NOVELTY - An electrochemical system comprises energy storage system having regenerative fuel cell system which generates electrical energy and reactant product from fuel and oxidizer in fuel cell mode; fuel storage vessel in fluid communication with regenerative fuel cell system, where regenerative fuel cell system comprises stacks of regenerative fuel cells (10); and energy source electrically connected to regenerative fuel cell system, to provide electrical energy to regenerative fuel cell system operating in electrolysis mode.    USE - As an electrochemical system used as fuel cell for converting energy stored in fuel to electrical energy, and generate electricity for hydrogen/bromine fuel cell to power airborne vehicle such as rocket, airplane, helicopter or blimp, water based vehicle such as ship or submarine, land based vehicle such as car, truck, motorcycle, tank or train, chemical reaction in chemical manufacturing process such as semiconductor manufacturing or chemical production process, or heating system of building such as commercial building, including office buildings, factories and hospitals and residential buildings.    ADVANTAGE - The electrochemical system is capable of conducting both hydrogen evaluation reactions and hydrogen oxidation reactions in same system. The electrochemical system has low cost, fast response time, and acceptable life and performance. The electrochemical systems achieve high power for fuel cell technology, while achieving low cost storage price due to the implementation of low cost reactant products.    DETAILED DESCRIPTION - An electrochemical system comprises energy storage system having regenerative fuel cell system which generates electrical energy and reactant product from fuel and oxidizer in fuel cell mode, and which generates fuel and oxidant from reactant product and electrical energy in electrolysis mode, fuel storage vessel in fluid communication with the regenerative fuel cell system, and at least one reactant product storage vessel in fluid communication with regenerative fuel cell system, where the regenerative fuel cell system comprises stacks of regenerative fuel cells (10), where regenerative fuel cells comprising housing; solid electrolyte membrane having first surface and second surface, disposed in the housing to partition it into anode side and cathode side; an anode formed on first surface so as to connect the first surface to anode side; cathode formed on second surface so as to connect second surface to cathode side, where anode comprising support and catalyst dispersed on it, where cathode comprising support and a catalyst optionally dispersed on it; where the catalyst dispersed on the anode and the catalyst optionally dispersed on the cathode are the same or different and are capable of catalyzing, in the presence of an acid or mixture of acids, or halogen ion or mixture of halogen ions, a charging reaction in electrolysis mode and a discharging reaction in fuel cell mode in the regenerative fuel cells; and energy source electrically connected to the regenerative fuel cell system, to provide electrical energy to regenerative fuel cell system operating in electrolysis mode, where energy source is selected from electrical grid, electrical generator, or renewable energy source selected from photovoltaic or solar power source, or wind power source. An INDEPENDENT CLAIM is included for operating an electrochemical system containing a regenerative fuel cell system involving providing an energy storage system; cyclically operating the regenerative fuel cell system in a fuel cell mode to generate electrical energy and reactant product from fuel and oxidizer and in an electrolysis mode to generate the fuel and oxidant from the reactant product and electrical energy; and providing electrical energy, from an energy source electrically connected to the regenerative fuel cell system, to the regenerative fuel cell system operating in the electrolysis mode.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of mechanically connected fuel cell stack in series.    Fuel cell stacks (10)    Inlet (20)    Outlet (25)    Feed (30)    Tubing (40)