The core of a polymer electrolyte membrane fuel cell stacks are membrane electrode assemblies (MEAs). Manufacturing processes for MEA have significant impact on their performance and durability. The authors have attempted an ultrasonic spray technique for the fabrication of membrane electrode assemblies.Ultrasonic vibration of a liquid surface attached to substrate causes the formation of surface capillary waves. As the amplitude increases, the rupture of capillary surface waves occur and liquid droplets are subsequently ejected from the surface. To realize the ultrasonic spray process, the atomized droplets using ultrasonic atomizer can be shaped and moved around with low velocity air or another carrier gas. Applications of ultrasonic spray technology range from ordinary liquids to molten metal for air conditioning, drug delivery, powder production, combustion, textile coating, solar cell manufacturing, and lately fuel cell manufacturing. The major advantages of ultrasonic spray include uniform spray with narrow distribution of droplet sizes, controllable mean droplet size diameter by ultrasonic forcing frequency, low droplet velocity (soft spray) that minimize splashing and material waste, silent, low energy consumption (for atomization and spray shaping), and scalable.The authors applied an ultrasonic spray process to fabricate membrane electrode assemblies.A commercial ultrasonic spray robot from Sono-Tek ® was used for this purpose. The spray parameters, such as generator frequency, ink feed rates, heater temperature, stand-off distance, etc., were investigated for the production of membrane electrode assembly with uniform and functional-gradient Pt/C catalyst layers. X-Y-Z robot Ultrasonic spray head Heating platen Ventilated enclosureFigure 1 (Top) The ultrasonic spray system and (Bottom) the 25-cm 2 MEA.The electrochemical performance of the ultrasonic sprayed MEAs was characterized by fuel cell testing. A wide range of catalyst loading can be realized, the MEAs tested so far contain approximately 0.3~0.4 mgPt/cm 2 on either side of the electrolyte.The representative performance of the ultrasonic sprayed MEA is shown in Figure 2. By optimizing the spray parameters, current density reached over 2 A/cm 2 at 0.6 V with hydrogen and oxygen (no back pressure).The ultrasonic spray technique was found to be a promising technique for producing membrane electrode assemblies with very high electrochemical performance. 0.3 0.4 0.5 0.6 0.7 0.8 0.9
The development of dual phase composite catalyst supports from carbon nanotubes and metal oxides can resist performance degradation. Carbon nanotubes provide a conductive framework and titanium oxides can form a strong bond with platinum catalysts. The combination of this composite material has been shown to offer some improvements in the performance durability of this electrocatalyst in the cathode of a polymer electrolyte fuel cell.
In polymer electrolyte cells, an approach is shown for construction of resilient electrocatalysts. In the anode where the hydrogen oxidation reaction is subject to poisoning from fuel impurities like carbon monoxide (CO), increased tolerance and stability for the catalyst is revealed by modification of the support structure and properties. A carbon nanotube framework serves as the foundation for metal oxide addition, namely titania and niobium doped form. The corrosion resistant transition metal oxides form a strong bond with platinum catalysts through unique electronic interactions, measured by XPS. Several other material characterizations are also included to make comparisons between composites. Composite supports contribute improved reactivity towards oxidation of CO, especially in reduced titania. Carbon corrosion resistance is also measured and shown to be the greatest for this support. Synergistic combination of effects is observed directly by preparation of electrocatalysts into working membrane electrode assemblies measured for their performance & durability.
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