The Economist:

WHEREVER automotive engineers gather, some wag will sooner or later announce that hydrogen is the fuel of the future—and always will be. The hydrogen-powered car has been just around the corner for decades. However, judging from announcements by Honda, Hyundai and Toyota at last week’s motor shows in Los Angeles and Tokyo, hydrogen cars will be hitting the showrooms from spring 2014 onwards. It seems the future is about to arrive.
 Hydrogen’s attraction as a transport fuel is that, unlike petrol, diesel, kerosene, natural gas and every other hydrocarbon fuel, it contains, well, no carbon. Burning it therefore creates no carbon-based greenhouse gases—at least, not in the engine. However, if air is used as the oxidiser instead of pure oxygen, burning hydrogen produces all the noxious oxides of nitrogen that fossil fuels generate. These are an even bigger curse than carbon dioxide as far as damaging greenhouse gases are concerned.
 That is why work on using hydrogen as a fuel for a modified internal-combustion engine has been more or less abandoned, even though getting such a power unit into production was considered cheaper than any of the clean alternatives. BMW built a couple of hydrogen-powered supercars, only to find them no cleaner than clunkers from the days before catalytic converters.