NOVELTY - A proton-conducting thin layer comprises a nano-scale ceramic material with a relative density of 90% or more and a grain size less than 30 nm. The layer is functional at temperatures below 100 degrees C, in the presence of water vapor. The ceramic is zirconium dioxide, cerium dioxide, yttrium aluminum garnet, aluminum oxide, other functional oxides or their combinations. It is yttrium oxide-stabilized zirconium dioxide, gadolinium-doped cerium dioxide or samarium-doped cerium oxide.    USE - A proton-conducting thin layer comprising dense nanoscale ceramic material. The electrochemical device made from the layer is a fuel cell, battery, hydrogen-separating thin layer, positron emission tomography (PET) scintillator or thin layer reactor (all claimed).    ADVANTAGE - The ceramic material is produced in the finest grain sizes using a high pressure modification of the spark plasma sintering (SPS) method. Suitable starting materials are commercially available. Short thermal cycles, less than 10 minutes in duration, are accompanied by rapid pressure increases of to up to 1 GPa. This results in a high degree of consolidation, and very limited grain growth. 98% relative density or more is achievable. Detailed discussion and extensive literature references are included. Further widely-ranging applications are cited, e.g. in lasers, chemical sensors and hydrogen generators.    DETAILED DESCRIPTION - The zirconium dioxide stabilized with yttrium oxide is cubic. The samarium oxide-doped cerium oxide, has the following empirical formula Ce0.80Sm0.20O2.0. The dense nanoscale ceramic, has a relative density of 95%-98% or more. Its grain size is 10 nm-5 nm or less. The operational temperature is 75 degrees C-50 degrees C or less. An electrochemical device includes an anode, cathode and the layer described. It is capable of functioning at below 100 degrees C, in the presence of water vapor.    DESCRIPTION OF DRAWING(S) - The drawing shows high pressure molding equipment used to make dense ceramics in accordance with the disclosure.    tungsten carbide (1)    thermocouple (2)    silicon carbide (3)    sample (4)