Nuclear quantum effects in ab initio dynamics: theory and experiments for lithium imide

Owing to their small mass, hydrogen atoms exhibit strong quantum behavior even at room temperature. Including these effects in first principles calculations is challenging, because of the huge computational effort required by conventional techniques. Here we present the first ab-initio application of a recently-developed stochastic scheme, which allows to approximate nuclear quantum effects inexpensively. The proton momentum distribution of lithium imide, a material of interest for hydrogen storage, was experimentally measured by inelastic neutron scattering experiments and compared with the outcome of quantum thermostatted ab initio dynamics. We obtain favorable agreement between theory and experiments for this purely quantum mechanical property, thereby demonstrating that it is possible to improve the modelling of complex hydrogen-containing materials without additional computational effort

Ammonia decomposition catalysis using lithium–calcium imide

Lithium–calcium imide is explored as a catalyst for the decomposition of ammonia. It shows the highest ammonia decomposition activity yet reported for a pure light metal amide or imide, comparable to lithium imide–amide at high temperature, with superior conversion observed at lower temperatures. Importantly, the post-reaction mass recovery of lithium–calcium imide is almost complete, indicating that it may be easier to contain than the other amide–imide catalysts reported to date. The basis of this improved recovery is that the catalyst is, at least partially, solid across the temperature range studied under ammonia flow. However, lithium–calcium imide itself is only stable at low and high temperatures under ammonia, with in situ powder diffraction showing the decomposition of the catalyst to lithium amide–imide and calcium imide at intermediate temperatures of 200–460 °C.</p

Enhancement of the Catalytic Activity of Lithium Amide towards Ammonia Decomposition by Addition of Transition Metals

The catalytic decomposition of ammonia to hydrogen is a vital process in the use of ammonia as a zero-carbon energy store. However, the viability of current catalyst systems in terms of operating conditions, versatility, and cost efficiency has proven an issue. Catalytic and gravimetric studies were conducted considering mechanistic uncertainty surrounding the enhanced catalytic activity reported for lithium amide and imide composited with transition metals (Cr, Mn, Fe). Gas flow in excess of ammonia decomposition was quantified and used to differentiate the extent of formation of non-stoichiometric lithium imide amide from other competing processes. This analysis showed the initial compositional transition from lithium amide to an imide-rich phase was reduced in temperature by compositing with Mn and Cr, but not with Fe. The system is, therefore, best considered as promoted lithium imide, with Cr and Mn acting to reduce the formation temperature of the active imide-rich phase such that the catalytic activity is enhanced

Hydrogen storage: the major technological barrier to the development of hydrogen fuel cell cars

In this paper, we review the current technology for the storage of hydrogen on board a fuel cell-propelled vehicle. Having outlined the technical specifications necessary to match the performance of hydrocarbon. fue1, we first outline the inherent difficulties with gas pressure and liquid hydrogen storage. We then outline the history of transition metal hydride storage, leading to the development of metal hydride batteries. A viable system, however, must involve lighter elements and be vacuum-tight. The first new system to get serious consideration is titanium-activated sodium alanate, followed by the lithium amide and borohydride systems that potentially overcome several of the disadvantages of alanates. Borohydrides can alternatively produce hydrogen by reaction with water in the presence of a catalyst but the product would have to be recycled via a chemical plant. Finally various possible ways of making magnesium hydride decompose and reform more readily are discussed. The alternative to lighter hydrides is the development of physisorption of molecular hydrogen on high surface area materials such as carbons, metal oxide frameworks, zeolites. Here the problem is that the surface binding energy is too low to work at anything above liquid nitrogen temperature. Recent investigations of the interaction mechanism are discussed which show that systems with stronger interactions will inevitably require a surface interaction that increases the molecular hydrogen–hydrogen distance

