The possibility of using ammonia as a hydrogen carrier is discussed. Compared to other hydrogen storage materials, ammonia has the advantages of a high hydrogen density, a well-developed technology for synthesis and distribution, and easy catalytic decomposition. Compared to hydrocarbons and alcohols, it has the advantage that there is no CO2 emission at the end user. The drawbacks are mainly the toxicity of liquid ammonia and the problems related to trace amounts of ammonia in the hydrogen after decomposition. Storage of ammonia in metal ammine salts is discussed, and it is shown that this maintains the high volumetric hydrogen density while alleviating the problems of handling the ammonia. Some of the remaining challenges for research in ammonia as a hydrogen carrier are outlined.

Ammonia for hydrogen storage: challenges and opportunities

Materials for hydrogen-based energy storage – past, recent progress and future outlook

Hydrogen storage alloys are of particular interest as a novel group in functional materials owing to their potential and practical applications in Ni/MH rechargeable batteries. This review is devoted to the specific alloy families developed for high-energy and high-power Ni/MH batteries in the last decades, especially for EV, HEV and PHEV applications. The scope of the work encompasses principles of Ni/MH batteries, electrochemical hydrogen storage thermodynamics and kinetics, prerequisites for hydrogen storage electrode alloys and recent advances in hydrogen storage electrode alloys. Rare earth AB5-type alloys, Ti- and Zr-based AB2-type alloys, Mg-based amorphous/nanocrystalline alloys, rare earth-Mg–Ni-based alloys and Ti–V-based alloys are highlighted. Additionally, the challenges met in developing advanced hydrogen storage alloys for Ni/MH rechargeable batteries are pointed out and some research directions are suggested.

Advanced hydrogen storage alloys for Ni/MH rechargeable batteries

Magnesium based materials for hydrogen based energy storage: Past, present and future

Nanocrystalline metal hydrides

The thermal expansion of the Mg-5.6 wt.% NiH 2 and Mg-55wt.% NiH 2 systems was examined by in situ X-ray powder diffraction using a specially designed apparatus for measurements at 500 °C and a hydrogen pressure of 5 MPa. The temperature dependence of the unit cell parameters of each alloy and hydride phase and the volume change associated with each phase transformation were determined. The thermal expansion data are summarized. The volume expansions of magnesium to MgH 2 , Mg 2 Ni to high temperature Mg 2 NiH 4 and high temperature Mg 2 NiH 4 to low temperature Mg 2 NiH 4 were calculated from these data as 30.4% (0.2%) at 340 °C, 27.8% (0.2%) at 320°C and 0.3% (0.1%) at 234 °C respectively (the standard deviations are given in parentheses).

Phase transformation and thermal expansion of MgNi alloys in a hydrogen atmosphere

Crystal structure, phase abundance and electrode performance of Laves phase compounds (Zr, A)V0.5Ni1.1Mn0.2Fe0.2 (A Ti, Nb or Hf)

Two experimental apparatuses — one based on a steady state method and the other based on a non-steady state method — were constructed for measuring the effective thermal conductivity of metal hydrides. The experimental and analytical procedures for the steady state method are described in detail and data are presented for the TiMn1.5 hydride as a function of hydrogen gas pressure, temperature and hydride composition.

Experimental measurements of thermal conductivity

Crystal structure of multiphase alloys (Zr,Ti)(Mn,V)2

The high temperature form of Mg 2 Ni hydride has an f.c.c. lattice with the metal atoms occupying antifluorite positions. It transforms to the low temperature form at 235°C. In earlier work the structure of the low temperature form was determined from its X-ray powder diffraction data. It can be described as a monoclinic distortion of the high temperature cubic lattice. However, the low temperature phase is usually contaminated with a small amount of an unknown phase which shows a similar powder pattern to the high temperature form. In the present investigation it was revealed that this unknown phase is another polymorph of Mg 2 Ni hydride which is easily transformed into the low temperature form by grinding the powder in a mortar at room temperature. This intermediate phase was found to be stabilized by substituting cobalt for 10 wt.% Ni. This structure can also be described as a distortion of the high temperature cubic lattice and the main peaks were indexed with an orthorhombic cell with a = 6.572 A , b = 4.520 A and c = 4.560 A . However, the presence of several weak peaks indicates that longer-range ordering exists and that the true unit cell is larger than this.

Y. Ishido

Papers

Phase transformation and thermal expansion of MgNi alloys in a hydrogen atmosphere

Crystal structure, phase abundance and electrode performance of Laves phase compounds (Zr, A)V0.5Ni1.1Mn0.2Fe0.2 (A Ti, Nb or Hf)

Experimental measurements of thermal conductivity

Crystal structure of multiphase alloys (Zr,Ti)(Mn,V)2

Phase transformations among three polymorphs of Mg2NiH4