Elucidating the Role of Temperature and Pressure to the Thermodynamic Stability of Charged Defects in Complex Metal-Hydrides: A Case Study of NaAlH4


Complex metal hydrides are one of the most technologically important hydrogen storage materials because of its huge applications as clean energy alternatives. In this class of materials, hydrogen related point defects have been shown to play a crucial role in catalyzed dehydrogenation. Accurate modeling of such cases is extremely challenging as materials property changes under operational environmental conditions. In this work, our aim is to elucidate the importance of environmental effects (i.e. finite temperature, hydrogen partial pressure and doping) to find the thermodynamic stability of different defects. Our material of choice is NaAlH4 - an important hydrogen storage material and a widely studied prototypical system. Our methodology employs density-functional theory (DFT) combined with ab initio atomistic thermodynamics, where the free energy of formation due to vibration of phonons is duly considered under harmonic approximation. We show that to understand the thermodynamic stability of various defects and its self-diffusion, the contribution of environmental effect to the free energy of formation is absolutely indispensable. DFT, with appropriate exchange and correlation functional, fails to predict the stable phases even at moderately low temperature. Finally, amongst various transition metal dopants, we find Ti and Sc are the most suitable for improving the reaction kinetics for fast dehydrogenation in NaAlH4.

Contact details: 

Ekta Arora and Saswata Bhattacharya

Department of Physics