Hydrogen sorption by porous materials composed of one to three elements selected from boron, carbon and nitrogen and metal modification to enhance the sorption

Article abstract of DOI:10.1016/j.jallcom.2013.02.136

In order to develop novel hydrogen storage materials with high hydrogen capacity, layered compounds with high specific surface areas and large pore volumes, referred to as BN's, CN's and C's, were synthesized from B-, C- and/or N-containing substances thr

Full text of DOI:10.1016/j.jallcom.2013.02.136

Journal of Alloys and Compounds 580 (2013) S305–S308
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Hydrogen sorption by porous materials composed of one to three
elements selected from boron, carbon and nitrogen and metal
modification to enhance the sorption
b
a
a
Nobuyuki Nishimiya a, , Yusuke Date , Yoshiyuki Kojima , Takeshi Toyama
⇑
a College of Science and Technology, Nihon University, Tokyo 101-8308, Japan
b Yonago National College of Technology, Tottori 683-8502, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 7 March 2013
In order to develop novel hydrogen storage materials with high hydrogen capacity, layered compounds
with high specific surface areas and large pore volumes, referred to as BN’s, CN’s and C’s, were synthe-
sized from B-, C- and/or N-containing substances through specified wet processing and optimized calci-
nations. Hydrogen contents at 77 K under 0.8 MPa of hydrogen increased as specific surface areas
increased independently of the formulations of the samples, and almost all of them exceeded the pre-
dicted values for two dimensional condensation of hydrogen, that is, 2.34 mass% per 1000 m2 gÀ1. While
modification with Pd did not increase the hydrogen capacity, Pt-modification brought about substantially
higher hydrogen capacity at 77 K. Ni-modification also imparted higher hydrogen capacity to C’s pre-
pared by calcination of electrospun polyacrylonitrile fibers, but formation of mesopores on destroying
micropores through excessive modification would reduce the hydrogen capacity.
Keywords:
BN
CN
Nanocarbon
Hydrogen storage material
High specific surface area
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
attempts to enlarge the surface area would make sense for higher
hydrogen capacity. Narrowing the range of the search for novel
Nanocarbons have been expected to be applicable as hydrogen
storage media. One of the authors, N.N., published a work on sin-
gle-walled carbon nanotubes, where the hydrogen capacity was
about 1 mass% at room temperature and 2.5 mass% at liquid nitro-
gen temperature (77 K) under 100 kPa of hydrogen [1]. Since the
specific surface area of the employed sample was about
500 m2 gÀ1, it was thought that some novel materials with high
specific surface area of a few thousand m2 gÀ1 were to be utilized
for practical hydrogen storage, hopefully at room temperature.
However, nanocarbons are now regarded as inferior to metal–
organic frameworks (MOFs) in respect of hydrogen capacity both
per volume and per weight, and sorption under cryogenic condi-
tions is believed as a matter of course [2]. The superiority of MOFs
was clarified on the basis of hydrogen uptake–specific surface area
relation [3]. It was claimed that the hydrogen capacity at 77 K sim-
ply increased with the specific surface area independently of what
the tested materials were [4]. If such a relation is generally valid,
hydrogen molecules would occupy a certain area irrespectively of
states of matrices. Trials to enhance the hydrogen capacity by
designing novel matrices would be thus of no sense and only the
matrices would not inspire the development of hydrogen storage
materials based on some new concepts. As a matter of fact, the
independence of hydrogen capacity of sorts of materials has not
yet been precisely evidenced. The first objective of the present
study is thus to test the predicted independence using analogous
materials comprising B, C and/or N. Another objective is concern-
ing with the effects of metal-modification. It was pointed out that
purified single-walled carbon nanotubes containing no metals did
not adsorb hydrogen [5], and since then the effects of metal-
modification on hydrogen sorption has been widely studied
[6–10]. There would presumably exist specific metal–matrix
combinations of high hydrogen capacity. The second objective is
thus to search for novel metal-modification effects. Hydrogen
capacity will be hereinafter discussed at 77 K, since the one at
room temperature is too small (less than 0.1 mass%) to be practi-
cally applicable.
2. Experimental
Melamine diborate was prepared from melamine and boric acid in an aqueous
mixture with a boric acid/melamine molar ratio of 3.0 (excessive in boric acid), was
filtered and dried, and was calcined at 723 K in air for 1.8 h and at 1173 K in N2 for
1 h to obtain BN. Both melamine and boric acid were purchased from Kanto Chem-
ical Co., Inc. and employed without purification. The preparative method was essen-
tially the same as described in the literature [11]. The product was already
identified and reported as B4N3O2H. This compound is referred to as BN in the pres-
ent work. When Pd-modification was intended, an acetone solution of palladium
⇑
Corresponding author. Address: College of Science and Technology, Nihon
University, 1-8-14 Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8308, Japan. Tel.: +81
3 3259 0797; fax: +81 3 3293 7572.
E-mail address: nishimiya.nobuyuki@nihon-u.ac.jp (N. Nishimiya).
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.02.136

