Structural Changes and Formation Energy Study in BCC Type Hydrogen Storage Materials
Y. Tanaka, M. Katagiri, H. Onodera, H. Ogawa
National Institute for Material Science, JP
Keywords: BCC, Hydrogen, electronic structure
Abstract:Considering sustainability, hydrogen (H) can be a candidate of alternative energy resource for its harmlessness. The problem to use H is how to transport and store it. Hydrogen storage metallic compounds have a potential to store H more efficiently than liquid hydrogen in terms of volumetric density. Materials whose structure is Body-Centered-Cubic (BCC) type have been emphasized for their easy manageability. Among them, first, we focus on Vanadium (V), since V is the only pure metal which can allow absorption and desorption of hydrogen at near ambient temperature and pressure. These conditions are the important criterion for the design of hydrogen storage materials. But there is an issue on this V-H system, which is not the uptake but the release of hydrogen at near ambient temperature and pressure conditions. The limitation of the released H attributes to the different plateau pressure of H in V. The logarithmic plateau pressure is proportional to the chemical potential of gaseous H. The chemical potential is equal to that of H in the host material. When phase transition happens, there is no difference of H chemical potential as shown in Fig.1. Also, during phase transition, plateau region appears in Pressure-Composition-Temperature (PCT) curves. As shown in Fig. 1, V-H system goes through two phase transitions, from alpha to beta and from beta to ganma phase. The hydrogen storage materials which are categorized as BCC type have common phase transition and therefore can be characterized by the beta phase. The beta phase causes change of the chemical potential of H in host materials and determines the amount of released H. Here is the significance for investigating beta phase. In our previous work, we showed the stability of beta phase around VH component. Whether we can increase the amount of released H or not depends on the success to release H from the VH composition. Necessary condition is to raise the chemical potential of H around VH composition. In order to control the chemical potential of H, we also study the doping effect of additional elements in V. One approach to solve this issue is to add hydrogen-phobic elements in V. We elucidate the doping effect on H enthalpy by DFT method. When Chromium is doped in V, the formation energy with H becomes higher than that of V. That result implies the raised H chemical potential in the host material. We finally, discuss the possibility of increasing the amount of released H.