Nanocatalyst doping of Zn(BH4)2 for on-board hydrogen storage

Article abstract of DOI:10.1016/j.jallcom.2007.08.028

In this work, we report the synthesis and characterization of Zn(BH₄)₂, a new class of complex borohydrides for on-board hydrogen storage. The thermal decomposition of Zn(BH₄)₂ comprises of not only the evolution of H₂, but also an appreciable amount of B-H (borane) compounds. Lowering the decomposition temperature by catalytic doping may lead to negligible release of boranes. An amount of 1.5 mol% nanoNi was estimated and found to be the optimum concentration for nanocatalyst doping of Zn(BH₄)₂. Significance of the nanoNi doping, lowers the melting and thermal decomposition temperatures (at least 20-40 °C) of Zn(BH₄)₂ as evidenced from the calorimetric analysis. At these low temperatures, the nanocatalyzed Zn(BH₄)₂ exhibits reduction in the amount of borane gases released by a factor of 20 as compared to the undoped sample.

Full text of DOI:10.1016/j.jallcom.2007.08.028

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Journal of Alloys and Compounds 462 (2008) 294–302
Nanocatalyst doping of Zn(BH4)2 for on-board hydrogen storage
Sesha Srinivasan∗, Diego Escobar, Michael Jurczyk, Yogi Goswami, Elias Stefanakos
Clean Energy Research Center, College of Engineering, University of South Florida, Tampa, FL 33620, USA
Received 20 April 2007; received in revised form 9 August 2007; accepted 10 August 2007
Available online 19 August 2007
Abstract
In this work, we report the synthesis and characterization of Zn(BH4)2, a new class of complex borohydrides for on-board hydrogen storage.
The thermal decomposition of Zn(BH4)2 comprises of not only the evolution of H2, but also an appreciable amount of B–H (borane) compounds.
Lowering the decomposition temperature by catalytic doping may lead to negligible release of boranes. An amount of 1.5 mol% nanoNi was
estimated and found to be the optimum concentration for nanocatalyst doping of Zn(BH4)2. Significance of the nanoNi doping, lowers the melting
and thermal decomposition temperatures (at least 20–40 ◦C) of Zn(BH4)2 as evidenced from the calorimetric analysis. At these low temperatures,
the nanocatalyzed Zn(BH4)2 exhibits reduction in the amount of borane gases released by a factor of 20 as compared to the undoped sample.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Zinc borohydrides; Hydrogen storage; Mechanochemical synthesis; Thermogravimetric analysis; Complex hydrides; Dehydrogenation; Nanocatalyst
doping
1. Introduction
A new transition metal assisted complex borohydride,
Zn(BH4)2, was recently reported to have a high theoretical
hydrogen storage capacity of 8.4 wt.% at the thermal decom-
position temperature of 85 ◦C [14]. Zn(BH4)2, a new class of
complex borohydrides is not only having highly chemoselective
reduction properties, but is also considered to be the promising
material for on-board storage of hydrogen [15]. Zinc borohy-
dride is unique because (i) Zn2+ is a soft Lewis acid as compared
to Ca2+, Li+ and Na+ and (ii) Zn2+ has a better coordinating abil-
ity and is thus expected to impart selectivity in hydride transfer
reactions. The wet chemical synthesis by metathetic reaction of
freshly fused ZnCl2 and Na(Li)BH4 in ultra dry THF, results in
the formation of Zn(BH4)2 (see Eq. (1)) solutions [16].
Light weight, low cost, highly reversible hydrogen storage
systems are indeed essential for realizing the low temperature
PEM fuel cell powered vehicles [1,2]. The breakthrough dis-
covery of Ti-catalyzed NaAlH4 [3,4] for the reversible on-board
hydrogen storage may not be the ideal system to attain the DOE
2010 and FreedomCAR technical targets [5,6]. This is because
for NaAlH4, the usable hydrogen storage capacity achievable
is 5.4 wt.%, which is well below the set DOE goals. On the
other hand, the borohydride complexes NaBH4 and LiBH4 pos-
sess high hydrogen storage capacity of 13.0 wt.% and 19.6 wt.%,
respectively [7]. However, the release of hydrogen from NaBH4
is possible only by hydrolysis (reaction with H2O) and this pro-
cess is irreversible [8]. For the case of LiBH4, the catalytic
addition of SiO2, significantly enhances its thermal desorption
at 200 ◦C [9]. The addition of various metal oxides such as TiO2,
V2O5, etc. has been shown to improve the reversible hydrogen
sorption characteristics of LiBH4 [10]. In general, the thermal
decomposition and/or recombination of hydrogen either from
NaBH4 or LiBH4 are difficult to achieve because of the thermo-
dynamic stability due to strong B–H interactions [11–13].
2Na(Li)BH4 + ZnCl2 → Zn(BH4)2 + 2Na(Li)Cl
(1)
However, the active hydrogen content is below 6.5 wt.% due
to partial release of hydrogen during the THF extraction pro-
cess. An inexpensive mechanochemical approach of ball milling
the stoichiometric mixtures of ZnCl2 and NaBH4 yields the
formation of Zn(BH4)2 in the solvent free solid state [17–19].
The present paper aims to systematically study the synthesis
and characterization of new Zn-based complex borohydrides,
Zn(BH4)2, for the efficient hydrogen storage. The active hydro-
gen content of 8.4 wt.% has been obtained at 100 ◦C for the
hydride mixture (2NaBH4 + ZnCl2) milled for only 30 min; this
∗
Corresponding author. Tel.: +1 813 974 0759; fax: +1 813 974 2050.
E-mail address: sesha@eng.usf.edu (S. Srinivasan).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.08.028

