Microstructure and hydrogen storage properties of (Ti0.32Cr0.43V0.25) + x wt% La (x = 0–10) alloys

The composite alloy of Ti_0.32Cr_0.43V_0.25 with x wt% La (where x = 0–10) was prepared by arc melting technique. The effect on hydrogen storage capacity, flatness of the plateau pressure, and residual hydrogen was investigated in La added Ti_0.32Cr_0.43V_0.25. Crystalline phase and microstructure of the prepared composite alloy were investigated and characterized by XRD, SEM and TEM. The crystal structure was refinement using Rietveld analysis. The effective hydrogen storage capacity of the composite alloy was found comparable to the parent alloy, when 5 wt% La was added. The effective hydrogen capacity (∼2.31 wt%) was close to that of the parent alloy (2.35 wt%) and the plateau slope was significantly improved from 30.5 of the parent alloy to 14.6. Appropriate mechanisms associated with the improved flatness by the La addition has been discussed in terms of the refined crystalline structure. Using TG/DTA method we have shown the differences in the interaction of residual hydrogen with the BCC phase of both parent alloy and 5 wt % La mixed alloy.     

La2−xSrxCoO4−δ (x = 0.9, 1.0, 1.1) Ruddlesden-Popper-type layered cobaltites as cathode materials for IT-SOFC application

La_2−xSr_xCoO_4−δ (x = 0.9, 1.0, 1.1) compounds with Ruddlesden-Popper K₂NiF₄-type structure have been investigated as potential cathode materials for IT-SOFC application. Materials have been prepared by citrate-nitrate combustion method. Structural evolution analysed by XRD shows a shortened Co–O–Co bond length within the perovskite layer as Sr substitution increases, while the interlayer distance at the same time increases. An oxygen stoichiometry close to 4 has been found for all compositions at room temperature. Thermal expansion coefficients have been obtained from temperature-dependent XRD analysis and show large values (>20 × 10⁻⁶ K⁻¹) compared to the currently utilized electrolyte materials. Electrochemical characterisation has been performed by means of impedance spectroscopy on symmetric cells with CGO electrolyte. Pure electrodes have a high Area Specific Resistance, probably due to limited oxygen ion diffusion. By using composite electrode (50 wt.% CGO), an Area Specific Resistance of 0.25 Ω cm² is obtained at approximately 700 °C for all the three compounds suggesting promising electrochemical properties for IT-SOFCs.     

Synthesis and characterization of BaIn0.3−xYxCe0.7O3−δ (x = 0, 0.1, 0.2, 0.3) proton conductors

The morphological and electrical properties of yttrium (Y) and indium (In) doped barium cerate perovskites of the form BaIn_0.3−xY_xCe_0.7O_3−δ (with x = 0–0.3) prepared by a modified Pechini method were investigated as potential high temperature proton conductors with improved chemical stability and conductivity. The sinterability increased with the increase of In-doping, and the perovskite phase was found in the BaIn_0.3−xY_xCe_0.7O_3−δ solid solutions over the range 0 ≤ x ≤ 0.3. The conductivities decreased from x = 0.3 to 0 while the tolerance to wet CO₂ improved for BaIn_0.3−xY_xCe_0.7O_3−δ samples with an increase of In-doping. BaIn_0.1Y_0.2Ce_0.7O_3−δ was found to have relatively high conductivity as well as acceptable wet CO₂ stability.     

