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Solar power on the Moon suffers from the month-long Lunar rotation which necessitates enough energy storage to last through the 2 week Lunar night.

An alternative to battery storage would be a closed-circuit hydrolysis/fuel cell with H2 storage.

Electrolyzers sometimes use proton-exchange membranes, as do fuel cells.

If a combined electrolyzer/fuel cell were paired with solar cells and metal hydride H2 storage, it could function as a “hydrogen battery”. For a given charging rate, the H2 battery capacity would be proportional to the H2 storage capacity. The longer the storage period, the more this is potentially a cost/weight advantage. There could also be benefits of integrating with other aspects of a Lunar “Hydrogen Economy” such as propellant production or use as a reducing agent in manufacturing.

Another potential application is on the long, inactive period in an interplanetary Cycler orbit.

Question: Do reversible proton-exchange membrane hydrolizer/fuel cells exist? Have they been incorporated into a “Hydrogen Battery”?

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  • $\begingroup$ Hydrogen alone would not work, you need oxygen too for the fuel cell. If water is electrolyzed, hydrogen and oxygen is generated and needs to be stored both. $\endgroup$
    – Uwe
    Nov 26, 2021 at 0:46
  • $\begingroup$ Yes, oxygen is needed. The closed cycle oscillates between H2O and H2 + O2 with the proton-exchange acting as a hydrolizer in one direction and a fuel cell in the other. All three need to be stored. $\endgroup$
    – Woody
    Nov 26, 2021 at 0:51
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    $\begingroup$ Please try to avoid adding a bunch of single-use tags. $\endgroup$
    – Organic Marble
    Nov 27, 2021 at 18:05
  • $\begingroup$ I first removed the hydrolysis tag then after thinking further I rolled back my edit because "water-splitting" is going to be an important thing in the future of spaceflight whenever one needs to make rocket fuel or keep those pesky humans alive! It's right up there with CO2 splitting, but I don't know a fancy/general name for that. But if we're serious about maintaining the tag, it would probably have to be added to a lot older questions. $\endgroup$
    – uhoh
    Nov 28, 2021 at 1:26
  • $\begingroup$ So I've just asked "Can I offer you a hydrolysis tag?" It's delicious and essential to future spaceflight, but can we maintain such a tag? $\endgroup$
    – uhoh
    Nov 28, 2021 at 1:26

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While I am not aware of this being done with a proton-exchange membrane fuel cell (it might be, just haven't heard about it if it is), there IS something akin to a "Hydrogen Battery" that generates, then consumes its own hydrogen in a rechargeable battery, and has a long history of use in space - the nickel-hydrogen battery.

Nickel-hydrogen batteries are related to the relatively-common nickel-metal hydride batteries, but use pressurized, gaseous hydrogen in place of the metal hydrides used in NiMH. They were, and continue to be, commonly used on spacecraft where cycle life is a major factor - something they do well at. Their energy density, however, is relatively poor compared to other batteries - hydrogen gas, after all, is anything but dense.

The pressure of hydrogen in the cell varies with state of charge, and measuring this pressure provides a reliable way to measure state of charge.

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