NOVELTY - The device comprises a dioxygen and dihydrogen source (42), a high pressure reservoir (52) that is adapted for storing the dioxygen and dihydrogen and fluidly connected to the source, a derivation line (60) connecting an outlet of the dioxygen and dihydrogen source to an outlet of the dioxygen and dihydrogen producing and storing device, a device (70) that is adapted for measuring the concentration of the dihydrogen and dioxygen produced by the source and is disposed on the derivation line, and a low pressure line (56).    USE - The device is useful in a fuel cell system for producing and storing dioxygen and/or dihydrogen (claimed).    ADVANTAGE - The device is capable of economically producing and storing dioxygen and/or dihydrogen without risk of gas explosion.    DETAILED DESCRIPTION - The device comprises a dioxygen and dihydrogen source (42), a high pressure reservoir (52) that is adapted for storing the dioxygen and dihydrogen and fluidly connected to the source, a derivation line (60) connecting an outlet of the dioxygen and dihydrogen source to an outlet of the dioxygen and dihydrogen producing and storing device, a device (70) that is adapted for measuring the concentration of the dihydrogen and dioxygen produced by the source and is disposed on the derivation line, and a low pressure line (56) fluidly connecting the high pressure reservoir to the outlet of the dioxygen and dihydrogen producing and storing device. The derivation line is supplied through a pressure regulator (62) to reduce the pressure in the derivation line, and emerges from the low pressure line. The low pressure line comprises a low pressure reservoir adapted for storing the dioxygen and dihydrogen passing through the derivation line. The dioxygen and dihydrogen source is an electrolyzer. INDEPENDENT CLAIMS are included for:    (1) a fuel cell system; and    (2) a process for producing and storing dioxygen and dihydrogen.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a power supply device of a fuel cell system.    Dioxygen and dihydrogen source (42)    High pressure reservoir (52)    Low pressure line (56)    Derivation line (60)    Pressure regulator (62)    Measuring device. (70)