Liquid Water Accumulation Research Articles

Microporous Layer Degradation in Polymer Electrolyte Membrane Fuel Cells

In this work, microporous layer (MPL) degradation was investigated through the characterization of polymer electrolyte membrane (PEM) fuel cell performance and in situ liquid water visualizations. While both the MPL and carbon fiber substrate underwent ex situ carbon corrosion-based degradation, the degradation of the MPL has the most significant impact on the electrochemical performance and the liquid water distribution within the operating PEM fuel cell. Specifically, MPL degradation resulted in larger quantities of liquid water accumulation within the gas diffusion layer (GDL), and we attributed this accumulation to the loss of MPL hydrophobicity caused by the carbon corrosion-based degradation process. The increased liquid water accumulation led to increased mass transport resistances and performance losses at high operating current densities (> 1.5 A/cm2). With increasing current density, the liquid water saturation profile converged to an upper threshold within GDLs with MPLs, whereas an upper liquid water saturation threshold was not observed for GDLs without MPLs. Predictions of long-term performance characteristics of PEM fuel cells should be informed by the proneness of the MPL to carbon corrosion degradation.

Investigation of PEMFC fault diagnosis with consideration of sensor reliability

Despite the wide range of applications for the polymer electrolyte membrane fuel cell (PEMFC), its reliability and durability are still major barriers for further commercialization. As a possible solution, PEMFC fault diagnosis has received much more attention in the last few decades. Due to the difficulty of developing an accurate PEMFC model incorporating various failure mode effects, data-driven approaches are widely used for diagnosis purposes. These methods depend largely on the quality of sensor measurements from the PEMFC. Therefore, it is necessary to investigate sensor reliability when performing PEMFC fault diagnosis.In this study, sensor reliability is investigated by proposing an identification technique to detect abnormal sensors during PEMFC operation. The identified abnormal sensors will be removed from the analysis in order to guarantee reliable diagnostic performance. Moreover, the effectiveness of the proposed technique is investigated using test data from a PEMFC system, where fuel cell flooding is observed. During the test, due to accumulation of liquid water inside the PEMFC, the humidity sensors will give misleading readings, and flooding cannot be identified correctly with inclusion of these humidity sensors in the analysis. With the proposed technique, the abnormal humidity measurements can be detected at an early stage. Results demonstrate that by removing the abnormal sensors, flooding can be identified with the remaining sensors, thus reliable health monitoring can be guaranteed during the PEMFC operation.

Challenges in Computational Modeling of Two-Phase Transport in Polymer Electrolyte Fuel Cells Flow Channels: A Review

Excess of liquid water in gas channels of polymer electrolyte fuel cells is responsible for malfunctioning of these devices. Not only it decreases their efficiency via partial blockage of reactants and pressure drop, but it can also lead to the irreversible damage due to oxygen starvation in case of complete channel flooding or full coverage of the gas diffusion layer with a liquid film. Liquid water evacuation is carried out via airflow in gas channels. Several experimental and computational techniques have been applied to date for the analysis of the coupled airflow–water behavior in order to understand the impact of fuel cell design and operation regimes upon the liquid water accumulation. Considerable progress has been achieved with the development of sophisticated computational fluid dynamics (CFD) tools. Nevertheless, the complexity of the problem under consideration leaves several issues unresolved. In this paper, analysis techniques applied to liquid water–airflow transport in fuel cells gas channels are reviewed and most important results are summarized. Computationally efficient, yet strongly simplified analytical models are discussed. Afterwards, CFD approaches including the conventional fixed grid (Eulerian) and the novel embedded Eulerian–Lagrangian models are described. Critical comparative assessment of the existing methods is provided at the end of the paper and the unresolved issues are highlighted.

