Enhancing the sinterability and electrical properties of BaZr0.1Ce0.7Y0.2O3-δ proton-conducting ceramic electrolyte

A novel strategy was proposed to enhance the sinterability and electrical properties of BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY) proton-conducting electrolyte by adding 10 wt.% La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ (LSGM) to form a 90 wt.% BZCY–10 wt.% LSGM (BL91) composite electrolyte. XRD patterns showed that no reaction occurred between the BZCY and LSGM electrolytes after sintering at 1400°C, 1450°C, 1500°C, and 1550°C for 10 h. The BL91 composite electrolyte exhibited higher relative densities and Vickers hardness and excellent electrical properties compared with those of the BZCY electrolyte. A combined approach of equivalent circuit model and distribution of relaxation time analysis was used to distinguish the bulk and grain-boundary contributions to the total conductivity and electrode processes. The introduction of 10 wt.% LSGM serves as a grain-boundary pinning phase, which can reduce the mobility of grain boundaries, thereby increasing sintered density and enhancing conductivity in BL91. A solid oxide fuel cell with proton-conducting BL91 and BZCY membranes was tested, in which the former displayed higher power outputs than the latter. Ohmic and interfacial polarization resistances decreased by approximately 20%, thereby revealing the remarkable electrical properties of the BL91 electrolyte. Results demonstrated that BL91 composite is a development prospect proton-conducting electrolyte.     

Densification,morphological and transport properties of functional La1–xBaxYbO3–δ ceramic materials

The effective operation of protonic ceramic electrochemical cells requires the design of electrolytes having not only high ionic conductivity, but also excellent stability with respect to carbonisation. In the present work, the La-based oxides (La_1–xBa_xYbO_3–δ, 0.03 ≤ x ≤ 0.10) are proposed as a possible alternative to the convenient Ba(Ce,Zr)O₃-based electrolytes due to their high chemical stability. It was discovered that Ba-doping results in a deterioration of sintering behaviour; as a result, the relative density decreases and open porosity appears (for x = 0.10). A thorough analysis of transport properties by means of AC and DC measurement techniques enables a selection of the La_0.97Ba_0.03YbO_3–δ sample, which demonstrates the highest conductivity compared with those samples where x = 0.5 and 0.10. Due to its excellent densification behaviour, stability and ionic conductivity, La_0.97Ba_0.03YbO_3–δ can be considered as a promising proton-conducting electrolyte in the La-based family.     

Electrochemical performance and chemical stability of proton-conducting BaZr0.8−xCexY0.2O3−δ electrolytes

Protonic ceramic fuel cells (PCFCs) using BaZr_0.8−xCe_xY_0.2O_3−δ (BZCY) as electrolyte materials have attracted widespread attention because of their high performance at reduced temperature. However, there are few systematic studies on both the performance and stability of BZCY materials. In this paper, we report our work on the electrochemical performance and chemical stability of BaZr_0.8−xCe_xY_0.2O_3−δ (x = 0, 0.1, 0.3, 0.5, and 0.7) series. The results show that electronic hole conductivity decreases with increasing Ce⁴⁺ content, especially at high temperature. In addition, H₂ atmosphere reduces the conductive activation energy of BZCY. On the contrary, air atmosphere causes serious electronic leakage. These effects are also reflected in the operation of PCFCs, that is, the higher the Ce⁴⁺ content, the higher the open-circuit voltage and output power density. However, low Ce⁴⁺ content may stabilize the materials in CO₂ atmosphere. At 700°C, an anode-supported PCFC based on BaZr_0.1Ce_0.7Y_0.2O_3−δ electrolyte, using humid H₂ fuel, gives a peak power density of 1.0 W cm⁻². At 600°C, BaZr_0.8Y_0.2O_3−δ and BaZr_0.7Ce_0.1Y_0.2O_3−δ show a good stability in CO₂-containing atmosphere.     

