CC BY
31
On the spillover effect of the solid H2 intercalation
into GNF's
Yu. S. Nechaev1, V.P. Filippova1, A. A. Tomchuk1, A. Yurum2, Yu. Yurum3, T.N. Veziroglu4
1 Kurdjumov Institute of Metals Science and Physics, Bardin Institute for Ferrous Metallurgy,
Moscow, Russia
2Nanoechnology Research and Application Centre, Sabanci University, Istanbul, Turkey
3Falulty of Engineering and Natural Sciences, Sabanci University, Istanbul, Turkey
4International Association for Hydrogen Energy, Miami, FL 33155, USA
yuri1939@inbox.ru, varia.filippova@yandex.ru, tomchuk-a@yandex.ru, ayurum@sabanciuniv.edu, yyurum@sabanciuniv.edu, tnveziroglu@gmail.com
PACS 67.63-Cd, 68.43-Hn, 68.47-Mn DOI 10.17586/2220-8054-2016-7-1-204-209
Keywords: HOPG, epitaxial graphenes, graphite nanofibers (GNFs), solid H2 intercalation, spillover effect. Received: 20 November 2015
1. Introduction
As is known, the hydrogen spillover effect can be characterized by three major steps, the first being where molecular hydrogen is split via dissociative chemisorption into its constituent atoms on a transition metal catalyst surface, followed by migration from the catalyst to the substrate, culminating in their diffusion throughout the substrate surfaces and/or in the bulk materials. The mechanism behind the hydrogen spillover effect has been long disputed, up to currently.
As is noted, for instance in [1-10], the hydrogen spillover effect manifestation in carbon-based nanomaterials has been not studied thoroughly, which is particularly relevant for solving the current problems of on-board hydrogen storage efficiency and safety.
2. Experimental/methodology
A thermodynamic analysis approach [11,12] for the related experimental data (including Figs. 1 - 4 from [12]) has been used.
3. Results and discussion
3.1. The physics for the intercalation of high density H2 gaseous nanophase into
graphene nanoblisters in HOPG and epitaxial graphenes (under atomic hydrogen treatment)
Figure 1 shows the two steps ((a) and (b)) of hydrogenation (at 300 K and the atomic hydrogen pressure P(Hgas) ~ 1 ■ 10-4 Pa, without any catalyst) of surface graphene layers of a highly oriented pyrolytic graphite (HOPG) resulted in intercalation of a high density H2 gaseous nanophase into surface graphene nanoblisters.
If we assume that the nanoblister approximates a semi-elliptical form, we obtain a blister area of Sb ~ 2.0 ■ 10-11 cm2 and a volume of V ~ 8.4 ■ 10-19 cm3. The amount of retained hydrogen in this sample becomes Q ~ 2.8 ■ 1014 H2/cm2 and the number of hydrogen molecules captured inside the blister becomes n & (QSb) & 5.5 ■ 103. Thus, within the
ideal gas approximation, and accuracy of one order of the magnitude, the internal pressure of molecular hydrogen in a single nanoblister at near-room temperature (T & 300 K) becomes PH2 & {kB(QSb)T/Vb} & 1 ■ 108 Pa. The hydrogen molecular gas density in the blisters (at T & 300 K and PH2 & 1 ■ 108 Pa) can be estimated as pH2 & {(QMH2Sb)/Vb} & 0.045 g/cm3, where MH2 is hydrogen's molecular mass.
Fig. 1. Model (from STM and AFM data} showing the hydrogen accumulation (intercalation) in HOPG, with the formation of blister-like surface nanostructures; hydrogénation was done at 300 K and the atomic hydrogen pressure P(Hgas) ~ 1 ■ 10-4 Pa, without any catalyst. (a) Pre-atomic hydrogen penetration (intercalation) step. (b) Molecular gaseous hydrogen, "captured" inside the surface graphene nanoblisters (at P(H2gas) ~ 1 ■ 108 Pa), after the intercalation step. Sizes are not drawn exactly in scale. The hydrogen compression effect is of 12 orders (from
P
(Hgas)
1 10 - 4 Pa to P(
\H2gas) & 1 ■ 108 Pa). According to analysis [11,12], it occurs at the expense of the energy of association of hydrogen atoms to the "captured" molecules, Eqs. (1), (2)
These data (Fig. 1) can be quantitatively described, with an accuracy of one order of magnitude, and interpreted within the thermodynamic approach [11,12], by using the conditions of thermo-elastic equilibrium for the reaction of (2H(gas) ^ H2(gas_ira_b1isiers)), as follows:
(PH2/PH2) = (Ph/PH)2exp {[AHdis - PH2AV]/kBT}, (1)
where P*H2 is related to the blister "wall" back pressure (caused by PH2) - the so called surface pressure (P*H2 & PH2 & 1 ■ 108 Pa), PH is the atomic hydrogen pressure corresponding to the atomic hydrogen flux (PH & 1 ■ 10-4 Pa), PH2 = PH = 1 Pa is the standard pressure, AHdis = 4.6 eV is the dissociation energy (enthalpy) of one molecule of gaseous hydrogen (at room temperature), ASdis = 11.8kB is the dissociation entropy, AV & (Sbrb/n) is the apparent volume change, rb is the radius of curvature of nanoblisters at the nanoblister edge (rb & 30 nm, Fig. 1), Na is Avogadro's number, and T is the temperature (T & 300 K). The quantity of
(P*H2AV) is related to the work of the nanoblister surface increasing with an intercalation of 1 molecule of hydrogen.
