NOVELTY - A fuel cell stack comprises at least one membrane electrode assembly (1); and a fuel inlet. It further comprises a catalyst that can promote a water gas shift reaction at ambient to 150degreesC, and the catalyst is located such that fuel supplied at fuel inlet will contact the catalyst before reaching the anode electrocatalyst layer.    USE - Used as a fuel cell stack.    ADVANTAGE - The water gas shift (WGS) reaction does not require oxygen, so no air bleed is necessary. It produces hydrogen, which can be used in the fuel cell electrocatalytic reactions, and it is less exothermic, so will not release significant amounts of heat into the cell. WGS catalysts are active at temperatures found in fuel cells and can be used to reduce carbon monoxide (CO) concentration when incorporated in a fuel cell. The catalyst promotes the WGS reaction at the operating temperature of the fuel cell, so that the CO concentration of the fuel is reduced before it reaches the anode electrocatalyst layer and CO poisoning of the anode electrocatalyst layer is alleviated.    DETAILED DESCRIPTION - A fuel cell stack comprises at least one membrane electrode assembly comprising a polymer electrolyte membrane (3) interposed between an anode and a cathode, where the anode comprises an anode gas diffusion substrate (5) and an anode electrocatalyst layer (4); and fuel inlet through which fuel is supplied to the fuel cell stack. The fuel cell stack further comprises a catalyst that can promote the water gas shift (WGS) reaction at ambient to 150degreesC, and the catalyst is located such that fuel supplied at the fuel inlet will contact the catalyst before reaching the anode electrocatalyst layer.    An INDEPENDENT CLAIM is also included for a method of operating a fuel cell stack comprising supplying a fuel comprising hydrogen and carbon monoxide at the fuel inlet; passing the fuel and water over the water gas shift catalyst, to provide a fuel with a reduced concentration of carbon monoxide; and providing the fuel with a reduced concentration of carbon monoxide to the anode electrocatalyst layer.    DESCRIPTION OF DRAWING(S) - The figure shows a portion of a fuel cell stack.    Membrane electrode assembly (1)    Polymer electrolyte membrane (3)    Electrocatalyst layer (4)    Gas diffusion substrate (5)    Field flow plate (6)