NOVELTY - A fuel cell (10) comprises an anode half-cell (20) having an electrocatalytic anode (24) and a liquid anolyte (22) comprising a reductant dissolved in an aqueous solution that supports electrical conduction; a cathode half-cell (30) having an electrocatalytic cathode (34) and a liquid catholyte (32) comprising an acid and an oxidant dissolved in an aqueous solution that supports electrical conduction; and a proton exchange membrane (40) separating the anode half-cell from the cathode half-cell.    USE - As a fuel cell for producing electricity (claimed) useful for underwater vehicles and applications, such as unmanned underwater vehicles (UUVs).    ADVANTAGE - The fuel cell employs liquid fuels that can be stored externally to the cell stack. The fuel cell does not require molecular oxygen as a chemical component. The fuel cell avoids the crossover of an anolyte across the fuel cell membrane lowering cell voltages and fuel efficiency; and carbon monoxide (CO) catalyst poisoning. Multiple fuel cells can be combined in series in a stacked formation producing a higher voltage, or combined in parallel increasing the amperage. The fuel cell can be refueled using conventional liquid fueling systems. Adoption of this system enables rapid turnaround and lower operating costs. The reductant can be stored as a neat liquid, a concentrated liquid, or mixed and metered as needed. Similarly, the oxidant can be stored as a neat liquid, a concentrated liquid, or mixed and metered as needed. In the case of hydrogen peroxide, the hydrogen peroxide is preferably stored as a concentrated aqueous solution, which is mixed and metered as needed. The fuel and oxidizer can be stored at ambient temperatures as liquids, and as such provide a system having greater safety and handling, than one that uses a compressed gas or a cryogenic liquid. The fuel cell can be configured such that it can operate with a minimal exhaust stream.    DETAILED DESCRIPTION - A fuel cell (10) comprises an anode half-cell (20) having an electrocatalytic anode (24) and a liquid anolyte (22) comprising a reductant dissolved in an aqueous solution that supports electrical conduction; a cathode half-cell (30) having an electrocatalytic cathode (34) and a liquid catholyte (32) comprising an acid and an oxidant dissolved in an aqueous solution that supports electrical conduction; and a proton exchange membrane (40) separating the anode half-cell from the cathode half-cell. The proton exchange membrane is made of a polymeric film material that is resistant to oxidation. The aqueous solution that supports electrical conduction is seawater, or a solution of water containing a salt selected from sodium, potassium, calcium, magnesium and lithium. The anode half-cell is replenished as needed with liquid reductant from a reservoir. The cathode half-cell is replenished as needed with hydrogen peroxide oxidant from a reservoir. The anolyte and cathode are re-circulated. The anode half-cell has a vent for releasing carbon dioxide gas. An INDEPENDENT CLAIM is included for producing electricity involving electrochemically reacting in the anode half-cell having the electrocatalytic anode, liquid anolyte, comprised of a reductant dissolved in seawater, with the liquid catholyte, comprised of an acid and an oxidant dissolved in seawater in the cathode half-cell having the electrocatalytic cathode, where the anode half-cell and the cathode half-cell share common proton exchange membrane; replenishing the liquid anolyte as needed; and replenishing the liquid catholyte as needed.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a fuel cell.    Fuel cell (10)    Anode half-cell (20)    Anolyte (22)    Anode (24)    Cathode half-cell (30)    Catholyte (32)    Cathode (34)    Proton exchange membrane (40)