This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 8889–8893 8889 Cite this: Phys. Chem. Chem. Phys., 2011, 13, 8889–8893 Investigation of hydrogen absorption in Li 7 VN 4 and Li 7 MnN 4 Guang He, a J. F. Herbst,* b T.N. Ramesh, a F. E. Pinkerton, b M. S. Meyer b and Linda Nazar* a Received 16th December 2010, Accepted 1st February 2011 DOI: 10.1039/c0cp02892d The hydrogen storage properties of Li 7 VN 4 and Li 7 MnN 4 were investigated both by experiment and by density functional theory calculations. Li 7 VN 4 did not sorb hydrogen under our experimental conditions. Li 7 MnN 4 was observed to sorb 7 hydrogen atoms through the formation of LiH, Mn 4 N, and ammonia gas. An applied pressurized mixture of H 2 /Ar and H 2 /N 2 gases was helpful to mitigate the release of NH 3 but could not prevent its formation. The introduction of N 2 also caused weight gain of the sample by re-nitriding the absorbed products LiH and Mn 4 N, which correlated with the presence of Li 2 NH, LiNH 2 , and Mn 2 N detected by X-ray diffraction. While our observed results for Li 7 VN 4 and Li 7 MnN 4 differ in detail, they are in overall qualitative agreement with our theoretical work, which strongly suggests that both compounds are unlikely to form quaternary hydrides. Introduction Onboard hydrogen storage has become a critical technology for the development of fuel cell vehicles (FCVs). 1–4 When various storage systems are compared, solid-state materials are advantageous as they possess a higher volumetric energy density than either compressed gas or liquid hydrogen systems. However, despite decades of studies, no solid state hydrogen storage materials satisfactorily meet the requirements for application to FCVs, such as high storage capacity and fast sorption kinetics in the range of 1–10 bar and 25–120 1C. 5 Conventional transition metal alloys, including the La–Ni (AB 5 ) system, Ti–Cr (AB 2 ) system and Ti–Fe (AB) system, 6 have the desired intermediate thermodynamic affinities for hydrogen. The hydrogen gas molecules split into atoms at the surface of the metal and then enter the metallic lattice in the atomic form, diffuse through the metal, jump between interstitial sites; and finally form a hydride phase with a more or less ordered hydrogen sublattice. The hydrogen s electron can be partly donated to the metal conduction band if the metallic lattice contains d or f electron states at the Fermi level. Hence these bare protons can move relatively freely through the metal lattice due to the electrostatic force screened by electrons at the metal Fermi surface. In the desorption process, the opposite takes place; i.e. two hydrogen atoms recombine to form H 2 again. These systems have better kinetics than complex hydrides, but their low gravimetric capacities and high costs prevent vehicular applications. In recent years alternative hydrogen storage systems have been examined, consisting of light alkali and alkaline earth metal nitrides and/or borides with improved storage capacity. A well-known and highly promising example is lithium nitride, which remarkably stores up to 11.4 wt% hydrogen according to the equation: 7 Li 3 N + 2H 2 2 Li 2 NH + LiH + H 2 2 LiNH 2 + 2LiH. One of the major limitations of this system is that the high storage capacity can only be achieved at a temperature of 400 1C, however the reaction tempera- ture can be deceased by incorporating other light elements such as Mg, B, Al, and K into this system. 8–11 Recently, Langmi et al. 12 have reported a reversible hydrogen storage system based on a Li–Fe–N phase, revealing the possible potential of hydrogen storage by lithium transition metal nitrides. Such materials might be expected to function midway between intermetallic alloy systems and the complex hydrides. Interesting new lithium transition metal nitrides have also been recently discovered that are considered as potential hydrogen storage materials. 13 In this article we report on two lithium metal nitrides—Li 7 VN 4 and Li 7 MnN 4 , as candidate materials for hydrogen storage. The electrochemical behaviour of these compounds as Li-ion battery cathodes have been studied extensively, 14–16 but this is the first report of their hydrogen storage properties, examined both experimentally and computationally. Experimental and computational details Synthesis Li 7 VN 4 and Li 7 MnN 4 were synthesized by a high temperature solid-state reaction of Li 3 N and V/Mn metal. Typically, Li 3 N (0.600 g; 10% molar excess) and V powder (325 mesh, 99.5%) a Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada. E-mail: [email protected]b Chemical Sciences and Materials Systems Laboratory, General Motors Research and Development Center, Mail Code 480-106-224, 30500 Mound Road, Warren, MI 48090-9055, USA PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by University of Waterloo on 07 March 2012 Published on 01 April 2011 on http://pubs.rsc.org | doi:10.1039/C0CP02892D View Online / Journal Homepage / Table of Contents for this issue
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This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 8889–8893 8889
Investigation of hydrogen absorption in Li7VN4 and Li7MnN4
Guang He,aJ. F. Herbst,*
bT.N. Ramesh,
aF. E. Pinkerton,
bM. S. Meyer
band
Linda Nazar*a
Received 16th December 2010, Accepted 1st February 2011
DOI: 10.1039/c0cp02892d
The hydrogen storage properties of Li7VN4 and Li7MnN4 were investigated both by experiment
and by density functional theory calculations. Li7VN4 did not sorb hydrogen under our
experimental conditions. Li7MnN4 was observed to sorb 7 hydrogen atoms through the formation
of LiH, Mn4N, and ammonia gas. An applied pressurized mixture of H2/Ar and H2/N2 gases was
helpful to mitigate the release of NH3 but could not prevent its formation. The introduction of
N2 also caused weight gain of the sample by re-nitriding the absorbed products LiH and Mn4N,
which correlated with the presence of Li2NH, LiNH2, and Mn2N detected by X-ray diffraction.
