You could only get out of burning hydrogen with oxygen to make water, at the very most, as much energy as you put into splitting teh water up to make hydrogen and oxygen.

But it doesn't have to all happen in the same place - so you could use the energy in a power station (perhaps using renewables) to split the water, and then burn the hydrogen in a car engine miles away, using oxygen from the air. Which would be a lot easier than carrying the power station around with you.