Understanding how water produced in the cathode catalyst layer is removed during proton exchange membrane fuel cell (PEMFC) operation is critical for optimization of materials and model development. The present work combines in situ and ex situ experiments designed to elucidate the dominant water discharge mechanism when considering capillary and vapor transport at normal PEMFC operating conditions. The flux of water vapor driven by the thermal gradient in the cathode diffusion layer can alone be sufficient to remove product water at high current densities even with saturated gas in the delivery channels. The role of an intermediate microporous layer and its impact in vapor vs liquid transport is also considered. We propose that the primary role of the microporous layer is to prevent condensed water from accumulating on and blocking oxygen access to the cathode catalyst layer.

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