>> You could store hydrogen over water with  floating tank with no 
>> bottom
>> or pump water in and out (lots of water) - maybe to a second tank 
>> or
>> use a bladder or bag. The latter is doable but liable to have a
>> shorter lifetime and Hydrogen diffusion is liable to be an issue.

> Would the Hydrogen tend to dissolve in the water?

Not to any significant extent.

> What would be a suitable material to stop the Hydrogen making its 
> way through the tank itself?

Anything with a decent density that's not chemically reactive with 
Hydrogen would do. Thin or semiporous low density materials allow 
Hydrogen diffusion at a distressing rate at low pressures. Normal 
party balloons will lose an appreciable amount of Hydrogen through the 
rubber in 12 hours. A balloon that floats in the evening will often be 
on the floor by morning. Helium is not as bad and the balloons sold 
for Helium use have less permeable rubber. A bit hard to make Helium 
by electrolysis though and it doesn't burn very well in most gases :-)

Seams and joins are an issue for Hydrogen permeation - it treats 
almost anything as an open doorway. A well built tank of quite 
lightweight metal would do well enough over the time spans liable to 
be relevant. There are almost certainly plastics that would be 
suitable at a reasonable price. One thing to be watched is reverse 
diffusion of Oxygen!. Much slower but it happens. This is something 
the Hydrogen airship people tended to keep a watch on. Understandably.

A floating metal or plastic shell gasometer in water would do a good 
job. It needs to be twice as tall as required capacity or to have 
folding skirts or to have a "slot" for the storage float to fit into 
when empty. Weighting the float could provide pressure if desired, but 
it's probably better to build it to work at minimum pressure and pump 
the gas.

Gasometer description here

        http://en.wikipedia.org/wiki/Gasometer

or operating principle only picture here

        http://upload.wikimedia.org/wikipedia/en/f/f3/Gasometer.png

        from the above page.

Nice big one (larger version of one on above page)

        http://en.wikipedia.org/wiki/Image:Gasometer_in_East_London.jpg

Gasometers at the oval - large picture

        http://upload.wikimedia.org/wikipedia/en/4/4a/Gasholders_at_the_Oval.JPG

I had no idea where "The Oval" was geographically until I saw the 
famed Millenium Wheel in the background.






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