In this chapter, short descriptions and experimental conditions for common characterization techniques of M-B-N-H type compounds will be provided. Then, various synthesis approaches of these compounds described in the literature will be summarized followed by the procedures and the equipment utilized during this thesis work.
Electron, neutron and x-ray diffraction are the main techniques that can provide crystallographic information on the compounds. X-rays interact with the electrons of the atom while neutron are only scattered by its nuclei. Conversely, electron scattering can be influenced by both electrons and the nuclei of the atoms. Each technique has its advantages and disadvantages.
Electron diffraction requires a Transmission Electron Microscope (TEM) and time-consuming sample preparations since the sample has to be electron transparent (very thin). It has advantages of focusing the electron beam on a nanometer sized crystallites. Obtaining crystal symmetry information on this scale combined with high resolution imaging TEM can be very powerful technique for specific applications. However, it has limited accuracy for calculation of unit cell parameters and the atomic positions compared to neutron and x-ray diffraction. For these experiments unless the nanocrystallites in the samples can be grown big enough for single crystal diffraction experiments, the typical sample is in powder form.
As the atomic number increases so does its number of electrons, hence heavier atoms scatter more strongly the x-rays than light atoms. On the other hand, neutron scattering cross-section is almost random and does not show any correlation with the atomic number. Hence, different isotopes can have wildly different neutron scattering power. Although 1H scatters neutron very strongly, the big part of this scattering is incoherent (becomes flat background noise in diffraction experiments). This deficiency can be overcome with increasing the flux of neutrons source or accumulation time during the diffraction analysis. A much better approach is to synthesize the deuterated samples using 2H which has lower ratio of incoherent to coherent scattering coefficient. Light elements such as alkali metals, boron, nitrogen and others are often used to synthesize hydrogen storage materials with high gravimetric capacity. Unfortunately, 6Li and 10B isotopes are neutron absorbing elements. Non-enriched samples contain 7.5 % 6Li and 20 % 10B. This means that for accurate neutron diffraction analysis, the samples have to be prepared using 7Li and 11B isotopes.
Atomic nuclei being much smaller than neutron wavelength act as point scattering sources, while electron density cloud being comparable to x-ray wavelength are not point sources. This translates into a decrease in x-ray scattering power with increasing scattering angle (atomic scattering factor). This problem can be partially eliminated by increasing the intensity of the x-ray source. On the other hand, neutron are scattered equally in all scattering angles. The diffraction information at high scattering angles is important for precise determination of the atomic positions in the crystal structure. Neutron diffraction does not suffer from intensity loss at higher scattering angles; consequently, precise experimental determination of the hydrogen atom positions becomes possible. A popular hydrogen storage material, NH3BH3 had a disputed crystalline structure for a long time because X-ray diffraction (XRD) analysis can only see scattered x-rays from boron and nitrogen atoms and hydrogen atoms are essentially transparent to it. Using neutron diffraction, not only hydrogen atoms provide extra diffraction information, but due to the higher contrast in neutron scattering between boron and nitrogen atoms, their atomic position in crystal structure became distinct and clear. This allowed accurate measurement of B-H and N-H bonding lengths in ammonia borane .
A popular source of x-rays is the synchrotron radiation that allows production of high flux beam with tunable wavelength. The large 2D x-ray detectors in synchrotron facilities can provide almost instantaneous data acquisition, making it ideal for in-situ dehydrogenation or hydrogenation experiments under varying temperature and hydrogen pressure conditions. Besides XRD measurements, x-ray near edge absorption spectroscopy (XANES) can be performed to study the oxidation state of the heavy atoms and their atomic neighborhood in hydride complexes. Small angle x-ray (SAXS) and neutron scattering (SANS) measurements can provide the information regarding size and distribution of hydride phases and the catalyst additives. Finally, an atom being mostly an empty space, neutrons can penetrate much deeper into the heavy materials before they encounter a nuclei to scatter off. This allows tomography and internal imaging of pressurized hydrogen containers, as well as evolution of dehydrogenated phases in these containers . Nevertheless, neutron experiments require fission or spallation sources and therefore their access is generally limited to few researchers. Hence, the main advantage of XRD measurements is its ease of use in laboratory conditions, minimal sample preparation requirements and the fast data processing options for identification of known phases, especially for powder samples.
