

In Finland and Sweden the KBS-3 concept has been chosen for the disposal of spent nuclear fuel in crystalline rock. Recent experiments have shown that heterogeneity of rock may play a major role in the transport of radionuclides. Autoradiographic methods have been proven to be able to assist the characterization of heterogeneous structures. In this study we tested a novel filmless autoradiographic device called BeaverTM which applies a micro patterned gaseous detector in order to quantitatively map beta emissions from C-14 atoms. The studied samples were impregnated with C-14-labelled methylmethacrylate (C-14-MMA) and polymerized to C-14-PMMA with thermal initiator. The BeaverTM was then used to determine the spatial distribution of the C-14-PMMA by measuring the C-14 emissions. The porosity is determined from the amount of C-14-PMMA in the rock sample and results were compared to ones from phosphor imaging plate autoradiography. The resulting images show a heterogeneous distribution of porosity which arises from the different minerals. The samples were chosen from three sites that have been used recently for in situ diffusion experiments: Olkiluoto (Finland), Äspö (Sweden) and Grimsel (Switzerland).

Heterogeneous porosity distributions of polymer electrolyte membrane fuel cell gas diffusion layer materials with rib-channel compression

In this study, X-ray Computed Tomography (X-CT) is used to study the structure of the bi-layered gas diffusion layer (GDL) of polymer electrolyte membrane (PEM) fuel cells. This work presents a unique, calibrated segmentation procedure developed in-house to identify the distinct components of the bi-layer GDL, isolating the carbon fibre, the microporous layer and the void regions as individual phases. The novel use of the areal mass and areal volume of the GDL facilitates calibrated, reliable and repeatable multi-component segmentation and removes the high degree of subjectivity that might otherwise be encountered in typical thresholding procedures. Samples with and without MPLs are studied for the impact of rib – channel compression on porosity profiles. Under the channel, the porosity profile is nearly identical to the uncompressed profile. Under the land region, there is a significant decrease in the GDL thickness. It is also observed that the majority of the compression is exhibited by the substrate region while the MPL undergoes significantly reduced compression.

Delineation of macroporous zones in the unsaturated portion of the Miami Limestone using ground penetrating radar, Miami Dade County, Florida

Summary The Atlantic Coastal Ridge in Dade County is an ooid shoal formed during the late Pleistocene. The ridge is composed of the eogenetic karst Miami Limestone, characterized by a heterogeneous distribution of porosity that can manifest as large touching-vug macroporous features with sizes in the meters and tens of meters. Direct evidence for the presence of such large dissolution features is only visible when exposed to the surface which may be problematic in urbanized areas. For that reason it is critical to detect any potential precursors for such dissolution features at the subsurface level. The purpose of this study was to investigate the ability of ground penetrating radar to detect areas of macroporosity by quantifying changes in electromagnetic wave velocity and relate them to changes in porosity after application of the complex refractive index model (CRIM). In order to constrain volumetric water content (VWC) in the CRIM, an experiment using Miami Limestone samples was designed to understand: (1) the range of typical VWCs; and (2) the effect of capillary fringe on water table elevation. The results show several areas where increases in EM wave travel time associated with contrasts in porosity exceeding 40% cannot be explained by changes in VWCs typically shown in the Miami Limestone. This study may help understand porosity variability in the unsaturated part of the Miami Limestone and if expanded to larger scales may aid groundwater flow models by better capturing distributions of macroporous areas that may contribute to direct recharge of the Biscayne aquifer.

X-ray Tomographic Analysis of Porosity Distributions in Gas Diffusion Layers of Proton Exchange Membrane Fuel Cells

This paper describes a method to characterize the structure of polytetrafluoroethylene (PTFE) treated gas diffusion layers (GDLs) with and without microporous layers (MPLs) using 3D X-ray micro computed tomographic (μCT) microscopy. In this work, the structure of single and dual layer GDLs is evaluated via μCT for various GDL samples (such as Toray TGP-H-060 and AvCarb EP40) loaded with different MPLs. A new method is presented for separating, or segmenting, the various phases of the GDL, i.e., void space, carbon fiber (including binder and PTFE), and MPL. Through analysis, it was found that the variation in bulk porosity and the average pore diameter of the GDLs depends highly on the GDL series manufacturing and treatment processes. Using advanced image analysis techniques, routines were developed to accurately segment the GDL fibers (including binder/PTFE) and the MPL. The percentage of the intruding MPL material into the carbon fiber paper as a function of the GDL thickness was successfully found for dual layer GDLs, with varying PTFE content and areal weight loading in the MPL. This analysis provides invaluable insight into the physical microstructure of paper-based GDLs, emphasizing the heterogeneous porosity distribution of single layer GDLs and the interaction of the MPL with the carbon fiber paper of dual layer GDLs.

