A solution-phase sodium hypochlorite (NaOCl) cathode for use in an aluminum-based seawater battery is presented. This catholyte attains a high power density, a positive redox potential, and a high Faradaic current capacity. NaOCl is also inexpensive, stable, and environmentally safe/friendly. Four aluminum alloys and 99.999% pure aluminum, serving as anodes, were tested in 3.0, 5.0, and 8.0 M NaOH solutions at the temperatures 45, 55, 65, and 80 degrees C. The catholyte concentration and temperature effects were optimized, and the best electrocatalyst was chosen to carry out the NaOCl reduction. Optimization experiments were carried out on a half-cell basis using a potentiostat in a three-electrode configuration with Ag/AgCl as the reference electrode and a carbon rod as the counter electrode. Full-cell testing incorporating the optimum conditions was performed with a flowing electrolyte apparatus. A catholyte concentration of 0.70 M NaOCl proved optimum in combination with 3.0 M NaOH at the temperature 55 degrees C and 5.0 M NaOH at the temperature 65 OC. The temperature 45 degrees C was not high enough for aluminum activation, the 80 OC data did not show an improvement over the 65 degrees C data, and polarization was not dramatically enhanced in 8.0 M NaOH. Nickel foil and porous nickel separately electroplated with palladium were found to be the most effective electrocatalysts. Full-cell data demonstrates a successful electrochemical system with current densities approaching 1200 mA/cm(2) and voltages above 1.0 V.