In a substantial improvement over present alkaline systems, an advanced hybrid bipolar plate for a unitized fuel cell/ electrolyzer has been developed. This design, which operates on pure feed streams (H2/O2 and water, respectively) consists of a porous metallic foil filled with a polymer that has very high water transport properties. Combined with a second metallic plate, the pore-filled metallic plates form a bipolar plate with an empty cavity in the center.
In electrolyzer mode, this cavity fills with water, which cools the stack, and provides the water for the electrolysis. The water passes through the polymer-filled pores under an RH gradient and feeds the electrolysis reaction. Under fuel-cell mode, the water is vacuumed out of the chamber with vacuum being continuously applied to remove water from the fuel-cell reaction. This evaporative cooling also provides heat removal from the stack.
At 80 °C, electrolyzer performance was superior to that of flowing water in the hydrogen chamber up to 400 mA/cm2. Above this current density, the membrane begins to dry out as water cannot be carried to the oxygen electrode fast enough. Similar behavior was seen when operating under fuel-cell mode. The current invention outperformed the traditional flow-through fuel cell up to 300 mA/cm2. Above this current density, the oxygen chamber begins to flood.
When operating in electrolyzer mode, the hybrid plate generates H2 and O2 at much lower water contents than traditional electrolysis cells. This greatly simplifies drying of the product gases. Because the water is the only product from the reaction, the feed gases can be operated under “dead-ended” conditions; thus, eliminating the need for saturation, recirculation, and water/gas separation systems for fuel-cell operation. In both fuel-cell and electrolyzer mode, this advanced, unitized cell shows equal or superior performance to discreet systems. This design also allows for simple high-pressure operation with a high differential pressure.
Keeping all feed reactants and products in the vapor phase leads to a system simplification. This eliminates the biggest challenge to unitized systems (water management), allowing the weight savings of a second stack. A study has been carried out and has successfully demonstrated proof-of-concept. More design work has to be done to translate this concept into a full system.
This work was done by Cortney K. Mittelsteadt and William Braff of Giner Electrochemical Systems, LLC for Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18269-1.