Hujun Cao

Effects of Al-based additives on the hydrogen storage performance of the Mg(NH2)2–2LiH system

The Mg(NH2)2-2LiH composite is a promising hydrogen storage material due to its relatively high reversible hydrogen capacity (~5.6 wt%) and suitable thermodynamic properties that allow hydrogen sorption conducting at temperatures below 90 °C. However, the presence of a severe kinetic barrier inhibits its low-temperature operation. In the present work, Li3AlH6 was introduced to the Mg(NH2)2-2LiH system. Experimental results show that a 3.2% mol Li3AlH6-modified Mg(NH2)2-2LiH sample released hydrogen at a rate ca. 4.5 times as fast as that of the Li3AlH6-free sample at 140 °C. The enhancement of desorption kinetics was simultaneously demonstrated by activation energy (Ea) of ca. 96.3 ± 9 kJ mol(-1) which was significantly decreased by 31 kJ mol(-1) from that of the Li3AlH6-free sample. The interaction of Li3AlH6 and Mg(NH2)2 during ball milling results in the formation of LiAl(NH)2, LiNH2 and Mg3N2. LiAl(NH)2 was actually the active species for the enhancement of dehydrogenation/re-hydrogenation kinetics of the system.

Effective thermodynamic alteration to Mg(NH2)2–LiH system: achieving near ambient-temperature hydrogen storage

A strategy for thermodynamic improvement of 2Mg(NH2)2–3LiH composite via “stabilizing” the dehydrogenated product LiNH2 was developed. By introducing LiI, LiBr and LiBH4 to the composite, hydrogen release at 0.1 MPa equilibrium pressure is thermodynamically allowed at 333, 320 and 337 K, respectively, which are ten degree Celsius lower than the pristine composite. Moreover, the dehydrogenation kinetics and reversibility are significantly improved.

Controllable synthesis of activated graphene and its application in supercapacitors

Activated graphene has been considered as an ideal electrode material for supercapacitors. In order to reveal the relationship between activated graphene and its precursor and controllably synthesize activated graphene, the structural parameters of the precursor (reduced graphene oxide, RGO) such as crystallinity and attached oxygen-functional groups were controllably adjusted during the synthesis of activated graphene and the effects of the precursor structure on the microstructure of activated graphene were investigated. The activation results reveal that the structure of RGO obviously affects the porous structure of activated graphene. Specifically, the crystallinity and oxygen-functional groups play an important role in the porosity development of activated graphene. By combining the simplified Brodie method and the subsequent post-oxidation process, porous activated graphene with a specific surface area of as high as 2406 m2 g−1 and high pore volume has been successfully prepared. The as-prepared activated graphene exhibits good capacitive characteristics and delivers high energy density (55.7 W h kg−1) when measured in a two-electrode cell with the EMIMBF4 ionic liquid as the electrolyte. The results demonstrate that the obtained activated graphene can be considered as a candidate for advanced electrode materials for supercapacitors.

Edge-enriched porous graphene nanoribbons for high energy density supercapacitors

A simple solution-based oxidative process and subsequent chemical activation combination method has been developed to prepare edge-enriched porous graphene nanoribbons (GNRs) as a high-performance electrode material for supercapacitors. The precursor aligned carbon nanotubes are cut longitudinally and unzipped by a modified Brodie method to form tube-like GNRs with abundant edges. The intermediate GNRs were subsequently chemically activated using KOH to generate a suitable porosity and create more edge sites. These edge sites contribute a larger capacitance than the basal plane of graphene and the nanopores facilitate the fast immigration of ions. As a result, the edge-enriched GNRs exhibit a capacitance uptake per specific surface area almost two times higher than that of conventional activated graphene sheets, which gives rise to the high energy density of the porous GNR electrode. The highly efficient utilization of the edge planes and easy, low-cost scale-up production will make porous GNRs potentially applicable to high-performance supercapacitors.

Improved kinetics of the Mg(NH2)2–2LiH system by addition of lithium halides

The Mg(NH2)2–2LiH composite is a promising on-board hydrogen storage material due to its high reversible hydrogen capacity and suitable thermodynamic properties. However, the severe kinetic barrier inhibits its low temperature operation. In the present work, the additive effects of lithium halides on the Mg(NH2)2–2LiH system were studied systematically. Experimental results showed that, among all those lithium halides, the LiBr doped Mg(NH2)2–2LiH composite exhibited the best dehydrogenation performance. The hydrogen sorption and desorption rates of the Mg(NH2)2–2LiH–0.2LiBr sample are ∼3 and 2 times, respectively, faster than that of the pristine sample at 140 °C. At the same time, enhanced kinetics for hydrogen desorption was observed from an activation energy (Ea) of ca. 92 ± 9 kJ mol−1 which was significantly decreased by 35 kJ mol−1 compared with the pristine sample. Subsequently, a plausible mechanism for the modified dehydrogenation/re-hydrogenation process was proposed.

