NOVELTY - An air-breathing fuel cell stack comprises a membrane electrode assembly (10) including an anode, a cathode, and an electrolyte; a fuel supplier to supply a fuel to the anode; a cathode current collector (20) disposed on the cathode; a cathode end plate (40) to support cathode current collector; and a filter (30) positioned disposed between the cathode current collector and the cathode end plate, to control the moisture content of the membrane electrode assembly. The air flow rate of the filter is 10 l/minute cm2, and the moisture flow rate of the filter is 100-150 Ml/minute cm2.    USE - As a fuel cell stack for a fuel cell system useful as power generation systems for large buildings, electric vehicles, portable applications, etc.    ADVANTAGE - The air-breathing fuel cell stack stably maintains output performance, by supplying clean air to cathode, and by controlling the release of moisture from the stack. The filter permits an air flow of greater than or equal to 10 Ml/minute cm2, and a moisture flow rate of 100-150 Ml/minute cm2. The filter is capable of controlling the inflow of the ambient air and the moisture release of the MEA, within a constant range, thus improving the output and operating time of the air-breathing fuel cell stack. The filter prevents the drying of the cathode and/or membrane, and suppresses the vaporization of moisture from the cathode side. The anode end plate and the middle plate store fuel, therefore a separate fuel tank is not required, so a fuel cell system can be more compact. The filter prevents the cathode and the electrolyte from drying out. The filter controls the moisture content of the membrane electrode assembly (MEA). The filter prevents pollution in ambient air from reaching the cathode. The filter has characteristics that allow for a sufficient flow of the ambient air to the MEA, and that limit the discharge of moisture from the MEA, to prevent the drying of the cathode and/or membrane, when the air-breathing fuel cell stack is operated.    DETAILED DESCRIPTION - An air-breathing fuel cell stack comprises a membrane electrode assembly (10) including an anode, a cathode, and an electrolyte disposed between the anode and the cathode; a fuel supplier coupled to the anode, to supply a fuel to the anode; a cathode current collector (20) disposed on the cathode; a cathode end plate (40) to support the cathode current collector; and a filter (30) positioned disposed between the cathode current collector and the cathode end plate, to control the moisture content of the membrane electrode assembly. The air flow rate of the filter is 10 l/minute cm2, and the moisture flow rate of the filter is 100-150 Ml/minute cm2. The filter comprises pores having a size of 5-20 mu m. The filter is a micro filter comprising a hydrophobic polymer. The filter includes a basic surface to neutralize an acidic pollutant selected from sulfur oxide, nitrogen oxide, hydrogen sulfide, hydrogen chloride, a volatile organic acid and/or non-volatile organic acid; or an acidic surface to neutralize a basic pollutant selected from ammonia, amine, amide, sodium hydroxide, lithium hydroxide, potassium hydroxide, a volatile organic base, and/or non-volatile organic base.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of the air-breathing fuel cell stack.    Membrane electrode assembly (10)    Cathode current collector (20)    Filter (30)    Cathode end plate (40)    Anode separator (50)    Anode end plate (60)    Cathode gasket (70)    Anode gasket. (70a)