Oxygen transmuting to Hydrogen
This is worse than filing the corners off 50p's to get 10p's.
The binding energy per nucleon in Oxygen is 8MeV, for Hydrogen it is zero.
So, per gram of oxygen, or of hydrogen, you need 8M x e x Ea = 7.68 x 10^11 Joules
- or about 200kWh per gram, in money terms (at 10p per unit) £21,000.
The economics are, take 200kG of water plus 4 billion quid and you have 200kg of Hydrogen to sell.