# Filter to separate hydrogen from oxygen

**URL:** <https://boards.straightdope.com/t/filter-to-separate-hydrogen-from-oxygen/1003492>\
**Category:** Factual Questions\
**Created:** [June 29, 2024, 3:25pm UTC](https://boards.straightdope.com/t/filter-to-separate-hydrogen-from-oxygen/1003492 "2024-06-29T15:25:27Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\
**Post date:** [June 29, 2024, 3:25pm UTC](https://boards.straightdope.com/t/filter-to-separate-hydrogen-from-oxygen/1003492/1 "2024-06-29T15:25:27Z")

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Here’s a story about making green hydrogen (gift link). After electrolyzing the water, it says there’s a filter to separate the two gases. What kind of filter would this be?

> **[How water could be the future of fuel](https://www.washingtonpost.com/climate-solutions/interactive/2024/green-hydrogen-water-clean-fuel-process/?pwapi_token=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJyZWFzb24iOiJnaWZ0IiwibmJmIjoxNzE5NjMzNjAwLCJpc3MiOiJzdWJzY3JpcHRpb25zIiwiZXhwIjoxNzIxMDE1OTk5LCJpYXQiOjE3MTk2MzM2MDAsImp0aSI6ImUzOWFkN2U1LTRkNzUtNGVhYi04NzU4LTljMTU1ZmEwYmU0MCIsInVybCI6Imh0dHBzOi8vd3d3Lndhc2hpbmd0b25wb3N0LmNvbS9jbGltYXRlLXNvbHV0aW9ucy9pbnRlcmFjdGl2ZS8yMDI0L2dyZWVuLWh5ZHJvZ2VuLXdhdGVyLWNsZWFuLWZ1ZWwtcHJvY2Vzcy8ifQ.KJtCyjZXDcsF5miRTZqgsR4QxQ8oC3NhwLQ_ZdtUrfg&itid=gfta)**
>
> A new generation of fuels could power planes and ships without warming the planet.

When we electrolyzed H in high school chem class, we just put the electrodes far enough apart that we could put the collection container above the cathode. I guess that doesn’t work in an industrial electrolyzer.

BTW, the hydrogen from this is not shipped to users, but rather combined with CO2 to make green diesel and naphtha. The green diesel sells for only twice regular diesel. Which is a deal considering that green hydrogen sells for about 6 times black hydrogen.

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**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\
**Post date:** [June 29, 2024, 3:28pm UTC](https://boards.straightdope.com/t/filter-to-separate-hydrogen-from-oxygen/1003492/2 "2024-06-29T15:28:21Z")

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Probably a [molecular sieve](https://en.wikipedia.org/wiki/Molecular_sieve).

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**Author:** ![CalJake](https://avatars.discourse-cdn.com/v4/letter/c/58f4c7/32.png) [@CalJake](https://boards.straightdope.com/u/CalJake)\
**Post date:** [June 29, 2024, 7:29pm UTC](https://boards.straightdope.com/t/filter-to-separate-hydrogen-from-oxygen/1003492/3 "2024-06-29T19:29:13Z")

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I routinely use hydrogen generators in a lab setting. The hydrogen and oxygen are separated by a “Proton Exchange Membrane” (PEM). Here’s a good description from a manufacturer of lab hydrogen generators:

[[How does a Hydrogen generator work?](https://www.peakscientific.com/discover/articles/hydrogen-gas-generator-how-gc/)](PEM description)

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**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\
**Post date:** [June 30, 2024, 1:41pm UTC](https://boards.straightdope.com/t/filter-to-separate-hydrogen-from-oxygen/1003492/4 "2024-06-30T13:41:42Z")

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The trick with electrolysis of water is efficiency. Running current through a body of water is just going to waste energy heating the water, so you want to reduce the gap the current has to run through, and thus its resistance.  
The proton exchange membrane provides the shortest possible gap between electrodes, and in a manner that still allows protons to flow - which will become your hydrogen.  
Efficiency still isn’t fabulous. But it will be a lot better than the usual high school demonstration.

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**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\
**Post date:** [June 30, 2024, 4:48pm UTC](https://boards.straightdope.com/t/filter-to-separate-hydrogen-from-oxygen/1003492/5 "2024-06-30T16:48:41Z")

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Thanks @CalJake, that explains the filter. And thanks @Francis_Vaughan for the explanation of why the need for the filter.

Also, welcome to the StraightDopeMB to CalJake.