Fuel cells have very high theoretical energy densities because concentrated liquid fuels. The theoretical energy density of pure methanol is 6100 Whr/kg whereas lead-acid and nickel-cadmiun batteries offer 30-85 Whr/kg, and lithium ion are generally between 110-160 Whr/kg. Only a fraction of the theoretical energy density is available for conversion in a fuel cell. A full passive fuel cell must deliver fuel, minimize cross-over, and vent carbon dioxide produced at the anode without loss of fuel. For the membrane, there are two primary energy loss mechanisms which must be considered and mitigated in order to achieve long operating life: resistive losses and crossover. In this talk, the design of low-power, high-efficienty, passive fuel cells will be discussed.