in theory, you should be able to separate a water molecule. If you were able to, you would form O2 and H+ and you could release the 02 and capture the H+Ions. Sometimes also, you can make several different reactions that separate the hydrogen and the oxygen atoms and bond with the 02 ions and Hydrogen is given off. That being said, hydrogen is much more likely to be pulled from that molecule because it only has 1 electron and oxygen needs two, therefore it is H20. There are ways to do it, like maybe bombarding the water with beta particles and causing it to separate, and forming two different products.
EDIT: i looked at my Chem book and Natural gas contains mostly Methane (CH4), with small amounts of Ethane(C2H6), and propane(C3H8) and Butane (C4H10)
The fuel value of natural gas is 49 kJ/g whereas Hydrogens fuel value is 142 kJ/g. Our gasoline contains 48kJ/g. If we were able to produce Hydrogen from a mixture of all of these gases would "should" have enough Hydrogen to support us, although like any other non renewable energy source, like gas and oil, this method would eventually run out. So i still think we need to pursue means of producing hydrogen from water, because this would yield a renewable energy source and would be clean. Take water, insert into tank, drive car, water is produced, refill with water. Wouldnt that be neat?
im tired now, so im gonna go study for my calculus 2 test on wednesday and my chemistry 1 test for tomorrow.