Simultaneous characterization of oxygen transport resistance and spatially resolved liquid water saturation at high-current density of polymer electrolyte membrane fuel cells with varied cathode relative humidity

In this study, the impacts of relative humidity on liquid water accumulation and mass transport resistance at high current densities were investigated for polymer electrolyte membrane (PEM) fuel cell cathode gas diffusion layers (GDLs). Through-plane liquid water saturation distributions were measured in situ using synchrotron X-ray radiography while simultaneously performing limiting current-based characterizations of oxygen transport resistance. In experiments with inlet cathode reactant relative humidity ranging from 0% to 100%, high local saturations (>0.6) were consistently observed in the GDL region adjacent to the flow field lands when the fuel cell reached limiting current. High land-region saturation is a possible indication of lower local temperatures. In the carbon fiber substrate portion of the GDL (i.e. excluding the MPL), the liquid water volume in the regions adjacent to the flow field channels was consistently small relative to the water volume adjacent to the land regions. We observed a trend of increasing liquid water saturation of the GDL adjacent to the channels with increasing levels of cathode inlet relative humidity, along with corresponding small increases in total oxygen transport resistance.

A Comparison of Felt-Type and Paper-Type Gas Diffusion Layers for Polymer Electrolyte Membrane Fuel Cell Applications Using X-Ray Techniques

This work presents a comparison between carbon felt-type and paper-type gas diffusion layers (GDLs) for polymer electrolyte membrane (PEM) fuel cells in terms of the similarities and the differences between their microstructures and the corresponding manner in which liquid water accumulated within the microstructures during operation. X-ray computed tomography (CT) was used to investigate the microstructure of single-layered GDLs (without a microporous layer (MPL)) and bilayered GDLs (with an MPL). In-operando synchrotron X-ray radiography was used to visualize the GDL liquid water accumulation during fuel cell operation as a function of current density. The felt-type GDLs studied here exhibited a more uniform porosity in the core regions, and the carbon fibers in the substrate were more prone to MPL intrusion. More liquid water accumulated in the felt-type GDLs during fuel cell operation; however, when differentiating between the microstructural impact of felt and paper GDLs, the presence of an MPL in bilayered GDLs was the most influential factor in liquid water management.

Distributed parameter model-based control of water activity and concentration of reactants in a polymer electrolyte membrane fuel cell

Water management is still a key challenge for optimal performance and durability of polymer electrolyte membrane (PEM) fuel cells. Water levels along the channel in a PEM fuel cell present important spatial variations that should be taken into account to avoid both local flooding and local drying. In this work, a decentralised model predictive control scheme is designed to maintain the water activity on both anode and cathode sides of the PEM at appropriate levels. The proposed strategy tackles the accumulation of liquid water on the surface of the catalyst layers, and the possibility of local drying, by controlling observed water activity spatial profiles. Classic PEM fuel cell issues like reactant starvation are also considered. High control performance is achieved. The strategy is applied to a validated distributed parameter PEM fuel cell model. Results show increased cell power density in comparison to non-spatial control strategies.

Visualization of Liquid Water Distribution at Interfaces of PEFC by Using Parallel-Beam Soft X-ray Radiography

Because of increasing expectation of the use of PEFC, further improvement of the performance is still demanded. Since the liquid water generation and its accumulation inside the PEFC is one of the most influential phenomena for the electric generation performances, it is required to obtain knowledge about the actual liquid water distribution inside the PEFC and its generation/drainage process in order to promote the efficiency of the PEFC. Particularly, the detailed liquid water behavior at the interfaces has not been provided in spite of its large impact on the liquid water drainage performances. By using soft X-ray radiography, our laboratory has reported liquid water distribution inside the PEFC. In principle, however, the conventional cone beam type X-ray used so far has difficulty to capture the interfaces because of the widening effect of the X-ray beam. This problem can be solved by parallel type X-ray beam. In this study, we introduce a parallel beam type X-ray microscope to improve the accuracy of the interface measurement. We especially focus on the liquid water distribution at the GDL/MPL/CL/membrane interfaces. In this study, in-plane measurement is performed to obtain two-dimensional visualization having rib/channel configuration. The visualization result operated at 30℃ cell temperature, 80% relative humidity and 0.6A/cm2 current density, is shown in fig.1. In the wide region on the cathode rib, accumulation of liquid water is observed same as reported in some other previous studies. Also, it is clearly shown that, in the CL/MPL interface of cathode side, film-like water distribution is created due to capillary pressure gap. In the presentation we will also focus on the generation/drainage process of the liquid water including intermittent behavior on the interfaces. This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Figure 1

