NOVELTY - A nano-structured catalyst coating is deposited on a porous electrode or solid electrolyte membrane substrate from a precursor catalyst material from metal, metal alloy, metal compound, or ceramic material, by heating and at least partially vaporizing the precursor material to provide a stream of nanometer-sized vapor clusters of the material in a chamber, and introducing a stream of a carrier gas into the chamber.    USE - For depositing a nano-structured catalyst coating on a porous electrode substrate for fuel cell applications.    ADVANTAGE - The material does not need post-fabrication mixing treatment, provides good catalytic effects, is fast and effective method, provides high mass activity, produces catalytic coatings on a substrate cost-effectively, is simple and easy to operate, and does not require the use of heavy and expensive equipment such as laser or vacuum-sputtering unit.    DETAILED DESCRIPTION - Deposition of a nano-structured catalyst coating on a porous electrode substrate or solid electrolyte membrane substrate from a precursor catalyst material from metal, metal alloy, metal compound, or ceramic material, involves operating an ionized arc nozzle to heat and at least partially vaporize the precursor catalyst material for providing a stream of nanometer-sized vapor clusters of the precursor material into a chamber in which a porous electrode substrate or solid electrolyte membrane substrate is disposed, and introducing a stream of a carrier gas into the chamber to impinge upon the stream of precursor vapor clusters to produce deposits of nano clusters that are carried by the carrier gas to deposit on a first side of the substrate for forming the coating. The ionized arc nozzle comprises a consumable electrode, a non-consumable electrode, and a working gas flow to form an ionized arc between the consumable and non-consumable electrodes. The consumable electrode provides the precursor catalyst material vaporizable by the ionized arc.    INDEPENDENT CLAIMS are also included for:    (a) a porous gas diffusion electrode for a fuel cell, comprising a gas permeable electrically conductive porous substrate layer of a known thickness having a gas-receiving face and an opposing catalytic-supporting face; a nano-structured catalyst coating supported on the catalyst-supporting face and comprising a catalytic material in finely divided form in electrical contact with the catalyst-supporting face and residing in a catalyst region less than half of the layer thickness into the substrate layer from the catalyst-supporting face;    (b) a fuel cell membrane electrode assembly comprising an ion-conducting electrolyte membrane comprising an ion-conducting electrolyte membrane sandwiched between two electrodes;    (c) a fuel cell comprising a porous gas diffusion electrode; and    (d) a catalytic solid electrolyte membrane for a fuel cell comprising an ion-conducting solid electrolyte substrate layer, and a nano-structured catalyst coating supported on the first face of the substrate and comprising a catalytic material in finely divided form in electrical contact with the first face and residing in a catalytic region less than a quarter of the layer thickness into the substrate layer from the first face.    The catalytic material is present in the form of ultra-fine particles having a diameter of 2-10 nm and the loading of the catalyst is 0.1-1 mg/cm2. The particles of catalytic material have been deposited by the inventive method. At least one electrode is a porous gas diffusion electrode and the nano-structured catalytic coating is in electric contact with the electrolytic membrane.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic of fuel cell.    Anode (10)    Cathode (12)    Electrocatalyst layer (18)    Bulb (20)    Current collector plate (21)