Chemical Reactions of Amides and Hydrides-A Methodology for
Synthesis of Hydrogen Storage Materials
XIONG Zhitao, WU Guotao, HU Jianjiang, LIU Yongfeng, CHEN Ping*
Physics Department, National University of Singapore
2 Science Drive 3, Singapore 117542
email: [email protected]
Introduction
There are strong pushes towards hydrogen economy worldwide. To overcome one of the technical barriers - onboard hydrogen storage,
great efforts have been devoted to the development of solid-state materials. NITRIDE and IMIDE possess strong affinity towards hydrogen,
which enables them as potential storage medias. Our discovery on amide-hydride system designed for reversible hydrogen storage was
regarded as a big step forward over the promise of clean hydrogen energy.
First amide-hydride system, Li-N-H system, was proposed for hydrogen
storage based on the discovery of hydrogenation of Li3N
Hydrogenation Li3N + 2H2→ LiNH2 + 2LiH
11.4wt%
Dehydrogenation LiNH2 +2LiH ↔ Li2NH + LiH + H2
5.41wt%
Dehydrogenation 2LiNH2 + MgH2→ Li2Mg(NH)2 + 2H2
5.56wt%
Hydrogenation & dehydrogenation cycle
thus, starting from hydrogenation of Li2NH
Li2NH+ H2 ↔ LiNH2 (amide) + LiH
Second amide-hydride system, Li-Mg-N-H system, was found by substituting
lithium hydride with magnesium hydride and so far it’s the most promising
system for hydrogen storage
6.89wt%
(hydride)
Li2Mg(NH)2 + 2H2 ↔ Mg(NH2)2 (amide) + 2LiH (hydride)
5.88wt%
of 1bar, temperature of 285oC is applied.
Advantage: Temperature for hydrogen desorption decreased to 180oC with
plateau pressure enhanced to above 20bar. Fast kinetics, 80% of H2 can be
hydrogenated & dehydrogenated in 1 hour.
P. Chen, Z. Xiong, et.al., Nature 420 (2002) 302-304
Z. T. Xiong, G. T. Wu, J. J. Hu and P. Chen, Adv. Mater. 2004, 16, 1522
Drawback: to reach the plateau pressure for hydrogen desorption
half hydrogenated
Abs
Des
1
0.1
100
a
Abs
10
20
30
40
10
1
0.1
20
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
2 Theta
40
60
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
80
2 Theta
H: Li3N / Li2NH
Other successful amide-hydride system including:
H: Li2Mg(NH)2
Pressure-Composition (P-C)
Isotherms of Li2Mg(NH)2
samples at temperature 180oC.
XRD patterns of
hydrogenated&dehydrogenated
Li2Mg(NH)2 sample
Pressure-Composition (P-C)
Isotherms of Li3N (a) and Li2NH
(b) samples at temperature 250oC.
XRD patterns of
hydrogenation&dehydrogenation
over Li3N
100
dehydrogenation
0.1
50
hydrogenation
Des
1
Pristine Li3N
1000
Li2Mg(NH)2
Mg(NH2)2
LiH
Pressure (psi)
fully hydrogenated
10
Intensity (a.u.)
dehydrogenated at 270oC
10
b
100
Pressure (psi)
Intensity (a.u.)
1000
Li3N
Li2NH
LiNH2
LiH
Simple mechanism proposed to explain interactions between Amides and Hydrides
δ+
H2 amount for cycle
Amide
Hydride
LiNH2
CaH2
2.71wt%
Mg(NH2)2
NaH
2.17wt%
δ-
H
H─M’
M─N
MM’NH + H2
H
LiNH2
LiAlH4
Mg(NH2)2
MgH2
H2 can only be released
Mg(NH2)2
CaH2
H2 can only be released
2.3wt%
H atoms attached to N normally possess positive charges, however, H in ionic
hydrides have negative one. The strong chemical potential for the combination of H+
and H- is one of the important driving forces!
Z. T. Xiong, G. T. Wu, J. J. Hu and P. Chen, Adv. Mater. 2004, 16, 1522
By changing amide or hydride, new reactions and new materials may
be discovered.
Z. Xiong, J. Hu, G. Wu and P. Chen, J. Alloy. Compd, 2005, 395, 209-212
c
d
0 100 200 30 0 40 0 5 00 6 00
Temperature
(oC)
Temperature Programmed
Desorption (TPD) spectra of post
milled Ca(NH2)2-NaH 1/1 a) H2
signal; b) NH3 signal with c) pure
Ca(NH2)2 and d) NaH included as
comparison
0.8
DSC
&
the heat of desorption was
calculated to be ~55 kJ/mol-H2
Release
0.6
*
&
&
&
&
NaH
NaNH2
CaNH-like
hydrogenation
*
*
*
3230
3210
c
b
a
3143
3074
dehydrogenation
0.4
0.2
3260
3294
& Ca(NH2)2
Intensity (a.u.)
b
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
-0.9
1.0
Intensity (a.u.)
a
H2 amount (wt%)
Intensity (a.u.)
210
(mW/mg)
←Endo Exo→
Ca(NH2)2-NaH system was investigated recently for reversible hydrogen storage
Soak
0.0
0
100
200
Temperature
300
20
40
60
80
2 Theta
(oC)
DSC and Volumetric measurement of
hydrogen desorption/absorption over
Ca(NH2)2-NaH system
XRD patterns of hydrogenated & dehydrogenated
Ca(NH2)2-NaH sample
Reaction formula can be written as:
1H atom per 1Ca(NH2)2-1NaH
molecule can be reversibly stored
Ca(NH2)2 + NaH ↔ NaNH2 + Ca-N-H solid solution
3600
3400
3200
Wave Number
3000
(cm-1)
FTIR spectra of: a) dehydrogenated b)
subsequently hydrogenated and c) again
dehydrogenated Ca(NH2)2-NaH sample
Dehydrogenation:
NaNH2 with signal at 3260, 3210, 3074 cm-1, CaNH-like structure 3143cm-1
Hydrogenation:
Ca(NH2)2 with characteristic N-H vibration at 3294 and 3230cm-1
Acknowledgement
This work is financially supported by Agency for Science, Technology and Research (A*STAR), Singapore.