A comparative investigation on protonic ceramic fuel cell electrolytes BaZr0.8Y0.2O3-δ and BaZr0.1Ce0.7Y0.2O3-δ with NiO as sintering aid

Solid oxide fuel cells based on proton-conducting ceramic electrolytes, i.e., protonic ceramic fuel cells (PCFCs), are promising in operating at intermediate to low temperature. BaZr_0.8Y_0.2O_3-δ (BZY) and BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY) are two typical electrolyte materials for PCFCs. However, there is still a lack of basis for making a choice between the two materials. In this paper, we present a comparison investigation on practical BZY and BZCY electrolytes with NiO of 2 mol.% as sintering aid. Their crystal structure, sinterability, microstructure, and electrical conductivity in humid air and hydrogen (3% H₂O) are measured and analyzed. Anode-supported PCFCs based on the two electrolyte materials are prepared and their electrochemical performances are tested and analyzed in association with an examination on their microstructure. The results show that both materials can be densified after sintered with NiO aid at 1400 °C for 6h. Ni is doped into the interstitial of BZY while it occupies the B site of perovskite lattice of BZCY. The sintered BZY has small grains and many grain boundaries while BZCY has large grains and much fewer grain boundaries, resulting in lower conductivity of BZY than that of BZCY. A PCFC with BZY electrolyte gives a peak power density of 360 mW cm^?2 at 700 °C, while this value for a PCFC with BZCY is 855 mW cm^?2. Although the performances of BZCY seems much better than those of BZY, a stability test in 10% CO₂-containing Ar at 650 °C shows BZY is stable while BZCY reacts with CO₂ to form BaCO₃ and CeO₂.     

晶粒尺寸对BaZr0.2Ti0.8O3陶瓷介电性能的影响

通过固相反应工艺制备锆钛酸钡Ba(Zr_(0.2)Ti_(0.8))O_3陶瓷,通过控制晶粒尺寸的大小研究晶粒尺寸对介电性能的影响。利用X射线衍射仪、扫描电子显微镜和阻抗分析仪对样品晶体结构、相组成、微观形貌和晶粒尺寸、介电性能进行了分析。结果表明:所有陶瓷样品均为钙钛矿结构无杂峰出现,晶粒尺寸不大于5μm时随着晶粒尺寸的增大介电常数逐渐提高,当晶粒尺寸在5μm时介电常数达到最大值12 293,介质损耗仅为0.009 95,然后随晶粒尺寸的增大介电常数降低。     

Predicting grain size distributions in perovskite-structured Ba0.5Sr0.5Co0.8Fe0.2O3–δ oxygen transport membranes

ABSTRACT

This study is conducted over a 3?×?3 time–temperature matrix on Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) ceramics, and sintered bodies above 93% dense are obtained. The electron backscatter diffraction band contrast micrographs of the polished sintered samples are analysed for characterising the grain size distributions (GSDs). This study develops an algorithm for predicting the GSDs of BSCF dependence of sintering condition (time and temperature). In addition, the GSDs predicted by the algorithm agree reasonably with those experimentally observed. When individual grain size is non-dimensionalised by the median grain size, the GSDs data of all BSCF samples reduce to a single self-similar GSD curve. The median grain size is predicted by the classical kinetics equation, D^n?=?tK₀exp(?Q/RT).     

Structural properties of 0.8CaZrO3–0.2CaFe2O4 composite

A composite with the 0.8CaZrO₃–0.2CaFe₂O₄ general formula was prepared by means of a double sintering process. Temperature of 1400 °C was found suitable to obtain the composite based on scanning electron microscopy and X-ray diffraction analysis. Phase composition, crystal structure, Mössbauer effect, microstructure and dielectric properties were studied. The influence of Fe ions on the CaZrO₃ structure has been reported. It was found that Fe substituted Zr in CaZrO₃ and vice versa in CaFe₂O₄ in the amount about 1.5 at% in both cases. The approximated formula of the obtained composite was 0.8Ca(Zr_0.9Fe_0.1)O₃–0.2Ca(Fe_0.8Zr_0.2)₂O₄. The SEM observations revealed that liquid CaFe₂O₄ occurred as a filling phase between CaZrO₃ grains which significantly enhanced densification of the material and influence on its properties. The dielectric constant reached the value of 170 at 1 MHz and the accompanying dissipation factor was very low – 0.005.     

