Like a fictional El Dorado glimmering in the distance, affordable hydrogen-powered fuel cells have been an elusive goal for a generation of researchers. Hydrogen is appealing because it can be produced domestically and burns cleanly, and fuel cell vehicles are two or three times more efficient than gasoline-powered ones. What's held them back has been the cost of building the cells themselves and a network of fueling stations to distribute the hydrogen. As a result, small fleets of FCVs are being tested by manufacturers, but no fuel cell vehicles have reached the consumer market. Two notable models in limited tests: the Honda FCX Clarity, and the 2012 Mercedes-Benz F-cell (above), which gets 52 miles per kg of hydrogen (roughly equivalent to a gallon of gasoline).