Patent ID: 7687163

Claim:
A method of operating an electrochemical conversion assembly comprising a plurality of electrochemical conversion cells, each of said cells comprising a membrane electrode assembly positioned between first and second reactant flowfields, wherein said membrane electrode assembly comprises a proton exchange membrane and said method comprises: executing an assembly warm-up operation by increasing the temperature T STACK of said membrane electrode assembly; initiating stoichiometry-based control of the relative humidity (RH) in one of said reactant flowfields when said temperature T STACK exceeds a threshold temperature T 0 , wherein said stoichiometry-based RH control comprises a reduction in said relative humidity from a value RH WET exceeding 100% relative humidity to a value RH DRY less than 100% relative humidity, said relative humidity value RH DRY is sufficiently low to permit reduction of an initial membrane hydration λ WET in said membrane electrode assembly, said reduction in said relative humidity to RH DRY is achieved by controlling the stoichiometry of said reactant flowfield and the temperature of said membrane electrode assembly such that said reduction in said relative humidity to RH DRY decreases substantially continuously, relative to portions of the RH profile of said reactant flowfield prior to initiation of said stoichiometry-based control; and transitioning from said stoichiometry-based RH control to temperature-based RH control when membrane hydration in said membrane electrode assembly falls below a target membrane hydration value λ DRY , wherein said temperature-based RH control is characterized by temperatures exceeding at least some of the temperatures characterizing said stoichiometry-based RH control.