Liuting Zhang

Novel AgPd hollow spheres anchored on graphene as an efficient catalyst for dehydrogenation of formic acid at room temperature

Highly dispersed AgPd hollow spheres anchored on graphene (denoted as AgPd-Hs/G) were successfully synthesized through a facile one-pot hydrothermal route for the first time. The fabrication strategy was efficient and green by using L-ascorbic acid (L-AA) as the reductant and trisodium citrate dihydrate as the stabilizer, without employing any seed, surfactant, organic solvent, template, stabilizing agent, or complicated apparatus. The as-synthesized AgPd-Hs/G catalyst exhibits a sphere-shaped hollow structure with an average diameter of about 18 nm and a thin wall of about 5 nm. The hollow architecture with a thin wall and excellent dispersion on the graphene ensure that most of the atoms are located on the surface or sub-surface, which provides reactive catalytic sites for the dehydrogenation of formic acid. Therefore, a superior catalytic effect was achieved compared with other catalysts such as Pd/G and AgPd/C. The as-synthesized AgPd-Hs/G exhibits a catalytic activity with an initial turnover frequency (TOF) value as high as 333 mol H2 mol−1 catalyst h−1 even at room temperature (25 °C) toward the decomposition of formic acid. The present AgPd-Hs/G with efficient catalysis on the dehydrogenation of formic acid without any CO generation at room temperature can pave the way for a practical liquid hydrogen storage system and therefore promote the application of formic acid in fuel cell systems.

Remarkable hydrogen desorption properties and mechanisms of the Mg2FeH6@MgH2 core–shell nanostructure

Mg2FeH6@MgH2 dual-metal hydrides with a core–shell nanostructure were synthesized via ball-milling and heat treatment methods using Mg and Fe as raw materials assisted by diethyl ether addition. Systematic investigations of the association between the microstructure and hydrogen desorption properties of the Mg2FeH6@MgH2 core–shell hydride were performed. It is found that the as-synthesized Mg2FeH6@MgH2 is comprised of the Mg2FeH6-core with a particle size of 40–60 nm and the MgH2-shell with a thickness of 5 nm. The hydrogen desorption of the Mg2FeH6@MgH2 core–shell nanoparticle starts at 220 °C, which is ∼45 °C lower than that of the Mg2FeH6/MgH2 micrometer particle. Compared to the as-synthesized Mg2FeH6/MgH2 micrometer particle, the Mg2FeH6@MgH2 core–shell sample exhibited faster hydrogen desorption kinetics, which released more than 5.0 wt% H2 within 50 min at 280 °C. The desorption activation energy of the core–shell Mg2FeH6@MgH2 was reduced to 115.7 kJ mol−1 H2, while the desorption reaction enthalpy and entropy were calculated to be −80.6 ± 7.4 kJ mol−1 H2 and −140.0 ± 11.9 J K−1 mol−1 H2, respectively. It is proposed that the improvements of both hydrogen desorption kinetics and thermodynamics are due to the special core–shell nanostructure of Mg2FeH6@MgH2. More remarkably, it is demonstrated that the core–shell nanostructure could be recovered after rehydrogenation, leading to excellent cycling hydrogen desorption properties of Mg2FeH6@MgH2. In addition, the suggested dehydrogenation mechanism involves the dehydrogenation of the MgH2-shell followed by the decomposition of the Mg2FeH6-core into Mg and Fe according to the three-dimensional phase-boundary process.

Enhanced hydrogen storage properties of MgH2 with numerous hydrogen diffusion channels provided by Na2Ti3O7 nanotubes

Na2Ti3O7 nanotubes (NTs) with a uniform diameter of 10 nm and Na2Ti3O7 nanorods (NRs) with a diameter of 100–500 nm were synthesized via a hydrothermal method and a solid-state method, respectively, and then introduced into MgH2 by ball milling to catalyze the hydrogenation/dehydrogenation process. The MgH2–Na2Ti3O7 NT and MgH2–Na2Ti3O7 NR composites can desorb 6.5 wt% H2 within 6 min and 16 min at 300 °C, respectively, while the bulk MgH2 hardly releases any hydrogen even over a much longer time. In addition, isothermal rehydrogenation measurements show that the MgH2–Na2Ti3O7 NT composite can absorb 6.0 wt% H2 within 60 s at 275 °C and can even absorb 1.5 wt% H2 within 30 min at a temperature as low as 50 °C. TEM and HRTEM analyses indicate that the Na2Ti3O7 NTs are homogeneously distributed in MgH2, which catalyze the de-/rehydrogenation of MgH2 and meanwhile offer numerous diffusion channels to significantly accelerate the transportation of hydrogen atoms. Moreover, compared with bulk MgH2 and the MgH2–Na2Ti3O7 NR composite, the activation energy of the MgH2–Na2Ti3O7 NT composite is significantly decreased to 70.43 kJ mol−1. Such Na2Ti3O7 NTs with a unique morphology of the catalyst being distributed as nanotubes in MgH2 are believed to pave the way for the future design of hydrogen storage materials with excellent hydrogen storage performances.

