Nd2−xGdxZr2O7 electrolytes: Thermal expansion and effect of temperature and dopant concentration on ionic conductivity of oxygen

The effect of temperature and complex dopant composition on oxygen ion conductivity in solid oxide electrolyte fuel cells was investigated by atomistic molecular dynamics simulation. A new electrolyte (Nd_2−xGd_xZr₂O₇) was selected to study oxygen ion conductivity using three Gd compositions (x = 0.8, 1.0, and 1.2) in a wide range of temperature (T = 1273 K–1873 K). MSD results of cations showed these groups of electrolyte are stable at high operating temperature. The first composition (x = 0.8) had the highest ionic conductivity that was in good agreement with the experimental data. A simple effective model that works with configuration energy of the oxygen crossing plate was applied to explain the observed conductivity trend. The model illustrated the point as well. Increasing Gd concentration decreases existence probability of easy crossing plate. Radial distribution function analysis also confirmed results. Thermal expansion of the electrolyte has a major effect on the selecting of the electrolyte materials; thus, this important factor was also studied. Results showed the first composition had the greatest thermal expansion.     

Sm0.2(Ce1−xTix)0.8O1.9 modified Ni-yttria-stabilized zirconia anode for direct methane fuel cell

Sm_0.2(Ce_1−xTi_x)_0.8O_1.9 (SCTx, x = 0-0.29) modified Ni-yttria-stabilized zirconia (YSZ) has been fabricated and evaluated as anode in solid oxide fuel cells for direct utilization of methane fuel. It has been found that both the amount of Ti-doping and the SCTx loading level in the anode have substantial effect on the electrochemical activity for methane oxidation. Optimal anode performance for methane oxidation has been obtained for Sm_0.2(Ce_0.83Ti_0.17)_0.8O_1.9 (SCT0.17) modified Ni-YSZ anode with SCT0.17 loading of about 241 mg cm⁻² resulted from four repeated impregnation cycles. When operating on humidified methane as fuel and ambient air as oxidant at 700 °C, single cells with the configuration of SCT0.17 modified Ni-YSZ anode, YSZ electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O₃-Sm_0.2Ce_0.8O_1.9 (LSCF-SDC) composite cathode show the polarization cell resistance of 0.63 Ω cm² under open circuit conditions and produce a peak power density of 383 mW cm⁻². It has been revealed that the coated Ti-doped SDC on Ni-YSZ anode not only effectively prevents the methane fuel from directly impacting on the Ni particles, but also enhances the kinetics of methane oxidation due to an improved oxygen storage capacity (OSC) and redox equilibrium of the anode surface, resulting in significant enhancement of the SCTx modified Ni-YSZ anode for direct methane oxidation.     

Preparation and characterization of Ce1−x(Gd0.5Pr0.5)xO2 electrolyte for IT-SOFCs

The Ce_1−x(Gd_0.5Pr_0.5)_xO₂ (x = 0–0.24) compositions were synthesized through the sol–gel process followed by low temperature combustion. X-ray diffraction data analysis showed that all the samples exhibit a cubic structure with single phase. The lattice parameter was calculated by rietveld refinement of XRD patterns. Dense ceramics were prepared by sintering the pellets at 1300 °C. The relative density of the samples was over 98%. The surface morphology was studied by Scanning electron microscopy (SEM). Chemical composition was analyzed by Energy dispersive spectroscopy (EDX). A.C. impedance spectroscopy measurements were carried out to study the grain, grain boundary and total ionic conductivity of co-doped ceria samples in the temperature range 150–700 °C. The Ce_0.84(Gd_0.5Pr_0.5)_0.16O₂ composition showed highest grain ionic conductivity i.e., 1.059 × 10⁻² S/cm at 500 °C which is 11.5% higher than the Ce_0.9Gd_0.1O₂ (with an activation energy 0.62 eV). At intermediate temperatures, the Ce_1−x(Gd_0.5Pr_0.5)_xO₂ materials were found to be ionic in nature.     

