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Thermal neutron radiography of a passive proton exchange membrane fuel cell for portable hydrogen energy systems

A proton exchange membrane fuel cell (PEMFC) for low power and portable applications is studied with thermal neutron radiography. The PEMFC operates under full passive conditions, with an air-breathing cathode and a dead-end anode supplied with static ambient air and dry hydrogen, respectively. A columnar cathodic plate favors the mobility of water drops over the cathode surface and their elimination. Thermal neutron images show liquid water build up during operation with the cell in vertical and horizontal positions, i.e. with its main plane aligned parallel and perpendicular to the gravity field, respectively. Polarization curves and impedance spectroscopy show cell orientation dependent response that can be related with the water accumulation profiles. In vertical position, lower water contents in the cathode electrode is favored by the elimination of water drops rolling over the cathode surface and the onset of natural convection. As a consequence, oxygen transport is improved in the vertical cell, that can be operated full passive for hours under ambient conditions, providing steady peak power densities above 100 mW/cm2. Gravity and natural convection are less effective in horizontal position, leading to a 17% decrease in peak power density due to oxygen transport losses. The horizontal position is especially adverse if the upper electrode is the cathode, because of anode flooding causing cell failure after production of a small amount of water (5 mg/cm2). The combined information from thermal neutron radiography and cell response characteristics explains the important influence of cell orientation on the performance of a fully passive air-breathing PEMFC for portable applications.