Microwave sintering of high-performance BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) electrolytes for intermediate-temperature solid oxide fuel cells

As a promising electrolyte material for solid oxide fuel cells (SOFCs), BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ (BZCYYb) often surfers from its high sintering temperature, which causes Ba evaporation and sluggish grain growth, thus reducing the electrical conductivity. In this work, densified BZCYYb electrolytes were fabricated at temperatures as low as 1400 °C using the microwave sintering technique. Comparing with the conventional sintered ones, a temperature decrease of 150 °C is achieved. The Ba evaporation is effectively suppressed, and large grain sizes of ～4 μm are obtained. The total conductivity for microwave sintered symmetric cell measured in wet air at 700 °C is 3.8 × 10^?2 S cm^?1, benefiting from both enhanced bulk conductivities by 1–2 times and grain boundary conductivities by 50 times. With the microwave sintered BZCYYb as electrolyte, an anode-supported cell reaches a maximum power density of 0.64 W cm^?2 at 700 °C.     

An easily controllable flash sintering process for densification of electrolyte for application in solid oxide fuel cells

A step-wise current-limiting flash sintering process (SCFS process) was proposed to densify La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ (LSGM) electrolyte. Well-densified microstructures of LSGM samples were obtained at 690 °C under an electric field of 100 V cm⁻¹. Compared with the current-limiting flash sintering process, the spike in power dissipation was avoided, the shrinkage rate of LSGM samples was moderate, and the microstructures of LSGM samples were uniform. The conductivity of LSGM samples sintered via SCFS process at 0.9 A for 30 min reached 0.072 S cm⁻¹ at 850 °C, and this was approximately the same as the value of conventionally sintered LSGM samples at 1400 °C for 10 h. These results proved that SCFS process achieved easy controllability, and it is expected that SCFS process can be applied in industry.     

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.     

The effect of Fe2O3 sintering aid on Gd0.1Ce0.9O1.95 diffusion barrier layer and solid oxide fuel cell performance

Solid oxide fuel cells (SOFCs) with the Gd_0.1Ce_0.9O_1.95 (GDC) diffusion barrier layer require the densification of GDC to improve the performance of the cells. In this work, the addition of 0.5 mol% Fe₂O₃ in GDC diffusion barrier layer as sintering aid is studied. The symmetrical cell and the fuel cell are fabricated by hot-pressing, co-sintering and screen-printing technologies. It is found that the addition of Fe₂O₃ can make GDC barrier layer denser at a reduced sintering temperature of 1250 °C and prevent diffusion of Sr to form ionic insulating interface between YSZ (Y₂O₃ stabilized ZrO₂) and La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF). The fuel cell based on the GDC-Fe₂O₃ barrier layer achieves better maximum power density of 0.89 W cm⁻² at 800 °C than that of 0.68 W cm⁻² without Fe₂O₃ addition. No obvious degradation was observed on fuel cell based on the GDC-Fe₂O₃ diffusion barrier layer after a stability test at 750 °C for 100 h under 0.75 V and the thermal cycling between 750 and 400 °C. The results indicate that the addition of Fe₂O₃ sintering aid in GDC diffusion barrier layer can promote the densification of GDC and exhibit good long-term stability and thermal cycle stability.     

Sintering behavior of BaCe0.7Zr0.1Y0.2O3-δ electrolyte at 1150 °C with the utilization of CuO and Bi2O3 as sintering aids and its electrical performance

BaCe_0.7Zr_0.1Y_0.2O_3-δ (BCZY) is one of the promising electrolytic candidate for solid oxide fuel cell (SOFC) due to its good proton conductivity and better stability. Herein, the effect of dual sintering aids such as CuO-Bi₂O₃ upon the sinterability at low temperature, improved electrochemical properties, and thermo-chemical changes about proton-conducting BaCe_0.7Zr_0.1Y_0.2O_3-δ electrolyte were investigated in detail. FESEM micrographs and shrinkage curves revealed significant improvement in sinterability and densifications of BCZY electrolyte. The dense pellets were sintered with CuO-Bi₂O₃ (2–3 mol %) as sintering aids at a temperature of 1150 °C for 5 h. The perfectly uniform distribution of sintering aids increased the linear shrinkage of BCZY from 5% till 19–21%. The crystallite size and grain growth within the structure was enhanced due to the formation of the melting phase of Bi₂O₃ and Cu²⁺ incorporation in the perovskite structure. The elevated and improved electrochemical measurement for BCZY with 2 mol% of CuO-Bi₂O₃ as sintering aid categorized it well suited for solid oxide fuel cells.     