Novel Nd2WO6-type Sm2−xAxM1−yByO6−δ (A = Ca, Sr; M = Mo, W; B = Ce, Ni) mixed conductors

In the present work, we have explored novel Nd₂WO₆-type structure Sm_2−xA_xM_1−yB_yO_6−δ (A = Ca, Sr; M = Mo, W; B = Ce, Ni) as precursor for the development of solid oxide fuel cells (SOFCs) anodes. The formation of single-phase monoclinic structure was confirmed by powder X-ray diffraction (PXRD) for the A- and B-doped Sm₂MO₆ (SMO). Samples after AC measurements under wet H₂ up to 850 °C changed from Nd₂WO₆-type structure into Sm₂MoO₅ due to the reduction of Mo^VI that was confirmed by PXRD and is consistent with literature. The electrical conductivity was determined using 2-probe AC impedance and DC method and was compared with 4-probe DC method. The total electrical conductivity obtained from these two different techniques was found to vary within the experimental error over the investigated temperature of 350-650 °C. Ionic and electronic conductivity were studied using electron-blocking electrodes technique. Among the samples studied, Sm_1.8Ca_0.2MoO_6−δ exhibits total conductivity of 0.12 S cm⁻¹ at 550 °C in wet H₂ with an activation energy of 0.06 eV. Ca-doped SMO appears to be chemically stable against reaction with YSZ electrolyte at 800 °C for 24 h in wet H₂. The ionic transference number (t_i) of Sm_1.9Ca_0.1MoO_6−δ in wet H₂ at 550 °C (pO₂ = 10^−25.5 atm) was found to be about 0.012 after subtraction of electrical lead resistance from the 2-probe AC data and showed predominate electronic conductors.     

Investigation on electrochemical performances of melt-spun nanocrystalline and amorphous Mg2Ni1−xMnx (x = 0–0.4) electrode alloys

In order to enhance the glass forming ability of the Mn₂Ni-type electrode alloy, Ni in the Mg₂Ni compound is partially substituted by Mn. The nanocrystalline and amorphous Mg₂Ni-type alloys with nominal compositions of Mg₂Ni_1−xMn_x (x = 0, 0.1, 0.2, 0.3, 0.4) are fabricated 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 successfully obtained. The microstructures of the as-spun alloy ribbons are characterized by XRD, SEM and TEM. The electrochemical hydrogen storage characteristics of the as-spun alloy ribbons were tested by an automatic galvanostatic system. The electrochemical impedance spectra (EIS) are plotted by an electrochemical workstation (PARSTAT 2273). The hydrogen diffusion coefficients in the alloys are calculated by virtue of potential-step method. The results show that no amorphous structure is detected in the as-spun Mn-free alloy, whereas the as-spun alloys containing Mn display a nanocrystalline and amorphous structure. The amorphization degree of the alloy increases with rising spinning rate, suggesting that the substitution of Mn for Ni facilitates the glass formation in the Mg₂Ni-type alloy. The substitution of Mn for Ni markedly improves the electrochemical hydrogen storage performances of the Mg₂Ni-type alloy, enhancing both the discharge capacity and the electrochemical cycle stability. 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 increasing amount of Mn substitution.     

Kinetic properties of La2Mg17–x wt.% Ni (x = 0–200) hydrogen storage alloys prepared by ball milling

The as-cast La₂Mg₁₇ with different amount of Ni powders were mixed through ball milling to produce a new type of La₂Mg₁₇–x wt.% Ni (x = 50, 100, 150, 200) alloy. The microstructures of the alloys were characterized by XRD technique, the results show that the crystal structure transfers to amorphous one with the increasing amount of Ni powders. La₂Mg₁₇–50 wt.% Ni alloy reaches the highest hydrogen absorption capacity of 5.13 wt.% at 300 °C under 2 MPa hydrogen pressure due to its amorphous structure. Furthermore, La₂Mg₁₇–50 wt.% Ni alloy expresses fast hydriding kinetics and absorbs 4.99 wt.% hydrogen gas in 200 s. The hydrogen desorption ability described as discharge capacity during electrochemical reaction is fade next to La₂Mg₁₇–200 wt.% Ni alloy, attributed to the less Mg₂NiH₄ with lower enthalpies and easier to release H₂. The maximum discharge capacity of La₂Mg₁₇–200 wt.% Ni alloy reaches to exciting 980.90 mAh/g, while the La₂Mg₁₇ alloy is only 18.10 mAh/g with inconspicuous improvement of cycle stability. These dramatic difference in electrochemical performance reflect the consequence of sluggish dehydriding process of La₂Mg₁₇–50 and 100 wt.% Ni alloys again. Whereas La₂Mg₁₇–200 wt.% Ni alloy has lower resistance both on alloy surface and in the bulk.     