PROPERTIES AND STRUCTURES OF Li-N BASED HYDROGEN STORAGE MATERIALS

Traditional transportation fuel, petroleum, is limited and nonrenewable, and it also causes pollutions. Hydrogen is considered one of the best alternative fuels for transportation. The key issue for using hydrogen as fuel for transportation is hydrogen storage. Lithium nitride (Li3N) is an important material which can be used for hydrogen storage. The decompositions of lithium amide (LiNH2) and lithium imide (Li2NH) are important steps for hydrogen storage in Li3N. The effect of anions (e.g. Cl-) on the decomposition of LiNH2 has never been studied. Li3N can react with LiBr to form lithium nitride bromide Li13N4Br which has been proposed as solid electrolyte for batteries. The decompositions of LiNH2 and Li2NH with and without promoter were investigated by using temperature programmed decomposition (TPD) and X-ray diffraction (XRD) techniques. It was found that the decomposition of LiNH2 produced Li2NH and NH3 via two steps: LiNH2 into a stable intermediate species (Li1.5NH1.5) and then into Li2NH. The decomposition of Li2NH produced Li, N2 and H2 via two steps: Li2NH into an intermediate species --- Li4NH and then into Li. The kinetic analysis of Li2NH decomposition showed that the activation energies are 533.6 kJ/mol for the first step and 754.2 kJ/mol for the second step. Furthermore, XRD demonstrated that the Li4NH, which was generated in the decomposition of Li2NH, formed a solid solution with Li2NH. In the solid solution, Li4NH possesses a similar cubic structure as Li2NH. The lattice parameter of the cubic Li4NH is 0.5033nm. The decompositions of LiNH2 and Li2NH can be promoted by chloride ion (Cl-). The introduction of Cl- into LiNH2 resulted in the generation of a new NH3 peak at low temperature of 250 °C besides the original NH3 peak at 330 °C in TPD profiles. Furthermore, Cl- can decrease the decomposition temperature of Li2NH by about 110 °C. The degradation of Li3N was systematically investigated with techniques of XRD, Fourier transform infrared (FT-IR) spectroscopy, and UV-visible spectroscopy. It was found that O2 could not affect Li3N at room temperature. However, H2O in air can cause the degradation of Li3N due to the reaction between H2O and Li3N to LiOH. The produced LiOH can further react with CO2 in air to Li2CO3 at room temperature. Furthermore, it was revealed that Alfa-Li3N is more stable in air than Beta-Li3N. The chemical stability of Li13N4Br in air has been investigated by XRD, TPD-MS, and UV-vis absorption as a function of time. The aging process finally leads to the degradation of the Li13N4Br into Li2CO3, lithium bromite (LiBrO2) and the release of gaseous NH3. The reaction order n = 2.43 is the best fitting for the Li13N4Br degradation in air reaction. Li13N4Br energy gap was calculated to be 2.61 eV

Structural and property investigations into low-doped lithium nitridometallates Li3-x-yMxN, x = Co, Ni and Cu

Bulk samples of the lithium nitridometallates of the general form Li3-xMxN and Li3-x-yMxN, where M = Co, Ni and Cu, x ≤ 0.1 and y = vacancy, have been synthesised. The products of the syntheses have been characterised by Powder X-ray Diffraction (PND), Constant Wavelength Powder Neutron Diffraction (CW PND), Time of Flight Powder Neutron Diffraction (ToF PND), SEM and SQUID magnetometry. The transition metal entirely substituted for the Li(1) at the interplanar site and resulted in a retention of the Li3N-type structure, P6/mmm. These materials displayed similar structural trends as seen in lithium nitridometallates with higher transition metal levels, and with vacancies, which were also dependent on reaction time and temperature at this low level. For longer reacted lithium nitridometallates a large concentration of vacancies was obtained despite a small quantity of dopant transition metal. The resultant charge was deemed too high to be solely balanced by the transition metal, as is usual in these materials. A charge compensation mechanism involving the nitrogen ion was assumed for the high-vacancy materials. The materials displayed some interesting forms of magnetism such as spin-glass magnetic behaviour and also Pauli paramagnetism. Li2.95Ni0.05N and Li2.90Co0.1N displayed a high specific capacity upon electrochemical testing when applied as a potential anode material, and gave better charge capacities than that of the previous best lithium anode material and showed signs of improvement upon cycling. Preliminary investigations into hydrogen adsorption of the materials had been attempted, with one particular material, Li2.95Ni0.05N, giving a hydrogen adsorption of 7.839 wt % over 90 hours with a hydrogen pressure of 20 bar and temperature of 250 oC. A corresponding 3.5 wt % loss was achieved upon desorption at the same temperature

Neutron diffraction and gravimetric study of the manganese nitriding reaction under ammonia decomposition conditions

Manganese and its nitrides have recently been shown to co-catalyse the ammonia decomposition reaction. The nitriding reaction of manganese under ammonia decomposition conditions is studied in situ simultaneously by thermogravimetric analysis and neutron diffraction. Combining these complementary measurements has yielded information on the rate of manganese nitriding as well as the elucidation of a gamut of different manganese nitride phases. The neutron diffraction background was shown to be related to the extent of the ammonia decomposition and therefore the gas composition. From this and the sample mass, implications about the rate-limiting steps for nitriding by ammonia and nitriding by nitrogen are discussed

Exploring N-rich phases in LixNy clusters for hydrogen storage at nano-scale

We have performed cascade genetic algorithm and ab initio atomistic thermodynamics under the framework of first-principles density functional theory to study the (meta-)stability of a wide range of LixNy clusters. We found that hybrid xc-functional is essential to address this problem as a local/semi-local functional simply fails even to predict a qualitative prediction. Most importantly, we find that though in bulk Lithium Nitride, Li rich phase, i.e. Li3N, is the stable stoichiometry, in small LixNy clusters N-rich phases are more stable at thermodynamic equilibrium. We further show a that these N-rich clusters are promising hydrogen storage material because of their easy adsorption and desorption ability at respectively low (< 300K) and moderately high temperature (> 600K).Comment: 5 pages, 4 figure