report
5.4.3.2 Hydrogen Production & Storage

Hydrogen production

Most developments of nanotechnology in hydrogen production are performed for the photo-electrolysis cells. Indeed, photo-electrode materials and their associate process displaying great performances (high efficiency) and longevity (corrosion-resistance) are required. At first, one method of hydrogen production by photo-electrolysis is the light-induced photo-oxidation of water, where metal oxides are often employed as the photocatalyst. TiO₂ is an effective catalyst for hydrogen production from water but its efficiency needs to be improved. The global idea is to use nanostructured catalyst to improve the efficiency such as vanadium oxide, which exhibited a high quantum efficiency of ~38.7 % when synthesized as nanorods. The implementation of aligned vanadium oxide nanorods into thin film allows tuning the hydrogen production by varying the incident angle of UV light on the films. Then, a high rate is obtained under UV light. It allows therefore possible commercial applications of this material as photoassisted hydrogen generators.

Hydrogen storage

Nanotechnologies are useful essentially for solid-state hydrogen storage. Some other usages of nanotechnologies can be considered for example to improve the mechanical strength of the hydrogen storage tank. Here the report focused on the use of nanotechnologies in solid-state storage. Solid-state storage seems to be the most appropriate storage means as it requires low pressure and is supposed to be a low volume storage mean.

The most important criteria taken into account to define the ability of a material to store hydrogen are the following: the hydrogen storage capacity, the number of time the storage can be done reversibly and the kinetics and temperature of the hydrogen adsorption or desorption. These criteria are strongly linked to the structure of the alloy. It seems therefore that powder or particles having nanostructured features have advantages.

Metal Hydrides

Metal hydrides seem to offer the best compromise weighing both safety and cost among all the known hydrogen storage methods. The volumetric hydrogen density of LaNi₅H₆ is 115 kg/m³ which is much higher than that of compressed or liquid hydrogen. The most interesting ones have been classified thanks to the following symbols AB₅ (e.g. LaNi₅), AB (e.g. FeTi), A₂B (Mg₂Ni) and AB₂ (e.g. ArV₂).

Concerning hydrides, it is then necessary to reduce the absorption/desorption temperature, to improve the kinetic of the reactions and to keep or get a high storage capacity.

A focus has been made on magnesium hydride and magnesium-based alloy. They are indeed considered as a promising hydrogen storage material thanks to magnesium hydride nominal capacity of 7.6 wt-% higher than those of other metal hydrides. The limitation of the bulk material is the slow absorption/desorption of hydrogen, the high temperature required for dehydrogenation and the durability, i.e. the resistance to cyclic hydrogenation/dehydrogenation.

The addition of a catalyst[i], the ball milling to reduce the crystallite and particle size[ii], mechanochemical methods[iii] have been developed to increase the hydrogen absorption rate and to reduce the desorption temperature.

It has to be noted that the small size of nanostructured materials has a strong influence on the rate of hydrogen absorption and dissociation as the diffusion rate indeed increases and the diffusion length decreases. For example, Nanocrystalline Mg obtained by ball milling exhibit much faster hydrogen sorption rates (several minutes) than its bulk counterparts (several hours) at relatively low temperatures. The limitation is the decrease of reversibility of the reaction due to a reduction of intra-grain volume[iv].

The addition of a small amount of catalysts (e.g. nanoparticles of Pd or Fe) could help the dissociation of H2 molecules or the recombination of H atoms, improving as a consequence the kinetics of sorption. Catalyst addition offsets the negative effects of surface oxidation and eliminates the need for activation.[v] It can possibly reduce absorption temperature and allow desorption at not too high temperature whereas pressure is maintained at low level[vi]. For example, hydrogen can be absorbed at 523 K with an increase capacity from 1.02 wt-% to 2.13 wt-% and can be desorbed at 613 K with an increase capacity from 0.5 wt-% to 5.12 wt-%.

The use of vanadium powder mixed with mechanically milled magnesium hydride leads to the possibility to desorbed hydrogen at low temperature (200 °C) and to obtain a rapid re-absorption of hydrogen at room temperature.[vii]

Nanocomposite materials of two or more hydrides are also proposed as a solution for efficient hydrogen storage. They show indeed improved hydrogen storage performances by combining attractive properties of components. An interesting ability is that the hydriding properties of nanocomposites exceed those of the components alone.

Adsorbents

The scope of candidates goes from activated carbon to carbon nanotubes and metal oxide frameworks.

