I’m currently working on an underwater robot and was hoping to use the gas bubbles produced by water electrolysis for buoyancy control. The operating depths range from 200–1000 m.
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The higher pressure itsels doent make it much more difficult. But...
For water electrolysis you need high overpotential, preferably large electrodes with small electrode gap between them. You want clear, filtered water and no fishes electrocuted, burning down the whole submarine due to an accidental short.
And you will need plenty of gas for buoyancy control, especially at $200{-}1000\ \mathrm{m}$ depths, therefore you need a lot of electricity:
According to Faraday's law $$m=(Q/F)(M/z)$$ where $F = 96485\ \mathrm{C/mol}$ you can make like $4.5\ \mathrm{l}$ of hydrogen gas (1 atm, 25 °C) with a $10\ \mathrm{A}$ current in 1 hour! That is like $0.04\ \mathrm{l}$ at $1000\ \mathrm{m}$ ($100\ \mathrm{atm}$) of a potentially explosive gas.
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1$\begingroup$ Does the overpotential increase with water pressure? $\endgroup$– user8697Aug 12, 2014 at 11:02
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$\begingroup$ Would the use of sea water facilitate the gas production process ??? $\endgroup$– user8697Aug 12, 2014 at 11:05
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$\begingroup$ @user8697 The better conduction (i.e. the salts in water) help the electrolysis. You cannot electrolize pure water, you have to add some ionic compound to it to improve conduction. I have no idea how big change of overpotential can be observed, but I would not expect much. $\endgroup$– GregAug 12, 2014 at 11:31
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$\begingroup$ Does the salinity increase with depth ? Would increased salinity facilitate the electrolysis ? $\endgroup$– user8697Aug 12, 2014 at 13:06