The authors review available experimental information on the existence, thermodynamic stability and physical properties of hydrides formed by the absorption of hydrogen gas in intermetallic compounds of two transition metals. The emphasis is on stability. It is shown that empirical models for the stability of ternary hydrides can be reconciled with ideas based on the results of band structure calculations of (binary) metallic hydrides. It is concluded that metallic hydrides can be looked upon as alloys of metallic hydrogen. In addition to the thermodynamic properties of ternary metallic hydrides the authors discuss experimental information on electronic properties (magnetic, super-conductivity, band structure features) and on crystallographic and metallurgical properties (neutron scattering, nuclear and electron spin resonance, Mossbauer spectroscopy, diffusion). Applications are briefly reviewed.

https://iopscience.iop.org/article/10.1088/0034-4885/45/9/001/pdf

Hydrides formed from intermetallic compounds of two transition metals: a special class of ternary alloys

Storing hydrogen in materials is based on the observation that metals can reversibly absorb hydrogen. However, the practical application of such a finding is found to be rather challenging especially for vehicular applications. The ideal material should reversibly store a significant amount of hydrogen under moderate conditions of pressures and temperatures. To date, such a material does not exist, and the high expectations of achieving the scientific discovery of a suitable material simultaneously with engineering innovations are out of reach. Of course, major breakthroughs have been achieved in the field, but the most promising materials still bind hydrogen too strongly and often suffer from poor hydrogen kinetics and/or lack of reversibility. Clearly, new approaches have to be explored, and the knowledge gained with high-energy ball milling needs to be exploited, i.e. size does matter! Herein, progress made towards the practical use of magnesium as a hydrogen store and the barriers still remaining are reviewed. In this context, the new approach of tailoring the properties of metal hydrides through size restriction at the nanoscale is discussed. Such an approach already shows great promise in leading to further breakthroughs because both thermodynamics and kinetics can be effectively controlled at molecular levels.

Hydrogen in magnesium: new perspectives toward functional stores

Effects of different carbon materials on MgH2 decomposition

Using light metal hydrides as hydrogen carriers is of particular interest for safe and compact storage of hydrogen. Magnesium hydride (MgH2) has attracted significant attention due to its 7.6 wt% hydrogen content and the natural abundance of Mg. However, bulk MgH2 is stable (ΔHf ∼ 76 kJ mol−1) and releases hydrogen only at impractically high temperatures (>300 °C). Herein, we demonstrate a first attempt to achieve ambient-temperature reversibility of hydrogen storage for MgH2 by fabricating non-confined ultrafine nanoparticles. Taking advantage of the big discrepancies in the solubility of metal hydrides and chlorides in THF, a novel metathesis process of liquid–solid phase driven by ultrasound was proposed. Ultrafine MgH2 nanoparticles predominantly of around 4–5 nm in size were successfully obtained without scaffolds or supports. A reversible hydrogen storage capacity of 6.7 wt% at 30 °C was measured, which has never been achieved before, thanks to thermodynamic destabilization and decreased kinetic barriers. The bare nanoparticles exhibited a stable and rapid hydrogen cycling behaviour in 50 cycles at 150 °C, a remarkable improvement compared with bulk MgH2. Our finding brings MgH2 a step closer to practical applications and the methodology presented here opens new pathways for fabricating sensitive nanoparticles.

Realizing 6.7 wt% reversible storage of hydrogen at ambient temperature with non-confined ultrafine magnesium hydrides

