NOVELTY - An electrochemical conversion assembly (10) comprises reactant supply providing humidified reactant to cathode flowfield portion, which defines reactant input (36) and reactant output (38); and a coolant supply providing a cooling fluid to a coolant flowfield portion, which defines a coolant input, a coolant output, and an operating coolant temperature profile having areas of relatively low and high coolant temperature. The cathode and coolant flowfield portions are configured such that reactant input and output are positioned closer to areas of relatively low and high coolant temperature.    USE - Used as a source of motive power for a vehicle.    ADVANTAGE - The overall water mass balance within the assembly is improved.    DETAILED DESCRIPTION - An electrochemical conversion assembly comprises an electrochemical conversion cell(s) converting first and second reactants to electrical energy; a reactant supply providing a humidified reactant to a cathode flowfield portion of the electrochemical conversion assembly; and a coolant supply providing a cooling fluid to a coolant flowfield portion of the electrochemical conversion assembly. The cathode flowfield portion defines a reactant input and a reactant output. The coolant flowfield portion defines a coolant input, a coolant output, and an operating coolant temperature profile having areas of relatively low coolant temperature (T-MIN) and areas of relatively high coolant temperature (T-MAX). The cathode flowfield portion and the coolant flowfield portion are configured such that the reactant input and the reactant output are positioned closer to the areas of T-MIN than the areas of T-MAX. INDEPENDENT CLAIMS are included for:    (1) a scheme for operating an electrochemical conversion assembly; and    (2) a vehicle comprising the electrochemical conversion assembly.    DESCRIPTION OF DRAWING(S) - The figure is a schematic illustration of an electrochemical conversion assembly.    Electrochemical conversion assembly (10)    Reactant input (36)    Reactant output (38)