NOVELTY - A fuel cell (1) comprises anode (11) having inner face and outer face fluidly communicable with fuel; cathode (12) having inner face ionically communicable with and physically separated from anode inner face, and having outer face fluidly communicable with oxidant; and movable guard(s) (15) movable over the anode outer face, cathode outer face, anode inner face and/or cathode inner face. The cathode inner face is ionically communicable with anode inner face by electrolyte.    USE - A fuel cell used for portable electronic devices.    ADVANTAGE - When guard is used to control effective active area of fuel cell to 0% of maximum potential active area, this has the effect of turning off the fuel cell, which provides advantage of limiting or preventing fuel crossover on shut-down. The guard may be configured to allow gradual changes in effective active area of fuel cell, and to allow step-wise changes in effective active area of fuel cell.    DETAILED DESCRIPTION - A fuel cell (1) comprises anode (11) having inner face and outer face fluidly communicable with fuel; cathode (12) having inner face ionically communicable with and physically separated from anode inner face, and having outer face fluidly communicable with oxidant; and movable guard(s) (15) movable over the anode outer face, cathode outer face, anode inner face and/or cathode inner face. The cathode inner face is ionically communicable with anode inner face by electrolyte. The guard has structure sufficient to block at least part of anode's communication with fuel, cathode's communication with oxidant, and ionic communication between anode and cathode, thus reducing maximum potential active area of electrode surface to effective active area of electrode surface, such that power output of fuel cell is reduced. INDEPENDENT CLAIMS are included for:    (1) a fuel system; and    (2) method of controlling active area of fuel cell.    DESCRIPTION OF DRAWING(S) - The drawings show schematic side sectional views of a membraneless direct liquid fuel cell having movable guard in a partially closed and a fully opened position.    Fuel cell (1)    Anode (11)    Cathode (12)    Movable guard (15)    Diffusion barrier (24)