NOVELTY - Cooling system contains coolant circulating in thermal contact with reactor (10) generating electrical and heat energies, at coolant pressure. Coolant has preset dielectric constant, resistivity and boiling point, at preset conditions. A portion of coolant, which is in liquid phase when initially contacted with reactor, is vaporized to vapor by absorbing and removing excess heat energy via specific heat transfer to coolant.    USE - For cooling electrochemical reactor, such as electrochemical fuel cell e.g. proton exchange membrane fuel cell (all claimed), electrolyzer and chemical reactor, used in transportation and stationary power plants.    ADVANTAGE - The cooling system is compact, eco-friendly and economical, and effectively enables cooling of electrochemical reactor. The coolant is non-toxic, non-flammable, and has relatively short atmospheric lifetime and low global warming potential. The fuel cell has improved durability, efficiency, corrosion resistance, and stack performance. Stack shunt currents, which cause premature degradation of fuel cell stack, are eliminated.    DETAILED DESCRIPTION - A cooling system comprises a coolant circulating in direct or indirect thermal contact with an electrochemical reactor (10) at a coolant pressure. The reactor generates electrical energy and heat energy, and operates at reactor temperature. The coolant has a dielectric constant of less than 10 at reactor temperature and coolant pressure, resistivity of greater than 10 to the power 3 ohm-cm, when circulated in thermal contact, and boiling point below the reactor temperature at the reactor pressure. A portion of the coolant, which is in liquid phase when initially contacted with reactor, is vaporized to a vapor by absorbing and removing excess heat energy from the reactor via sensible heat transfer and latent heat transfer to the coolant.    INDEPENDENT CLAIMS are included for the following:    (1) cooling of electrochemical reactor, which involves operating the reactor at reactor temperature, circulating the coolant in direct or indirect thermal contact with the reactor at coolant pressure, heating the coolant by absorbing and removing excess heat energy from reactor via sensible heat transfer to the coolant, and vaporizing a portion of coolant to vapor by absorbing and removing excess heat energy from reactor via latent heat transfer to the coolant;    (2) usage of coolant to absorb and remove excess heat energy generated by the electrochemical reactor; and    (3) operation of fuel cell stack, which generates electrical energy and excess heat energy, at a desired operating temperature. The coolant is circulated in direct or indirect thermal contact with the fuel cell stack at a varying pressure. The coolant is then heated by absorbing and removing excess heat energy, such that a portion or all of the coolant is vaporized. The pressure of the coolant is then adjusted such that the desired operating temperature of the reactor is obtained.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of cooling system of electrochemical reactor.    electrochemical reactor (10)    cooling channel (11)    pump (12)    inlet pipe (14)    heat exchanger/condenser (18)