Improvement in Hydrogen Desorption from b- and g-MgH 2 upon Transition-Metal Doping Tanveer Hussain,* [a, b, d] Tuhina Adit Maark, [c] Sudip Chakraborty, [a] and Rajeev Ahuja [a, b] 1. Introduction The storage of hydrogen (H 2 ), which is an excellent alternate fuel to hydrocarbon-based energy sources, remains a key chal- lenge for the research community. For materials-based storage, a sufficiently high gravimetric H 2 -storage capacity and absorp- tion/desorption at feasible operating conditions are desired. Among the various candidate materials for solid-state H 2 stor- age, MgH 2 is a promising choice, owing to its high storage ca- pacity (7.6 wt %), abundance, low cost, and H 2 absorption/de- sorption reversibility. [1–3] However, although the strong Mg ˇH bonds ensure high stability (DH = ˇ75 kJ mol ˇ1 ), they also demand high desorption temperatures ( ⇡ 570 K), which limit the use of MgH 2 for practical applications. Slow hydriding/de- hydriding rates are also an issue associated with MgH 2 . [4, 5] Therefore, the efficient utilization of MgH 2 for reversible H 2 storage under suitable conditions depends on improving its thermodynamics and kinetics. Many strategies have been em- ployed in this regard, including nanostructuring, doping with foreign atoms, and the application of mechanical strain. [6–12] The bulk of the recent research work describing the H 2 -stor- age properties of MgH 2 has focused on doping with transition metals (TMs). On the experimental side, Liang et al. [13] explored the catalytic behavior of selected TMs (Ti, V, Mn, Fe, and Ni) mechanically milled into MgH 2 nanostructures. The Ti and V dopants significantly reduced the H 2 desorption energies and their composites showed a rapid absorption/desorption of H 2 . By means of mechanical alloying, Shang et al. [14] doped MgH 2 with Al, Ti, Fe, Ni, Cu, and Nb to study its H 2 -storage activities. TM doping (8 %) caused a rapid reduction of the hydrogen-de- sorption temperature in the following decreasing order: Ni > Al > Fe > Nb > Ti > Cu. Bassetti et al. [15] investigated the catalytic effect of Fe in MgH 2 –Fe nanostructures formed by ball milling, with the objective of improving the H 2 -desorption kinetics of MgH 2 . They concluded that for the rapid desorption of hydro- gen from the doped nanostructures, the catalyst should be dis- tributed uniformly in the MgH 2 structure. Similarly on the theoretical frontier, numerous studies have focused on examining the strategy of TM doping for address- ing the H 2 -absorption/desorption issues relating to MgH 2 . Xiao et al. [16] performed a first principles study based on density functional theory (DFT) to shed light on the electronic proper- ties and energetics of TM doping in MgH 2 at a significant doping concentration ( ⇡ 12 %). The TM dopants considered were Sc, Ti, V, Y, Zr, and Nb. The calculated formation enthal- pies showed that the Mg 7 TMH 16 hydrides are less stable than pure MgH 2 . It was predicted that the TMs formed strong inter- actions with H atoms, causing a destabilization of MgH 2 , which ultimately reduces the desorption temperatures. In another study, Shelyapina et al. [10] illustrated that mono- and co-doping of TMs (Ti, V, and Nb) resulted in the generation of Mg 7 TMH 16 and Mg 6 TM 2 H 16 phases, which facilitated hydrogen desorption at lower temperatures by improving the kinetics. Tao et al. [17] employed DFT calculations to analyze the hydrogen-diffusion path and calculate the activation barriers for various MgH 2 structures. They also investigated the effect of hydrogen va- A thorough study of the structural, electronic, and hydrogen- desorption properties of b- and g-MgH 2 phases substituted by selected transition metals (TMs) is performed through first- principles calculations based on density functional theory (DFT). The TMs considered herein include Sc, V, Fe, Co, Ni, Cu, Y, Zr, and Nb, which substitute for Mg at a doping concentra- tion of 3.125 % in both the hydrides. This insertion of TMs causes a variation in the cell volumes of b- and g-MgH 2 . The majority of the TM dopants decrease the lattice constants, with Ni resulting in the largest reduction. From the formation- energy calculations, it is predicted that except for Cu and Ni, the mixing of all the selected TM dopants with the MgH 2 phases is exothermic. The selected TMs also influence the sta- bility of both b- and g-MgH 2 and cause destabilization by weakening the Mg ˇH bonds. Our results show that doping with certain TMs can facilitate desorption of hydrogen from b- and g-MgH 2 at much lower temperatures than from their pure forms. The hydrogen adsorption strengths are also studied by density-of-states analysis. [a] Dr. T. Hussain, Dr. S. Chakraborty, Prof. R. Ahuja Condensed Matter Theory Group, Department of Physics and Astronomy Box 516, Uppsala University, 75120 Uppsala (Sweden) E-mail : tanveer.hussain@physics.uu.se [b] Dr. T. Hussain, Prof. R. Ahuja Applied Materials Physics, Department of Materials and Engineering Royal Institute of Technology (KTH), S-100 44 Stockholm (Sweden) [c] Dr. T. A. Maark School of Engineering, Brown University, Providence, RI 02912 (USA) [d] Dr. T. Hussain Centre for Theoretical and Computational Molecular Science Australian Institute for Bioengineering and Nanotechnology The University of Queensland, Brisbane, Qld 4072 (Australia) ChemPhysChem 2015, 16, 2557 – 2561 ⌫ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2557 Articles DOI: 10.1002/cphc.201500238