The effects of graphite on the reversible hydrogen storage of nanostructured lithium amide and lithium hydride (LiNH2 + 1.2LiH) system

The addition of 5 wt.% of graphite was incorporated into the (LiNH2 + 1.2LiH) hydride system in order to study its effect on the prevention of LiH from hydrolysis/oxidation which leads to the escape of NH3. The composite hydride system was processed by ball milling for 25 h. Thermal behavior in DSC up to 500 °C and isothermal desorption in a Sieverts-type apparatus were carried out. XRD was used to obtain information about phase changes. It is found that after ball milling graphite becomes amorphous. DSC analysis shows that for the mixture ((LiNH2 + 1.2LiH) + 5 wt.% graphite) graphite can prevent or at least substantially reduce the oxidation/hydrolysis of LiH since no melting peak of retained LiNH2 is observed. Both the DSC and Sieverts-type tests show that the addition of graphite increases the apparent activation energy of desorption from the ∼57–58 to ∼85–90 kJ/mol range. On the other hand, the graphite additive increases measurably the desorbed/absorbed capacity of hydrogen at 275, 300 and 325 °C. The ((LiNH2 + 1.2LiH) + 5 wt.% graphite) system is fully reversible desorbing/absorbing ∼5 wt.% H2 at 325 °C in the following reaction: (LiNH2 + LiH ↔ Li2NH + H2). Step-wise pressure–composition–temperature (PCT) tests show that the enthalpy and entropy change of this reversible reaction is −62.4 and −61.0 kJ/mol H2 and 117.8 and 115.8 J/mol K for undoped and 5 wt.% G doped (LiNH2 + 1.2LiH) system, respectively. It shows that within an experimental error there is no measurable effect of graphite additive on the thermodynamic properties. The Van’t Hoff analysis of the obtained thermodynamic data shows that the equilibrium temperature at atmospheric pressure of hydrogen (1 bar H2) is 256.8 and 253.9 °C for the undoped and 5 wt.% G doped (LiNH2 + 1.2 LiH) system ball milled for 25 h, respectively. Such high equilibrium temperatures render it rather obvious that both of these hydride systems cannot be employed for hydrogen desorption/absorption below 100 °C as required by the DOE targets for the automotive hydrogen storage materials.

► It is shown that, indeed, the graphite additive prevents LiH from being hydrolyzed/oxidized. The addition of graphite slightly increases the apparent activation energy of desorption and the desorbed hydrogen capacity. Furthermore, step-wise pressure–composition–temperature (PCT) tests show that the enthalpy and entropy change of the reversible reaction is −62.4 and −61.0 kJ/mol H2 and 117.8 and 115.8 J/mol K for undoped and 5 wt.% G doped (LiNH2 + 1.2LiH) system, respectively.