Intermediate temperature ionic conduction in Sn1−xGaxP2O7

A novel series of samples Sn_1−xGa_xP₂O₇ (x = 0.00, 0.01, 0.03, 0.06, 0.09, 0.12, 0.15) are synthesized by solid state reaction. XRD patterns indicate that the samples of x = 0.00 − 0.09 exhibit a single cubic phase structure, and the doping limit of Ga³⁺ in Sn_1−xGa_xP₂O₇ is x = 0.09. The protonic and oxide-ionic conduction in Sn_1−xGa_xP₂O₇ are investigated using some electrochemical methods at intermediate temperatures (323-523 K). It is found that the samples exhibit appreciable protonic conduction in hydrogen atmosphere, and a mixed conduction of oxide-ion and electron hole in dry oxygen-containing atmosphere. The highest conductivities are observed for the sample of x = 0.09 to be 4.6 × 10⁻² S cm⁻¹ in wet H₂ and 2.9 × 10⁻² S cm⁻¹ in dry air at 448 K, respectively. The H₂/air fuel cell using x = 0.09 as electrolyte (thickness: 1.45 mm) generates a maximum power density of 19.2 mW cm⁻² at 423 K and 22.1 mW cm⁻² at 448 K, respectively.     

Structural and conductivity study of the proton conductor BaCe(0.9−x)ZrxY0.1O(3−δ) at intermediate temperatures

The perovskite BaCe_(0.9−x)Zr_xY_0.1O_(3−δ) is prepared by solid-state reaction at 1400 °C and sintering at 1700 °C. It is characterised using X-ray diffraction, Raman spectroscopy and electrical measurements. A distortion from the cubic structure at room temperature is noticeable in the Raman spectra for 0.2 < x < 0.8, but not in the X-ray diffraction patterns. This work points out the rhombohedral nature of this distortion. Phase transitions are studied up to 600 °C. The direct current conductivity is measured as a function of oxygen partial pressure, and at a water vapour partial pressure of 0.015 atm. The total conductivity is resolved into an ionic and a p-type component using a fitting procedure appropriate to the assumed defect model. The first contribution is useful for estimating the proton transport number, while the value of the second one should not be too high not to deteriorate the electrodes performance.     

Reducibility of Ce1−xGdxO2−δ in prospective working conditions

Nanocrystalline powders of CGO materials with different contents of Gd were prepared by a freeze-drying method and used to prepare dense ceramic samples. This method allowed one to obtain good quality samples to re-examine the reducibility of CGO materials. These materials were characterized by a combination of coulometric titration, impedance spectroscopy and ion blocking measurements to evaluate changes in point defect chemistry and mixed conducting properties. We have examined the onset of n-type conductivity as a function of temperature and oxygen partial pressure, and this information was used to re-examine the mixed transport properties in reducing conditions imposed by fuels, and also the OCV obtained with CGO solid electrolyte cells under air/CGO/fuel gradients. The polaron mobility was also evaluated and was found to depend slightly on temperature and to decrease with increasing oxygen deficiency. This confirmed that the electronic conductivity is mainly dependent on reducibility.     

Ionic conduction in Sn1−xScxP2O7 for intermediate temperature fuel cells

A novel series of mixed ion conductors, Sn_1−xSc_xP₂O₇ (x = 0.03, 0.06, 0.09, 0.12), were synthesized by a solid-state reaction method. The conduction behaviors of the ion conductors in wet hydrogen atmosphere were investigated by some electrochemical methods including AC impedance spectroscopy, gas concentration cells in the temperature range of 323-523 K. It was found that the doping limit of Sc³⁺ in SnP₂O₇ was between 9 mol% and 12 mol%. The highest conductivity was observed to be 2.76 × 10⁻² S cm⁻¹ for the sample of x = 0.06 under wet H₂ atmosphere at 473 K. The ionic conduction was contributed mainly to proton and partially to oxide ion in wet hydrogen atmosphere from 373 K to 523 K. The H₂/air fuel cells using Sn_1−xSc_xP₂O₇ (x = 0.03, 0.06, 0.09) as electrolytes (1.7 mm in thickness) generated the maximum power densities of 11.16 mW cm⁻² for x = 0.03, 25.02 mW cm⁻² for x = 0.06 and 14.34 mW cm⁻² for x = 0.09 at 423 K, respectively. The results indicated that Sn_1−xSc_xP₂O₇ is a promising solid electrolyte system for intermediate temperature fuel cells.     

