Hydrogen storage and electrochemical properties of mechanically alloyed La1.5-xGdxMg0.5Ni7 (0 ≤ x ≤ 1.5)

RE-Mg-Ni-based alloys with their unique superlattice structures have been considered to be possible candidates for hydrogen storage containers as well as electrodes in Ni-MH_x batteries. In this study, mechanical alloying with subsequent annealing in the argon atmosphere at 1123 K for 0.5 h, was applied to produce the La_1.5-xGd_xMg_0.5Ni₇ (0 ≤ x ≤ 1.5) alloys. Hydrogen storage and electrochemical properties of the synthesized material have been investigated. For example, at the 50^th cycle the La_1.5Mg_0.5Ni₇ material shows a much lower reversible electrochemical capacity than La_1.25Gd_0.25Mg_0.5Ni₇ alloy. Additionally, the partial substitution of La by Gd improves the kinetics of hydrogen absorption. On the other hand, the stability of the electrochemical discharge capacity increases with the increasing value Gd up to x = 1.0. However, a significant reduction in the discharge capacity was observed for the Gd content above x = 0.25. From the application point of view, only La_1.25Gd_0.25Mg_0.5Ni₇ alloy show great potential in the future application as electrode material in Ni-MH_x batteries.     

Microstructure and hydrogen storage properties of Zr0.8Ti0.2Co1−xFex (x=0, 0.1, 0.2, 0.3) alloys

In the present study, Zr_0.8Ti_0.2Co_1?xFe_x (x = 0, 0.1, 0.2 and 0.3) alloys were prepared by arc melting method. The effect of Fe substitution on microstructure and hydrogen storage properties was studied systematically. The phase structure and hydrogen storage properties were characterized by X-ray diffraction (XRD), Electron Probe Micro-analysis (EMPA) and Sievert's type volumetric apparatus. XRD and EPMA analysis show that Zr_0.8Ti_0.2Co alloy forms cubic phase ZrCo and traces of ZrCo₂, while the alloys of composition with x = 0.1, 0.2 and 0.3 form cubic phase ZrCo with the secondary Laves phases Zr(Co,Fe)₂ and Zr₂Co. The cell volumes and content of the secondary phase increase gradually as the content of Fe substitution increases. The hydrogen storage experiment shows that Fe substitution for Co ameliorates initial hydriding kinetic property and shortens the incubation duration of the Zr_0.8Ti_0.2Co_1?xFe_x (x = 0.1, 0.2 and 0.3) alloys, compared with Zr_0.8Ti_0.2Co alloy. The improved kinetic property is due to the catalyst effect of the secondary phase, which makes it favorable for the application in International Thermonuclear Experimental Reactor (ITER).     

Electrochemical evaluation of double perovskite PrBaCo2-xMnxO5+δ (x = 0, 0.5, 1) as promising cathodes for IT-SOFCs

Mn-substituted double perovskites, PrBaCo_2-xMn_xO_5+δ (x = 0, 0.5, 1), are evaluated as cathode materials for intermediate-temperature solid oxide fuel cells. The effects of Mn substitution content on their structural and electrochemical properties including crystal structure, thermal expansion coefficient, and cathodic interfacial polarization resistance are investigated. The PrBaCo_2-xMn_xO_5+δ samples exhibit structural changes with increasing Mn contents from tetragonal (x = 0) to cubic (x = 0.5 and 1.0) symmetry. The thermal expansion coefficient decreases with the increasing Mn content while the cathodic performance increases with the increment of Mn content from x = 0 to x = 0.5 then decreases with the further increment of Mn content from x = 0.5 to x = 1.0. When using La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O₃ with 300 μm thickness as electrolyte and Sr₂Fe_1.4Ni_0.1Mo_0.5O_6-δ as anode, the maximum powder density of the x = 0.5 composite is 0.638 W cm^?2, which is higher than that of the other two samples with x = 0 (0.474 W cm^?2) and x = 1.0 (0.371 W cm^?2) at 800 °C.     

(Pr0.9La0.1)2(Ni0.74Cu0.21Nb0.05)O4+δ-Ce0.9Gd0.1O2−δ (GDC) as an active and CO2-tolerant nano-composite cathode for intermediate temperature solid oxide fuel cells

Nano composite of Ruddlesden-Popper electrocatalyst-oxygen ionic conductor, (Pr_0.9La_0.1)₂(Ni_0.74Cu_0.21Nb_0.05)O_4+δ (PLNCN)-Ce_0.9Gd_0.1O_2?δ (GDC), is developed as composite cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs) via a infiltration way. The electrochemical behavior of PLNCN-GDC nanostructured electrode is assessed with respect to infiltration loading and oxygen partial pressure. The optimized PLNCN loading can improve charge transfer dynamics for electrochemical oxygen reduction reaction, thus promoting cathode performance. Importantly, the encouraging results of single cell highlight high activity and good CO₂ tolerance of PLNCN-GDC nanostructured cathode.     

