NOVELTY - A cathode catalyst (A1) comprises a ruthenium-selenium alloy having an average particle size of less than or equal to6 nm.    USE - As cathode catalyst for fuel cell in fuel cell system (claimed) e.g. polymer electrolyte membrane fuel cell (PEMFC), a direct oxidation fuel cell (DOFC), or a mixed reactant fuel cell.    ADVANTAGE - The cathode catalyst exhibits a high catalytic activity. The Ru-containing catalyst has excellent activity and stability for an oxygen reduction reaction. The Ru-Se (ruthenium-selenium) catalyst has improved catalytic activity in comparison to a platinum-based catalyst since the Ru-Se catalyst has no reactivity for an oxidation reaction of a fuel, particularly a hydrocarbon fuel. The amorphous Ru-Se catalyst has a more improved catalytic activity than a crystalline Ru-Se catalyst. The smaller particle diameter provides an increase in the catalyst active surface area, and thus improves catalyst activity.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) preparing a catalyst involving drying a ruthenium solution including a water soluble ruthenium precursor to obtain a first dried product; subjecting the first dried product to a first heat treatment to obtain a heat-treated product; adding a selenium (Se) solution including a water-soluble Se precursor to the heat-treated product to obtain a mixture; drying the mixture to obtain a second dried product including ruthenium and Se; and subjecting the second dried product to a second heat-treatment;    (2) a cathode comprising cathode catalyst (A1);    (3) a membrane-electrode assembly (A2) comprising an anode and a cathode containing cathode catalyst (A1) facing each other; and a polymer electrolyte membrane interposed between the anode and cathode; and    (4) a fuel cell system (100) comprising an electricity generating element (19) comprising a membrane-electrode assembly (A2) (20); a fuel supplier (1) for supplying the electricity generating element with a fuel; and an oxidant supplier (5) for supplying the electricity generating element with an oxidant.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of the fuel cell system.    fuel supplier (1)    oxidant supplier (5)    electricity generating element (19)    membrane-electrode assembly (20)    fuel cell system. (100)