NOVELTY - A gas source (22) in fluid communication with anode or cathode compartment (14, 16) of each electrochemical cell (12), includes two components that are mutually selectively reactive, depending on electric potential between anode and cathode. A constant current between the anode and cathode causes the electric potential to oscillate autonomously, while the gas components are directed through the anode compartment or cathode compartment, the oscillation in potential causing autonomous oscillation of selective reaction of the gas components    USE - Purification of hydrogen gas used as fuel source in fuel cells such as proton-exchange membrane (PEM) fuel cells, phosphoric acid fuel cell, solid oxide fuel cell, alkaline fuel cell and molten carbonate fuel cell. Purification is effected by electrochemical preferential oxidation of carbon monoxide contained in impure reformate, to remove carbon monoxide which is a poison to electrocatalysts used in proton exchange membrane fuel cells.    ADVANTAGE - Purification of hydrogen gas is performed effectively.    DETAILED DESCRIPTION - The electrochemical device comprises:    (A) a first electrochemical reactor including single or multiple electrochemical cells, where each cell includes anode and cathode compartments separated by an ion-selective partition, first gas inlet and outlet in fluid communication with the anode compartment of each cell, second gas inlet and outlet in fluid communication with the cathode compartment of each cell, and a galvanostat in electrical communication with the anode and cathode of each electrochemical cell;    (B) a gas source (22) in fluid communication with the anode compartment or cathode compartment (14,16) of each electrochemical cell (12), including at least two components that are selectively reactive to each other, depending on the electric potential between anode and cathode, whereby a constant current between the anode and cathode causes the electric potential to oscillate autonomously, while the gas components are directed through the anode compartment or cathode compartment, the oscillation in potential causing autonomous oscillation of selective reaction of the gas components; and    (C) a fuel cell system that includes a single fuel cell or stack of fuel cells, each of the fuel cells including an anode compartment, a cathode compartment and a proton-exchange membrane between the anode and cathode compartments, where the first or second gas outlet of the electrochemical device is in fluid communication with the fuel cell system.    An INDEPENDENT CLAIM is also included for gas purification method.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the electrochemical device.    electrochemical device (10)    electrochemical cell (12)    anode compartment (14)    cathode compartment (16)    gas source (22)