Designing multivalent NiMn-based layered nanosheets with high specific surface area and abundant active sites for solid-state hydrogen storage in magnesium hydride

Abstract

Catalytic doping of magnesium hydride (MgH₂) to improve its hydrogen ab/desorption kinetic properties is considered to be an effective and feasible method. In solid-phase catalysis, the extent of contact between the catalyst and the substrate determines the catalytic reaction in a great sense. With large specific surface area and abundant active sites, two-dimensional (2D) nanomaterials are promising catalysts for MgH₂ via providing numerous pathways for the diffusion and dissociation of hydrogen. In this regard, 2D NiMn-based layered double hydroxide and layered metallic oxide (LMO) are designed and introduced into MgH₂ to improve its hydrogen storage properties. Simultaneous enhancement in interfacial contact, desorption temperature and kinetics are achieved. The MgH₂+9wt% Ni₃Mn-LMO composites begin to discharge hydrogen at only 190 °C and 6.10wt% H₂ could be charged in 600 s at 150 °C. The activation energy for de/hydrogenation is reduced by 42.43% and 46.56%, respectively, compared to pure MgH₂. Even at a low operating temperature of 235 °C, the modified system was still able to release 4.44wt% H₂ in an hour, which has rarely been reported in previous studies. Microstructure observations and density functional theory calculations revealed that first, the hydrogen pumping effect of Mg₂Ni/Mg₂NiH₄ promotes the adsorption and desorption of hydrogen molecules on the surface of MgH₂, second, MnO_x drew electrons from Mg₂Ni, producing a new Density of State structure with a lower d-bond center. This unique change further strengthens the Mg₂Ni/Mg₂NiH₄ pump effect on MgH₂. Our work indicates that the application of 2D metal-based catalysts is a feasible and promising approach towards MgH₂ for solid-state hydrogen storage to meet technical and scientific requirements.