Specific Grain Boundary Conductivity Research Articles

ZnO sintering additive without negative impact on proton-conducting SrHf0.8Sc0.2O3-δ electrolyte

Many proton-conducting oxides are refractory, making it difficult to fabricate dense electrolyte membranes for electrochemical devices. The use of a sintering additive is a common strategy, although it often has a negative effect on proton transport. We found that the addition of ZnO (2 and 4 mol%) successfully contributes to densification of SrHf0.8Sc0.2O3-δ ceramics at 1450 °C without negative consequences for the ionic transport. The chemical stability of the materials, high levels of hydration and proton conductivity were confirmed. ZnO is partially (less than 0.3 mol%) incorporated into the crystal lattice of the perovskite and partially accumulates at grain boundaries gradually volatilizing during high-temperature treatments. As a result, ZnO sintering additive does not influence the bulk and specific grain-boundary conductivities, ionic transference numbers and hydration ability, maintaining the excellent electrolytic properties of SrHf0.8Sc0.2O3-δ ceramics.

Grain boundary conduction behaviors of ultra-fine grained CeO2/BaCeO3 based electrolytes

CeO 2 /BaCeO 3 based electrolytes, mixtures of oxygen ion and proton conductors, are promising in the application of solid oxide fuel cells (SOFCs). However, their grain boundary conduction behaviors are still unclear. Herein, Sm, Bi co-doping CeO 2 /BaCeO 3 electrolytes (80 wt% Ce 0.8 Sm 0.1 Bi 0 · 1 O 2-δ - 20 wt% BaCe 0.8 Sm 0.1 Bi 0 · 1 O 3-δ , BiSDC-BCSBi) with ultra-fine grained (110–220 nm) and micron (1–1.8 μm) structures were prepared. In the ultra-fine grained structure, specific grain-boundary conductivities measured at 350 °C and 400 °C are 1–2 orders of magnitude higher than micron structures, thus resulting in dramatically enhanced electrical performances, which can be attributed to two aspects. One is the decrease of space charge potential Δ φ (0) (0.165 V for ultrafine-fine grained ones, 0.396 V for micron ones). The other is the dilution of impurities (the impurity blocking term ω/d g is 0.94 for ultrafine-fine grained ones, and 0.53 for micron ones). In the ultra-fine grained electrolytes, no extra electronic conduction is introduced, and the ion migration number of O 2− is higher than that of H + .

Enhanced sintering and electrical properties of proton-conducting electrolytes through Cu doping in BaZr0.5Ce0.3Y0.2O3-δ

To obtain BaZrO3-based electrolyte materials with both good sinterability and high proton conductivity, a Cu-doped B-site BaZr0.5Ce0.3Y0.2-xCuxO3-δ (BZCYCux; x = 0, 0.05, 0.1) system is designed and synthesized. The effects of Cu doping on the crystal structure, sintering and electrical properties of BZCYCux are systematically investigated. The BZCYCu0.05 sample demonstrates the highest sintering density and total conductivity among the studied materials. BZCYCu0.05 can be densified at 1250 °C, which is 300 °C lower than that for unmodified BZCY. The calculated results suggest that the increased conductivity in BZCYCu0.05 benefits from a high apparent specific grain-boundary conductivity, which originates from a small space-charge potential and a large clean part of impurities in the electrolyte. A synthetic analysis based on impedance spectroscopy and distribution of relaxation time further confirms that BZCYCu0.05 is more conductive. A single cell built using the BZCYCu0.05 (∼46 μm) electrolyte shows an excellent power density of 258 mW cm−2 at 700 °C. These results indicate that BZCYCu0.05 is a potential proton conductor for use in protonic ceramic fuel cells.

