NOVELTY - An electrochemical and/or catalytic composition comprises reactive metal particles and a substrate(s) that has lesser reactivity than the reactive metal particles and that has a high surface area relative to its volume. A portion(s) of a surface of the substrate comprises an interior surface within an outer dimension of the substrate. A portion(s) of the reactive metal particles resides proximate to a portion of the interior surface.    USE - The composition is for use in electrochemical and/or catalytic applications. It is used in an electrochemical component for coupling to a current collector for providing a portion of a circuit that permits an electrical connection between the component and a second component to transmit current between them, useful for an electrode, preferably gas diffusion electrode for use in an electrical energy generating device, where energy may be provided in a controlled fashion; a fuel cell for consuming a fuel to generate electricity; a hydrogen generator for electrolyzing water to yield oxygen and hydrogen; and a sensor (i.e. electrochemical sensor, preferably a biosensor) for detecting a presence of a gas and for detecting an analyte capable of undergoing an electrochemical reaction at the sensor (all claimed). The electrode is used as anode and/or cathode useful in metal-air batteries (e.g. zinc/air battery), hydrogen fuel cells, direct methanol fuel cells, direct oxidation fuel cells, alkaline fuel cell system, and other air or oxygen breathing electrochemical systems. It is used in liquid diffusion systems or in sensors for electrochemical active species, e.g. oxygen in a reducing environment or for detecting electrochemically active species in a liquid phase, e.g. in water testing.    ADVANTAGE - The composition is capable of being maintained in a stable environment to permit controlled oxidation of a portion(s) of the reactive metal particles; provides higher power capability through the use of nanoparticle catalyst, which can replace or eliminate platinum as a catalyst for electrochemical reactions; and uses lower cost materials as catalysts in electrodes. The use of reactive metal particles increases the performance of electrochemical cells, which equates to an increased amount of available energy available for the end user; and increases electrochemical cell power and/or longevity. The superior catalytic activity of the electrode provides good sensitivity.    DETAILED DESCRIPTION - The substrate comprises a material having affinity for the reactive metal particles such that when the reactive particles are brought into contact with the substrate the particles may become associated with the substrate. It is highly porous, and comprises highly porous particles. The substrate comprises carbon. A portion(s) of the reactive metal particles comprises nanoparticles having a diameter of less than 1 mu m, preferably less than 10 nm. A portion(s) of the nanoparticles comprises nanoparticles having an oxide shell. The reactive metal particles comprise a metal from groups 3-16, lanthanides, and/or their alloys. An INDEPENDENT CLAIM is included for a method of manufacturing an electrochemical and/or catalytic composition comprising contacting in an anoxic fluid, reactive metal particles and substrate.