Catalytic Effect of Nb2O5 in MgH2-Nb2O5 Ball-Milled Composites

Abstract

We report a study on the desorption properties, crystallography and chemical state of MgH 2 and 1 mol% Nb 2 O 5 ball-milled composites. Desorption temperatures of the composites decreased with increase of ball-milling time. Size of MgH 2 crystallites decreased during ball-milling. Reduction of Nb 2 O 5 after ball-milling was confirmed by tracing the chemical state of Nb and was further supported by TEM observation. The reduced phases may act as more effective catalysts improving the desorption properties.

The Improvement in Hydrogen Storage Performance of MgH2 Enabled by Multilayer Ti3C2

“…Therefore, the dehydrogenation temperature of MgH 2 is higher than 300 • C. In addition, the adsorption and desorption kinetics of MgH 2 are poor, resulting in the slow reaction rate of absorption and desorption. The activation energy (E a ) of MgH 2 is ~143.0-160.6 kJ/mol [3][4][5][6][7]. Therefore, improving the thermodynamic and kinetic properties of hydrogen absorption/desorption reactions of Mg-based hydrogen storage materials is the key aim of current research.…”

Mechanically-Induced Catalyzation of MgH2 Powders with Zr2Ni-Ball Milling Media

“…Severe plastic deformation (SPD) technique [15], including high energy ball milling [16], cold rolling (CRing) [17,18], high pressure torsion (HPT) [19] and equal channel angular pressing (ECAP) [20] were employed to improve the hydrogen storage properties of MgH 2 . Apart from the mechanical treatment approach, catalyzation MgH 2 powders with proper volume fractions of catalytic agents [21], including pure metals [22], intermetallic compounds [23][24][25][26], metal-oxides [27,28], metal/metal oxide composites [5,29], -carbides [30,31], metallic glassy alloys [32][33][34] and chlorides [35] have shown outstanding effects on enhancing the hydrogen storage properties of MgH 2 and infer its apparent activation energy of decomposition [36]. More recently, self-assembled MgH 2 and metal borohydride nanoparticles on graphene technique has shown great enhancement of the behavior of the metal hydride phase [37][38][39].…”

Computational Investigation of MgH2/NbOx for Hydrogen Storage

“…In the literature, partially reduced Nb 2 O 5 , labeled as NbOx, has been proposed as an active phase to explain the improvement of MgH 2 dehydrogenation. − For instance, NbOx may serve as the site to attract surface hydrogen or introduce the NbO-MgH 2 interface, highlighting the importance of Nb oxidation states . Recent experimental effort has been made to control it, but it is very challenging given that the preparation of MgH 2 agents and the following hydrogenation/​dehyrogenation process is complicated.…”

“…Interestingly, the morphology of Nb 2 O 5 additives can also affect the performance. , Most recently, hollow Nb 2 O 5 has been employed as active catalysts (10 wt %), promoting MgH 2 dehydrogenation with the onset temperature down to 195 °C . Tailoring the electronic state of Nb is also promising to optimize the catalysis performance, such as Nb 2 O 5 -Nb mixing with different ratios, or combining with N-modified Nb 2 O 5 (10 wt %). , To understand these superior performances associated with Nb 2 O 5 , various active phases have been proposed, including ultrasmall NbO particles, reduced Nb 2 O 5 , ,− ultrasmall MgH 2 or NbOx, − new phases (Mg-Nb-O , ), etc. Even so, however, the catalysis mechanism has not been fully understood and several key questions need better clarification, including (i) whether bulk-doped and interface-distributed Nb performs differently; (ii) whether oxygen contributes to the promotion effect, and if so, is there any synergetic effect between Nb and O; and (iii) what the essential role is played by Nb/O in the process.…”

“…A typical challenge is that a reliable comparison between different samples is hardly approachable, under which theoretical calculations have been employed to examine the mechanism associated with size effect, metal oxide additive, and new phases . In principle, Mg–H bonding may be weakened if Nb 2 O 5 can compete with Mg as electron donor, under which reduced Nb 2 O 5 , showing as Nb or NbOx, can be particularly relevant for studying the impact of the cations. − In addition, recent N-Nb 2 O 5 catalyst vividly demonstrates the synergetic role between Nb and N, highlighting the importance of anions. , Experimentally, it had been reported that ppm-level N 2 or O 2 can affect hydrogen adsorption and desorption of MgH 2 samples . Using oxygen as an example, Mg–O bonding (85.6 kcal/mol) is much stronger than Nb–H (31.6 kcal/mol), meaning that O or N dopants can remarkably change local charges of Mg and H. Such an anion-associated mechanism had been employed earlier, − using B/N dopants to reduce the dehydrogenation temperature.…”