Effect of the addition mechanism of ZnO sintering aid on densification, microstructure and electrical properties of Ba(Zr,Y)O3-δ proton-conducting perovskite

We explore three different potential mechanisms to introduce 4 mol% ZnO sintering additive to the promising yttrium-doped barium zirconate (Ba(Zr,Y)O3-δ, BZY) proton conductor. The mechanisms involve Zn substitution for Y, Zr, or B-site cation excess. The addition of ZnO promotes high densification levels (up to 98% of the theoretical value) at 1300 °C, irrespective of the mechanism. However, scanning electron microscopy shows that the B-site cation excess mechanism leads to an impaired grain growth compared to the other mechanisms. Rietveld refinement of the lattice-parameters and scanning transmission electron microscopy-energy dispersive X-ray spectroscopy indicates that Zn resides in both grains and grain boundaries in all cases. Determination of partial conductivities demonstrates that the Zr substitution mechanism provides slightly higher values of bulk protonic conductivity, as well as a higher hydration enthalpy. In contrast, the B-site excess mechanism provides the highest specific grain-boundary conductivity, as a result of greater Zn segregation to the grain boundary.

Modifying Grain Boundary Ionic/Electronic Transport in Nano-Sr- and Mg- Doped LaGaO3-δ by Sintering Variations

Perovskite La0.9Sr0.1Ga0.9Mg0.1O3-δ (LSGM) is one of the fastest known oxide ion conductors, with reported enhanced p-type electronic transference numbers at grain boundaries, attributed to space charge effects. As this material is applied as a solid oxide fuel/electrolysis cell electrolyte, it is of interest to learn how its mixed conductivity may be tailored. Field assisted sintering technique/spark plasma sintering (FAST/SPS) and conventional sintering without field or pressure were employed to prepare pellets with various grain sizes, in order to systematically assess the influence of processing route on the mixed conductivity. AC-impedance spectroscopy and the brick layer model were applied to determine local conductivities as a function of temperature, oxygen partial pressure, and dc bias. With increasing sintering temperature and grain size, the following trends were observed: larger electrical grain boundary (GB) widths, higher GB potentials, lower specific GB conductivity, greater dc-bias dependence of GB conductivity, higher pO2-dependence of GB conductivity indicating higher electronic transference numbers, and lower pre-exponential factor for specific GB conductivity. These results suggest an increasing GB space charge effect with increasing sintering temperature/grain size, which coincided with increasing compositional uniformity by TEM and EDS. The results confirm that sintering route is an important variable for tailoring mixed conduction.

Effect of sintering conditions on the electrical-transport properties of the SrZrO3-based protonic ceramic electrolyser membrane

Abstract The effects of sintering temperature and addition of 4 mol.% ZnO as sintering additive on the crystal structure, microstructure and electrical properties of SrZr 0.9 Y 0.1 O 3-δ are reported. The presence of ZnO as sintering aid brings about high densification at 1300 °C (relative density ∼97%); gas-tightness is not achieved for ZnO-free samples sintered below 1600 °C. Bulk conductivity (σ B ) is considerably higher in wet and dry O 2 on doping with ZnO, but only slight variations of σ B with sintering temperature are observed for the Zn-containing phases. Similarly, the apparent grain-boundary conductivities are much greater for the Zn-doped samples. The grain-boundary volume and accompanying resistances are much reduced on sintering at 1500 °C with ZnO addition in comparison to Zn-modified samples sintered below 1500 °C, with only minor changes in grain-boundary relaxation frequency observed. Conversely, in comparison to the undoped sample with sintering temperature of 1600 °C, there is an enormous improvement in the specific grain-boundary conductivity of two orders of magnitude for the ZnO-containing samples. Analysis on the basis of the core space-charge-layer model relates the enhancement of the grain-boundary transport to a higher concentration of charge carriers in the space-charge layer and associated lower potential barrier heights.

Electrical conductivity and grain boundary composition of Gd-doped and Gd/Pr co-doped ceria

We characterize electrical conductivity, microstructure, nano-scale grain boundary structure and chemistry of ceria electrolytes with nominal compositions of Gd0.2Ce0.8O2-δ (GDC) and Gd0.11Pr0.04Ce0.85O2-δ (GPDC). The electrolytes are fabricated using mixed oxide nanopowders synthesized by spray drying. AC impedance spectroscopy was performed from 150°C to 700°C in air to determine grain-interior electrical conductivity. Grain-boundary conductivity was determined below 300°C. The grain-interior conductivity of the GPDC was higher than that of GDC by as much as 10 times, depending on the temperature. The GPDC specific grain-boundary conductivity was measured to be approximately 100 times higher than that of GDC. Energy dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) confirmed the grain-to-grain compositional uniformity of both materials following heat treatments. Grain boundaries were free of glassy intergranular phases; dopant concentration and Ce oxidation state were found to vary significantly near grain boundaries. Boundary core composition was estimated from STEM EELS to be Gd0.62Ce0.38O2-δ, and Gd0.29Pr0.16Ce0.55O2-δ in GDC and GPDC, respectively. Pr segregation to grain boundaries in the GPDC is hypothesized to enhance conductivity by both decreasing oxygen vacancy migration energy, and inducing mixed ionic–electronic conductivity in the near-boundary region.

