Hydrogen-storage materials for mobile applications

Abstract

Energy can be stored in different forms: as mechanical energy (for example, potential energy or rotation energy of a flywheel); in an electric or magnetic field (capacitors and coils, respectively); as chemical energy of reactants and fuels (batteries, petrol or hydrogen); or as nuclear fuel (uranium or deuterium). Chemical and electric energy can be transmitted easily because they both involve electronic Coulomb interaction. Chemical energy is based on the energy of unpaired outer electrons (valence electrons) eager to be stabilized by electrons from other atoms. The hydrogen atom is most attractive because its electron (for charge neutrality) is accompanied by only one proton. Hydrogen thus has the best ratio of valence electrons to protons (and neutrons) of all the periodic table, and the energy gain per electron is very high.Hydrogen is the most abundant element on Earth, but less than 1% is present as molecular hydrogen gas H 2 . The overwhelming majority is chemically bound as H 2 O in water and some is bound to liquid or gaseous hydrocarbons. The clean way to produce hydrogen from water is to use sunlight in combination with photovoltaic cells and water electrolysis (see review in this issue by Grätzel, pages 338-344). Other forms of primary energy and other water-splitting processes are also used: the hydrogen consumed today as a chemical raw material (about 5 ǂ10 10 kg per year worldwide) is to a large extent produced using fossil fuels and the reaction of hydrocarbon chains (-CH 2 -) with H 2 O at high temperatures, which produces H 2 and CO 2 . Direct thermal dissociation of H 2 O requires temperatures higher than 2,000 ᑻC (>900 ᑻC with a Pt/Ru catalyst).The chemical energy per mass of hydrogen ) is at least three times larger than that of other chemical fuels (for example, the equivalent value for liquid hydrocarbons is 47 MJ kg -1 ). Once produced, hydrogen is a clean synthetic fuel: when burnt with oxygen, the only exhaust gas is water vapour, but when burnt with air, lean mixtures have to be used to avoid the formation of nitrogen oxides. Whether hydrogen can be considered a clean form of energy on a global scale depends on the primary energy that is used to split water 1 . The availability of free energy is often unsafe. The mechanical energy of a 1,000-kg car running out of control at 40 km h -1 can kill pedestrians. The process of burning hydrogen can be done in an efficient and controlled way to liberate energy at a desirable rate, or in an uncontrolled way with the potential to cause damage. For historical reasons hydrogen has a bad reputation, which is not altogether justified: a more recent analysis 2 of the Hindenburg catastrophe shows that the air ship caught fire because of a highly flammable skin material and not because of the hydrogen gas it contained. The safety of hydrogen relies on its high volatility and non-toxicity.Today, many scientists and engineers, some companies, governmental and non-governmental agencies and even finance institutions are convinced that hydrogen's physical...

Boron substituted graphene: energy landscape for hydrogen adsorption

“…Early euphoric reports of over 60 wt% storage of hydrogen in carbon nanofibers at ambient temperatures and 112 bar pressure (Chambers et al 1998) and of up to 20 wt% in alkali-doped nanotubes at 1 bar pressure (Chen et al 1999) have been however scaled down by subsequent studies. Proven storage for carbon nanotubes remains at less than 8 wt% of hydrogen even at cryogenic conditions of 77 K, and a meager 1.5 wt% at ambient temperature (Schlapbach and Zuttel 2001;Pfeifer et al 2008;Poirier et al 2004). Similar conclusion comes out from numerical simulation (Wang and Johnson 1999;Bathia and Myers 2006;Gigras et al 2007).…”

Storage of Hydrogen in the Pure Form

“…Another comparison of volumetric versus gravimetric storage density including solid storage systems is given in Figure 1.25 [26]. It is obvious that, with regard to volumetric storage density, storage of hydrogen in compounds has the greater potential.…”

Clathrate Hydrates

“…As explained above, scientists do not even agree yet on the number of guest molecules occupying the cages of hydrates. This is why Schlapbach and Zuttel [50] suggest that the key to the fabrication of proper hydrogen storage materials lies in further characterization of the fundamental nature and strength of hydrogen bonding interactions with a variety of host materials. Fortunately, with the discovery of hydrogen clathrate hydrates in 1999 and the spark of the idea of hydrogen storage in hydrates, the current decade is witnessing some activity by various groups.…”