Enhanced sintering of Ce0.8Nd0.2O2−δ-La0.8Sr0.2Ga0.8Mg0.2O3−δ using CoO as a sintering aid

Ce_0.8Nd_0.2O_1.9 (NDC) and La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ (LSGM) electrolytes were prepared using a sol-gel method. NDC-LSGM composite electrolytes were subsequently prepared by adding 5% (w, mass fraction) precalcined LSGM powders to NDC sols. The electrolyte materials of NDC-Co and NDC-LSGM-Co were obtained by adding 1 mol% CoO to NDC sols and NDC-LSGM composite electrolytes, respectively. The microstructure and phase composition of the pellets were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and energy dispersive X-ray spectroscopy (EDS). The electrical conductivities of the pellets were measured using alternative current (AC) impedance spectroscopy. The results indicate that a single perovskite phase is observed for the LSGM ceramic, while NDC-Co, NDC-LSGM and NDC-LSGM-Co have a cubic fluorite structure similar to that of NDC. As a sintering aid, CoO can further promote grain growth and increase relative density (＞95%) of the NDC-LSGM composite electrolyte. The enhancement of the total conductivity is primarily attributed to the large increase in the conductivity of the grain boundary. However, the slight decrease of the grain boundary conductivity of the NDC-LSGM-Co electrolyte is caused by the presence of trace amounts of impurity phases in the grain boundaries.     

Effect of the reactive surface area of proton-conducting NiBa0.8Sr0.2Ce0.6Zr0.2Y0.2O3-δ anodes on cell performance

To determine the optimal combination of NiO and Ba_0.8Sr_0.2Ce_0.6Zr_0.2Y_0.2O_3-δ (BSCZY) for fabricating anode materials, Ni-BSCZY samples were prepared using the solid state reaction process. The porous structure of anode substrates not only provides mechanical strength to the fuel cells to enable fuel gases to flow to the electrolyte membrane but also creates an excess surface area on which to form a larger triple-phase boundary when NiO is added to the anode sample. The effect of NiO content on the microstructures, surface area, and electric conductivity of these Ni-BSCZY (NiO55-BSCZY, NiO60-BSCZY, and NiO65-BSCZY) anode materials were systematically investigated using X-ray diffraction, scanning electron microscopy, an analytic technique based on the Brunauer–Emmett–Teller surface area theory, and four-probe conductivity analysis. In addition, three anode-supported cells containing identical electrolytes but various combinations of NiO and BSCZY anode materials were fabricated and used for performance and electrochemical impedance measurement. The results revealed that the reactive surface area of the anode in contact with the electrolyte plays a crucial role in total cell performance. The cell containing the anode material (NiO60-BSCZY) with the highest surface area of 6.91 m² g⁻¹ and the lowest total resistance of 2.19 Ω cm² exhibited the highest power density of 169.2 mW cm⁻² at 800 °C.     

Phase interaction and oxygen transport in La0.8Sr0.2Fe0.8Co0.2O3–(La0.9Sr0.1)0.98Ga0.8Mg0.2O3 composites

Composite ceramics made of two perovskite-type compounds, (La_0.9Sr_0.1)_0.98Ga_0.8Mg_0.2O_3−δ (LSGM) and La_0.8Sr_0.2Fe_0.8Co_0.2O_3−δ (LSFC) mixed in the ratio 60:40 wt.%, possess relatively high oxygen permeability limited by both bulk ionic conduction and surface exchange at 700−950 °C. Sintering at elevated temperatures (1320–1410 °C) necessary to obtain dense materials leads to fast interdiffusion of the components, forming almost single perovskite phase ceramics with local inhomogeneities. This phase interaction decreases the oxygen ionic transport in the composites, where the level of ionic conductivity is intermediate between those of LSGM and LSFC. The scanning electron microscopy (SEM) suggests a presence of Ga-enriched domains, probably having a high ionic conductivity. The size and concentration of these domains can be increased by decreasing sintering temperature or using preliminary coarsened LSGM powders. The maximum oxygen permeability is thus observed for the composite prepared under minimum sintering conditions sufficient to obtain gas-tight ceramics, including the use of LSGM, preliminary passivated at 1150 °C, and sintered at 1320 °C. The activation energy values for total conductivity, which is predominantly p-type electronic and slightly decreases due to component interaction, vary in the narrow range from 24.0 to 26.2 kJ/mol at 25–575 °C. The average thermal expansion coefficients (TECs) of LSGM-LSFC composites, calculated from dilatometric data in air, are (12.4–13.5)×10⁻⁶ K⁻¹ at 100–650 °C and (17.8–19.8)×10⁻⁶ K⁻¹ at 650–1000 °C.     

