Статья поступила в редакцию 13.12.10. Ред. рег. № 916
The article has entered in publishing office 13.12.10. Ed. reg. No. 916
УДК 541.67:541.142
О ВЫСОКОПЛОТНОМ «МЕГАБАРНОМ» ВОДОРОДЕ В ГРАФАНОПОДОБНЫХ ПОЛИСЛОЙНЫХ УГЛЕРОДНЫХ НАНОСТРУКТУРАХ
Ю.С. Нечаев
ФГУП «ЦНИИчермет им. И. П. Бардина», Институт металловедения и физики металлов им. Г.В. Курдюмова 105005 Москва, ул. 2-я Бауманская, д. 9/23 Тел./факс: 8 (495) 4910262; e-mail: yuri1939@inbox.ru
Заключение совета рецензентов: 23.12.10 Заключение совета экспертов: 25.12.10 Принято к публикации: 28.12.10
Рассматриваются теоретические (термодинамические) и экспериментальные основы создания намного более простого, технологичного и эффективного способа (по сравнению с известными способами мегабарного динамического и статического сжатия) получения высокоплотного «мегабарного» водорода посредством интеркаляции водорода между графе-новыми (графаноподобными) слоями в углеродных наноматериалах при технологичных температурах и давлениях. Показано, что один из рассматриваемых процессов хемосорбции водорода в графите и углеродных наноструктурах может быть связан с образованием графаноподобных (карбогидридных) областей. Посредством обработки гравиметрических, волью-мометрических и электронно-микроскопических данных определена плотность (рн = 0,7±0,2 г/см3) «мегабарного» водорода, интеркалированного в графитовые нановолокна (в количестве > 15 масс. % так называемого «обратимого» водорода). Это намного более приемлемая и эффективная технология хранения водорода (в отношении известных требований Министерства энергетики США) по сравнению с используемыми в настоящее время технологиями хранения водорода в композитных баллонах (при давлении около 80 МПа) и космическими криогенными технологиями хранения водорода «на борту автомобиля».
Ключевые слова: углеродные наноматериалы; высокоплотный («мегабарный») водород; графаноподобные области; хранение водорода
ON THE HIGH-DENSITY "MEGABAR" HYDROGEN IN GRAPHANE-LIKE CARBONACEOUS MULTILAYER NANOSTRUCTURES
Yu.S. Nechaev
Bardin Institute for Ferrous Metallurgy, Kurdjumov Institute of Metals Science and Physics 9/23 Vtoraya Baumanskaya str., Moscow, 105005, Russia Tel.: 0-007-495-4910262. E-mail: yuri1939@inbox.ru
Referred: 23.12.10 Expertise: 25.12.10 Accepted: 28.12.10
Theoretical (thermodynamic) and experimental backgrounds are considered to develop a much more simple, technological and effective method (in comparison with the known megabar compression dynamic and static ones) of producing high-density ("megabar") hydrogen carrier by means of the hydrogen intercalation in carbonaceous nanomaterials between graphene (graphane-like) layers at relevant temperatures and pressures. As is shown, one of the considered processes of chemisorption of hydrogen in graphite and carbonaceous nanostructures can be related to formation of graphane-like (carbohydride-like) regions. By using the gravimetric, volumetric and electron microscopy data, the density value (pH = 0.7±0.2 g/cM3) of the intercalated "megabar" hydrogen in graphite nanofibers (> 15 mass% of the "reversible" hydrogen) has been defined. It is much more acceptable and efficient technology (with respect to the known U.S. DOE requirements), in comparison with the composite vessels with high hydrogen pressure (about 80 MPa) and the space cryogenic technologies of the hydrogen on-board storage used nowadays.
Keywords: carbon-based nanomaterials; high-density ("megabar") hydrogen carrier; graphane-like regions; hydrogen storage.
1. Introduction
As is noted in [1], the main problem related to the use of hydrogen as a fuel is the gaseous nature of hydrogen molecules, giving the necessity of compression it in heavy and somewhat unsafe pressurized gas
cylinders to enable transport and storage of the fuel. As is also noted in [1], the possible method of hydrogen storage using absorption in metal hydrides may be useful in some cases, but it is not a general solution to the problem of storage and distribution of hydrogen because of the large mass of the absorbent. If hydrogen should
become the fuel or energy carrier of the future, these problems must be solved. The energy content of hydrogen is the highest of any fuel yet exists (except nuclear fuel) for vehicles, rockets and other applications relative to the mass of hydrogen alone [1, 2]. Thus, as is emphasized in [1], it appears the necessity to find forms of hydrogen that can be stored and transported without much overhead, while still retaining the high energy content of hydrogen gas. In a series of articles ([1, 3, 4] and some others), a novel method (based on theoretical predictions [5]) is described for producing an atomic hydrogen material of high density of 0.5-0.7 g/cm3 at low pressure, but only in microscopic amounts. In this method, hydrogen gas is absorbed in a K-promoted iron oxide catalyst (a hydrogen-abstract catalyst) and desorbs as clustors containing H atoms at low pressure and at a temperature of < 900 K. The clustors are of the so-called Rydberg matter type, with a final interatomic distance of 150 pm, which is found by Coulomb explosion measurements. This bond distance corresponds to the material density of 0.5-0.7 g/cm3, depending upon the exact structure. The atomic hydrogen material thus formed is concluded to be a metallic quantum liquid, mainly, by comparison with the shock-wave (dynamic) or static (in the diamond-anvil cell) "megabar" compression experiments ([6-9] and others). As is noted in [1], the stability against transformation of this material to hydrogen gas is not known, but the atomic condensed hydrogen may become an important future energy carrier.