An investigation on electrochemical and gaseous hydrogen storage performances of as-cast La1−xPrxMgNi3.6Co0.4 (x = 0–0.4) alloys

The as-cast RE–Mg–Ni-based AB₂-type La_1−xPr_xMgNi_3.6Co_0.4 (x = 0–0.4) alloys were prepared by vacuum induction furnace with a high purity helium gas as the protective atmosphere. The phase composition and microstructure of the as-cast alloys were characterized by XRD, SEM equipped with EDS. The results indicate that the as-cast alloys consist of two phases of LaMgNi₄ and LaNi₅. The measurements of the electrochemical properties show that the discharge capacity of the alloys slightly decreases with Pr content rising. As the Pr content grows from 0 to 0.4, the maximum discharge capacity decreases from 347.0 to 310.4 mAh/g. However, the cycle stability and the high-rate dischargeability of the alloy obviously augment with the Pr content increasing. Furthermore, the measurements of the electrochemical hydrogen storage kinetics reveal that the limiting current density (I_L) first increases then decreases whereas the exchange current density I₀ of the alloys first decreases then increases with the rising amount of Pr substitution, which indicates that the electrochemical dynamic of the alloy electrode are jointly dominated by the charge-transfer resistance and diffusion ability of hydrogen atoms. The measuring of the gaseous hydrogen storage reveals two pressure plateaus appear on each pressure–concentration–isotherm (P–C–T) curve of the as-cast alloys, which correspond to the LaMgNi₄ and LaNi₅ phases. Furthermore, we note that the pressure plateau of the P–C–T curve visibly rises with Pr content increasing.     

Preparation and evaluation of Ca3−xBixCo4O9−δ (0< x ≤ 0.5) as novel cathodes for intermediate temperature-solid oxide fuel cells

The misfit compounds Ca_3−xBi_xCo₄O_9−δ (x = 0.1–0.5) were successfully synthesized via conventional solid state reaction and evaluated as cathode materials for intermediate temperature-solid oxide fuel cells. The powders were characterized by X-ray diffraction, scanning emission microscopy, X-ray photoelectron spectroscopy, thermogravimetry analysis and oxygen-temperature programmed desorption. The monoclinic Ca_3−xBi_xCo₄O_9−δ powders exhibit good thermal stability and chemical compatibility with Ce_0.8Sm_0.2O_2−γ electrolyte. Among the investigated single-phase samples, Ca_2.9Bi_0.1Co₄O_9−δ shows the maximal conductivity of 655.9 S cm⁻¹ and higher catalytic activity compared with other Ca_3−xBi_xCo₄O_9−δ compositions. Ca_2.9Bi_0.1Co₄O_9−δ also shows the best cathodic performance and its cathode polarization resistance can be further decreased by incorporating 30 wt.% Ce_0.8Sm_0.2O_2−γ. The maximal power densities of the NiO/Ce_0.8Sm_0.2O_2−γ anode-supported button cells fabricated with the Ce_0.8Sm_0.2O_2−γ electrolyte and Ca_2.9Bi_0.1Co₄O_9−δ + 30 wt.% Ce_0.8Sm_0.2O_2−γ cathode reach 430 and 320 mW cm⁻² at 700 and 650 °C respectively.     

Characterization of layered perovskite oxides NdBa1−xSrxCo2O5+δ (x = 0 and 0.5) as cathode materials for IT-SOFC

The crystal structure, thermal expansion, and electrochemical properties of the layered perovskite series NdBa_1−xSr_xCo₂O_5+δ (x = 0 and 0.5) were investigated to study the effects of substituting Sr for Ba on a layered perovskite oxide.     