High lithium ionic conductivity in the garnet-type oxide Li7−X La3(Zr2−X, NbX)O12 (X = 0-2)

Lithium garnet-type oxides Li_7−XLa₃(Zr_2−X, Nb_X)O₁₂ (X = 0-2) were synthesized by a solid-state reaction, and their lithium ion conductivity was measured using an AC impedance method at temperatures ranging from 25 to 150 °C in air. The lithium ion conductivity increased with increasing Nb content, and reached a maximum of ∼0.8 mS cm⁻¹ at 25 °C. By contrast, the activation energy reached a minimum of ∼30 kJ mol⁻¹ at the same point with X = 0.25. The potential window was examined by cyclic voltammetry (CV), which showed lithium deposition and dissolution peaks around 0 V vs. Li⁺/Li, but showed no evidence of other reactions up to 9 V vs. Li⁺/Li.     

Single-phase ceramics with La1−xSrxGa1−yMgyO3−δ composition from precursors obtained by mechanosynthesis

Dense ceramics with La_0.80Sr_0.20Ga_0.85Mg_0.15O_2.825 and La_0.80Sr_0.15Ga_0.85Mg_0.20O_2.825 compositions have been prepared by sintering of mechanosynthesized precursors. The perovskite is synthesized after 85 h of milling in a planetary mill. Single phases have been obtained at conditions that are not possible if traditional solid-state reaction (SSR) method is used. The influence of milling time and composition in the reactivity of the precursors is studied. Highest purity is obtained in Sr = 0.15 and Mg = 0.20 composition, with relative density higher than 97%. The total elimination of typical secondary phases for these compositions, as SrLaGaO₄ and SrLaGa₃O₇, allows the total conductivity of the ceramics to be improved. The influence of the grain size and the nature of the grain boundaries on the electrical characteristic of the ceramics are also discussed.     

Minimization of the cation mixing in Li1+x(NMC)1−xO2 as cathode material

Li_1+x(Ni_1/3Mn_1/3Co_1/3)_1−xO₂ layered materials were synthesized by the co-precipitation method with different Li/M molar ratios (M = Ni + Mn + Co). Elemental titration evaluated by inductively coupled plasma spectrometry (ICP), structural properties studied by X-ray diffraction (XRD), Rietveld analysis of XRD data, scanning electron microscopy (SEM) and magnetic measurements carried out by superconducting quantum interference devices (SQUID) showed the well-defined α-NaFeO₂ structure with cationic distribution close to the nominal formula. The Li/Ni cation mixing on the 3b Wyckoff site of the interlayer space was consistent with the structural model [Li_1−yNi_y]_3b[Li_x+yNi_{(1−x)/3−y}Mn_(1−x)/3Co_(1−x)/3]_3aO₂ (x = 0.02, 0.04) and was very small. Both Rietveld refinements and magnetic measurements revealed a concentration of Ni²⁺-3b ions lower than 2%; moreover, for the optimized sample synthesized at Li/M = 1.10, only 1.43% of nickel ions were located into the Li sublattice. Electrochemical properties were investigated by galvanostatic charge-discharge cycling. Data obtained with Li_1+x(Ni_1/3Mn_1/3Co_1/3)_1−xO₂ reflected the high degree of sample optimization. An initial discharge capacity of 150 mAh g⁻¹ was delivered at 1 C-rate in the cut-off voltage of 3.0-4.3 V. More than 95% of its initial capacity was retained after 30 cycles at 1 C-rate. Finally, it is demonstrated that a cation mixing below 2% is considered as the threshold for which the electrochemical performance does not change for Li_1+x(Ni_1/3Mn_1/3Co_1/3)_1−xO₂.     

