????8. 燃料電池

????經濟實惠的氫動力燃料電池對整整一代研究人員來說就像傳說中在遠方閃耀的金山（El Dorado）一樣，是一個充滿誘惑力的目標。氫氣之所以這么吸引人，是因為它能在國內生產，而且燃燒清潔，同時燃料電池汽車的能效要比汽油車高兩到三倍。妨礙它推廣的主要因素是電池生產成本較高，加注氫氣的站點也太少。結果，現在汽車廠商正在測試小型燃料電池車車隊，但這類車還無法投放普通消費者市場。目前兩款有名的車型正在接受極限測試：一個是本田FCX Clarity，另一個是2012款梅賽德斯奔馳F-Cell（見上圖），這輛車每公斤氫氣（大概相當于一加侖汽油）可行駛52英里。

????8. Fuel cells

????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).