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N. Nishimiya et al. / Journal of Alloys and Compounds 580 (2013) S305–S308
acetate was mixed with melamine diborate so as to distribute 1 Pd atom per 1 nm2
of melamine diborate. Palladium acetate was purchased from Sigma–Aldrich Co.
and employed without purification.
reflection were similarly observed as described in the literature
[11]. FT-IR bands at 800 and 1400 cmÀ1 for BN, 1620 cmÀ1 for
NH and 3500 cmÀ1 for OH were similarly found as illustrated in
the literature.
CN’s were prepared by calcination of melamine in the presence of NH4Cl, pur-
chased from Kanto Chemical Co., Inc., in air at 723 K for 1 h and successively at
973 K for 1 h. The mass ratio of NH4Cl to melamine was 0.33 and the both ingredi-
ents were tightly mixed using formaldehyde, purchased from Kanto Chemical Co.,
Inc. Here the condition of calcination was the same as described in the literature
[12]. When Pd- or Pt-modification was intended, an aqueous solution of palladium
acetate or dinitrodiammineplatinum (II) was respectively mixed with melamine so
as to distribute 1 metal atom per 50 nm2 of predicted product. Dinitrodiammine-
platinum (II) was purchased from Sigma–Aldrich Co. and employed without
purification.
C’s were prepared by calcination of electrospun polyacrylonitrile (PAN) fibers
with diameters of 0.5–1 lm at 1073 K in Ar-10% H2 for 2 h. Nanofiber Electrospin-
ning Unit manufactured by Kato Tech Co. Ltd. was used with dimethylformamide
solutions of PAN supplied. When Ni-modification was intended, 0.045–0.9 g of Ni
(II) acetylacetonate was added to 20 mL of 5.0 mass% PAN solutions. PAN was pur-
chased from Sigma–Aldrich Co., and dimethylformamide and Ni (II) acetylacetonate
from Kanto Chemical Co., Inc.
Hydrogen sorption isotherms at 77 K showed substantially
reversible nature as shown in Fig. 2. Each isotherm for the first
hydriding–dehydriding cycle in Fig. 2 coincided with the one for
the respective second cycle. The hydrogen contents increased as
specific surface areas increased. The variation of the former with
the latter is illustrated in Fig. 3, where other data points for CN’s
and C’s are also included. Hydrogen contents of BN depicted by
open circles exceeded the predicted values for two dimensional
condensation of hydrogen, that is, 2.34 mass% per 1000 m2 gÀ1
Here the stripped domain phase with a density enhancing factor
of 1.126 [13] was assumed. If hydrogen commensurately con-
.
q
denses, 1 g of a sorbent with 1000 m2 gÀ1 of specific surface area
would adsorb 6.35 Â 1021 hydrogen molecules with 0.1575 nm2 of
occupying area to reach 2.08 mass% of hydrogen content. This va-
lue is enhanced to 2.34 mass% through formation of the stripped
domain phase. The reason why the hydrogen concentrations of
the BN compounds exceeded the predicted values has not been
clarified. Existence of more crowded phases, multilayered adsorp-
tion and exotic sorption in micropores are plausible. It is to be