Powder X-ray diffraction
The typical Powder X-ray Diffraction (XRD) experiment is performed using Bruker D8. A special XRD sample holder is used due to the air sensitive (possible pyrophoric) nature of the hydrogen storage materials. The sample holder has a silicon substrate that suppresses background noise in 2θ=20−120° range for Cu Kα wavelength (1.5406 Å). The sample holder is prepared in glovebox and it is sealed using Teflon O-ring and 8μm thick polyimide (Kapton) film that is transparent to x-rays.
For known crystalline phases, EVA database is used to identify them. High resolution measurements are performed to analyze the unknown phases. Phase analysis was performed using the structure models (crystal information file – cif) taken from the databases (Crystallography Open Database) or literature. Possible solutions to unit cell type and parameters were determined using DICVOL and the most likely space group symmetry was determined by CHEKCELL software. Le Bail fit analysis using HighScore software was performed to fit standard peak shape parameters and Chebyshev polynomial of 15th order for background signal. The unit cell parameters were calculated for each compound with reasonable accuracy (goodness of fit below 0.01). The standard deviation in unit cell volume is calculated by manually changing unit cell parameters until the goodness of fit started to increase. Rietveld refinement analysis was performed using both HighScore and FullProf programs.
Various hydrogen storage materials discussed in Chapter 1 require the thermal energy as the main energy source to release the hydrogen gas. Hence, investigation of thermodynamic parameters such as the heat and the kinetics of de/rehydrogenation reactions, mass loss corresponding to volatile decomposition products, types of gases that are released and the quantification of the amount of each released gas necessitate multiple characterization equipment to provide a full picture.
Differential Scanning Calorimetry – Thermogravimetry
Differential Scanning Calorimetry (DSC) and Thermogravimetry (TG) experiments usually are performed together using Setaram Sensys Evo. DSC provides the heat flow information that can help to explain various phase transition events such as melting, boiling, decomposition reactions etc. Simultaneously performed TG experiment provides mass loss information which coincides with each phase transition. This extra information for example helps to differentiate the melting process from endothermal decomposition process.
Typically, 15-30mg of powder is transferred to an aluminum crucible in glovebox and sealed with a perforated aluminum cap. The cap restricts the sample exposure to air during the transfer from glovebox. A small hole on the cap helps to avoid built-up of overpressure during the decomposition process. The typical experiment is performed under 20ml/min He flow and the samples is heated up to 500°C with the rate of 2°C/min.
Sievert type volumetry
Gravimetric analysis can provide a complete picture of the decomposition process if only one type of volatile species is released. In all the other cases, the TG result needs to be complemented with a volumetric analysis to provide a more quantitative assessment. Hence, by decomposing the sample under similar conditions to TG in a precisely known reservoir volume (Vr) and measuring the increase in pressure (ΔP) we can calculate the mole amount (n) of various gases. Compressibility factor (Z) for real gases is used to take into account the intermolecular attraction of gas molecules and their molecular volume:
ΔPTPD = ( Vr ) ∑(niZi)
ΔmTG = (mTPD) ∑(niMi)
In these equations, ΔmTG is the weight loss measured by TG while mTG is the initial mass of the powder, similarly mTPD is the initial mass of the same sample powder used in volumetric measurement, ni and Mi are the molar quantity and the molar mass of the gas i released during decomposition. Since there are two independent equations, the molar quantity of only up to two different gas mixtures can be quantified by solving these equations. Theoretically, by changing the reservoir temperature (Tr) or volume more independent equations can be obtained to solve for more than one impurity gases. The compressibility factor Zi(T, P) for gases depends on the pressure and the temperature. Hence, care must be given to use correct compressibility factor for chosen experimental conditions. For example, compressibility factor of hydrogen and ammonia under 1 bar and 25 °C conditions are Z(H2) = 1.0006 and Z(NH3) = 0.98946.
