Elsevier

Electrochimica Acta

Volume 256, 1 December 2017, Pages 279-290
Electrochimica Acta

Investigating Phase‐Change‐Induced Flow in Gas Diffusion Layers in Fuel Cells with X‐ray Computed Tomography

https://doi.org/10.1016/j.electacta.2017.10.012Get rights and content

Abstract

The performance of polymer‐electrolyte fuel cells is heavily dependent on proper management of liquid water. One particular reason is that liquid water can collect in the gas diffusion layers (GDLs) blocking the reactant flow to the catalyst layer. This results in increased mass‐transport losses. At higher temperatures, evaporation of water becomes a dominant water‐removal mechanism and specifically phase‐change‐induced (PCI) flow is present due to thermal gradients. This study used synchrotron based micro X‐ray computed tomography (CT) to visualize and quantify the water distribution within gas diffusion layers subject to a thermal gradient. Plotting saturation as a function of through‐plane distance quantitatively shows water redistribution, where water evaporates at hotter locations and condenses in colder locations. The morphology of the GDLs on the micro‐scale, as well as evaporating water clusters, are resolved, indicating that the GDL voids are slightly prolate, whereas water clusters are oblate. From the mean radii of water distributions and visual inspection, it is observed that larger water clusters evaporate faster than smaller ones.

Introduction

The performance of polymer‐electrolyte fuel cells (PEFCs) and other multiphase flow technologies is significantly dependent on liquid‐water management [1], [2], [3], [4]. This is particularly true for PEFCs at low operating temperatures and during startup operations due to hindered reactant delivery by water in the cathode [5], [6], [7], [8], [9]. Because of the exothermic oxygen reduction reaction (ORR) at the cathode, a thermal gradient develops during operation in the through‐plane direction, with the hottest location in the catalyst layer (CL). At higher temperatures (∼80 °C), this thermal gradient, in combination with the dependence of vapor pressure on temperature, promotes removal of water in a vapor form [1], [4], [5], [10], [11], [12], [13]. Water vapor within the CL travels through the gas diffusion layer (GDL) to the gas channels (GCs) (see Fig. 1), where it condenses due to the decrease in temperature. This type of flow, which is due to the evaporation and condensation of water, is known as phase‐change‐induced (PCI) flow [4], [5], [14]. Wang and Wang [4] was the first study in the PEFC field to investigate this phenomenon using a multi-dimensional model that incorporated electrochemical heat generation, phase-change, two-phase flow, non-constant gas phase pressure, and the entire PEFC domain. By doing so, the authors in [4] also clearly identified the importance of thermal gradients as the driving force for PCI flow within the porous media of the fuel cell. Although water is removed in the vapor phase, depending on a PEFC’s operating temperature, a fraction of the total water has to still be removed in the liquid phase. Thus PEFCs experience two‐phase water flow and, consequently, substantially coupled heat and mass transport. As such, effective water management requires an understanding of the interaction between pressure‐driven, capillary‐driven, and PCI water transport [4], [5]. Phase change is not a drive potential or force like pressure and capillary forces, however, the term “PCI” has become the common name in literature for heat‐driven mass transport of water by evaporation and condensation in a temperature gradient.

The GDL is a porous fibrous component of PEFCs responsible for the transport of electrons, water byproduct, gaseous reactants, and heat [15]. It is made from carbon fibers which are assembled to form either nonwoven paper, woven cloth, or felt. With pores on the order of 10 μm, these materials have porosities typically ranging from 65 % to 90 % and thickness around 200–400 μm [16], [17]. Generally, cell compression influences the GDL’s structure [15] and performance during operation. Because carbon fibers are naturally hydrophilic, GDLs are typically treated with 5–20 % of polytetrafluoroethylene (PTFE). Due to non‐uniformities in the coating, there is a mix of hydrophilic and hydrophobic pores, which causes the overall structure to possess mixed wettability [18], [19]. As with most porous media, heat and mass transport properties depend on local morphology in addition to bulk material properties.

Most scientific work concerning transport in porous media has been conducted in the fields of civil and petroleum engineering [20], [21], [22], [23], [24]. Although this provides a starting point, there are a number of notable differences between the systems studied in those fields and thin materials such as GDLs and CLs. It is necessary to re‐examine each of the various transport mechanisms as they pertain to engineered systems [18]. To this end, much has already been accomplished for transport mechanisms guided by capillary, convection, and gravitational forces. Non‐isothermal phenomena, on the other hand, remain an area that is not well researched [1], [5], [18]. Amongst existing non‐isothermal studies, most do not address multiphase flow; let alone phase change [1]. Furthermore, those that do address multiphase flow are typically simulation‐based [18] due to difficulties with an experimental approach [1].