In the following sections, the theoretical (thermodynamic) and experimental backgrounds are described (in the light of results [10-12]) to develop a much more simple, technological and effective (in comparison with [1-9]) method of producing high-density (~ 0.7 g/cm3) "megabar" hydrogen carrier intercalated at relevant temperatures and pressures in carbonaceous nanomaterials between graphane-like [13, 14] regions. It may be a real rout for a general solution to the problem of storage and distribution of hydrogen. And these results on the "megabar" hydrogen carrier and graphane-like [13, 14] regions in carbon-based nanostructures are seems rather new compared to previous studies.
2. The intercalation of high-density hydrogen carrier into near-surface graphane-like layers
A real possibility of hydrogen intercalation into (between) near-surface graphene layers of highly oriented pyrolytical graphite (HOPG) has been shown in some experimental studies [15-20]. It has been analyzed in [10-12] with taking into account also results [21-40]. In the present study, some new very important aspects of this problem are considered. Hence, some self-plagiarism of [10-12] in the present article is necessary.
Study [15] of atomic hydrogen accumulation in HOPG samples and etching their surface on hydrogen thermal desorption (TD) have been performed using scanning tunneling microscope (STM) and atomic force
microscope (AFM). STM investigations revealed that the surface morphology of untreated reference HOPG samples was found atomically flat, with a typical periodic structure of graphite. Exposure (treatment) of the reference HOPG samples (30-125 min at the external atomic hydrogen pressure PH ~ 1 Pa and near-room temperature) to different atomic hydrogen doses has drastically changed the initially flat HOPG surface into a rough surface, covered with bumps-blisters, with the average height of about 4 nm [15]. According to [15], bumps found on the HOPG surface after atomic hydrogen exposure is simply monolayer graphite (graphene) blisters, containing inside hydrogen gas in molecular form. As is supposed in [15], atomic hydrogen intercalates between layers in the graphite net through holes in graphene hexagons (due to a small diameter of atomic hydrogen in comparison with the hole size) and later on being converted to H2 gas form which makes it captured inside the graphene blisters (due to a relatively large kinetic diameter of hydrogen molecules).
In [15], it was found that an average blister radius of 25 nm and a height of 4 nm. Then considering the blister as a semi-ellipse, the blister area (Sb ~ 2.0-10-11 cm2) and its volume (V ~ 8.4-10-19 cm3) were found [15]. The amount of retained hydrogen in this sample was Q ~ ~ 2.8-1014 H2/cm2 and the number of hydrogen molecules captured inside the blister turned out as (Q Sb) ~ 5.5-103 [15]. Thus (within the ideal gas approximation [21]) the internal pressure of the molecular hydrogen into the single bluster at room temperature (T) is of PH2 ~ k(QSb)T/Vb ~ ~ 2.5-107 Pa, k - being the Boltsman constant; the estimated accuracy is not higher than the order of magnitude. During TD heating, for instance, at 1,000 K the pressure can reach a value of PH2 ~ 8.5-107 Pa (within the ideal gas approximation [21]), which can be enough for some blisters to get punctured (or tensile ruptured), may be due to some defects in their roofs (walls).
In [15], the pressure values are compared with known experimental values of tensile and compressive strengths for graphite - 107 Pa and 3-107 Pa, respectively. But it seems more reasonable to take into account the recent data on elastisity, strength and toughness of carbon nanorods and nanotubes, for instance, [22, 23], data [24] on stress-strain state of multiwall carbon nanotube under internal pressure, data [25] on carbon onions as nanoscopic pressure cells for diamond formation, and data [26] on the elastic properties and intrinsic strength of monolayer graphene. As is noted in [26], their experiments establish graphene (a defect-free monolayer sheet) as the strongest material ever measured. In studies [22-26] much higher values (by several orders of magnitude, in comparison with graphite) of modulus of elastisity, modulus of elongation and tensile strength are declared. Hence, it follows: (
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