NOVELTY - Composite electrocatalyst powders are produced by generating an aerosol of droplets from a precursor liquid.    USE - The method is used for producing composite electrocatalyst particles used in the fabrication of energy devices, e.g. batteries or fuel cells.    ADVANTAGE - The method permits control over the crystallinity by controlling the reaction temperature and/or residence time. It enables good control over the oxidation state of the metal. It can produce catalyst morphologies and active species structures which yield improved catalytic performance.    DETAILED DESCRIPTION - Production of composite electrocatalyst particles involves generating an aerosol of droplets from a precursor liquid; moving the droplets in a carrier gas; and heating the droplets to remove liquid and react at least one of the first and second precursors to form composite electrocatalyst particles where the active species phase is dispersed on the support phase. The precursor liquid comprises at least first precursor to a support phase and at least a second precursor to an active species phase.    INDEPENDENT CLAIMS are also included for the following:    (A) a powder batch comprising composite electrocatalyst particles having a support phase and an active species phase dispersed on the support phase, where the support phase comprises primary support particles having an average size of 10-100 nm and where the average cluster size of the active species phase is not more than 20 nm and the electrocatalyst particles have a surface area of at least 90 m2/g;    (B) an electrode assembly comprising: a current collector disposed on a gas diffusion layer; and an electrocatalyst layer comprising carbon particles and electrocatalyst particles dispersed throughout a polymer matrix where the hydrophobicity of the polymer matrix decreases with increased distance from the current collector; and    (C) a method for making particles including a polymer phase comprising: forming a liquid-containing precursor comprising at least a first precursor component and a polymer emulsion; generating an aerosol of precursor droplets from the liquid-containing precursor; and heating the aerosol of precursor droplets to remove liquid and form the particles.