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So I understand the question is probably vague to a scientist, but this is all the specifics I could mustard up with my non-scientific background.

So I understand the basics of the process and have used it occationally in a garage environment to fiddle around, but never really thought about the applications until a particular Indiegogo campaign claimed to have created artificial gills small enough to hold in your mouth and generating 45 minutes of breathable air.
The campaign have gotten a wide spread of media attention and it got me thinking, how much wattage would be required to produce 6L of air over the course of a second which happens to be a rough average of a adults lung capacity.

I understand that submarines use this process to create breathable air, but as someone mentioned they have a nuclear reactor onboard and "it would not be wise to attach that to your face".

Would shortening a Lithium Ion/Polymer battery be sufficient to create such a quick reaction? Or are we talking megawatts in order to produce such a volume of air in that time span?

Update to the numbers used:

Pointed out by @DavePhD, you don't use 6L per breath, rather 6L on average per minute.
Taking into account that this would be a physical exercise under water, you'd normally use anything between 14-17L per minute, on average 16L.

Also noted is that I should have used P instead of the term wattage, thanks @Loong

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  • $\begingroup$ Please note that the quantity electric power shall not be called “wattage”. $\endgroup$
    – user7951
    Commented Mar 29, 2016 at 11:03
  • $\begingroup$ @Loong What's it called? Sorry for being ignorant and novice but would Joule be better? Oh wait, it's probably not as easy as that heh. Man I'm out on thin ice here concerning my knowledge in the area and should probably not keep on guessing! $\endgroup$
    – Torxed
    Commented Mar 29, 2016 at 11:04
  • $\begingroup$ Now you are mixing up quantities and units. The name of the quantity is “electrical power” (symbol: $P$). The name of the unit in this case is “watt” (symbol: $\mathrm W$); however, you could use any other unit of power. Since quantities are themselves always independent of the unit in which they are expressed, a quantity name shall not reflect the name of any corresponding unit. Therefore, you should avoid the name “wattage”. $\endgroup$
    – user7951
    Commented Mar 29, 2016 at 11:22
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    $\begingroup$ Instead of 6L per second, people breath about 6L per minute. 6L is the lung capacity, but a typical breath is much less and the interval between breathes is about 4s. $\endgroup$
    – DavePhD
    Commented Mar 29, 2016 at 12:14
  • $\begingroup$ @DavePhD Makes sense, since it takes roughly 1-2 seconds to breathe out the carbon dioxide. And you're right, come to think of my average air consumption while diving back in the day was 16L per minute (remember, it's a physical exercise). I'm glad you corrected me on this one! $\endgroup$
    – Torxed
    Commented Mar 29, 2016 at 12:27

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I guess it would have to be the amount energy released by the reaction $$\ce{2H2 + O2 -> 2H2O}$$ where you have four liters of hydrogen gas and two liters of oxygen gas, all divided by one second (or less, increasing the wattage as you decrease the time).

You asked about "air" — but you can't get normal air from electrolysis of water.

So, the heat of combustion for hydrogen is $286\ \mathrm{kJ/mol}$. $$286\ \frac{\mathrm{kJ}}{\mathrm{mol}} \times \frac{4\ \mathrm L}{22.4\ \frac{\mathrm L}{\mathrm{mol}}} = 51.1\ \mathrm{kJ}$$ and that is assuming atmospheric pressure which is certainly low, but how low depends on your depth below the surface of the water. So with the "barely under the surface" energy of $51.1\ \mathrm{kJ}$, we can then divide that by seconds: $$\frac{51.1\ \mathrm{kJ}}{1\ \mathrm s} = 51.1\ \mathrm{kW}$$ so you would need a bare minimum of $51.1\ \mathrm{kW}$ of power to convert water into a 67:33 mixture of $\ce{H2:O2}$. That seems like a lot of energy in a small amount of time to me: think about a six-liter balloon with the perfect combustion ratio of hydrogen and oxygen. That reaction is a lot faster than one second, but it's still a lot of energy. Tough to get into a small, portable battery and have it fill a capacitor with that amount of e energy every few seconds.

I'll be honest, I'm just doing these calculations on-the-fly, and I may have made a mistake or five, but it seems reasonable to me. If anyone takes any issue with my answer, please, feel free to edit/give suggestions.

Edit: This Wikipedia page has some information on hydrogen-oxygen diving air. They use a 96:4 ratio. Not sure why, but if that's the best ratio, this would take significantly more energy per breath, since a lot of oxygen would just get dumped.

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  • $\begingroup$ Based on this information I've calculated that you would need roughly 5 LiPo 12S batteries of 60C with 5Ah capacity (which produce 300A over 44.4V each continiously) with a potential burst of up to 600A each for shorter durations. The size of these batteries would be roughly 10cm wide and 26cm heigh and 32cm long, It would be a odd formfactor to carry around and by the design schematics of the Indiegogo device I'd say impossible. But Plausable in say a back pack. Drawing that much current would not last 45 min either from this setup. $\endgroup$
    – Torxed
    Commented Mar 29, 2016 at 11:05
  • $\begingroup$ @Torxed I understood very little of that, but I can agree that carrying around with those dimentions would be awkward. You might have an easier time getting dissolved gasses out of solution from the water... Like fish. $\endgroup$
    – SendersReagent
    Commented Mar 29, 2016 at 11:10
  • $\begingroup$ Sorry for the code language, basically "5000mAh 60C" Lithium Polymer battery, 60C standing for the discharge rate the material can handle ((60*5000)/1000) is the calculation for the ammount of Amps that particular battery can dish out). Hehe indeed it would be an aswkard thing to carry around on your face. It would however be less clumbsy than a regular diving tube that you would spend hours to pressurize. I'm surprised that no one has done this yet if this all adds upp. $\endgroup$
    – Torxed
    Commented Mar 29, 2016 at 11:15
  • $\begingroup$ "gills" means taking dissolved oxygen out of water, not performing electrolysis. The OP is referring to this concept: techinsider.io/… $\endgroup$
    – DavePhD
    Commented Mar 29, 2016 at 12:12
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    $\begingroup$ @DavePhD I'm aware of that. But he also talks about submarines and making breathable air from water. I answered that part, and it's more feasable to make work than the dissolved oxygen thing, apparently, it's just still terrible. $\endgroup$
    – SendersReagent
    Commented Mar 29, 2016 at 12:30

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