While our observed results for Li7VN4 and Li7MnN4 differ in detail, they are in overall
qualitative agreement with our theoretical work, which strongly suggests that both compounds
are unlikely to form quaternary hydrides.
Introduction
Onboard hydrogen storage has become a critical technology
for the development of fuel cell vehicles (FCVs).1–4 When
various storage systems are compared, solid-state materials are
advantageous as they possess a higher volumetric energy density
than either compressed gas or liquid hydrogen systems. However,
despite decades of studies, no solid state hydrogen storage
materials satisfactorily meet the requirements for application to
FCVs, such as high storage capacity and fast sorption kinetics in
the range of 1–10 bar and 25–120 1C.5 Conventional transition
metal alloys, including the La–Ni (AB5) system, Ti–Cr (AB2)
system and Ti–Fe (AB) system,6 have the desired intermediate
thermodynamic affinities for hydrogen. The hydrogen gas
molecules split into atoms at the surface of the metal and then
enter the metallic lattice in the atomic form, diffuse through the
metal, jump between interstitial sites; and finally form a hydride
phase with a more or less ordered hydrogen sublattice. The
hydrogen s electron can be partly donated to the metal conduction
band if the metallic lattice contains d or f electron states at the
Fermi level. Hence these bare protons can move relatively freely
through the metal lattice due to the electrostatic force screened by
electrons at the metal Fermi surface. In the desorption process, the
opposite takes place; i.e. two hydrogen atoms recombine to form
H2 again. These systems have better kinetics than complex
hydrides, but their low gravimetric capacities and high costs
prevent vehicular applications.
In recent years alternative hydrogen storage systems have
been examined, consisting of light alkali and alkaline earth
metal nitrides and/or borides with improved storage capacity.
A well-known and highly promising example is lithium nitride,
which remarkably stores up to 11.4 wt% hydrogen according
to the equation:7 Li3N + 2H2 2 Li2NH + LiH + H2 2
LiNH2 + 2LiH. One of the major limitations of this system is
that the high storage capacity can only be achieved at a
temperature of 400 1C, however the reaction tempera-
ture can be deceased by incorporating other light elements
such as Mg, B, Al, and K into this system.8–11 Recently,
Langmi et al.12 have reported a reversible hydrogen storage
system based on a Li–Fe–N phase, revealing the possible
potential of hydrogen storage by lithium transition metal
nitrides. Such materials might be expected to function midway
between intermetallic alloy systems and the complex hydrides.
Interesting new lithium transition metal nitrides have also
been recently discovered that are considered as potential
hydrogen storage materials.13 In this article we report on
two lithiummetal nitrides—Li7VN4 and Li7MnN4, as candidate
materials for hydrogen storage. The electrochemical behaviour
of these compounds as Li-ion battery cathodes have been
studied extensively,14–16 but this is the first report of their
hydrogen storage properties, examined both experimentally
and computationally.
