Terry D. Humphries

Crystal structure and in situ decomposition of Eu(BH4)2 and Sm(BH4)2

Synthesis of halide free rare earth metal (RE) borohydride complexes is demonstrated by the metathesis reaction of trivalent RE metal chlorides and LiBH4 in ethereal solution, combined with solvent extraction using dimethyl sulfide. The crystal structures of Eu(BH4)2 and Sm(BH4)2 are orthorhombic (space group Pbcn) and are shown to be related to the structure of Sr(BH4)2 by Rietveld refinement. Further, the thermal decomposition of these materials has been studied by in situ synchrotron radiation powder X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, mass spectrometry and Sieverts measurements. The decomposition pathway of these solvent extracted materials has been compared against materials prepared by mechano-chemistry, the process of which is simplified by the absence of chloride impurities.

Hydrogen cycling in γ-Mg(BH4)2 with cobalt-based additives

Magnesium borohydride (Mg(BH4)2) is an attractive candidate as a hydrogen storage material due to its high hydrogen content and predicted favorable thermodynamics. In this work we demonstrate reversible hydrogen desorption in partially decomposed Mg(BH4)2 which was ball milled together with 2 mol% Co-based additives. Powder X-ray diffraction and infrared spectroscopy showed that after partial decomposition at 285 °C, amorphous boron-hydride compounds were formed. Rehydrogenation at equivalent temperatures and hydrogen pressures of 120 bar yielded the formation of crystalline Mg(BH4)2 in the first cycle, and amorphous Mg(BH4)2 with other boron–hydrogen compounds upon the third H2 absorption. Reversibility was observed in the samples with and without Co-based additives, although the additives enhanced hydrogen desorption kinetics in the first cycle. X-ray absorption spectroscopy at Co K-edge revealed that all the additives, apart from Co2B, reacted during the first desorption to form new stable species.

Enhanced tunability of thermodynamic stability of complex hydrides by the incorporation of H– anions

First-principles calculations were employed to investigate hypothetical complex hydrides (M,M′)4FeH8 (M = Na, Li; M′=Mg, Zn, Y, Al). Besides complex anion [FeH6]4–, these materials contain two H– anions, which raise the total anionic charge state from tetravalent to hexavalent, and thereby significantly increasing the number of combinations of countercations. We have determined that similar to complex hydrides (M,M′)2FeH6 containing only [FeH6]4–, the thermodynamic stability is tuned by the average cation electronegativity. Thus, the chemical flexibility provided by incorporating H– enhances the tunability of thermodynamic stability, which will be beneficial in obtaining optimal stability for hydrogen storage materials.

Fluoride substitution in sodium hydride for thermal energy storage applications

The solid-state solutions of NaHxF1−x (x = 1, 0.95, 0.85, 0.5) have been investigated to determine their potential for thermal energy applications. Thermal analyses of these materials have determined that an increase in fluorine content increases the temperature of hydrogen release, with a maximum rate of desorption at 443 °C for NaH0.5F0.5 compared to 408 °C for pure NaH, while pressure–composition–isotherm measurements have established a ΔHdes of 106 ± 5 kJ mol−1 H2 and ΔSdes of 143 ± 5 J K−1 mol−1 H2, compared to 117 kJ mol−1 H2 and 167 J K−1 mol−1 H2, respectively, for pure NaH. While fluorine substitution actually leads to a decrease in the stability (enthalpy) compared to pure NaH, it has a larger depressing effect on the entropy that leads to reduced hydrogen equilibrium pressures. In situ powder X-ray diffraction studies have ascertained that decomposition occurs via enrichment of fluorine in the NaHxF1−x composites while, unlike pure NaH, rehydrogenation is easily achievable under mild pressures. Further, cycling studies have proven that the material is stable over at least seven hydrogen sorption cycles, with only a slight decrease in capacity while operating between 470 and 520 °C. Theoretically, these materials may operate between 470 and 775 °C and, as such, show great potential as thermal energy storage materials for concentrating solar thermal power applications.

In situ high pressure NMR study of the direct synthesis of NaAlH4

The direct synthesis of NaAlH4 has been studied, for the first time, by in situ (27)Al and (23)Na wide-line NMR spectroscopy using high pressure NMR apparatus. Na3AlH6 formation is observed within two minutes of hydrogen addition, while NaAlH4 is detected after a total of four minutes. This indicates the formation of the hexahydride does not proceed to completion before the formation of the tetrahydride ensues.

