The TM single-atom catalytic system bidirectionally enhances the hydrogen absorption/desorption kinetics of Mg/MgH2: An insight into the synergetic enhancement mechanism and underlying principle

Abstract

Mg/MgH₂ has garnered significant attention primarily due to its abundant availability and high gravimetric density. Nevertheless, its practical implementation hindered by its high thermodynamic stability and sluggish kinetics. Fortunately, the introduction of transition metal single atom (TM SA) catalysts has emerged as an effective method to enhance the hydrogen storage properties of Mg/MgH₂. Among these catalysts, the synergistic effect of nanoconfinement and TM SAs plays a pivotal role in the hydriding/dehydriding kinetics of Mg/MgH₂. However, the effects of varying TM SAs interacting with N modified confined materials on H₂ adsorption and desorption and underlying mechanisms remain enigmatic. Leveraging DFT calculations, we investigated the potential of combining TM SA catalysts with N-modified Carbon nanomaterials (CNT) to enhance the hydrogenation/dehydrogenation of Mg/MgH₂. TM SA NCNTs-Mg/MgH₂ heterojunction systems encompassing ten 3d/4d transition metals were designed and constructed. We systematically investigated the impact of TM SA NCNTs on the hydrogen absorption and desorption properties of Mg/MgH₂ by examining parameters such as the electronic localization function (ELF), distorted charge density distributions, adsorption energies, dissociation energies, electronegativity, and the d-band center. Notably, the energy barriers for Mg/MgH₂ hydrogenation and dehydrogenation were significantly reduced by 0.2–0.7 eV and 1.6–2.2 eV, respectively, through the catalytic promotion of TM SA NCNTs. Herein, a novel “electronic-ropeway” effect was proposed to elucidate the underlying mechanism responsible for enhancing the hydrogen absorption and desorption kinetics in Mg/MgH₂. Specifically, the contribution degree of TM SA NCNTs and system electronegativity emerged as effective descriptors for predicting the reduced hydrogenation/dehydrogenation energy barriers. It is anticipated that elucidating the role of TM SA-N-CNTs will pave the way for developing innovative strategies to enhance the hydrogen absorption and desorption kinetics of Mg/MgH₂ systems, thereby providing valuable design principles for the construction of novel Mg/MgH₂ hydrogen storage materials.