•Pt–Co NPs are embedded in PG, forming a well dispersed catalyst.•Pt–Co@PG catalyst presents excellent catalytic activity for AB hydrolytic.•Hydrolysis reveal high TOF 461.17 molH2 min−1 molPt−1 and low Ea 32.17 kJ mol−1.•Pt–Co@PG catalyst exhibits good reusability with 81.2% after five runs.Nanoporous graphene (PG) supported Pt–Co bimetallic nanoparticles were prepared and their catalytic activity in hydrogen generation from hydrolysis of NH3·BH3 solution were examined. The synthesized Pt–Co@PG with a loading amount of 30 wt% Pt–Co (atomic ratio 1:9) exhibited a superior TOF value of 461.17 molH2 min−1 molPt−1 and an activation energy (Ea) value of 32.79 kJ mol−1 for NH3·BH3 hydrolysis. This remarkably enhanced activity was ascribed to the charge interaction between Pt–Co NPs and PG support. The defects and holes on PG acting as the anchoring sites for Pt–Co NPs was beneficial for achieving a uniform distribution and a decreased particle size for the NPs. The Pt–Co@PG catalysts also showed a well-established reusability, with 81.2% of their initial catalytic activity after five runs of reactions, demonstrating that they had high durability.

•Three-dimensional graphene oxide is used as catalyst carrier.•3DGO facilitates to form highly dispersed cobalt.•Increasing alkalinity promotes hydrolysis but deteriorates recyclability.Highly dispersed cobalt nanoparticles supported on three-dimensional graphene oxide particles (Co@3DGO) have been successfully prepared by a chemical reduction method. Cobalt presents a dispersive morphology with an average diameter of 20 nm. 3DGO exhibits a stereo lamellar structure with oxygen containing functional groups which increases effective contact area and anchors the cobalt ions. Co carried on 3DGO induces a higher hydrogen generation rate (HGR) in NaBH4 hydrolysis up to 4394 mLH2min−1gcobalt−1 at 298 K. As well as the reaction activation energy is 37 kJ mol−1. Moreover, it was found that recycling performance is inverse to the concentration of NaOH. After five consecutive cycles, the recyclability retention percentage increases from 54% to 70% when the concentration of NaOH was reduced from 0.25 M to 0.05 M.Download high-res image (121KB)Download full-size image

•A novel PAMAM dendrimers-encapsulated Ag–Co catalyst is prepared.•Poly(amidoamine) dendrimers benefit AgCo NPs formation and dispersibility.•Catalytic activity of AgCo/PAMAM NPs is largely enhanced on AB hydrolytic reaction.•Initial turnover frequency (TOF) value reaches to 15.84 molH2·min−1·molM−1.•Synergistic effect of PAMAM-encapsulated AgCo on AB hydrolysis reaction is studied.Multifunctional poly(amidoamine) (PAMAM) dendrimers-encapsulated Ag–Co bimetallic nanoparticles (Ag–Co/PAMAM NPs) have been prepared via a facile co-complexation chemical reduction method. By exploiting the well-defined dendritic spatial construction of PAMAM dendrimers as NPs support and capping ligand, the as-synthesized NPs show unconspicuous agglomeration and uniformly distribution with average diameter of 5 nm. The Ag–Co/PAMAM NPs show the composition-dependent catalytic activity in catalytic dehydrogenation of ammonia borane (AB), with NPs in 30 atom.% Ag exhibiting the superior activity and yielding an initial turnover frequency (TOF) as high as 15.84 molH2·min−1·molM−1. Combined with the electro–transfer interaction between Ag and Co, the effective control of the NPs dispersibility and the electro-donating ability of PAMAM dendrimers with amounts of amide and amine groups facilitate the catalytic performances of Ag–Co/PAMAM catalyst on the hydrolysis of AB. The work also includes the kinetic studies on zero-order with respect to substrate concentration and first-order reaction with respect to catalyst concentration, as well as temperature effect to determine the apparent activation energy of the reaction (Ea = 35.66 kJ mol−1). Furthermore, the reusability tests reveal Ag–Co/PAMAM NPs still show good catalytic activity and magnetically reusability in successive runs, which make these dendrimers-stabilized bimetallic nanoparticles promising heterogeneous catalysts in practical application.

