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  • How do you humidify PEM fuel cell at start-up when membrane humidifier used?

    Posted by Goktug Gonel on April 5, 2023 at 07:45

    Hello,

    Thank you for creating such community to connect and learn more about Hydrogen!

    For the hydrogen PEM fuel cells, I am noticing membrane humidifiers to be more commercially available. When we start a fuel cell, the MEA will be dry. With the membrane humidifiers air only gets humidified by exhaust damped air from fuel cell. What is the humidification strategy to humidify Hydrogen PEM Fuel Cells with membrane humidifiers used?

    thank you,

    Goktug

    Goktug Gonel replied 1 year ago 2 Members · 5 Replies
  • 5 Replies
  • Roger Penn

    Member
    April 10, 2023 at 15:31

    With a start-up (SU) from ambient temperatures, the goal is to get to the stack operating temperature as quickly as possible for humidification of dry air (the humidifier will transfer water vapour much more efficiently than liquid water). The stack will have to warm up from ~20 °C to 55-60 °C and as it transitions through the warm-up region (30-40 °C) there will be more liquid water (which can also cause issues such as cell flooding). The goal is to pass through this warm-up region quickly. One strategy is to draw load from the stack to warm up quickly, even if the load isn’t being demanded by the vehicle (or application). For example, can put current into a coolant heater (4-8 kW) which does two things:

    1. Heats up the stack due to current draw.

    2. Heats the coolant going into the stack to warm up even faster.

    Once operating temperatures are reached the humidification should be fine (there may a short phase of under-humidification, but if you transition through this quickly, the stack won’t dry out too much and can re-humidify to optimal conditions at normal operation conditions). This is one strategy, there are other strategies as well…

    • Goktug Gonel

      Member
      May 9, 2023 at 13:18

      Thank you for your reply Roger!

      However I am still a bit lost. Let’s assume car is at full stop has not been working. I assume that the nafion will be dry. Even if you start the car fuel cell will not be working, because dry Nafion will not be able to let H+ to go to the cathode side. Membrane based humidifier will not be useful in this case. Because there is no counter humid air flow to humidify the “dry” air pumped from atmosphere.

      Are you saying that the membrane will not ever get fully dry by just leaving the car not working for a couple of days? Maybe I was thinking an extreme, rare condition.

  • Roger Penn

    Member
    April 10, 2023 at 15:35

    The other (obvious) advantage of current draw to a coolant heater (or other consumer) is that the current draw creates product water that will come out the cathode exhaust and enter the humidifier to humidify the incoming (dry) air going to the stack.

  • Roger Penn

    Member
    May 16, 2023 at 16:26

    If you stop a car and let it sit for a few days the membrane will still be humidified. The cathode blocking valves close and there is no flow of gas through the stack to dry it out. So when you start again, there is sufficient proton conduction to draw current. As soon as you draw current you are making water at the cathode catalyst layer and fully hydrating the membrane.

    The only time the membrane is truly dry, is when the stack is brand new and has never been operated before. At that time, a proper Conditioning Procedure must be followed to hydrate the stack (and do a few other things like remove Platinum oxide, re-orient the ionomer, etc.) for normal operation. This Conditioning Procedure is a slow process and must be done carefully, without drawing too much load.

    If for some reason the stack is dried out in operation (e.g. by a purge that goes on too long or global overheating), then it must be carefully rehydrated. Some proton conduction will remain, you just can’t pull full load until the rehydration is complete. This would be an unusual situation due to a failure of some kind (purge algorithm incorrect, coolant circuit controls failure, etc.).

    • Goktug Gonel

      Member
      June 6, 2023 at 08:14

      Thank you again for your reply. This was very enlightening.

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