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5Статья 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: (i) that the blister formation in HOPG [15] at room temperatures can occur within the elastic deformation conditions, and (ii) that a counteraction blister pressure (of the fugacity order) should be taken into account (within both the mechanical and thermodynamic considerations).
International Scientific Journal for Alternative Energy and Ecology № 10 (90) 2010
© Scientific Technical Centre «TATA», 2010
By using the above obtained stress (pressure in a blister) value as ~108 Pa, and finding from data [15] the local elastic deformation degree of a blister roof as ~ 0.1, one can estimate (within Hooke's law) modulus of elastisity of the defect surface graphene layer in HOPG as ~109 Pa. This value is about two orders higher than the known quantity for graphite, but about two orders lower than the quantity for more perfect graphene in carbon nanotubes, nanorods and monolayer graphene (a defect-free sheet) [22-26].
It is also consistent with the thermodynamic estimation of the equilibrium hydrogen fugacity (fH2) in the blisters in [15] which can be performed by using the "acting masses law" for the "reaction" of 2H(gas)^H2(gas_m_blisters), as follows (Eq. (1)): (fe/P0) ~ « (PH/P0)2exp{[AHdis-TASdis-fm(AV/n)]/RT}, where PH ~ 1 Pa is the atomic hydrogen pressure in the atomizer in [15], P0 = 1 Pa is the standard pressure, AHdis = 448 kJ/mol(H2) is the known experimental value of the dissociation energy (enthalpy) of one mole of gaseous hydrogen (at room temperatures), ASdis ~ 0 (in the used approximation) is the dissociation entropy, AV ~ Vb, n ~ (Q Sb)/NA, Na being the Avogadro number, R is the gas constant, T~ 300 K in [15]; hence, fH2 ~ 109 Pa. Such a value can be related to the saturation stage in [15] for Q > 4-1014 H2/cm2.
It is necessary to emphasize that the adsorbed hydrogen amount Q (localized, mainly, in the blisters in the graphite monolayer [15]) corresponds to a relatively low average hydrogen/carbon atomic ratio in the monolayer (i.e., between the two graphene layers), namely: (H/C) ~ (2Q/ Nc) ~ 0.1, where Nc being the number of carbon atoms per 1 cm2 of two graphene layers.
On the other hand, the hydrogen volumetric (mass) density in the blisters in [15] can be estimated as p ~ (Q MH2 Sb)/Vb ~ 0.045 g/cm3, where MH2 being the hydrogen molecule mass; this is close (within the errors) to the liquid hydrogen mass density (0.071 g/cm3 at 21.2 K and 0.1 MPa).
By using the experimental and estimates' results considered above, one can conclude on some three-dimensional clustering of hydrogen molecules in the graphite monolayer (between the two graphene layers), that is on formation and growth of liquid-like three-dimensional nanoclustors under the conditions given in [15].
In this connection, it is relevant to point, for instance, to results [27] of molecular dynamics simulations of liquefaction of hydrogen molecules upon exterior (deformed) surfaces of single-walled carbon nanotube (SWNT) bundles (at 80 K and 10 MPa). Those studies were carried out in relevance to the experimental finding on some hydrogen phase transitions in SWNT bundles [28].
It is also relevant to compare data [15] with related results [20] of studying the graphite surface modifications induced by interaction of hydrogen atoms (respectively deuterium) with perfectly crystalline HOPG surfaces. The surface properties were probed [20] with high-resolution electron-energy-loss spectroscopy
(HREELS) revealing the formation of C-H units with different vibrational energies. Comparison (in [20]) the results with the density functional theory (DFT) calculations [29] led to establish the models of the hydrogen adsorption processes at the graphite surface. It was shown in [20] that the vibration at 295 meV was due to a single H atom bonding to graphite (C-H), while vibrations at 331 meV and 345 meV (and higher energy losses) were respectively related to formation of dimmer and quartet, or more generally, a higher number of clustering atoms (i.e., hydrogen clustors formation). Subsequently, studies were performed in [20] by using scanning tunneling microscopy (STM). On the electronic point of view in [20], as hydrogen locally disturbed the electronic density near the Fermi level, charge density confinement was observed between the hydrogen clustors, particularly when using low tip-sample bias voltage.
