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IntroductionNowadays a wide interest in the scientific community is directed towards metal nanoparticles dispersed in polymeric matrix for their unique physical and chemical properties, [1][2][3] which are of particular interest for different applications in optical, [4] electrical, [5] magnetic [6] and catalytic [7][8][9] systems. Among the various nanocomposites, the preparation of platinum clusters dispersed in polymers for application in heterogeneous catalysis has been extensively studied. [10][11][12][13][14][15] The use of platinum as basic component of catalytic converters for gasoline engines exhaust system, fuel cell catalysts, etc. has pushed the research to cost reduction, specifically to decrease the platinum amount required for the highest process efficiency, and has stimulated the development of new methods for nanosized catalyst preparation. [16] The use of nanosized platinum clusters improves the catalytic activity per platinum mass due to the higher surface areato-volume ratio, [17][18][19][20] and therefore reduce the amount of the expensive metal needed. The development of high

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High performance electrocatalytic hydrocarbon thin films containing Pt nano-clusters are deposited by simultaneous plasma polymerization of ethylene and sputtering of a Pt target. The proposed synthetic approach largely simplifies the PEMFC electrocatalyst fabrication process with respect to conventional methods. The deposition of the hydrocarbon matrix provides the mechanical support and electronic continuity, ensuring a uniform metal dispersion, avoiding Pt nanoparticle agglomeration. The Pt catalyst is dispersed as small vertically stacked clusters with size less than 10 nm in columnar thin (250-500 nm) films. PEMFC testing with plasma deposited 500 nm thick film and 0.513 mg cm À2 Pt load as anodic electrocatalyst led to a maximum reproducible power density as high as 300 mW cm À2 .