The main objective of this work is to predict the breakthrough pressure of liquid water transport through the gas diffusion layer (GDL) and/or micro porous layer (MPL) used in polymer electrolyte membrane fuel cells. The integration of structural GDL and MPL with Lattice Boltzmann Method is primary focused. The numerical predictions are also compared with experimental data. The interaction between liquid phase and different surface treatments of solid structures controls the evolution of liquid water and the change of capillary pressure. The geometries of GDLs and MPLs were obtained by three dimensional reconstructed micro-structure images from both nanometer and micrometer-scaled high spatial resolution X-ray computed tomography (CT). The predictions of water breakthrough pressure agree with the data observed in the experiment. They also reveal that the breakthrough pressure and liquid water evolution inside the GDL samples are different when the wetting properties of GDL and/or MPL are changed. The detailed microporous property can be obtained using high spatial resolution image from nanometer-scaled X-ray CT, a.k.a. Nano X-ray CT. Meanwhile, images from micrometer-scaled X-ray CT, a.k.a. Micro X-ray CT, give proper field of view to cover complete vision of porous materials, including cracks in the MPL. The enhancement of the transport of liquid water and reactant gases in polymer electrolyte membrane fuel cells (PEMFCs) is a critical subject that has been greatly studied due to its critical importance to fuel cell performance at high current densities.1-3 The liquid transport and the concurrent two-phase flow in the gas diffusion layer (GDL) and micro porous layer (MPL) are the most widely studied. In general GDL materials for PEMFCs are made of carbon fiber based cloths and paper. These materials are typically porous to allow for the transport of reactant gases to the catalyst layer as well as transport of condensed water from the catalyst layer. To accelerate the removal of liquid water, GDLs are usually impregnated with non-wetting polymer such as polytetrafluoroethylene (PTFE) to create hydrophobic characteristics. Further, a thin MPL which consists of carbon powder and polymer binders is applied to the GDL side facing the catalyst layer in the membrane electrode assembly (MEA) to further increase cell performance and mechanical stability. The complex structural and chemical heterogeneity of the GDL and MPL make studying the transport of liquid water and obtaining the solution for mass transport losses substantially complicated. 4,5 Many researchers have studied the transport of liquid water through the GDL and MPL in order to develop an understanding of the resistance of reactant gas transport due to water accumulation.6-27 These included the observation of vapor condensation and liquid breakthrough in a GDL using an environmental scanning electron microscope (SEM).13,14 They proposed a treelike transport mechanism in which micro-droplets condensed from vapor agglomerate to form macro-droplets which even...

Interfacial and bulk properties between the catalyst layer and the porous transport layer (PTL) restrict the iridium loading reduction for proton exchange membrane water electrolyzers (PEMWEs), by limiting their mass and charge transport. Using titanium fiber PTLs of varying thickness and porosity, the bulk and interface transport properties are investigated, correlating them to PEMWEs cell performance at ultra‐low Ir loadings of ≈0.05 mgIr cm−2. Electrochemical experiments, tomography, and modeling are combined to study the bulk and interfacial impacts of PTLs on PEMWE performance. It is found that the PEMWE performance is largely dependent on the PTL properties at ultra‐low Ir loadings; bulk structural properties are critical to determine the mass transport and Ohmic resistance of PEMWEs while the surface properties of PTLs are critical to govern the catalyst layer utilization and electrode kinetics. The PTL‐induced variation in kinetic and mass transport overpotential are on the order of ≈40 and 60 mV (at 80 A mgIr−1), respectively, while a nonnegligible 35 mV (at 3 A cm−2) difference in Ohmic overpotential. Thus at least 150 mV improvement in PEMWE performance can be achieved through PTL structural optimization without membrane thickness reduction or advent of new electrocatalysts.

In this paper, direct-modeling-based Lattice Boltzmann Method (LBM) combined with in-situ flow visualization, is used to explore fundamentally the transport of liquid-water inside the gas-diffusion layers (GDLs) used in polymer electrolyte fuel cells. Studies of the water evolution, water saturation, and breakthrough pressure inside a GDL with single and multiple injection points under land and channel geometries are explored. The model and experiment demonstrate good agreement between geometries of GDLs provided in this study which were obtained by a three-dimensional (3-D), reconstructed micro-structure from micro X-ray computed tomography (CT). The overall predictions of water evolution within the GDL agree well with the data visualized from the X-ray CT experiment for all cases studied. It also reveals that the liquid-water saturation profiles inside the GDL and breakthrough pressure are different when the location of the water injection point is altered, thereby providing analysis as to the impact of microporous layers or catalyst-layer functioning. Moreover, the uncompressed GDL undergoes a significantly different mechanism of water transport than that of the compressed GDL. Furthermore, the predictions show that the wettability variation is one of the key factors of the saturation characteristics.