The volumetric decomposition experiments (Temperature Programmed Desorption – TPD) are conducted using Setaram PCT Pro. Typically, 100-200mg of powder is loaded into the steal sample holder in glovebox, sealed and connected to the machine. This steal reactor is certified up to 200 bar pressure and sealed using copper rings that can handle up to 250 °C temperature without risking the hydrogen leakage. Sample and the system is purged several types with helium gas and evacuated to achieve lowest possible vacuum. Then, the large reservoir (Vr = 1L) and the sample holder are filled with 1 bar hydrogen to obtain similar decomposition conditions as in TG experiments. The pressure, sample and reservoir temperature values are typically recorded with time interval range of 1-100s depending on the kinetics and total duration of the decomposition reaction. Once the data acquisition is started, the sample holder is dynamically heated up to 250 °C with external electrical heating element (average heating rate in 25°C/min). Precise volume calibration in volumetric measurements is very important for improving the accuracy of the calculation, especially for hydrogenation experiments that can be performed with hydrogen pressures up to 50 bar. The “dead volume” is referred to any volume that is outside the known PCT Pro internal volume. This volume is usually minimized by filling the empty areas of the sample holder as much as possible with steal pieces that fit tightly. The rest of the dead volume is calibrated using helium gas, which has closest real gas properties to hydrogen. This procedure is performed both at initial and final sample temperatures of the typical experiment since the calibration process is temperature sensitive.
The molar quantity calculations using TPD and TG experiments are only possible if the nature of the volatile decomposition product are known. For an unknown or new compounds, the qualitative analysis of the volatile species needs to be performed beforehand. Additionally, to identify amorphous phases in synthesized samples or in decomposition by-products spectroscopic analysis tools are necessary.
The various spectroscopy experiments can be performed in transmission mode as absorption spectroscopy or in reflection mode as emission spectroscopy. As a general description, a spectroscopic analysis concerns the interaction of an electromagnetic radiation with the matter. The energy of the excitation source determines the type of the interaction. X-rays have high enough energy to excite the electrons from core shells of the atoms. Ionizing radiation (x-ray and ultraviolet) can also eject electrons from the atom creating photoelectrons which can be used to study surface chemical composition of samples, oxidation state of the atoms (XPS). For ultraviolet/visible light, due to lower energy, only outer shell electrons of the atoms can be excited. The visible/infrared radiation can induce vibration of atoms in the molecular compounds.
For elemental analysis, atomic spectroscopy techniques require an extra energy source for vaporization, atomization and ionization of the samples. This energy source could be flame, laser, plasma, electric arc and others. Since hydrogen storage materials release volatile gases upon heating, it is possible to combine DSC-TG assembly with Mass Spectrometry (MS) to analyze the nature of the released gases. Most of these techniques provide a qualitative information on the analyzed sample. However, calibration and quantitative analysis is also possible. For example, inductively coupled plasma atomic emission spectroscopy ICP-AES can provide precise elemental concentrations in a given sample. For this analysis the sample needs to be prepared by digestion in aqueous acidic solution. It is also widely used for characterization of hydrogen storage materials in order to verify the purity of the synthesized compounds, especially for poorly crystalline samples or samples that might contain amorphous phases . Unfortunately, ICP-AES technique is not the suited for quantification of nitrogen atoms. For that purpose, ion chromatography is used to quantify nitrogen content by analyzing the diffusion of [NH4]+ cations through an ion exchange membrane .
Mass Spectrometry (MS) ionizes a molecule by bombarding it with electron beam. This process splits the molecule to its fragments that will have various mass and charge values. These ionized species are further accelerated by electric field and to separate them, a magnetic field deflects them onto an electron multiplier detector. The signal detected for each species is integrated for various mass-to-charge ratios which provides a unique chemical fingerprint that can identify each molecule.
