Li 1s core exciton in LiH studied by x-ray Raman scattering spectroscopy.

The Li 1s core excitation spectra in LiH was studied by means of x-ray Raman scattering (XRS) spectroscopy in a wide range of momentum transfers q. The analysis of the near-edge region of the measured spectra in combination with q-dependent ab initio calculations of XRS spectra based on the Bethe-Salpeter equation (BSE) reveals that the prominent peak at the excitation onset arises from two main contributions, namely a pre-edge peak associated to a p-type core exciton and strong transitions to empty states near the bottom of the conduction band, which is in contrast to previous experimental studies that attributed that feature to a single excitonic peak. The p-like angular symmetry of the core exciton is supported by BSE calculations of the relative contributions to the XRS spectra from monopole and dipole transitions and by the observed decrease of its normalised intensity for increasing momentum transfers. Higher energy spectral features in the measured XRS spectra are well reproduced by BSE, as well as by real-space multiple-scattering calculations.

Evaluation of the formation and carbon dioxide capture by Li4SiO4 using in situ synchrotron powder X-ray diffraction studies.

Carbon capture and storage using regenerable sorbents are an effective approach to reduce CO2 emissions from stationary sources. In this work, lithium orthosilicate (Li4SiO4) was studied as a carbon dioxide sorbent. For a deeper understanding of the synthesis and carbonation mechanism of Li4SiO4, an in situ synchrotron radiation powder X-ray diffraction technique was used. The Li4SiO4 powders were synthesized by a combination of ball milling of a Li2CO3 and SiO2 mixture followed by a thermal treatment process at low temperature. In situ studies showed that formation of Li4SiO4 from the as-milled 2Li2CO3-SiO2 mixture involves decomposition of Li2CO3 by reaction with SiO2via Li2SiO3 as an intermediate compound. No evidence of Li2Si2O5 formation was obtained, in spite of thermodynamic predictions. The CO2 capture by Li4SiO4 was evaluated dynamically over a wide temperature range, reaching a maximum weight increase of 34 wt% and good cyclability after about 10 cycles. By thermogravimetric and microstructural analyses in combination with ex situ and in situ measurements, a two step carbonation mechanism and its influence on the final CO2 capture was clearly elucidated. Under dynamical conditions up to 700 °C, the lower number of Li2CO3 nuclei initially formed retards the double shell formation and the nucleation and growth of the Li2CO3 particles remains the controlling step up to higher CO2 capture capacity. Isothermal carbonation at 700 °C favours the formation of a higher number of Li2CO3 nuclei that creates a thin carbonate shell. The CO2 diffusion through this shell is the limiting step from the beginning and further carbonation is hindered as the reaction progresses.

Clarifying the dehydrogenation pathway of catalysed Li4(NH2)3BH4-LiH composites.

The effect of different metal oxides (Co3O4 and NiO) on the dehydrogenation reaction pathways of the Li4(NH2)3BH4-LiH composite was investigated. The additives were reduced to metallic species i.e. Co and Ni which act as catalysts by breaking the B-H bonds in the Li-B-N-H compounds. The onset decomposition temperature was lowered by 32 °C for the Ni-catalysed sample, which released 8.8 wt% hydrogen below 275 °C. It was demonstrated that the decomposition of the doped composite followed a mechanism via LiNH2 and Li3BN2 formation as the end product with a strong reduction of NH3 emission. The sample could be partially re-hydrogenated (∼1.5 wt%) due to lithium imide/amide transformation. To understand the role of LiH, Li4(NH2)3BH4-LiH-NiO and Li4(NH2)3BH4-NiO composites were compared. The absence of LiH as a reactant forced the system to follow another path, which involved the formation of an intermediate phase of composition Li3BN2H2 at the early stages of dehydrogenation and the end products LiNH2 and monoclinic Li3BN2. We provided evidence for the interaction between NiO and LiNH2 during heating and proposed that the presence of Li facilitates a NHx-rich environment and the Ni catalyst mediates the electron transfer to promote NHx coupling.

Effective participation of Li4(NH2)3BH4 in the dehydrogenation pathway of the Mg(NH2)2-2LiH composite.

Lithium fast-ion conductors have shown positive effects on the hydrogen storage properties of the Li-Mg-N-H system. In the present work, Li4(NH2)3BH4 doped Mg(NH2)2-2LiH was formed by milling the 2LiNH2-MgH2-0.2LiBH4 composite and posterior annealing under hydrogen pressure to reduce the kinetic barrier of the Li-Mg-N-H system. The effect of repetitive dehydrogenation/rehydrogenation cycles on the kinetic and thermodynamic performance was evaluated. The dehydrogenation rate in the doped composite was twice that in the un-doped sample at 200 °C, while hydrogenation was 20 times faster. The activation energy decreases by 9% due to the presence of Li4(NH2)3BH4 compared to the un-doped composite, evidencing its catalytic role. The presence of Li4(NH2)3BH4 in the composite stabilized the hydrogen storage capacity after successive sorption cycles. Thermodynamic studies revealed a variation in the pressure composition isotherm curves between the first dehydrogenation cycle and the subsequent. The Li4(NH2)3BH4 doped composite showed a sloped plateau region at higher equilibrium pressure in regard to the flat plateau of the un-doped composite. Detailed structural investigations revealed the effective influence of Li4(NH2)3BH4 in different reactions: the irreversible dehydrogenation in the presence of MgH2 and the reversible hydrogen release when it reacts with Li2Mg2(NH)3. The role of Li4(NH2)3BH4 in improving the dehydrogenation kinetics is associated with the weakening of the N-H bond and the mobile small ion mass transfer enhancement.

Equipment for hydrogen absorption-desorption cycling characterization of hydride forming materials.

Hydrogen storage materials suffer different degradation processes when they are cycled, forming and decomposing their associated hydride phases. In order to study these processes, we have designed and built an automated apparatus specifically developed for cycling samples of hydride forming materials by changing the external hydrogen pressure. Instead of the standard open configuration involving a high-pressure, high-quality gas bottle and a vacuum pump, the equipment uses another hydride forming material (in our case LaNi5) as a source and sink of hydrogen. The resulting closed-loop configuration eliminates hydrogen waste and ensures that extremely high purity gas is used during the whole experiment, thanks to the purifying properties of the selected hydride as source/sink. Hydrogen pressure is set by changing the source/sink temperature. Cycles can be performed as fast as one cycle every 5 min, a period comparable with typical good hydride forming material kinetics. An example of application of the apparatus is given for 1000 absorption/ desorption cycles on a Mm0.8Ca0.2Ni5 sample.