NOVELTY - Energy recovery method for a vehicle, comprises: (a) converting kinetic energy, preferably in a braking rate of the vehicle into electrical energy, and (b) using the electrical energy for the electrolysis of water, which contains membrane electrode unit of a fuel cell system in a membrane (1) and is changeable between a fuel cell operation and electrolysis operation, preferably by operating the membrane electrode unit in the electrolysis operation. The membrane electrode unit comprises at least one hydrogen-side electrode (2) and at least one oxygen-side electrode (3).    USE - The method is useful for recovering energy for a vehicle.    ADVANTAGE - The method and system: are used in vehicles without battery hybridization, and with a completely charged battery; recover braking energy; does not require an additional water tank; and have less space requirement in vehicle.    DETAILED DESCRIPTION - Energy recovery method for a vehicle, comprises: (a) converting kinetic energy, preferably in a braking rate of the vehicle into electrical energy, and (b) using the electrical energy for the electrolysis of water, which contains membrane electrode unit of a fuel cell system in a membrane (1) and is changeable between a fuel cell operation and electrolysis operation, preferably by operating the membrane electrode unit in the electrolysis operation. The membrane electrode unit comprises at least one hydrogen-side electrode (2) adjacent to the membrane in hydrogen path (4) of the fuel cell system and at least one oxygen-side electrode (3), which is arranged in the oxygen path (5) of the fuel cell system. An INDEPENDENT CLAIM is also included for an energy recovery system for a vehicle, and for carrying out an energy recovery method, comprising a recuperation device for converting kinetic energy, preferably at the vehicle braking rate into electric energy, a fuel cell system with membrane electrode unit, which is changeable between a fuel cell operation and electrolysis operation, where the membrane electrode unit comprises at least one hydrogen-side electrode, at least one oxygen-side electrode and at least one water-containing membrane arranged between the hydrogen-side electrode and oxygen-side electrode, and the electrical energy converted from the recuperation device is fed into the membrane electrode unit for the electrolysis of water present in the membrane of the membrane electrode unit.    DESCRIPTION OF DRAWING(S) - The figures 1 and 2 show a schematic sectional views of the energy recovery system.    Membrane (1)    Hydrogen-side electrode (2)    Oxygen-side electrode (3)    Hydrogen path (4)    Oxygen-path (5)