Abstract
Proton exchange membrane fuel cells (PEMFCs) require mechanical compression to ensure structural integrity, prevent leakage, and to minimize the electrical contact resistance. The mechanical properties and dimensions of the fuel cell vary during assembly due to manufacturing tolerances and during operation due to both temperature and humidity. Variation in stack compression affects the interfacial contact pressures between components and hence fuel cell performance. This paper presents a one-dimensional equivalent stiffness model of a PEMFC stack capable of predicting independent membrane and gasket contact pressures for an applied external load. The model accounts for nonlinear component compression behavior, thickness variation due to manufacturing tolerances, thermal expansion, membrane expansion due to water uptake, and stack dimensional change due to clamping mechanism stiffness. The equivalent stiffness model is compared to a three-dimensional (3D) finite element model, showing good agreement for multicell stacks. Results demonstrate that the correct specification of gasket thickness and stiffness is essential in ensuring a predictable membrane contact pressure, adequate sealing, and avoiding excessive stresses in the bi-polar plate (BPP). Increase in membrane contact pressure due to membrane water uptake is shown to be significantly greater than the increase due to component thermal expansion in the PEMFC operating range. The predicted increase in membrane contact pressure due to thermal and hydration effects is 18% for a stack containing fully hydrated Nafion® 117 membranes at 80 °C, 90% relative humidity (RH) using an eight bolt clamping design and a nominal 1.2 MPa assembly pressure.
Highlights
Proton exchange membrane fuel cells (PEMFCs) have seen increased use in recent years as an alternative clean energy source in multiple applications ranging from consumer electronics to automotive powertrains
The contact pressure of the gasket is significantly higher than the membrane electrolyte assembly (MEA) due to the gasket assembly being stiffer than the MEA (GDL-proton exchange membrane (PEM)-gas diffusion layer (GDL))
An equivalent stiffness model of a PEM fuel cell stack has been presented, which acts as a useful tool for the development of contact pressure distribution in fuel cell stack designs at a low computational cost
Summary
Proton exchange membrane fuel cells (PEMFCs) have seen increased use in recent years as an alternative clean energy source in multiple applications ranging from consumer electronics to automotive powertrains. The purpose of the clamping load in a fuel cell stack is to ensure adequate gas tight sealing, reduce electrical and thermal contact resistance between the different components, and provide some structural integrity [1,2]. Wen et al [4] and Lee et al [3] used polarization curves to demonstrate how different clamping pressures, applied through varying bolt torque, influenced the cell performance. The six-bolt design at the maximum tested torque of 16 NÁm gave the best overall performance in a ten-cell stack. Lee et al [3] used a 10 cm single cell design to observe the cell performance at different clamping loads with different types of gas diffusion layer (GDL). An optimum tightening torque was observed with performance decreasing at high bolt torques, most likely due to overcompression of the GDL inhibiting the transport of reactant gases and product water
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