Hydrogen adsorption and phase transitions in fullerite

Article abstract of DOI:10.1063/1.1312857

Hydrogen desorption and adsorption properties of the fullerene materials C₆₀, C₇₀, and fullerite (a mixture of C₆₀ and C₇₀) were measured volumetrically using a Sievert's apparatus. Over several cycles of isotherm measurements at 77 K, the hydrogen storage capacities of one of the fullerite samples increased from an initial value of 0.4 wt% for the first cycle to a capacity of 4.4 wt% for the fourth cycle. Correspondingly, the surface area of this sample increased from 0.9 to 11 m²/g, and there were changes in its x-ray powder diffraction pattern. In comparison, two other fullerite samples, prepared by a different procedure showed no such behavior. Pure C₆₀ and pure C₇₀ were also cycled and exhibited small and constant capacities of 0.7 and 0.33 wt%, respectively, as a function of number of cycles. The enhanced storage capacity of fullerite material is tentatively attributed to the presence of C₆₀ oxide.

Full text of DOI:10.1063/1.1312857

Hydrogen adsorption and phase transitions in fullerite
Y. Ye, C. C. Ahn, B. Fultz, J. J. Vajo, and J. J. Zinck
Citation: Applied Physics Letters 77, 2171 (2000); doi: 10.1063/1.1312857
View online: http://dx.doi.org/10.1063/1.1312857
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APPLIED PHYSICS LETTERS
VOLUME 77, NUMBER 14
2 OCTOBER 2000
Hydrogen adsorption and phase transitions in fullerite
Y. Ye, C. C. Ahn,a) and B. Fultz
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125
J. J. Vajo and J. J. Zinck
HRL Laboratories, LLC, Malibu, California 90265
͑Received 25 May 2000; accepted for publication 1 August 2000͒
Hydrogen desorption and adsorption properties of the fullerene materials C60 , C70, and fullerite ͑a
mixture of C60 and C70) were measured volumetrically using a Sievert’s apparatus. Over several
cycles of isotherm measurements at 77 K, the hydrogen storage capacities of one of the fullerite
samples increased from an initial value of 0.4 wt % for the first cycle to a capacity of 4.4 wt % for
the fourth cycle. Correspondingly, the surface area of this sample increased from 0.9 to 11 m2/g, and
there were changes in its x-ray powder diffraction pattern. In comparison, two other fullerite
samples, prepared by a different procedure showed no such behavior. Pure C60 and pure C70 were
also cycled and exhibited small and constant capacities of 0.7 and 0.33 wt %, respectively, as a
function of number of cycles. The enhanced storage capacity of fullerite material is tentatively
attributed to the presence of C60 oxide. © 2000 American Institute of Physics.
͓S0003-6951͑00͒03039-4͔
There has been widespread interest in recent claims that
certain carbon structures such as nanofibers and nanotubes
can store much more hydrogen than conventional forms of
carbon.1–3 Crystalline fcc C60 , however, has been observed
to absorb H2 in octahedral interstices, providing a small stor-
age capacity of only 0.28 wt % at 40 °C.4
In this letter, we present the results of studies on the
sorption of H2 by fullerenes. In the fullerene family, the
closed-cage, nearly spherical C60 and the ellipsoidal rugby-
ball-shaped C70 are the most stable molecules5 and are thus
relatively abundant. We performed measurements on pure
C60 , pure C70, and three sets of a mixture of these two com-
monly referred to as ‘‘fullerite.’’ We report a surprising in-
crease in the amount of H2 adsorption for one of these ful-
lerites. The isotherms show the characteristics of
physisorption, so we note that this work is different from
hydrogenation studies of fullerenes6,7 involving chemisorp-
tion of atomic hydrogen.
Several different commercially available fullerene
samples were obtained from Alfa Aesar and MER Corp. All
these fullerenes were made by a carbon arc discharge in he-
lium ͑Hoffman–Dratchshermer method͒. The carbon soot
was then treated in toluene. The fullerite is soluble in toluene
at the limit of 2 g/l and for the first fullerite sample ͑denoted
fullerite No. 1͒ from MER Corp. and obtained in 1992, the
solution was spread onto a teflon tray and evaporated at
room temperature. For the second fullerite sample from Alfa
Aesar ͑denoted fullerite No. 2͒ and obtained in 1999, the
precipitate from the toluene solution was obtained after slow
evaporation of the toluene. The precipitate was then washed
with petroleum ether and underwent drying to remove the
rest of the solvent. A third sample of fullerite was obtained
from MER in 1999 ͑denoted as fullerite No. 3͒ and was
processed similarly to fullerite No. 2. All fullerite samples
typically contained about 75% C60 , 22% C70 , with the bal-
ance being higher fullerene molecules. The pure C60 ͑or C70)
fullerenes were obtained by chromatographic purification,
and the purity was 99.9% for C60 and ϳ98% for C70 .
Desorption and some adsorption isotherms were mea-
sured with a computer-controlled Sieverts’ apparatus ͑i.e., a
volumetric system for quantitative measurement of gas ab-
sorption and desorption by solids͒ on 600 mg samples at 300
and 77 K. After vacuum degassing at 200 °C for 10 h, the
measurement temperature was attained and hydrogen gas of
99.9999% purity was admitted into the reactor to a maxi-
mum pressure of about 120 bar. This pressure was main-
tained for 15 h to allow the adsorption to equilibrate and to
check for leaks in the system. To correct for instrumental
effects, identical volumetric measurements were performed
on an empty reactor after each sample measurement. This
procedure was used for every adsorption/desorption cycle,
even when multiple cycles were taken on the same sample.
Surface area was measured by the Micromeritics Corp.
by Brunauer–Emmett–Teller ͑BET͒ surface area analysis us-
ing nitrogen gas. X-ray powder diffractometry was per-
formed on as-received and cycled samples, using an Inel
CPS-120 powder diffractometer using Co K␣ radiation. High
performance liquid chromatography ͑HPLC͒ measurements
were performed at MER Corp.
Table I summarizes the results from the hydrogen
adsorption/desorption and BET measurements. The hydro-
gen storage capacities of pure C60 and C70 are reproducible
and are consistent with results of others.4 On the other hand,
fullerite No. 1 exhibited unusual isotherm behavior. As
shown in Fig. 1͑a͒, the capacity during the first desorption
run on the as-received fullerite No. 1 ͑labeled 1st run͒ is
small, comparable to the capacity of pure C60 or pure C70 .
The capacity then increased dramatically with each subse-
quent isotherm cycle. In the fourth cycle ͑labeled 4th run͒, it
reached a maximum of 4.4 wt % at 120 bar at 77 K. This
value is consistent with complete H2 adsorption onto the sur-
faces of the fullerene molecules, assuming that H2 molecules
a͒
Electronic mail: cca@caltech.edu
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0003-6951/2000/77(14)/2171/3/$17.00 2171 © 2000 American Institute of Physics
202.28.191.34 On: Fri, 19 Dec 2014 23:55:24