noted that the upward deviations of the hydrogen contents were
observed where hydrogen sorption was not yet saturated.
Characterization of the prepared samples was performed by using Topcon SM-
300 scanning electron microscope (SEM), Micromeritics ASAP 2010 nitrogen
adsorption apparatus, Rigaku MultiFlex Cu K
a X-ray diffractometer (XRD), Shima-
dzu FTIR-8400S Fourier transform infrared spectrometer (FT-IR) and Shimadzu
ESCA-850 X-ray photoelectron spectrometer (XPS), and hydrogen sorption charac-
teristics were assessed by a typical volumetric method at 77 K and at room temper-
ature using Suzuki Shokan PCT-2ST hydrogen sorption apparatus.
3. Results and discussion
Pd-modification did not bring about higher hydrogen capacity
as shown by filled circles in Fig. 3. For example, a typical Pd-
modified sample of 814 m2 gÀ1 had a little bit smaller hydrogen
capacity, 1.762 mass%, than a pristine sample with less specific
surface area of 714 m2 gÀ1, 1.868 mass%. The SEM appearances of
both the samples were similar as shown in Fig. 1c and d, and pore
volumes determined by nitrogen adsorption were also similar. The
micropore volume of the Pd-modified sample of 814 m2 gÀ1 was
0.38 cm3 gÀ1 as estimated by a Dubinin–Radushkevich plot, and
the mesopore volume was 0.06 cm3 gÀ1 as calculated from the step
width of the hysteresis. The corresponding values for the pristine
sample were 0.36 cm3 gÀ1 and 0.07 cm3 gÀ1, respectively. Neither
the presence of Pd nor the slightly larger micropore volume en-
hanced hydrogen sorption. The chemical state of the loaded Pd
would be metallic. The 3p3/2 binding energy as determined by
XPS was 533 eV for as-prepared samples and was not altered
through several hydriding–dehydriding cycles at 623 K under
3.1. BN compounds
Melamine diborate was precipitated in pillar forms as shown in
Fig. 1a, and calcined BN particles were also in similar appearances
as shown in Fig. 1b. While the thickness of the latter was reduced
to a few to several tenth of the one of the former, the specific sur-
face area significantly increased from 0.7 m2 gÀ1 for melamine
diborate in Fig. 1a to 340 m2 gÀ1 for BN in Fig. 1b. This suggested
that the calcined BN sample had some porous structures. When
melamine diborate with higher specific surface area as large as
3 m2 gÀ1 was calcined, BN with higher specific surface area was ob-
tained as shown in Fig. 1c. The sizes of pillars were smaller than
those in Fig. 1b and the specific surface area was enlarged to
714 m2 gÀ1. The specific surface area of the BN compound was thus
controllable by changing the starting specific surface area of mela-
mine diborate. Typical XRD peaks at around 2h = 26.5° for the
(002) reflection and at around 2h = 43° for the (10) turbostratic
(a)
(b)
(c)
(d)
Fig. 1. SEM secondary electron images of (a) melamine diborate, 0.7 m2 gÀ1, (b) BN, 340 m2 gÀ1, (c) BN, 714 m2 gÀ1 and (d) Pd-modified BN, 814 m2 gÀ1
.

