Corrosion-Resistant Tantalum Coatings for PEM Fuel Cell Bipolar Plates
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Corrosion-Resistant Tantalum Coatings for PEM Fuel Cell Bipolar Plates Leszek Gladczuk*, Chirag Joshi*, Anamika Patel*, Jim Guiheen#, Zafar Iqbal**, Marek Sosnowski* * Department of Electrical Engineering and ** Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, USA # Honeywell International Inc, Morristown, New Jersey 07960, USA Abstract: Tantalum is a tough, corrosion resistant metal, which would be suitable for use as bipolar plates for proton exchange membrane (PEM) fuel cells, if it was not for its high weight and price. Relatively thin tantalum coatings, however, can be deposited on other inexpensive and lighter weight metals, such as aluminum and steel, providing a passive protection layer on these easily formed substrates. We have successfully deposited, high quality α (body-centered-cubic, bcc) and β (tetragonal) phase tantalum coatings that were a few micrometers thick by dc magnetron sputtering on steel and aluminum. The growth of the thermodynamically preferred body-centered-cubic (bcc) tantalum phase was induced by a choice of deposition conditions and substrate surface treatment. The microstructure and corrosion resistance of the α-phase in an environment approximately simulating the electrochemical conditions used in a PEM fuel cell were investigated under potentiodynamic conditions. Preliminary potentiostatic measurements of a β-phase sample are also presented. INTRODUCTION Bipolar plates are critical components of a proton exchange membrane (PEM) fuel cell, which enable patterned fuel and oxidant gas flow, electrical charge transport and current collection, cooling and water management. A 10 kW fuel cell stack requires approximately 120 bipolar plates at typical cell efficiencies and footprint levels. For the fuel cell stack to be lightweight and low volume, bipolar plates must combine intrinsic lightness with their ability to be fabricated in thin sheets. Typical metals such as aluminum and steel can easily meet the requirement that they be of the order of 0.05 inches thick when formed for use as regular bipolar plates, and 0.1 inches thick when formed as coolant bipolar plates [1]. Bipolar plates must also have high chemical and corrosion stability, mechanical strength and low electrical resistance. Corrosion resistance is required because the plates are in constant contact with acidic water at pH ≈ 5 that is generated under the operating conditions of the stack. Oxides are formed during corrosion, which can migrate and poison the catalyst. Oxides also increase the electrical resistivity of the plates resulting in a drop in fuel cell performance. In order to minimize ohmic losses the specific resistance of the plates need to be maintained at values below 0.1 ohm.cm2. Hydrogen and oxygen permeation across the bipolar plate also compromises the efficiency of the fuel cell and in addition, results in an unsafe operating condition – hence hydrogen-air permeation rates -10 2 of not more than about 7.5 x 10 mol/cm -sec must be maintained [1].
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