Two-step sintering ceria-based electrolytes

Yttrium and gadolinium-doped ceria-based electrolytes (20 at% dopant cation) with and without small Ga₂O₃-additions (0.5 mol%) were fired at peak temperatures of 1250 and 1300 °C, or following a two-step sintering profile including one peak temperature and subsequent dwell at 1150 °C, 10 h. All materials were characterized by scanning electron microscopy, X-ray diffraction and impedance spectroscopy in air, in the temperature range 200–800 °C. Average grain sizes in the range 150–250 nm and densifications up to about 94% were found dependent on the sintering profile and presence of Ga. The grain boundary arcs in the impedance spectra increased significantly with Ga-doping, cancelling the apparently positive role of Ga on bulk transport, evidenced mostly in the case of yttrium-doped materials.     

Role of gas-phase composition on the performance of ceria-based composite electrolytes

Ceria-based composites (ranging from 30 to 70 vol%) including a mixture of Li and Na carbonates (1:2 M ratio, respectively) were sequentially exposed to pure CO₂, O₂ and a mixture of 10 vol% H₂ diluted in N₂, at temperatures ranging from 300 to 580 °C, to study the role of composite/gas interaction on electrical performance. Impedance spectroscopy measurements performed under these working conditions were complemented by microstructural and structural characterization. Changes in conductivity and electrode impedance when changing the atmosphere from CO₂ to O₂ and to H₂ confirmed changes in concentration and/or in mobility of charge carriers and electroactive species. Novel species identified either by X-ray diffraction (hydrated carbonates, only present at the surface of the samples), or by infrared spectroscopy (hydrogen carbonate ions), might be related to the observed conductivity enhancement under H₂.     

Electrical properties of Mg-doped Gd0.1Ce0.9O1.95 under different sintering conditions

Ce_0.9Gd_0.1O_1.95 with various Mg doping contents was synthesized by citric acid-nitrate low temperature combustion process and sintered under different conditions. The crystal structures, microstructures and electrical properties were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and ac impedance spectroscopy. Low solubility of Mg²⁺ in Ce_0.9Gd_0.1O_1.95 lattice was evidenced by XRD and FESEM micrographs. The samples sintered at 1300 °C exhibited the higher total conductivity than those sintered at 1100 and 1500 °C, with the maximum value of 1.48 × 10⁻² S cm⁻¹ (measured at 600 °C) at the Mg doping content of 6 mol%, corresponding to the minimum total activation energy (E_tol) of 0.84 eV (150–400 °C). The effect of Mg doping on the electrical conductivity was significant particularly at higher sintering temperatures. At the sintering temperature of 1500 °C, the addition of Mg (10 mol%) enhanced the grain boundary conductivity by over 10² times comparing with that of undoped Ce_0.9Gd_0.1O_1.95, which may be explained by the optimization of space charge layer due to the segregation of Mg²⁺ to the grain boundaries.     

Electrical properties of ceria Co-doped with Sm and Nd

Ceria co-doped with Sm³⁺ and Nd³⁺ powders are successfully synthesized by citric acid–nitrate low-temperature combustion process. In order to optimize the electrical properties of the series of ceria co-doped with Sm³⁺ and Nd³⁺, the effects of co-doping, doping content and sintering conditions on grain and grain boundary conductivity are investigated in detail. For the series of Ce_0.9(Sm_xNd_1−x)_0.1O_1.95 (x = 0, 0.5, 1) and Ce_1−x(Sm_0.5Nd_0.5)_xO_δ (x = 0.05, 0.10, 0.15, 0.20) sintered under the same condition, Ce_0.9(Sm_0.5Nd_0.5)_0.1O_1.95 exhibits both higher grain and grain boundary conductivity. Compared with Ce_0.9Gd_0.1O_1.95 and Ce_0.8Sm_0.2O_1.9, Ce_0.9(Sm_0.5Nd_0.5)_0.1O_1.95 sintered at 1350–1400 °C shows higher total conductivity with the value of 1.0 × 10⁻² S cm⁻¹ at 550 °C. In addition, it can be found the trends of grain and grain boundary activation energies of Ce_1−x(Sm_0.5Nd_0.5)_xO_δ are both consistent with those of Ce_1−xNd_xO_δ, but different from those of Ce_1−xSm_xO_δ, which can be explained as: the local ordering of oxygen vacancies maybe occurs more easily in Nd-doped ceria than in Sm-doped ceria; the segregation amount of Sm³⁺ is more than that of Nd³⁺ to the grain boundaries in ceria co-doped with Sm³⁺ and Nd³⁺, which is confirmed by X-ray photoelectron spectroscopy (XPS).     

