IN SITU SYNTHESIS OF LiNH2 FROM Li METAL AND LIQUID NH3 IN
NANOPOROUS CARBON: A NEW NANOCONFINEMENT APPROACH
Natchapol Poonyayant1,*, Vitalie Stavila2,#, Natee Angboonpong1, Pasit Pakawatpanurut1,3,
Lennie Klebanoff2
1
Physical Chemistry Program, Center for Alternative Energy and Department of Chemistry,
Faculty of Science, Mahidol University, Bangkok, Thailand
2
Sandia National Laboratories, Livermore, CA 94551
3
Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University,
Bangkok, Thailand
*e-mail: natchapolp@gmail.com, #e-mail: vnstavi@sandia.gov
Abstract
LiNH2 is one of the promising hydrogen storage materials with high hydrogen
capacity and potential for reversibility. However, the bulk material desorbs hydrogen only at
high temperatures and also releases ammonia as a by-product. Nanoconfinement of LiNH2 in
nanoporous carbon was hypothesized to solve these problems by improving the kinetics of
hydrogen release while also suppressing the ammonia formation. A novel in situ synthesis of
LiNH2 confined in nanoporous carbon was demonstrated in this study. Powder X-Ray
Diffraction indicates that the only crystalline phase present is LiNH2. Surface area
measurements by porosimetry confirm the confinement of LiNH2 inside the pore of
nanoporous carbon. Elemental analysis showed a chemical composition of Li0.7N1H2.94 which
suggests that the product was solvated with ammonia. Thermal decomposition of
nanoconfined LiNH2 was monitored using the Thermo Gravimetric Analysis/Differential
Scanning Calorimetry/Mass Spectrometry analysis. The nanoconfined LiNH2 releases
hydrogen at 75oC lower than bulk. This work demonstrates a new versatile approach towards
metal hydride nanoconfinement which does not involve melting or vaporizing the material.
Keywords: LiNH2, nanoconfinement, in situ synthesis, hydrogen storage
Introduction
As one of the cleanest alternative energy source, hydrogen energy has gained
increased attention during the last decade. The only product of hydrogen combustion is water.
It is also compatible with existing engine technologies such as internal combustion engines or
fuel cells. The most challenging obstacle for hydrogen energy in terms of mobile application
is the storage of hydrogen. The conventional hydrogen storage methods such as compressed
or liquefied hydrogen do not provide sufficient gravimetric or volumetric hydrogen density,
resulting in too heavy or too large storage systems [1]. Moreover, those methods require
extremely high tank strength or cryogenic condition making them challenging for mobile
applications. One of the storage methods is to use solid-state materials such as metal
hydrides, which release hydrogen upon thermal decomposition.
Lithium amide (LiNH2) is one of the candidate materials for hydrogen storage, as it
exhibits high hydrogen capacity and has potential for reversibility when used with binary
metal hydrides such as LiH or MgH2. LiNH2 has 9.78 or 6.26 wt% theoretical hydrogen
content, relatively. It was studied by several groups, including Chen et al., who showed
promising hydrogen storage performance of LiN3, decomposition product of LiNH2 and LiH
[2]. In addition to possessing high hydrogen capacity, LiNH2 also showed high reversibility.
Unfortunately, bulk LiNH2 by itself is not a promising hydrogen storage material. Although it
displays high hydrogen content, rather high temperatures are required. The decomposition of
LiNH2 occurs as high as 450oC, yet the rate of decomposition is still rather slow. The material
is also known to release ammonia along with hydrogen, which is detrimental to both capacity
and purity of hydrogen stream.
In order to improve the kinetics of the metal hydride decomposition reaction, an
efficient approach is to increase the surface area of the solid to generate more reacting
surfaces. To push the limits of this improvement, the hydride particle size should be reduced
down to the nanometer scale, sometimes less than 10 nm. This raises a question on how to
create reacting surfaces at nanoscale. One approach is to use an appropriate nanoscale
framework or host. To date solution impregnation and melt infiltration are the methods of
choice to achieve a nanostructured metal hydride inside the pores of a nanoscale framework.
In the work of Z. Li and coworkers, ammonia borane was nanoconfined in porous metalorganic framework (MOF) JUC-32-Y [3]. Ammonia borane was dissolved in methanol and
was impregnated as a solution in the MOF pores. The confined ammonia borane decomposes
70oC lower than bulk material. JUC-32-Y nanoscale template was found to prevent the
formation of ammonia, diborane, and borazine from ammonia borane during hydrogen
desorption. Stavila et al. demonstrated melt infusion of NaAlH4 into MOF-74(Mg) at 195oC
under 25 MPa H2 pressure [4]. The high H2 pressure was used to avoid the decomposition of
NaAlH4 upon heating. The MOF-confined NaAlH4 particles showed large improvement in
desorption kinetics compared to bulk NaAlH4. Vapor deposition can also be used, for
example Stavila et al. showed that TiCl4 can penetrate the MOF-74(Mg) pores in vapor phase
and acts as a catalyst for the rehydrogenation of NaAlH4 [4].
