V2.38Nb10.7O32.7: A V2O5–Nb2O5 mixed oxide tunnel structure related to the tetragonal tungsten bronzes

In situ synchrotron radiation investigation of V2O5-Nb2O5 metastable compounds: transformational kinetics at high temperatures with a new structural solution for the orthorhombic V4Nb20O60 phase

Due to the considerable interest in vanadium niobium oxides as a lithium storage material, the kinetics and transformation processes of the V2O5-5Nb2O5 system have been investigated by in situ synchrotron powder X-ray diffraction. The diffraction data after the thermal treatments selected with a view on the most significant features were supplemented with specific ex situ experiments conducted using a laboratory rotating anode X-ray diffractometer. The morphological changes of the mixed powders assuming an amorphous and nanocrystalline solid solution structure as a function of the temperature were inspected by scanning electron microscopy observations. The structural solution of the powder diffraction pattern of the phase recorded in situ at a temperature of about 700 °C was compatible with an orthorhombic crystal structure with the space group Amm2. The obtained lattice parameters for this structure were a = 3.965 Å; b = 17.395 Å, c = 17.742 Å, and the cell composition was V4Nb20O60, Pearson symbol oA84, and density = 4.10 g cm-3. In this structure, while the niobium atoms may be four-, five-, and six-fold coordinated by oxygen atoms, the vanadium atoms were six-fold or seven-fold coordinated. At the temperature of 800 °C and just above, the selected 1 : 2 and 1 : 3 V2O5-Nb2O5 compositions, respectively, returned mostly a tetragonal VNb9O25 phase, in line with earlier observations conducted for determination of the stability phase diagram of such quasi-binary systems.

Probing Mg Intercalation in the Tetragonal Tungsten Bronze Framework V4Nb18O55

While commercial Li-ion batteries offer the highest energy densities of current rechargeable battery technologies, their energy storage limit has almost been achieved. Therefore, there is considerable interest in Mg batteries, which could offer increased energy densities in comparison to Li-ion batteries if a high-voltage electrode material, such as a transition-metal oxide, can be developed. However, there are currently very few oxide materials which have demonstrated reversible and efficient Mg²⁺ insertion and extraction at high voltages; this is thought to be due to poor Mg²⁺ diffusion kinetics within the oxide structural framework. Herein, the authors provide conclusive evidence of electrochemical insertion of Mg²⁺ into the tetragonal tungsten bronze V₄Nb₁₈O₅₅, with a maximum reversible electrochemical capacity of 75 mA h g^-1, which corresponds to a magnesiated composition of Mg₄V₄Nb₁₈O₅₅. Experimental electrochemical magnesiation/demagnesiation revealed a large voltage hysteresis with charge/discharge (1.12 V vs Mg/Mg²⁺); when magnesiation is limited to a composition of Mg₂V₄Nb₁₈O₅₅, this hysteresis can be reduced to only 0.5 V. Hybrid-exchange density functional theory (DFT) calculations suggest that a limited number of Mg sites are accessible via low-energy diffusion pathways, but that larger kinetic barriers need to be overcome to access the entire structure. The reversible Mg²⁺ intercalation involved concurrent V and Nb redox activity and changes in crystal structure, as confirmed by an array of complementary methods, including powder X-ray diffraction, X-ray absorption spectroscopy, and energy-dispersive X-ray spectroscopy. Consequently, it can be concluded that the tetragonal tungsten bronzes show promise as intercalation electrode materials for Mg batteries.

Synergistic Effect of New ZrNi5/Nb2O5 Catalytic Agent on Storage Behavior of Nanocrystalline MgH2 Powders

Due to its availability and high storage capacity, Mg is an ideal material in hydrogen storage applications. In practice, doping Mg/MgH2 with catalyst(s) is necessary in enhancing the de/rehydrogenation kinetics and minimizing both of decomposition temperature and its related apparent activation energy. The present study proposed a new heterogeneous catalytic agent that consisted of intermetallic compound (ZrNi5)/metal oxide (Nb2O5) binary system for using with different concentrations (5−30 wt%) to improve MgH2. Doping MgH2 powders with low concentration (5, 7, 10 wt%) of this new catalytic system led to superior absorption/desorption kinetics, being indexed by the short time that is required to absorb/desorb 4.2−5.6 wt% H2 within 200 s to 300 s. Increasing the doping dose to 15–30 wt% led to better kinetic effect but a significant decrease in the hydrogen storage capacity was seen. The dependent of apparent activation energy and decomposition temperature of MgH2 on the concentration of ZrNi5/Nb2O5 has been investigated. They tended to be linearly decreased with increasing the catalyst concentrations. The results elucidated the crucial role of catalytic additives on the disintegration of MgH2 into ultrafine powders (196 nm to 364 nm diameter). The formation of such nanoparticles enhance the hydrogen diffusion and shorten the time that is required for the hydrogenation/dehydrogenation process. Moreover, this refractory catalytic system acted as a grain growth inhibitor, in which Mg/MgH2 powders maintained their submicron level during the cycle-life-test that was extended to 100 h at 200 °C.

Remarkable hydrogen storage properties of MgH2 doped with VNbO5

The present work concerns the catalytic effect of VNbO₅, a ternary oxide prepared via a solid-state route, on the sorption performance of MgH₂. Three doped systems, namely 5, 10 and 15 wt% VNbO₅-MgH₂ have been prepared by ball milling and thoroughly characterized. Hydrogen sorption, evaluated by temperature programmed desorption experiments, revealed a significant reduction of the desorption temperature from 330 °C for the un-doped sample (prepared and tested for comparison) to 235 °C for the VNbO₅-doped sample. Furthermore, more than 5 wt% of hydrogen can be absorbed in 5 minutes at 160 °C under 20 bar of hydrogen, which is remarkable compared to the 0.7 wt% achieved for the un-doped system. The sample doped with 15 wt% of additive, showed good reversibility: over 5 wt% of hydrogen with negligible degradation even after 70 consecutive cycles at 275 °C and 50 cycles at 300 °C. The kinetics analysis carried out by Kissinger's method exhibited a considerable reduction of the activation energy for the desorption process. Finally, pressure-composition-isotherm experiments conducted at three different temperatures allowed estimating the thermodynamic stability of the system and shed light on the additive role of VNbO₅.