Fighting the Noise: Towards the Limits of Subsecond X-ray Tomographic Microscopy of PEFC

Management and removal of liquid water is essential to maintain and improve the overall performance of polymer electrolyte fuel cells (PEFC). X-ray tomographic microscopy (XTM) of PEFCs has been proven as a valuable tool in order to understand accumulation of liquid water in the gas diffusion layer (GDL), both with in-situ [1-3] and operando setups [4-8]. Progress in operando XTM of PEFCs has paved the way for 4D imaging studies of the water distribution in the GDL [9]. In order to capture the water dynamics at high current density operation a further decrease of the scan time towards 0.1 s is aspired and the consequences of the reduced signal to noise ratio in the XTM data on water detectability need to be quantified. In this work different imaging parameters and their consequences on the contrast-to-noise ratio (CNR) of water versus void have been studied with an ex-situ XTM experiment at the TOMCAT beamline of the Swiss Light Source (SLS) at Paul Scherrer Institut (PSI). The cathode channels of a double channel XTM cell [8] have been filled with liquid water, while keeping the anode channels empty, as shown in Figure 1. The CNR(H2O/Void) in the gas channels serves as an indicator for contrast between liquid water and void in the GDL to derive optimized imaging parameters for operando conditions. Over a range of monochromatic beam energies (13.5 – 21 keV) and scan times (0.2 – 10 s) a beam energy of 13.5 keV was found to be mostly appropriate as it provides the highest CNR value. The presentation will focus on the consequences of sub-second XTM acquisition time on the image processing pipeline for the detectability of small water droplets and their connecting paths.

The Impacts of Microporous Layer Degradation on Liquid Water Distributions in Polymer Electrolyte Membrane Fuel Cells Using Synchrotron Imaging

The polymer electrolyte membrane (PEM) fuel cell, featuring high power density and zero-local emissions, is a favorable candidate for replacing the traditional internal combustion engine in automotive vehicles. The gas diffusion layer (GDL) in the PEM fuel cell facilitates water management and transport of reactant gases. The GDL is typically coated with a microporous layer (MPL) to improve water management and minimize contact resistance at the catalyst layer/GDL interface, further enhancing overall cell performance (1, 2). Achieving long-term durability is one of the major obstacles to the realization of commercial success for PEM fuel cells. The majority of PEM fuel cell durability research has been focused on the impacts of catalyst layer and membrane degradation on the cell performance, rather than the GDL. However, high quantities of liquid water accumulation in the GDL were reported in a fuel cell with aged (degraded) GDLs compared to a fuel cell with pristine GDLs, leading to significant performance losses through increased mass transport resistance (3). Furthermore, the MPL experienced up to twice the mass loss from the carbon substrate attributed to a carbon-corrosion based accelerated degradation protocol (4). Therefore, a clear understanding of the effects of MPL degradation on the liquid water behavior via in operando visualization is essential. In this study, as-received pristine GDLs were immersed into a heated hydrogen peroxide solution to perform an accelerated degradation process. The porosity distributions of pristine and degraded GDLs were characterized using desktop micro-computed tomography. Pristine and degraded GDLs were then assembled identically in a customized fuel cell for performance comparison and in operando liquid water visualizations conducted using synchrotron X-ray radiography. Synchrotron X-ray radiography has been demonstrated as a highly precise and accurate tool for in operando visualizing liquid water distribution in PEM fuel cells (5). It was found that the presence of the MPL was beneficial for water management in the GDL, further enhancing the cell performance. However, the MPL itself was more prone to degradation (compared to the substrate), leading to significant performance losses. Figure 1 presents the experimental through-plane liquid water distributions in pristine and degraded Sigracet 29 BC GDLs at an operating current density of 2.0 A/cm2. The leftmost location (x = 0 μm) represents the through-plane position of the catalyst layer/MPL interface and the rightmost location (x= 162.5 μm) represents the through-plane position of GDL/flow field interface. Degradation of the MPL resulted in higher quantities of liquid water in the MPL region, as well as in the carbon substrate region. The impacts of MPL degradation on liquid water behavior will be presented in this work, further highlighting impacts of degradation on liquid water management in long-term fuel cell operation. Reference: 1. J. Lee, J. Hinebaugh and A. Bazylak, J.Power Sources., 227(2013). 2. I. Zenyuk, E. Kumbur and S. Litster, J.Power Sources., 241(2013). 3. H. Liu, M. G. George, N. Ge, R. Banerjee, S. Chevalier, J. Lee, P. Shrestha, D. Muirhead, J. Hinebaugh, R. Zeis, M. Messerschmidt, J. Scholta and A. Bazylak, ECS Trans., 75,14(2016). 4. Liu H., George M.G., Messerschmidt M., Zeis R., Kramer D., Scholta J. and A. Bazylak, Journal of the Electrochemical Society.,(Submitted). 5. S. Chevalier, N. Ge, J. Lee, R. Banerjee, H. Liu, G. George, P. Shrestha, D. Muirhead, J. Hinebaugh and Y. Tabuchi, J.Power Sources., (2016). Figure 1