2172
Appl. Phys. Lett., Vol. 77, No. 14, 2 October 2000
Ye et al.
TABLE I. H2 storage capacities and BET surface areas of fullerenes.
Surface area
(m2/g)
H2 wt %
͑300 K͒
H2 wt %
͑77 K͒
Composition
Sample
͑HPLC͒
Abs
Des
Abs
0.83
Des
As rec.
Cycled
C60
C70
99.9ϩ%
98ϩ%
0.07
0.12
0.08
0.12
0.70
0.33
Fullerite
No.1
Fullerite
No.2
Fullerite
No.3
75%C60 22%C70
1.5%C60 oxides
75%C60 22%C70
0.2%C60 oxides
77%C60 21%C70
0.6%C60 oxides
4.00
0.58–4.38
0.9
4
11
3
0.38–0.60
0.80–0.99
0.2–0.3
of diameter 3.5 Å form a close-packed shell around each
fullerene molecule. This implies that further improvements
in the hydrogen adsorption capacities of mixed fullerite ma-
terials may be difficult to achieve. At the fifth cycle, the
capacity decreased to about 2.5 wt %. Samples of the same
material were analyzed for their surface area by the BET
method. The BET surface area changed from 0.9 m2/g for the
as-received fullerite No. 1 to 11 m2/g after five isotherm
cycles.
To verify these results, we tested a second sample of
fullerite No. 1, and obtained the same isotherm behavior as a
function of cycle number. These results are presented in Fig.
1͑b͒. A total of seven cycles was performed on the second
sample of fullerite No. 1 and cycles five to seven showed
identical isotherm traces. On the other hand, a similar
Sievert’s apparatus was used to measure one point of the
isotherm at 77 K at about 50 bar, but this measurement
showed only 0.05 wt % hydrogen sorption. Sample handling
procedures were similar although not identical, and we are
still investigating this discrepancy in these measurements.
Fullerite No. 2 behaved similarly to the samples of pure C60
or pure C70 and showed only small differences as a function
of cycle number as shown in Table I. Fullerite No. 3 which
was obtained recently from MER and used as a control
sample, showed even smaller desorption capacities than ful-
lerite No. 2 at 77 K. For comparison, we also measured the
hydrogen desorption of high surface area saran carbon.8
HPLC measurements of these materials showed only
slight variations in the C60–C70 ratio. Therefore, we believe
that the important difference between fullerite Nos. 1, 2, and
3, is the amount of oxidized C60 found in as-received fuller-
ite No. 1. The oxidized C60 accounts for ϳ2% of the as-
received fullerite. The oxidized component was absent in the
material after it had been cycled with H2 gas. The oxidation
of fullerite No. 1 occurred presumably over the 7 year time
span from its synthesis to the time the desorption experi-
ments were performed.
X-ray diffraction patterns of the samples are presented in
Fig. 2. The diffraction pattern of the pure C60 is that of the
expected fcc structure with aϭ14.2 Å.9 For fullerite
samples, the broad peaks between the C60 ͑220͒ and ͑311͒
FIG. 1. Desorption isotherms of composition vs pressure at 77 K for two
different samples of fullerite No. 1 of C60–C70 fullerite materials. The upper
set includes a trace for Saran carbon. The lower set shows identical isotherm
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behavior as a function of adsorption/desorption cycle number.
FIG. 2. X-ray diffraction patterns of fullerenes.
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Appl. Phys. Lett., Vol. 77, No. 14, 2 October 2000
Ye et al.
2173
peaks are contributed by phases of C70 . The structural phase
transitions and orientational ordering in C70 at different tem-
perature and pressure have been studied extensively for the
past decade.10–12 Although there is still no agreement about
the details of the structures, there is general agreement as to
the temperatures of the phase transitions. Five phases have
been observed, including face-centered-cubic ͑fcc͒,12
rhombohedral,10 ideal hexagonal-close-packed ͑hcp͒ (c/a
ϭ1.63),12,10 deformed hcp (c/aϭ1.82),13,14 and a mono-
clinic phase.10 Some phase transitions occur over a wide
temperature range and exhibit large hysteresis which de-
pends on the thermal history of the sample.10,15
gen sorption/desorption cycle. The x-ray diffraction patterns
of fullerite No. 1 also showed a change in structure, and this
sample also showed an unusual increase in surface area ͑al-
though the increase in surface area is itself too small to ac-
