Managing MEA hydration cycling life
View Patent ↗In accordance with one embodiment of the present invention, a method of operating an electrochemical conversion cell is provided wherein the method comprises the steps of (i) initiating a membrane dehydration sequence when the membrane is characterized by an initial membrane hydration λ WET and (ii) maintaining the membrane dehydration sequence until the membrane is characterized by a target membrane hydration λ DRY . According to the method, the membrane dehydration sequence is characterized by a drying rate that varies in a manner that substantially corresponds to a fatigue life contour map of the membrane. Additional methods and corresponding systems are contemplated.
1. A method of operating an electrochemical conversion cell 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:
initiating a membrane dehydration sequence when said membrane is characterized by an initial membrane hydration λ WET ; and
maintaining said membrane dehydration sequence until said membrane is characterized by a target membrane hydration λ DRY , wherein
said initial membrane hydration λ WET is substantially larger than said target membrane hydration λ DRY , and
said membrane dehydration sequence is characterized by a drying rate that varies in a manner which substantially corresponds to a fatigue life contour map of said membrane, wherein said dehydration sequence is characterized by a drying rate that decreases as membrane hydration approaches said target membrane hydration λ DRY .
2. A method of operating an electrochemical conversion cell as claimed in claim 1 wherein said membrane is dehydrated by controlling one or more of the temperature of the membrane, the humidity in the first reactant flowfield, the humidity in the second reactant flowfield, the flow rate in the first reactant flowfield, the flow rate in the second reactant flowfield, the gas pressure in the first reactant flowfield, and the gas pressure in the second reactant flowfield.
3. A method of operating an electrochemical conversion cell as claimed in claim 1 wherein said dehydration rate varies such that it tracks portions of said fatigue life contour map that are characterized by an expected cycle life value that exceeds an expected cycle life associated with substantially constant dehydration at a rate substantially greater than a majority of those tracked in said fatigue life contour map.
4. A method of operating an electrochemical conversion cell as claimed in claim 1 wherein said drying rate varies such that it tracks portions of said fatigue life contour map that are characterized by an expected cycle life value that exceeds about 6,000 cycles, where said cycle life value represents a point at which a substantial increase in the leak rate of reactants across said membrane is initiated, and a cycle represents humidification cycling of said membrane from a relatively hydrated state, to a relatively dehydrated state, and back to said relatively hydrated state.
5. A method of operating an electrochemical conversion cell as claimed in claim 1 wherein said drying rate decreases continuously, intermittently, or continuously and intermittently, in a uniform or non-uniform manner.
6. A method of operating an electrochemical conversion cell as claimed in claim 1 wherein said target membrane hydration λ DRY is below about λ=8, where λ represents the water content of said membrane in terms of the number of water molecules per acid site.
7. A method of operating an electrochemical conversion cell as claimed in claim 1 wherein said fatigue life contour map of said membrane is constructed using a fatigue life curve of said membrane and a calculated, measured, approximated or otherwise determined membrane stress.
8. A method of operating an electrochemical conversion cell as claimed in claim 7 wherein said fatigue life curve comprises a plot representative of points at which a substantial increase in the leak rate of reactants across said membrane is initiated.
9. A method of operating an electrochemical conversion cell as claimed in claim 8 wherein said points are a function of stress in said membrane.
10. A method of operating an electrochemical conversion cell as claimed in claim 8 wherein said membrane stress is determined from a stress model representative of said membrane.
11. A method of operating an electrochemical conversion cell as claimed in claim 1 wherein said membrane dehydration sequence is initiated upon detection of a trigger signal representative of an operating condition of said electrochemical conversion cell.
12. A method of operating an electrochemical conversion cell as claimed in claim 11 wherein said trigger signal is representative of the water content of said membrane, the relative humidity in the reactant flowfield, the power demand on said cell, the initiation or termination of a shut-down or start-up sequence in said cell, the completion of a hydration cycle of said cell, or combinations thereof.
13. A method of operating an electrochemical conversion cell as claimed in claim 11 wherein said trigger signal represents a point at which the water content of said membrane reaches a maximum value.
14. A method of operating an electrochemical conversion cell as claimed in claim 11 wherein said trigger signal is representative of a water content of said membrane exceeding about 12 water molecules per acid site.
15. A method of operating an electrochemical conversion cell as claimed in claim 1 wherein said membrane dehydration sequence is terminated upon detection of a termination signal representative of an operating condition of said electrochemical conversion cell.
16. A method of operating an electrochemical conversion cell as claimed in claim 15 wherein said termination signal is representative of the water content of said membrane, the relative humidity in the reactant flowfield, the power demand on said cell, the initiation or termination of a shut-down or start-up sequence in said cell, the completion of a dehydration cycle of said cell, or combinations thereof.
17. A method of operating an electrochemical conversion cell as claimed in claim 15 wherein said termination signal is representative of a water content of said membrane below about 12 water molecules per acid site.
18. A system comprising at least one electrochemical conversion cell and a cell controller, wherein:
said electrochemical conversion cell comprises a membrane electrode assembly positioned between first and second reactant flowfields;
said membrane electrode assembly comprises a proton exchange membrane; and
said cell controller is configured to initiate a membrane dehydration sequence when said membrane is characterized by an initial membrane hydration λ WET , and maintain said membrane dehydration sequence until said membrane is characterized by a target membrane hydration λ DRY , wherein said initial membrane hydration λ WET is substantially larger than said target membrane hydration λ DRY and said membrane dehydration sequence is characterized by a drying rate that varies in a manner which substantially corresponds to a fatigue life contour map of said membrane, wherein said dehydration sequence is characterized by a drying rate that decreases as membrane hydration approaches said target membrane hydration λ DRY .