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In this paper, the substitution of different amounts of Zr for Ti in the hydrogen storage electrode alloy Mg35Ti10Ni55 has been carried out for the purpose of further improving the electrochemical performance of the electrode alloy. The electrochemical properties were tested on the purposely prepared Mg35Ti10−xZrxNi55 (x=1, 3, 5, 7, 9) electrode alloys. The results showed that all alloys substituted with Zr possessed a higher cycling capacity retention and a larger initial discharge capacity (except Mg35Ti1Zr9Ni55) than the original alloy Mg35Ti10Ni55. Among these alloys Mg35Ti5Zr5Ni55 exhibited the highest discharge capacity and cycling stability. It was found from X-ray Photoelectron Spectroscopy (XPS) surface analysis that the Zr content reduced the degree of oxidation of Mg. The higher the Zr content, the lower is the degree of oxidation of Mg. Auger electron spectroscopy (AES) analysis revealed that the thickness of the passive oxide film coating on the alloy surfaces became first reduced as the Zr content increased but subsequently became augmented as the Zr content exceeded 5 at.%. The study indicated that the corrosion inhibiting effect of Zr alone, without the presence of Ti was weak. The synergetic effect of Zr and Ti made the passive film more compact, less pervious and more protective than by Ti or Zr alone. The thickness of the passive film agreed well with the polarization resistance of the alloys obtained from electrochemical impedance spectroscopy (EIS) Nyquist diagrams, suggesting that the surface polarization resistance RP being closely related to the film thickness and the exchange current density on the surface of the alloy varied accordingly. The conclusion of this study is that 5 at.% Zr substitution in Mg35Ti10Ni55 leads to the formation on the surface of a good and thin passive film, which retards the corrosion of the alloy and at the same time leads to the least polarization resistance and highest exchange current in this series of alloys studied. [Copyright &y& Elsevier]

The phase structures and electrochemical behaviors of the V2.1TiNi0.5Hf0.05Crx (x=0, 0.074, 0.113 and 0.152) alloys have been investigated. It is found that the addition of Cr gives rise to a decrease in the amount of the V-based solid solution main phase and an increase in the amount of C14-type secondary phase without any change in the pattern of the three-dimensional network structure. On adding Cr, the unit cell of the main phase contracts, while that of the secondary phase significantly expands. Electrochemical measurements show that the maximum discharge capacity of the Cr-added alloys (x=0.074–0.152) is only 368–395 mAh/g, much less than that of the V2.1TiNi0.5Hf0.05 alloy (444 mAh/g), but their high-rate dischargeability and cycle stability are markedly improved. For the alloy with the Cr content of x=0.152, the best high-rate dischargeability and best cycle stability are obtained. [Copyright &y& Elsevier]

The mechanically alloyed (MA) Mg35Ti10−xCrxNi55 (x=5, 7, 9) electrode alloys were investigated for their electrochemical performance. It was found that when the Cr content was low, with the increase of Cr, the maximum discharge capacity decreased gradually, while the cycling capacity retention was improved significantly. X-ray photoelectron spectroscopy (XPS) surface analysis revealed that the Cr content in alloys effectively depressed the oxidation of metallic elements behind the passivation layer, which was built up during corrosion at the surface. Further XPS and Auger electron spectroscopy (AES) studies revealed that on the external side of the surface of Mg35Ti5Cr5Ni55 a multi-component (Cr2O3)k(TiO2)l(NiO)m(Mg(OH)2)n composite passivation layer was formed, which was much thinner (nearly 150 nm) and more compact than the layer formed on the surface of Mg35Ti10Ni55. The corrosion current density Icorr of the alloys decreased also with the augmentation of Cr content. However, Cr in the alloy was found to cause an increase of the reaction resistance at the surface and at the same time a reduction in the surface reaction activty. For the optimum performance of the alloy, the amount of Cr substitution was suggested to be around 5 at.%. At this level of Cr substitution, Icorr is still too high for practical usage. The formation a more compact, less pervious to OH− and electrochemically less active surface passivation layer due to the introduction of Cr is believed to be the main cause for the improvement of the cycling stability, the reduction of exchange current density and the deterioration of surface reaction activity. [Copyright &y& Elsevier]

The phase structures and electrochemical properties of the V2.1TiNi0.5Hfx (x=0–0.25) alloys were investigated. It is found that the addition of Hf (x≥0.05) into the V2.1TiNi0.5 alloy changes the secondary phase from the b.c.c. TiNi-based phase to a C14 Laves phase with hexagonal structure, in the form of a three-dimensional network along the grain boundaries of the main phase. The amount of the C14 Laves phase increases with increasing Hf content. The discharge capacity of the Hf added alloys remains high. The V2.1TiNi0.5Hf0.05 alloy electrode has a discharge capacity of 444 mAh/g (discharge current: 25 mA/g), exceeding that of the V2.1TiNi0.5 alloy. Proper Hf content (x≤0.12) is advantageous for improving the high-rate dischargeability of V2.1TiNi0.5Hfx alloys. The presence of Hf always leads to a degradation in cycle stability of the alloys, mainly due to the increase of the V content in the secondary phase and the increasing tendency to form cracks on the alloy surface in cycling. [Copyright &y& Elsevier]

Abstract An original electrode, 3D TiO 2 @C core/shell nanobelt arrays with controllable carbon coating thickness, was successfully constructed in this work. The 3D TiO 2 nanobelts were directly grown on Ti foil, and then were coated with a layer of metal-organic framework (MOF). After thermal annealing, a porous carbon layer with tunable thickness was coated onto the 3D TiO 2 nanobelts through changing the MOF thickness. The as-prepared TiO 2 @C-2 h with 5-nm-thick carbon layer presented an excellent discharge capacity of 210.5 mAh g−1 after 100 cycles at a current density of 50 mA g−1, which is 38% higher than that of pure TiO 2 electrode without carbon coating. Moreover, it can maintain a reversible capacity of 141 mAh g−1 after 1000 cycles at a current density of 200 mA g−1, and a good rate performance with 33% of capacity retention when the current density increasing from 50 to 5000 mA g−1. All the results indicate that the TiO 2 @C core/shell nanobelt array is a promising anode for sodium-ion batteries. Highlights • 3D TiO 2 nanobelt arrays possess short ion transport distance. • Coating of porous carbon layer can solve low rate capability and poor stability. • Carbon coating can be tuned by MOF thickness to get optimal performance. • Ideal 3D TiO 2 @C nanobelt-SIB exhibit superior reversible capacity and rate capability. [ABSTRACT FROM AUTHOR]

In this paper, Ti-based AB2 Laves phase hydrogen storage electrode alloys are prepared and their electrochemical cycle stability is studied, including Ti0.7Zr0.5V0.2Mn1.8−xNix (x=0.4, 0.8, 1.1, 1.4 and 1.7) and Ti1.0Zr0.2Cr0.4Ni0.8V0.8−xAlx (x=0.0, 0.2, 0.4 and 0.6) alloys. Energy disperse spectroscopy (EDS) and X-ray photoelectron energy spectroscopy (XPS) were used to analyze the changes of the elements on the alloy grain surface before and after charging–discharging cycles in a KOH solution in order to understand the nature of the decrease of discharge capacity with cycling. The experimental results show that the linear-polarization resistance increases when cycling. The segregation, oxidization and dissolution of some elements in the alloy are key factors influencing the cycle stability and are also a main problem that needs to be overcome in order to use these materials as negative electrode in a commercial nickel–metal hydride rechargeable battery. [Copyright &y& Elsevier]