Enhanced ionic conductivity of Ce0.8Sm0.2O2−δ by Sr addition

Sm and Sr co-doped ceria-based electrolyte with compositions of Ce0.8(Sm1−xSrx)0.2O2−δ (x=0, 0.3, 0.5, 0.7) are synthesized and investigated with the aim of improving the electrical properties of Ce0.8Sm0.2O2−δ. X-ray diffraction (XRD) and electron microscope (SEM and TEM) techniques are employed to characterize the microstructure of powders and sintered pellets. The ionic conductivity has been examined by the A.C. impedance spectroscopy in air. The Ce0.8(Sm0.7Sr0.3)0.2O2−δ exhibits the highest bulk conductivity among the series, which can be mainly ascribed to the increase of oxygen vacancy concentration. The specific grain-boundary conductivities are observed to increase with the Sr doping content up to x=0.5. Further increase in Sr concentration will lead to reduced specific grain-boundary conductivities. The total conductivities of all Sm and Sr co-doped ceria are higher than that of Ce0.8Sm0.2O1.9. The results indicate that Sr co-doping opens a new avenue to improve ionic conductivity in Ce0.8Sm0.2O1.9.

Comparative study of Ce 0.85Sm 0.075Nd 0.075O 2− δ electrolyte synthesized by different routes

In this work, four different methods, including polyvinyl alcohol (PVA)-assisted sol–gel process, polyethylene glycol (PEG)-assisted sol–gel process, citrate sol–gel process and oxalate coprecipitation process (OCP) are employed to synthesize the Sm and Nd co-doped ceria electrolyte with the composition of Ce 0.85Sm 0.075Nd 0.075O 2− δ (SNDC). The phase structure of the powders can be well indexed with the fluorite-type CeO 2 structure. The morphology of sintered samples indicates that the ceramics can be highly densified. The relative density and the average grain size vary with the synthesis processes and the sintering temperatures. The bulk conductivities are quite close and the OCP–SNDC yields highest grain-boundary conductivities and total conductivities. The results indicate that the OCP process for the powder synthesis results in higher relative density and conductivities, lower grain-boundary resistance and activation energy. Grain-boundary space charge potentials for different specimens are calculated based on the Mott–Schottky model. The synthesis process and sintering temperature have significant effect on the space charge potential and the specific grain-boundary conductivity.

Electrical properties of grain boundaries and size effects in samarium-doped ceria

Ce 0.85Sm 0.15O 1.925 samples have been prepared using the glycine-nitrate process. The effects of microstructure on their electrical properties were investigated by X-ray diffraction, density measurements, scanning electron microscopy and AC impedance spectroscopy. The grain-boundary conductivities and total conductivities increase with decreasing grain size for sintered densities of 95% or greater. The bulk conductivities are nearly independent of grain size for sintered densities of 95% or greater. It is found that the space-charge potential is nearly independent of grain size and the increase of the conduction path width is responsible for the increase of the apparent specific grain-boundary conductivity. The power densities and current densities of single cells based on Ce 0.85Sm 0.15O 1.925 electrolytes with different grain size are evaluated.

Enhanced ionic transport in fine-grained scandia-stabilized zirconia ceramics

In this work, the transport properties of fine-grained scandia-stabilized zirconia ceramics with low Si content have been investigated. These materials were prepared from ZrO 2–6 mol% Sc 2O 3 nanopowders synthesized by a nitrate-lysine gel-combustion route. High relative densities and excellent electrical properties were obtained, even for sintering temperatures as low as 1350 °C. Our electrochemical impedance spectroscopy study showed that both the volume fraction of grain boundaries and the specific grain-boundary conductivity are significantly enhanced with decreasing grain size, resulting in a higher total ionic conductivity.

