[Reddwarf4ever (OP)]

I hear the UK Gov is planning to use excess electricity to produce Hydrogen from water, to use as a fuel mix with natural gas to reduce CO2 emissions. My Question is I know our planet has a lot of water, but if this becomes widespread, will it have any environmental impacts  by converting massive quantities of water into H2 ? Also when the H2 is burnt in Air, how much water is produced, in particular in a ratio of water used to produce the H2 ? Also when the H2 is produced from water, is the Oxygen also extracted ? Or is this lost to the atmosphere in the process ?.
Although possibly not safe, but if the H2 removed from water was burnt with O2 wouldn’t that be more efficient than burning in air ?
Hope I have made sense with my question, thank you

[Bored chemist]

Also when the H2 is burnt in Air, how much water is produced,

Exactly as much as was used to make the hydrogen in the first place.

Problem solved.
We don't "use up" the water, we just borrow a little of it.

[Reddwarf4ever (OP)]

That’s the problem with the internet, so much is incorrect, I read that a lot less is recovered from burning than initially used, so that’s good news. Is there a ratio of power used and power recovered by burning the H2 ?

I also read that Oxygen recovery is expensive as the electrode uses a very rare metal, the rarest on earth, in the most efficient process ? So can the H2 be extracted without also extracting the O2
Thanks

[chiralSPO]

A lot less energy is recovered than went in (but it can be released whenever and wherever it is needed, which is more useful than being tied to daylight or wind or a waterfall, etc.). But there is precisely a 1-to-1 correspondence to water converted to hydrogen and oxygen, and water produced by recombining them.

[evan_au]

There are many different processes that can be used to turn water into hydrogen & oxygen.
- One method is to use solar cells/wind energy to produce electricity, which is then used to produce hydrogen & oxygen
- Another method is to use sunlight to directly convert water into hydrogen & oxygen

And there are many different processes that can turn hydrogen & oxygen back into water.
- The simplest method is to burn it - but this is very inefficient, typically retrieving only 30-40% of the theoretical energy content
- There are more complex methods using fuel cells which can be more efficient.

Naturally, scientists are always looking for better ways...

uses a very rare metal

The conventional petrol engine also uses a very rare metal (platinum) as a catalyst to break down pollutants in the exhaust gas.
- At least a hydrogen-fuelled vehicle should have fewer pollutants.

[alancalverd]

Carbon electrodes work well and are very cheap. You still get hydrogen from the cathode but chlorine is released at the anode if you electrolyse sea water, and chlorine is a very useful industrial chemical. I'm sure other correspondents will tell us what happens to the oxygen!

[Reddwarf4ever (OP)]

Yes would be good to know…..

Thanks

[evan_au]

Yes would be good to know…..

Atomic oxygen plus carbon electrode = Carbon dioxide.

One goal of the hydrogen economy is to be less reliant on CO₂ production as a source of energy.

[alancalverd]

Not in practice, according to the manufacturers. Oxidation of pure carbon electrodes is negligible at low temperatures.

In an acidic solution you get classical dissociation and reformation into molecular hydrogen and oxygen. It is depressing how many illustrations of this textbook experiment show equal volumes of both, but that's a matter for another grumble.

In a saline solution, chlorine is preferentially liberated (equal volumes of H2 and Cl2) and the electrolyte turns alkaline with NaOH replacing NaCl as the solute. Now that is a Good Thing because alkaline sea water absorbs carbon dioxide!

[Bored chemist]

Now that is a Good Thing because alkaline sea water absorbs carbon dioxide!

Except that, in effect, sooner or later the chlorine ends up in the ocean as HCl where it reacts with carbonate to give CO2.
It's a sort of benefit, but not as big as you think.

[alancalverd]

Except that, in effect, sooner or later the chlorine ends up in the ocean as HCl where it reacts with carbonate to give CO2.

Possibly, but I'd prefer to sell it for oxidising bacteria, bleaching fabrics, making dry cleaning liquid, and as a feedstock in the rubber and plastics industry. Or pump it back into the sea to react with the NaOH we just made.

[Bored chemist]

Possibly, but I'd prefer to sell it for oxidising bacteria,

The chlorine oxidises the bacteria and is, in return, reduced to HCl.

Or pump it back into the sea to react with the NaOH we just made.

That's my point; it's difficult to "make acid" without also making an alkali. One big exception is burning fossil fuel.
You could have used that NaOH to strip CO2 from the air, but in the grand scheme of things, it ends up back with the chlorine.

[alancalverd]

Possibly, but I'd prefer to sell it for oxidising bacteria,

The chlorine oxidises the bacteria and is, in return, reduced to HCl.

which every swimming pool and hospital cleaner thinks is a Good Thing.

Or pump it back into the sea to react with the NaOH we just made.

That's my point; it's difficult to "make acid" without also making an alkali. One big exception is burning fossil fuel.
You could have used that NaOH to strip CO2 from the air, but in the grand scheme of things, it ends up back with the chlorine.

So we start with water and NaCl, and end up with hydrogen, oxygen and NaCl if we can catalyse the decomposition of NaOCl.