Electrical conductivity and related properties of molten carbonates coexisting with ceria-based oxide powder for hybrid electrolyte

The electrical, thermal and structural properties of composite electrolyte containing Ce_0.9Gd_0.1O_1.95 (GDC) powder and (Li_0.52Na_0.48)₂CO₃ eutectics are investigated by AC impedance, differential thermal analysis and polarized Raman scattering spectroscopy. The system shows a dependence of the electrical conductivity upon the temperature. The transition point varies with the apparent average thickness of the liquid phase, while the activation energy, ΔE_a, remains constant at any distance from the solid phase. Higher electrical conductivity was obtained for the GDC/(Li_0.52Na_0.48)₂CO₃ composite than that for α-Al₂O₃/(Li_0.52Na_0.48)₂CO₃. Even in the N₂ or Air gas flow, the weight loss caused by decomposition of CO₃²⁻ ion based on Lux–Flood equilibrium was rarely observed. The symmetric stretching mode of the polarized Raman spectra shows that carbonate ion maintains its D_3h symmetry in the presence of ceria. A constant value of the depolarization ratio of the ν₁(A′₁) mode with regard to the apparent average thickness confirms that the symmetry of carbonate ions in the molten state is not altered by the presence of ceria powder. It was more stable than that for the system containing α-Al₂O₃ as a reference sample. These findings contribute to the understanding of the properties of ceria-based carbonate electrolyte for intermediate temperature solid oxide fuel cells.     

Effect of nano-grain size on the ionic conductivity of spark plasma sintered 8YSZ electrolyte

Densification and micro-structural development of ultra fine 8 mol% yttria stabilized zirconia (8YSZ) nano powder were investigated systematically by varying the SPS sintering temperature at constant applied pressure of 50 MPa. A hundred fold decrease in average grain size ranging from 10 μm to 80 nm is observed on decreasing the SPS sintering temperature from 1200 °C to 1050 °C with >99% of theoretical densities. Impedance measurements on the samples indicated an enhancement in the ionic conductivity at 700 °C from 0.004 S/cm to 0.018 S/cm with decrease in grain size from 10 μm to 0.51 μm and a significant increase in conductivity from 0.018 S/cm to 0.068 S/cm on further reduction of grain size to 80 nm. A significant change in the grain-boundary conductivity is noticed on reducing the grain sizes to nano regime. The diverse microstructure with ultra fine grain size resulting from SPS at 1050 °C could contribute to the enhanced ionic conductivity, which is supported by the activation energy data.     

Thermodynamic predictions of the impact of fuel composition on the propensity of sulphur to interact with Ni and ceria-based anodes for solid oxide fuel cells

Thermodynamic calculations have been made to predict the stability of solid oxide fuel cell (SOFC) anode materials when exposed to hydrogen sulphide (H₂S) in hydrogen (H₂) over a range of partial pressures of sulphur (pS₂) and oxygen (pO₂) representative of fuel cell operating conditions. The study focussed on the behaviour of nickel and ceria, which form the basis of nickel–gadolinium-doped ceria (Ni-CGO) anodes, often used as an active layer within SOFCs. The reaction of Ni with sulphur is predicted to become more favourable as temperature and hydrogen partial pressure (pH₂) decrease. Ceria is shown to become increasingly non-stoichiometric (CeO_n, n < 2) as pO₂ decreases and temperature increases, and it is predicted that its reaction with sulphur becomes more favourable under these conditions.     