The Impacts of Microporous Layer Degradation on Liquid Water Distributions in Polymer Electrolyte Membrane Fuel Cells Using Synchrotron Imaging

In this work, commercial gas diffusion layers (GDLs) both with and without microporous layers (MPLs) were degraded through an accelerated degradation process. Pristine and degraded GDLs were each assembled in a customized fuel cell to investigate MPL degradation impacts on the distribution of liquid water via synchrotron X-ray radiography. The presence of an MPL reduced the liquid water accumulation within the GDL, which provided benefits for the performance of the fuel cell. However, the MPL was more strongly affected by the degradation process compared to the carbon fiber substrate, leading to an increased mass transport resistance and a notable performance degradation. In the degraded GDL with an MPL, higher quantities of liquid water accumulated within the GDL, as well as at the catalyst layer/MPL, MPL/carbon fiber substrate, and GDL/flow field interfaces.

Visualization of Liquid Water Distribution at Interfaces of PEFC by Using Parallel-Beam Soft X-ray Radiography

Liquid water behavior inside polymer electrolyte fuel cell (PEFC) is investigated by using X-ray radiography. In the present study, liquid water distribution in regions close to interfaces is accurately visualized by using parallel beam type X-ray microscope and computer image processing. Long interval imaging shows liquid water accumulation at an interface between catalyst layer (CL) and micro porous layer (MPL). It is also clearly shown that volume of liquid water inside MPL is much smaller than that inside gass diffusion layer (GDL). Short interval imaging reveals behavior of temporal development of liquid water accumulation.

Fighting the Noise: Towards the Limits of Subsecond X-ray Tomographic Microscopy of PEFC

Management and removal of liquid water is essential to maintain and improve the overall performance of polymer electrolyte fuel cells (PEFC). X-ray tomographic microscopy (XTM) of PEFCs has been proven as a valuable tool in order to understand accumulation of liquid water in the gas diffusion layer (GDL), both with in-situ and operando setups. Progress in operando XTM of PEFCs has paved the way for 4D imaging studies of the water distribution in the GDL. In order to capture the water dynamics at high current density operation a further decrease of the scan time towards 0.1 s is aspired. In this work different imaging parameters, filters and their consequences on the contrast-to-noise ratio (CNR) of water versus void and water feature detectability have been studied with an ex-situ XTM experiment at the TOMCAT beamline of the Swiss Light Source (SLS) at Paul Scherrer Institut (PSI).