Solid solution limits and electrical properties of scheelite SryLa1-yNb1-xVxO4-δ materials for x = 0.25 and 0.30 as potential proton conducting ceramic electrolytes

The proton conductivity and solid solubility limits of acceptor strontium doped vanadium stabilised lanthanum niobate (Sr_yLa_1?yNb_1-xV_xO_4-δ, x = 0.25, 0.30 and y = 0 to 0.10) were explored as potential proton conducting ceramic electrolytes. All samples were synthesized via a solid-state method. The phase purity, microstructure and thermal expansion behaviour of the materials were studied using powder X-ray diffraction, scanning electron microscopy and dilatometry, respectively. A maximum solid solution limit of 5% Sr in the A-site of Sr_yLa_1?yNb_1-xV_xO_4-δ samples is observed for a vanadium content of x = 0.25, while further increases in the Sr or vanadium contents lead to the presence of Sr₃(VO₄)₂ as a secondary phase. This acceptor dopant content of 5%Sr in the current scheelite material exceeds that possible in the parent vanadium-free fergusonite Sr_yLa_1?yNbO_4-δ material by a factor of 5. All Sr doped scheelite materials show linear thermal expansion behaviour, successfully avoiding the scheelite to fergusonite structural phase change during thermal cycling. The average grain size is shown to be increased by increasing vanadium content. In humid conditions, all phase pure samples show predominantly proton conductivity at lower temperatures, while p-type conductivity is noted at higher temperatures under dry oxidising conditions. In the low temperature range, the Sr_0.05La_0.95Nb_0.75V_0.25O_4-δ sample, containing the largest acceptor dopant concentration, exhibits slightly higher bulk and specific grain boundary conductivities in comparison to other phase pure compositions.     

Preparation and characterization of ceramic interconnect La0.8Ca0.2Cr0.9M0.1O3−δ (M = Al, Co, Cu, Fe) for IT-SOFCs

The lattice parameters, electrical conductivity, activation energy, mechanical properties, and microstructure of (La_0.8Ca_0.2)CrO_3−δ-based specimens were investigated systematically in this paper. The tolerance factors for (La_0.8Ca_0.2)CrO_3−δ-based specimens were all greater than 0.9, indicating the perovskite was not distorted with different cations (Al³⁺, Co³⁺, Cu²⁺, Fe³⁺) substitution for B site of (La_0.8Ca_0.2)CrO_3−δ. (La_0.8Ca_0.2)Cr_0.9Co_0.1O_3−δ specimen revealed the maximum electrical conductivity, σ_850 °C = 59.59 S/cm with minimum activation energy, E_a = 11.2 kJ/mol among (La_0.8Ca_0.2)CrO_3−δ-based specimens. The grain size seemed dependent on doping species and the grain sizes were distributed in the range of 2.4-5.6 μm for (La_0.8Ca_0.2)CrO_3−δ-based specimens. The rate of grain growth was proportional to the boundary mobility M_b, which was related to the diffusion coefficient of doping cation. (La_0.8Ca_0.2)CrO_3−δ-based specimens revealed variety in microhardness, in the range of 4.33-9.85 GPa and the fracture toughness were distributed in the range of 3.52-4.33 MPa m^1/2. Based on the results in terms of grain size and mechanical properties, we concluded that the microhardness and fracture toughness were dependent on the dopant ions. The (La_0.8Ca_0.2)Cr_0.9Co_0.1O_3−δ specimen shows high electrical conductivity and mechanical properties Consequently, it is a promising candidate as an interconnect material for intermediate temperature solid oxide fuel cell (IT-SOFC) applications.     

Cermets Ni/(Ce0.9Ln0.1O1.95) (Ln = Gd,La, Nd and Sm) prepared by solution combustion method as catalysts for hydrogen production by partial oxidation of methane

Catalysts based on Ni/(Ce_0.9Ln_0.1O_1.95) (Ln = Gd, La, Nd and Sm) have been developed and tested for hydrogen production by partial oxidation of methane. The synthesis method (SCS, solution combustion synthesis) produces macroporous composite materials composed of ceramic (cer, Ce_0.9Ln_0.1O_1.95) and metallic (met, Ni) phases, without the need of an activation stage prior to the catalytic reaction. The catalysts have been characterized by different techniques: X-ray diffraction, N₂ adsorption-desorption, Hg porosimetry, Scanning Electron Microscopy, Temperature Programmed Reduction, H₂ and O₂ pulse chemisorption, X-ray photoelectron spectroscopy and Raman spectroscopy. With the exception of the lanthanum-loaded catalyst, the catalysts are highly active, selective and stable; being the one doped with gadolinium the most efficient. Correlations structure-activity point out that the excellent catalytic performance is related to the high catalytic surface area per unit mass of catalyst and to an appropriate balance of nickel dispersion to oxygen vacancies of the support.     