N. Nishimiya et al. / Journal of Alloys and Compounds 580 (2013) S305–S308
S307
1
0.1
250
714 m2 g-1
229 m2 g-1
200
150
100
50
Modified with “0.045 g Ni”
904 m2 g-1
“0.90 g Ni”
0.01
0.001
Not modified
,
,
,
,
: adsorption
: desorption
77 K
0
0
0.2
0.4
0.6
0.8
1
0
0.5
1
1.5
2
2.5
Relative pressure / -
Hydrogen concentration / mass%
Fig. 4. Nitrogen adsorption isotherms at 77 K for electrospun C and Ni-modified C.
Open: adsorption, filled: desorption.
Fig. 2. Hydrogen sorption isotherms at 77 K for BN’s with different specific surface
areas.
had micropores correspondingly to their specific surface areas,
none of them had mesopores.
Ni-modified C
BN
Pt-modified CN
From reversible isotherms similar as those in Fig. 2, the data
points represented by triangles in Fig. 3 were obtained. Hydrogen
storage capacity at 77 K increased with specific surface area, and al-
most all triangles deviated to the higher hydrogen capacity side
compared with the prediction line based on the two dimensional
condensation of hydrogen with the stripped domain phase, but
the Pd-modification did not bring about higher hydrogen capacity
as shown by a filled triangle. Also in the case of Pt-modification
illustrated by half filled triangles, higher hydrogen capacity was
not attained compared with pristine CN’s plotted by open triangles
in a specific surface area range between 200 and 400 m2 gÀ1. On the
other hand, significant enhancement of the hydrogen capacity was
CN
Pd-modified BN
C
Pd-modified CN
Fig. 3. Variation of hydrogen content at 77 K under 0.8 MPa of hydrogen with
specific surface area for BN, Pd-modified BN, CN, Pd- or Pt-modified CN, C and Ni-
modified C. The straight line denotes a predicted one assuming 2.34 mass% per
observed in a lower specific surface area range below 100 m2 gÀ1
.
1000 m2 gÀ1
.
This would be a symptom for high activity of Pt at 77 K. The
chemical states of Pd and Pt in the modified samples were not
known since XPS spectra were not accurately recorded. The in-
tended modification of 1 metal atom per 50 nm2 of CN would bring
about too small abundance of the metals for the XPS measurement.
0.8 MPa of hydrogen and vacuum. Since the Pd-modification was
adjusted to distribute 1 Pd atom per 1 nm2 of melamine diborate,
the final abundance of Pd would be 1 Pd atom per 60–80 nm2 of
the BN compound, and the mass% of Pd would be about 0.2%. This
estimation is based on the fact that the yield of B4N3O2H from mel-
amine diborate was 48% and that the typical specific surface area
around 600–800 m2 gÀ1 had been enhanced 180–210 fold on the
calcination. For further discussions, intended variation of the Pd
loading would be necessary. Since the relative ratio of the meso-
pore volume to the micropore volume was found to increase
through intended formation of BCN compounds on calcination of
melamine diborate in the presence of magnesium, respective roles
of mesopores and micropores for hydrogen sorption would be sep-
arately discussed. The BCN compounds herein represent B–C–N
ternary compounds with layered structures that contain 10–
17 mass% of C depending on the Mg/B molar ratio of 0.3–2.0 during
the calcination.
3.3. Electrospun fiber derived nanocarbons
An open square in Fig. 3 corresponds to a nanocarbon sample
derived from electrospun fibers and those samples modified with
Ni by adding 0.045 g and 0.90 g of the Ni complex correspond to
filled squares. In addition, nitrogen adsorption isotherms are
shown in Fig. 4. The specific surface area of the unmodified sample
was 210 m2 gÀ1, the micropore volume 0.10 cm3 gÀ1, and the
hydrogen content 0.78 mass% at 77 K. The corresponding values
for Ni-modified sample with 0.045 g of the complex were quite
well as 458 m2 gÀ1, 0.23 cm3 gÀ1 and 1.40 mass%, but those with
0.90 g of the complex were 244 m2 gÀ1
,
0.10 cm3 gÀ1 and
0.66 mass%, and 0.04 cm3 gÀ1 of mesopores were formed instead.
Formation of mesopores on destroying micropores through an
excessive Ni-modification would somehow reduce the hydrogen
capacity.
3.2. CN compounds
The C/N mass ratios of synthesized CN compounds determined
by elemental analyses were around 1.4, which was much higher
than the expected value, 0.64, for C3N4. As a matter of fact, a C/N
mass ratio of 0.64 could be reached when formaldehyde was not
used for tight mixing of melamine and NH4Cl prior to calcinations.
During the search for the optimum conditions to obtain the highest
specific surface area, the C/N ratio deviated to the carbon-rich side.
The dependence of the specific surface area on the portion of form-
aldehyde was not monotonous. The expected XRD reflection for the
(002) spacing of g-C3N4 appeared at 2h = 27.5–25.9°, and the spe-
cific surface area increased as 2h decreased. CN stretching band
was always observed by FT-IR. While the prepared CN compounds
4. Conclusion
Hydrogen storage capacity at 77 K increased with specific sur-
face area for BN’s, CN’s and C’s, irrespectively of the elemental
composition, and essentially exceeded the prediction based on
the assumption of the reported two dimensional condensation of
hydrogen with the stripped domain phase, that is, 2.34 mass%
per 1000 m2 gÀ1. This empirical theorem was not valid prema-
turely at 0.8 MPa, where saturation of hydrogen sorption was not
attained, and was not to be applied to estimate the upper limit
of the hydrogen capacity of high specific surface area materials.
While modification with Pd did not increase the hydrogen capacity

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