As wiki states under Thermolysis water heated to around 2500 C will break the bonds of H20 into H2 and O2.
If you reached this temperature instantly would the H2O just change into a gas (that would look like thin air) and could the H2 be extracted and cooled out to some storage unit, like a compressor if you were using something like tungsten pipes and tanks?
What would happen to H2 at such temperatures?
After some more reading I found an Israel team The Weizmann team already started to achieve this back in 1995 (read here). Now they have a brochure claiming a two-step H2O-splitting thermochemical cycle based, in which they have patented it already so hope they are not to greedy.
This video of a team in Arizona, which is what got me wondering about this subject in the first place, showing a solar furnace melting steel shows that this team is not far off either.
btw I have posted this on another forum already, but reading their rules since it covers highly explosive material, I may not be there for long.