Fast hydrogen release under moderate conditions from NaBH4 destabilized by fluorographite

A simple approach to dramatically enhance the dehydrogenation properties of sodium borohydride is achieved by ball milling NaBH4 with fluorographite (FGi). It was found that the ball-milled NaBH4–FGi composite starts to release hydrogen without impurity gas at a lower temperature of 125 °C, and obtains a hydrogen desorption capacity of ca. 4.8 wt% below 130 °C in seconds, which is improved markedly compared to the ball-milled pristine NaBH4. The significant thermodynamic and kinetic improvement of the NaBH4–FGi composite can be ascribed to the reaction between NaBH4 and FGi as well as the formation of micro-scale NaBH4. Moreover, since the dehydrogenation process of NaBH4–FGi composite is exothermal, the fully reverse reaction is not feasible. In-depth investigations show that the partial rehydrogenation is due to the formation of Na2B12H12 and another new borohydride.

Significantly improved hydrogen storage properties of NaAlH4 catalyzed by Ce-based nanoparticles

NaAlH4, a prototypical high energy density complex hydride, possesses a favorable thermodynamics and high hydrogen storage capacity. However, the poor kinetics and degradation of cycling stability retard its practical application. To ease these problems, CeB6, CeF3 and CeO2 nanoparticles with a size of about 10 nm are synthesized by the wet-chemistry method and introduced into NaAlH4 systems as additives in this work. The results show that all of the nanoparticles are effective in improving the hydriding–dehydriding kinetics of NaAlH4, and nano-CeB6 possesses the highest catalytic activity. The rehydrogenation of dehydrogenated NaAlH4 doped with nano-CeB6 can be accomplished in less than 20 min with a high capacity of 4.9 wt%, which shows a 20% increase in capacity compared to that of chloride-doped NaAlH4. Due to the structural stability and good dispersion of nano-CeB6 and nano-CeF3, a favorable cycling stability with high capacity retention is achieved for their doped samples. Moreover, hydrogen can be released from the hydrogenated sample doped with nano-CeB6 at a temperature as low as 75 °C, fulfilling the operation temperature of a PEM fuel cell. In the nano-CeO2 doped NaAlH4 system, CeO2 is first reduced to CeH2.51. In the subsequent cycles, the formed CeH2.51 gradually transforms into Ce–Al, and simultaneously the kinetics of the doped system is further enhanced. It is believed that the utilization of Ce-based nanoparticles as catalysts would substantially improve the practical applications of NaAlH4 for hydrogen storage.

Facile synthesis of bowl-like 3D Mg(BH4)2–NaBH4–fluorographene composite with unexpected superior dehydrogenation performances

A remarkable improvement in the hydrogen desorption performance of Mg(BH4)2–NaBH4 eutectic composite is achieved by simply ball-milling with fluorographene (FG). It is found that the desorption temperature of Mg(BH4)2–NaBH4 (∼200 °C) is still too high and liquid phase appears during dehydrogenation. Particularly, the novel bowl-like 3D Mg(BH4)2–NaBH4–FG composite can be formed after ball-milling. The novel Mg(BH4)2–NaBH4–FG exhibits a low dehydrogenation temperature of 114.9 °C with 6.9 wt% of pure hydrogen in seconds. In-depth investigations show that such greatly improved hydrogen desorption properties of the Mg(BH4)2–NaBH4–FG composite could be ascribed to both the “novel bowl-like 3D structure” with large specific surface area and the “reactant destabilized modification” owing to the decrease of the reaction enthalpy caused by the formation of NaMgF3. This finding provides a facile preparation of complex borohydride composites with low dehydrogenation temperature and fast rate, which accelerates its practical application in fuel cells.

Enhanced hydrogen storage properties of a dual-cation (Li+, Mg2+) borohydride and its dehydrogenation mechanism

In this paper, we present a new method to synthesize a dual-cation (Li+, Mg2+) borohydride. It is found that Li–Mg–B–H is formed by mechanical milling a mixture of LiBH4 and MgCl2 with a molar ratio of 3 : 1 in diethyl ether (Et2O) and a subsequent heating process. The morphology and structure of the as-prepared Li–Mg–B–H compound are studied by SEM, XRD, FTIR and NMR measurements. Further experiments testify that Li–Mg–B–H can release approximately 12.3 wt% of hydrogen under 4 bar initial hydrogen pressure from room temperature to 500 °C and reach a maximum desorption rate of 13.80 wt% per h at 375 °C, which is 30 times faster than that of pristine LiBH4. Thermal analysis indicates that the decomposition process of the new compound involves three steps: (1) Li–Mg–B–H first decomposes into LiBH4 and MgH2 and synchronously releases a number of H2 molecules; (2) MgH2 decomposes to Mg and H2; (3) LiBH4 reacts with Mg, generating H2, MgB2 and LiH. Moreover, Li–Mg–B–H is proved to be partially reversible, which can release 5.3 wt% hydrogen in the second dehydrogenation process. The strategy of altering the χp of metal ions in borohydrides may shed light on designing dual-cation borohydrides with better hydrogen storage performance.