To improve the electrochemical properties of rare-earth-Mg-Ni-based hydrogen storage alloys, the effects of stoichiometry and Cu-substitution on the phase structure and thermodynamic properties of the alloys were studied. Nonsubstituted Ml0.80Mg0.20(Ni2.90Co0.50-Mn0.30Al0.30)x (x = 0.68, 0.70, 0.72, 0.74, 0.76) alloys and Cu-substituted Ml0.80Mg0.20(Ni2.90Co0.50−yCuyMn0.30Al0.30)0.70 (y = 0, 0.10, 0.30, 0.50) alloys were prepared by induction melting. Phase structure analysis shows that the nonsubstituted alloys consist of a LaNi5 phase, a LaNi3 phase, and a minor La2Ni7 phase; in addition, in the case of Cu-substitution, the Nd2Ni7 phase appears and the LaNi3 phase vanishes. Thermodynamic tests show that the enthalpy change in the dehydriding process decreases, indicating that hydride stability decreases with increasing stoichiometry and increasing Cu content. The maximum discharge capacity, kinetic properties, and cycling stability of the alloy electrodes all increase and then decrease with increasing stoichiometry or increasing Cu content. Furthermore, Cu substitution for Co ameliorates the discharge capacity, kinetics, and cycling stability of the alloy electrodes.

•Ce2Ni7-type single phase La1.6Mg0.4Ni7 alloy is obtained by annealing treatment.•Cell volume change rate of [La1.22Mg0.78Ni4] is larger than of [LaNi5] (I and II).•Ce2Ni7-type phase decomposes into amorphous La and Mg, nano Ni and LaNi5.•Phase decomposition contributes to decrease of discharge capacity.•Single phase alloy electrode has superior discharge capacity and cycling stability.The Ce2Ni7-type (hexagonal, 2H) single phase La1.6Mg0.4Ni7 alloy has been obtained by annealing the induction melting as-cast sample at 1223 K for 12 h. The relationship between phase structural stability and volume change rate of the three kinds of slabs in Ce2Ni7-type structure is studied. It is found that the volume change rate of Mg-containing [La1.22Mg0.78Ni4] slab after hydrogenation/dehydrogenation is larger than that of [LaNi5] I (outer) and [LaNi5] II (inner) slabs, and the consecutive cell volume change of [La1.22Mg0.78Ni4] slab ultimately results in the decomposition of Ce2Ni7-type phase La1.6Mg0.4Ni7 to amorphous La and Mg phases, nanocrystalline Ni, and CaCu5-type LaNi5 phases, as well as the reduction of electrochemical discharge capacity. Electrochemical studies show that the single phase alloy electrode possesses good discharge capacity (400 mAh g−1) and cycling stability (84.2% after 100 cycles). The improvement in phase structure stability and the cycling stability of the superlattice structure alloys can be achieved by inhibiting the significant volume change of Mg-containing slabs during hydrogenation/dehydrogenation.

•La–Mg–Ni-based alloy with only 2H- and 3R-type (La,Mg)2Ni7 phase was created.•Transformation of 2H- to 3R-type phase was realized by annealing and LaNi5 addition.•We found that 3R-type (La,Mg)2Ni7 phase had better HRD than 2H-type.•LaNi5 addition increased HRD but worsened cycling stability in A2B7-type alloys.La0.75Mg0.25Ni3.5 alloys with hexagonal (2H-) and rhombohedral (3R-) (La,Mg)2Ni7 phase were created by powder metallurgy. Partial crystal transformation of 2H- into 3R-type allotropes was realized by heat treatment and introducing LaNi5 compound. It was found that the alloy annealed within 1073–1223 K kept (La,Mg)2Ni7 phase and obvious crystal transformation of 2H- into 3R-type occurred as annealing temperature reached 1223 K. Electrochemical study showed similar discharge capacity and degradation behavior for La0.75Mg0.25Ni3.5 alloys with different amounts of 2H- and 3R-type allotropes while HRD was promoted by increasing 3R-type phase abundance. Introducing LaNi5 into La0.75Mg0.25Ni3.5 alloy increased 3R- to 2H-type phase ratio and led to an additional plateau in P–C isotherms. LaNi5 introduction improved HRD, however it accelerated cycling degradation. Rietveld analysis indicated that after hydrogenation, the cell expansion of 2H- and 3R-type (La,Mg)2Ni7 phase was similar while the cell expansion of LaNi5 phase was smaller than that of (La,Mg)2Ni7 phase. This caused discrete cell expansion between (La,Mg)2Ni7 and LaNi5 phases, leading to severe pulverization and oxidation.

