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 degrees 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 similar to 57-58 to similar 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 degrees C. The ((LiNH2 + 1.2LiH) + 5 wt.% graphite) system is fully reversible desorbing/absorbing similar to 5 wt.% H-2 at 325 degrees C in the following reaction: (LiNH2 + LiH <-> Li2NH+H-2). 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 H-2 and 117.8 and 115.8 J/molK 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 H-2) is 256.8 and 253.9 degrees 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 degrees C as required by the DOE targets for the automotive hydrogen storage materials. (C) 2011 Elsevier B. V. All rights reserved.