Enhanced hydrogen storage performance for MgH2–NaAlH4 system—the effects of stoichiometry and Nb2O5 nanoparticles on cycling behaviour

Abstract

Nowadays, the technological utilization of reactive hydride composites (RHC) as promising hydrogen storage materials is hampered by their reaction kinetics. In the present work, effects of reactant stoichiometry on ensuing hydrogen sorption properties and pathway of the MgH 2 -NaAlH 4 (mole ratios 1 : 2, 1 : 1 and 2 : 1) system, both undoped and doped with Nb 2 O 5 nanoparticles, were investigated. It was found that the as-prepared reactant stoichiometry of MgH 2 /NaAlH 4 system had a profound impact on its dehydrogenation kinetics and reaction mechanism. Variable temperature dehydrogenation data revealed that undoped binary composites possessed enhanced hydrogen desorption properties compared to that of pristine NaAlH 4 and MgH 2 . The use of Nb 2 O 5 displayed superior catalytic effects in terms of enhancing dehydriding/rehydriding kinetics and reducing the dehydrogenation temperature of MgH 2 -NaAlH 4 system. Isothermal volumetric measurements at 300 uC revealed that enhancements arising upon adding Nb 2 O 5 were almost double that of undoped MgH 2 -NaAlH 4 composites. The apparent activation energies for NaAlH 4 , Na 3 AlH 6 , MgH 2 , and NaH relevant decompositions in doped composite were found to be much lower than that for the undoped one. Moreover, Nb 2 O 5 doping also markedly enhanced the reversible capacity of MgH 2 -NaAlH 4 composites under moderate conditions, persisting well during three de/ rehydrogenation cycles. XRD, XPS, and FESEM-EDS analyses demonstrated that reduction of Nb 2 O 5 during first desorption was coupled to the migration of reduced niobium oxide species from the bulk to the surface of the material. It was suggested that these finely dispersed oxygen-deficient niobium species might contribute to kinetic improvement by serving as the active sites to facilitate hydrogen diffusion through the diffusion barriers both during dehydrogenation and rehydrogenation.

The influence of ball-milling time on the dehydrogenation properties of the NaAlH4–MgH2 composite

“…It is not clear yet why the reaction (6) was not observed in the NaAlH 4 eMgH 2 composites with the molar ratio of 1:1 [13,14] but only in the mixtures with excess MgH 2 [12,14]. Furthermore, the decomposition of the excess MgH 2 into Mg and hydrogen as well as reaction (4) take place.…”

“…The investigation of NaAlH 4 eMgH 2 [14] revealed two possible reaction schemes (1:4 and 1:2 vs. 1:1), unlike LiAlH 4 eMgH 2 (1:1, 1:2 and 1:4) [11]. The NaAlH 4 eMgH 2 (1:1) composition [13,14] is expected to form the dehydrogenation product Na 3 AlH 6 and follow the anticipated reactions (1), (2) and (4) [11,13,14].…”

“…The NaAlH 4 eMgH 2 (1:1) composition [13,14] is expected to form the dehydrogenation product Na 3 AlH 6 and follow the anticipated reactions (1), (2) and (4) [11,13,14]. Further decomposition of NaH and residual MgH 2 leads to hydrogen gas and respective metals without the formation of a NaeMg alloy.…”

Hydrogen-storage properties of MgH2–10Ni–2NaAlH4–2Ti prepared by reactive mechanical grinding

“…The effects of reactant stoichiometry on ensuing hydrogen sorption properties and pathway of the MgH 2 -NaAlH 4 (mole ratios 1:2, 1:1, and 2:1) system were investigated by Rafi-ud-din et al [21]. Variable temperature dehydrogenation data revealed that binary composites possessed enhanced hydrogen desorption properties compared with that of pristine NaAlH 4 and MgH 2 .…”

Metallic glassy Ti2Ni grain-growth inhibitor powder for enhancing the hydrogenation/dehydrogenation kinetics of MgH2

“…6,7 Despite these attractive properties, magnesium hydride (MgH 2 ) in its pure form is very stable and has very slow hydrogenation/dehydrogenation kinetics at temperatures less than 350 C. 8,9 Such serious drawbacks are considered as major barriers preventing the use of such an attractive metal hydride material for potential use in fuel cell applications. 10,11 Since the 1990s, enormous efforts have been dedicated for improving the hydrogenation/dehydrogenation behaviors of MgH 2 , using pure metal catalysts, 12,13 intermetallic compounds, 14 metal oxides, 15,16 carbides 17 and different families of composite and nanocomposite materials. [18][19][20][21] Recently, Shinde et al 22 demonstrated that successful self-assembly of MgH 2 nanoparticles in activated carbon led to the improvement of the physical and chemical behavior of the hydride phase for achieving reversible hydrogen storage.…”