It is also necessary to take into account the experimental data [16-19] on hydrogen thermo-desorption (TD) from highly oriented pyrolytic graphite (HOPG) exposed to atomic hydrogen (as in [15]) at near-room temperatures, and their constructive (thermo-dynamic) analysis [10-12]. As was shown in [16-19], such a treatment resulted in emergence of surface nano-hillocks (nano-blisters) with heights of 3-5 nm and diameters of 40-75 nm, the most of which disappeared after hydrogen thermo-desorption that proceeded as a first-order reaction [17, 18]. The TD measurements in [16-19] (at a heating rate of u = 25 K/s) revealed two TD peaks (processes): a peak a centered at Ta ~ 1123 K (ATa ~ 180 K - its width at half of its height, SJS^ ~ 0.45
- its fraction of the total spectrum area, Qa ~ 230 kJ/mol
- the activation energy of the process) and a TD peak p centered at Tp ~ 1523 K (ATp ~ 250 K, Sp/SE ~ 0.55, Qp ~ 385 kJ/mol).
Using analyses results [10-12] and results of the above consideration, one can attribute the TD process (peak) a with process III of dissociative chemisorption of hydrogen between graphene layers (Table 1 in [12], model F* in Fig. 1). The rate-controlling stage of the process (peak) a can be attributed to diffusion of hydrogen atoms (between the two surface graphene layers from the nearest graphene blisters to a "punctured" one (accompanying with the diffusant reversible trapping, i.e., C-H bonding at chemisorption centers in the graphene layers (model F* in Fig. 1). The diffusion characteristics of the processes a and III are as follows: the diffusion activation energy Qa ~ QIII « 250 kJ/mol (H), the pre-exponential factor of the diffusion coefficient D0a « D0III = 3-10-3 cm2/s (Table 1 in [12]), and the diffusion coefficient Da ~ DIII « 7-10-15 cm2/s. Hence, the diffusion length (Da ATa / u)12 is of order of 1-10 nm, i.e., as the separation between the walls of the neighboring blisters (in [15]). It can be related to study results given in [20] (considered above) on the vibration contribution at 295 meV due to a single H atom bonding to graphite (C-H).
Рис. 1. (Модифицированная версия из обзора [12]; оригинал
представлен в [34].) Теоретические модели хемосорбции водорода на графите (в графеновой плоскости). Модель F*, характеризуемая энергией связи атома водорода с графитом около -2.5 эВ/атом, может соответствовать графаноподобной (карбогидридной) ситуации; комплекс (C-H, C-H, C-H*) в моделях F(F*) может отвечать критическому одномерному зародышу графана [10, 41, 42] Fig. 1. (The modified figure from review [12]; the origin one is
shown in [34].) Theoretical models of hydrogen atom chemisorption on graphite (in the graphene plane). Model F* (characterized by the binding energy of hydrogen atom with graphite of about -2.5 eV/atom) can correspond to graphane-like (carbohydride-like) situation; complex (C-H, C-H, C-H*) in models F(F*) can correspond to a critical 1D-nucleus of graphane [10, 41, 42]
In the same way, one can attribute the TD process (peak) p with process IV of dissociative chemisorption of hydrogen between graphene layers with some defects, for instance, as dislocation loops (Table 1 in [12], models C and/or D in Fig. 1). The rate-controlling stage of the process (peak) p can be attributed to diffusion of hydrogen atoms (between the two surface graphene layers) from the available graphene blisters to a "punctured" one (accompanying with the diffusant reversible trapping, i.e., C-H bonding at chemisorption centers in the defects' regions of the graphene layers (models C and/or D in Fig. 1). The diffusion characteristics are as Qp « QIV « 365 kJ/mol (H), D0p ~ D0IV = 6-102 cm2/s (Table 1 in [12]), and Dp = Av = 2-1010 cm2/s; the diffusion length (Dp ATp / v)m is of order of 102 nm, i.e., as the separation between the neighboring etch-pits (in [15]). The defects of the dislocation loop type can be formed (created) during (and/or due to) "shrinking" and/or disappearing a number of the blisters.