Invited: Advanced Visualization Tools to Investigate PEM Fuel Cell Materials

The commercialization of polymer electrolyte membrane fuel cells (PEMFCs) has been hindered by durability and cost issues that could be overcome with an improved understanding of the water transport mechanism in PEMFC operations. To better understand this phenomenon, various techniques have been employed as powerful diagnostic tools to investigate PEMFC components in relations to their role in water management issues.A key component of interest for visualization is the gas diffusion layer (GDL), which provides passages for electron conduction, fuel transport, heat conduction, and water removal. X-ray imaging, due to its high sensitivity to carbon and non-destructive nature, is well suited for the study of the GDL microstructures. Using conventional desktop micro-computed tomography (micro-CT), with a spatial resolution of approximated 5 µm, ex-situ analyses of the heterogeneous porosity distributions of various GDLs were performed in both the through-plane and in-plane directions [1]. GDL materials show a linear transitional region near the outer surfaces which led to high overall bulk porosities. GDLs treated with micro-porous layers (MPLs) were also visualized to determine the porosity distributions of the GDL microstructure and the MPL coating independently. In general, it was found that MPL penetration into the GDL highly depended on local through-plane GDL porosity [2]. Other investigations utilizing the micro-CT examined the effect of rib and channel compression on the GDL porosity, and enabled the measurement of water content in the GDL microstructure at various current densities [3].Synchrotron X-ray radiography provides another effective visualization tool. In particular, due to the high intensities that the parallel monochromatic beam the synchrotron can provide, this technique suitably lends itself to the typically challenging task of visualizing the dynamic fuel cell operations. In-situ studies of water management in the microstructure of PEM fuel cells were previously performed at the Biomedical Imaging and Therapy Beamline (BMIT-BM) at the Canadian Light Source (Saskatoon, Canada). The facility provided image acquisitions with an effective spatial resolution of 10 µm and a temporal resolution of 3 seconds per frame. Applying the principle of the Beer-Lambert law, raw images were processed to measure the water thickness distributions within the fuel cell in the in-plane and through-plane directions [4]. The effect of MPL thickness and channel wettability on the overall performance and the liquid water saturation within the microstructures were examined [5].Additional visualization tools which have shown to provide invaluable insight in the microstructure of PEM fuel components include atomic force microscopy (AFM), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), and nano-computed tomography (nano-CT). AFM has been used to examine the surface morphology of GDL fibres, in order to determine the effective GDL thermal conductivity. Measurement of heterogeneous through-plane distribution of polytetrafluoroethylene (PTFE) within the GDL can be achieved through SEM and EDS imaging. Finally, nano-CT provides the means to visualize the sub-micron pores within the MPL, which are undetectable with traditional micro-CT scanners.Z. Fishman, J. Hinebaugh, and A. Bazylak. Microscale tomography investigations of heterogeneous porosity distributions of PEMFC GDLs. Journal of the Electrochemical Society, 157 (11) B1643-B1650 (2010).Z. Fishman and A. Bazylak. Heterogeneous through-plane porosity distributions for treated PEMFC GDLs. II Effect of MPL cracks. Journal of the Electrochemical Society, 158 (8), B846-B851 (2011).R. Yip and A. Bazylak. Investigation of liquid water content of a compressed PEMFC GDL using micro-computed tomography. Proceedings of the ASME 2012 6th International Conference on Energy Sustainability & 10th Fuel Cell Science, Engineering and Technology Conference, FuelCell2012-91446, 473-477 (2012).J. Lee, J. Hinebaugh, and A. Bazylak. Synchrotron X-ray radiographic investigations of liquid water transport behavior in a PEMFC with MPL-coated GDLs. Journal of Power Sources, 227, 123-130 (2013).J. Lee, P. Antonacci, N. Ge, R Yip, T. Kotaka, Y. Tabuchi, and A. Bazylak. Impact of MPL thickness on water management of PEMFC by synchrotron X-ray radiography. ECS Orlando.