A typical ATG-MS study is conducted by Netzsch STA 449 – QMS 403C equipment under 20ml/min Ar flow and heating rate of 10 °C/min. Sample preparation was carried similar to DSC-TG experiments.
Table of contents :
I. State of the art
I.2. Hydrogen economy
I.2.1. Bigger Picture
I.3. Physical hydrogen storage
I.3.1. Compressed hydrogen
I.3.2. Liquefied hydrogen
I.3.3. Hydrogen in porous compound
I.4. Chemical hydrogen storage
I.4.1. Hydrogen containing gases
I.4.2. Liquid organic hydrogen carriers
I.4.3. Solid-state hydrogen storage
I.4.3.1. Metal hydrides
I.4.3.2. Complex hydrides
I.4.3.3. Metal amides
I.4.3.4. Ammonia Borane
I.4.3.5. Metal amidoboranes
I.4.3.6. Metal borohydrides
I.4.3.7. Metal borohydride derivatives
I.4.3.8. Composite Reactive hydrides
I.4.3.10. Direct hydrogenation catalysts
I.4.3.11. Frustrated Lewis Pairs
II.1. Crystallographic analysis
II.1.1. Powder X-ray diffraction
II.2. Thermodynamic analysis
II.2.1. Differential Scanning Calorimetry – Thermogravimetry
II.2.2. Sievert type volumetry
II.3. Spectroscopic analysis
II.3.1. Mass Spectrometry
II.3.2. Raman and Infra-red Spectroscopy
II.3.3. Nuclear Magnetic Resonance Spectroscopy
II.4. Synthesis process
II.4.2. Review of mechanochemical synthesis of metal borohydrides
II.4.3. Review of solvothermal synthesis of metal borohydrides
II.4.4. Solvothermal synthesis equipment
III. Synthesis and characterization of ammine zinc borohydrides
III.2. Literature review on synthesis of zinc borohydride solvates
III.3. Road to scalable synthesis
III.3.1. Synthesis procedure
III.4. Experimental results
III.4.1. Crystallographic analysis
III.4.2. Thermodynamic analysis
III.4.3. Spectroscopic analysis
III.5. Discussion of results
IV. Optimizing hydrogen storage capacity of Zn-B-N-H compounds
IV.2. Ammonium metal borohydrides
IV.2.1. Ammonium Zinc borohydride
IV.2.1.1. Crystallographic analysis
IV.2.1.2. Raman analysis
IV.2.1.3. Thermodynamic analysis
IV.2.2. Overview of ammonium metal borohydrides
IV.3. Ammonia borane destabilization
IV.4. Ammine zinc borohydride – Ammonia borane complex
IV.4.1. Metal Amidoboranes
IV.4.2. Zinc amidoborane
IV.5. Hydrogen storage capacity of Zn-B-N-H system
V. Mg-B-N-H system and exploration of reversibility
V.1. Drawback of Zn-B-N-H system as practical hydrogen storage system
V.3. Synthesis of Mg-B-N-H compounds
V.4. Hydrogen storage properties of Mg-B-N-H system
V.5. Regeneration of spent M(BNHx)m fuel
V.5.1. Direct hydrogenation of M(BNHx)m
V.5.2. Metal recovery from M(BNHx)m
V.5.3. Full regeneration of Mg(BNHx)2
V.5.3.1. B-N bond cleavage
V.5.3.2. B-F bond reduction
VI. General Conclusions and Perspectives
VII. Résumé détaillé des chapitres
VII.1. Chapitre I – L’état de l’art
VII.2. Chapitre II – Méthodologie
VII.3. Chapitre III – Synthèse et caractérisation de borohydrures d’amines de zinc
VII.4. Chapitre IV – Optimiser la capacité d’hydrogène des composés Zn-B-N-H
VII.5. Chapitre V – Système Mg-B-N-H et exploration de la réversibilité