(Invited) Open-Source Macro- and Meso-Scale Modeling of PEFCs

Since the early polymer electrolyte fuel cell (PEFC) mathematical modeling studies of Springer and Bernardi and Verbrugge [1], a variety of steady-state and transient PEFC models have emerged in the literature, see [2] and references within. Fuel cell modeling results have highlighted the importance of multi-dimensional effects (e.g., dead-zones under the land area [3]), coupled physical processes (e.g., heat-pipe effects [4]), liquid water accumulation, and interfacial effects (e.g., water/channel/porous media interactions [5]). Even though substantial progress has been made in developing meso- and macro-scale models, a generally accepted PEFC model does not yet exist, even for dry conditions. The absence of such model is due to: a) multiple scales under consideration, i.e., from nanometers in the catalyst layer to millimeters in the channel; b) the complex heterogeneous materials used; and, c) the coupled physical processes occurring inside the PEFC. The small number of experimental validation techniques used, and the limited number of comparative numerical modeling studies has also contributed to the disagreement between modeling approaches. A common platform for numerical modeling of fuel cells is essential in order to integrate new models, compare and assess the validity of existing ones on a one-to-one basis, and eventually, to perform fuel cell design and optimization studies. Our research group has developed an open-source mathematical modeling platform for the analysis of polymer electrolyte fuel cells, i.e., OpenFCST [6]. This modeling platform has been implemented using object-oriented concepts in order to develop a toolbox with multitude of physical models, such as multi-step reaction kinetics, gas, liquid and ion transport equations. It is also capable of handling various theoretical representations of the main layers in a fuel cells, for example, the catalyst layer can be analyzed with either a macro-homogeneous or an agglomerate model. Users can easily modify the mathematical formulation for an individual physical model while maintaining all other models and parameters the same. In this talk, the general architecture of OpenFCST and the new models implemented since its first release in 2013 [6] will be discussed. OpenFCST will be used to analyze several recently proposed mathematical models for fuel cells. Critical to improving PEFC mathematical models is the development of: 1) methodologies to estimate macro-scale reaction and transport properties based on meso- and micro-scale information of the heterogeneous materials, such as micro-scale imaging, electrode kinetic mechanisms, and pore-size distributions; 2) accurate transport models for multi-component gas transport, and multi-phase flow; and, 3) improved models for the interfaces between layers in the membrane electrode assembly (MEA) and also between the MEA and the channel. In this talk, new advances in micro-scale simulations using FIB-SEM data in OpenFCST that include voxel meshing and a morphological image opening algorithms will be presented and used to estimate effective transport properties of CL under dry and wet conditions [8]. A multi-dimensional, two-phase, non-isothermal membrane electrode assembly model in OpenFCST that includes a dual wettability pore-size distribution model will be described and compared to: a) experimental performance, water fluxes and temperature and saturation distributions, and b) to the common saturation-based, two-phase flow transport model in porous media. The pore-size distribution model is shown to be able to identify when liquid water would start leaving the cell, and possible discontinuities in the saturation profiles inside the MEA. A key issue in two-phase flow models is the channel/GDL interface. A two-phase flow finite element formulation that uses two independent meshes for air and liquid will also be described and used to study droplet dynamics in a channel [5].

Understanding the Role of the Micro-Porous Layer on Fuel Cell Performance Using a Non-Isothermal, Two-Phase Model

