Abstract
Hydrogen is a promising and clean fuel for transportation and domestic applications, but is difficult to store. Many systems have been investigated in order to improve the maximum hydrogen storage capacity (reversibility), high kinetics, moderate equilibrium pressure and/or decomposition temperature, and better cyclability. In this chapter, a review of studies related to stability of Zr-based Laves phase system as well as in-situ neutron diffraction investigation, the kinetics of TiFe, surface treatment of LaNi5 system, mechanically alloyed Mg-based hydrides, and graphite nanofibers are reported.
Top1. Introduction
In the search for alternative fuels, hydrogen is the ideal candidate as a clean energy carrier for both transportation and stationary applications. The storage of hydrogen in the form of metallic hydrides is the safest method and presents several advantages, e.g.: i) relatively large hydrogen storage capacity compared with the gaseous, liquid and even the solid form (see Table 1); and ii) good reversibility of hydrogenation/dehydrogenation with many metallic hydrides.
Table 1. Density of hydrogen in selected compounds
| Compound | Number of H atoms / cm3 (x 1022) |
| Liquid Hydrogen (20 K) | 4.2 |
| Solid Hydrogen (4.2 K) | 5.3 |
| TiH2 | 9.2 |
For optimum hydrogen storage, metal hydrides should meet the following criteria: high storage capacity, low dissociation temperature, moderate dissociation pressure, low heat of formation in order to minimise the energy necessary for hydrogen release (and also less heat to dissipate during the exothermic hydride formation); low cost, light weight (in particular for transport applications such as electric vehicles), and high stability against O2 and moisture for long cycle life.
The major challenges in the development of new hydrogen storage materials, with particular reference to batteries and fuel cells, are improved energy storage density, kinetics, cycle life, using readily available elements at a reasonable cost. Many studies have been devoted to the increase of the storage capacity by alloying modifications, whereas kinetics was improved via both alloying and processing. Different types of materials have been investigated for the purpose, including LaNi5 and TiFe compounds, Zr- and Ti-based Laves phases, Mg2Ni and Mg based materials, and composites. Recently, carbon nanostructures (nanotubes, nanofibers) and complex light metals hydrides (such as sodium alanates) have also attracted attention as promising materials for hydrogen storage. Table 2 reports some characteristics of some selected hydrides. This report outlines the pros and cons and promises of these groups of materials, with an aim of providing a balanced view of this rapidly developing technology.
Table 2. Characteristics of selected hydrides
| Material | Hydrides | Theoretical capacity (wt.%) | Equilibrium conditions
P/T (atm/°C) | Heat of formation
kJ/mole | Activation
conditions | Kinetics | Price |
| Zr | ZrH2 | 2.14 | P=1 / T=881 | -217 | average, needs high purity H2 | good at high temperatures | high |
| Ti | TiH2 | 4.01 | P=1 / T=643 | -164 | average, heat up to 400-600°C | average | low |
| V | VH2 | 3.78 | P=1 / T=20 | -40.12 | very difficult | good at high temperatures | high |
| Y | YH2
YH3 | 2.20
3.26 | /
/ | -72.0
-69.0 | /
/ | /
/ | very
high |
| Mg | MgH2 | 7.60 | P=1 / T=295 | -75.24 | 350°C, 30 atm
30 hours | very slow | low |
| FeTi | FeTiH2 | 1.89 | P=1 / T=0 | -24.66 | 400°C, 10 atm
10-20 hours | quite fast | quite
low |
| LaNi5 | LaNi5H6 | 1.37 | P=1 / T=10 | -31.8 | 25°C, 10 atm
1 hour | fast | high |
| ZrCr2 | ZrCr2H3.4 | 1.71 | P=1 / T=166 | -45.2 | heat to 500°C under vacuum | quite fast | average |
| TiCr1.8 | TiCr1.8H3.5 | 2.4 | P=1 / T=-91 | -20.2 | difficult | / | cheap |
| Mg2Ni | Mg2NiH4 | 3.59 | P=1 / T=253 | -64.37 | difficult | slow | average |
| Mg2Cu | MgH2+Mg2Cu | 1.44 | P=1 / T=290 | -66.0 | difficult | slow | average |
| ZrNi | ZrNiH3 | 1.96 | P=1 / T=292 | -76.85 | easy | fast | high |
| NaAlH4 | 3NaAlH4 →
Na3AlH6 + 2Al + 3H2
Na3AlH6 →
3NaH + Al + 3/2H2
NaH →
Na + 1/2H2 | 3.7
1.85
1.85
total: 7.4 | T=185 °C
T=230 °C
T=260 °C | /
/
/ | /
/
/ |
very slow |
low |
| SWNT | depend on purity
(see Table 3) | 8.4 * | P=0.75 /
T=-191 | / | / | / | very high |
*: Measured value.
Key Terms in this Chapter
Hydrogen Storage Capacity: The amount of H 2 absorbed/adsorbed by a material under specific conditions (temperature and pressure).
Intermetallics: Alloys of at least 2 elements that combine to form a material with a defined crystal structure, such as ZrCr 2 , LaNi 5 , TiFe, etc.
Kintetics: Behaviour of a material during hydrogenation and dehydrogenation.
Hydrogenation and Dehydrogenation: When a material absorbs (uptake) OR desorbs (release) a certain amount of H 2 under specific temperature and pressure.
Light-Complex Hydrides: Ionic hydrides containing usually alakali metals such as NaBH 4 , LiAlH 4 , etc.
Thermodynamics: The interaction of a material with hydrogen (H 2 ) at specific temperature, where H 2 uptake (released) depends on H 2 pressure. It is usually expressed by Van’t Hoff equation.
Mg-Compounds: Compounds that contains Mg element such as Mg 1-x M x , Mg 1-x M x H 2 , Mg 2 Ni, etc.
Thin Films: Layer of materials deposited onto substrate, classified as 2 dimension materials.
Carbon Nanostructures: All types of carbon based materials that possess a microstructure at the nanoscale, such as nanotubes, nanofibers, graphene, etc.