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.     

Chemical Expansion of Yttrium‐Doped Barium Zirconate and Correlation with Proton Concentration and Conductivity

Y‐doped BaZrO₃ (BZY) is a promising candidate as an electrolyte in fuel cells, and attracts increasing attention. In this work, a systematic investigation was performed on microstructure, proton concentration, proton conductivity, and hydration induced chemical expansion in Y‐doped BaZrO₃. The results revealed that the bimodal microstructure in BaZr_0.85Y_0.15O_3?δ was composed of large grains with composition close to the nominal value, and fine grains with large compositional discrepancy. This property is considered to be one of the evidences of phase separation at lower temperature than sintering temperature (1600°C), which hinders the grain growth. Thermal expansion coefficient of BZY was measured for various dopant level, and was determined to be around 10^?5 K^?1 in wet and dry argon atmosphere. In addition, chemical expansion effect due to hydration was confirmed by HT‐XRD in dry and wet Ar atmospheres, and suggests an interesting relationship between the lattice change ratio and proton concentration, in the BZY system with different Y content. The change ratio of lattice constant due to hydration increased obviously with the proton concentration for the sample containing the Y content of 0.02 and 0.05, but only changed slightly when the Y content was increased to 0.1 and 0.15. However, when the Y content was further increased over 0.2, the change ratio of lattice constant due to hydration starts to increase obviously again. Such results indicate a high possibility that the stable sites of protons in BZY changed with the variation in Y content.     

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.     

Proton conducting solid oxide fuel cells with chemically stable BaZr0.75Y0.2Pr0.05O3-δ electrolyte

A BaZrO₃-based electrolyte with low Pr-doping concentration is proposed as electrolyte for proton-conducting solid oxide fuel cells (SOFCs). The new material BaZr_0.75Y_0.2Pr_0.05O_3-δ (BZYP5) shows a good chemical stability against CO₂. In addition, the low doping concentration of Pr in BaZrO₃ improves the sinterability of BaZrO₃ and also allows its structure to remain stable even in the reducing atmosphere, which is critical for fuel cell applications. The cell with BZYP5 as electrolyte shows maximum power densities of 124, 70, and 43 mW cm⁻² at 600, 550, and 500 °C, respectively, which are larger than that for the cell with conventional high Pr-doping BaZrO₃ electrolyte reported previously. Electrochemical analysis indicates that the BZYP5 electrolyte shows a good ionic conductivity. These results suggest that the low Pr-doping strategy presented in this study promotes the densification for BaZrO₃ and the good electrolyte conductivity of BaZrO₃ is maintained which could be the reason for the improved cell performance, suggesting BZYP5 is a promising electrolyte for proton-conducting SOFCs.     

Dopant Site Occupancy and Chemical Expansion in Rare Earth‐Doped Barium Zirconate

Rare earth‐doped BaZrO₃ is a very attractive material in electrochemical applications due to its proton conductive property. In this work, powder X‐ray diffraction patterns of BaZr_0.8M_0.2O_3?δ (M = Sc, Eu, Sm, Dy) were collected using synchrotron radiation, and also using characteristic X‐ray of CuKα in dry and wet atmospheres at high temperature. Then, a combined interpretation of the diffraction patterns was established by using Rietveld refinement. The results revealed that an obvious lattice expansion was observed for BaZr_0.8M_0.2O_3?δ (M = Sc, Eu, Sm, Dy) in wet O₂ compared with the case in dry condition, indicating a chemical expansion effect on lattice volume by incorporating water into lattice. Eu, Sm, and Dy cations occupied both A‐ and B‐sites of BaZrO₃ crystalline lattice, whereas Sc cations were determined to occupy B‐site only. These results indicate clearly an increasing tendency toward A‐site occupation for the rare earth cations in BaZrO₃ with an increasing radius.     