Zeolite-templated IrxRu1−xO2 electrocatalysts for oxygen evolution reaction in solid polymer electrolyte water electrolyzers

Ir_xRu_1−xO₂(1 ≥ x ≥ 0) with nanorod structure were successfully synthesized by employed pre-filling the Ir and/or Ru guest species into the peripheral-pore of NH₂-modified as-synthesized SBA-15 and explored as electrocatalyst for oxygen evolution reaction (OER) in water electrolyzers. Various physicochemical parameters for zeolite template and/or Ir_xRu_1−xO₂ were obtained by SEM, TEM, XRD, EDX and N₂ gas absorption/desorption measurements. The morphology for prepared Ir_xRu_1−xO₂ samples with individual and/or cluster nanorods was changed with the component difference. Cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy, steady state polarization curves and stability tests were performed to investigate the catalytic activity and stability of these electrocatalysts for OER. The cell with catalyst RuO₂ showed best catalytic performance with the lowest onset potential (1.374 V at 10 mA cm⁻²), which may be ascribed to regular nanoclusters and larger outer active surface area. Meanwhile, the cell stability tests suggested that the addition of IrO₂ in Ir_xRu_1−xO₂ improved the stability of the RuO₂ catalyst.     

Bulk conduction and relaxation in [(ZrO2)1−x(CeO2)x]0.92(Y2O3)0.08 (0 ≤ x ≤ 1) solid solutions at intermediate temperatures

Bulk conduction and relaxation of the [(ZrO₂)_1−x(CeO₂)_x]_0.92(Y₂O₃)_0.08 (0 ≤ x ≤ 1) solid solutions were studied using impedance spectroscopy at intermediate temperatures (200-500 °C). The bulk conductivity as a function of x shows a “V-shape” variation which is a competitive effect of the defect associates and the lattice parameter. In the ZrO₂-rich region (x < 0.5) CeO₂ doping increases the concentration of defect associates which limits the mobility of the oxygen vacancies; in the CeO₂-rich region (x > 0.5) the increase of x increases the lattice parameter which enlarges the free channel for oxygen vacancy migration. Further analysis indicates the ionic radius of the tetravalent dopant determines the composition dependence of the ionic conductivity of the solid solutions. When doping YSZ with other tetravalent dopant with similar ionic radius with Zr⁴⁺, e.g., Hf⁴⁺, such “V-shape” composition dependence of the bulk conductivity cannot be observed.     

Influence of CaO on structure and electrical conductivity of pyrochlore-type Sm2Zr2O7

(Sm_1−xCa_x)₂Zr₂O_7−x (0 ≤ x ≤ 0.100) ceramics were prepared by a solid state reaction process at 1973 K for 10 h in air, and were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). (Sm_1−xCa_x)₂Zr₂O_7−x (0 ≤ x ≤ 0.025) ceramics have a single phase of pyrochlore-type structure; however (Sm_1−xCa_x)₂Zr₂O_7−x (0.050 ≤ x ≤ 0.100) consist of pyrochlore phase and a small amount of perovskite-like CaZrO₃. The electrical conductivity of (Sm_1−xCa_x)₂Zr₂O_7−x ceramics was investigated by complex impedance spectroscopy over a frequency range of 0.1 Hz to 20 MHz in the temperature range of 573–873 K. The measured electrical conductivity obeys the Arrhenius relation. Both the activation energy and pre-exponential factor for grain conductivity increase with increasing the CaO content; however, electrical conductivity of (Sm_1−xCa_x)₂Zr₂O_7−x decreases with increasing the CaO content, which is due to the increase in structural disordering at 0 ≤ x ≤ 0.025 and the presence of the poorly conducting CaZrO₃ phase at 0.050 ≤ x ≤ 0.100, respectively.     