We decided to explore the hydrogen storage characteristics of nanoconfined LiNH2.
Unfortunately, the common synthetic approaches described to date are impractical for LiNH2,
as the material is non-volatile and does not dissolve in common solvents. Even if it could be
dissolved, the remaining solvent would be too hard to remove without decomposing the
hydride. LiNH2 has a high melting point, not to mention the need for high hydrogen pressure
to suppress hydrogen desorption. This inspired us to seek for new approach for
nanoconfinement of LiNH2.
Lagowski showed that Li could be dissolved in ammonia in form of Li+(NH3)x + e(NH3)x [5]. It is known that Li metal slowly reacts with ammonia to form LiNH2. We
hypothesized that Li could be dissolved and impregnated in a porous template; LiNH2 could
be synthesized in situ from Li metal and liquid ammonia inside the pores of an appropriate
framework. The host selected for this purpose was the chemically stable nanoporous carbon
(npC). The advantage of npC lies in its high thermal and chemical stability which prevents
any side reactions with the hydride or hydrogen upon heating. npC pore of ~2 nm was
expected to be small enough to provide nanoconfinement effect. Interestingly, liquid
ammonia served as both solvent for Li and reactant for the LiNH2 synthesis simultaneously.
Moreover, ammonia could be removed from the reaction simply and efficiently at room
temperature resulting in a pure reaction product. We hypothesized that LiNH2 confined in
npC (LiNH2@npC) will exhibit improvement of hydrogen storage properties from
nanoconfinement effect such as lower decomposition temperature and suppression of
ammonia desorption.
Methodology
Synthesis of nanoporous carbon: 90 g of tetraethyl orthosilicate was added in small
portions to a suspension of 45 g poly(ethylene glycol)-block-poly(propylene glycol)-blockpoly(ethylene glycol) (Pluronic P123) in 300 mL of 0.5 M hydrochloric acid solution and the
mixture was stirred for 4 hours. The resulting gel was aged at 60oC for 16 hours and heated in
the oven at 90oC for 4 hours. After this, the obtained solid was calcined in an oven in air at
600oC for 16 hours to decompose the triblock copolymer and form mesoporous silica. The ascalcined silica was added to 200 mL 50 wt% solution of sucrose in water and the mixture was
stirred for 30 min. The solvent was removed using a rotary evaporator and all the resulting
solid was calcined at 600oC under nitrogen for 16 hours to form a solid block which was
fractured into smaller pieces. In a plastic beaker, the resulting mesoporous silica/carbon
composite was treated with excess 2 wt% HF solution in water to dissolve the silica. The
solid was washed 5 times with EtOH to remove excess acid. Finally, the product was dried in
vacuum (20 mTorr) at 250oC for 16 hours to yield activated nanoporous carbon.
In situ synthesis of nanoconfined LiNH2: In a typical synthesis of LiNH2@npC,
0.360 g of Li metal (99%, Aldrich) was placed in a Schlenk tube containing 3.109 g activated
npC inside of a glove-box. The tube was then cooled in dry ice/acetone bath. In continuous
flow of Ar, anhydrous ammonia gas (Matheson, Ultra-High purity) was fed into the tube until
it condensed ~20 mL liquid and covered all Li and npC solid. The bath was kept cold for 30
min. After that, the reaction flask was left under a constant Ar flow so that liquid ammonia
could slowly evaporate while the temperature increased to room temperature overnight.
Excess ammonia gas was trapped with HCl acid before it was vented. LiNH2@npC was
recovered and weighted in a glove-box.
Characterization: The samples were grounded and loaded into glass capillaries
which were then sealed with vacuum grease in a glove-box for Powder X-Ray Diffraction
(PXRD) characterization. The measurements were performed on a Panalytical Empyrean
diffractometer equipped with a Cu target. Gas adsorption experiments were performed on a
QuantaChrome Quadrasorb-SI porosimeter. Simultaneous Thermo Gravimetric
Analysis/Differential Scanning Calorimetry/ Mass Spectrometry (TGA/DSC/MS)
experiments were performed on Mettler TGA/DSC-1 STARe system equipped with Pfeiffer
Mass Spectrometer. In each TGA/DSC/MS experiment ~ 10 mg of sample was sealed in
aluminum crucible under Ar atmosphere. It was heated from 30 – 600oC with 5oC/min
heating rate under Ar flow of 5 ml/min. Elemental analysis was done at ALS Environmental.