Transport properties of proton conductive Y-doped BaHfO3 and Ca or Sr-substituted Y-doped BaZrO3

An electrolyte in fuel cells requires not only high ionic conductivity, but also high transport numbers of ionic conduction. Although Y-doped BaZrO₃ is regarded to be the most promising candidate as the electrolyte in protonic ceramic fuel cells (PCFCs), significant hole conduction generates in wet oxygen at high temperatures. With the aim to increase the transport number of ionic conduction, in this work, Sr and Ca were introduced to partially substitute Ba in BaZr_0.8Y_0.2O_3-δ. The results revealed that a single cubic perovskite phase was obtained for Ba_0.95Ca_0.05Zr_0.8Y_0.2O_3-δ and Ba_1-xSr_xZr_0.8Y_0.2O_3-δ (x = 0.05, 0.10, 0.15, 0.20 or 0.40). However, replacing Ba with Sr resulted in almost no increase in the transport number of ionic conduction in wet oxygen atmosphere, but drastic decrease in proton conductivity at all replacement levels. In addition, Ba_0.95Ca_0.05Zr_0.8Y_0.2O_3-δ shows no meaningful change in the transport number of ionic conduction, compared with BaZr_0.8Y_0.2O_3-δ. Incorporating Ca or Sr into the Ba-site of BaZr_0.8Y_0.2O_3-δ appears to impart no positive influence on electrochemical properties. These interesting results also indicate that the hole conductivity decreases with the decrease in proton conductivity, and will aid to consider the hole conduction mechanism. BaHfO₃ doped with 10 and 20 mol% Y was also prepared. A bimodal microstructure was observed for BaHf_0.9Y_0.1O_3-δ, whereas BaHf_0.8Y_0.2O_3-δ shows uniform grain size after sintering at 1600°C for 24 hours. The transport numbers of ionic conduction and bulk conductivity in such Y-doped BaHfO₃ samples are close to those of BaZrO₃ doped with the same amount of Y.     

A novel approach of synthesizing nano Y2O3 powders for the fabrication of submicron IR transparent ceramics

A facile methodology of synthesizing highly reactive, round-edged, Sulfur–free nano Y₂O₃ powders to fabricate submicron IR transparent yttria ceramics having a unique combination of superior optical and mechanical properties are reported for the first time. Dispersion of yttrium hydroxide into aqueous sol and addition of seed particles produced near-spherical yttria powders having non – aggregated particles with narrow size distribution. The powder exhibited excellent sinterability reaching near-theoretical density at temperatures around 1400 °C in air. Effective inter-particle coordination and traces of Al additives assisted achieving superior densification. Sintered specimens showed average grain sizes closer to 700 nm. Post-sinter hot isostatic pressing eliminated the residual porosity from the sintered samples leading to exhibit IR transmissions up to 84% in the 2.0–9.0 μm regions, equivalent to single crystal Y₂O₃. Achieving densification through solid-state sintering and retaining the sintered grain sizes in the submicron regions significantly enhanced the mechanical properties. Sintered and HIPed Y₂O₃ specimens were further characterized for their thermal properties at temperature regions between ambient to 950 °C.     

Guidelines for processing of porous barium zirconate-based ceramic electrolytes for electrochemical solid oxide cell applications

Efficient use of barium-containing electrocatalysts for NO_x decomposition in electrochemical solid oxide cells requires good chemical compatibility of these materials with the electrolyte in a temperature range 300–600 °C and imply the use of porous electrolytes allowing significant gas flows across the cell. BaZr_0.85Y_0.15O₃ electrolyte has good chemical compatibility with Ba-containing materials and potentially appropriate level of ionic conductivity. This work is focused on guidelines for processing BaZr_0.85Y_0.15O₃ ceramics with cellular porosity (i.e., with spherical pores and with developed gas interconnections), using emulsification of paraffin in water-based ceramic suspension. Porosity of sintered at 1500 °C ceramics is practically open and varies in the range of 70–84%, with average pore sizes in the range ~3–30 µm. The type of pore size distribution is close to log-normal. Microstructure, average cell size and level of gas permeability mainly depends on surfactant content, while total porosity is mostly determined by paraffin content.     

Synthesis and properties of core–shell structured BaCe0.9Y0.1O2.95:BaZr0.9Y0.1O2.95

The perovskite proton conductor BaZr_0.9Y_0.1O_2.95 (BZY10) shows better chemical stability but lower conductivity than BaCe_0.9Y_0.1O_2.95 (BCY10). In this paper we attempted to synthesize BCY10:BZY10 core–shell materials in which BCY10 particles prepared by solid reaction were wrapped by a sol–gel deposited thin layer of BZY10 with ZnO as sintering aid to improve the sinterability of the materials. The effects of the BCY10/BZY10 ratios on the phase purity, microstructure, chemical stability and electrical conductivity of the samples were characterized by XRD, TEM, SEM, TGA and electrochemical impedance spectroscopy. A dense core–shell structure was formed after being sintered at 1300 °C for 10 h. The core–shell samples displayed improved stability against CO₂ and water vapor at high temperature. With BCY10/BZY10 ratio varying from 9:1 to 7:3, the core–shell samples became more stable, and the total conductivities decreased.     