The in [15] considered mechanism of etching (i.e., creating etch-pits on the surface due to departure together with some (a small amount) hydrogen accompanying carbon atoms (obviously, as hydrocarbons complexes) from the holes edges (in punctured graphene blisters), resulting in bigger sizes of the holes, can be attributed to process II of dissociative-
associative chemisorption of hydrogen molecules (Table 1 in [12], model H in Fig. 1). As has been shown in [12], only process II (from the considered ones I-IV in Table 1 in [12]) is characterized by an accompanying (initiated by the process) occurrence of a fairly small amount of hydrocarbons (CH4 and others) in the thermal desorption (TD) spectra. The explanation of this phenomenon is that the energy (-AH(12)II = 560 kJ/mol (H2), Table 1 in [12]) of desorption (detachment) of two hydrogen atoms from the carbon atom of the sorption center (model H in Fig. 1) is much higher than the energy (-AHC-C « 485 kJ/mol) of detachment of this carbon atom from its two nearest carbon neighbors. It can be related to results in [20] on the vibration contribution at 331 meV due to a dimmer of hydrogen atoms bonding to graphite.
As has been noted in [15], the results lead to the assumption that atomic hydrogen could be accumulated in closed graphite nanotubes, through graphene sheet walls of the nanotubes, in H2 gas form and this storage would be stable.
In this connection it is relevant to note results [30-33] on interactions of low-kinetic-energy hydrogen atoms (0.5-30 eV) with single-walled carbon nanotubes (SWNT) based on the molecular dynamics and, particularly, ab initio calculations. According to finding [30], hydrogen atoms with energy of 16-25 eV are characterized by a high probability of penetrating through the side faces of the closed SWNTs and accumulating inside these capsules in the form of hydrogen molecules. Owing to the high mechanical strength of nanotubes, hydrogen molecules can be concentrated therein up to volume densities exceeding by far that observed upon capillary condensation [30].
According to estimates in [31], the pressure of molecular hydrogen embedded into SWNT can reach a value as high as 60 GPa. This hydrogen condensate inside individual SWNT, depending on hydrogen volume density and pressure, can undergo several phase transitions giving rise to different crystal lattices built up of hydrogen molecules [32]. If the pressure of hydrogen inside a SWNT is high enough [32], the bulk sorption capacity can reach the value of 0.063 g/cm3, which meets the requirements [37]. In [33, 38] potential ways for a practical achieving of such high hydrogen sorption levels by bundles of closed SWNTs have been suggested as well as methods for hydrogen withdrawal through the walls of the nanotubes. As a matter of fact, recently, it was experimentally proven [39, 40] that 5.1 wt% of hydrogen storage was obtained by hydrogenation of single-wall carbon nanotubes with atomic hydrogen using core-level photo electron spectroscopy and X-ray absorption spectroscopy.
Thus, there are reasons to believe that the above thermodynamic theory (description) of results [15-20] is consistent with results [30-40].
And as it can be shown by comparing results [10-12] and [13, 14], there are serious reasons to suppose that the TD peak a in [17, 18], corresponding to process III of dissociative chemisorption of hydrogen between
International Scientific Journal for Alternative Energy and Ecology № 10 (90) 2010
© Scientific Technical Centre «TATA», 2010
graphene layers (Table 1 in [12], model F* in Fig. 1) can be related to the formation of graphane-like [13, 14] regions in the material. As is estimated in [13], the standard energy (enthalpy) of formation of graphane from graphite and molecular gaseous hydrogen is about -0.1 eV/atom. Hence, by using the known value of the dissociation energy of molecular gaseous hydrogen, one can define the binding energy of atomic hydrogen with graphane as about -2.5 eV/atom. This quantity is coincided with the related characteristics of process III (Table 1 in [12]) considered above. Therefore, from the thermodynamic point of view, process III is related to hydrogen thermodesorption from the graphane-like [13, 14] near-surface layers (in the blisters in [15-20]) in the graphite material. The further consideration of this open question is continued in the next sections.
3. The necessary conditions for forming the high-density (~0.7 g/cm3) "megabar" hydrogen carrier intercalated between graphane-like regions in carbonaceous nanomaterials
By using Figs. 2 and 3, one can estimate the external pressure of the molecular gaseous hydrogen of PH2 «
= 1108 Pa (at room temperature) corresponding to the hydrogen density of p~ 0.045 g/cm3. It is the density of hydrogen intercalated in near-surface blisters in HOPG at the pressure of the atomic gaseous hydrogen of PH « 1 Pa and room temperature (as in [15]). The value of PH2 «
« 1108 Pa is in accordance with the above estimated value of the internal pressure of the molecular gaseous hydrogen of PH2 « 3-107 Pa in the blisters in [15]. And
this accordance points to the quantitative character of the thermodynamic estimations (the order of magnitude accuracy) with using Eq. (1).