Results
Nanoporous Carbon (npC) was used as a host for LiNH2 mainly due to its chemical
inertness. It comprises of more than 90 wt% carbon making it stable up to temperatures in
excess of 600oC. It has a higher chemical and thermal stability compared to MOFs. This
allows us to synthesize LiNH2 with npC without the doubt if there would be any side
reactions with the template. It also has highly porous structure with the pore size of ~ 2 nm in
diameter, which makes it convenient to study the nanoconfinement effects in metal hydrides.
LiNH2 was synthesized in situ from Li metal and liquid ammonia with the presence of
npC as shown in Scheme 1. Li metal was dissolved by liquid ammonia and was absorbed into
the nanoporous structure of the npC.
Scheme 1 Reaction of Li metal and liquid ammonia in the presence of npC
The initial calculated Li metal amount was 20.56% of total pore volume and was equivalent
to the volume of LiNH2 that can fill 30.62% of npC pore volume. This was done to minimize
the formation of bulk LiNH2 outside of npC pores. Liquid ammonia was maintained for 30
minutes and the Schenk tube was shook occasionally to make sure Li was completely
dissolved. When liquid ammonia was evaporated, solvated Li(NH3)x was converted into
LiNH2 in the npC pores yielding nanoconfined LiNH2. Initial Li metal pieces were not
observed in the final product which is a sign that Li is infiltrated in the npC pores. Also the
nanoconfined npC was more brittle than original npC which might be resulted from LiNH2
penetration into npC structure. The surface of npC was lightly covered with white powder
that is possibly LiNH2. PXRD patterns of nanoconfined npC (Figure 1) showed LiNH2 peaks
as the only crystalline phase in the sample. The PXRD results confirm the successful
Intensity (a.u.)
synthesis of LiNH2 in the presence of npC. The PXRD pattern of nanoconfined LiNH2
matched that of bulk LiNH2. This crystalline peaks could be from ordered LiNH2 inside the
pores or alternatively from material outside the pores.
(b)
(a)
10
20
30
40
50
2θ
60
70
80
90
Figure 1 PXRD patterns of (a) LiNH2 and (b) LiNH2@npC
To determine where LiNH2 is localized, gas adsorption experiments were performed
and results are shown in Table 1. From these results that the surface area and pore volume of
npC decreased by 77% indicates that the synthesized LiNH2 was mostly in the pore of npC.
Due to the fact that interior surface area of npC is tremendously larger than its exterior
surface area, it is reasonable to say that most of LiNH2 formed in the porous structure of npC
rather than forming as bulk LiNH2. Technically LiNH2 was supposed to fill only 30% of total
pore volume according to initial Li loading. A reasonable explanation behind this discrepancy
can be that the final product LiNH2 only filled the npC pores partially and blocked some
accessible pore volume, leaving some void in the pores, which could result in more pore
volume filling than expected. Loading of LiNH2 was calculated from the summary of Li, N,
and H wt% from elemental analysis (Table 1). Empirical formula of Li-N-H compound in the
nanoconfined sample was also derived from elemental analysis. The ratio of N to Li is more
than 1:1 suggesting that there could be some solvated NH3 left in the sample. The temperature
for removing the solvated NH3 should not release hydrogen from LiNH2@npC. To
investigate the LiNH2@npC decomposition, simultaneous TGA/DSC/MS was used.
Table 1 BET surface area and total pore volume from gas adsorption experiments, loading and empirical
formula from elemental analysis
Sample
Activated npC
LiNH2@npC
BET surface area
(m2/g)
1058.1
241.2
Total pore volume
(mL/g)
1.062
0.238
Loading
(wt%)
28.33
Empirical
Formula
Li0.70N1H2.94
Thermal decomposition of nanoconfined sample was studied in comparison to bulk
LiNH2 using TGA/DSC/MS (Figure 2). This simultaneous technique allows us to correlate
sample weight loss, enthalpy of reaction, and type of released gas. There was no weight loss
from bulk LiNH2 until 330°C where it started losing 2.2 wt% as NH3. DSC showed sharp and
large endothermic peak at 362°C for this weight loss. The loss of large 23.8 wt% in the
temperature range of 455 °C to 600°C was from H2 release along with NH3. This step started
with a sharp endothermic peak followed by multiple exothermic peaks.
