Comment by pjc50

Comment by pjc50 2 days ago

This is under-explained, isn't it? The reaction has to be endothermic, so it must be taking in ambient heat. Would be useful if someone dug up the actual paper rather than the press release.

One aspect of these miracle solutions to watch out for: the catalyst is often very expensive and has a finite lifespan.

Edit: actual paper https://www.science.org/doi/full/10.1126/sciadv.ads4443

Edit: got to the bit in the paper where they describe the process; "contact electrification". This appears to be an electrostatic phenomenon like tribocharging (the old "rub a balloon on your hair" trick). Water droplets hitting the catalyst generates enough potential at the surface to trigger a reaction. So I suppose the energy input is actually in the spray+pump of the experiment, or wind in the outdoor example.

The resulting output is extremely dilute. Raising the concentration is likely to consume more energy for generating an actually useful output.

blueflow 2 days ago

There is the smoking gun:

> resulting in ammonia concentrations ranging from 25 to 120 μM in 1 hour

Not usable as fuel. You'd need to separate the ammonium from the water using a energy intensive process (cooking or such).

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NortySpock 2 days ago

At that point in the presentation, I'd probably sarcastically ask if they were accidentally measuring how many dogs mark their territory in a 100 foot radius of the device, per hour, via their collector.

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- amluto 2 days ago
  
  They did attempt to control for the ammonia concentration in the collected water without their catalyst. But they did not try to calculate the equilibrium concentration of ammonia in water exposed to the atmosphere.
  
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smaudet 2 days ago

Direct solar energy output, perhaps. Seems potentially simple to boil it off with heat.

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cyberax 2 days ago

You can catalytically oxidize ammonia in water solution. The energy generated is more than enough to overcome the energy released during solvation.

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amluto 2 days ago

I find it surprising that the paper has no discussion whatsoever of the thermodynamics of the process. The overall reaction is very endothermic (you can burn ammonia in oxygen as fuel!), so the only way it’s happening at all is that it’s approaching equilibrium, presumably driven by the increase of entropy available by creating a low concentration of ammonia in whatever weird phase it’s created in. Getting high concentrations from a similar process is going to need some energy-consuming step to shift that equilibrium.

Worse, they seem to be using some chilled object to condense ammonia solution from the air, so you’re also paying the energy cost of keeping it cold, which means you’re paying the full cost of producing a lot of water from atmospheric water vapor. Maybe a future improvement could start with liquid water.

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pjc50 a day ago

The droplet size appears to be critical here.

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