ON THE EXPERIMENTAL AND THEORETICAL BASIS OF DEVELOPING A SUPER HYDROGEN
CARBONACEOUS ADSORBENT FOR FUEL-CELL-POWERED VEHICLES
i i
Yu. S. Nechaev, O. K. Alexeeva1, G. A. Filippov, A. L. Gusev2 2
c
a u
I. P. Bardin Central Research Institute of Ferrous Metallurgy |
2-nd Baumanskaja Str., 9/23, Moscow, 105005, RUSSIA J
1 Institute of Hydrogen Energetics and Plasma Technologies, £
Russian Research Center "Kurchatov Institute"
Kurchatova pl., 1, Moscow, 123182, RUSSIA £
a
2 Russian Federal Nuclear Center — All-Russian Research Institute of Experimental Physics e
37 Mira pr., Sarov, Nizhniy Novgorod Region, 607188, RUSSIA
Results of experimental and theoretical investigations into hydrogen sorption by various carbon nanostructures, including fullerenes, single-walled and multi-walled nanotubes, nanofibers and nanographite-based composites are surveyed. Results of a thermodynamic analysis of the most significant experimental data are presented. The emphasis is placed on the studies reporting the extremum sorption parameters. The thermodynamic and kinetic (diffusion) parameters and equations describing the sorption processes are refined. The prospects of the applications of novel carbon nanomaterials for hydrogen storage in automotive industry are discussed.
Introduction
Hydrogen storage is a key enabling technology for the advancement of hydrogen and fuel cell power technologies in transportation, stationary and portable applications. The use of hydrogen will diversify energy sources, and reduce pollution and greenhouse gas emissions. The most available technologies for the on-board hydrogen storage are compared in Table 1, [1], which explains the renewed interest in the use of carbon-based materials for hydrogen storage. As is seen from the recent reviews [2, 3], a number of papers reporting theoretical and experimental hydrogen storage data in novel carbon nanomaterials have been published. But the discrepancies in the results reported by different laboratories are so large [2,3], that no positive conclusion can be made by the majority of researchers. As is noted in the analytical review [4] of 2001, a solution of the problem in question is related to optimizing and understanding the interaction between hydrogen and carbon materials. In order to optimize a carbon-based hydrogen-storage system, it is necessary [4] to obtain a better understanding of the adsorption mechanism and to determine the precise adsorption sites in the carbon network that are responsible for the more promising adsorption properties. But the situation of 2001 [1, 4] with respect to the optimizing and interaction understanding has retained near the same and nowadays [5, 6]. There-
fore, the urgent "open" questions can be formulated, as follows: (1) physical reasons of an anomalous scattering (of 2—3 orders of the magnitude) and non- or poor reproducibility of experimental data on the hydrogen sorption by novel carbon na-nomaterials (CNs); (2) a poor knowledge on characteristics and nature of the hydrogen sorption by CNs; (3) a poor knowledge on characteristics and mechanisms of hydrogen diffusion — the rate-controlling stage of a number of the desorption process.
Methodology & Results
In the present study, the thermodynamic analysis [3, 6, 7] is performed of the most significant experimental data on the hydrogen sorption by graphites and related novel carbon-based nanoma-
terials at room temperatures and technological pres- *
sures (fullerenes, single- and multi-walled nano- ft
tubes (SWNT, MWNT), graphite nanofibers I
(GNF), nanostructured graphites). The thermo- S
dynamic and kinetic (diffusion) characteristics of 1
sorption processes are refined and compared with 1
the theoretical quantities (Table 2), for optimizing I
and a better understanding the interaction between P
hydrogen and carbon materials. The attention is ^
LT
concentrated on a series of the Rodriguez-Baker § unique studies [8] (it is also discussed in [1—4, 6, ® 7]) of graphite nanofibers charged at hydrogen pressure of 11 MPa, where a super hydrogen adsorption capacity (H/C « 6—8) has been found. As
Статья поступила в редакцию 08.11.2005. The article has entered in publishing office 08.11.2005.
Ю. С. Нечаев, О. К. Алексеева, Г. А. Филиппов, А. Л. Гусев Теоретико-экспериментальная база для создания углеродистых адсорбентов с
высоким содержанием водорода для транспортных средств на топливных элементах
Table 1
Comparison of Hydrogen Storage Technologies (2001), [1]
Compressed Gas Liquid Hydrogen Metal Hydride Storage in Carbon
Advantages Good technology base. Fair gravimetric and volumetric densities. Good technology base. Good gravimetric and volumetric densities. Excellent volumetric density. High degree of safety. Potentially high gravimetric densities.
Disadvantages Cost of compression. Bulky. Safety concerns. Cost of liquefaction. Boil-off losses. Safety concerns. Alloy cost. Poor gravimetric densities. Unknown.
Future prospects Goal of 12 wt. % H2 will be met or exceeded. Mature technology. Complex hydrides could improve gravimetric density. Uncertain. High risk research. Possibly high pay-off.
