NOVELTY - A fuel cell has a fuel vessel having a source of fuel; an oxidant vessel having a source of oxidant; a reaction zone having cathode(s), anode(s) and electrolyte; a closed water vessel; a fuel conduit that connects the fuel vessel and the reaction zone; an oxidant conduit that connects the oxidant vessel and the reaction zone; and electrical conductors connected to anode and cathode.    USE - For a power supply or source (claimed) useful for commercial energy generation, powering electric vehicles, and powering other equipment, e.g. communication and control equipment located in remote areas away from commercially available power sources, e.g. equipment used downhole in oil and gas wells, or beneath the surface of the sea adjacent to offshore wells.    ADVANTAGE - The fuel cell provides a simple, reliable and efficient way of generating electrical power in a downhole or subsea environment. It also provides power without the need for any moving parts, decreasing the chance of a mechanical failure. It further provides power for remote power applications, e.g. power for geo-sensors at the surface of the earth.    DETAILED DESCRIPTION - A fuel cell comprises:    (a) a fuel vessel (12) comprising a source of fuel;    (b) an oxidant vessel (14) comprising a source of oxidant;    (c) a reaction zone (16) comprising cathode(s), anode(s) and electrolyte in-between each anode and cathode;    (d) a closed water vessel (54) that is connected to the reaction zone by at least one capillary flow path;    (e) a fuel conduit (18) that connects the fuel vessel and the reaction zone;    (f) an oxidant conduit (20) that connects the oxidant vessel and the reaction zone; and    (g) electrical conductors connected to anode and cathode and adapted to conduct electricity to an external device.    The fuel conduit comprises a fuel pressure control apparatus adapted to maintain a static pressure of fuel in the reaction zone. The oxidant conduit comprises an oxidant pressure control apparatus adapted to maintain a static pressure of oxidant in the reaction zone. INDEPENDENT CLAIMS are also included for the following:    (A) a downhole assembly comprising a downhole structure in a subterranean borehole and a fuel cell as above, attached to the downhole structure;    (B) a power supply system comprising a fuel cell; and a rechargeable battery electrically connected to the fuel cell in such a way that the fuel cell recharges the rechargeable battery during periods on non-use;    (C) a method for supplying power to a well comprising providing a fuel cell in or near the well and connecting the fuel cell to battery to form a hybrid power source;    (D) a method for completing a wellbore comprising providing a fuel cell as above; connecting the fuel cell to an electrical device; and inserting the fuel cell and electrical device into the wellbore; and    (E) a method of supplying power to an electrical circuit of a downhole tool comprising providing a fuel cell as above; electrically connecting the fuel cell to the electrical circuit of the downhole tool; inserting the downhole tool and fuel cell into a wellbore; and generating electricity within the wellbore from the fuel cell and supplying at least some of the electricity to energize the electrical circuit of the downhole tool.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional drawing of a fuel cell apparatus comprising two fuel cells in a stack configuration.    Fuel vessel (12)    Oxidant vessel (14)    Reaction zone (16)    Fuel conduit (18)    Oxidant conduit (20)    Bipolar plate (34)    Water vessel (54)