DOE OSTI2013
Over the past 20 years, great technical progress has been made in improving power density and durability of fuel cell stacks. Yet, operating durably at high performance levels under very dry conditions, e.g., < 20% RH at 80 °C or above, remains beyond even the best fuel cell membrane electrode assemblies. Thus, today it is essential to humidify the gases supplied to the fuel cell inlets. In this work, we have produced a new, inexpensive, composite membrane capable of very high water vapor transport and low air cross-over. The composite structure consists of a very thin ionomer layer (e.g., < 5 μm) sandwiched between two microporous polymer layers. The thin ionomer layer facilitates the rapid water transport and provides an impermeable layer to prevent gas cross-over. Such an approach reduces cost, but maintains performance. The microporous layer protects the thin ionomer layer from mechanical damage during handling; confers strength to the thin layer allowing it to be more durable during use; and allows it to withstand higher automotive pressures and temperatures. The composite structure will therefore allow lower total cost while still meeting automotive humidifier water transport and durability targets. Because the transport rates of these new materials are so high, existing planar membrane humidifier module designs available at the start of the program were incapable of efficiently utilizing the high rates. Therefore, the assembled team designed, tested and demonstrated an innovative, low-cost humidifier module with customized channel geometries that can take advantage of the high the water transport rates. Program Results: The objectives of the program have been fully met. The optimized membrane produced in the program has very high transport rates, nearly twice that of the closest competitive option, a homogeneous perfluorosulfonic acid (PFSA) membrane. Furthermore, the composite structure imparts significant durability advantages, allowing the membrane to remain gas impermeable during 20,000 relative humidity (RH) cycles, unlike the closest competitive options that fail very rapidly. It does suffer from a loss of performance with time at temperature because of inherent properties of the PFSA polymer, but this degradation is manageable through relatively simple system design changes. The final membrane of this program has been produced on manufacturing level equipment in a roll to roll form in the scale of hundreds of square meters. The process is readily scalable to automotive level volumes to allow production of a low-cost membrane required for this application. A full scale automotive air-to-air humidifier module has been designed, tested and built. The final module meets the design specifications required for automotive fuel cell use, and exceeds all the DOE 2017 targets for a cathode humidification module as determined in independent testing at Ford Motor Company. It has a small volume, less than 5 liters; a high water transfer rate, significantly greater than 5 g/s at the DOE specified conditions; acceptably low pressure drop; and is projected to maintain these properties over 5000 hours. Additionally, high volume cost projections suggest that the module can be produced for less than $100 per unit in a scale of 500,000 units per year.