Regeneration of sodium alanate studied by powder in situ neutron and synchrotron X-ray diffraction

The regeneration pathway of sodium alanate has been studied in detail by in situ synchrotron powder X-ray diffraction (SR-XRD) and powder neutron diffraction (PND). Rietveld refinement of the data has accurately determined the composition of all crystalline phases during the reaction process and shows definitively that Al initially reacts with NaH to form Na3AlH6, followed by the formation of NaAlH4 (before the total consumption of NaH) in two indiscrete reactions. During hydrogenation, an expansion of 0.6% of the Na3AlH6 unit cell is observed indicating towards the inclusion of Ti within the crystal lattice. This study promotes the recent development of next-generation sample holders and detectors that now enable the in situ diffraction measurement of hydrogen storage materials under relatively high gas pressures (>100 bar) and temperatures.

Thermal optimisation of metal hydride reactors for thermal energy storage applications

Metal hydrides (MHs) are promising candidates as thermal energy storage (TES) materials for concentrated solar thermal applications. A key requirement for this technology is a high temperature heat transfer fluid (HTF) that can deliver heat to the MHs for storage during the day, and remove heat at night time to produce electricity. In this study, supercritical water was used as a HTF to heat a prototype thermochemical heat storage reactor filled with MgH2 powder during H2 sorption, rather than electrical heating of the MH reactor. This is beneficial as the HTF flows through a coil of tubing embedded within the MH bed and is hence in better contact with the MgH2 powder. This internal heating mode produces a more uniform temperature distribution within the reactor by increasing the heat exchange surface area and reducing the characteristic heat exchange distances. Moreover, supercritical water can be implemented as a heat carrier for the entire thermal energy system within a concentrating solar thermal plant, from the receiver, through the heat storage system, and also within a conventional turbine-driven electric power generation system. Thus, the total system energy efficiency can be improved by minimising HTF heat exchangers.

Novel synthesis of porous Mg scaffold as a reactive containment vessel for LiBH4

A novel porous Mg scaffold was synthesised and melt-infiltrated with LiBH4 to simultaneously act as both a confining framework and a destabilising agent for H2 release from LiBH4. This porous Mg scaffold was synthesised by sintering a pellet of NaMgH3 at 450 °C under dynamic vacuum. During the sintering process the multi-metal hydride, decomposed to Mg metal and molten Na. The vacuum applied in combination with the applied sintering temperature, created the ideal conditions for the Na to vaporise and to gradually exit the pellet. The pores of the scaffold were created by the removal of the H2 and Na from the body of the NaMgH3 pellet. The specific surface area of the porous Mg scaffold was determined by the Brunauer–Emmett–Teller (BET) method and from Small-Angle X-ray Scattering (SAXS) measurements, which was 26(1) and 39(5) m2 g−1 respectively. The pore size distribution was analysed using the Barrett–Joyner–Halenda (BJH) method which revealed that the majority of the pores were macropores, with only a small amount of mesopores present in the scaffold. The melt-infiltrated LiBH4 was highly dispersed in the porous scaffold according to the morphological observation carried out by a Scanning Electron Microscope (SEM) and also catalysed the formation of MgH2 as seen from the X-ray diffraction (XRD) patterns of the samples after the infiltration process. Temperature Programmed Desorption (TPD) experiments, which were conducted under various H2 backpressures, revealed that the melt-infiltrated LiBH4 samples exhibited a H2 desorption onset temperature (Tdes) at 100 °C which is 250 °C lower than the bulk LiBH4 and 330 °C lower than the bulk 2LiBH4/MgH2 composite. Moreover, the LiH formed during the decomposition of the LiBH4 was itself observed to fully decompose at 550 °C. The as-synthesised porous Mg scaffold acted as a reactive containment vessel for LiBH4 which not only confined the complex metal hydride but also destabilised it by significantly reducing the H2 desorption temperature down to 100 °C.

Complex hydrides as thermal energy storage materials: characterisation and thermal decomposition of Na2Mg2NiH6

Complex transition metal hydrides have been identified as being materials for multi-functional applications holding potential as thermal energy storage materials, hydrogen storage materials and optical sensors. Na2Mg2NiH6 (2Na+·2Mg2+·2H−·[NiH4]4−) is one such material. In this study, the decomposition pathway and thermodynamics have been explored for the first time, revealing that at 225 °C, hydrogen desorption commences with two major decomposition steps, with maximum H2 desorption rates at 278 and 350 °C as measured by differential scanning calorimetry. The first step of decomposition results in the formation of Mg2NiHx (x < 0.3) and NaH, before these compounds decompose into Mg2Ni and Na, respectively. PCI analysis of Na2Mg2NiH6 has determined the thermodynamics of decomposition for the first step to have a ΔHdes and ΔSdes of 83 kJ mol−1 H2 and 140 J K−1 mol−1 H2, respectively. Hydrogen cycling of the first step has been achieved for 10 cycles without any significant reduction in hydrogen capacity, with complete hydrogen desorption within 20 min at 395 °C. Despite the relatively high cost of Ni, the ability to effectively store hydrogen reversibly at operational temperatures of 318–568 °C should allow this material to be considered as a thermal energy storage material.