The main electrical power source for the two-man Gemini vehicle was an ion-exchange membrane fuel cell built by General Electric under contract by McDonnell Aircraft Corporation (Oster, 1962). The hydrogen and oxygen reactants were stored cryogenically, and the water product was used as drinking water for the crew on long missions. The first mission with the fuel cell was Gemini 5, which flew August 21-29, 1965.
The power system consisted of three stacks of fuel cells, each with 32 membrane and electrode assemblies (Cohen, 1966). The membrane was a polystyrene sulfonic acid electrolyte mixed with a Kel-F® (a registered trademark of 3M Company until 1995; polychlorotrifluoroethylene) backbone. The membrane was covered on both sides by titanium screen electrodes, which were coated with platinum catalyst. The anode side of the membrane and electrode assembly was enclosed by a titanium sheet bonded at the edge of the membrane, forming a manifold for hydrogen gas. The cathode side was left open for the oxygen gas. On the outer face of the titanium sheet were two loops of tubing for the coolant, and in between each pass of the tubing were wicks to remove the product water from the cell. (The water moved by capillary action within the wicks to a collecting point made of felt, and the water was separated from the oxygen by a porous ceramic plate and stored in a tank.) The cells were stacked so that the tubing and wicks of one cell made contact with the oxygen electrode of the adjacent cell. Three stack modules were encapsulated by a vessel that was insulated with foam for vibration and noise damping and for temperature control.
The system operated at low temperatures of 21 °C (70°F) and low pressures. The hydrogen gas was pressurized at 0.12 bar (1.7 psi) above the water vapor pressure (water used for humidification), and oxygen was at 0.035 bar (0.5 psi) above the hydrogen pressure. The reactant gases were humidified prior to reaching the membrane because the conductivity of the polystyrene sulfonic acid membrane was dependent on water content. Besides being added to the system, water was also produced in the reaction, accumulating within the pores of the electrode, flooding them, and decreasing the fuel cell performance. Therefore, the electrodes were made wetproof by PTFE, which was also used to bind the platinum catalyst to the titanium screens, and wicks were inserted into the electrodes to pull water away from the cell. The difficult management of water in the system was a reason for the selection of an alternative fuel cell technology over the solid polymer fuel cell for the later space programs (Warshay and Prokopius, 1990).
The fuel cell stacks produced 1 kW (620 W average). In tests for durability, the cell voltage decayed at a rate of 1 to 5 mV per hour mostly because of degradation within the membrane. The average performance of one of the stack modules during the mission was 26.5 V at 16 A on Day 1, rising to 27 V at 16 A on Day 8. The lower voltage on the first day was attributed to the water imbalance caused by the low current used when the fuel cell was in standby mode before the launch.
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