Multicomponent hydrogen storage systems

2) shows us that AG will be zero for an equilibrium pressure of 1 atm. Hence Eq. (17.1) rearranges to  

Given that AS is very similar for all the solid hydride dehydrogenations, because these reactions normally liberate one mole of gas from a solid, Eq. (17.4) illustrates the importance of the enthalpy of dehydrogenation upon 7(1 bar), i.e. a larger dehydrogenation endotherm results in a higher 7(1 bar). 

This chapter will first explain the theory of destabilisation and then move on to look at specific examples starting with the destabilisation of complex hydrides with separate sections devoted to different classes of destabilisation agents first metal hydrides, then non-hydride systems (where either an element or alloy which does not hydride or an inorganic salt is added as the destabilisation agent) and finally attempts to use another complex hydride to form a destabilised multicomponent system. The penultimate section summarises the work reported on other multicomponent hydrogen storage systems, predominantly the destabilisation of binary hydrides by other elements, and the chapter is concluded with an outlook of potential future research. 

The underlying principle to destabilisation is best illustrated using an enthalpy diagram such as that shown in Fig. 17.1. The example on the left would be illustrative of destabilisation of a binary hydride, YH2. Without the addition of a second phase, YH2 will decompose, forming the elements with a change in enthalpy of A7/a (i.e. -A//f(YH2), where A//f is the enthalpy of formation). Introducing a second phase, Z, to YH2 now allows an alternative reaction to occur where Y can combine with Z to form YZ, which has a lower change in enthalpy, A7/b (i-e. A//b = + A7/f(YZ)). For example Si can be added to 

MgH2 (Vajo et al, 2004) to reduce the enthalpy of dehydrogenation via the following reaction  

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These examples with carbon illustrate how difficult it is to predict the reaction path that a multicomponent hydrogen storage system will follow. A number of ternary phase multicomponent hydrogen storage systems have been proposed (Alapati et al, 2007b), but these will not be discussed as there will be even greater uncertainty that the proposed reaction will occur and the kinetics are likely to be even worse as such systems will require mass transport between three phases rather than two. 

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