Ln0.6Sr0.4Co1−yFeyO3−δ (Ln = La and Nd; y = 0 and 0.5) cathodes with thin yttria-stabilized zirconia electrolytes for intermediate temperature solid oxide fuel cells

The electrochemical performances of the solid oxide fuel cells (SOFC) fabricated with Ln_0.6Sr_0.4Co_1−yFe_yO_3−δ (Ln = La, Nd; y = 0, 0.5) perovskite cathodes, thin yttria-stabilized zirconia (YSZ) electrolytes, and YSZ–Ni anodes by tape casting, co-firing, and screen printing are evaluated at 600–800 °C. Peak power densities of ∼550 mW cm⁻² are achieved at 800 °C with a La_0.6Sr_0.4CoO_3−δ (LSC) cathode that is known to have high electrical conductivity. Substitution of La by Nd (Nd_0.6Sr_0.4CoO_3−δ) to reduce the thermal expansion coefficient (TEC) results in only a slight decrease in power density despite a lower electrical conductivity. Conversely, substitution of Fe for Co (La_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ or Nd_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ) to reduce the TEC further reduces the cell performance greatly due to a significant decrease in electrical conductivity. However, infiltration of the Fe-substituted cathodes with Ag to increase the electrical conductivity increases the cell performance while preserving the low TEC.     

Hydrogen storage properties of Ti1.4V0.6Ni + x Mg (x = 1–3, wt.%) alloys

We prepared Ti_1.4V_0.6Ni ribbons by arc-melting and subsequent melt-spinning techniques. Ti_1.4V_0.6Ni + x Mg (x = 1, 1.5, 2, 2.5 and 3, wt.%) composite alloys were obtained by the mechanical ball-milling method. The structures and hydrogen storage properties of alloys were investigated. Ti_1.4V_0.6Ni + x Mg composite alloys contained icosahedral quasicrystalline phase, Ti₂Ni-type phase, β-Ti solid-solution phase and metallic Mg. The electrochemical and gaseous hydrogen storage properties of alloys were improved with Mg addition. Ti_1.4V_0.6Ni + 2 Mg alloy showed maximum electrochemical discharge capacity of 282.5 mAh g⁻¹ as well as copacetic high-rate discharge ability of 82.3% at the discharge current density of 240 mA g⁻¹ compared with that of 30 mA g⁻¹, and the cycling life achieved above 200 mAh g⁻¹ after 50 consecutive cycles of charging and discharging. The hydrogen absorption/desorption properties of Ti_1.4V_0.6Ni + x Mg (x = 1, 2 and 3, wt.%) alloys were better than Ti_1.4V_0.6Ni. Ti_1.4V_0.6Ni + 3 Mg alloy also exhibited a favorable hydrogen absorption capacity of 1.53 wt.%. The improvement in the hydrogen storage characteristics caused by adding Mg may be ascribed to better hydrogen diffusion and anti-corrosion ability.     

Structure and electrochemical properties of (La1−xDyx)0.8Mg0.2Ni3.4Al0.1 (x = 0.0–0.20) hydrogen storage alloys

The structure and electrochemical characteristics of (La_1−xDy_x)_0.8Mg_0.2Ni_3.4Al_0.1 (x = 0–0.20) hydrogen storage alloys have been investigated. Dysprosium was adopted as a partial substitution element for lanthanum in order to improve electrochemical properties. The XRD, SEM and EDX results showed that the alloys were composed of (La, Mg)₂Ni₇, LaNi₅ and (La, Mg)Ni₂ phases. The introduction of Dy promoted the formation of (La, Mg)₂Ni₇ phase which possesses high hydrogen storage capacity, and controlling dysprosium content at 0.05 can obtain the maximum (La, Mg)₂Ni₇ phase abundance in the alloys. The maximum discharge capacity was heightened from 382.5 to 390.2 mAh/g, which was ascribed to (La, Mg)₂Ni₇ phase abundance increasing from 54.8% to a maximum (60.5%). Also, the biggest discharge capacity retention remained 82.7% after 100 cycles at discharge current density of 300 mA/g.     

