Aimy Bazylak

Microscale Tomography Investigations of Heterogeneous Porosity Distributions of PEMFC GDLs

A first comparison is made of the heterogeneous porosity distributions of various polymer electrolyte membrane fuel cell (PEMFC) gas diffusion layer (GDL) materials. Microscale computed tomography imaging is performed for three main categories of commercially available GDL materials: carbon fiber paper, felt, and cloth. The methodology to analyze the through-plane and in-plane porosities of various materials is presented, and the relationship between heterogeneous porosity distributions and manufacturing techniques is discussed. This work also provides insight into the manufacturing process employed for GDLs.

Unstructured Pore Network Modeling with Heterogeneous PEMFC GDL Porosity Distributions

This is the first investigation of the liquid water saturation profile dependence on empirically determined heterogeneous polymer electrolyte membrane fuel cell (PEMFC) gas diffusion layer (GDL) porosity distributions. An unstructured, two-dimensional pore network model using an invasion percolation algorithm is presented. Random fiber placements are based on the heterogeneous porosity distributions of six commercially available GDL materials recently obtained through X-ray-computed tomography visualizations. The pore space is characterized with a Voronoi diagram, and simulations are performed with a single inlet liquid water cluster. Saturation profiles are also computed for GDLs with uniform, sinusoidal, and square-wave porosity distributions. Liquid water tends to accumulate in regions of high porosity due to the associated lower capillary pressures. The results of this work suggest that GDLs tailored to have smooth porosity distributions have fewer pockets of high saturation levels within the bulk of the material. Finally, a study on theoretical surface modifications demonstrates that low porosity surface treatments at the catalyst layer|GDL interface result in greatly reduced overall saturation levels of the material.

Natural Convection in an Enclosure with Distributed Heat Sources

ABSTRACT Presented in this article is a computational analysis of the heat transfer due to an array of distributed heat sources on the bottom wall of a horizontal enclosure. The heat sources are modeled as flush-mounted sources with prescribed heat flux boundary conditions. Optimum heat transfer rates and the onset of thermal instability triggering various regimes are found to be governed by the length and spacing of the sources and the width-to-height aspect ratio of the enclosure. With respect to source spacing, we found that spacing equal to that of the source length provides effective convective heat transfer, and increasing the source spacing further does not result in significant improvements. The transition from a conduction-dominated regime to a convection-dominated regime is found to be characterized by a range of Rayleigh numbers, in contrast to the classical bottom wall heating problem. The range of Rayleigh numbers at which transition takes place decreases as the source length increases. At the transition region for very small source lengths, the Rayleigh-Bénard cell structure grows significantly to form fewer and larger cells, which accounts for higher heat transfer rates compared to configurations with longer heat sources where the cell structure remains the same throughout transition. Following the transition to a convection-dominated regime, bifurcations in the Rayleigh-Bénard cell structures as well as further regime changes are observed, reflecting the instabilities in the physical system.

Heterogeneous Through-Plane Distributions of Tortuosity, Effective Diffusivity, and Permeability for PEMFC GDLs

In this work, we applied the Tomadakis and Sotirchos transport model to heterogeneous porosity distributions measured from high resolution microscale computed tomography visualizations of polymer electrolyte membrane fuel cell (PEMFC) gas diffusion layers (GDLs). We calculated the corresponding local distributions of tortuosity ( T ), relative diffusivity [f(e)], and single phase permeability (K). The heterogeneous distributions of relative diffusivity and permeability strongly correspond to the measured heterogeneous porosity distributions, exhibiting transitional surface and core regions. Reasonable agreement was found between the average properties of the GDL core region with bulk values from literature. This work provides insight into the heterogeneity of important properties, which should be considered when modeling transport phenomena in the PEMFC GDL.

Condensation in PEM Fuel Cell Gas Diffusion Layers: A Pore Network Modeling Approach

A two-dimensional dynamic pore network model is developed and employed to simulate the through-plane transport of liquid water originating from condensation in hydrophobic gas diffusion layers (GDLs) for polymer electrolyte membrane (PEM) fuel cells. The model tracks viscous and capillary forces over a range of specified condensation rates and nucleation positions. A simplified mass transport assumption of a uniform water vapor flux between the cathode catalyst layer and the liquid water cluster allows for a computationally inexpensive model. Stochastically generated steady-state saturation profiles are compared to investigate the effects of nucleation position, channel rib presence, coalescence assumptions, and condensation rates on liquid water distribution. Results indicate that GDL saturation conditions become increasingly more desirable as nucleation sites are placed further away from the catalyst layer, and saturation profiles are significantly higher when nucleation is adjacent to a hydrophobic rib compared to the gas channel or a hydrophilic rib. Meanwhile, the trapping assumption that affects liquid water coalescence in small throats has little impact on saturation patterns but has a large impact on the systems viscous forces. Finally, the model can predict the limiting water cluster growth rates of capillary dominated growth for a given pore network.

Accounting for low-frequency synchrotron X-ray beam position fluctuations for dynamic visualizations.