Synthesis and sintering of Gd-doped CeO2 electrolytes with and without 1 at.% CuO dopping for solid oxide fuel cell applications

Nano-sized Ce_0.8Gd_0.2O_2−δ and Ce_0.79Gd_0.2Cu_0.01O_2−δ electrolyte powders were synthesized by the polyvinyl alcohol assisted combustion method, and then characterized by powder characteristics, sintering behaviors and electrical properties. The results demonstrate that the as-synthesized Ce_0.8Gd_0.2O_2−δ and Ce_0.79Gd_0.2Cu_0.01O_2−δ possessed similar powder characteristics, including cubic fluorite crystalline structure, porous foamy morphology and agglomerated secondary particles composed of gas cavities and primary nano crystals. Nevertheless, after ball-milling these two powders exhibited quite different sintering abilities. A significant reduction of about 400 °C in densification temperature of Ce_0.79Gd_0.2Cu_0.01O_2−δ was obtained when compared with Ce_0.8Gd_0.2O_2−δ. The Ce_0.79Gd_0.2Cu_0.01O_2−δ pellets sintered at 1000 °C and the Ce_0.8Gd_0.2O_2−δ sintered at 1400 °C exhibited relative densities of 96.33% and 95.7%, respectively. The sintering of Ce_0.79Gd_0.2Cu_0.01O_2−δ was dominated by the liquid phase process, followed by the evaporation-condensation process, Moreover, Ce_0.79Gd_0.2Cu_0.01O_2−δ shows much higher conductivity of 0.026 S cm⁻¹ than Ce_0.8Gd_0.2O_2−δ (0.0065 S cm⁻¹) at a testing temperature of 600 °C.     

Effects of rapid process on the conductivity of multiple elements doped ceria-based electrolyte

The citric acid-based combustion technique (SV) for powder preparation and the rapid microwave sintering (MW) process are used to lower the synthesizing temperature and to shorten the processing time then to modify the grain boundary resistance and oxygen vacancies mobility in multiple elements doped ceria-based electrolyte (LSBC). Nanoparticles of less than 50 nm with a pure fluorite structure are prepared by SV method at a low temperature of 600 °C. Microwave sintering lowers the sintering temperature to 1400 °C from the conventional sintering (CS) temperature of 1500 °C needed for solid-state (SS) prepared LSBC, and requires only 15 min of sintering time. The SV sample conventionally sintered at 1400 °C-6 h reaches a conductivity of 0.006 S cm⁻¹. When the SV samples are microwave sintered at 1400 °C-15 min, they achieve a conductivity as high as 0.01 S cm⁻¹ measured at 600 °C. Microwave sintering reduces the grain boundary resistance of both SS and SV samples. The migration enthalpy (H_m) of 0.66 eV in the SS-MW and SV-MW samples is similar to that of the fully densified SS-CS sample. The Schottky barrier height can be adjusted by SV powder preparation and by the MW process using a slightly lower sintering temperature and with a shorter processing time for multiple elements doped solid electrolyte.     

Addition effects of erbia-stabilized bismuth oxide on ceria-based carbonate composite electrolytes for intermediate temperature-−solid oxide fuel cells

