Over the past few years, the importance of water management to the successful operation of polymer electrolyte membrane (PEM) fuel cells has stimulated an extensive research focus on liquid water ...transport and its effect on performance and durability. Empirical methods employed to investigate water transport in the fuel cell have the potential to provide useful feedback for developing empirical correlations and validating numerical models for fuel cell research and development. In this paper, a literature review is provided for the experimental techniques that have been applied to visualize liquid water in operating hydrogen PEM fuel cells and flow fields. The main hypotheses that have been proposed to describe liquid water transport in the gas diffusion layer (GDL) and current challenges will also be discussed.
This paper presents an overview of polymer electrolyte membrane fuel cell (PEMFC) stack testing. Stack testing is critical for evaluating and demonstrating the viability and durability required for ...commercial applications. Single cell performance cannot be employed alone to fully derive the expected performance of PEMFC stacks, due to the non-uniformity in potential, temperature, and reactant and product flow distributions observed in stacks. In this paper, we provide a comprehensive review of the state-of-the art in PEMFC testing. We discuss the main topics of investigation, including single cell vs. stack-level performance, cell voltage uniformity, influence of operating conditions, durability and degradation, dynamic operation, and stack demonstrations. We also present opportunities for future work, including the need to verify the impact of stack size and cell voltage uniformity on performance, determine operating conditions for achieving a balance between electrical efficiency and flooding/dry-out, meet lifetime requirements through endurance testing, and develop a stronger understanding of degradation.
For the first time, pore network modelling is applied to polymer electrolyte membrane (PEM) electrolyzers. Realistic sintered titanium powder-based porous transport layers (PTLs) are generated via ...stochastic modelling, and we determine the influence of PTL-catalyst coated membrane (CCM) contact, pore and throat sizes, and porosity on two-phase transport. The PTL-CCM interfaces exhibit large open pore spaces, which are catastrophic for effective reactant transport due to the localized preferential accumulation of oxygen gas. As expected, increasing the sizes of the pores and throats results in higher oxygen gas saturations; however, we concurrently observe a surprisingly dominant mass transport effect whereby liquid water permeability is enhanced. The numerically generated PTLs are further tailored for enhanced mass transport by imposing a porosity gradient. By increasing the porosity from the PTL-CCM interface to the PTL-flow field interface, lower gas saturations and higher liquid water permeabilities are observed. With the opposite porosity gradient, the majority of pores adjacent to the high porosity CCM region become invaded with oxygen gas. We recommend a backing layer (microporous layer) at the CCM-PTL interface that improves the contact and permeation of product oxygen away from the CCM, thereby enhancing the performance of the PEM electrolyzer.
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•First use of pore network modelling for polymer electrolyte membrane electrolyzers.•Large open pore spaces are catastrophic for effective liquid water transport.•Despite higher oxygen saturation larger pores and throats yield higher permeability.•PTL porosity gradients reduce gas saturations and improve permeabilities.•A backing layer (microporous layer) will improve contact and reactant permeation.
Synchrotron X-ray radiography was employed to visualize the distribution of liquid water throughout an operating polymer electrolyte membrane fuel cell (PEMFC). The behavior of liquid water in the ...fuel cells assembled with and without micro-porous layers (MPLs) was compared. Through-plane images under various operating conditions combined with PEMFC performance revealed that, under wet conditions, the MPL was associated with an increased number of breakthrough locations, resulting in higher operating potentials. With these results, it is suggested that the MPL prevents liquid water from agglomerating into larger clusters at the catalyst layer/MPL interface, thereby reducing the number of blocked pathways for oxygen transport.
► Synchrotron X-ray radiography was used to visualize dynamic liquid water transport in a PEMFC. ► Breakthrough locations for PEMFCs with and without MPLs were compared. ► MPLs prevent the formation of large connected clusters of liquid water in the GDL.
Stochastic modeling of GDL structures requires a detailed characterization of the constituent elements of the material. In this work, a variety of imaging methods, including optical microscopy, ...microscale computed tomography, and scanning electron microscopy were used to characterize seven commercially available gas diffusion layers (GDLs). The result is a catalogue of the following geometrical characteristics: fiber diameter, fiber pitch and co-alignment, areal weight and volume, and microporous layer (MPL) crack size and frequency. This catalogue, when combined with previous GDL characterizations, is expected to provide enough information to create representative, predictive, stochastic models of the GDL.