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295
capacity however decreases with increased milling duration.
Besides, nanocatalyst doping is carried out in order to lower
the decomposition temperature of Zn(BH4)2. For the case of
the complex mixture, doped with 1.5 mol% nanoNi, a com-
plete weight loss of 10 wt.% is achieved at temperatures below
91 ◦C. The thermal decomposition characteristics of Zn(BH4)2
exhibit an endothermic reaction process due to melting transi-
tion and to release of hydrogen at this temperature range. The
structural, bonding, microstructural and chemical characteris-
tics of Zn(BH4)2 have been explored by XRD, FTIR, SEM and
EDS techniques. The gas quantification analysis at the thermal
decomposition temperatures of the samples were examined via
GC/MS technique.
was added during the milling process for the synthesis of nanocatalyst doped
Zn(BH4)2.
2.2. XRD and FTIR characterization
The powder X-ray diffraction of the mechano-chemically milled complex
borohydride has been carried out by the Philips X’pert diffractometer with Cu
˚
K␣ radiation of λ = 5.4060 A. The incident and diffraction slit width used for the
measurements are 1◦ and 2◦ respectively. The Zn-borohydride sample holder has
been covered with polyethylene tape (foil) with O-ring seal in an N2 filled glove
box in order to avoid the O2/moisture pickup during the XRD measurements.
The diffraction from the tape was calibrated without the actual sample and found
to be occurring at the 2θ angles of 22◦ and 24◦, respectively. The XRD phase
identification and particle size calculation has been carried out using PANalytical
X’pert Highscore software, version 1.0f [21].
The B–H bond stretch of the NaBH4 and Zn(BH4)2 were compared via
Perkin-Elmer Spectrum One FTIR spectrometer. This instrument operates in a
single-beam mode and is capable of data collection over a wavenumber range
of 370–7800 cm−1 with a resolution of 0.5 cm−1. The complex borohydride
samples were pelletized and sealed in a specially designed KBr cell [22] for
Infrared measurements.
2. Experimental details
2.1. Materials and method
Starting materials such as NaBH4 (98%) and ZnCl2 (99%) are obtained
from Sigma–Aldrich and Nano Nickel (hereafter nanoNi; 99.999%; 3–10 nm;
Fig. 1) is obtained from Quantum Sphere Inc., they were used without fur-
ther purification. High purity H2 (99.9999%), N2 (99.99%) and He (99.99%)
are procured from Airgas Inc. for the synthesis and analytical measurements.
All chemical reactions and operations are performed in a nitrogen filled glove
box (Innovative Technologies Inc.). NaBH4 and ZnCl2 with 2:1 mole ratio was
mixed in a stainless steel bowl (80 ml) and a lid sealed with viton O-ring in
the glove box. The bowl was then evacuated for at least 1 h to remove the
residual oxygen and moisture down to ppm levels. A specially designed lid
[20] with inlet and outlet valves were used for this purpose. The mechano-
chemical process employing high energy milling has been carried out by Fritsch
pulversette planetary mono mill, P6 in an inert atmosphere. The milling param-
eters such as ball to powder weight ratio and milling speed are optimized to
20:1 and 300 rpm, respectively. Milling duration has been varied as 20 min,
30 min, 1 h, 2 h, 4 h and 8 h. Thus, mechanochemically processed complex
hydrides were immediately transferred to the glove box for further character-
ization measurements. In a similar way, few mole concentrations of nanoNi
2.3. Microstructural and chemical analysis
The microstructures of the Zn(BH4)2 in the different stages (before and
after hydrogen sorption) were observed by Hitachi S800 scanning electron
microscope (SEM) and local phase composition was determined in the energy
dispersive X-ray spectrometry (EDS) mode using the same instrument. A fixed
working distance of 5 mm and a voltage of 20 kV were used. Samples prepara-
tion for the SEM measurement was carried out inside the glove box by covering
the sample holder with parafilm for minimal exposure to oxygen while transfer-
ring in to the secondary emission chamber. EDAX Genesis software was used
to analyze the SEM images and EDS spectra.
2.4. Simultaneous gravimetric and calorimetric measurements
The simultaneous DSC and TGA (SDT) analysis pertaining to the weight
loss and the heat flow for the reaction enthalpy during thermal decomposition
of undoped and nanocatalyst doped Zn(BH4)2 was estimated by using the TA
instrument’s SDT-Q600 analytical tool. The calibration of SDT was performed
in four steps with empty pan and standard sapphire disc. The four calibration
subroutines such as TGA weight, DTA baseline, temperature and DSC heat flow
were carried out before an actual measurement of the sample. A pre-weighed
sample was loaded into the ceramic pan and covered with the ceramic lid inside
thegloveboxtopreventthemoisturefromgettingintothesampleduringtransfer.
The ramp rate of 5 ◦C/min was used for all the measurements. TA’s Universal
Analysis 2000 software program was used to analyze the TGA and DSC profiles.
2.5. Dehydrogenation kinetics measurements
The isothermal volumetric measurements were carried out by Hy-Energy’s
PCTPro 2000 sorption equipment. This fully automated Sievert’s type instru-
ment uses an internal PID controlled pressure regulator with maximum pressure
of 170 bar. It also includes five built-in and calibrated reservoir volumes of 4.66,
11.61, 160.11, 1021.30and1169.80 ml. Thevolumecalibrationwithoutandwith
the sample was performed at a constant temperature with an accuracy of 1 ◦C
using a helium gas. The software subroutines for hydrogen purging cycles, leak
test, kinetics, PCT and cycling etc were performed by the HyDataV2.1 Lab-
View program. The data collected for each run were analyzed using the Igor Pro
5.03 program with a built in HyAnalysis Macro.
2.6. Dehydrogenation and gas analysis measurements
Temperature programmed desorption (TPD) technique was carried out by
Quantachrome Instrument’s Autosorb-1 equipment. The carrier gases used for
the TPD and TPR measurement were nitrogen and argon:hydrogen (95:5%)
Fig. 1. Transmission electron micrograph of nickel nanoparticle supplied by
Quantum Sphere Inc. (photo: courtesy by Quantum Sphere Inc.).