Experimental and theoretical studies on static and dynamic pressure–concentration isotherms of MmNi5−xAlx (x = 0, 0.3, 0.5 and 0.8) hydrides

The static and dynamic pressure–concentration isotherms (PCIs) of MmNi_5−xAl_x (x = 0, 0.3, 0.5 and 0.8) hydrides were measured at different temperatures using volumetric method. The effect of Al substitution on PCI and thermodynamic properties were studied. The plateau pressure and maximum hydrogen storage capacity decreased with Al content whereas reaction enthalpy increased. The plateau pressure, plateau slope and hysteresis effect was observed more for dynamic PCIs compared to static PCIs. Different mathematical models used for metal hydride-based thermodynamic devices simulation are compared to select suitable model for static and dynamic PCI simulation of MmNi₅-based hydrides. Few important physical coefficients (partial molar volume, reaction enthalpy, reaction entropy, etc.) useful for development of thermodynamic devices were estimated. A relation has been proposed to correlate aluminium content and physical coefficients for the prediction of unknown PCI. The simulated and experimental PCIs were found matching closely for both static and dynamic conditions.     

Conductivity study of dense BaCexZr(0.9−x)Y0.1O(3−δ) prepared by solid state reactive sintering at 1500 °C

A cost and time effective process was used to prepare the solid solutions BaCe_xZr_(0.9−x)Y_0.1O_(3−δ) (0 ≤ x ≤ 0.4). 98% dense samples were obtained by solid state reactive sintering at 1500 °C for 4 h, with the addition of 1 wt% of NiO to the quantity of synthesized/sintered compound. Scanning electron micrographs reveal polygonal grains of 1–5 microns, whose size increases from the compound with no cerium (BCZY09) to the samples containing cerium (BCZY18–BCZY45). The conductivity, measured in wet reducing atmosphere (9% H₂ in N₂, p(H₂O) = 0.015 atm) by impedance spectroscopy, increases with the cerium content. Some samples have also been prepared using barium sulfate (BaSO₄) as barium precursor (instead of barium carbonate BaCO₃) due to its non toxicity. The corresponding samples (prepared at 1575 °C) showed similar properties as the ones prepared with barium carbonate. Furthermore, different geometries (rods, tubes, pellets) could be made.     

Preparation and characterization of La0.9Sr0.1Ga0.8Mg0.2O3 − δ thin film on the porous cathode for SOFC

A dense and crack-free La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 − δ thin film has been prepared by RF magnetron sputtering. The XRD, FESEM, XPS and four-probe technique are employed to characterize the La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 − δ film. Results show that after annealing at 1000 °C, the La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 − δ film presents a polycrystalline perovskite structure with grain size of 100–300 nm. XPS data show that both La and Ga are in their +3 state. Sr element has two chemical states which are related to Sr²⁺ in the perovskite lattice and SrO_1 − δ suboxide. The O 1s spectrum also shows two chemical states which can be assigned to molecularly adsorbed O₂ species and O²⁻ in the lattice. The electrical conductivity reaches to 0.093 S cm⁻¹ at 800 °C. The microstructure and conductivity analysis indicates that the La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 − δ thin film prepared by RF magnetron sputtering is suitable for intermediate temperature Solid oxide fuel cell.     