Summary Understanding the relationships between porous transport layer (PTL) morphology and oxygen removal is essential to improve the polymer electrolyte water electrolyzer (PEWE) performance. Operando X-ray computed tomography and machine learning were performed on a model electrolyzer at different water flow rates and current densities to determine how these operating conditions alter oxygen transport in the PTLs. We report a direct observation of oxygen taking preferential pathways through the PTL, regardless of the water flow rate or current density (1-4 A/cm 2 ). Oxygen distribution in the PTL had a periodic behavior with period of 400 μm . A computational fluid dynamics model was used to predict oxygen distribution in the PTL showing periodic oxygen front. Observed oxygen distribution is due to low in-plane PTL tortuosity and high porosity enabling merging of oxygen bubbles in the middle of the PTL and also due to aerophobicity of the layer.

Enhancement of fuel cell performance at high current densities is essential to improve the overall power density and to reduce the cost of proton exchange membrane fuel cell (PEMFC) systems. Mass transport over-potential is the major barrier to achieving high performance at a high current density. Condensed water, specifically in the gas diffusion layer (GDL), reduces oxygen transport to the oxygen reduction reaction (ORR) area. Experimental investigations of oxygen transport are limited by an inability to resolve the water saturation-dependent properties. The alternative approach to understand and overcome transport resistances, particularly inside the GDL, is to use state-of-the-art mathematical modeling. This work shows the successful development of a multi-scale calculation technique with co-simulation approach that incorporates a detailed structure of each scale dimension for every component of a fuel cell. The flow-field bipolar plates and membrane electrode assembly (MEA) models are calculated using traditional computational fluid dynamics (CFD) method with existing PEMFC model; whereas the detail structured GDLs are numerically predicted by Lattice Bolzmann method (LBM). This technique can be used to develop material and design solutions to improve the mass transport; which is the most critical for high end performance and operational robustness.

The direct modeling-based Lattice Boltzmann Agglomeration Method (LBAM) is used to explore the electrochemical kinetics and multi-scalar/multi-physics transport inside the detailed structure of the porous and catalyst layers inside polymer electrolyte membrane fuel cells (PEMFCs). The complete structure of the samples is obtained by both micro-and nano-X-ray computed tomography (CT). LBAM is able to predict the electrochemical kinetics in the nanoscale catalyst layer and investigate the electrochemical variables during cell operation. This work shows success in integrating the lattice elements into an agglomerate structure in the catalyst layer. The predictions of LBAM were compared with a macro-kinetics model and experimental data. The overall predictions reveal that the local saturation of liquid water, distributions of electrochemical variables, and mass fraction across the samples can be controlled by the regulation of operating conditions. LBAM is a highly effective method of predicting the partial flooding issue, understanding the transport resistance, and investigating transport inside the porous transport layer that affects the overall cell performance in the PEMFC. The outcome of this work will be used for the optimization of porous structure design, durability, and water management improvement, for novel porous materials, particularly in the catalyst layer.

Gas diffusion layers (GDLs) are porous carbonaceous layers that are widely used in energy conversion and storage devices. Simulation of water transport through GDLs, in a polymer electrolyte fuel cell (PEFC), for example, typically uses goniometer-measured external contact angles. Until now, there is no well-developed method to obtain contact angles inside the GDLs. AlRatrout et al. developed an open-source code to compute local contact angles at triple-phase contact points from segmented micro-X-ray computed tomography (X-ray CT) images of porous rocks. We apply it, for the first time, to micro-X-ray CT images of water-filled commercial GDLs and compute local contact angles at internal GDL fiber−water−air triple-phase contact points. We obtain a state of mixed wettability (hydrophilic and hydrophobic) inside all GDL samples, with a broad range of contact angles, instead of one hydrophobic contact angle found from goniometer experiments. Lattice Boltzmann water transport simulations performed with these distributed contact angles produce results that are in better agreement with experimental data. We also obtain high-resolution X-ray photoelectron spectroscopy (XPS) data of the GDL samples and find that the concentration of oxide species correlates strongly with the measured hydrophilicity. The method introduced here can help rationally design GDLs and directly quantify their internal surface wettability that is needed for accurate predictions of their functionality in energy technology devices.

Proton exchange membrane fuel cells (PEMFC) require a gas diffusion layer (GDL) to aid in the transport of liquid fuel to the catalyst layer. In this work, direct modeling using the Lattice Boltzmann Method (LBM) was applied to X-ray CT scans of four different carbon gas diffusion layers to understand the mass transport properties through the samples. Three injection orientations were used to study local saturation levels, water evolution through the sample, and mass transport behavior at breakthrough conditions. The LBM, combined with computational fluid dynamic modeling techniques, can accurately predict liquid saturation at the macro and micro scale, which provides more insight into the mass transport phenomena through the GDL. The change of pore structure and orientation in both the in-plane and through-plane determines the path that liquid water must take, which could aid or impact PEMFC performance. The outcomes from this work will also benefit any research that needs knowledge of internal mass transport qualities of gas diffusion media.