Mitigating fuel cell start up/shut down degradation

A method of operating an electrochemical conversion assembly comprising a membrane electrode assembly, an anode flow field, and a cathode flow field, wherein:

• said membrane electrode assembly comprises a proton exchange membrane separating an anode and cathode of said assembly;
• said proton exchange membrane comprises a material characterized by enhanced proton conductivity under wet conditions;
• and
• said method comprises
• initiating a start up sequence where a hydrogenous front is driven through said anode flow field,
• operating said electrochemical conversion assembly under wet conditions by driving a hydrogenous gas through said anode flow field and an oxygen-containing gas through said cathode flow field, and
• initiating a shut down sequence comprising driving a substantially dry oxygen-containing reactant gas through said cathode flow field for an amount of time sufficient to reduce the water content λ of said proton exchange membrane to below about 5, where λ corresponds to an H2O/SO3H ratio in said membrane, wherein said substantially dry oxygen-containing reactant gas is initially introduced at a first temperature characterized by a relatively high vapor capacity and is subsequently introduced at a second, cooler temperature sufficient to reduce the temperature of said membrane electrode assembly;
• interrupting a flow of said substantially dry oxygen-containing reactant gas when the water content of said proton exchange membrane is reduced;
• supplying said hydrogenous gas through said anode flow field after interrupting said flow of said substantially dry oxygen-containing reactant gas for a time sufficient to dilute an oxygen content of said cathode flow field;
• and introducing a substantially dry air purge into said anode flow field following dilution of said oxygen content.

A method of operating an electrochemical conversion assembly comprising a membrane electrode assembly, an anode flow field, and a cathode flow field, wherein:

• said membrane electrode assembly comprises a proton exchange membrane separating an anode and cathode of said assembly;
• said membrane electrode assembly comprises a catalyst supported by a relatively high surface area support material;
• said proton exchange membrane comprises a material characterized by enhanced proton conductivity under wet conditions;
• and
• said method comprises
• initiating a start up sequence,
• operating said electrochemical conversion assembly under wet conditions by driving a hydrogenous gas through said anode flow field and an oxygen-containing gas through said cathode flow field, and
• initiating a shut down sequence comprising driving a substantially dry oxygen-containing reactant gas through said anode flow field for an amount of time sufficient to reduce the water content of said proton exchange membrane to a level sufficient to suppress catalyst dissolution and corrosion in said membrane electrode assembly, wherein said substantially dry oxygen-containing reactant is initially introduced at a first temperature characterized by a relatively high vapor capacity and is subsequently introduced at a second, cooler temperature sufficient to reduce the temperature of said membrane electrode assembly;
• interrupting a flow of said substantially dry oxygen-containing reactant gas when the water content of said proton exchange membrane is reduced;
• supplying said hydrogenous gas through said anode flow field after interrupting said flow of said substantially dry oxygen-containing reactant gas for a time sufficient to dilute an oxygen content of said cathode flow field;
• and introducing a substantially dry air purge into said anode flow field following dilution of said oxygen content.

An electrochemical conversion assembly comprising a membrane electrode assembly, an anode flow field, a cathode flow field, a source of substantially dry gas, and an assembly controller, wherein:

• said membrane electrode assembly comprises a proton exchange membrane separating an anode and cathode of said assembly;
• said membrane electrode assembly comprises a catalyst supported by a relatively high surface area support material;
• said proton exchange membrane comprises a material characterized by enhanced proton conductivity under wet conditions;
• said support material and said electrode catalyst are subject to corrosion and catalyst dissolution as a result of relatively high voltages developed on said cathode of said membrane electrode assembly;
• said electrochemical conversion assembly is optimized for operation under wet conditions with a hydrogenous gas driven through said anode flow field and an oxygen-containing gas driven through said cathode flow field;
• said assembly controller is programmed to initiate a shut down sequence where a substantially dry oxygen-containing reactant gas from said dry gas source is driven through said cathode flow field for an amount of time sufficient to reduce the water content of said proton exchange membrane to a level sufficient to suppress said catalyst dissolution and said corrosion in said membrane electrode assembly, wherein said substantially dry oxygen-containing reactant gas is initially introduced at a first temperature characterized by a relatively high vapor capacity and is subsequently introduced at a second, cooler temperature sufficient to reduce the temperature of said membrane electrode assembly;
• a flow of said substantially dry oxygen-containing reactant gas is interrupted when the water content of said proton exchange membrane is reduced;
• said hydrogenous gas is supplied through said anode flow field after said flow of said substantially dry oxygen-containing reactant gas is interrupted for a time sufficient to dilute an oxygen content of said cathode flow field;
• and introducing a substantially dry air purge into said anode flow field following dilution of said oxygen content.