NOVELTY - The system (100) has an ozone generator (114) for introducing ozone into coolant in the system. A deionization apparatus (104) is coupled to a fuel cell stack (102). A bypass conduit (116) is arranged in parallel with the deionization apparatus. A controller (106) controls flow of the coolant to the fuel cell stack through the deionization apparatus or the bypass conduit based on operating state of the ozone generator. The controller is configured such that coolant passes through the bypass conduit when a level of ozone in the coolant is determined to be above a predetermined threshold.    USE - Fuel cell system for use in a vehicle (claimed).    ADVANTAGE - The system allows the ozone to disinfect coolant by reducing bacteria/microbes from the coolant. The system removes unwanted ions from the coolant which detrimentally affect the operation of the fuel cell stack, while ensuring chance of the ozone contacting the ion exchange resin in the deionization apparatus is small or zero. The system allows a lower stack voltage to indicate that ozone is required to improve coolant purity. The system allows the controller to control the coolant flow to the fuel cell stack by controlling the position of a non-return valve that is positioned between the deionization apparatus and the fuel cell stack to prevent back flow of coolant from the fuel cell stack into the deionization apparatus, while improving efficiency of ion exchange and eliminating a need to require the resin to be replaced frequently.    DETAILED DESCRIPTION - The coolant is water.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of fuel cell system comprising fuel cell stack, coolant reservoir, ozone generator, deionization apparatus, bypass conduit and controller with coolant flow directed through the deionization apparatus.    Fuel cell system (100)    Fuel cell stack (102)    Deionization apparatus (104)    Controller (106)    Coolant conduit (108)    Ozone generator (114)    Bypass conduit (116)    Connection (120)