In examining the potential role of hydrogen in the energy economy of the future, we take an optimistic view. All the technology required for implementation is feasible but a great deal of development and refinement is necessary. A pessimistic approach would obviously discourage further thinking about an important and perhaps the most reasonable alternative for the future. We have considered a limited number of alternative energy systems involving hydrogen and have shown that hydrogen could be a viable secondary source of energy derived from nuclear power; for the immediate future, hydrogen could be derived from coal. A hydrogen supply system could have greater flexibility and be competitive with a more conventional all-electric delivery system. Technological improvements could make hydrogen as an energy source an economic reality. The systems examined in this article show how hydrogen can serve as a general-purpose fuel for residential and automotive applications. Aside from being a source of heat and motive power, hydrogen could also supply the electrical needs of the household via fuel cells (19), turbines, or conventional "total energy systems." The total cost of energy to a residence supplied with hydrogen fuel depends on the ratio of the requirements for direct fuel use to the requirements for electrical use. A greater direct use of hydrogen as a fuel without conversion to electricity reduces the overall cost of energy supplied to the household because of the greater expense of electrical transmission and distribution. Hydrogen fuel is especially attractive for use in domestic residential applications where the bulk of the energy requirement is for thermal energy. Although a considerable amount of research is required before any hydrogen energy delivery system can be implemented, the necessary developments are within the capability of present-day technology and the system could be made attractive economically .Techniques for producing hydrogen from water by electrolysis, from coal, and directly from thermal energy could be found that are less expensive than those now available; inexpensive fuel cells could be developed, and high-temperature turbines could be used for the efficient conversion of hydrogen (and oxygen) to electricity. The use of hydrogen as an automotive fuel would be a key factor in the development of a hydrogen energy economy, and safe storage techniques for carrying sufficient quantities of hydrogen in automotive systems can certainly be developed. The use of hydrogen in automobiles would significantly reduce urban pollution because the dispersed fossil fuel emissions would be replaced by radioactive wastes generated at large central stations. The conversion of internal or external combustion engines for combustion of hydrogen fuel would probably have less economic impact on the automotive industry than the mass introduction of electric automobiles. However, this is a subject that requires more detailed study. All of the safety aspects of hydrogen utilization will have to be examined, especially the problems of safety in the domestic use and the long distance transport of hydrogen in pipelines at high pressures. It is our opinion that the various energy planning agencies should now begin to outline the mode of implementing hydrogen energy delivery systems in the energy economy. The initial transition to hydrogen energy derived from available fossil fuels such as coal should be considered together with the long range view of all the hydrogen being derived eventually from nuclear energy. By the year 1985 when petroleum imports may be in excess of the domestic supply, these plans could set the stage for the transition period from fossil to a predominantly nuclear energy economy able to supply abundant synthetic fuels such as hydrogen. Synthetic fuels will obviously be more expensive than fuels now derived from petroleum; however, there may be no other viable choice. Thus, it is essential that the analysis and technological feasibility of a hydrogen energy system be considered now. It is of vital importance to the nation to develop some general-purpose fuel that can be Produced from a variety of domestic energy sources and reduce our dependence on imported oil.

Hydrogen: Its Future Role in the Nation's Energy Economy.

Metal Hydride Storage for Mobile and Stationary Applications

Metal Hydrides as a Source of Fuel for Vehicular Propulsion

Iron titanium hydride as a source of hydrogen fuel for stationary and automotive applications

Metal hydrides offer the possibility of a convenient and safe method for the storage of hydrogen. These compounds provide for compact storage in a form that is equal to or better than cryogenic liquid hydrogen on a volume basis. Considerable research has gone into the study of hydrides derived from rare earth, iron-titanium and magnesium alloys. The formation of these compounds is reversible and the chemistry of relevant hydrides has been discussed. Heat must be provided to decompose these compounds and release the hydrogen, while heat is liberated when the compounds are formed and must be removed to allow the hydriding reactions to proceed to completion. The iron-titanium and magnesium alloys are especially promising hydride storage media, the former in stationary applications, or where weight is not a limiting consideration, and the latter for mobile applications, Each of these materials has unique pressure-temperature characteristics and reaction kinetics which must be considered in the design of a hydrogen storage system. These special characteristics are discussed for particular applications. The results of recent work on hydrogen storage development and the engineering design of storage systems are reviewed.

K. C. Hoffman

Papers

Metal Hydride Storage for Mobile and Stationary Applications

Metal Hydrides as a Source of Fuel for Vehicular Propulsion

Iron titanium hydride as a source of hydrogen fuel for stationary and automotive applications

Metal hydride storage for mobile and stationary applications

Metal hydrides as a source of hydrogen fuel