Carbon nanostructures such as carbon nanofibers, nanotubes or fullerenes were supposed to have very high hydrogen storage capability: 10wt-% hydrogen at room temperature for CNTs and 5.3 wt-% hydrogen at 77 K for activated carbon. But it seemed that it was much a speculative value: it has been proven that only an undetected amount of hydrogen at least stored. Since then, studies are still performed because of the great theoretical potential of these structures. A solution could be the use of catalyst such as Ni to improve the storage capacity.

Several studies on single walled nanotubes and multi walled nanotubes have been performed. A record of 8 wt-% hydrogen capacity for purified single walled nanotubes[viii] has been reported but has never been reached again. For now, it seems that the hydrogen storage capacity of carbon materials depend on their specific surface area but not on their nanostructure.[ix] The hydrogen storage capacity can be improved by the addition of an alkali metal, by activation of the carbon[x] or by spillover.

High value of hydrogen adsorption in graphite nanofibers was indeed reported in 1998 (60wt-%)[xi] but has never been reached again.[xii]

Metal oxide frameworks (MOF) are networks of transition metal atoms bridged by organic ligands. Those ligands have been used to as structured nanoporous materials for hydrogen storage.[xiii] The main advantages are that they present large overall pore volumes and surface areas, adjustable pore sizes, the adsorption is tuneable thanks to the functionalisation of the ligand and the choice of the metal, and at least, the bulk volume is totally accessible. Thanks to these structures, the adsorption is better.

[i] "Tailoring Hydrogen Storage Materials Towards Application"; M. Dornheim, N. Eigen, G. Barkhordarian, T. Klassen & R. Bormann: Advanced Engineering Materials 2006, 8

[ii] "Particle size effects on the desorption properties of nanostructured magnesium dihydride (MgH2) synthesized by controlled reactive mechanical milling (CRMM)"; R.A. Varin, T. Czujko, Ch. Chiu & Z. Wronski: Journal of Alloys and Compounds 424 356 (2006)

[iii] "Improvement of hydrogen-storage properties of Mg by reactive mechanical grinding with Fe₂O₃"; I.-H. Kwon, J.-L. Bobet, J.-S. Baec & M.-Y. Song: Journal of Alloys and Compounds 396 264 (2005)

[iv] "Recent progress in hydrogen storage"; P. Chen & M. Zhu: Materials Today 11, 36 (2008)

[v] "Nanocrystalline magnesium for hydrogen storage"; A. Zaluska, L. Zaluski & J.O. Ström-Olsen: Journal of Alloys and Compounds 288 217 (1999)

[vi] "Improving hydrogen storage/release properties of magnesium with nano-sized metal catalysts as measured by tapered element oscillating microbalance"; X. Xu, C. Song: Applied Catalysis A: General 300, 130 (2006)

[vii] "Hydrogen desorption kinetics of a mechanically milled MgH 15at.%V nanocomposite"; G. Liang, J. Huot, S. Boily & R. Schulz: Journal of Alloys and Compounds 305 239 (2000)

[viii] "Hydrogen adsorption and cohesive energy of single-walled carbon nanotubes"; Y. Ye, C. C. Ahn, C. Witham, B. Fultza, J. Liu, A. G. Rinzler, D. Colbert, K. A. Smith & R. E. Smalley: Applied Physics Letter 74, 2307 (1999)

[ix] "Hydrogen adsorption in different carbon nanostructures"; B. Panella, M. Hirscher & S. Roth: Carbon 43, 2209 (2005)

[x] "Hydrogen storage on chemically activated carbons and carbon nanomaterials at high pressures"; M. Jordá-Beneyto, F. Suárez-García, D. Lozano-Castelló , D. Cazorla-Amoró s & A. Linares-Solano: Carbon 45, 293 (2007)

[xi] "Hydrogen Storage in Graphite Nanofibers"; A. Chambers, C. Park, R.T.K. Baker & N.M. Rodriguez: The Journal of Physical Chemistry B 102, 4253 (1998)

[xii] "Studies on synthesis and hydrogenation behaviour of graphitic nanofibres prepared through palladium catalyst assisted thermal cracking of acetylene"; B.K. Gupta, R. S. Tiwari & O. N. Srivastava: Journal of Alloys and Compounds 381, 301 (2004)

[xiii] "Hydrogen storage in metal-organic frameworks"; D.J. Collins & H.-C. Zhou: Journal of Materials Chemistry 17, 3154 (2007)