Welcome! Let’s grow the hydrogen economy together. Let’s grow the hydrogen community! Questions & Answers Can wet hydrogen (~50 ppm H2O) be used in PEM fuel cells?

  • Can wet hydrogen (~50 ppm H2O) be used in PEM fuel cells?

    Posted by jon on May 8, 2024 at 14:28

    Industry standards for water in hydrogen for PEM fuel cells is 5 ppm. However PEM also has humidification as a part of the process. I am trying to understand the difference, and why the strict limitation on water vapor given the need for internal hydration.

    Roger Penn replied 1 month ago 2 Members · 3 Replies
  • 3 Replies
  • Roger Penn

    Member
    May 12, 2024 at 17:10

    The very low quantity of water in the hydrogen purity standards is not for the stack. It is for the hydrogen refueling station (HRS) and hydrogen storage system (tank system). In the HRS the compression heats up the gas and so it gets cooled down again to be delivered to the vehicle. If it is sub-zero after cooling (or the ambient is sub-zero), then a bit of water could freeze and form ice on a valve seat or other place where it isn’t wanted.

    If a tank valve at 700 bar can’t close when it is supposed to because of ice that would be very bad (severe failure mode in the tank system). Even a system isolation valve (at 10-20 bar) must not have ice preventing it from closing when it must (severe failure mode in the fuel cell system).

    So to guard against such failure modes, the hydrogen purity specification (ISO/SAE) have very low water content to ensure no ice formation in unwanted locations.

    The fuel cell stack itself requires water on the anode to function (the protons need to form a hydronium ion, H3O+, to cross the membrane to the cathode). Without water on the anode proton conduction is impeded or stopped and the fuel cell wouldn’t function.

    So in the anode recirculation loop, there can a very high percentage of water vapour (50%-90%). Liquid water is knocked out at the stack outlet, but the goal is to try to keep water in vapour form and bring it around the anode loop to hydrate the dry H2 coming in. The humidified hydrogen then goes back to the stack inlet. Many fuel cell stack operating conditions promote water cross-over from cathode (where product water is produced) back to the anode to ensure sufficient water on the anode side of the membrane.

    If you perform a water balance on a fuel cell stack, many people are surprised to find more water comes out of the anode than the cathode, but it is usually designed that way by the operating conditions (temperature, pressure, relative humidity, stoich, differential temperature/dT, etc.).

    The H2 purity specification is what comes out of the nozzle at the HRS and goes into the vehicle (or other application). For the water/hydration levels inside the fuel cell stack, one must refer to the stack and system operating conditions.

  • jon

    Member
    May 12, 2024 at 23:58

    All answers should strive to be as helpful as this one. Thank you very much for the thoughful response.

  • Roger Penn

    Member
    May 16, 2024 at 17:11

    You are most welcome Jon Bakken.

    Just to clarify – other contaminants listed in ISO/SAE hydrogen purity standard do affect the stack (e.g. carbon monoxide and sulfur compounds can readily cause performance loss in a fuel cell stack). It is the water 5 ppm item that is for the refueling station and/or tank system.

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