NOVELTY - Operation of cooling sub-system of an electrochemical fuel cell system during start-up by: (a) directing a start-up coolant through an electrochemical fuel cell stack; and (b) directing a standard coolant through the fuel cell stack when the temperature of the fuel cell stack or the start-up coolant reaches a first predetermined temperature. The heat capacity of the start-up coolant is different from the heat capacity of the standard coolant.    USE - For operating a cooling sub-system of electrochemical fuel cell system.    ADVANTAGE - Provides a more efficient method of efficiently starting a fuel cell stack at low and sub-freezing temperatures.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a cooling subsystem for an electrochemical fuel cell system (200) having an electrochemical fuel cell stack (210), comprising:    (a) a coolant reservoir (230) fluidly connected to the fuel cell stack, where the coolant reservoir is configured to allow separation of a first coolant fluid and a second coolant fluid (232) contained in the coolant reservoir, and where the coolant reservoir comprises a first coolant fluid outlet (233) and a second coolant fluid outlet; and    (b) a standard coolant loop fluidly connected to the fuel cell stack and both the first coolant fluid outlet and the second coolant fluid outlet of the coolant reservoir, the standard coolant loop comprising a standard pump (220).    A start-up coolant comprising first coolant fluid and optionally second coolant fluid is directed through the fuel cell stack during start-up of the fuel cell system. A standard coolant, comprising the second coolant fluid, is directed through the fuel cell stack when the temperature of either the fuel cell stack or the start-up coolant reaches a first predetermined temperature.    DESCRIPTION OF DRAWING(S) - The drawing shows a cooling sub-system for electrochemical fuel cell system.    Fuel cell system (200)    Fuel cell stack (210)    Standard pump (220)    Coolant reservoir (230)    Second coolant fluid (232)    First coolant fluid outlet (233)    Heat exchanger (260)    Start-up pump (270)