Effect of BaOB2O3 composite sintering aid on sinterability and electrical property of BaZr0.85Y0.15O3-δ ceramic

Yttrium-doped barium zirconate (BZY) has been extensively studied as a promising electrolyte material for protonic ceramic fuel cells. However, inferior sinterability is a major barrier in BZY development. In our research, the effect of BaOB₂O₃ composite sintering aid on the sintering behavior and electrical property of BaZr_0.85Y_0.15O_3-δ (BZY15) are examined. BaOB₂O₃ addition reduces the sintering temperature of BZY15 by approximately 200 °C via the liquid-phase sintering mechanism. The corresponding bulk and grain boundary conductivities are prominently improved (<3 wt% BaOB₂O₃ addition), whereas the further addition of sintering aid decreases the grain boundary conductivity. Notably, the scanning electron microscope (SEM) and electrochemical impedance spectroscopy (EIS) analyses suggest that the enhanced conductivity may be related to the temperature dependence of Ba evaporation.     

Effect of CO2 Exposure on the Chemical Stability and Mechanical Properties of BaZrO3‐Ceramics

The reactivity of BaZrO₃ with CO₂ has been addressed as one of the major challenges with BaZrO₃‐based electrolytes in protonic ceramic fuel cells. Here, we present a study of the effect of CO₂ exposure on BaZrO₃‐materials at elevated temperatures. Dense BaZr_1?xY_xO_3?x/2 (x = 0, 0.05, 0.1, 0.2) and BaCe_0.2Zr_0.7Y_0.1O_2.95 ceramics were prepared by sintering of powder prepared by spray pyrolysis. The Vickers indentation method was used to determine the hardness and estimate the fracture toughness of pristine materials as well as the corresponding materials exposed to CO₂. Formation of BaCO₃ on the surface of exposed ceramics was confirmed by X‐ray diffraction and electron microcopy. The reaction resulted in formation of Ba‐deficient perovskite at the exposed surface. The reaction with CO₂ was most pronounced at 650°C compared to the other temperatures applied in the study. The reactivity was also shown to depend on the Y‐content and the grain size and was most pronounced for BaZr_0.9Y_0.1O_2.95. The reaction with CO₂ was observed to have a profound effect on the fracture toughness of the ceramics, demonstrating a depression of the mechanical stability of the materials. The results are discussed with respect to the chemical and mechanical stability of BaZrO₃ materials, with particular emphasis on the composition and grain size.     

Doping effect of divalent cations on sintering of polycrystalline yttria

The sintering behavior of Y₂O₃ doped with 1 mol% of Ca²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Sr²⁺ or Zn²⁺ was investigated by pressureless sintering in air at a sintering temperature in the range 900–1600 °C. The sintering temperature required for full densification in Y₂O₃ was reduced by 100–400 °C by the cation doping, while undoped Y₂O₃ was densified at 1600 °C. The most effective dopant among the examined cations was Zn²⁺. The grain growth kinetics of undoped and cation-doped Y₂O₃ was described by the parabolic law. The grain boundary mobility of Y₂O₃ was accelerated by doping of the divalent cations. High-resolution transmission electron microscopy (HRTEM) observations and nano-probe X-ray energy dispersive spectroscopy (EDS) analyses confirmed that the dopant cations tended to segregate along the grain boundaries without forming amorphous layers. The improved sinterability of Y₂O₃ is probably related to the accelerated grain boundary diffusion owing to the grain boundary segregation of the dopant cations.     

Influences of Bi0.75Y0.25O1.5 addition on the microstructure and ionic conductivity of Ce0.8Y0.2O1.9 ceramics

A second phase of Y₂O₃-stabilized Bi₂O₃ (Bi_0.75Y_0.25O_1.5,YSB) is introduced into Y₂O₃-doped CeO₂ (Ce_0.8Y_0.2O_1.9,YDC) as a sintering additive and the composite ceramics of YDC-xYSB (x = 0, 5, 10, 20, 30, 40 wt%) are prepared through sintering at 1100°C for 6 h in air atmosphere. The YDC-xYSB ceramics are composed of both YDC and YSB with cubic fluorite structure, and no other impurity phases are detected in XRD patterns. The relative density of YDC-xYSB rises firstly for x ≤5 wt%, and then it declines with YSB addition from 5 to 40 wt%. The average grain size of YDC decreases from 270 nm to 85.7 nm with YSB addition from 0 to 40 wt%. The YSB phase segregates at the grain boundaries of YDC based on the TEM analysis result. The ionic conductivity of YDC-xYSB (x ≥5 wt%) is lower than that of YDC in the test temperature of 200°C–500°C, while it gradually exceeds that of YDC in 500°C–750°C. At 750°C, the conductivity of YDC-30%YSB (6.22 × 10⁻² S/cm) is 1.35 times higher than that of YDC (4.6 × 10⁻² S/cm). The YSB addition can improve the ionic conductivity of YDC in 500°C–750°C and decrease its sintering temperature.     