Enhancing sinterability and electrochemical properties of Ba(Zr0.1Ce0.7Y0.2)O3-δ proton conducting electrolyte for solid oxide fuel cells by addition of NiO

The influence of NiO on the sintering behavior and electrical properties of proton conducting Ba(Zr_0.1Ce_0.7Y_0.2)O_3-δ (BZCY7) as an electrolyte supporter for solid oxide fuel cells is systematically investigated. SEM images and shrinkage curve demonstrate that the sinterability of the electrolyte pellets is dramatically improved by doping NiO as a sintering aid. The sintering aid amount and sintering temperature are optimized by analyzing the linear shrinkage, grain size and morphology for a series of sintered BZCY7 electrolyte pellets. Almost full dense electrolyte pellets are successfully formed by using 0.5–1.0 wt% NiO loading after sintering at 1400 °C for 6 h. The linear shrinkage of 0.5 wt% NiO modified BZCY7 sample is about 14.25% higher than that without NiO addition (4.81%). Energy dispersive X-ray spectroscopy analysis indicate that partial NiO might dissolve into the perovskite lattice structure and the other NiO react with BZCY7 to form BaY₂NiO₅ secondary phase as a sintering aid. Excessive NiO is especially detrimental to the electrical properties of BZCY7 and thus lower the open circuit voltage. The electrochemical performance for a series of single cells with different concentration NiO modified BZCY7 electrolyte are measured and analyzed. The optimized composition of 0.5 wt% NiO modified BZCY7 as an electrolyte support for solid oxide fuel cell demonstrates a high electrochemical performance.     

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.     

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.     

Highly conductive proton-conducting electrolyte with a low sintering temperature for electrochemical ammonia synthesis

BaCe_0·7Zr_0.1Gd_0.2O_3-δ (BCZG) powder is synthesized by a citrate sol-gel method, and different amounts of Li₂CO₃ are introduced to lower the sintering temperature. The densification temperature of BCZG ceramic is decreased drastically to 1250 °C by using Li₂CO₃ as sintering aid. BCZG with 2.5 wt% of Li₂CO₃ (BCZG-2.5L) can not only remarkably promote the sintering process of BCZG but also enhance its electrical conductivity. The total ionic conductivity of BCZG-2.5L attains to 1.9 × 10⁻² S cm⁻¹ at 600 °C in a wet H₂ atmosphere. Ammonia synthesis at atmospheric pressure is conducted on (2K, 10Fe)/Ni-BCZG | BCZG-2.5L | Ni-BCZG electrolytic cell with an applied voltage of 0.2–1.6 V at a temperature of 450–600 °C. The highest NH₃ formation rate of 1.87 × 10⁻¹⁰ mol s⁻¹ cm⁻² and the highest current efficiency of 0.53% is achieved at 500 °C with an applied voltage of 0.8 V.     

Fabrication of anode-supported thin BCZY electrolyte protonic fuel cells using NiO sintering aid

A thin and fully dense BaCe_0.6Zr_0.2Y_0.2O_3-δ (BCZY) electrolyte for the use of anode-supported protonic fuel cells has been successfully prepared by spin coating using NiO sintering aid. The effects of NiO addition on the electrolyte microstructures and fuel cell performances are also investigated. An appropriate NiO addition has a significant positive contribution to the densification and grain growth of thin BCZY electrolytes. However, too much NiO addition gives rise to NiO aggregation in BCZY electrolyte and deteriorates the cell performance. The enhanced sintering mechanism can be mainly attributed to the oxygen vacancies generated from the NiO decomposition and bulk diffusion of Ni into BCZY perovskites. The fuel cell with a BCZY-3%NiO electrolyte exhibits the highest maximum power density of ~106.6 mW/cm² at 800 °C among all fuel cells in this study. The electrochemical impedance characteristics of thin BCZY electrolyte fuel cells are further discussed under open circuit conditions.     

Low-temperature sintering and electrical properties of strontium- and magnesium-doped lanthanum gallate with V2O5 additive

The effects of a V₂O₅ additive on the low-temperature sintering and ionic conductivity of strontium- and magnesium-doped lanthanum gallate (LSGM: La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8) are studied. The LSGM powders prepared by the glycine nitrate method are mixed with 0.5-2 at.% of VO_5/2 and then sintered at 1100-1400 °C in air for 4 h. The apparent density and phase purity of the LSGM specimens are increased with increasing sintering temperature and VO_5/2 concentration due to the enhanced sintering and mass transfer via the intergranular liquid phase. The 1 at.% VO_5/2-doped LSGM specimen sintered at 1300 °C exhibits a high oxide ion conductivity of ∼0.027 S cm⁻¹ at 700 °C over a wide range of oxygen partial pressure (PO₂=10⁻²⁷−1 atm), thereby demonstrating its potential as a useful electrolyte for anode-supported solid oxide fuel cells (SOFCs) without the requirement for any buffer layer between the electrolyte and anode.     

