[nudephil (OP)]

Here's a question from listener Gary:

When using hydrogen as an energy source, to obtain the hydrogen we have to split H2O - eg the hydrogen from water. In doing so, at a very large rate and over time, wouldn't water become scarce and the atmosphere become supercharged with oxygen?

Can anyone help?

[Halc]

When using hydrogen as an energy source, to obtain the hydrogen we have to split H2O - eg the hydrogen from water. In doing so, at a very large rate and over time, wouldn't water become scarce and the atmosphere become supercharged with oxygen?

Separating hydrogen from water takes a lot of energy, which is given back only partially when the hydrogen is burned as fuel, such as a fuel cell or the space shuttle engines. The space shuttle (but not the two boosters) is a glorified water rocket, expelling pretty much pure water.
The only way to get net energy from hydrogen is via fusion, and fusion to date has probably burned less than a few kilos of hydrogen. In this age of global sea level rise, I'd think the absence of a few liters of water would be appreciated.
And the kiddies get some balloons out of the deal!

[chiralSPO]

You aree correct that splitting water into hydrogen and oxygen would use up water and produce oxygen (in addition to the hydrogen that would presumably be stored). But for that hydrogen to ever be useful as a fuel, it would need to be recombined with that oxygen, reforming the water. If you lose containment of the hydrogen, some of it might escape into space, preventing recombination with oxygen to form water. But no business is likely to waste their resources doing that.

Storing energy by splitting water does require some amount of water (and high purity water at that!), but compared to present-day grid-scale technological solutions (pumped hydro) electrolysis is very efficient in terms of water needs.

Pumped hydro is used today for storing large amounts of energy: You need a reservoir at the top of a mountain, and one at the bottom. To "charge" it with energy for later, one must simply use that energy to pump the water up to the top reservoir. Then, when the energy is required, you leet the water flow back down, and through a hydroelectric turbine. This has about a 90% round-trip efficiency (ie you get back 90% of the energy you put in), which is good; but to store 1 MWh of energy (3.6×10⁹ J), one would need to pump a million liters of water up to an altitude of 360 meters (double the change in height to halve the volume of water or vice versa).

On the other hand, one could electrolyze the water using the energy that needs to be stored, generating hydrogen and oxygen, and then release the usable energy by recombining the oxygen and hydrogen using a fuel cell (about 70% round trip efficiency) or by combustion (about 20% round trip efficiency). So the efficiency is not great, but there is a much smaller water requirement: to store 1 MWh of energy (3.6×10⁹ J), one would only need to split about fifteen thousand liters of water (less than 1/60 the volume needed with the 360 meter tall pumped hydro system).

[evan_au]

one would only need to split about fifteen thousand liters of water

15 tons of hydrogen gas takes a lot of space - unless you expend a lot of energy compressing it.

[Bored chemist]

one would only need to split about fifteen thousand liters of water

15 tons of hydrogen gas takes a lot of space - unless you expend a lot of energy compressing it.

If you are going to quote me, at least give some context.

[chiralSPO]

one would only need to split about fifteen thousand liters of water

15 tons of hydrogen gas takes a lot of space - unless you expend a lot of energy compressing it.

'Twas me who said this!

And yes, I agree whole heartedly. I think that rechargeable zinc-oxygen batteries (fuel cells) are the way to go (at least for stationary storage of huge amounts of energy.) While splitting water to make hydrogen and oxygen provides fuel with a favorable specific energy (up to about 142 MJ per kg of hydrogen, assuming its being re-converted to liquid water by reaction with oxygen), its energy density is not great (is only about 0.01 MJ per L of uncompressed hydrogen; up to 8.5 MJ/L if liquified).

In comparison splitting zinc oxide provides zinc metal, which has only modest specific energy (5.3 MJ/kg, assuming it's being reconverted to zinc oxide). But with an energy density of 38 MJ per/L of zinc, that's a much smaller footprint (and easier storage). If the oxygen is coming from the air, you can have a very compact fuel cell. But if the oxygen must be high purity, or is being stored, much of that improved energy density is lost, because of the volume of the oxygen gas.

[evan_au]

fifteen thousand liters of water...15 tons of hydrogen gas

Oops! bad chemistry....
fifteen thousand liters of water is 15 tonnes of water.
But the hydrogen only makes up 1/9 of the mass of the water = 1.7 tonnes...