Highly conductive Er_0.2Bi_0.8O_1.5 (ESB) and rare-earth doped ceria solid oxide electrolytes (SOEs) at intermediate temperature (IT) continue to suffer disadvantages in terms of thermodynamic instability and significant electronic conduction, respectively, at low oxygen partial pressure for solid oxide fuel cell (SOFC) operations. It is therefore necessary to improve the low-temperature ionic conductivity in order to enhance the electrolytic domain of these materials and thereby mitigate cell efficiency dissipation by electronic conduction. In this work, an advanced multiphase carbonate composite material based on ceria has been developed to overcome this IT-SOE challenge. This advanced electrolyte is comprise of nanostructured neodymium-doped ceria (NDC) and 38 wt% (Li–0.5Na)₂CO₃ carbonate with a small amount of ESB phase. The addition of 2 wt% ESB in ceria-based materials decreases the grain boundary resistance of the SOEs in the IT range. Further, a small amount of highly conducting ESB phase in the NDC/[(Li–0.5Na)₂CO₃] composite electrolyte increases the overall conductivity of the composite SOEs. The NDC electrolyte containing 38 wt% carbonate shows the highest conductivity of 0.104 Scm⁻¹ at 600 °C, while the conductivity is increased to 0.165 Scm⁻¹ by the addition of 2 wt% ESB. In addition, the activation energy of the multiphase composite electrolytes (0.52 eV) is lower than that of the NDC/carbonates (0.65 eV) in the IT range. This is attributed to the effect of the physical properties of the NDC sample, induced by the light ESB doping, on the ionic conductivity, and this effect is closely associated with the grain boundary property. Furthermore, the interfacial effects of the multiphase materials also contribute to the improved conductivity of this advanced composite electrolyte.     

Synthesis of doped ceria-zirconia core-shell nanocomposites via sol-gel process

In order to introduce more conductive interfaces, the doped ceria-zirconia core-shell nanocomposites are synthesized via a simple and low-cost sol-gel process. Nitrates, citric acid and polyethylene glycol (PEG) are used as starting materials, and the compositions of the core and the shell are Ce_0.9Gd_0.1O_1.95 (GDC) and 8 mol% Sc₂O₃ doped ZrO₂ (ScDZ), respectively. The room-temperature ammonia co-precipitation method is used to prepare the core materials. X-ray diffraction (XRD) indicates that the average grain sizes of the core and shell materials are about 6 nm and 8 nm. The core-shell nanostructure with about 60 nm diameter GDC core (approximate) and about 20 nm thick ScDZ shell, is supported by the field-emission scanning electron microscopy (FESEM) and the transmission electron microscopy (TEM) results. The effects of PEG and the mechanisms during the process of forming the core-shell nanocomposites are discussed in detail.     

Comparative study of the nano-composite electrolytes based on samaria-doped ceria for low temperature solid oxide fuel cells (LT-SOFCs)

Ceria-based electrolyte materials have great potential in low and intermediate temperature solid oxide fuel cell applications. In the present study, three types of ceria-based nano-composite electrolytes (LNK-SDC, LN-SDC and NK-SDC) were synthesized. One-step co-precipitation method was adopted and different techniques were applied to characterize the obtained ceria-based nano-composite electrolyte materials. TGA, XRD and SEM were used to analyze the thermal effect, crystal structure and morphology of the materials. Cubic fluorite structures have been observed in all composite electrolytes. Furthermore, the crystallite sizes of the LN-SDC, NK-SDC, LNK-SDC were calculated by Scherrer formula and found to be in the range 20 nm, 21 nm and 19 nm, respectively. These values emphasize a good agreement with the SEM results. The ionic conductivities were measured using EIS (Electrochemical Impedance Spectroscopy) with two-probe method and the activation energies were also calculated using Arrhenius plot. The maximum power density was achieved 484 mW/cm² of LNK-SDC electrolyte at 570 °C using the LiCuZnNi oxide electrodes.     

Improved electrical conductivity of Ce0.9Gd0.1O1.95 and Ce0.9Sm0.1O1.95 by co-doping

Solid electrolytes are the most important and indispensable part of a solid oxide fuel cell (SOFC) where hydrogen is used as one of the fuels to obtain electricity. Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 were chosen to be the base electrolytes. The effects of MgO and Nd₂O₃ as co-dopants on the electrical conductivity were investigated, respectively. For 4 mol% Mg-doped Ce_0.9Gd_0.1O_1.95 or Ce_0.9Sm_0.1O_1.95, MgO phases were detected by FESEM micrographs, which showed a very low solubility of Mg²⁺ in ceria lattice. The existence of MgO phases was observed to have negligible effect on the grain conductivity, but improve the grain boundary conductivity measured by ac impedance spectroscopy. However, when Nd₂O₃ was used as a co-dopant, XRD patterns and FESEM both indicated a pure cubic phase. Ce_0.9Gd_0.05Nd_0.05O_1.95 and Ce_0.9Sm_0.05Nd_0.05O_1.95 were found to exhibit higher grain conductivity, comparing with single-doped ceria.     