Water management is a critical factor in improving fuel cell performance at high current densities [1]. Under dry conditions, keeping the ionomer phase in polymer electrolyte membrane (PEM) and catalyst layers (CLs) sufficiently hydrated is critical to maintaining high protonic conductivity and reducing the ohimc losses. When the cell is operating at high relative humidity, removing the excessive water generated in the electrodes is of importance in order to avoid liquid water accumulation and achieve high performance. An efficient membrane electrode assembly (MEA) design therefore requires an extensive understanding of water management in fuel cells. The use of a micro-porous layer (MPL) in fuel cells is known to improve water and heat management resulting in a better performance. Experimental evidence has shown that remarkable fuel cell performance improvements are still possible by modifying the MPL composition [2] and micro-structure, for instance, by milling holes [3]. In situ heat and water flux measurements conducted by Thomas et al. [4] showed that by inserting an MPL between CL and gas diffusion layer (GDL), the temperature in the electrodes increased by at least 1° C at high current density. The warmer electrode therefore, results in a higher evaporation which also facilitates the diffusive transport of water vapour by creating a higher concentration gradient. Based on experimental observations from the ex situ diffusive vapour flux and liquid permeation flux measurements as well as in situ electrochemical performance with varying MPLs, Owejan et al. [5] hypothesized the role of MPL is to prevent the condensed water in the GDL from forming a liquid film at the GDL/CL interface and creating an in-plane diffusive path for reactant. In their study, the thermal conductivity in porous layers demonstrated a significant impact on improving the performance by creating a higher temperature driven diffusive flux. In order to understand the effects of MPL on capillary-driven flow and phase change induced flow, a multi-dimensional, non-isothermal, two-phase numerical model is developed in OpenFCST [6]. The porous media transport properties for two-phase flow are estimated using a micro-scale mathematical pore size distribution model [1] which is capable of accounting for the layer mixed wettabilities and micro-structure. Experimental validation of two-phase flow models is rarely performed even though it is of importance. In this study, the electrochemical performance of an MEA with a SGL 24BA and SGL 24BC is measured in our laboratory at varying operating conditions and also predicted using the numerical two-phase model. Membrane water transport, water fluxes in liquid and vapour form at cathode boundary, and the phase change induced flow are analyzed for studies with and without an MPL. The performance analysis under hot/dry condition indicates that adding an MPL in the dry condition results in an excessive protonic transport loss in the membrane, especially at high current density. Under hot/wet condition, the additional heat preserved on the electrodes by adding the MPL leads to a substantial reduction in water accumulation. Simulation results at the cold/wet condition highlight that adding an MPL not only leads to a decrease in water accumulation in the electrode but also creates an in-plane diffusion pathway for gas transport in the cathode. The increase in temperature in the electrode also results in a decrease in relative humidity, especially at the anode. This leads to a higher membrane water content gradient between the fully saturated cathode and the anode which results in a higher back diffusion. A parametric study of MPL thermal conductivity suggests that the excessive water in the cathode can be removed as water vapour by decreasing the MPL thermal conductivity under fully humidified conditions. However, an extremely low MPL thermal conductivity can lead to a significant deterioration of performance even at high relative humidity due to the membrane dehydration. The paramteric study highlights the optimal MPL conducivity for our cell. An optimal MPL design requires a comprehensive water balance between membrane hydration and sufficient electrode water evaporation.

The experimental analysis of a dead-end H2/O2 PEM fuel cell stack with cascade type design

Proton exchange membrane fuel cells (PEMFCs) with a dead-ended anode and cathode can reach high hydrogen and oxygen utilization by a relatively simple system. Nevertheless, the accumulation of the water in the anode and cathode channels can lead to a local fuel starvation deteriorating the performance and the durability of PEMFCs. In this study, a novel design for a polymer electrolyte membrane (PEM) fuel-cell stack was presented which could achieve higher fuel utilization without using hydrogen and oxygen recirculation devices such as hydrogen pumps or ejectors that consume parasitic power and require additional control schemes. The basic concept of the innovatively proposed design was to divide the cells of a stack into several stages by conducting the outlet gas of each stage to a separator and reentering it into the next stage; thereby, a multistage anode and cathode system was prepared. In this relatively ingenious design, a higher gaseous flow rate was maintained at the cell outlet, even under dead-end conditions resulted in a reduced purge-gas emission by avoiding the accumulation of liquid water in the cells. The results revealed that proposed design had the same polarization curve as the open-end mode, leading to an enhanced PEMFC performance.

Investigating the effects of gas diffusion layer substrate thickness on polymer electrolyte membrane fuel cell performance via synchrotron X-ray radiography

Synchrotron X-ray radiography was used to visualize the liquid water accumulation in polymer electrolyte membrane (PEM) fuel cells to compare the impact of carbon substrate thickness on water management. A differential fuel cell with an active area of 0.68cm2 and rib/channel width of 0.2mm was custom-made to provide 1-dimensional (1D) conditions over the active area. The fuel cell with the thin substrate (TGP-H-030) outperformed the fuel cell with the thick substrate (TGP-H-060). The fuel cell with the thinner substrate exhibited a higher limiting current density, less liquid water in the microporous layer (MPL)-substrate transition region, and reduced oxygen transport resistance measured through electrochemical impedance spectroscopy (EIS). The compression behaviour of each GDL was also investigated through two consecutive fuel cell assemblies. The pressure in the second assembly was lower than that for the initial assemblies for both GDLs, and this significant change in assembly pressure was more pronounced for the thinner GDL (TGP-H-030). The resulting interfacial contact between the catalyst layer and the GDL was degraded, which manifested in the microscale displacement of fuel cell materials during operation (detected as a negative liquid water thickness). While the thinner GDL provided superior performance, the long term effects of material deformation may exacerbate a heterogeneous distribution of liquid water that could also impact the performance.