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296
S. Srinivasan et al. / Journal of Alloys and Compounds 462 (2008) 294–302
pre-mixed gases, respectively. The thermal desorption/reduction profiles were
recorded and analyzed using TPRWIN software package.
The thermal decomposition behaviors of the samples and gas quantification
analysis were examined by a Varian Saturn triple quadrupole (320-MS) GC/MS
upon heating up to 150 ◦C with a heating rate of 5 ◦C/min under a helium flow.
The gas analysis and plotting of the curves were carried out by Saturnview
Version 5.52 software.
3. Results
3.1. XRD and SEM-EDS of undoped and nanoNi doped
Zn(BH4)2
Fig. 2 represents the powder X-ray diffraction patterns of the
complex mixture (2NaBH4 + ZnCl2) mechanically milled under
a N2 ambient for different time durations of 30 min, 2 h, 4 h and
8 h. The major peaks correspond to NaCl and Zn(BH4)2. It is
interesting to note that no parent compounds corresponding to
NaBH4 or/and ZnCl2 are present in the mechano-chemically
milled complex mixture. This ensures the formation of a new
complex borohydride structure Zn(BH4)2 with a milling dura-
tion of 30 min. The full width at half maximum (FWHM) of the
NaCl reflection increases with an increase in milling duration
from 30 min to 8 h. Scherrer’s equation was used to determine
the crystallite sizes in the range 25–40 nm, depending on the ball
milling duration. The disappearance of Zn-complex borohydride
peaks at higher milling duration confirms the partial desorption
of hydrogen during the ball milling process.
The Zn(BH4)2 + NaCl mixture (see Eq. (1)) after filtration
and further purification under THF allow us to extract NaCl from
the mixture. The XRD of the extracted salt (see Fig. 3) exhibits
the pure crystalline phases of NaCl and no complex borohydride
peaks. This is further confirmed by the comparative elemental
analysisbeforeandafterpurification, bySEM-EDSandisshown
in Fig. 4. This suggests that 100% pure Zn-borohydride could
be possible to synthesize by our mechano-chemical approach
followed by subsequent purification. Zn(BH4)2 after dehydro-
genation and rehydrogenation cycles shows the presence of only
Zn peaks and no boron either in nanocrystalline or amorphous
forms. This indicates the boron is consumed during the decom-
Fig. 3. X-ray diffraction patterns of (i) Zn(BH4)2/NaCl mixture obtained after
ball milling 20 min, (ii) after NaCl extraction and (iii) after PCT dehydrogenation
position process to release B2H6 gases as per the Eq. (2):
Zn(BH4)2 → Zn + B2H6 + H2
(2)
The XRD profiles shown in Fig. 5 correspond to the mixture
of 2NaBH4 + ZnCl2 + X mol% nanoNi (X = 0.5, 1.0, 1.5, 2.0, 2.5)
ball milled under an ambient of N2 gas for 20 min. The presence
of NaCl and Zn(BH4)2 remains the same at the different con-
centrations and no presence of Ni was found, except for the
2.5 mol% nanoNi doped sample. The XRD pattern exhibits a
small peak at 43.8◦ and is attributed to the presence of the Ni
dopant having the grain size ∼3–10 nm.
3.2. FTIR spectra of undoped and nanoNi doped Zn(BH4)2
The B–H bonding environment of Zn(BH4)2 was determined
by FTIR spectroscopic technique as shown in Fig. 6. The FTIR
−
spectra of BH4 ion in NaBH4 has a characteristic band at
2283 and 2217 cm−1, whereas in the Zn(BH4)2/NaCl ball milled
mixture, it occurs at around 2089 and 2450 cm−1, respectively.
These values (wavenumbers) correspond to the bonding and
terminal B–H bands in Zn(BH4)2 structure.
The FTIR spectra shown in Fig. 6 also correspond to the
mixture of the undoped and doped with 1.5 mol% nanoNi
Zn(BH4)2/NaCl. The FTIR profile for the doped sample is
almost the same as the undoped sample, showing the presence
of a band from B–H stretching between 2000 and 2500 cm−1
,
a Zn–H stretching around 1400 cm−1, and a BH2 deformation
around 1100 cm−1, these bands confirm that no major change in
the bonding environment after nanocatalyst doping.
3.3. Thermo gravimetric and calorimetric analysis of
undoped and nanoNi Zn(BH4)2
The TGA and DSC of the undoped complex hydride
mechano-chemically prepared with different milling durations
are shown in Figs. 7 and 8, respectively. A theoretical hydrogen
storage capacity of 8.4 wt.% at 100 ◦C has been obtained for
Zn(BH4)2 milled only for 20–30 min. However, this capacity
decreases as the milling duration increases. This is apparently
Fig. 2. X-ray diffraction patterns of a Zn(BH4)2/NaCl mixture obtained after
milling under N2 ambient for 30 min, 2 h, 4 h and 8 h.