Previous studies have shown PCI flow to be a significant contributor to overall water transport within PEFCs. For instance, Weber and Newman [1], through use of one‐dimensional simulations, showed that non‐isothermal effects are significant when feed gas flows are or become saturated. According to their results, net evaporation/condensation accounts for only 2.6 % of overall heat generation within a fuel cell. However, the heat generated/consumed by each individually is approximately 100 times that of the net contribution. Additionally, their work shows that a thermal gradient of only a few degrees is required across the GDL to completely remove product water from the CL, with larger thermal gradients needed at lower temperatures. As noted by Kumbur and Mench [18], the GDL provides one of the largest thermal resistances in a PEFC and therefore may experience a temperature gradient in excess of 5 °C. Kim and Mench [5] conducted an experimental study of PCI flow in which they tested various membrane‐GDL combinations. It was found that PCI flow does dominate net water flux at high temperatures (80 °C). Furthermore, it was shown that incomplete saturation of the porous media is key to determining whether or not PCI flow will occur.

Over the last several years, there has been a significant effort in characterizing morphology and water distribution within the pores of the GDL by using X‐ray computed tomography (CT) [25], [26], [27], [28], [29], [30], [31], [32]. Micro‐CT, with a resolution of 1.3 μm, is well fit to non‐destructively visualize three‐dimensional GDL structures and water filling of GDL pores [15], [27], [33]. Recent studies indicate that, during PEFC operation, liquid water occupies less than 50 % [34] of the GDL pore volume because of the GDL’s hydrophobic treatments, and, in the absence of temperature gradients, capillary fingering is the predominant liquid‐water‐transport mechanism [32], [35]. Previously, X‐ray CT was used to study the evaporation of water within GDLs under constant temperature. It was found that evaporation rates at water saturations higher than 10 % scale with the surface area of water and are diffusion limited [15].

In this study, a novel X‐ray CT technique to explore PCI flow within a PEFC is presented. Coupled measurements of temperature, thermal gradients, and thermal conductivity are combined with visualizations of GDL morphology and water distribution. The overall results of this study contribute to the general understanding of evaporation phenomena in porous media pertaining to PEFCs.

Section snippets

Sample Apparatus

A custom apparatus (Fig. 2), was designed and fabricated to conduct the experiment at the synchrotron X‐ray CT beamlines. The design aimed to control sample compression, temperature, temperature gradient, and water capillary pressure. The apparatus adheres to size restrictions for the two different X‐ray CT beamlines where data was collected and achieves a balance between the needs for structural stability and an un‐obstructed view of the sample. The sample sits inside a polyetheretherkeytone

Thermal Conductivity

Assuming one‐dimensional heat flow though the setup, heat flux and temperature are coupled by the following formulation of Fourier’s Law.q=kΔTΔxwhere q is heat flux per unit area, k is thermal conductivity, ΔT is temperature difference, and Δx is position difference. Thermal resistance, R [K W−1], can be used to rearrange Eq. (1) as follows were Q is heat flux and A is cross‐sectional area:ΔT=QRR=ΔxkA=ΔTQFig. 3c shows relevant temperature locations. Taking T3/T4 to be at 0 mm along their

Thermal Considerations

Fig. 4 shows mean temperature of the GDL, temperature drop across it, and calculated thermal conductivity as a function of heat flux. Per each heat flux value, multiple measurements are shown. These are measurements for different times as plotted by Fig. 5. Fig. 4a–b clearly show the same trends observed in Fig. 5a–b. As heat flux increases, so does the mean temperature of the sample. On the other hand, the temperature drop does not show a clear trend. Fig. 4c shows a gradual decrease in

Conclusion

X‐ray computed tomography (CT) and a custom sample apparatus were used to examine phase‐change‐induced (PCI) flow within SGL10BA. A thermal gradient was imposed on the stack of two GDLs and water was injected to emulate the operating conditions within a polymer‐electrolyte fuel cell (PEFC). Once a pseudo‐steady thermal state was reached, the sample was scanned 4–5 times per heat flux chosen to collect the necessary tomographic data. Tomographic scans were conducted at several heat flux values

Acknowledgements

The authors would like to acknowledge support from the National Science Foundation under CBET Award 1605159. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract

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