TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Parameter: The control system is configured such that the relative humidity in the electrochemical conversion assembly can exceed 100%, but the relative humidity within the second flowfield portion of the electrochemical conversion assembly is maintained at or below 100%. The humidification range extends from 75-100%. The second reactant supply is configured to supply 1.9 or greater than 1.7 times a stoichiometric amount of air to the cathode side of the electrochemical conversion assembly for operation of the system at current densities above 0.2 or above 0.3 A/cm2, respectively. It is configured to provide the second reactant at a pressure below 165, preferably 150 kPa. The second reactant supply is configured to increase a stoichiometric amount of the second reactant supplied to the cathode side of the electrochemical conversion assembly by 10% in response to a 10% increase in the cathode-side RH signal.Preferred Component: An upper bound of the humidification range varies inversely with the degree to which the vapor barrier layers inhibit the migration of water molecules from the membrane electrode assembly. The second reactant supply is configured to increase a stoichiometric amount of the second reactant supplied to the cathode side of the electrochemical conversion assembly, in response to increasing relative humidity as indicated by the cathode side RH signal; and to decrease a stoichiometric amount of the second reactant supplied to the cathode side of the electrochemical conversion assembly in response to an decrease in the cathode-side RH signal. It is configured to increase a volumetric flow rate of the second reactant to the cathode side of the electrochemical conversion assembly, and to increase a pressure at which the second reactant is supplied to the cathode side of the electrochemical conversion assembly in response to increasing relative humidity as indicated by the cathode-side RH signal.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Component: The first reactant comprises hydrogen; and the second reactant comprises air or oxygen.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Component: The humidity sensor is configured to provide the cathode-side RH signal to the second reactant supply. It is configured to provide the cathode-side RH signal to the temperature controller. The temperature controller comprises fluid cooling loop thermally coupled to the electrochemical conversion assembly. It is configured to increase the temperature of the electrochemical conversion assembly in response to increasing relative humidity as indicated by the cathode side RH signal; and to decrease the temperature of the electrochemical conversion assembly in response to a decrease in the cathode-side RH signal. The humidity sensor is configured to measure relative humidity at an output of the second flowfield portion.Preferred Parameter: The temperature controller is configured to control the operating temperature according to uniform progression extending from 59 or 62degreesC, for operation of the electrochemical conversion assembly at a current density of 0.05 or 0.2 A/cm2 to 67 or 65degreesC, at a current density of 0.8 or 0.7 A/cm2, respectively. It is configured to increase the temperature of the electrochemical conversion assembly by 1-5degreesC, in response to a 10% increase in the cathode-side RH signal.    TECHNOLOGY FOCUS - POLYMERS - Preferred Parameter: The microporous vapor barrier layers define a mean pore size diameter below 10, preferably 1 microns. They define respective thicknesses of below 100 microns, a mean pore size diameter of below 1 microns, and a through plane permeability of less than 0.01 Frazier (cubic feet per minute (CFM)/square feet) at a differential pressure of 0.5 inch water (H20)).Preferred Component: The microporous vapor barrier layers comprise expanded polytetraflouroethylene (PTFE) defining a mean pore size diameter below 1 microns.