Abstract
The success of PEMFCs in heavy duty applications depends on achieving high power densities while maintaining exceptional durability. Under high current density operating conditions, the transport of oxygen from the gas channel via the GDL and MPL to the active Pt-particles in the catalyst layer becomes performance limiting. In the different layers, oxygen transport takes place via various transport processes, such as Knudsen, molecular and ionomer diffusion. Gas transport in a fuel cell is influenced to varying degrees by the operating conditions, geometry of the flow channel, and the properties of the porous media. Porosity and tortuosity of the diffusion media depend on the local clamping force and the amount of liquid water present. In a large-active-area cell, the clamping force and liquid water as well as oxygen concentration, pressure and temperature can vary significantly between the inlets and the exits, which results in a spatially varying oxygen transport resistance and local performance along the channel. To enable targeted optimization of the operating strategy and component design for high current density operation, a comprehensive understand of the local diffusion processes and their dependencies is essential.In our study, limiting current measurement is used as a diagnostic tool in combination with a segmented cell to determine the local oxygen transport resistance [1, 2]. In the measurement setup, the clamping force is manually set for each segment and recorded continuously throughout the experiment. By systematically varying the operating conditions (p, RH, T), clamping force and cathode gas composition (N2/He), we aim to deconvolute each transport mechanism and the allocation of the transport resistance to each individual layer. The local operating conditions, such as pressure and temperature, are simulated using empirical models based on local measurement data. The combined results allow us to obtain the local oxygen transport resistance as a function of local operating conditions and the clamping force along the flow channel.In this study, the distribution of different gas diffusion processes and their impact on fuel cell performance along the channel are presented. In particular, the effect of the clamping force and the formation of liquid water along the channel on local porosity and tortuosity in the GDL are discussed. Furthermore, our results demonstrate how the overall performance of the cell can be optimized by adjusting the clamping force distribution along the flow channel.
Published Version
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