New device can make hydrogen when dunked in salt water

Interesting, but the energy required to desalinate water for use in a conventional electrolyser is minute compared with the energy required to electrolyse the water.

The real decider will be whether this process is cheaper than a combination of a desalinator and conventional electrolysis.

Sylvia.

Actually, on reflection, I'm a bit suspicious.

Seems to me that, in theory at least, electrolysis is 100% reversible - you can combine the oxygen and hydrogen, and get back all the energy you used to split them, obtaining pure water in the process. But separating water from salt unavoidably requires energy, because you can use pure water and salt water to obtain energy.

So if the process they're describing really were to produce oxygen and hydrogen from salt water for the same energy as required to electrolyse pure water, then it would violate the laws of thermodynamics. More likely, the difference in energy is such a small fraction of the total that they failed to measure it.

Sylvia.

A 9-volt battery?

Trouble is, in salt water, while that releases hydrogen, instead of oxygen, it releases chlorine.

Sylvia.

The last statement isn't quite right.

Dissolving common salt (and a few other inorganic compounds like ammonium nitrate) in water requires energy and cools the water.

Separating the mixture is something of a problem. Sulphates accumulating and gumming up the semi permeable membrane is much bigger problem.

Ice + salt is endothermic (hence freezing mixture).

The same is true of water + salt too so the thing is OK thermodynamically and is just a way of using a semipermeable membrane to allow only water and not hydrated ions to cross from the seawater into the electrolyte. Interesting trick but how robust is it?

Electrolysing brine was and is the basis of the chloralkali industry which in its day was a major player with the likes of Brunner Mond and Solvay (the latter still in business) exploiting it.

The world isn't short of non-sea water either so the trick may still help avoid poisoning electrolytes. I think it may have already been used for a long time to make sodium hydroxide. They have another trick to take out sulphate impurities that would otherwise accumulate.

Chlorine is a valuable commodity chemical for PVC manufacture.