One of the technologies being touted as the next great thing for our cars is the hydrogen fuel cell. If youve heard anything about them, its that there are no harmful emissions, the only by-product is pure water, straight from your tailpipe. Of course, thats only part of the story. While it is true that your exhaust will be clean, thats only because hydrogen in a cell is not a source of energy the way gasoline naturally is—its a carrier, like a battery. The energy to be stored in the cell has to come from somewhere else. Right now, the sources are the same as theyve always been, relying heavily on fossil fuels. The emissions are simply moved from your exhaust to a power plant.

But what if the hydrogen could be produced with alternative energy sources?

Launch the slideshow to see how titanium oxide reacts with light to zap dirt at the molecular level

Not so long ago, chemical engineers discovered how to use titanium dioxide to keep buildings free of discoloring pollution.