圖1. 用於儲存潔淨能源之高度多孔、可程式控制的人造海綿。 (圖援用自原文)
A new metal-organic framework material based on aluminium can store large amounts of hydrogen and methane at relatively low pressures. The material might be used to carry clean energy in fuel cell-powered vehicles according to the researchers at Northwestern University in the US who developed it.
一種以鋁為基礎的金屬-有機結構物材料，在相對低的壓力下，能儲存大量氫氣及甲烷。根據美國西北大學，研發此種材料之研究人員們的說法，這種材料可能被用來攜帶，於燃料電池驅動之車輛中的潔淨能源。
In 2017, transport vehicles (including cars, trucks, planes, trains and boats) overtook power plants as the largest source of greenhouse gas emissions in the US. The share of transport-based emissions increased still further in 2018, and the trend is expected to continue – making the search for transport-friendly alternative energy sources ever more important.
在2017年，運輸車輛(包括汽車、卡車、飛機、火車及船舶)超越了發電廠，成為美國最大溫室氣體排放來源。在2018年，以運輸為基礎的排放部分仍然進一步增加，且預期趨勢持續下去。因此，使得找尋運輸友善的替代能源更為重要。
Methane and hydrogen are often touted as potential replacements for diesel and petrol fuels in vehicles. While methane is considered a “transition” fuel, since its combustion still emits carbon dioxide (albeit less than that of petrol), hydrogen has been hailed as “the fuel of the future” since burning it produces neither carbon dioxide nor particulate-based pollution.
甲烷及氫氣經常被吹捧為交通工具中，柴油及汽油燃料的潛在替代物。不過，甲烷被認為是“過渡”燃料。因為，甲烷燃燒時，仍然排放二氧化碳(儘管比汽油少)。氫氣一直被譽為“未來燃料”，因為其燃燒既不會產生二氧化碳，也不會產生基於微粒的污染。
The problem is that because both hydrogen and methane are gases at ambient temperatures, they need to be compressed and kept at high pressures (of 700 bar and 250 bar respectively) whenever they are transported and stored. Since such high pressures can create hazards for drivers and others involved in handling stored fuel, the maximum storage pressure limit for real-world vehicles has been set to 100 bar – significantly reducing the amount of gas that can be stored in a given space.
問題是，因為在環境溫度下，氫氣及甲烷兩者皆是氣體。無論何時，運輸及存儲它們皆必需經壓縮，並保持於分別是700及250巴(壓力單位)的高壓下。由於如此高的壓力，會對駕駛及其他涉及處理儲存燃料的人員造成危險，實際車輛的最大存儲壓力限制，已經被設定為100巴。這大大減少了，能被儲存於一定空間的氣體數量。
In recent years, researchers have investigated high-surface-area porous adsorbent materials as a means of increasing the amounts of gas that can be stored in a given volume without increasing the pressure.
在最近幾年，研究人員們已經調查研究了，大表面積的多孔吸附材料作為一種，在不增加壓力情況下，增加能被儲存於一定體積中之氣體數量的方法。
Metal-organic frameworks (MOFs), which have surface areas of 2000m2/g or more, are considered promising candidates. These highly crystalline materials are made up of organic molecules and metal ions or clusters that self-assemble into multidimensional structures, and are easy to design thanks to their tailorable pore chemistry and shape.
具有2000m2/g或更多表面積的金屬-有機結構物(MOFs)，被認為是有指望的候選物。此些高度結晶的材料是由，有機分子及自行組裝成多維結構的金屬離子或簇所組成。由於其可定製的多孔化學物質及形狀，因此是易於設計的。
The Northwestern team, led by Omar Farha, used molecular simulations to inform the design of ultraporous MOFs based on trinuclear clusters, called NU-1501-M (where M is Al or Fe).
該由美國西北大學Omar Farha領導的團隊，使用了諸多分子模擬，來引導以三核團簇為基礎，被稱為NU-1501-M(在此M是鋁或鐵)之超多孔MOFs的設計。
圖2. 以氧為中心的三核簇結構 (圖援用自：https://www.researchgate.net/figure/Schematic-representation-of-the-oxo-centered-trinuclear-clusters-structure-M-3-ORCOO-6_fig1_329224139)
The researchers found that NU-1501-Al boasted high gravimetric (mass) and volumetric (size) storage performances for hydrogen and methane. Indeed, the material proved capable of storing 0.66 g of methane per gram of material at 100 bar and 270K – a value that exceeds the 0.5 g/g target set by the US Department of Energy (DOE) for developing the next generation of clean-energy automobiles.
此些研究人員發現，就氫氣及甲烷而言，NU-1501-Al具有大重量(質量)及體積(尺寸)的儲存性能。實際上，該種材料經證實，能夠在壓力100巴及絕對溫度270度下，每克能儲存0.66 g甲烷。這是一個超出，由美國能源部，就開發下一代潔淨能源汽車，所設定0.5 g/g的目標值。
The material also has a high deliverable storage capacity of 14% by weight for hydrogen, which means it can store 14% of its own mass of hydrogen. While this figure seems low compared to its ability to store methane (66% by weight), it again surpasses the DOE target for 2020 of 4.5% by weight.
根據氫氣重量，這種材料也具有14%之高可運載的儲存容量。這意味著，此種材料能儲存14%氫氣本身的質量。雖然相較於儲存甲烷的能力(根據重量66％)，此數字似乎很低。不過，它再次超越了美國能源部，2020年根據重量的4.5％目標。
These high values are possible thanks to the material’s tiny pores, which measure less than 2.5nm across and thus offer a very high surface area for gas adsorption. As Farha notes, a one-gram sample of the material, with a volume equivalent to six M&M candies, has enough surface area to cover 1.3 American football fields.
由於這種材料直徑小於2.5奈米(nm)，因此為氣體吸附提供了非常大之表面積的細小孔洞，上述高的數值是可能的。如同Farha特別提及的，該種材料具有體積等同於，六顆M&M巧克力糖的一克重樣本，具有足以覆蓋1.3個美式足球場的表面積。
This extensive surface area means that the team’s MOFs can store “tremendous” amounts of hydrogen and methane within their pores, Farha says, adding that the materials could deliver either gas to a car engine at lower pressures than are needed for current fuel-cell vehicles.
Farha表示，這種廣大的表面積意味著，該團隊的MOFs能在其孔洞中，儲存“大量”的氫氣及甲烷。附言，這種材料可能以比目前燃料電池汽車所需更低的壓力，將任一種上述氣體輸送到汽車引擎中。
原文網址：https://physicsworld.com/a/ultraporous-metal-organic-frameworks-could-make-clean-energy-carriers/
翻譯：許東榮
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