Jianjiang Hu

Synthesis of sodium amidoborane (NaNH2BH3) for hydrogen production

The prospect of building a future energy system on hydrogen has stimulated much research effort in developing hydrogen storage technologies. One of the potential materials newly developed is sodium amidoborane (NaNH2BH3) which evolves ∼7.5 wt% hydrogen at temperatures as low as 91 °C. In this paper, two methods of synthesizing pure NaNH2BH3 were reported. One method is by reacting NaH and ammonia borane in THF at low temperatures, and the other is by reacting NaNH2 and ammonia borane in THF at ambient temperature. Non-isothermal testing on the thermolysis of solid NaNH2BH3 showed that hydrogen evolution was composed of two exothermic steps. More than 1 equiv. H2 was evolved rapidly at temperatures below 87 °C. After evolving 2 equiv. H2, NaH was identified in solid products and coexisted with amorphous BN.

The kinetic properties of Mg(BH4)2 infiltrated in activated carbon.

A wet incipient impregnation procedure was developed to infiltrate Mg(BH4)2 into the voids of pre-treated activated carbon with a pore diameter of < 2 nm. The thermal data of the composite material showed a strong broadening of the signals. The peak decomposition temperature was shifted to lower values by the infiltration. A Kissinger analysis of bulk Mg(BH4)2 and the nanocomposite revealed a high activation barrier for the first step of the dehydrogenation of the bulk material, which was lowered by a factor of two for the nanoconfined hydride.

Effects of triphenyl phosphate on the hydrogen storage performance of the Mg(NH2)2-2LiH system

Mg(NH2)2–2LiH is an attractive system because of its high reversible hydrogen capacity (∼5.6 wt%) and suitable thermodynamic parameters that allow operation below 100 °C. However, a relatively high kinetic barrier in the hydrogen desorption blocks its application at low temperature. In this work, a small amount of additive, triphenyl phosphate (TPP), was introduced and its effects on hydrogen desorption/absorption in the Mg(NH2)2–2LiH system were studied. Experimental results showed that TPP can prevent aggregation/crystallization during the cycling tests and thus achieve an enhanced kinetic performance. Complete dehydrogenation and hydrogenation can be successfully carried out at temperatures below 150 °C. Moreover, a significant reduction of the entropy change of hydrogen desorption (ΔSdes) was found in the TPP-doped system compared with the pristine Mg(NH2)2–2LiH system, thought to be due to the persistence of amorphous Mg(NH2)2 in the TPP-doped sample during dehydrogenation and hydrogenation cycling, thereby greatly affecting the equilibrium hydrogen pressure.

Beneficial effects of stoichiometry and nanostructure for a LiBH4–MgH2 hydrogen storage system

The hydrogen storage system [MgH2–2LiBH4] shows attractive properties such as favorable thermodynamics, high hydrogen capacity and reversibility. However, there exists an incubation period that amounts up to 10 hours in the dehydrogenation steps, which restricts this system as a practical material. In this study, the influences of stoichiometry and the nanoscale MgH2 were investigated for the system. Considerably shortened incubation times were achieved with deficit amounts of LiBH4 or by using nanoscale MgH2. In addition, the application of nanoscale MgH2 prevented or suppressed the formation of [B12H12]2− in the dehydrogenation, which is otherwise an issue concerning the re-cyclability.

Mechanistic insights into the remarkable catalytic activity of nanosized Co@C composites for hydrogen desorption from the LiBH4–2LiNH2 system

In this work, we demonstrate a first attempt at understanding the catalytic mechanism of nanosized Co in reducing the dehydrogenation temperature of the Li–B–N–H hydrogen storage system by experimental observation and theoretical calculation. A nanosized Co@C composite (Co particles <10 nm) is successfully synthesized by casting a furfuryl alcohol-filled, Co-based metal–organic framework, MOF-74(Co), at 700 °C. Adding small quantities of the prepared nanosized Co@C composite significantly reduces the dehydrogenation temperature of the LiBH4–2LiNH2 system. The 5 wt% Co@C-containing sample releases approximately 10.0 wt% hydrogen at 130–230 °C with a peak temperature of 210 °C, which is reduced by 125 °C from that of the pristine sample. During hydrogen desorption, nanosized Co remains in the metallic state and only works as a catalyst to reduce the kinetic barriers of hydrogen release from the LiBH4–2LiNH2 system. Ab initio calculations reveal that the presence of a Co catalyst induces a redistribution of charge, which not only weakens the chemical H–B bonding but also enhances the electrostatic interactions between Hδ+ in the NH2 groups and Hδ− in the BH4 groups, consequently reducing the energy barriers for the formation of H2 molecules. This explains the low-temperature dehydrogenation behaviour of the Co-catalysed Li–B–N–H systems.

Improved overall hydrogen storage properties of a CsH and KH co-doped Mg(NH2)2/2LiH system by forming mixed amides of Li–K and Cs–Mg

A CsH and KH co-doped Mg(NH2)2/2LiH composite was prepared with a composition of Mg(NH2)2/2LiH–(0.08 − x)CsH–xKH, and the hydrogen storage characteristics was systematically investigated. The results showed that the presence of KH further improved the reaction thermodynamics and kinetics of hydrogen storage in a CsH-containing Mg(NH2)2/2LiH system. A sample with 0.04 mol CsH and 0.04 mol KH had optimal hydrogen storage performance; its dehydrogenation could proceed at 130 °C and hydrogenation at 120 °C with 4.89 wt% of hydrogen storage capacity. At 130 °C, a 25-fold increase in the dehydrogenation rate was achieved for the CsH and KH co-doped sample. More importantly, the CsH and KH co-doped sample also had good cycling stability because more than 97% of the hydrogen storage capacity (4.34 wt%) remained for the Mg(NH2)2/2LiH–0.04CsH–0.04KH sample after 30 cycles. A structural characterization revealed that added CsH and KH participated in the dehydrogenation and hydrogenation reactions by reversibly forming mixed amides of Li–K and Cs–Mg, which caused the improved hydrogen storage thermodynamics and kinetics.

Vanadium oxide nanoparticles supported on cubic carbon nanoboxes as highly active catalyst precursors for hydrogen storage in MgH2

Magnesium hydride (MgH2) has attracted intense interest as a high-capacity hydrogen storage material. However, high thermal stability and slow kinetics limit its practical applications. Herein, vanadium oxide nanoparticles supported on cubic carbon nanoboxes (nano-V2O3@C) are synthesized successfully by using MIL-47(V) as a precursor, and superior catalytic effects derived from the nano-V2O3@C composite towards the hydrogen storage reaction of MgH2 are demonstrated. The MgH2-9 wt% nano-V2O3@C sample starts releasing hydrogen at 215 °C, which is 60 °C lower than that of the additive-free MgH2. At 275 °C, approximately 6.4 wt% of hydrogen is released from the MgH2-9 wt% V2O3@C sample within 20 min. The dehydrogenated sample absorbs hydrogen even at room temperature under 50 bar of hydrogen pressure, and rehydrogenation is complete within 700 s at 150 °C. XRD and XPS measurements identify the existence of metallic V after ball milling, and its presence remains nearly constant in the subsequent dehydrogenation/hydrogenation process upon heating. Further ab initio calculations reveal that the presence of V facilitates the breaking of the Mg–H bond of the MgH2 unit, which is reasonably responsible for the significantly reduced operating temperatures and improved kinetics of the V-catalysed MgH2.