Nitrogen-doped porous graphene–activated carbon composite derived from “bucky gels” for supercapacitors

A simple method has been developed to prepare nitrogen-doped porous graphene–activated carbon (AC) composites as high-performance electrode materials for supercapacitors. The graphene-based “bucky gels”, prepared by simple mixing and grinding of graphene in ionic liquids (ILs), are carbonized to form an “untractable char” intermediate product, and finally converted to the nitrogen-doped porous graphene–AC composite by chemical activation using KOH. Results demonstrate that the introduction of graphene sheets into the composite not only effectively enhance the specific surface area and conductivity of graphene–AC composite, but also enlarge the pore size in the electrode material compared with pure AC. In addition, the nitrogen-doping can further improve the kinetics for both charge transfer and ion transport throughout the electrode. Its found that the composite has a large specific surface area of 2375.2 m2 g−1, and also contains plenty of mesopores and appreciable nitrogen-doping amount. It exhibits a specific capacitance up to 145 F g−1 at 20 mV s−1 in 6 M KOH electrolyte, and the specific capacitance decreases by only 1.6% after 5000 cycles. This kind of nitrogen-doped composite represents an alternative promising candidate as electrode material for supercapacitors.

Ternary Amides Containing Transition Metals for Hydrogen Storage: A Case Study with Alkali Metal Amidozincates

The alkali metal amidozincates Li4 [Zn(NH2)4](NH2)2 and K2[Zn(NH2)4] were, to the best of our knowledge, studied for the first time as hydrogen storage media. Compared with the LiNH2-2 LiH system, both Li4 [Zn(NH2)4](NH2)2-12 LiH and K2[Zn(NH2)4]-8 LiH systems showed improved rehydrogenation performance, especially K2[Zn(NH2)4]-8 LiH, which can be fully hydrogenated within 30 s at approximately 230 °C. The absorption properties are stable upon cycling. This work shows that ternary amides containing transition metals have great potential as hydrogen storage materials.

Hydrogen storage over alkali metal hydride and alkali metal hydroxide composites

Alkali metal hydroxide and hydride composite systems contain both protic (H bonded with O) and hydridic hydrogen. The interaction of these two types of hydrides produces hydrogen. The enthalpy of dehydrogenation increased with the increase of atomic number of alkali metals, i.e., −23 kJ/molH2 for LiOH-LiH, 55.34 kJ/molH2 for NaOH-NaH and 222 kJ/molH2 for KOH-KH. These thermodynamic calculation results were consistent with our experimental results. H2 was released from LiOH-LiH system during ball milling. The dehydrogenation temperature of NaOH-NaH system was about 150 °C; whereas KOH and KH did not interact with each other during the heating process. Instead, KH decomposed by itself. In these three systems, NaOH-NaH was the only reversible hydrogen storage system, the enthalpy of dehydrogenation was about 55.65 kJ/molH2, and the corresponding entropy was ca. 101.23 J/(molH2·K), so the temperature for releasing 1.0 bar H2 was as high as 518 °C, showing unfavorable thermodynamic properties. The activation energy for hydrogen desorption of NaOH-NaH was found to be 57.87 kJ/mol, showing good kinetic properties.

KNH2–KH: a metal amide–hydride solid solution

We report for the first time the formation of a metal amide-hydride solid solution. The dissolution of KH into KNH2 leads to an anionic substitution, which decreases the interaction among NH2- ions. The rotational properties of the high temperature polymorphs of KNH2 are thereby retained down to room temperature.

A new potassium-based intermediate and its role in the desorption properties of the K–Mg–N–H system

New insights into the reaction pathways of different potassium/magnesium amide-hydride based systems are discussed. In situ SR-PXD experiments were for the first time performed in order to reveal the evolution of the phases connected with the hydrogen releasing processes. Evidence of a new K-N-H intermediate is shown and discussed with particular focus on structural modification. Based on these results, a new reaction mechanism of amide-hydride anionic exchange is proposed.