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S. Srinivasan et al. / Journal of Alloys and Compounds 462 (2008) 294–302
297
Fig. 4. SEM-EDS profiles of Zn(BH4)2 before and after THF extraction and purification.
Table 1
Gravimetric weight loss with milling time of Zn(BH4)2
Ball milling time
Weight loss (%)
20 min
30 min
2 h
4 h
8 h
8.5
8.45
6.674
6.382
5.22
duetothe partialreleaseofhydrogenduringthe extendedmilling
process; say from 4 to 8 h. The weight loss due to thermal
decomposition of the Zn(BH4)2 ball milled with different time
durations is given in Table 1. An endothermic reaction due to
melting transition and gas decomposition were observed from
the DSC profiles as represented in Fig. 8.
The TGA and DSC patterns for the undoped and nanoNi
doped samples are given in Figs. 9 and 10, respectively. The
TGA/DSC analysis exhibits an alteration and lowering in the
Fig. 5. XRDpatternsofZn(BH4)2/NaClmixture, dopedwithX mol%ofnanoNi,
(X = 0.5, 1.0, 1.5, 2.0 and 2.5) ball milled 20 min.
Table 2
DSC and TGA analysis of undoped and nanoNi doped Zn(BH4)2
X mol% nanoNi doped Zn(BH4)2
DSC melting
transition (◦C)
TGA on-set decomposition
temperature (◦C)
TGA peak decomposition
temperature (◦C)
Weight loss (%)
Undoped
0.5
1.0
1.5
2.0
93
93
92
90
86
86
100
75
80
70
72
70
115
98
94
91
92
90
9.5
10.5
8.5
10.0
7.4
2.5
9.5