Microstructures and electrochemical characteristics of the La0.75Mg0.25Ni2.5Mx (M=Ni, Co; x=0–1.0) hydrogen storage alloys

In order to investigate the influences of the stoichiometric ratios of B/A (A: gross A-site elements, B: gross B-site elements) and the substitution of Co for Ni on the structure and the electrochemical performances of the _AB2.5–3.5${\mathrm{AB}}_{2.5–3.5}$-type electrode alloys, the La–Mg–Ni–Co system _{La0.75Mg0.25Ni2.5Mx}${\mathrm{La}}_{0.75}{\mathrm{Mg}}_{0.25}{\mathrm{Ni}}_{2.5}{\mathrm{M}}_x$ (M=Ni$\mathrm{M}=\mathrm{Ni}$, Co; x=0$x=0$, 0.2, 0.4, 0.6, 0.8, 1.0) alloys were prepared by induction melting in a helium atmosphere. The structures and electrochemical performances of the alloys were systemically measured. The obtained results show that the structures and electrochemical performances of the alloys are closely relevant to the M content. All the alloys exhibit a multiphase structure, including _LaNi2${\mathrm{LaNi}}_2$, (La,Mg)_Ni3$(\mathrm{La},\mathrm{Mg}){\mathrm{Ni}}_3$ and _LaNi5${\mathrm{LaNi}}_5$ phases, and the major phase in the alloys changes from _LaNi2${\mathrm{LaNi}}_2$ to (La,Mg)_Ni3+_LaNi5$(\mathrm{La},\mathrm{Mg}){\mathrm{Ni}}_3+{\mathrm{LaNi}}_5$ with the variety of M content. The electrochemical performances of the alloys, involving the discharge capacity, the high rate discharge (HRD) ability, the activation capability and the discharge potential characteristics, significantly improve with increasing M content. When M content x$x$ increases from 0 to 1.0, the discharge capacity rises from 177.7 to 343.62  mAh/g for the alloy (M=Ni)$(\mathrm{M}=\mathrm{Ni})$, and from 177.7 to 388.7 mAh/g for the alloy (M=Co)$(\mathrm{M}=\mathrm{Co})$. The cycle stability of the alloy first mounts up then declines with growing M content. The substitution of Co for Ni significantly ameliorates the electrochemical performances. For a fixed M content (x=1.0)$(x=1.0)$, the substitution of Co for Ni enhances the discharge capacity from 343.62 to 388.7 mAh/g, and the capacity retention ratio (_S100)$(S_{100})$ after 100 charging–discharging cycles from 51.45% to 61.1%.     

Characterization of perovskite-type cathode,La0.75Sr0.25 Mn0.95−xCox Ni0.05O3+δ (0.1 ≤ x ≤ 0.3), for intermediate-temperature solid oxide fuel cells

Phase evolution, structure, thermal property, morphology, electrical property and reactivity of a perovskite-type cathode system, La_0.75Sr_0.25 Mn_0.95−xCo_xNi_0.05O_3+δ (0.1 ≤ x ≤ 0.3), are reported. The samples are synthesized using metal acetates by the Pechini method. A perovskite-type phase is formed after calcination at ∼700 °C and a rhombohedral symmetry of R – 3c space group is stabilized at ∼1100 °C. An increase in x decreases the unit cell volume linearly, accompanying with a linear decrease in bond lengths and tilt angle. The differential thermal analysis suggests the phase stabilization for a temperature range, 50–1100 °C. The thermo-gravimetric, thermal expansion, and electrical and ionic conductivities studies suggest presence of a Jahn–Teller transition at ∼260–290 °C. The samples with x = 0.1 mol exhibit electrical conductivity of ∼55 S cm⁻¹ at ∼600 °C, activation energy of ∼0.13 eV, coefficient of thermal expansion of ∼12 × 10⁶ °C⁻¹, crystallite size of ∼45 nm, Brunauer–Emmett–Teller (BET) surface area of 1.26 m² g⁻¹ and average particle size of ∼0.9 μm. A fairly high ionic conductivity, 5–9 × 10⁻² S cm⁻¹ makes the sample with x = 0.1 mole suitable for intermediate-temperature solid oxide fuel cell cathode applications. The experimental results are discussed with the help of the defect models proposed for La_1−xSr_xMnO_3+δ.