LiNH2@npC released NH3 at low temperature ranged from 65 – 350 °C correspond to
8.8 wt% loss from TGA. Considering 28.33 wt% LiNH2 loading, the weight loss was
equivalent to 31.06 wt% from only the LiNH2 component. The broad peak and the low
temperature of desorption suggested the NH3 loss should be from solvated NH3. In addition,
if it were a reaction, there should have been a corresponding DSC peak present. Surprisingly,
some H2 gradually evolved at this rather low temperature region as well. The reason for this
low temperature H2 release may be the altered thermodynamics of the reaction due to
nanoconfinement effects. Interestingly, LiNH2@npC released H2 at 380 – 485 °C along with
NH3. This desorption started 75°C lower than bulk LiNH2, and finished more than 100 °C
lower than bulk. Moreover, the total weight loss in 380 – 485 °C was 2.2 wt%, which was
equivalent to 11.30 wt% loss from only LiNH2. Compared to bulk LiNH2, the weight loss
was reduced by about half. This suggests that not only nanoconfinement can improve kinetics
of LiNH2 decomposition, but it might also reduce the ammonia desorption. The DSC features
of the LiNH2@npC sample were weaker than those of bulk LiNH2, which may be due to high
npC content; however, it still showed signs of exothermic reactions in the temperature range
corresponding to H2 desorption from LiNH2@npC.
Weight Loss (wt%)
100
95
(a)
90
85
80
75
Heat Flow (Wg^-1)
oC
70
oC
0 5 50 15100 25150 35200 45 250 55 300 65 350 75 400 85 450 95 500 105550 115600
2 ^ exo
0
-2
(b)
-4
-6
-8
o
-10
0 5 50 15100 25150 35200 45250 55300 65 350 75 400 85 450 95 500 105550 115600
C
oC
0
Intensity (a.u.)
(c)
o
5 50 15100 25150 35200 45250 55300 65350 75400 85450 95500 105550 115600
C
oC
(d)
o
0
5 50 15100 25150 35200 45250 55300 65350 75400 85450 95500 105550 115600
C
oC
Figure 2 (a) TGA (b) DSC (c) MS m/z = 2 (d) MS m/z = 17 of LiNH2 (solid) and LiNH2@npC (dash);
the heating rate is 5°C/min
Discussion and Conclusion
Nanoporous carbon was found to be a suitable host for the in situ synthesis of
nanoconfined LiNH2 due to its high thermal and chemical stability. The pore size of npC was
small enough to provide nanoconfinement effect in the scale at which LiNH2 reaction
differed largely from bulk. In situ synthesis of LiNH2 from Li metal and liquid NH3 in the
presence of npC was successful with PXRD results showing LiNH2 was present. LiNH2 was
confirmed to be nanoconfined within npC pores from gas adsorption results showing decrease
in total pore volume. Elemental analysis and TGA/DSC/MS measurements indicate that there
is solvated ammonia remaining in the nanoconfined material. The change in both H2 and NH3
desorption profiles suggests that the nanoconfinement in npC could significantly improve the
kinetics of LiNH2 decomposition and reduces the H2 desorption temperature by 75oC.
TGA/DSC/MS data show a 11.30 wt% loss from LiNH2@npC, approx. half of the wt% from
bulk LiNH2. This clearly indicates a decrease in the amount of ammonia desorbed as a result
of nanoconfinement. However, quantification of H2 and NH3 needs to be done in order to
determine hydrogen storage capacity and ammonia concentration in the hydrogen stream.
This proposed synthetic approach represents a new synthetic pathway towards
nanoconfinement. The new in situ method could be applied for the nanoconfinement of other
amide materials in appropriate hosts.
References
1. Klebanoff L, editor. Hydrogen storage technology. Boca Raton: Taylor and Francis; 2012.
2. Chen P, Xiong Z, Luo J, Lin J, Tan KL. Interaction of hydrogen with metal nitrides and imides. Nature.
2002;420(6913):302-4.
3. Li Z, Zhu G, Lu G, Qiu S, Yao X. Ammonia Borane Confined by a Metal−Organic Framework for
Chemical Hydrogen Storage: Enhancing Kinetics and Eliminating Ammonia. J Am Chem Soc.
2010;132(5):1490-1.
4. Stavila V, Bhakta RK, Alam TM, Majzoub EH, Allendorf MD. Reversible Hydrogen Storage by NaAlH4
Confined within a Titanium-Functionalized MOF-74(Mg) Nanoreactor. ACS Nano. 2012;6(11):9807-17.
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Acknowledgements: The authors acknowledge Ken Stewart and Joe Cordaro (Sandia
National Laboratories) for their technical assistance. Poonyayant, Angboonpong, and
Pakawatpanurut acknowledge support from the Development and Promotion of Science and
Technology talents project, the Center of Excellence for Innovation in Chemistry (PERCHCIC), and the Distinction Program, Faculty of Science, Mahidol University. This work was
supported by the U.S. Department of Energy, Office of Efficiency and Renewable Energy,
under Award Nos. DE-AC04-94AL8500. Partial support for this work from The Boeing
Company is also greatly appreciated. The authors thank Joe Breit from Boeing for his
support of this work.