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4. ALKALINE FUEL CELL The Alkaline Fuel Cell (AFC) was one of the first modern fuel cells to be developed, beginning in 1960. The application at that time was to provide on-board electric power for the Apollo space vehicle. Desirable attributes of the AFC include excellent performance compared to other candidate fuel cells due to its active O2 electrode kinetics and flexibility to use a wide range of electro-catalysts. The AFC continues to be used: it now provides on-board power for the Space Shuttle Orbiter with cells manufactured by UTC Fuel Cells. The AFC developed for space application was based, in large part, on work initiated by F.T. Bacon (1) in the 1930s. By 1952, construction and performance testing of a 5-kW alkaline fuel cell, operating on H2 and O2, was completed. The fuel cell developed by Bacon operated at 200 to 240 oC with 45 percent KOH electrolyte. Pressure was maintained at 40 to 55 atm to prevent the electrolyte from boiling. At this relatively high temperature and pressure, performance of the cell was quite good (0.78 volts at 800 mA/cm2). The anode consisted of a dual-porosity Ni electrode (two-layer structure with porous Ni of 16 μm maximum pore diameter on the electrolyte side and 30 μm pore diameter on the gas side). The cathode consisted of a porous structure of lithiated NiO. The three-phase boundary in the porous electrodes was maintained by a differential gas pressure across the electrode, since a wetproofing agent was not available at that time, i.e., PTFE (polytetrafluoroethylene) as a wetproofing material did not exist, and it would not have been stable in the high temperature alkaline solution (2). The kinetics of O2 reduction in alkaline electrolytes are more favorable than in phosphoric acid electrolyte. Consider a Pt cathode (0.25 mg/cm2) in 30 percent KOH at 70 °C and in 96 percent phosphoric acid at 165 °C. The cathode potentials (vs. RHE - Reversible Hydrogen Electrode) at 100 mA/cm2 in these two electrolytes are 0.868 and 0.730 V, respectively, according to data reported by Appleby (Figure 2.15-1 in Reference 3). Various explanations have been advanced for the higher O2 reduction rates in alkaline electrolytes (4). The practical consequence of the higher performance of Pt cathodes in alkaline electrolytes is that AFCs are capable of higher efficiencies than PAFCs at a given current density, or higher power densities at the same efficiency. Bockris (2) estimates that the efficiency of AFCs fueled by pure H2 is about 60 percent HHV, and that of PAFCs is about 50 percent HHV. The high performance of the alkaline cell relative to phosphoric acid and other cells leads to the plausibility of developing the technology for terrestrial application. The leading developer of alkaline technology for space application, UTC Fuel Cells, investigated adaptating the 4-1PDF Image | Fuel Cell Handbook (Seventh Edition)
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