Balancing Thermodynamics, Kinetics, and Reversibility in Ti-Doped MgB2H8: A First-Principles Assessment of a Practical Solid-State Hydrogen Storage Material

TL;DR

This study uses first-principles calculations to evaluate Ti-doped MgB2H8, a promising solid-state hydrogen storage material with improved thermodynamics and kinetics over the pristine compound.

Contribution

It demonstrates that Ti doping enhances hydrogen capacity, reduces desorption enthalpy, and improves diffusion barriers, making MgB2H8 more practical for hydrogen storage.

Findings

Pristine MgB2H8 has 14.9 wt% hydrogen capacity but high desorption enthalpy.

Ti doping maintains high capacity (~10.4 wt%) and lowers desorption enthalpy (~36 kJ/mol H2).

Diffusion barriers decrease to 0.38 eV with Ti doping, improving kinetics.

Abstract

Hydrogen storage remains a key challenge for the development of a sustainable hydrogen energy system, where materials must satisfy requirements on storage capacity, thermodynamics, kinetics, and reversibility. Complex borohydrides are attractive due to their high hydrogen density, but their practical use is limited by slow hydrogen diffusion and unfavorable desorption thermodynamics. In this work, we present a first-principles study of pristine and Ti-doped MgB2H8 as a solid-state hydrogen storage material. Density functional theory calculations show that pristine MgB2H8 has a high gravimetric hydrogen capacity of about 14.9 wt percent, but also a relatively high hydrogen desorption enthalpy of about 42 kJ per mol H2 and diffusion barriers around 0.5 eV, which limit its performance at moderate temperatures. Substitutional doping with Ti at the Mg site improves these properties while maintaining structural stability. The doped system retains a high hydrogen capacity of about 10.4 wt percent and shows a reduced desorption enthalpy of about 36 kJ per mol H2, placing it within a favorable thermodynamic range for hydrogen release. Nudged elastic band calculations show a reduction in hydrogen migration barriers to about 0.38 eV, indicating improved diffusion kinetics. Phonon and elastic analyses confirm that Ti doping preserves stability. Electronic structure analysis shows that Ti 3d states near the Fermi level weaken B-H bonding and stabilize intermediate hydrogen configurations, explaining the improved behavior. These results identify Ti-doped MgB2H8 as a promising hydrogen storage material.