First-principles study on the dehydrogenation thermodynamics and kinetics of Ti, Zr, V and Nb doped MgH2

Understanding the desorption thermodynamics and kinetics of MgH2 is crucial for its engineering application. Using first -principles calculations, we investigated H -related point defects in bulk MgH2 and the role of Ti, Zr, V and Nb doping on hydrogen desorption from a defect point of view. It was found that charged, rather than neutral, H -related defects appear in the desorption process, but the four dopants prefer to substitute Mg with the neutral charge state. Electronic structure analysis reveals that covalent and ionic bonds coexist between H and the dopants. Therefore, the four dopants can weaken the ionic interaction of Mg-H. Ti, Zr, V and Nb doping not only resultes in a decrease in the microscopic removal energy for both neutral and charged nearby H atoms but also reduces the macroscopic hydrogen desorption temperature. Desorption kinetics is studied by examining the energy barriers for charged hydrogen to leave away from the dopants along several pathways. Mass transport of hydrogen in pure MgH2 is mediated by positively charged vacancy or negatively charged interstitial. In doped MgH2, it is very hard for charged hydrogen interstitials to escape from the doping atoms, but both positively and negatively charged H vacancies are easy to leave away from the doping atoms, implying these dopants cannot hinder mass transport of hydrogen in the desorption process. Our theoretic study on H -related defects reveals that Ti, Zr, V and Nb doping is beneficial for hydrogen desorption from MgH2.