A case study of mechanical properties of perovskite-structured Ba0.5Sr0.5Co0.8Fe0.2O3−δ oxygen transport membrane

This study has investigated mechanical properties of perovskite-structured Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) oxygen transport membrane. The Young’s modulus and fracture toughness are determined by both macroscopic-scale and microscopic-scale methods. Both three-point and ring-on-ring bending tests as macroscopic-scale methods produce broadly similar results with a Young’s modulus, which is lower than that measured from micro-indentation method under a 10 N load. Young’s modulus and fracture toughness of BSCF show strongly dependent of the porosity. However, the fracture toughness of BSCF is independent of grain size. The fracture toughness determined by macroscopic-scale method is similar with that measured by microscopic-scale method. The crack shape of BSCF under a 10 N load is determined to be a median-radial mode. The intrinsic Young’s modulus and fracture toughness are determined to be 105.6 GPa and 1.49 MPa m^0.5, respectively, according the Minimum Solid Area (MSA) model. Annealing decreases the fracture toughness of BSCF between RT and 800 °C.     

A novel BaCe0.5Fe0.3Bi0.2O3–δ perovskite-type cathode for proton-conducting solid oxide fuel cells

A perovskite-type BaCe_0.5Fe_0.3Bi_0.2O_3-δ (BCFB) was employed as a novel cathode material for proton-conducting solid oxide fuel cells (SOFCs). The single cells with the structure of NiO-BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY7) anode substrate|NiO-BZCY7 anode functional layer|BZCY7 electrolyte membrane|BCFB cathode layer were fabricated by a dry-pressing method and investigated from 550 to 700 °C with humidified hydrogen (~3% H₂O) as the fuel and the static air as the oxidant. The low interfacial polarization resistance of 0.098 Ω cm² and the maximum power density of 736 mW cm⁻² are achieved at 700 °C. The excellent electrochemical performance indicates that BCFB may be a promising cathode material for proton-conducting SOFCs.     

A permeation model study of oxygen transport kinetics of BaxSr1-xCo0.8Fe0.2O3-δ

Ba_xSr_1-xCo_0.8Fe_0.2O_3-δ (BSCF) materials with different Ba doping amounts are widely applied as membranes for oxygen separation and electrodes in solid oxide cells. Different opinions were put forward about the effect of Ba contents on the oxygen transport kinetics. In this work, an oxygen permeation model was first used to investigate the effect of Ba content in BSCF (x = 0, 0.1, 0.3, 0.5, 0.7) on the oxygen transport kinetics. The permeation resistance constants were obtained separately through fitting the condition experimental data to the model. With the increase of Ba content, the interfacial oxygen exchange resistance constants of both sides decrease quickly and then increase slightly, while bulk diffusion resistance remains monotone decreasing. The improvement of oxygen permeation flux is mainly contributed by the enhancement of interfacial oxygen exchange kinetics. It is inferred that Ba element may play an important role in the interfacial oxygen reactions.     

High temperature mechanical properties of Al2O3/TiC micro–nano-composite ceramic tool materials

A type of Al₂O₃-based composite ceramic tool material simultaneously reinforced with micro-scale and nano-scale TiC particles was fabricated by the hot-pressing technology with different contents of cobalt additive. The effects of cobalt on the ambient temperature mechanical properties and high temperature flexural strength were investigated. The flexural strength and fracture toughness of the composite with 3 vol% cobalt as a function of temperature were investigated. Cobalt greatly enhanced the ambient temperature flexural strength and fracture toughness, while further increasing the content of cobalt led to a dramatic strength degradation, especially at high temperature. The flexural strength of the composite containing 3 vol% cobalt decreased as the temperature increased from 20 to 1200 °C, and the fracture toughness decreased as a function of the temperature up to 1000 °C but increased at 1200 °C. The degradation of high temperature flexural strength was ascribed to the change of the fracture mode, the grain and grain boundary oxidation, the decrease of elastic modulus and the grain boundary sliding.     

Moderate temperature sintering of BaZr0.8Y0.2O3-δ protonic ceramics by A novel cold sintering pretreatment

The state-of-the-art protonic ceramic conductor BaZr_0.8Y_0.2O_3-δ (BZY20) requires an extremely high sintering temperature (≥1700 °C) to achieve the desired relative density and microstructure necessary to function as a proton conducting electrolyte. In this work, we developed a cold sintering pretreatment assisted moderate-temperature sintering method for the fabrication of high-quality pure BZY20 pellets. BZY20 pellets with high relative density of ~94% were fabricated with a final sintering temperature of 1500 °C (200 °C lower than the traditional sintering temperature). A comparison with BZY20 control samples indicated that the proper amount of BaCO₃ introduced on the BZY20 particle surface and the high green density achieved by cold sintering pretreatment were the main drivers for lowering the sintering temperature. The electrical conductivity measurement by electrochemical impedance spectroscopy showed that the as-prepared BZY20 pellets have a proton conductivity comparable to the state-of-the-art values. The cold sintering pretreatment outlined in this work has the potential to lower the sintering temperatures for similar types of protonic ceramic materials under consideration for a wide range of energy conversion and storage applications.     

