There is great interest today in producing "green" hydrogen by electrolysis. Then, the hydrogen (and maybe the oxygen) would be stored at high pressure, often at 5000 psi (35 MPa). The question is, which provides the thermodynamic advantage--(1) electrolize at low pressure, and then pressurize, or (2) perform the electrolysis at considerable pressure? Hmm, there is probably a paper on this somewhere.
Both ways have the same thermodynamics. The change in Gibb's free energy depends only on initial and final states,
Thanks, Yurii, that makes perfect sense. I think for equivalence, the post pressurization would need to be done at constant temperature, and I think that can be approximated by staged compression--but correct me if I am wrong.
The best way to produce H2 is steam-methane reforming/ The best way to produce O2 is air separation. Cryogenic for very high purities, membrane for lower purity product.
Where cheap electricity is available, electrolysis is preferred. Where there is cheap methane, steam reforming will be better. But there are also compression and transport costs.
I suggest looking through: High-pressure electrolysis - Wikipedia.
If the electrolyser separates H2 and O2 at high pressure it is only necessary to pump the electrolyte to this pressure. The energy for this is much less than for compression of the produced gases H2 and O2 to high pressure. The downside is a more exotic and therefore more expensive electrolyser.
Dave Limb You can do nothing against thermodynamics. If you produce pressurized H2 and O2, the concentrations of them in solution will be higher and you will need to apply higher overpotentials.
You are absolutely right - nothing is free. I guess the question is - are the thermodynamic losses in the HP electrolyser more or less than those required for external compression. There is a lot of effort going into HP electrolysis so there must be some potential benefit.
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