You hear a lot of people - especially car manufacturers - talking about how ten years from now all the cars are going to be running on hydrogen fuel cells. Of course they've been giving us the "ten years in the future" line for the past thirty years, but still, it sounds nice.
But the issue has always been this: where do you get all that free hydrogen? Hydrogen is not rare, of course; it's by far the most common substance in the universe. But on Earth most hydrogen atoms are hooked up, mostly with a couple of oxygens, making water. That's basically what a fuel cell does, so to get the fuel for the cell you have to separate the hydrogen from the water too, leading to the whole second law of thermodynamics, leaving you inevitably behind in the exchange.
Now, however, scientists are working on getting nitrogen-fixing bacteria to do the work for us. If they succeed, it could be a potentially unlimited source of renewable fuel. Cross your fingers.