Experimental and computational details
Synthesis
Li7VN4 and Li7MnN4 were synthesized by a high temperature
solid-state reaction of Li3N and V/Mn metal. Typically, Li3N
(0.600 g; 10% molar excess) and V powder (325 mesh, 99.5%)
aDepartment of Chemistry, University of Waterloo,200 University Avenue West, Waterloo, Ontario N2L 3G1,Canada. E-mail: [email protected]
bChemical Sciences and Materials Systems Laboratory, General MotorsResearch and Development Center, Mail Code 480-106-224,30500 Mound Road, Warren, MI 48090-9055, USA
PCCP Dynamic Article Links
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View Online / Journal Homepage / Table of Contents for this issue
This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 8889–8893 8891
two distinct 16h sites in tetragonal a-Li7VN4. Each struc-
ture was fully optimized with VASP and the enthalpy DH*
of hydride formation computed, e.g.:
DH*[Li7MnN4H(8e)] � E[Li7MnN4H(8e)]
� E(Li7MnN4). (2)
As Table 1 shows, we obtain large and positive values of DH*
for all five models, strongly suggesting that neither Li7MnN4
nor Li7VN4 is likely to form a quaternary hydride. The entries
for Li7MnN4Hn are undoubtedly also reflective of g-Li7VN4Hn
having the same cubic structure, and the DH* values for
a-Li7VN4 illustrate that not even the lower tetragonal symmetry
offers any prospect for changing the sign of DH*. As described
below, our experimental results confirm the inferences from our
theoretical modeling work.
Experimental studies
The X-ray powder diffraction patterns of the prepared Li7VN4
and Li7MnN4 are shown in Fig. 1. Previous studies29,30 have
classified the crystal structures of lithiated transition metal
nitrides of the first row into two groups: the ionic anti-fluorite
structure for elements in the early series where the metal is
present in its group oxidation state25,28,31,32 and a layered
structure based on a-Li3N for elements in the late series that
exist in the univalent state.33–35 In this paper, the studied
elements vanadium and manganese are within the range of
the early series, therefore they both form an anti-fluorite
structure. For the Li3N/ V system, Li7VN4 (P-43n) was obtained
as a major phase with trace quantities of VN and Li7VN4
(Pa3-). In the case of Li3N/Mn a similar XRD pattern is
observed, which can be assigned to Li7MnN4, an isostructure
of Li7VN4. It is notable that as manganese is in the middle
of the first row transition metals, its lithiated nitrides can
adopt anti-fluorite or layered structures. Moreover, a new
composition LixMn2�xN with an anti-rutile structure was also
prepared by Niewa’s group.36 The various possible structures
of the Li–Mn–N system can complicate the preparation of a
single Li7MnN4 phase, however no layered Li2[(Li1�xMx)N]
or anti-rutile LixMn2�xN structures are observed in the XRD
pattern. Therefore, a pure phase of Li7MnN4 was obtained by
a simple high temperature solid-state process.
Attempts were made to induce H2 absorption in both samples.
Fig. 2 shows the thermogravimetric profile of Li7VN4. The
slight increase in weight at the beginning of the temperature
ramp is a convection cell artifact of the high pressure TGA.
The change in weight during depressurization suggests that the
sample became less dense during the temperature profile.
Excluding these weight changes, only 1 wt% loss is observed
after 7 h, even with high temperature and pressure conditions
(600 1C and 83 bar). IGA measurement similarly observed a
weight loss of about 0.35 wt% even after 47 h at 425 1C in
20 bar H2. These results confirm that Li7VN4 is very stable and
hydrogen cannot be easily absorbed. A concurrent mass
spectrographic measurement shows the small weight change
might be due to the production of N2.
The thermogravimetric profile of Li7MnN4 is given in Fig. 3.
Instead of H2 absorption, there is a weight loss of roughly
26 wt% under 83 bar of H2. A slight weight gain of 2.6%
occurred near 350 1C, corresponding to an uptake of approxi-
mately 4 H atoms per formula unit associated with the
formation of LiH. Niewa et al.28 found that the reduction,
or decomposition reaction of Li7MnN4, took place in the
presence of Li above 250 1C, to produce a series of Li–Mn–N
compounds such as Li5[(Li1�xMnx)]3 and Li2[(Li1�xMx)N]. In
our case, H2 gas played a similar role as Li by reducing Mn(V)
in Li7MnN4 to a lower oxidation state. A quantity of LiH was
probably formed during this process, as some Li atoms were
released from Li7MnN4 with the reduction of Mn(V). The large
Table 1 Enthalpies of hydride formation DH* calculated [see eqn (2)]for quaternary hydrides modeled by inserting H atoms into theindicated vacancy sites in cubic Li7MnN4 (8 f.u. per unit cell) andtetragonal a-Li7VN4 (2 f.u. per primitive cell)
This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 8889–8893 8893
Acknowledgements
L.F.N. gratefully acknowledges General Motors Canada for
funding, and NSERC through its Discovery, Canada Research
Chair and Collaborative Research & Development programs.
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Fig. 6 XRD pattern of Li7MnN4 produced by decomposition in H2