•Gas diffusion layer materials are characterized for stochastic modeling purposes.•Fiber properties include: diameter, pitch, co-alignment frequency.•MPL properties include: areal weight, areal volume, crack size, crack frequency.
Proton exchange membrane water electrolysis (PEMWE) is the most promising technology for sustainable hydrogen production. However, it has been too expensive to compete with current state-of-the-art ...technologies due to the high cost of titanium bipolar plates (BPPs) and porous transport layers (PTLs). Here, we report a high-performance, durable and low-cost PEMWE cell with coated stainless steel (ss) BPPs and PTLs. When using uncoated ss components in the PEMWE cell, the cell depolarizes rapidly, reaching 2 V at only 0.15 A cm
−2
. After the application of non-precious metal coatings of Ti and Nb/Ti on the ss-BPP and ss-PTL, respectively, the current density can be increased by a factor of 13 while maintaining the same performance. Extensive physical and electrochemical characterization supported by pore network modelling shows that the Nb/Ti coating on the ss-PTL leads to efficient water and gas transport at the interface with the anode. The PEMWE cell with coated ss components was evaluated in an accelerated stress test (AST) for more than 1000 h. No sign of Fe contamination in either the membrane or the electrodes is observed at the end of the test. With our results, we demonstrate that PEMWE cells can be manufactured almost entirely in ss, facilitating an unprecedented cost reduction in this technology and advancing the widespread use of green H
2
.
A high-performance, durable and low-cost PEM electrolyser that can be produced almost entirely with coated stainless steel parts.
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.
In this work, the effective through-plane thermal conductivities for compressed, anisotropic and heterogeneous polymer electrolyte membrane (PEM) fuel cell gas diffusion layers (GDLs) were determined ...analytically from representative physical GDL models, which were informed by microscale computed tomography imaging of four commercially available GDL materials. The number of fibre-to-fibre contact points and corresponding contact areas were extracted from these physical models as inputs to a thermal resistance model. It was found that the effective thermal conductivity increased with increasing GDL thickness (with bulk porosity remaining almost constant). The analytical model was employed to determine the bulk thermal conductivity as well as the thermal conductivity of the core region of the material. By isolating the core region from the bulk, a better understanding of the effect of the heterogeneous porosity profiles on the through-plane thermal conductivity was determined and discussed. Unlike the bulk thermal conductivity, the thermal conductivity of the core region was not dependent upon the material thickness. It was also found that the surface transition regions of the porosity distributions have a dominating effect over the addition of PTFE in impacting the overall thermal conductivity.
► Effective thermal conductivity increased with increasing compression and GDL thickness. ► Core thermal conductivities are higher than bulk values by a factor of 2.87–7.79. ► Compared to PTFE addition, high porosity surfaces dominate thermal conductivity.
A stochastic modeling algorithm was developed that accounts for porosity distribution, fiber diameter, fiber co-alignment, fiber pitch, and binder and/or polytetrafluorethylene fractions. Materials ...representative of a commercially available GDL were digitally generated based on empirical measurements of these various properties. Materials made with varying fiber diameters and binder/fiber volume ratios were compared with a generated reference material through porosity heterogeneity calculations and mercury intrusion porosimetry simulations. Fiber diameters and binder/fiber ratios were found to be key modeling parameters that exhibited non-negligible impacts on the pore space. These key parameters were found to positively correlate with heterogeneity and mean pore diameter and exhibit a complementary relationship in their impact on the pore space. Because both parameters directly impacted the number of fibers added to the domain, modeling techniques and parameters pertaining to fiber count must be considered carefully.
•A stochastic modeling procedure for PEMFC GDL substrates is provided.•Models capture fiber pitch, fiber bundling, porosity distributions, binder fraction.•Models are characterized in terms of heterogeneity and pore size distribution.•Models are shown to be sensitive to binder fraction and fiber diameter.
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