Novel electrospun gas diffusion layers for polymer electrolyte membrane fuel cells: Part II. In operando synchrotron imaging for microscale liquid water transport characterization

Abstract This is the second paper in a two-part series in which we investigate the impact of the gas diffusion layer structure on the liquid water distribution in an operating polymer electrolyte membrane (PEM) fuel cell through the procedures of design, fabrication, and testing of novel hydrophobic electrospun gas diffusion layers (eGDLs). In this work, fibre diameters and alignment in eGDLs are precisely controlled, and concurrent synchrotron X-ray radiography and electrochemical impedance spectroscopy (EIS) are used to evaluate the influence of the controlled eGDL parameters on the liquid water distribution and on membrane liquid water content. For eGDLs with small fibre diameters (150–200 nm) and correspondingly smaller pore sizes, reduced liquid water accumulation under the flow field ribs is observed. However, more liquid water is pinned onto the eGDL – at the interface with flow field channels. Orienting fibre alignment perpendicular to the flow field channel direction leads to improved eGDL-catalyst layer contact and prevents rib-channel membrane deformation. On the other hand, eGDLs facilitate significant membrane dry-out, even under highly humidified operating conditions at high current densities.

Accelerated Degradation of Polymer Electrolyte Membrane Fuel Cell Gas Diffusion Layers

In this work, as-received pristine GDLs were degraded (aged) through an accelerated ageing process by immersion into a 35% wt. solution of H2O2 at 90°C for 12 hours. Pristine and degraded GDLs were assembled in custom-built fuel cells for in operando synchrotron X-ray visualization. Electrochemical impedance spectroscopy (EIS) measurements were obtained, while synchrotron X-ray radiography was performed to investigate the effect of ageing on liquid water transport behavior. In both pristine and aged GDLs, the liquid water profile converged to an upper (terminal) threshold in the saturation pattern as the current density increased. The terminal steady state liquid water profile was established at a lower current density for the aged GDLs compared to that of the pristine GDLs. Higher water quantities of liquid water accumulated at the GDL/flow field channel and MPL/carbon fiber substrate interfaces of the fuel cell with an aged GDL compared to the fuel cell with a pristine GDL. The aged GDLs exhibited more liquid water accumulation at lower current densities. Increases in liquid water accumulation and reduced performance were attributed to the reduction in hydrophobicity of the GDL.

Water generation and transport below Europa's strike-slip faults

Jupiter's moon Europa has a very young surface with the abundance of unique terrains that indicate recent endogenic activity. Morphological models as well as spectral observations suggest that it might possess shallow lenses of liquid water within its outer ice shell. Here we investigate the generation and possible accumulation of liquid water below the tidally activated strike-slip faults using a numerical model of two-phase ice-water mixture in two-dimensional Cartesian geometry. Our results suggest that generation of shallow partially molten regions underneath Europa's active strike-slip faults is possible, but their lifetime is constrained by the formation of Rayleigh-Taylor instabilities due to the negative buoyancy of the melt. Once formed, typically within a few million years, these instabilities efficiently transport the meltwater through the shell. Consequently, the maximum water content in the partially molten regions never exceeds 10% which challenges their possible detection by future exploration mission.

Water build-up and evolution during the start-up of a PEMFC: Visualization by means of Neutron Imaging

A commercial 50 cm2 PEM fuel cell with serpentine flow fields was operated at 2.0 bar (a) and 60 °C with different relative humidity (RH) values for the inlet reactants (a matrix of 3 RH for anode and 3 RH for cathode). Between each test the cell was decompressed and liquid water was thus flushed out. The liquid water build-up and the time evolution during each experiment were recorded by means of Neutron Imaging. A qualitative and quantitative analysis of the results is presented in this work. It was observed that the dynamics of water build-up comprises three main stages, where the major difference is the liquid water accumulation rate. The onset location for the water appearance in the flow field channels was found to be determined by the flow field design, gravity and gas flow direction along the serpentine path. The time evolution of the water progressive accumulation along the flow field channels and cell active area is discussed.