The regenerative hydrogen-oxygen ion-exchange membrane fuel cell is especially suitable for use in space power systems. During discharge, the anode reaction is H2+20H-Â¿2H2O+2e. The electrons pass out of the fuel cell at the anode to the load, then re-enter at the cathode. The cathode reaction is Â¿02+H2O+2e Â¿20H-. The hydroxyl ions pass through the membrane to the anode; the electrical circuit is thus completed. The cell reaction is H2+Â¿02Â¿H2O. During charge, electrical energy is furnished to the fuel cell and water is electrolyzed into hydrogen and oxygen. Single cells have been cycled continuously for several months without significant decreases in performance. The electrochemical efficiencies ampere-minutes out/ampere-minutes in are 95 to 100 per cent and the energy efficiencies watt-minutes out/watt-minutes in are as high as 50 per cent. Cells have been operated initially at current densities of 38 and 100 a per square foot at 0.8 and 0.6 v, respectively. Multicell units have been built and tested. Experimental fuel-cell batteries have been tested under zero-gravity conditions in aircraft and have been flown in missile flights. The results have shown that this fuel cell operates satisfactorily in the environment of outer space.