Table 2
Some characteristics and mechanisms of chemisorption and diffusion of hydrogen in isotropic graphite and related carbon nanostructures (processes I, II, III, IV), [3, 7]
Models of chemisorption and diffusion of hydrogen in the materials; energies of formation of the chemical bonds Type of the sorption isotherm
Chemisorption of hydrogen in sp2 carbon materials Characteristics of chemisorption and diffusion of hydrogen in the materials
Process III (thermo-desorption peak) in isotropic [10] (Fig. 8, [3]) and nanostruc-tured [11-13] (Figs. 7, 8, [12]) graphites, and in GNFs [8] Dissociative chemisorption of hydrogen between the graphene layers («reactions» (4)-(7) in [3]). Bulk diffusion of hydrogen atoms, accompanying with a reversible trapping the diffusant by the chemisorption "centers" on the garphene layers, model F* (Fig. 9, [3]); Aff(form.c-H)in « -243 kJ/mol(H). AH(7)III ~ (1/2 AH(diss.H2) + AH(form. c-H)iii) ~ -19 kJ/mol(H), AS^m /R » -14,7 (-15,4) (Xm max = 0,5 (1,0)); (Eqs. (8)-(10) [3]). Aii = Aim exp(-Qm /RT), Aciii » 310-3 cm2/s, Qm » (Qaatt.) - AH(form.c-H)iii ) « 250 kJ /mol(H); Q0att.) « 7 kJ/mol(H); (Eqs. (11), (12) in [3]) Sieverts-Langmuir (Eq. (9) in [3])
Process II (TPD peak) in isotropic [10] (Fig. 8, [3]) and nanostructured [11, 12] (Figs. 7, 8, [12]) graphites, in GNFs [8], in defected SWNTs [14] and MWNTs [15] «Dissociative-associative» chemi-sorption of H2 in the intergrain or defected (surface) regions ("reactions" (13)-(16) in [3]). Diffusion of H2 in these regions, accompanying with a reversible dissociation and trapping of the diffusant on the chemisorption "centers", model H (Fig. 9, [3]); AH(form.c=2H)ii « -570 kJ/mol(2H). AH(16)ii ~ (AH(diss.H2) + AH(form.c=2H)ii) ~ -120 kJ/mol(H2), AS(16)ii /R « -30 (for X max = 0,5 (0,25)); (Eqs. (25), (26) in [3]). Aii = Ami exp(-Qn /RT), D„n *> 1,8'103 cm2/s, Qii » (g(de£) - AH(16)ii ) » 120 kJ/mol(H2); Q(def) « 10 k J/mol(H2); (Eqs. (22), (24) in [3]) Henry-Langmuir (Eq. (25) in [3])
Process I (TPD peak) in isotropic graph-ite[10] (Fig. 8, [3]), in SWNTs [16, 17] (Figs. 1, 2, [17]) and in MWNTs [15] «Dissociative-associative» chemisorp-tion of H2 in surface layers of the material («reactions» (13)-(16) in [3]). Diffusion of H2 in these layers, accompanying with a reversible dissociation and trapping of the diffusant on the "centers", model G or F (Fig. 9, [3]); AH(form c=2H,c2=2H)i ~ -460 kJ/mol(2H). AH(16)i ~ (AH(diss.H2) + AH(form. c=2H, c2=2H)i) « -10 kJ/mol(H2), AS(16)i /R » -20 (for Xi max » 0,5 0,25)); (Eqs. (20), (21) in [3]). Di = A«exp(-Qi/RT), Dw » 3-10-3 cm2/s, Qi « (Q(Surf.) - AH(16)i) « 20 kJ /mol(H2); Q(Surf.) » 10 kJ /mol(H2); (Eqs. (17), (19)) Henry-Langmuir (Eq. (20) in [3])
Process IV (TPD peak) in isotropic [10] (Fig. 8, [3]), pyrolytic [18] and nanustructured [13] graphites Dissociative chemisorption of H2 in defected regions of graphite lattice («reactions» (4)-(7) in [3]). Bulk diffusion of H in the defected regions, with the trapping by the «centers», models C, D (Fig. 9, [3]); AH(&rm.c-H)iv « -364 kJ/mol(H). AH(7)iV ~ (1/2 AH(diss.H2) + AH(form.c-H)iV) « -140 kJ/mol(H); (Eq. (27) in [3]). Div = D„iv exp(-Qiv /RT), DWv » 6-102 cm /s, Qiv ~ -AH(form.c-H)iv ~ 365 kJ /mol(H); (Eq. (28) in [3]) Sieverts-Langmuir (Eq. as (9) in [3])
D0III , D0II , D0I , D0IV — the pre-exponential (entropie) factors of the hydrogen diffusion coefficients (D , D , DI , DIV) in the carbon materials corresponding to processes III, II, I, IV.
analysis [3, 6, 7] has shown, a considerable part of the anomalous non-reproduced data [8] (~30 % of the total storage amount) is in a satisfactory accordance with the most of other known data on the chemisorption, particularly, with the data [11, 19] on mechanical synthesis of hydrogen with graphite (charged at 1 MPa) and the recent data
[20] on graphite nanofibers and single-walled na-notubes (charged at 9 GPa). As is also shown [3, 6, 7], the hydrogen bulk and "grain boundary" concentrations in specimens [8] (after a fast release from them of ~70 % of adsorbed hydrogen) are about of a "carbohydride" value (H/C ~ 1); a similar situation is in specimens [11, 19, 20].
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The extreme anomalous part (~70 %) of the adsorbed hydrogen in specimens [8] might be related with an unknown physical-chemical mechanism of the adsorption. It is consistent with the neutron diffraction data [21] about a high density packing of hydrogen adsorbed on graphite (see Fig. 1 in [4]; as is noted [1], it is higher than in solid molecular hydrogen), and also with X-ray data [20] on an anomalous increase (~40 %) of the graphene inter-layer distance.
Conclusion
In a number of scientific and technological investigations, the hydrogen sorption processes in carbon nanomaterials are r
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