The value of the tensile stresses ab (caused by P*H2) in the graphene nanoblister "walls" with a thickness of db and a radius of curvature rb can be evaluated from another condition (equation) of the thermo-elastic equilibrium for the system in question, which is related to Eq. (1), as follows:
^b - Ph2 (rb/2db) & (ebEb), (2)
where eb is a degree of elastic deformation of the graphene nanoblister walls, and Eb is the Young's modulus for the graphene nanoblister walls.
Substituting in the first part of Eq. (2) the quantities of P^2 — 1 ■ 108 Pa, rb — 30 nm and db — 0.15 nm results in the value of ab — 1 ■ 1010 Pa.
The degree of elastic deformation for the graphene nanoblister walls, apparently reaches eb — 0.1 (Fig. 1). Hence, with Hooke's law of approximation, using the second part of Eq. (2), one can estimate, with the accuracy of one order of the magnitude, the value of the Young's modulus for the graphene nanoblister walls: Eb — (ab/eb) — 0.1 TPa. It is close (within reasonable error) to the experimental value [13, 14] for the Young's modulus of perfect (that is, without defects) graphene (Egraphene — 1.0 TPa).
Similar STM, AFM and other data from different researchers for epitaxial graphenes (for instance, Fig. 2) can be analyzed and interpreted in a similar manner [11, 12], within the same physical concept (Eqs. 1, 2).
In this connection, it is expedient to note that a number of researchers (for instance, [15, 16]) have not sufficiently considered the "thermodynamic forces" and/or energetics of formation (under the atomic hydrogen treatment) for graphene nanoblisters in the surface HOPG layers and epitaxial graphenes.
It is also expedient to note that very recent experimental data [17] show that a hydrogen atom can not pass through a perfect graphene network. Conversely, the analysis [11, 12] of a number of experimental data (including Fig. 1, 2) shows that a hydrogen atom can pass through permeable defects in graphene, for instance, through triple junctions of grain boundaries. In Fig. 2 a and b, one can imagine some grain boundary network substituted (obviously, in some nano-regions at grain boundaries) by some nano-protrusions.
Fig. 2. (a) STM image of hydrogenated graphene/SiC. (b) Same image as in (a) with inverted color scheme, giving emphasis to preferential hydrogen adsorption on the SiC surface. Hydrogen dose at T(beam) = 1600 K, t = 5 s, F = 1012 -1013 atoms/cm2- s (P(Hgas) « 1 ■ 10-4 Pa [11, 12])
3.2. The physics of intercalation of the solid H2 nanophase into hydrogenated graphite nanofibers (with metallic catalysts)
The physics for the intercalation of high density solid molecular hydrogen (pH2 ~ 0.5 g/cm3, Fig. 3) into closed (in the definite sense) nanoregions in hydrogenated GNFs (Fig. 4) is related to the same concept, Eqs. of type (1), (2) [11, 12].
1400
1200 1000 800
cC
600
400
200 0
0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5
p, gsm"3
Fig. 3. Data on isentropes (S/R) and isotherms for deuterium and protium. The density (p) of protium (H2, H) is increased by a factor of two (for the scale reasons). The experimental and theoretical isotherms show that at T = 300 K and the external compression pressure of P = 50 GPa hydrogen exists in a high density solid molecular state pH2 « 0.5 g/cm3 [12]
Obviously, it is a manifestation of the spillover effect, relevant to providing the necessary partial pressure of atomic gaseous hydrogen (with material hydrogenation at an initial molecular hydrogen pressure PH2 = 8 MPa).
4. Conclusions
The "thermodynamic forces" and energetics of forming of graphene nanoblisters (under atomic hydrogen treatment, without catalysts) in the surface HOPG layers (Fig. 1) and epitaxial
graphenes (Fig. 2) are quantitatively described, particularly, two conditions of the thermal-elastic thermodynamic equilibrium - Eqs. (1),(2) - are considered.
The physics for the intercalation of a high density gaseous H2 nanophase (pH2 « 0.045 g/cm3) into graphene nanoblisters (Figs. 1, 2) is considered - Eqs (1), (2). The hydrogen compression effect of 12 orders of magnitude (from P(Hgas) ~ 1 ■ 10-4 Pa to P(H2gas) « 1 ■ 108 Pa), at the expense of the energy of association of hydrogen atoms to the "captured" molecules, is shown.
The physics for the intercalation of the high density solid H2 nanophase (pH2 « 0.5 g/cm3, [18]) into hydrogenated graphite nanofibers with Pd-catalyst (Figs. 3, 4) is considered.
Fig. 4. Micrograph of hydrogenated graphite nanofibers (GNFs), with Pd-catalyst (hydrogenated at 300 K and an initial pressure of P(H2gas) — 8 MPa), after release from them, at 300 K, for 10 min, of the intercalated solid H2 nanophase (17 wt%) of a high density of pH2 — 0.5 g/cm3 (analysis [11, 12]). The arrows in the picture indicate some of the slit-like closed nanopores of the lens shape, where the solid H2 intercalated nanophase (under pressure of ~50 GPa, according to data on Fig. 3) was localized. Such a pressure level can be also evaluated by the consideration of the material deformation and the necessary stresses for forming the lens shape closed nanopores (at the expense of the energy of association of hydrogen atoms to molecules "captured" inside the nanopores, Eqs. of type (1), (2)
In the light of analysis [11,12], the spillover effect is obviously manifested in the extraordinary data [19].
This effect can be used for solving of the current problem of the efficient and safe hydrogen on-board storage [20].
Acknowledgements
This work has been supported by the RFBR (Project #14-08-91376 CT) and the TUBITAK (Project # 213M523).
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