Gradation of mechanical properties in gas diffusion electrode. Part 1: Influence of nano-scale heterogeneity in catalyst layer on interfacial strength between catalyst layer and membrane

Stress and plastic deformation analyses of catalyst layer have been conducted after experimentally investigating its mechanical properties at nano-scale. Interestingly, catalyst layer is found to have varying mechanical properties as a function of depth and therefore it is classified under graded material. Effect of gradation in catalyst layer on interfacial strength between membrane and catalyst layer is explained with the aid of numerical simulations. Stress redistribution near interface line is observed in graded model, while stresses are found to have concentrated at critical locations throughout the discrete model. However, it is outlined from this study that the gradation in catalyst layer leads to greater amount of plastic energy dissipation—an indication of enhanced ductility. An experimental coupled numerical approach is presented to characterize the effect of transitional variations of mechanical properties in catalyst layer on the interfacial line and membrane.     

Effect of conductive carbon material content and structure in carbon fiber paper made from carbon felt on the performance of a proton exchange membrane fuel cell

This study concerns the use of conductive carbon material with different content and structure to produce carbon fiber paper for use in proton exchange membrane fuel cells, and investigates how changes in the content and structure of the conductive carbon material influence fuel cell performance.In this study, phenolic resin is used as a conductive carbon material, and is subjected to heat treatment at temperatures of 700 °C, 1000 °C, and 1400 °C, which changes its structure. Before carbon fiber paper is prepared from carbon felt, the felt is treated with phenolic resin solutions with resin content of 5, 10, 15, 20, 25, and 30 wt%. During fuel cell testing, torsion of 40, 60, 80, 100, and 120 kgf-cm is applied. The study found that when the phenolic resin content is 15 wt%, the heat treatment temperature 1400 °C, the test area 25 cm², and the test temperature 65 °C, a fuel cell can achieve a current density of 2020 mA cm⁻² at 0.5 V and torque of 120 kgf-cm.     

Correlation between performance of polymer electrolyte membrane fuel cell and degradation of the carbon support in the membrane electrode assembly using image processing method

Long-time operation and various conditions cause the membrane electrode assembly (MEA) of polymer electrolyte membrane fuel cells (PEMFCs) to degrade, which results in decreased performance. The degradation of the MEA appears as various symptoms, such as the loss of carbon support and agglomeration of the Pt catalyst. In this paper, damage on the surface of the MEA by long-time operation and various conditions is induced intentionally by high-temperature conditions in a thermostat chamber. The MEA surface damage is photographed by scanning electron microscopy (SEM), and the loss of the carbon support that fixes the platinum catalyst is judged. Image processing is used to analyze damage on the MEA surface, and binarization processing is applied to the image processing method. SEM imagery is taken at magnifications of 100 × and the trends in quantified surface damage on the MEA according to the degradation temperature are analyzed. The correlation between the quantitative damage on the MEA surface and the performance of the PEMFC is checked. As a result, the tendency of decreasing PEMFC performance is derived from increasing quantified damage on the MEA surface.     

Studies of off-stoichiometric AB2 metal hydride alloy: Part 2. Hydrogen storage and electrochemical properties

In Part 2 of this two-part series of papers, gaseous hydrogen storage and electrochemical properties of three series of alloys with different combinations of Cr/Mn/Co ratios are studied and compared to the structural properties reported in Part 1. As the B/A stoichiometry in each series of alloys increases from 1.8 to 2.2, systematic trends in certain storage properties are found: the hydrogen dissociation pressure and heat of hydride formation increases; the alloy with a AB_2.0 stoichiometry has the highest electrochemical full capacity; and slightly higher and lower B-contents increase the electrochemical high-rate-dischargeability and gaseous phase maximum storage capacity, respectively. Stoichiometric or slightly hyper-stoichiometric AB₂ alloys have lower PCT hysteresis which are expected to reduce pulverization during cycling. The full and high-rate discharge electrochemical capacities correlate well with the maximum and reversible gaseous hydrogen storages, respectively. Slight hyper-stoichiometry increases the high-rate dischargeability. Open circuit voltage, an important parameter in high-power application, is also found to be more relevant to the surface reaction than to the bulk hydride stability.     