Synchrotron X-ray radiography on beamline 05B1-1 at the Canadian Light Source Inc. was employed to study dynamic liquid water transport in the porous electrode materials of polymer electrolyte membrane fuel cells. Dynamic liquid water distributions were quantified for each radiograph in a sequence, and non-physical liquid water measurements were obtained. It was determined that the position of the beam oscillated vertically with an amplitude of ~25 µm at the sample and a frequency of ~50 mHz. In addition, the mean beam position moved linearly in the vertical direction at a rate of 0.74 µm s(-1). No evidence of horizontal oscillations was detected. In this work a technique is presented to account for the temporal and spatial dependence of synchrotron beam intensity, which resulted in a significant reduction in false water thickness. This work provides valuable insight into the treatment of radiographic time-series for capturing dynamic processes from synchrotron radiation.

Calibrating the X-ray attenuation of liquid water and correcting sample movement artefacts during in operando synchrotron X-ray radiographic imaging of polymer electrolyte membrane fuel cells

Synchrotron X-ray radiography, due to its high temporal and spatial resolutions, provides a valuable means for understanding the in operando water transport behaviour in polymer electrolyte membrane fuel cells. The purpose of this study is to address the specific artefact of imaging sample movement, which poses a significant challenge to synchrotron-based imaging for fuel cell diagnostics. Specifically, the impact of the micrometer-scale movement of the sample was determined, and a correction methodology was developed. At a photon energy level of 20 keV, a maximum movement of 7.5 µm resulted in a false water thickness of 0.93 cm (9% higher than the maximum amount of water that the experimental apparatus could physically contain). This artefact was corrected by image translations based on the relationship between the false water thickness value and the distance moved by the sample. The implementation of this correction method led to a significant reduction in false water thickness (to ∼0.04 cm). Furthermore, to account for inaccuracies in pixel intensities due to the scattering effect and higher harmonics, a calibration technique was introduced for the liquid water X-ray attenuation coefficient, which was found to be 0.657 ± 0.023 cm(-1) at 20 keV. The work presented in this paper provides valuable tools for artefact compensation and accuracy improvements for dynamic synchrotron X-ray imaging of fuel cells.

OpenPNM: A Pore Network Modeling Package

Pore network modeling is a widely used technique for simulating multiphase transport in porous materials, but there are very few software options available. This work outlines the OpenPNM package that was jointly developed by several porous media research groups to help address this gap. OpenPNM is written in Python using NumPy and SciPy for most mathematical operations, thus combining Pythons ease of use with the performance necessary to perform large simulations. The package assists the user with managing and interacting with all the topological, geometrical, and thermophysical data. It also includes a suite of commonly used algorithms for simulating percolation and performing transport calculations on pore networks. Most importantly, it was designed to be highly flexible to suit any application and be easily customized to include user-specified pore-scale physics models. The framework is fast, powerful, and concise. An illustrative example is included that determines the effective diffusivity through a partially water-saturated porous material with just 29 lines of code.

Fractal Flow Patterns in Hydrophobic Microfluidic Pore Networks: Experimental Modeling of Two-Phase Flow in Porous Electrodes

Experimental two-phase invasion percolation flow patterns were observed in hydrophobic micro-porous networks designed to model fuel cell specific porous media. In order to mimic the operational conditions encountered in the porous electrodes of polymer electrolyte membrane fuel cells (PEMFCs), micro-porous networks were fabricated with corresponding microchannel size distributions. The inlet channels were invaded homogeneously with flow rates corresponding to fuel cell current densities of 1.0 to 0.1 A/cm2 (Ca 10e-7-10e-8). A variety of fractal breakthrough patterns were observed and analyzed to quantify flooding density and geometrical diversity in terms of the total saturation, St, local saturations, s, and fractal dimension, D. It was found that St increases monotonically during the invasion process until the breakthrough point is reached, and s profiles indicate the dynamic distribution of the liquid phase during the process. Fractal analysis confirmed that the experiments fall within the flow regime of invasion percolation with trapping. In this work, we propose to correlate the fractal dimension, D, to the total saturation and use this map as a parameter for modeling liquid water transport in the GDL.

Porous Transport Layer Related Mass Transport Losses in Polymer Electrolyte Membrane Electrolysis: A Review

The unintended accumulation of oxygen gas in polymer electrolyte membrane (PEM) electrolyzers has been recently identified as one of the main hurdles to achieving high cell efficiencies. Oxygen is a by-product of the electrochemical reaction used to produce hydrogen, and this oxygen must be removed in order to reduce mass transport losses. The porous transport layer (PTL) is a key component of the PEM electrolyzer which facilitates mass transport and electrical conductance. However, oxygen bubble accumulation potentially dominates the total mass transport losses during operation. Many experimental and computational studies have been performed in an attempt to understand the relationship between the morphology of the PTL and the voltage loss of the electrolyzer, but this relationship has yet to be fully defined. In this work, efforts towards identifying and understanding mass transport losses are discussed. PTL structural parameters that were shown to affect performance, such as bulk porosity, particle size, pore size, thickness, and permeability are reviewed. Visualization techniques that have been employed to investigate the behavior of oxygen bubbles are also discussed. This work presents a summary of the studies which have been performed to investigate the key parameters of the PTL that should be tailored for improved PEM electrolyzer performance.Copyright