A new way to keep hydrogen: Plumage power

    Hydrogen has long been touted as the future of energy. It is clean, efficient and the most abundant substance in the universe. It can be used to run an internal combustion engine in a car or power one using a fuel cell, with heat and water as the only emissions. But hydrogen is difficult to store because it is the lightest element. Filling a typical fuel tank of 75 litres-about 20 American gallons-with hydrogen at room temperature and pressure will take a hydrogen-powered car only about a kilometre or so. The gas can be compressed to take up less space, but that can be dangerous. It also uses energy, which removes some of the benefits.

    Another way to store hydrogen is to put something inside the tank which increases the total internal surface area to which the molecules of the gas can cling. This means more hydrogen can then be packed into a smaller volume. There has been some progress with materials that can do this, including specially engineered carbon nanotubes. But carbon nanotubes are very expensive to make, especially in large quantities. Richard Wool, a chemical engineer at the University of Delaware, estimates the cost of fitting a single car with a tank full of carbon nanotubes to be $5.5m. Other materials might do, but they could still end up costing over $20,000 a car.

    Dr Wool and a colleague, Erman Sen?z, think they have found a way to bring the price down to only around $200 a car by using chicken feathers. The fibres in feathers are almost entirely composed of keratin, a protein also found in hair and nails. When heated in the absence of oxygen （a process called pyrolysis）, keratin forms hollow tubular structures six millionths of a metre across and riddled with microscopic pores, much like carbon nanotubes.

    The researchers demonstrated how this can be done at the 13th Annual Green Chemistry and Engineering Conference, held recently in College Park, Maryland. To avoid melting the fibres and depriving them of their desirable structural properties, they first heat-treated the feathers to around 215°C. This strengthened their structure and allowed further heating to 400-450°C. At this point the material becomes more porous, increasing its surface area and its hydrogen-storing capacity.

    The substance they created is capable of holding 1.5% of its weight in hydrogen. Since about 4.5kg of the gas is needed to cover 480km （about 300 miles）, the typical range of a petrol-powered car, this would translate into a rather large 284-litre tank stuffed with some 300kg of carbonised chicken feathers, according to Mr Sen?z. This still falls short of the 6% hydrogen-storage target which has been set by America's Department of Energy to encourage innovation with alternative fuels. But the researchers think they can improve their material further by making it even more porous. And unlike rival technologies theirs is well on the way to meeting the department's cost criteria of a hydrogen system that costs $4 per kilowatt hour stored and less than $700 for installing it. Moreover, it could also help with another environmental problem: reducing the mountains of chicken feathers that the poultry industry has to dispose of every year.

    羽毛的力量：儲氫材料新發現

    長久以來氫氣一直被標榜為未來能源。氫氣潔凈、燃燒效率高而且是宇宙中最豐富的物質。氫氣可以用在汽車的內燃機引擎，或者以燃料電池的形式為汽車提供能源。使用氫氣能源的排放物僅僅是熱和水。但是，儲存氫氣很困難，這是因為氫氣是最輕的元素。通常情況下，一輛汽車的油箱容量為75升，約合20加侖，在室溫和常壓下，用氫氣作為能源填滿油箱的話，僅僅只需要約1公斤的氫氣。可以把氫氣壓縮以減少所占用的空間，但是這可能會有危險。而壓縮氫氣也需要能源，這也是它的弊端。

    儲氫的另外一種方法就是在油箱中放入一些提高總的內表面積的物質，氫氣就附著在這些物質的表面。這就意味著更多的氫氣可以壓縮到更小的空間里面。對這種材料的研究有了一些進展，其中包括特別加工過的碳納米管。但是碳納米管非常昂貴，尤其是大批量生產。Richard Wool是來自特拉華大學（University of Delaware）的一名化工工程師，他估計，單單把一輛汽車的油箱填滿碳納米管，其成本就高達550萬美元。其它的材料也許可行，但是僅僅對一輛汽車來說，其成本也超過2萬美元。

    Wool博士和他的同事Erman Sen?z認為，他們找到了一種方法使成本降到僅僅只有200美元，那就是使用雞毛。雞毛中的纖維幾乎全部由角蛋白構成，這種蛋白質在頭發和指甲中也有。當在沒有氧氣的情況下加熱（這個過程稱為熱分解）的時候，角蛋白會形成中空的管狀結構，管的寬度只有百萬分之六米，同時還形成許多的微孔，這種結構很像納米管。

    研究者們在第13屆年度綠色化學和工程會議上展示了他們的研究成果，會議在馬里蘭的大學公園舉行。為了避免融化纖維以及避免丟失所想要的結構特點，他們首先在約215°C下熱處理羽毛。這就強化了羽毛的結構，而且使得隨后在400---500°C的熱處理成為可能。經過這樣的處理，這種材料的孔結構增多，從而提高了表面積以及它的儲氫容量。

    Wool博士和他的同事所制備的材料能存儲其重量為1.5%的氫氣。燃燒石油的汽車在加滿油的情況下通常能跑480公里（約合300英里）,如果使用氫氣的話，大概需要4.5公斤，根據Sen?z的估計，這就意味著需要一個284升的油箱，在里面塞滿約300公斤的碳化雞毛。為了鼓勵替代能源的創新，美國能源部制定的儲氫標準是6%,該研究還遠遠低于這個標準。但是，研究者們認為，他們可以把這種材料的孔結構進一步增多，從而提高其儲氫能力。和其它競爭對手的技術不一樣的是，他們的技術正在接近能源部的氫氣系統成本標準，即每千瓦時4美元，安裝成本低于700美元。而且，這項技術還能幫助解決另外一個環境問題：減少每年家禽業必須處理的堆積如山的雞毛。

    Vocabulary:

    High-capacity:大容量

    Plumage: 羽毛

    Tout: 標榜；吹捧

    Abundant: 豐富的

    Internal: 內部的

    Combustion: 燃燒

    Benefit: 好處

    Cling: 附著

    Nanotube: 納米管

    Fibre: 纖維

    Keratin: 角蛋白

    Protein: 蛋白質

    Pyrolysis: 熱分解

    Tubular: 管狀的

    Melt: 融化

    Deprive: 剝奪

    Desirable: 值得要的

    Innovation: 創新

    Alternative: 替代的

    Poultry: 家禽

    Dispose: 處理