Characterization of one-step co-fired BaZr0.8Y0.2O3-δ-La2Ce2O7 composite electrolyte for low-temperature solid oxide fuel cells

Doped BaZrO₃ has outstanding stability and proton conductivity, and doped-ceria La₂Ce₂O₇(LCO) has mixed conductivities of free-electron, oxygen-ion and proton, which are both potential electrolyte materials. Herein, one-step-co-fired BaZr_0.8Y_0.2O_3-δ(BZY)-LCO composite electrolytes are prepared and characterized to give play to their respective advantages and avoid their shortcomings for low-temperature solid oxide fuel cells (LT-SOFCs). As for different components of BZY-LCO, the sintering activity increases with the increase of LCO content, however, the corresponding open circuit voltage decreases. Interestingly, the 30 %mol BZY-70 %mol LCO (3BZY-LCO) shows the best overall performance as functions of sintering activity, total conductivity, cell performance. As for the anode supported single cells, the maximum power density of 3BZY-LCO electrolyte can reach 0.581 W cm^?2 at 750 °C using humidified hydrogen (3% H₂O) as fuel. Preliminary experiment results suggest that the strategy of composite electrolyte can make up for the shortcomings of doped BaZrO₃ and doped-ceria to a certain extent for LT-SOFCs.     

Characterisation of BaZr0.9Y0.1O3‐δ Prepared by Three Different Synthesis Methods: Study of the Sinterability and the Conductivity

BaZr_0.9Y_0.1O_3‐δ has been synthesised by three different methods: the solid‐state reaction, the spray pyrolysis and the spray drying. Significantly different apparent lattice parameters (between 0.4192 nm for the sample prepared by the solid‐state reaction method and sintered at 1,500 °C and 0.4206 nm for the sample prepared by the solid‐state reaction method and sintered at 1,720 °C) are observed after calcination and sintering, depending on the synthesis method and the sintering temperature. The bulk conductivity values also vary over several orders of magnitude (–7.2< log σ_b <–3.6 at 300 °C) depending on the synthesis method and the sintering temperature. These variations of the bulk conductivity and also the activation energy are correlated with variations of the apparent lattice parameter. The influence of the preparation method on the electrical properties is discussed. The grain boundaries are more resistive than the bulk. The variation of the grain boundary conductivity could be correlated to the microstructure in terms of the grain size.     

Structure,mechanical properties,and thermal conductivity of BaZrO3 doped at the A-B site

BaZrO₃ with the ABO₃ perovskite structure can be doped at the A or B sites to obtain the corresponding properties. In this study, BaZrO₃ was doped with Ca²⁺, Sr²⁺, Ti⁴⁺, and Ce⁴⁺. The structure, phase composition, and mechanical and thermal properties of the composites were investigated. The sintered Ba_1-xCa_xZrO₃ samples with x = 0.2 and 0.4 exhibited relatively superior comprehensive behaviours; the cold modulus of rupture was increased by 127.36% and 134.59%, while that for BaZr_0·8Ti_0·2O₃ and BaZr_0·8Ce_0·2O₃ was increased by 99.30% and 112.37%, respectively. Further increases in strength and density were obtained in the Ba_0·8Ca_0·2Zr_0·8Ti_0·2O₃ and Ba_0·6Ca_0·4Zr_0·8Ti_0·2O₃ co-doped samples, and the corresponding apparent porosities were greatly decreased from 30.6% to below 1.0%. The thermal conductivities of the doped samples were generally low, achieving a minimum of 0.361 W/(m·K). The combination of high strength and low thermal conductivity demonstrate the significant application potential of BaZrO₃-based composite materials.     