count for significant physisorption͒.
We suggest two possible reasons for lower cohesive en-
ergy of fullerite No. 1 than fullerite No. 2. The microstruc-
tural distribution of C60 , C70 , and higher fullerene molecules
may differ owing to differences in material preparation,
causing differences in the structural transformations under
temperature and hydrogen pressure. Differences in the
breadth of the fcc C60 peaks in the diffraction patterns are
evidence for a microstructural difference between the fuller-
ites Nos. 1 and 2, and these peaks underwent an observable
change after fullerite No. 1 was cycled. The cohesive energy
could also differ between fullerite Nos. 1 and 2 because of
the more extensive oxidation of the C60 in fullerite No. 1.
The cyclic exposure to hydrogen gas reduced the oxidized
C60 in fullerite No. 1, perhaps causing an increase of its
cohesive energy and the observed reduction of hydrogen
storage capacity after four cycles. The C60 oxide was absent
after five cycles, but the hydrogen adsorption capacity of
fullerite No. 1 remained large, so we cannot attribute all of
the difference between fullerite samples to oxidation. We do
note, however, that oxidation has been observed to influence
Figure 2 shows that the diffraction pattern of C70 in ful-
lerite No. 1 changed after several isotherm cycles. Due to the
broadening and the low intensity of the C70 peaks, it is not
possible for us to identify reliably the phases in the samples.
However, the change in the diffraction pattern from qϭ1.3
to 1.4 ÅϪ1 indicates that some parts of the material have
undergone a phase change after hydrogen cycling. Further-
more, the diffraction peaks from the C60 in fullerite No. 1
became sharper after cycling. On the other hand, the C60
diffraction peaks from fullerite No. 2 sharpened after cy-
cling, but there was little change in the region of q
ϭ1.3–1.4 ÅϪ1
.
We believe the high hydrogen adsorption of fullerite No.
1 is a consequence of a hydrogen-induced structural transi-
tion in the fullerite much like that reported for carbon single-
walled nanotubes.8 Three energies are involved. One is the
energy of adsorption of the H2 molecule on the surface of the
carbon. For SWNTs this energy for hydrogen physisorption
was approximately Ϫ38 meV ͑characteristic of adsorption on
graphite͒, and we expect this adsorption energy to be similar
for hydrogen adsorption on C60 and C70 . Second is the van
der Waals energy of cohesion of the C60 and C70 crystals.
Evidently these van der Waals interactions in pure C60 and
C70 are sufficiently strong so that the crystals remain intact
and the hydrogen sorption is limited to absorption in inter-
stitial sites and adsorption on the relatively few surface sites.
The third ‘‘energy’’ is the chemical potential of the hydrogen
molecules, which increases with hydrogen gas pressure. It is
possible to reduce this contribution to the total free energy
by surface adsorption of some of the hydrogen. The phase
transition in the SWNT material was driven by this reduction
in hydrogen chemical potential during physisorption, which
was sufficient to overcome the van der Waals attraction be-
tween the tubes in a rope, separating them into individual
tubes with a large surface area for hydrogen adsorption.
Evidently this phase transition does not occur in the
samples of pure C60 or C70 , and these materials remain intact
because their van der Waals attractions are strong. The van
der Waals interaction and other electron–electron correlation
effects responsible for cohesion decrease rapidly with dis-
tance, however. Poor crystallinity in the C60–C70 fullerite
No. 1, perhaps induced by the phase transitions in the C70
regions, or by oxidation, could reduce the cohesive energy of
the fullerite so that hydrogen adsorption could occur with
structural dissociation. The shapes of the isotherms of fuller-
ite No. 1 were not reproducible until after the fifth cycle,
indicating that its cohesive energy was altered after a hydro-
16
structural phase transitions in pure C60 and mechanical
properties of C60.17
This work was supported by DOE through Basic Energy
Sciences Grant No. DE-FG03-00ER15035.
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