Hydrogen capacity

In the first three cycles, the capacity of the alanate is high for both materials, the titanium metal-doped and the nitride-doped material. However, after 15 cycles the hydrogen capacity decreases significantly below 4% for the titanium metal-doped sample, whereas for the nitride-doped sample the capacity remains high at about 5% hydrogen. 

High hydrogen capacity per unit mass and unit volume Low dissociation temperature... 

This reaction is not easily reversible onboard a vehicle. Hence, the by-products must be removed from the vehicle and regenerated offboard. The irreversible metal hydrides can include a transition metal, for example, Mg2FeH6 (5.5 wt% hydrogen capacity), a nontransition metal such as Be(BH4)2 (20.8 wt% hydrogen capacity), or NaAlH4. Various promising metal hydrides are discussed in the following sections. 

Fast adsorption/desorption kinetics and relatively small (<10 kj/mol) adsorption enthalpies are observed for hydrogen adsorption on many porous materials, which indicates that physisorption on porous materials is suitable for fast recharging with hydrogen [81,82], The narrowest pores make the biggest contribution to hydrogen-adsorption capacity, whereas mesopores contribute to total pore volume, but little to hydrogen capacity, and are detrimental for the overall volumetric capacity. Hence, porous materials with very narrow pores or pore-size distributions are required for enhanced hydrogen capacity at low pressures. 

With respect to the German case study used in Chapter 14 to discuss the build-up of a hydrogen infrastructure, Fig. 10.9 shows where surplus hydrogen capacities (from chlorine-alkali electrolysis) exist in Germany. If these capacities are added up, the resulting total amount is about 1 billion Nm3 per year (around 4% of total German hydrogen production). 

Figure 4. Diagram showing the enthalpy of formation and hydrogen capacity of known metal hydrides.

Graph 1. Hydrogen capacity for Pt-MnCb (bar of black check on white background) in comparison with theoretically calculated amount of gas adsorbed by Pt (plain bar) and support (bar with white diamonds on black background). 

Direct utilization of metallic Ti as dopant to prepare Ti-doped NaAlH4 offers a clear potential to achieve high hydrogen capacity. Material structural/compositional design is a feasible way to further improve hydrogen storage performance of Ti-NaAIRt system. 

Systems based on complex lithium tetraborohydrate are of interest because of its large hydrogen capacity (ca. 18.3 wt%). However the material requires large energy input to release stored hydrogen (AH = 66.9 kJ mol"1 H2 7). Work into the catalysis of the system has focused on chloride and oxide additions however these reduce the overall system capacity 7. 

Storage system Volumetric hydrogen capacity (kgH, m- ) Drawbacks... 

In the third method called mechanochemical activation synthesis (MCAS), a mixtnre of metal componnd, viz. metal chloride, is ball-milled to induce a reaction to yield a high-hydrogen capacity hydride. 

Hydrogen capacity H in Fig. 1.24 can be expressed in either atomic HIM ratio (H - number of H atoms, M - number of metal atoms) or weight percent (wt%), both of which are commonly used [14]. It must be noticed that calculating wt% both mass of hydrogen and mass of metal (not only mass of metal) must be considered in the denominator. 

Fig. 1.24 Schematic presentation of hydrogen capacity defined by various methods...

Capacity presentation in wt% is very useful from technological point of view, because it gives direct information on how much of hydrogen can be stored in a material. Regardless of the units, there are several ways to express hydrogen capacity. The reversible capacity is conservatively defined as the plateau... 

When change in hydrogen mass is known using (1.33) we can easily calculate hydrogen capacity in the investigated material. 

The difference in pressure between the initial pressure and that at the instant of time is taken for calculation of the mass of absorbed/desorbed hydrogen from (1.44) and the wt% of absorbed/desorbed hydrogen (capacity) from (1.33). 

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