Structural and electrical properties of (Ba1−xSrx)(Zr0.9M0.1)O3, M = Y, La, solid solutions

The aim of the work was to study the structural and electrical properties of the (Ba_1−xSr_x)(Zr_0.9Y_0.1)O₃ and (Ba_1−xSr_x)(Zr_0.9Y_0.1)O₃ solid solutions. The powders of different strontium content (x = 0, 0.03, 0.05 and 0.1) were prepared by a thermal decomposition of organo-metallic precursors containing ethylenediaminetetraacetate acid. Some parameters describing stability and transport properties of the perovskite structure, such as tolerance factor, specific free volume and global instability index, were calculated. It was found that the introduction of strontium into both solid solutions caused the increase of specific free volume and global instability index—these structures became a little less stable but, on the other hand, better ionic conductor. All samples were cubic perovskite and the substitution of strontium for barium caused the decrease of respective lattice parameters. Electrical conductivity measurements were performed by the d.c. four-probe method in controlled gas atmospheres containing Ar, air, H₂ and/or H₂O at the temperature from 300 to 800 °C. It was found that the conductivity depended on a chemical composition of the samples and the atmosphere. In general, the electrical conductivity was higher in wet atmospheres which contained oxygen, being in accordance with the model of a proton transport in the perovskite structure which assumed the presence of the oxygen vacancy. The solid solution containing 5 mol.% of strontium showed the highest conductivity and the lowest activation energy of conductivity regardless of the atmospheres.     

Stability and performance of infiltrated La0.8Sr0.2CoxFe1−xO3 electrodes with and without Sm0.2Ce0.8O1.9 interlayers

The chemical stability of composite electrodes produced by the infiltration of La_0.8Sr_0.2Co_xFe_1−xO₃ (LSCF) into a porous yttria-stabilized zirconia (YSZ) scaffold were investigated as a function of the Co:Fe ratio in the LSCF and the LSCF calcination temperature. XRD and impedance spectroscopy results indicate that for an LSCF calcination temperature of 1123 K, reactions between the LSCF and YSZ do not occur to a significant extent. Reactions producing La₂Zr₂O₇ and SrZrO₃ at the interface were observed, however, for a calcination temperature of 1373 K and x values greater than 0.2. In addition to determining the conditions for which reactions between LSCF and YSZ occur, the effectiveness of infiltrated SDC interlayers in preventing reactions at the LSCF-YSZ interface and their influence on the overall performance of LSCF/YSZ composite electrodes was studied.     

Synthesis and performance of Li3(V1−xMgx)2(PO4)3 cathode materials

In order to search for cathode materials with better performance, Li₃(V_1−xMg_x)₂(PO₄)₃ (0, 0.04, 0.07, 0.10 and 0.13) is prepared via a carbothermal reduction (CTR) process with LiOH·H₂O, V₂O₅, Mg(CH₃COO)₂·4H₂O, NH₄H₂PO₄, and sucrose as raw materials and investigated by X-ray diffraction (XRD), scanning electron microscopic (SEM) and electrochemical impedance spectrum (EIS). XRD shows that Li₃(V_1−xMg_x)₂(PO₄)₃ (x = 0.04, 0.07, 0.10 and 0.13) has the same monoclinic structure as undoped Li₃V₂(PO₄)₃ while the particle size of Li₃(V_1−xMg_x)₂(PO₄)₃ is smaller than that of Li₃V₂(PO₄)₃ according to SEM images. EIS reveals that the charge transfer resistance of as-prepared materials is reduced and its reversibility is enhanced proved by the cyclic votammograms. The Mg²⁺-doped Li₃V₂(PO₄)₃ has a better high rate discharge performance. At a discharge rate of 20 C, the discharge capacity of Li₃(V_0.9Mg_0.1)₂(PO₄)₃ is 107 mAh g⁻¹ and the capacity retention is 98% after 80 cycles. Li₃(V_0.9Mg_0.1)₂(PO₄)₃//graphite full cells (085580-type) have good discharge performance and the modified cathode material has very good compatibility with graphite.     

