NOVELTY - The electrochemical system comprises pressure sealed containment vessel (101) that contains a cell comprising a first positive electrode, a second negative electrode and a reaction zone between the electrodes, a reservoir (119) containing a first volume to selectively accumulate metal-halide aqueous electrolyte component and a second volume to selectively accumulate a liquefied chlorine reactant (126), a separation device separating the first volume from the second volume and a flow circuit to deliver the chlorine reactant and the electrolyte between the reservoir and the cell.    USE - The metal halogen electrochemical system such as flow battery is useful for off-peak energy storage in the power grid.    ADVANTAGE - The metal halogen electrochemical system is economical and efficient to manufacture, and has long cycle life, high stability and reliability, and low maintenance requirement.    DETAILED DESCRIPTION - The electrochemical system comprises pressure sealed containment vessel (101) that contains a cell comprising a first positive electrode, a second negative electrode and a reaction zone between the electrodes, a reservoir (119) containing a first volume to selectively accumulate metal-halide aqueous electrolyte component and a second volume to selectively accumulate a liquefied chlorine reactant (126), a separation device separating the first volume from the second volume and a flow circuit to deliver the chlorine reactant and the electrolyte between the reservoir and the cell, and electrodes or conductive plates electrically contacting the separation device to apply a voltage to the separation device to facilitate selective diffusion of the liquefied chlorine reactant through the separation device. The separation device has a higher permeability to the liquefied chlorine reactant than the metal-halide electrolyte component based on a molecular size and/or an electrical charge, and comprises a molecular sieve or a selective porous membrane, a porous container or tray having the molecular sieve, a sump plate, and a curved sheet with two openings. The molecular sieve or selective porous membrane comprises a molecular sieve consisting of granular natural or synthetic silica-alumina materials and/or a mesoporous carbon material. The curved sheet comprises a truncated cone or funnel shaped curved sheet having a first opening on its upper end and a second opening on its lower end, and a mesh, screen or perforated plate containing openings in its sidewall, is configured to facilitate the separation using a centrifugal force, and is tapered toward a lower portion and an upper portion of the reservoir. The first volume is located above the curved sheet and above the second volume in the reservoir, and has a higher concentration of the metal-halide electrolyte component than the second volume. The second volume is located below the curved sheet and below the first volume in the reservoir, and has a higher concentration of chlorine reactant than the first volume. An INDEPENDENT CLAIM is included for a method of operating an electrochemical system.    DESCRIPTION OF DRAWING(S) - The diagram shows a side cross section view of an electrochemical system.    Pressure containment vessel (101)    Stack (103)    Feed manifold (115)    Reservoir (119)    Return manifold (120)    Chlorine reactant. (126)