P, MPa
¡O2
P, GPa
2.5
p, g/cm3
Рис. 2. (Из обзора [10]; оригинал представлен в [6].) Зависимость давления от плотности протия вдоль линий изотерм и линии равновесия «жидкость - пар». Температуры изотерм (в K) показаны цифрами Fig. 2. (From review [10]; the origin one is shown in [6].) Dependence of the pressure from the density along isotherms and the curve of the liquid - vapor equilibrium for protium. The temperatures (in K) are pointed at the isotherms
0,05
0J0
0.15
0,2G
0.25
0,30
0.35
0,40 p, g/cm3
Рис. 3. (Из обзора [10]; оригинал представлен в [6].) Изотермическое (при T = 300 K) сжатие протия и дейтерия
Fig. 3. (From review [10]; the origin one is shown in [6].) Isothermal (T = 300 K) compression of protium and deuterium
From Figs. 4 and 5, one can estimate the external pressure value of P^ ~ 3-1011 Pa (several Mbar) corresponding to the high desired density of p « 0.7 g/cm3 of the intercalated hydrogen at room temperatures. Hence, by using Eq. (1), one can estimate the necessary value of Ph = 50 Pa.
P, GPa
200
0
Os» 3
,0
1.5
p, g/cm3
Рис. 4. (Модифицированная версия из обзора [10]; оригинал
представлен в [6].) Кривые квазиизоэнтропного и изотермического (утолщенная линия, 300 K) сжатия жидкого дейтерия и протия. Плотность протия увеличена вдвое Fig. 4. (The modified figure from review [10]; the origin one is shown in [6].) Curves of quasi-isentropic and isothermal (the thickened one, 300 K) compression of liquid deuterium and protium. The protium density is increased by two times
Рис. 5. (Модифицированная версия из обзора [10]; оригинал представлен в [6].) Фазовая диаграмма дейтерия;
3 - линия плавления Fig. 5. (The modified figure from review [10]; the origin one is shown in [6].) The phase diagram of deuterium; 3 - the melting curve
Furthermore, it should be also emphasized that the carbon nanotubes and nanofibers can stand (keep) such megabar pressure of the hydrogen intercalated between
closed graphene (or graphane-like) layers, due to their very high values of the elastisity modulus, modulus of elongation and tensile strength noted in [22-26].
Such conditions for forming high-density ("megabar") hydrogen carrier intercalated between graphane-like closed regions in graphite nanofibers (GNFs) can be created (in the light of results [10-14, 35, 36]), by using the catalyst method [41] of the hydrogen atomizing and closing of the edge regions of graphene (and/or graphane) layers in the material. The main results are shown in Figs. 6 and 7.
ïj;
473 673 873 1073 Т. К
Рис. 6. (Модифицированная версия из обзора [12]; оригинал представлен в [36].) Термодесорбция хемосорбированного водорода из графитных нановолокон (ГНВ). Термодесорбционный пик y (с энергией активации десорбции около 2,5 эВ/атом) может отвечать уходу водорода из полислойных графаноподобных (карбогидридных) областей в ГНВ [10, 41, 42] Fig. 6. (The modified figure from review [12]; the origin one is shown in [36].) Thermo-desorption of chemisorbed hydrogen from graphite nanofibers (GNFs). Peak y (with the desorption energy of about 2.5 eV/atom) can correspond to hydrogen release from multilayer graphane-like (carbohydride-like) regions in GNFs [10, 41, 42]
Рис. 7. (Из обзора [12]; оригинал представлен в [35].) Микрофотографии обезводороженных графитных нановолокон (ГНВ). Стрелками показаны некоторые из щелевидных нанопор, образующихся в ГНВ (очевидно, в полислойных графаноподобных (карбогидридных) областях) после ухода из них интеркалированного высокоплотного («мегабарного») водорода Fig. 7. (From review [12]; the origin one is shown in [35].) Micrographs of dehydrogenated graphite nanofibers (GNFs). The arrows indicate some of the slit-like nanopores formed in GNFs (obviously, in multilayer graphane-like (carbohydride-like) regions) after release from them the intercalated high-density ("megabar") hydrogen carrier [10, 41, 42]
The density of the intercalated hydrogen in the material (Fig. 7) can be estimated by using the relevant ordinary expressions as Eq. (2): Ph = Pc[(1-fH fmH ]/[(1-fmH) fvH], where pC is the known GNF density (pC = 2.3 g/cm3), f,H = [vH /(vH+vC)], vH and vC are the volumes of the "hydrogen" and carbon parts of GNFs, respectively, fvH is the volume fraction of the slit-like nanopores formed in GNFs (fvH = 0,4±0,1, from Fig. 7), fmH = = [mH /(mH+mC)], mH and mC are the masses of the hydrogen part and the carbon part of GNFs, respectively, fmH is the mass fraction of the intercalated hydrogen in the slit-like nanopores in GNFs in Fig. 7 (fmH = 0.17±0.01 is the experimental value from [35]).
