NOVELTY - A particle composite comprises a palladium or palladium alloy core coated with an atomically thin layer of platinum atoms.    USE - The particle composite is used in a suspension or dispersion or a catalyst useful for a fuel cell used for producing electrical energy. It is used for reducing oxygen gas. (all claimed)    ADVANTAGE - The particle composite achieves low platinum loading (greater than or equal to 7 times lower) along with increased electrocatalytic activity. The platinum monolayer-coated palladium nanoparticles have 5-8 times greater activity than the platinum nanoparticles.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a suspension or dispersion comprising a particle composite suspended or dispersed in a liquid phase;    (2) a catalyst comprising a particle composite bound to a support;    (3) a fuel cell comprising an oxygen-reducing cathode comprising an electrically conductive support bound to particle composites comprising a palladium or palladium alloy nanoparticle coated with an atomically thin layer of platinum atoms; an anode; an electrically conductive contact connecting the oxygen-reducing cathode with the anode; and an ion-conducting electrolyte in mutual contact with the oxygen-reducing cathode and the anode;    (4) a method of producing electrical energy, comprising providing a fuel cell; contacting the oxygen-reducing cathode with oxygen; and contacting the anode with a fuel source; and    (5) a method of reducing oxygen gas, comprising contacting a nanoparticle composite comprising a palladium or palladium alloy nanoparticle coated with an atomically thin layer of zerovalent platinum atoms with oxygen gas.    DESCRIPTION OF DRAWING(S) - The figure is a comparison of polarization curves for oxygen reduction on palladium nanoparticles, 10 nmol loading (left curve); commercial platinum nanoparticles, 10 nmol loading (second to left curve); and inventive platinum-coated palladium nanoparticles, 10 and 20 nmol palladium loadings (two right curves).