NOVELTY - Fuel cell system (10) comprises: an anode compartment (12) comprising substituted hydrazine compounds (I) as fuel, and a catalyst layer (50) comprising copper or a copper alloy; a cathode compartment (14) comprising an oxidant; and a separator (20) interposed between the cathode and the anode compartments.    USE - The fuel cell system is useful: in a power plant; for powering vehicles and electricity-consuming devices (all claimed) e.g. pager and personal computer; and e.g. as a component in a power source in e.g. a spacecraft.    ADVANTAGE - The fuel cell system: exhibits an open circuit potential higher than 1.5 volts and a power output of greater than or equal to 0.1 watt; is cost-effective; consists of relatively simple catalysts; exhibits improved performance and high level of stability of up to 90 hours work; and shows no anode poisoning.    DETAILED DESCRIPTION - Fuel cell system comprises: an anode compartment comprising substituted hydrazine compounds of formula (R1R2NNR3R4) (I) or their salts, hydrates or solvates as fuel, and a catalyst layer comprising copper or a copper alloy; a cathode compartment comprising an oxidant; and a separator interposed between the cathode and the anode compartments. INDEPENDENT CLAIMS are also included for:    (1) a method of operating the fuel cell system comprising contacting the anode compartment with an anode electrolyte solution comprising (I) and, at the same time, contacting the cathode compartment with a cathode electrolyte solution comprising the oxidant, and electrochemically reacting (I) and the oxidant;    (2) a method of producing electrical energy comprising electrochemically reacting (I) and the oxidant, where (I) is contacted with the anode compartment comprising an anode having the catalyst layer, and the oxidant is contacted with the cathode compartment;    (3) a power plant comprising the fuel cell system; and    (4) a vehicle fueled by the fuel cell system.    R1-R4 = H (preferred), alkyl, alkenyl, alkynyl, heteroalicyclic, alkoxy, carboxy, ketone, amide, hydrazide or amine.    Provided that at least one of R1-R4 is H.    DESCRIPTION OF DRAWING(S) - The figure is a schematic illustration of the fuel cell system.    Fuel cell system (10)    Anode compartment (12)    Cathode compartment (14)    Separator (20)    Catalyst layer (50)