Dopant-concentration dependence of grain-boundary conductivity in ceria: A space-charge analysis

Here we elucidate the origin of the characteristic development of grain-boundary conductivity and of its activation energy measured in Gd-doped ceria (GDC) ceramics upon change in the dopant concentration in the context of the space-charge effect. The grain-boundary conductivity in GDC was measured as a function of the dopant concentration (1–20 cat.%). The total conductivity of the sample with 1 cat.% dopant is lower than the bulk conductivity by several orders of magnitude largely due to exceedingly high grain-boundary resistance in the sample, consistent with the results previously reported in literature. The specific grain-boundary conductivity of GDC however rapidly increases with increasing dopant content to approach a plateau at concentrations above 15 cat.%, leading to the total conductivity being close to its bulk conductivity at the relatively high dopant level. On the other hand, the grain-boundary width estimated experimentally in GDC is found to decrease substantially as the dopant concentration increases, implying that the observed increase in the grain-boundary conductivity is correlated with such reduction in the grain-boundary width. In addition, the values of the grain-boundary width determined at each dopant concentration are in quantitative agreement with those calculated based on a space-charge model, in which the grain-boundary resistance is attributed to the Schottky-type potential barrier present at the grain boundaries. Furthermore, the values of activation energy of the grain-boundary conductivity measured at different dopant concentrations can also be reproduced quantitatively using the space-charge model. It is therefore verifed unambiguously that the characteristic increase in the grain-boundary conductivity measured in GDC upon increasing dopant concentration results from concurrent reduction in the height of the potential barrier formed inherently at the grain boundaries.

Improvement of Grain-Boundary Conductivity of Trivalent Cation-Doped Barium Zirconate Sintered at 1600°C by Co-doping Scandium and Yttrium

Scandium and yttrium co-doped barium zirconate [ ] have been investigated in terms of phase relationship, microstructures, and electrical conductivity. The bulk conductivity of the scandium and yttrium co-doped barium zirconate increased with the dopant ratio of yttria. had the highest grain-boundary conductivity among the scandium and yttrium co-doped barium zirconates in this study. But, , , , and consisted of a single cubic perovskite phase at 1600°C and their densities of grain-boundary were smaller than that of . From the observation of microstructure and results of grain boundary-conductivity measurement, we can say that yttrium is a dopant that increases specific grain-boundary conductivity and bulk conductivity, and scandium is a dopant that increases the grain size. Thus, there is a trade-off relation between the grain size and specific grain-boundary conductivity based on the mixing ratio of scandia to yttria. The total conductivity of at 600°C was estimated to be , which is the highest-class conductivity among reported trivalent cation-doped barium zirconates.

Blocking Grain Boundaries in Yttria‐Doped and Undoped Ceria Ceramics of High Purity

CeO2 samples doped with 10, 1.0, and 0.1 mol% Y2O3 and undoped CeO2 samples of high purity were studied by impedance spectroscopy at temperatures <800°C and under various oxygen partial pressures. According to microstructural investigations by SEM and analytical STEM (equipped with EDXS), the grain boundaries were free of any second phase, providing direct grain‐to‐grain contacts. An amorphous siliceous phase was detected at only a few triple junctions, if at all; as a result, its contribution to the grain‐boundary resistance was negligible. Nevertheless, the specific grain‐boundary conductivities were still 2–7 orders of magnitude lower than the bulk conductivities, depending on dopant concentration, temperature, and oxygen partial pressure. The charge carrier transport across the grain boundaries occurred only through the grain‐to‐grain contacts, whose properties were then determined by the space‐charge layer. The space‐charge potential in acceptor‐doped CeO2 was positive, causing the simultaneous depletion of oxygen vacancies and accumulation of electrons in the space‐charge layer. The very low grain‐boundary conductivities can be accounted for by the oxygen‐vacancy depletion; the accumulation of electrons became evident in weakly doped and undoped CeO2 at high temperatures and under low oxygen partial pressures.