Hydrogen absorption properties of Zr(V1−xFex)2 intermetallic compounds

The Zr(V_1−xFe_x)₂ (x = 0.02, 0.05, 0.10, 0.15, 0.25) alloys were prepared by the arc-melt method and annealed at 1273 K for 168 h in an argon atmosphere. Phase structure investigations of the as-cast and annealed Zr(V_1−xFe_x)₂ alloys indicate the annealing treatment can eliminate the minority phases originating from the non-equilibrium solidification of as-cast alloys. The ZrV₂-type phase becomes the dominant one in each annealed alloy. The substitution of Fe in V sites leads to the contraction of their lattice. For annealed Zr(V_1−xFe_x)₂ alloys, the P–t and PCT curves obtained between 673 K and 823 K give the evidence that the absorption process is controlled by a rate-controlling hydrogen diffusion. With the increase of iron, the equilibrium pressure and the plateau slope increase while the hydrogenation capacity and the absolute value of enthalpy and entropy decrease accordingly. The stability of metal hydride reduces gradually as the Fe content varies from x = 0.02 to 0.25 which promotes the hydrogen release and favors the practical applications of the Zr(V_1−xFe_x)₂ alloys.     

Physical and electrochemical properties of spherical Li1+x(Ni1/3Co1/3Mn1/3)1−xO2 cathode materials

A (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor with an uniform, spherical morphology was prepared by coprecipitation using a continuously stirred tank reactor method. The as-prepared spherical (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor served to produce dense, spherical Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ (0 ≤ x ≤ 0.15) cathode materials. These Li-rich cathodes were also prepared by a second synthesis route that involved the use of an M₃O₄ (M = Ni_1/3Co_1/3Mn_1/3) spinel compound, itself obtained from the carbonate (Ni_1/3Co_1/3Mn_1/3)CO₃ precursor. In both cases, the final Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ products were highly uniform, having a narrow particle size distribution (10-μm average particle size) as a result of the homogeneity and spherical morphology of the starting mixed-metal carbonate precursor. The rate capability of the Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ electrode materials, which was significantly improved with increased lithium content, was found to be better in the case of the denser materials made from the spinel precursor compound. This result suggests that spherical morphology, high density, and increased lithium content were key factors in enabling the high rate capabilities, and hence the power performances, of the Li-rich Li_1+x(Ni_1/3Co_1/3Mn_1/3)_1−xO₂ cathodes.     

Alloying effects on the hydride formation of Zr(Mn1−xCox)2

To study the alloying effects on ZrMn₂-H system, thermodynamic properties of Zr(Mn_1−xCo_x)₂ hydride were measured by volumetric method. ZrMn₂ gave a single plateau region in the pressure-composition isotherm. On the other hand, double plateaus were clearly observed in Zr(Mn_0.7Co_0.3)₂ and Zr(Mn_0.6Co_0.4)₂-H systems. The appearance of the double plateau characteristics would be explained in view of the hydrogen binding in the tetrahedral occupation sites in Zr(Mn_1−xCo_x)₂. Since the hydrogen binding in the tetrahedral 2ZrMnCo site would be less stable than that in the 2Zr2Mn site, the equilibrium pressure increases with increasing cobalt content. The appearance of the first plateau was ascribed the increase in the bonding of Mn-H in 2Zr2Mn site adjoining the 2ZrMnCo site.     

Determination of Cu(In1−xGax)3Se5 defect phase in MBE grown Cu(In1−xGax)Se2 thin film by Rietveld analysis

Quantitative phase analysis of Cu(In_1−xGa_x)Se₂ (CIGS) thin film grown over Mo coated soda lime glass substrates was studied by Rietveld refinement process using room temperature X-ray data at θ-2θ mode. Films were found to contain both stoichiometric Cu(In_1−xGa_x)Se₂ and defect related Cu(In_1−xGa_x)₃Se₅ phases. Best fitting was obtained using crystal structure with space group I-42d for Cu(In_1−xGa_x)Se₂ and I-42m for Cu(In_1−xGa_x)₃Se₅ phase. The effects of Ga/III (=Ga/In+Ga=x) ratio and Se flux during growth over the formation of Cu(In_1−xGa_x)₃Se₅ defect phase in CIGS was studied and the correlation between quantity of Cu(In_1−xGa_x)₃Se₅ phase and solar cell performance is discussed.