Role of temperature and pressure to the thermodynamic stability of charged defects in complex metal hydrides: A case study of NaAlH4
Ekta Arora* and Saswata Bhattacharya
Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016
*Ekta.Arora[at]physics.iitd.ac.in
Complex metal hydrides are considered one of the most technologically relevant source of molecular hydrogen, a clean and safe alternative of conventional energy resources. In this class of materials, dehydrogenation at moderate temperature is possible only in the presence of a suitable catalyst. However, the mechanism of catalysed dehydrogenation involves self-diffusion of hydrogen related point defects. In this work, we emphasize on the importance of environmental effects (viz. temperature and partial pressure of hydrogen and doping) on the thermodynamic stability of hydrogen related point defects. Our material of choice is NaAlH4, a viable hydrogen storage material and a widely studied prototypical system. Our approach employs density functional theory (DFT) combined with ab initio atomistic thermodynamics, where the free energy due to lattice vibrations is duly considered within harmonic approximation. We show here, in order to understand the relative thermodynamic stability of various charged defects in bulk complex metal hydrides, inclusion of environmental effects is absolutely indispensible. DFT analysis alone completely fails to predict the correct stable phases even at moderately low temperature [1]. We further show that amongst various transition metal dopants, Ti and Sc are the most suitable for improving the reaction kinetics in NaAlH4.
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