Controlled sintering and phase transformation of yttria-doped tetragonal zirconia polycrystal material

In this paper, 3 mol% yttria-doped tetragonal zirconia polycrystal material (3 mol% Y₂O₃–ZrO₂) was prepared using an optimised pressureless sintering process. The phase change and particle size distribution of Y₂O₃–ZrO₂ during sintering were studied, and the effect of sintering temperature on the properties of Y₂O₃–ZrO₂ was analysed. The raw materials and prepared samples were analysed using XRD, Raman spectroscopy, SEM, and Gaussian mathematical fitting. The results show that sintering encourages the transformation of the monoclinic phase into the tetragonal phase, thus improving the crystallinity of the sample. The relative content of the tetragonal phase in the sample increased from 57.43% to 99.80% after sintering at 1200 °C for 1 h. In the range of sintering temperatures studied in this paper (800–1200 °C), the zirconia material sintered at 1000 °C presented the lowest porosity and the best density.     

Sintering behavior of Y-doped BaZrO3 refractory with TiO2 additive and effects of its dissolution on titanium melts

In this study, the sintering behavior of Y-doped BaZrO₃ with TiO₂ additive and effects of its dissolution on titanium melts were investigated. In order to overcome the difficulty of Y-doped BaZrO₃ sintering performance, the 0-5 wt% TiO₂ was added into Y-doped BaZrO₃ and its densification was investigated by density analyzer, scanning electron microscope (SEM) and X-ray diffraction (XRD). Thereafter, the interface reaction between two crucibles (without and with 2 wt% TiO₂) and titanium alloys, the thermodynamics and kinetics of dissolution reaction were also investigated. The results showed that Y-doped BaZrO₃ was rarely dense without TiO₂ additive and its relative density was just 88%, while after doping 2 wt% TiO₂, the relative density was more than 97%. However, with the excessive TiO₂(>2 wt%) doping, the secondary phase was observed by SEM and XRD. After melting titanium-rich alloys (Ti₂Ni, 66 mol% Ti) by Y-doped BaZrO₃ crucibles without and with 2% TiO₂ additive, the erosion layer of the two crucibles was approximately 4000 and 1700 μm, respectively. It was also found that the dissolved reaction rate was related to the density and grain size of Y-doped BaZrO₃ ceramic; the higher density and larger grain size ceramics can effectively prevent the crucible from being eroded by Ti₂Ni melt.     

Flame Spray Synthesis of BaZr0.8Y0.2O3–δ Electrolyte Nanopowders for Intermediate Temperature Proton Conducting Fuel Cells

Flame Spray Synthesis (FSS) technique has been used for the preparation of BaZr_0.8Y_0.2O_3–δ (BZY20) nanoprecursors. The nanoprecursors were composed of a perovskite phase mixed with doped Zirconia and barium nitrate. Pure phase powder could be obtained after calcining the precursors at 1,200 °C. Both nanoprecursors and pure phase powder were then sintered at 1,600 °C to obtain dense specimen. AC impedance spectroscopy performed on the sintered samples allowed correlation of the electrical properties of the samples to their microstructures. The sintered nanoprecursors compared with the sintered pure phase powders showed enhanced grain growth associated with higher grain boundary conductivity. The influence of the reactive sintering on the enhanced grain growth and electrical properties in the nanoprecursors is discussed. The high total proton conductivity measured (7.7·10⁻³ S cm⁻¹ at 450 °C) promotes FSS as an effective powder synthesis method for the preparation of BZY20 electrolyte material for proton conducting fuel cells operating in the intermediate temperature range.     

Thermodynamics and kinetics of sintering of Y2O3

Surface energy (γ_S) and grain boundary energy (γ_GB) of yttrium oxide (Y₂O₃) were determined by analyzing the heat of sintering (ΔH_sintering) using differential scanning calorimetry (DSC). The data allowed quantification of sintering driving forces, which when combined with a thorough kinetic analysis of the process, provide better understanding of Y₂O₃ densification as well as insights into effective strategies to improve its sinterability. The quantitative thermodynamic study revealed moderate thermodynamic driving force for densification in Y₂O₃ (as compared to other oxides) represented by a dihedral angle of 152.7° calculated from its surface and grain boundary energies. The activation energy was determined as 307 ± 61 kJ/mol, consistent with activation energies previously reported for processes relevant to sintering of Y₂O_3, such as Y³⁺ diffusion and grain boundary mobility. Finally, we propose that a refined deconvolution study on the DSC curve for Y₂O₃ sintering, combined with the associated material's microstructure evolution, may help identify shifts in sintering mechanisms, and therefore, specific activation energies at increasing temperatures.