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Breaking down water into H2 and O2 through electrolysis takes 241.8 Kj/mol. According to howstuffworks.com, the average person uses 550L of pure oxygen per day, which equates to about 24.6 moles of pure O2.

The new Tesla car batteries are 85 kWh, which (in a perfect world) would be enough energy to extract about 1265 moles of O2 from water through electrolysis, enough oxygen to breathe for 51 days! Even a system at 1% efficiency would be able to provide enough oxygen to breathe for 12 hours. Current scuba tanks only last a fraction of that (using bottled air).

Why do scuba divers still use bottled air rather than generating their oxygen through electrolysis, in a world where batteries have enough capacity to generate plenty of oxygen through electrolysis?

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    $\begingroup$ Submarines and the ISS do this. I suspect carrying the quantity of batteries and kit required to do this would be impractical for a single diver. $\endgroup$
    – atom44
    Dec 2, 2016 at 10:43
  • $\begingroup$ This is an economics question, not an engineering question. And it ought to be a fairly obvious answer too. $\endgroup$
    – 410 gone
    Dec 2, 2016 at 12:07
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    $\begingroup$ Aren't you missing the glaringly obvious answer of "buoyancy?" That 85kWh battery weighs in at over 540kg. Strap that on to your back and go take a swim, let us know how well it goes. $\endgroup$
    – user16
    Dec 2, 2016 at 12:40
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    $\begingroup$ Surprised no other comments have addressed this yet. You don't want pure oxygen when you dive. You want compressed air or a similar mixture. Pure oxygen would be extremely dangerous and is only used in special circumstances if ever when diving. You can't reliably extract the other gasses you would need for an appropriate air mixture, so you're not really helping anything making oxygen yourself. $\endgroup$
    – JMac
    Dec 2, 2016 at 13:05
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    $\begingroup$ @JMac very much so. Read the original "The SIlent World" for Cousteau's description of the disasterous outcome of breathing compressed pure O2. $\endgroup$
    – Carl Witthoft
    Dec 2, 2016 at 14:15

2 Answers 2

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Bottled air is used for SCUBA because it is simple, reliable and relatively inexpensive and an open circuit system with a demand valve is pretty safe to use.

There are closed circuit systems (rebreathers) which use bottled pure oxygen and CO2 scrubbers, which give greatly extended endurance per weight, but they are significantly more complex than gas-mix SCUBA as the partial pressures of O2 and CO2 need to be carefully monitored and controlled and tend only to be used for specialist military and commercial applications as even small variations in gas mixture can be fatal.

If you are breathing a gas mixture which already has the correct partial pressure of oxygen then there is much less to go wrong. In this case only a small fraction of the oxygen carried is actually absorbed but having an open circuit means that you are always breathing the correct partial pressure of oxygen and exhaled CO2 just escapes as bubbles with each breath.

You also need at enough pressure in the system to ensure that you can actually inhgale against the water pressure at whatever depth you are at, again a high pressure cylinder with a demand valve is a failry simple way to achieve this.

Once you start using pure oxygen you need to use either a closed circuit system or prvide an inert gas to dilute it to the correct partial pressure.

There are two key things to consider here :

1) The partial pressure of oxygen needs to be right, too little and it can't be absorbed by osmosis, too much and it becomes toxic. Equally you only inhale a small amount of oxygen for each lungfull as it is absored passively across a concentration gradient.

2) It is just as important to be able to get rid of carbon dioxide as it is to absorb oxygen, indeed the breathing mechanism is driven by CO2 concentration which is why it is such a powerful asphixiant. In a sealed room you will die from CO2 asphixiation beore you run out of oxygen.

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A scuba diver needs more air per minute than only 7 to 8 liters, because he is not at rest. A diver swimming with moderate speed needs about 20 to 25 liters per minute. But this is the consumption at surface, in ten meters depth the necessary mass flow is doubled and in 20 meters tripled. Breathing pure oxygen is safe only in very shallow water, about 3 m depth. Using only the minimal mass flow of oxygen is possible only if a scrubber is used to remove the exhaled carbon dioxide within the closed loop. If pure oxygen is used in a open loop the minimum flow is again about 20 to 25 liters per minute.

Scuba diving with compressed air in bottles is the simplest and safest way to do it. Any kind of possible failure would be noticed by the diver immediately. In case of a failure, the diver is able to react and use an independent second system, share the air with his buddy or directly ascend to surface (if not to deep).

A closed loop oxygen rebreather may fail in several ways resulting in losing consciousness of the diver without warning before.

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