Hence, one can estimate the density value pH =
0.7±0.2 g/cM3, which in the case of the overall compression of the material (hydrogen) corresponds to megabar external pressures (Figs. 4, 5).
In our case, the external pressure can be, for instance, PH2 = 10 MPa (100 bar); but it should provide PH = 50 Pa at the material surface (the catalyst method [41]), the temperature being about 300 K, and desorption time about 10 min [35, 36, 10-12, 41].
It gives a real possibility of quenching the high-density ("megabar") hydrogen carrier and graphane-like regions in carbon-based nanomaterials for a sufficient long time, relevance for its detailed (stationary) studies and technological applications.
Hence, it also follows that in our case the local internal megabar pressures in the slit-like nanopores in graphane-like regions in GNFs (Fig. 7) have had place. It is confirmed by the fact of the observed definite swelling of the slit-like nanopores in their middle parts (Fig. 7),
1.e. the lentil-like form of the nanopores in GNFs.
Evaluations of the necessary local stresses for such
swelling of the nanopores, by using data [22-26] on the very high values of the elastisity modulus, modulus of elongation and tensile strength of the material (defect-free graphene), can give megabar quantities. Such high local stresses (internal pressures) are formed at the expense of the energy of the internal reactions (Eq. (1)). And this also points to the quantitative character of the thermodynamic estimations with using Eq. (1).
4. Discussion on the graphene-graphane problem
In [13], it has been predicted the stability of an extended two-dimensional hydrocarbon on the basis of first-principles total-energy calculations. This compound that has been called in [13] as graphane is (according to [13]) a fully saturated hydrocarbon derived from a single graphene sheet with formula CH. And as is also noted in [13]: "The graphane bonds are fully saturated and there is no opportunity for hydrogen bonding between the graphane sheets. The week van der Waals attraction contributes negligibly..."
But from results [13], it does not follow that multilayer graphane-like regions (with the week van der Waals attraction between layers) can derive from multilayer
International Scientific Journal for Alternative Energy and Ecology № 10 (90) 2010
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graphene sheets (with the week attraction between them), for instance, in graphite nanofibers (Fig. 7).
As is shown in Section 2 (with using data [13]), the binding energy of atomic hydrogen with graphane is AH ^ -2.5 eV/atom. But this quantity practically coincides with the binding energy of atomic hydrogen with graphite (model F* of the hydrogen chemisorption in Fig. 1; process III in Table 1 in [12]). Hence, the complex (C-H, C-H, C-H*) in models F(F*) in Fig. 1 can correspond to a critical one-dimensional nucleus of graphane in graphite [41]. Peak y in Fig. 6 (with the desorption energy of about 2.5 eV/atom) can correspond to hydrogen release from multilayer graphane-like (carbohydride-like) regions in graphite nanofibers (GNFs) [41].
As is noted in [14]: "Although graphite is known as one of the most chemically inert materials, we have found that graphene, a single atomic plane of graphite, can react with atomic hydrogen, which transforms this highly conductive semimetal (single-layer graphene) into an insulator (graphane)."
But it's not taken into account the hydrogen chemisorption by graphite and related carbon nanostructures (Figs. 1, 6; Table 1 in [12]). Obviously, graphite and related carbon nanostructures are chemically active (nearly, as graphene is), relevance to hydrogen. Hence, multilayer graphane-like (carbohydride-like) material can derive from multilayer graphene sheets, for instance, in graphite nanofibers (Fig. 7).