Desirable performance of intermediate-temperature solid oxide fuel cell with an anode-supported La0.9Sr0.1Ga0.8Mg0.2O3 − δ electrolyte membrane

An anode-supported La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 − δ (LSGM) electrolyte membrane is successfully fabricated by simple, cost-effective spin coating process. Nano-sized NiO and Ce_0.8Gd_0.2O_3 − α (GDC) powders derived from precipitation and citric-nitrate process, respectively, are used for anode support. The dense and uniform LSGM membrane of ca. 50 μm in thickness is obtained by sintering at relatively low temperature 1300 °C for 5 h. A single cell based on the as-prepared LSGM electrolyte membrane exhibits desirable high cell performance and generates high output power densities of 760 mW cm⁻² at 700 °C and 257 mW cm⁻² at 600 °C, respectively, when operated with humidified hydrogen as the fuel and air as the oxidant. The single cell is characterized by field-emission scanning electron microscope (FESEM), X-ray diffraction (XRD) and electrochemical AC impedance. The results demonstrate that it is feasible to fabricate dense LSGM membrane for solid oxide fuel cell by this simple, cost-effective and efficient process. In addition, optimized anode microstructure significantly reduces polarization resistance (0.025 Ω cm² at 700 °C).     

Electrical conductivity of Ce0.8Gd0.2−xDyxO2−δ (0 ≤ x ≤ 0.2) co-doped with Gd and Dy for intermediate-temperature solid oxide fuel cells

High-quality nano-sized Ce_0.8Gd_0.2−xDy_xO_2−δ (0 ≤ x ≤ 0.2) powders are synthesized by a solution combustion process. The calcined powders are composed of a ceria-based single phase with a cubic fluorite structure and are nanocrystalline nature, i.e., 15-24 nm in crystallite size. The addition of an intermediate amount of Dy³⁺ (0.03 ≤ x ≤ 0.16) for Gd³⁺ in Ce_0.8Gd_0.2O_2−δ decreases the electrical conductivity. On the other hand, the doping of a small amount of Dy³⁺ (0.01 ≤ x ≤ 0.02) and of a large amount of Dy³⁺ (0.17 ≤ x ≤ 0.19) leads to an increase in conductivity. The Ce_0.8Gd_0.03Dy_0.17O_2−δ shows the highest electrical conductivity (0.215 S cm⁻¹) at 800 °C.     

Microstructure and hydrogen storage properties of Ti10V84−xFe6Zrx (x = 1–8) alloys

Ti₁₀V_84−xFe₆Zr_x (x = 1, 2, 4, 6, 8) hydrogen storage alloys were prepared by induction melting with magnetic levitation, and the effects of Zr content on the microstructures and hydrogen storage properties have been investigated systematically. The results show that the alloy with x = 1 has a single V-based solid solution phase with BCC structure, while other alloys with x = 2–8 consist of a BCC main phase and a C14 type Laves secondary phase, and the abundance ratio of the secondary phase increases with increasing Zr content. As the Zr content in the alloy increases, the activation behavior is improved, but the hydrogen absorption and desorption capacities decrease gradually. For the alloy with the Zr content of x = 1, the best overall hydrogen storage properties are obtained.     

Catalytic activity of perovskite-type doped La0.08Sr0.92Ti1−xMxO3−δ (M = Mn,Fe, and Co) oxides for methane oxidation

Transition metal doped La_0.08Sr_0.92M_0.20Ti_0.80O_3−δ (M = Mn, Fe, and Co) perovskite oxides were synthesized by the Pechini method. The methane oxidation behavior and the polarization resistance of the solid oxide fuel cells (SOFCs) with the perovskite oxides as anode was subsequently measured as a function of operation temperature. Surface atomic concentrations of the perovskite oxides were evaluated using X-ray photoelectron spectroscopy (XPS) and their relationship to the catalytic activity were discussed with respect to the transition metal dopant. The complete oxidation of methane was predominant in the low-temperature region, while the partial oxidation of methane occurred at high temperatures. Fe- and Co-doped perovskites showed better catalytic activity for the methane oxidation reaction than Mn-doped powder. This phenomenon could be explained by the high atomic concentration with low oxidation states and the resulting high oxygen vacancy concentration in the Fe- and Co-doped perovskite powder samples.