•La–Mg–Ni-based alloys with specific phase composition were prepared.•LaNi5 reacts with LaMgNi4, generating (La,Mg)Ni3 and (La,Mg)2Ni7 phases.•Alloy with (La,Mg)2Ni7 main and (La,Mg)Ni3 minor phases has better stability.•Alloy with (La,Mg)2Ni7 main and LaNi5 minor phases has better kinetics.Starting from two precursors LaNi5 and LaMgNi4, four alloys with different phase structures were prepared by powder sintering technique. The results suggest that LaNi5 phase can consecutively react with LaMgNi4 phase generating (La,Mg)Ni3 and (La,Mg)2Ni7 phases, and the mole ratio of precursors LaNi5/LaMgNi4 (x) affects the phase structures of alloys significantly. XRD and Rietveld refinement results demonstrate that the alloys mainly consist of (La,Mg)Ni3 and (La,Mg)2Ni7 phases (x = 0.28 and 0.59) or (La,Mg)2Ni7 and LaNi5 phases (x = 0.87 and 1.47). When x increases from 0.28 to 0.59, the main phase becomes (La,Mg)2Ni7 phase with Ce2Ni7- and Gd2Co7-type from PuNi3-type (La,Mg)Ni3 phase. As x rises from 0.59 to 0.87, the secondary phase (La,Mg)Ni3 disappears with CaCu5-type LaNi5 phase emerging. When x grows from 0.87 to 1.47, the content of LaNi5 phase increases from 17.88 to 60.72 wt.% with (La,Mg)2Ni7 phase content declining. The alloy with (La,Mg)2Ni7 as the main phase and (La,Mg)Ni3 as the secondary phase is conducive to cyclic stability and the alloy with (La,Mg)2Ni7 as the main phase and LaNi5 as the secondary phase is beneficial to the high-rate dischargeability.

•MoS2 addition lowers the onset dehydrogenation temperature by 113 °C.•The total dehydrogenating amount increases from 9.26 wt.% to 10.47 wt.% at 500 °C.•Li2S and MoB2 act as catalysts and improve the hydrogen storage properties.•The addition of MoS2 accelerates the dehydrogenation rate.A 2LiBH4–MgH2–MoS2 composite was prepared by solid-state ball milling, and the effects of MoS2 as an additive on the hydrogen storage properties of 2LiBH4–MgH2 system together with the corresponding mechanism were investigated. As shown in the TG–DSC and MS results, with the addition of 20 wt.% of MoS2, the onset dehydrogenation temperature is reduced to 206 °C, which is 113 °C lower than that of the pristine 2LiBH4–MgH2 system. Meanwhile, the total dehydrogenation amount can be increased from 9.26 wt.% to 10.47 wt.%, and no gas impurities such as B2H6 and H2S are released. Furthermore, MoS2 improves the dehydrogenation kinetics, and lowers the activation energy (Ea) 34.49 kJ mol−1 of the dehydrogenation reaction between Mg and LiBH4 to a value lower than that of the pristine 2LiBH4–MgH2 sample. According to the XRD test, Li2S and MoB2 are formed by the reaction between LiBH4 and MoS2, which act as catalysts and are responsible for the improved hydrogen storage properties of the 2LiBH4–MgH2 system.