NOVELTY - A control device comprises electrochemical conversion cell comprising respective microporous vapor barrier layers; a second reactant supply configured to provide a second reactant to a cathode side of a membrane electrode assembly via a second flowfield portion; and a temperature controller coupled to an electrochemical conversion assembly and configured to control an operating temperature of the assembly.    USE - The control device is used for controlling an electrochemical conversion assembly useful for vehicle (claimed).    ADVANTAGE - The electrochemical conversion assembly can be operated without the presence of liquid water in particular flow field portions of the assemblies and, as a result, the deleterious effects often attributable to the presence of water in fuel cell structures can be avoided. It maintains humidity in the membrane electrode assembly without introducing liquid water or excessive humidity in targeted flow field portions of the assemblies. It enjoys the advantages of high humidity in the membrane electrode assembly while eliminating, or at least reducing, excessive humidity in flow field portions of the electrochemical conversion cells.    DETAILED DESCRIPTION - A control device comprises:    (a) a control system for controlling an electrochemical conversion assembly, the system comprising an electrochemical conversion assembly comprising electrochemical conversion cell(s) configured to convert first and second reactants to electrical energy, the electrochemical conversion cell comprising a membrane electrode assembly, first and second flowfield portions defined on opposite sides of the membrane electrode assembly, and respective microporous vapor barrier layers disposed between the membrane electrode assembly and the flowfield portions on opposite sides of the membrane electrode assembly, where microporous vapor barrier layers are configured to at least partially inhibit the migration of water molecules from the membrane electrode assembly;    (b) a first reactant supply configured to provide a first reactant to an anode side of the membrane electrode assembly via the first flowfield portion;    (c) a second reactant supply configured to provide a second reactant to a cathode side of the membrane electrode assembly via the second flowfield portion;    (d) a temperature controller coupled to the electrochemical conversion assembly and configured to control an operating temperature of the electrochemical conversion assembly; and    (e) a humidity sensor configured to measure relative humidity within the electrochemical conversion assembly and provide a corresponding cathode-side relative humidity (RH) signal indicative of the relative humidity of gas moving through the second flowfield portion, where the system is configured such that operation of one or both of the second reactant supply and the temperature controller responds to the cathode-side RH signal to a degree to maintain relative humidity within the second flowfield portion within humidification range, which is a function of the degree to which the vapor barrier layers inhibit the migration of water molecules from the membrane electrode assembly.    An INDEPENDENT CLAIM is also included for a method of controlling an electrochemical conversion assembly comprising:    (a) electrochemical conversion cell(s) configured to convert first and second reactants to electrical energy, comprising supplying a first reactant to an anode side of a membrane electrode assembly of the electrochemical conversion assembly;    (b) supplying a second reactant to a cathode side of the membrane electrode assembly;    (c) controlling an operating temperature of the electrochemical conversion assembly;    (d) measuring relative humidity within the electrochemical conversion assembly, and providing a corresponding cathode-side RH signal indicative of the relative humidity of gas moving through the second flowfield portion; and    (e) operating one or both of the second reactant supply and the temperature controller, such that it responds to the cathode-side RH signal to a degree to maintain relative humidity within the second flowfield portion within humidification range, which is a function of the degree to which respective microporous vapor barrier layers on the anode and cathode sides of the membrane electrode assembly inhibit the migration of water molecules from the membrane electrode assembly.    DESCRIPTION OF DRAWING(S) - The figure shows optimal control of RH conditions in a fuel cell stack.