Effect of sintering aids on the densification and electrical properties of SiO2 –containing Ce0.8Sm0.2O1.9 ceramic

In this work, five different metal-oxide additives (metal?=?Ba, Co, Fe, Li, and Mn) were examined as sintering aids and SiO₂ impurity scavengers for Ce_0.8Sm_0.2O_1.9 (SDC). 2?mol% additives were loaded into the SDC with ～150?ppm (moderately impure) and ～2000?ppm (highly impure) SiO₂. Ba-, Co-, Fe- and Mn-oxides showed comparative sintering-aid effect on both moderately- and highly-impure SDC specimens, but the sintering-assisting effect of Li-oxide was completely neutralized in highly impure SDC. Regarding electrical property, the deleterious effect of 2000?ppm SiO₂ impurity on the grain-boundary conduction of SDC can be effectively alleviated by adding Ba-, Co-, Fe-, or Mn-oxides. Microstructure analysis revealed that Ba-oxide reacted directly with SiO₂ and consequently enhanced grain-boundary conduction. By contrast, with the addition of Co-, Fe-, and Mn-oxides, the improved grain-boundary conductions of impure SDC were related to the scavenging reactions between Si, Ca (another original impurity) and Sm components.     

Low-temperature sintering and enhanced stability of fluorine-modified BaZr0.8Y0.2O3-δ synthesised by a sol-gel alkoxide route

Sol-gel synthesis using alkoxides is employed to obtain fluorine-modified BaZr_0.8Y_0.2O_3-δ nanopowders with Ba excess at 750 °C. Possible effects of F loss at high temperature were limited on densifying at the low temperature of 1200 °C for 4 h on addition of ZnO as sintering agent. The presence of F in sintered samples was confirmed by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry. XPS and Raman spectroscopy indicated that F doping of BZY inhibits carbonate formation. Conductivity measurements for prolonged periods in wet H₂-containing and CO₂-containing atmospheres at 500 °C also demonstrated chemical stability. The temperature dependence of the electrical conductivity of sintered samples in wet and dry N₂ and O₂ indicates that the grain-boundary dominates the electrical behaviour to 500 °C, above which the bulk process dominates. Ionic conduction is prevalent at low temperature, with a transition to a mixed ionic-electronic conducting behaviour at 450 °C.     

Effect of optimal GDC loading on long-term stability of La0.8Sr0.2Co0.2Fe0.8O3-δ/Gd0.2Ce0.8O1.9 composite cathodes

Three types of La_0.8Sr_0.2Co_0.2Fe_0.8O_3-δ/Gd_0.2Ce_0.8O_1.9 (LSCF/GDC) composite cathodes with different optimal GDC loading are fabricated through electrospinning, screen printing and solution infiltration method. Constant current polarization with current density of 100 mA cm^?2 at 750°C is applied to test the stability of LSCF/GDC composite cathodes. After constant current polarization for 144 h, the polarization resistance (R_p) of 280 nm-nanofiber skeletal LSCF/GDC composite cathode after pore-forming exhibits the minimum increase, from 0.062 Ω cm² to 0.098 Ω cm². Scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) results show that the microstructure and surface chemical composition of the cathode maintain stable during the constant current polarization. Combined with the X-ray diffraction (XRD) result, a relationship among GDC loading, stress, Sr surface segregation and long-term stability is established.     

Transport properties of BaZr0.5Ce0.3Y0.2O3−δ proton conductor prepared by spark plasma sintering

Dense BaZr_0.5Ce_0.3Y_0.2O_3−δ (BZCY532) proton conductors were prepared by a spark plasma sintering method. Their conductivities were determined in different atmospheres: dry air, wet N₂ and wet H₂. Moreover, the potential electronic conductivity contribution to the total conductivity was also identified by testing their total conductivities at different oxygen partial pressures (1–10⁻²⁴ atm) in combination with an XPS analysis. It is found that the prepared dense BZCY532 ceramics are good proton conductors at 600 °C. In addition, the Ce³⁺ concentration in the dense BZCY532 ceramics is around 3.5 atm% of the total Ce element, and the electronic contribution to the total conductivity can be neglected after a postheat treatment.