Enhanced hydrogen storage kinetics of nanocrystalline and amorphous Mg_2N-type alloy by substituting Ni with Co

In order to improve the hydrogen storage kinetics of the Mg2Ni-type alloys, Ni in the alloy was partially substituted with element Co. The Mg2Ni-type Mg2Ni1-xCox （x=0, 0.1, 0.2, 0.3, 0.4） alloys were fabricated by melt-spinning technique. The structures of the as-spun alloys were characterized by XRD and TEM. The gaseous and electrochemical hydrogen storage kinetics of the alloys was measured. The results show that the substitution of Co for Ni notably enhances the glass forming ability of the Mg2Ni-type alloy. The amorphization degree of the alloys visibly increases with rising of Co content. Furthermore, the substitution of Co for Ni significantly improves the hydrogen storage kinetics of the alloys. With an increase in the amount of Co substitution from 0 to 0.4, the hydrogen absorption saturation ratio of the as-spun （15 m/s） alloy increases from 81.2% to 84.9%, the hydrogen desorption ratio from 17.60% to 64.79%, the hydrogen diffusion coefficient increases from 1.07×10-11 to 2.79×10-11 cm2/s and the limiting current density increases from 46.7 to 191.7 mA/g, respectively.

Electrochemical hydrogen storage characteristics of nanocrystalline and amorphous Mg_2Ni-type alloys prepared by melt-spinning

The nanocrystalline and amorphous Mg2Ni-type alloys with nominal compositions of Mg2Ni1-xMnx （x=0, 0.1, 0.2, 0.3, 0.4） were synthesized by melt-spinning technique. The spun alloy ribbons with a continuous length, a thickness of about 30 μm and a width of about 25 mm are obtained. The structures of the as-spun alloy ribbons were characterized by XRD and HRTEM. The electrochemical hydrogen storage characteristics of the as-spun alloy ribbons were measured by an automatic galvanostatic system. The electrochemical impedance spectrums （EIS） were plotted by an electrochemical workstation. The hydrogen diffusion coefficients （D） in the alloys were calculated by virtue of potential-step measurement. The results show that all the as-spun （x=0） alloys hold a typical nanocrystalline structure, whereas the as-spun （x=0.4） alloy displays a nanocrystalline and amorphous structure, confirming that the substitution of Mn for Ni facilitates the glass formation in the Mg2Ni-type alloy. The substitution of Mn for Ni significantly improves the electrochemical hydrogen storage performances of the alloys, involving the discharge capacity and the electrochemical cycle stability. With an increase in the amount of Mn substitution from 0 to 0.4, the discharge capacity of the as-spun （20 m/s） alloy increases from 96.5 to 265.3 mA·h/g, and its capacity retaining rate （S20） at the 20th cycle increases from 31.3% to 70.2%. Furthermore, the high rate dischargeability （HRD）, electrochemical impedance spectrum and potential-step measurements all indicate that the electrochemical kinetics of the alloy electrodes first increases then decreases with raising the amount of Mn substitution.

Thermoelectric Properties of Ni-substituted Polycrystalline In_4Se_3

Thermoelectric （TE） materials can convert directly low-grade heat energy to electricity, and vice versa, which is highly expected to play an important role in the future energy management. The application practice demands efficient TE materials made of non-toxic and inexpensive components. Herein, we report a Ni substituted polycrystalline n-type bulk material In4.xNixSe3 （x = 0-0.1）. Based on density functional theory calculation, Ni tends to substitute at the In3 site in the In4Se3, which causes a monotonous unit cell volume reduction. At x=0.01, Ni substitution results in a sharp decrease in the carder concentration （he） in comparison with that of pure In4Se3, and then ne increases with the increase of Ni concentration. Ni substitution leads to a performance enhancement from 0.6 for pure In4Se3 to an optimum ZTvalue of 0.8 at 450℃.

Gaseous and Electrochemical Hydrogen Storage Kinetics of As-quenched Nanocrystalline and Amorphous Mg_2Ni-type Alloys

The nanocrystalline and amorphous Mg2Ni-type Mg2Ni1-xCox （x = 0, 0.1, 0.2, 0.3, 0.4） alloys were synthesized by melt quenching technology. The structures of the as-cast and quenched alloys were characterized by XRD, SEM and HRTEM. The gaseous hydrogen storage kinetics of the alloys was measured using an automatically controlled Sieverts apparatus. The alloy electrodes were charged and discharged with a constant current density in order to investigate the electrochemical hydrogen storage kinetics of the alloys. The results demonstrate that the substitution of Co for Ni results in the formation of secondary phases MgCo2 and Mg instead of altering the major phase Mg2Ni. No amorphous phase is detected in the as-quenched Co- ffee alloy, however, a certain amount of amorphous phase is clearly found in the as-quenched alloys substituted by Co. Furthermore, both the rapid quenching and the Co substitution significantly improve the gaseous and electrochemical hydrogen storage kinetics of the alloys, for which the notable increase of the hydrogen diffusion coefficient （D） along with the limiting current density （IL） and the obvious decline of the electrochemical impedance generated by both the Co substitution and the rapid quenching are basically responsible.

Co and Ni single sites on the(111)n surface ofγ-Al2O3–a periodic boundary DFT study

The influences of increasing the number of d-electrons in the single metal(Fe-like)substituted(111)n surface ofγ-Al2O3 on its possible catalytic effects were explored.The energetic properties,local structures,and in-site electron configurations of the most active tri-coordinated Co and Ni single-site(111)n surface ofγ-Al2O3 have been studied using the density functional theory(DFT)approach under periodic boundary conditions.The replacement of Al by a Co or Ni atom on the I position of the(111)n surface leads to significant elongations of metal–O distances.The energy released from the substitution process on the AlI site of the(111)n surface follows the sequence NiI(164.85 kcal mol−1)>CoI(113.17 kcal mol−1)>FeI(44.30 kcal mol−1).The triplet and quintet(ground state)of the CoI substituted complex are energy degenerate.Also,the doublet and quartet(ground state)of the NiI substituted complex have the same stable energy.This energy degeneracy comes from theα–βelectron flipping on the p-orbital of the neighboring O that is next to the substituted CoI or NiI site on the(111)n surface ofγ-Al2O3.Different from the FeI substituted single-site(111)n surface,in which the electron configuration of FeI varies according to its spin-multiplicity state,substituted NiI has a unique d8 electron configuration in all three spin states,and similarly,CoI has a unique d7 electron configuration in all three open shell spin states.An increase of the population of d-electrons in the single metal substituted(111)n surface ofγ-Al2O3 is likely to provide a more stable electron configuration in the metal catalytic center.