Simulation of thermal stresses in anode-supported solid oxide fuel cell stacks. Part II: Loss of gas-tightness, electrical contact and thermal buckling

Structural stability issues in planar solid oxide fuel cells arise from the mismatch between the coefficients of thermal expansion of the components. The stress state at operating temperature is the superposition of several contributions, which differ depending on the component. First, the cells accumulate residual stresses due to the sintering phase during the manufacturing process. Further, the load applied during assembly of the stack to ensure electric contact and flatten the cells prevents a completely stress-free expansion of each component during the heat-up. Finally, thermal gradients cause additional stresses in operation.The temperature profile generated by a thermo-electrochemical model implemented in an equation-oriented process modelling tool (gPROMS) was imported into finite-element software (ABAQUS) to calculate the distribution of stress and contact pressure on all components of a standard solid oxide fuel cell repeat unit.The different layers of the cell in exception of the cathode, i.e. anode, electrolyte and compensating layer were considered in the analysis to account for the cell curvature. Both steady-state and dynamic simulations were performed, with an emphasis on the cycling of the electrical load. The study includes two different types of cell, operation under both thermal partial oxidation and internal steam-methane reforming and two different initial thicknesses of the air and fuel compressive sealing gaskets.The results generated by the models are presented in two papers: Part I focuses on cell cracking. In the present paper, Part II, the occurrences of loss of gas-tightness in the compressive gaskets and/or electrical contact in the gas diffusion layer were identified. In addition, the dependence on temperature of both coefficients of thermal expansion and Young's modulus of the metallic interconnect (MIC) were implemented in the finite-element model to compute the plastic deformation, while the possibilities of thermal buckling were analysed in a dedicated and separate model.The value of the minimum stable thickness of the MIC is large, even though significantly affected by the operating conditions. This phenomenon prevents any unconsidered decrease of the thickness to reduce the thermal inertia of the stack. Thermal gradients and the shape of the temperature profile during operation induce significant decreases of the contact pressure on the gaskets near the fuel manifold, at the inlet or outlet, depending on the flow configuration. On the contrary, the electrical contact was ensured independently of the operating point and history, even though plastic strain developed in the gas diffusion layer.     

Characterization of transport properties in gas diffusion layers for proton exchange membrane fuel cells: 1. Wettability (internal contact angle to water and surface energy of GDL fibers)

This is the first in a series of papers in which we present state-of-the-art methods demonstrated at Case for the estimation of transport properties in gas diffusion layers (GDLs) for proton exchange membrane fuel cells (PEMFCs). Most of the methods used today for measuring wettability properties of GDLs are related to the external contact angle to water. The external contact angle however does not describe adequately capillary forces acting on the water inside the GDL pores. We show as well that the direct method of estimation of the internal contact angle using goniometry on micrographs is impractical. We propose and describe in this paper a method for estimating the internal contact angle to water and the surface energy of hydrophobic and hydrophilic gas diffusion media. The method was applied to GDLs having different contents of hydrophobic agent and carbon types. The method can be applied separately to different components of the GDL including macro-porous substrates and micro-porous layers. The uncertainty estimates using this method are usually within 3% of the measured value.     

Characterization and analysis methods for the examination of the heterogeneous solid oxide fuel cell electrode microstructure: Part 2. Quantitative measurement of the microstructure and contributions to transport losses

Advanced characterization and analysis of multifunctional materials, such as the materials found in heterogeneous solid oxide fuel cell (SOFC) electrode architectures, can help to provide a qualitative and quantitative understanding of how these structures respond to different manufacturing and operating practices. Dense, opaque materials, which have large X-ray mass absorption coefficients and features on sub-micrometer length scales, can make characterization difficult. Advances in tomographic X-ray imaging can permit this level of detailed characterization, and complement stereographic scanning electron microscope measurements that have also been reported. In this second part of a two-part study, details regarding quantitative characterization methods that have been used to examine the SOFC anode microstructure are reported. The detailed formulation and validation of a phase size distributions for the three constitutive phases, as well as resistive loss microstructure-induced resistive loss distributions in the nickel (Ni) and yttria-stabilized zirconia (YSZ) phases are provided in this section.     

Effective thermal conductivity and thermal contact resistance of gas diffusion layers in proton exchange membrane fuel cells. Part 2: Hysteresis effect under cyclic compressive load

Heat transfer through the gas diffusion layer (GDL) is a key process in the design and operation of a PEM fuel cell. The analysis of this process requires the determination of the effective thermal conductivity as well as the thermal contact resistance between the GDL and adjacent surfaces/layers. The Part 1 companion paper describes an experimental procedure and a test bed devised to allow separation of the effective thermal conductivity and thermal contact resistance, and presents measurements under a range of static compressive loads. In practice, during operation of a fuel cell stack, the compressive load on the GDL changes.In the present study, experiments are performed on Toray carbon papers with 78% porosity and 5% PTFE under a cyclic compressive load. Results show a significant hysteresis in the loading and unloading cycle data for total thermal resistance, thermal contact resistance (TCR), effective thermal conductivity, thickness, and porosity. It is found that after 5 loading-unloading cycles, the geometrical, mechanical, and thermal parameters reach a “steady-state” condition and remain unchanged. A key finding of this study is that the TCR is the dominant component of the GDL total thermal resistance with a significant hysteresis resulting in up to a 34% difference between the loading and unloading cycle data. This work aims to clarify the impact of unsteady/cyclic compression on the thermal and structural properties of GDLs and provides new insights on the importance of TCR which is a critical interfacial transport phenomenon.