In [14], the transformation of graphane into graphene has been obtained by annealing at T = 723 K in Ar atmosphere for t = 24 hours. Such a sluggish kinetics has not been interpreted and/or quantitatively described in [14].
But one can do it by using the known kinetic equation as Eq. 3: 1/t = v exp [AH / RT], with a typical frequency factor of v ~ 3-1012 c-1 and the quantity of AH = -240 kJ/mol (H).
In [14], the same hydrogenation procedures (2 hours exposure in cold hydrogen plasma) both for single-layer graphene samples and bilayer ones (on a substrate) have been applied. Bilayer samples exhibited significantly lower affinity to hydrogen, as compared to single-layer graphene samples. As has been supposed in [14], this observation is in agreement with the theory that hydrogen cannot be adsorbed on one side of a flat graphene, and with their conclusions that hydrogen adsorption for graphene on a substrate is facilitated by ripples. And as has been also supposed in [14], higher rigidity of bilayers suppresses their rippling, thus reducing the probability of hydrogen adsorption.
But these results [14] can be related to hydrogen absorption between the two graphene layers, which can be rate-controlling by diffusion there of hydrogen atoms with characteristics of process III (D0III and QIII; Table 1 in [12]) considered in Section 2 for the graphene (graphane) blisters in HOPG in [15]. Hence, the efficient hydrogenation regimes (conditions) for the bilayer samples in [14] can be found.
As it's concluded in [15]: "The successive hydrogen sorption-desorption cycles of HOPG lead to gradual erosion of upper graphite layers, making the material unfavorable for effective accumulation of hydrogen." But, as it's also noted in [15], this conclusion does not relate to carbon nanotubes. And obviously, it does not relate to multilayer graphene (or graphane-like) regions in graphite nanostructures (Fig 7).
5. Summary
Within this research, the following results have been obtained.
Firstly, the thermodynamic theory (description) of the process of high-density hydrogen intercalation into (between) near-surface graphene layers of highly oriented pyrolytical graphite has been developed. It includes the kinetic (diffusion) aspects, with taking into account several processes of hydrogen chemisorption, and particularly, a possibility of formation of the graphane-like (carbohydride-like) layers.
Secondly, the thermodynamic theory (description) of the process of high-density ("megabar") hydrogen intercalation into (between) closed graphene (graphane-like) layers of novel carbonaceous nanomaterials (graphite nanofibers and others) has been also developed. The value of atomic hydrogen external pressure (PH « 50 Pa), which is necessary for producing hydrogen of high density (p « 0.7 g/cm3) intercalated (at room temperatures) in carbonaceous nanostructures, has been estimated. As is shown, it corresponds to formation of a megabar local internal pressure of molecular hydrogen between closed graphene (graphane-like) layers at the expense of energy of the local association of hydrogen atoms. It confirms by the fact of local swelling of the material.
Thirdly, there are serious reasons to suppose that one of the considered processes of dissociative chemisorption of hydrogen between graphene layers in graphite and carbonaceous nanostructures can be related to formation of graphane-like (carbohydride-like) regions. It demands further studies.
Fourthly, the necessary conditions of formation of high-density ("megabar") hydrogen carrier intercalated between graphene (graphane-like) layers in carbonaceous nanomaterials have been defined.
Fifthly, 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 "reversible" hydrogen) has been defined. As is shown, in this case the external pressure of molecular hydrogen gas can be as Ph2 = 10 MPa (100 bar), providing Ph = 50 Pa at the material surface (the catalyst method [41, 42]), the temperature being about 300 K, and desorption time about 10 min.
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. This could be also beneficial for other industries which are working on the hydrogen generation from water splitting, by using either nuclear or tidal energy.
The physics, chemistry and technology aspects of the high-density ("megabar") hydrogen intercalation between graphene (graphane-like) layers in carbonaceous nanostructures should be, certainly, much more developed (in the light of the present results). Therefore, it seems useful to broaden and deepen the discussion in scientific publications of this actual open question, and/or the international cooperation. This is obviously the most efficient and economical way (route) of solution of the bottle-neck problem of the hydrogen storage in vehicles, rockets and others.
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