Effect of powder morphology on the microstructural characteristics of La0.6Sr0.4Co0.2Fe0.8O3 cathode: A Kinetic Monte Carlo investigation

Microstructural parameters such as triple phase boundary (TPB) density, surface area density, connectivity and tortuosity of different phases strongly influence the performance of solid oxide fuel cells (SOFCs). In this study, the effect of the powder morphology on the microstructural parameters of a La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) cathode is comprehensively examined using Kinetic Monte Carlo (KMC) simulations. A number of numerical samples consisting of spheres or clumped spheres are created using a Discrete Element Method (DEM), taking into account the powder morphology such as particle size, particle size distribution, particle aspect ratio and sphericity, and particle orientation. The DEM-generated numerical structures with different particle morphologies are submitted to the KMC simulations. Their effects on relative density, densification rate, surface area density, tortuosity factor of LSCF phase and tortuosity factor and connectivity of the pore phase are compared and analyzed.     

Synthesis and characterization of lanthanum silicate apatite by gel-casting route as electrolytes for solid oxide fuel cells

Lanthanum silicate oxyapatite, La₁₀Si₆O₂₇ is successfully synthesized by a water-based gel-casting technique. The effect of calcination and sintering temperatures on the conductivity is investigated in detail in the temperature range between 300 and 800 °C by the impedance spectroscopy. The highest oxygen ion conductivity is 1.50 × 10⁻³ S cm⁻¹ at 500 °C and 3.46 × 10⁻² S cm⁻¹ at 800 °C for an apatite electrolyte sintered at 1650 °C, which is one order of magnitude higher than that synthesized by the conventional solid state reaction route under the same sintering conditions. The thermal expansion coefficient (TEC) of the as-synthesized apatite is 9.7 × 10⁻⁶ K⁻¹. A solid oxide fuel cell using La₁₀Si₆O₂₇ as an electrolyte shows an open circuit potential of 1.06 V and power output of 7.89 mW cm⁻² at 800 °C. The results demonstrate the potential of the silicate oxyapatite materials synthesized by the gel-casting as an alternative electrolyte in solid oxide fuel cells.     

Sintering and ionic conductivity of 8YSZ and CGO10 electrolytes with small addition of Fe2O3: A comparative study

Two typical electrolytes, i.e., 8YSZ (8 mol% yttria-stabilized zirconia) and CGO10 (10 mol% Gd-doped ceria), with Si contents of ∼30 ppm and ≥500 ppm were prepared, whose grain-boundary (GB) conductivities should be controlled by intrinsic (space-charge layer) and extrinsic (resistive siliceous films) effects, respectively. 1 at% FeO_1.5 was loaded into these materials via a conventional mixed-oxide method. A comparative study was carried out to demonstrate how 1 at% Fe addition affected these materials with different levels of SiO₂ impurities with respect to sintering, GB and GI (grain interior) conductivities. FeO_1.5 was found to be a sintering promoter for both 8YSZ and CGO10 ceramics, but it is more effective to enhance the densification of ceria-based electrolytes. A reduction in sintering temperature of ∼200 °C for 1 at% Fe-doped CGO10 was achieved compared with ∼110 °C reduction for the 8YSZ with the same amount of Fe loading. The effect of FeO_1.5 loading on the electrical conduction was found to be different, depending significantly on the impurity level and the types of electrolytes. In general, the loading of FeO_1.5 is positive for ceria-based ceramics since FeO_1.5 has a scavenging effect on SiO₂ impurity with little effect on the GI conduction. Although the scavenging behavior of FeO_1.5 was also found in the impure 8YSZ, it led to a significant reduction in the GI conductivity.     

