Study on structure and electrochemical properties of the （LaGdMg）Ni3.35-xCoxAl0.15（x=0-2.0） hydrogen storage alloys

Hydrogen storage alloys(LaGdMg)Ni3.35-xCoxAl0.15(x=0,0.1,0.3,0.5,1.0,1.5,2.0) were prepared by induction melting followed by annealing treatment in argon atmosphere.The effects of partly replacing Ni by Co element in(LaGdMg)Ni3.35Al0.15 on the phase structure and electrochemical properties of(LaGdMg)Ni3.35-xCoxAl0.15 alloys were investigated.Structure analysis showed that the alloys consisted of Ce2Ni7-type(Gd2Co7-type),CaCu5-type,Pr5Co19-type,PuNi3-type phase structure.The addition of Co element obviously reduced the contents of CaCu5-type phase and increased the contents of Ce2Ni7-type phase.However,Pr5Co19-type and CaCu5-type phase obviously increased with the high content of Co.Rietveld analysis showed that the c-axis lattice parameters and cell volumes of the component phases increased with increasing Co content.The electrochemical measurements showed that as the Co content increased,the maximum discharge capacity and the cyclic stability of the annealed alloys both first increased then decreased.The(LaGdMg)Ni3.05Co0.3Al0.15 alloy electrode exhibited the maximum discharge capacity(392.92 mAh/g),and the(LaGdMg)Ni1.85Co1.0Al0.15 alloy electrode showed the best cyclic stability(S100=96.1%).     

Microstructure and electrochemical characteristics of La0.7Ce0.3Ni3.75Mn0.35Al0.15Cu0.75-x（V0.81Fe0.19）x hydrogen storage alloys

Microstructure and electrochemical characteristics of La0.7Ce0.3Ni3.75Mn0.35Al0.15Cu0.75-x(V0.81Fe0.19)x hydrogen storage alloys were investigated. XRD indicated that La0.7Ce0.3Ni3.75Mn0.35Al0.15Cu0.75-x(V0.81Fe0.19)x alloys consisted of a single phase with CaCu5-type structure, and the lattice parameter a and cell volume V increased with increasing x value. The maximum discharge capacity first increased from 319.0 (x=0) to 324.0 mAh/g (x=0.05), and then decreased to 307.0 mAh/g (x=0.20). The high-rate dischargeability at the discharge current density of 1200 mA/g first increased from 52.1% (x=0) to 59.1% (x=0.15), and then decreased to 55.4% (x=0.20). The hydrogen diffusion in the bulky alloy was responsible for the high-rate dischargeability. Cycling stability first increased with increasing x from 0 to 0.10 and then decreased when x increased to 0.20, which was resulted from the synthesized effect of the improvement of the pulverization resistance and the decrease of corrosion resistance.     

Effect of substitution of aluminum for nickel on electrochemical properties of La_(0.75)Mg_(0.25)Ni_(3.5–x)Co_(0.2)Al_x hydrogen storage alloys

La0.75Mg0.25Ni3.5–xCo0.2Alx (x=0–0.09) hydrogen storage alloys were prepared by induction melting and effect of Al substitution for Ni on phase constitution and electrochemical property was investigated.With the substitution of Al for Ni,LaNi5 and LaNi2 phases occurred and (La,Mg)2(Ni,Co,Al)7 phase with hexagonal Ce2Ni7-type structure replaced (La,Mg)2(Ni,Co)7 phase.The cell volumes of LaNi5 and (La,Mg)2(Ni,Co,Al)7 main phases increased with increasing Al content.Some electrochemical properties and kinetic parameters of the alloys,including discharge capacity,high rate discharge ability (HRD),loss angle (ψ),exchange current density (I0) and limiting current density (IL),decreased with increasing amount of substitution of Al for Ni.Substitution of Al for Ni could be favorable for positive shift in corrosion potential of the alloy electrode,and prolonged cyclic lifetime of La0.75Mg0.25Ni3.5–xCo0.2Alx (x=0–0.09) alloy electrodes.     

Structure and high-temperature electrochemical properties of La_（0.60）Nd_（0.15）Mg_（0.25）Ni_（3.3）Si_（0.10） hydrogen storage alloys

The structure and high-temperature electrochemical properties of the as-cast and annealed (940 °C, 8 h) La0.60Nd0.15Mg0.25Ni3.3Si0.10 hydrogen storage alloys were investigated. The X-ray diffraction revealed that the multiphase structure of the as-cast alloy with LaNi5 phase as the main phase was converted into a double-phase structure with La2Ni7 phase as the main phase after annealing. The surface morphology studied by scanning electronic microscope (SEM) showed that the annealed alloy had a much higher anti-corrosion ability than the as-cast alloy. Both alloys presented excellent activation characteristics at all test temperatures. The maximum discharge capacity of the as-cast alloy decreased when the test temperature increased, while the temperature almost had no effect on the annealed alloy. As the test temperature increased, the cyclic stability and charge retention of both alloys decreased, and these properties were improved significantly by annealing.     

Phase structure and hydrogen storage properties of REMg8.35Ni2.18Al0.21 （RE=La, Ce, Pr, and Nd） hydrogen storage alloys

REMg 8.35Ni2.18Al0.21 (RE=La, Ce, Pr, and Nd) alloys were prepared by induction melting and following annealing. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results showed that the alloys were composed of Mg2Ni, (La, Pr, Nd)Mg2Ni, (La, Ce)2Mg17 , (Ce, Pr, Nd)Mg12 and Ce2Ni7 phases. The above phases were disproportioned into Mg2NiH4 , MgH2 and REH x (x=2.51 or 3) phases in hydriding. CeH2.51 phase transformed into CeH2.29 phase in dehydriding, whereas LaH3 , PrH3 and NdH3 phases remained unchanged. The PrMg8.41Ni2.14Al0.20 alloy had the fastest hydriding kinetics and the highest dehydriding plateau pressure while the CeMg8.35Ni2.18Al0.21 alloy presented the best hydriding/dehydriding reversibility. The onset hydrogen desorption temperature of the CeMg8.35Ni2.18Al0.21 hydride decreased remarkably owing to the phase transformation between the CeH2.51 and the CeH2.29 .     

A study on hydrogen storage and electrochemical properties of La_（0.55）Pr_（0.05）Nd_（0.15）Mg_（0.25）Ni_（3.5） （Co_（0.5）Al_（0.5））_x （x=0.0, 0.1, 0.3, 0.5） alloys

The La0.55Pr0.05Nd0.15Mg0.25Ni3.5(Co0.5Al0.5)x(x=0.0, 0.1, 0.3, 0.5) alloys were prepared by magnetic levitation melting under an Ar atmosphere, and the effects of Co and Al on the hydrogen storage and electrochemical properties were systematically investigated by pressure composition isotherms, cyclic voltammetry, Tafel polarization and electrochemical impedance spectroscopy testing. The results showed that the alloy phases were mainly consisted of (La,Pr)(Ni,Co)5, LaMg2Ni9, (La,Nd)2Ni7 and LaNi3 phases, and the cell volumes of (La,Pr)(Ni,Co)5, LaMg2Ni9, (La,Nd)2Ni7 and LaNi3 phases expanded with Co and Al element added. The hydrogen storage capacity initially increased from 1.36 (x=0) to 1.47 wt.% (x=0.3) and then decreased to 1.22 wt.% (x=0.5). The discharge capacity retention and cycle stability of the alloy electrodes were improved with the increase of Co and Al contents. The La0.55Pr0.05Nd0.15Mg0.25Ni3.5(Co0.5Al0.5)0.3 alloy electrode possessed better electrochemical kinetic characteristic.