Finite temperature effects in complex metal hydrides: A case study of NaAlH₄

 

*Ekta Arora, Saswata Bhattacharya

 

E-mail:*Ekta.Arora[at]physics.iitd.ac.in

 

Department of physics, Indian Institute of Technology Delhi, India

 

Hydrogen being a clean carrier of energy can serve as an alternative of conventional sources of energy in order to meet the ever growing energy demands of the world. However, because of high diffusivity it expands to larger volumes and its cost effective storage becomes a challenge. Realizing hydrogen energy into vehicular applications calls for its storage into solid state materials. Complex metal hydrides are one of such candidate materials with high gravimetric storage capacity. The dehydrogenation of these materials exploits the mechanism of diffusion of H-related defects. In this talk, we will be emphasizing on the importance of environmental effects, i.e., temperature, pressure and doping on the relative stability of H-related defects in bulk NaAlH₄, which is a widely studied prototypical system. Our methodology includes ground state calculations using density functional theory (DFT) followed by inclusion of ab initio thermodynamics for finite temperature calculations. We have duly considered the free energy of lattice vibrations (phonons) within the harmonic approximation. We show that relative stability of such defects depends on the temperature and partial pressure of hydrogen to a great extent. We obtain totally different trend in the stability of H-related defects at 0 K and 300 K. We show that in order to capture the most favorable H-related defects in bulk NaAlH₄ at its operational temperature, inclusion of environmental effects is very crucial. DFT alone with appropriate functional is not sufficient to determine the defect stability trend in complex metal hydrides ^[1]. Further, we analyse the effect of transition metal atom dopants substituted at the sites of Na and Al on the self diffusion of H-related defects. We show that Ti and Sc are the best suited dopants towards increasing the diffusivity of the most prominent H-related defects. Hence, Ti and Sc should be the most favored dopants for improving the reaction kinetics of dehydrogenation in bullk NaAlH₄.

 

References:

1. E. Arora, S. Saini, P. Basera, M. Kumar, A. Singh, S. Bhattacharya J. Phys. Chem. C, 123, 62 (2019).