Palladium and ceria infiltrated La0.8Sr0.2Co0.5Fe0.5O3−δ cathodes of solid oxide fuel cells

La_0.8Sr_0.2Co_0.5Fe_0.5O_3−δ (LSCF) cathodes infiltrated with electrocatalytically active Pd and (Gd,Ce)O₂ (GDC) nanoparticles are investigated as high performance cathodes for the O₂ reduction reaction in intermediate temperature solid oxide fuel cells (IT-SOFCs). Incorporation of nano-sized Pd and GDC particles significantly reduces the electrode area specific resistance (ASR) as compared to the pure LSCF cathode; ASR is 0.1 Ω cm² for the reaction on a LSCF cathode infiltrated with 1.2 mg cm⁻² Pd and 0.06 Ω cm² on a LSCF cathode infiltrated with 1.5 mg cm⁻² GDC at 750 °C, which are all significantly smaller than 0.22 Ω cm² obtained for the reaction on a conventional LSCF cathode. The activation energy of GDC- and Pd-impregnated LSCF cathodes is 157 and 176 kJ mol⁻¹, respectively. The GDC-infiltrated LSCF cathode has a lower activation energy and higher electrocatalytic activity for the O₂ reduction reaction, showing promising potential for applications in IT-SOFCs.     

Design of experiment approach applied to reducing and oxidizing tolerance of anode supported solid oxide fuel cell. Part II: Electrical, electrochemical and microstructural characterization of tape-cast cells

One of the major limitations of the nickel (Ni) - yttria-stabilized zirconia (YSZ) anode support for solid oxide fuel cells (SOFC) is its low capability to withstand transients between reducing and oxidizing atmospheres (“RedOx” cycle), owing to the Ni-to-NiO volume expansion. This work presents results on different anode supports fabricated by tape casting. Three compositions are prepared, as the outcome of a preceding design of experiment approach. The NiO proportion is 40, 50 and 60 wt% of the anode composite.The anode support characteristics like shrinkage during sintering, in-situ conductivity at high temperature, electrochemical performance and tolerance against RedOx cycles have been measured. Performance up to 0.72 W cm⁻² (0.62 V, 800 °C) is recorded for the 60 wt% NiO sample on small cells. The open circuit voltage is maintained within ±5 mV after 10 full RedOx cycles at 800 °C and one at 850 °C. Performances tend to be stabilized after one or multiple RedOx cycles. The microstructural observations show round Ni particles after the first reduction; after a RedOx cycle, the Ni particles include micro-porosities that are stable under humidified reducing atmosphere for more than 300 h.     

Perovskite-related intergrowth cathode materials with thin YSZ electrolytes for intermediate temperature solid oxide fuel cells

The electrochemical performances of solid oxide fuel cells with thin yttria-stabilized zirconia (YSZ) electrolytes and YSZ/Ni anodes were studied with two intergrowth oxides cathodes (Sr_2.7La_0.3Fe_1.4Co_0.6O_7−δ and LaSr₃Fe_1.5Co_1.5O_10−δ) and the results compared to a related perovskite cathode (La_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ). It was found that cells produced with LaSr₃Fe_1.5Co_1.5O_10−δ exhibited peak power densities close to 0.75 W cm⁻², despite the relatively modest electrical conductivity of this compound. In contrast, cells produced with Sr_2.7La_0.3Fe_1.4Co_0.6O_7−δ and La_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ cathodes both exhibited peak power densities of less than 0.4 W cm⁻². The greater performance for the cells produced with LaSr₃Fe_1.5Co_1.5O_10−δ may be attributed to a higher catalytic activity for this compound or to an improved adhesion of the cathode to the interlayer/electrolyte.