Tailoring Reaction Enthalpies of Hydrides

Furthermore, to realize certain hydrogen release pressures the storage tank needs to be kept at a certain temperature level which again depends on the exact alloy/ composite composition. Therefore, there is a lower limit for the temperature of the generated heat so that it can be used directly without any additional heat pumps so on, for heat supply to the storage tank. This shows that the reaction enthalpy is a 

Only a few binary hydrides like VH2 or NbH2 show reasonable hydrogen desorption pressures at temperatures suitable for most technological applications. Therefore, thermodynamic tuning of metal hydrides was, and still is, an important challenge to be faced. 

The development of conventional room-temperature hydrides based on interme-tallic compounds led to a large number of storage materials exhibiting very favorable sorption enthalpies with values of around 25 kj (mol H2), which can be operated in combination with conventional (80 °C operation temperature) polymer electrolyte membrane (PEM) fuel cells. However, their gravimetric storage capacity is limited to less than 3 wt.% H2. 

The remaining challenge, therefore, is the development of lightweight hydrides or hydride composites with suitable kinetics in an appropriate temperature range as well as suitable thermodynamic properties. 

However, the temperatures of operation and the reaction heat are too high for most technical applications, for example, 200 °C and 75 kJ (mol H2) , respectively. 

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To conclude The functional anion concept opens up ne v possibilities to tailor reaction enthalpies of high capacity hydrides to vards potential applications. At present, ho vever, not much is known about how successful this method will eventually be and which material classes can be successfully altered by it. 

Another attractive alternative to tailor the reaction enthalpies of already known single component hydrides MH is to mix them with suitable reactants (A) which react exothermically with the hydrides during heating whereby hydrogen is released and one or many stable compounds are formed according to the scheme ... 

In addition to these single-phase hydrides multiphase mixed hydrides or so-called reactive composites attract increasing attention. By addition of another element or compound the reaction enthalpy of the hydride can be tailored very efficiently if a stable compound is formed from the hydride and the additive(s) upon desorption. Unfortunately, this approach leads to an unwanted decrease in hydrogen storage capacity. 

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