Hydrogen carrier density

Dehydrogenation activities, compared for tetralin and decalin [5,12] under the same superheated liquid-film conditions over the same Pt/C catalyst, exhibited around 3.9-63 times preference of tetralin (Table 13.3), which can certainly be ascribed to advantageous adsorption due to the a-bonding capability of its aromatic part [17-19]. It was, thus, confirmed experimentally that tetralin is superior to decalin as the organic hydrogen carrier for stationary applications in terms of rapid hydrogen supply or power density, provided that the density of fuel storage is unimportant. 

In the study by Johnson et al. (1986) it was shown by Hall effect measurements that the sheet carrier density was decreased and the mobility was increased for a thin w-type layer following exposure to a hydrogen plasma at 150°C. To explain the mobility increase it was argued that donor-H complexes were formed and that the concentration of ionized scattering centers was thereby decreased. On the basis of semiempirical calculations, a structural model was suggested for the donor-H complex in... 

Fig. 7.16 Conductivity a as a function of temperature T for a YH film at a series of charge carrier densities n (T = 0.35 K). The metal-insulator transition is tuned by a combination of hydrogen gas loading (curves up to 300 K) and UV illumination...

In this context, hydrogen carriers like alcohols feeding a direct alcohol fuel cell (DAFC) appear advantageous for two main reasons they are liquid (which simplifies the problems of storage and distribution) and their theoretical mass energy density is rather high, close to that of gasoUne (6.1 and 8.0 kWh/kg for methanol and ethanol, respectively [4]). The most studied alcohols are methanol [5], which is the simplest mono-alcohol, and ethanol [6]. 

A comparison of the gravimetric and volumetric density of various hydrogen carriers shows that liquid hydrocarbons have - apart from borohydrides - by far the best combined properties (see Figure 2.1). 

Another exciting feature is the possibility of observing a continuous decrease in the carrier density when absorbing hydrogen, which gives rise to a metal-insulator (M-I) transition somewhere in the interval between 2 and 3 atoms H per atom R. Moreover, temperature dependent M-I transitions have been observed in the p-phase between 250 and BOOK both in sub-stoichiometric LaHs and CeHs- as well as in super-stoichiometric YH2+J and in RH2+ (R=Gd, Ho, Er) with xf 0.1-0.3, which are driven by the order-disorder transformation in the H sublattice mentioned above. 

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