Improved polarization behaviour of nanostructured La0.65Sr0.3MnO3 cathode with engineered morphology

We report a simple synthesis of La_0.65Sr_0.3MnO₃ nanorods (LSM-R) through hydrothermal reaction followed by calcination at high temperature (700–850 °C). Thermogravimetric analysis and XRD study reveals that 850 °C is adequate for phase pure LSM-R formation. The microstructure of the powder has been clinically studied using FESEM and TEM. It is observed that the intermediate hydrothermal treatment plays key role in formation of such nanorods. Different bulk properties of LSM-R like sinteractivity, CTE, density, electrical conductivity etc. have been comprehensively studied. A maximum electrical conductivity of around 250 S/cm at 800 °C is obtained when LSM-R specimen is sintered at 1200 °C/2 h. The cathodic polarization of such LSM-R is measured using impedance analysis. It is observed that polarization value initially decreases attains minimum and then starts to increase with increase in cathode sintering temperature and dwelling time. A minimum cathodic polarization value of 0.32 Ω/cm² at 800 °C is obtained at an optimized cathode sintering condition of 1000 °C/2 h.     

High-conductivity electrolyte with a low sintering temperature for solid oxide fuel cells

Bismuth oxide and scandia co-doped zirconia (Sc₂O₃)_0.06(Bi₂O₃)_x(ZrO₂)_0.94–x (ScSZB, x = 0, 0.01, 0.03, 0.05, 0.07, 0.1) powders are prepared via a citrate sol-gel method. Bi₂O₃ promotes the sintering process of scandia stabilized zirconia (ScSZ) and increases electrical conductivity of system. A high conductivity of ～0.094 S/cm at 800 °C is achieved on 5 mol% Bi₂O₃ doped ScSZ (ScSZB05). X-ray Rietveld refinement and transmission electron microscope (TEM) analysis of the ScSZB05 reveal the formation of cubic phase and rhombohedral phase at room temperature. The electrolyte-supported cell constructed by the ScSZ electrolyte gives the maximum power density of 258.3 mW/cm² at 800 °C, while the cell with ScSZB05 electrolyte shows a higher value of 387.6 mW/cm². The performance obtained by theoretical simulation of the two electrolyte-supported cells is in good agreement with the experimental results.     

Fabrication of electrolyte supported micro-tubular SOFCs using extrusion and dip-coating

Dense electrolyte supported micro tubular solid oxide fuel cells (T-SOFCs) were prepared in this study. Green Zr_0.8Sc_0.2O_2 − δ (ScSZ) electrolyte micro-tubes with a thickness of 300 μm were successfully prepared by extrusion at room temperature. After firing at 1400 °C, the bare electrolyte micro-tube with a thickness of 210 μm, a diameter of 3.8 mm, and a length of 40 mm reached a relative density of 96.84% and a flexural strength of 202 MPa. To achieve a better sintering shrinkage match, the electrolyte micro-tubes were pre-sintered at 1100 °C, coated with NiO/ScSZ (60 vol.%:40 vol.%) in the inner surface of the micro-tubes by dip-coating method, and then co-fired at 1400 °C. After co-sintering, the interface of the porous anode layer and the dense electrolyte layer demonstrated good adhesion and mechanical integrity. Subsequently, a La_0.8Sr_0.2MnO_3 − δ (LSM)/Ce_0.8Gd_0.2O_2 − δ (GDC) (80 vol.%:20 vol.%) cathode layer was coated on the outer surface of the micro-tubes by dip-coating method and post-sintered at 1100 °C. The thicknesses of the anode and cathode layers read approximately 28 and 34 μm respectively. After thermal cycling between 30 °C and 800 °C under 97% N₂–3%H₂ on the anode and air on the cathode, the micro T-SOFCs showed no delamination and retained good adhesion based on the SEM results. The maximum power densities (MPD) of the single cell read 0.26 and 0.23 W cm⁻² respectively at 920 and 900 °C, and the open circuit voltage (OCV) reported the same 1.08 V.     

Thermogravimetric analysis of coking during dry reforming of methane

Nickel-based catalysts used for dry reforming of methane (DRM) suffer from coking and sintering, which hinders the broad application of the process in the industry. Thermogravimetric analysis was employed to investigate coking on a commercial nickel catalyst with an anti-coking additive (CaO). It was found that the catalyst sintered at temperatures between 850 and 900 °C, which resulted in permanent catalyst deactivation. For the tested Ni/CaO–Al₂O₃ catalyst, the coking and carbon gasification rates are equal at the temperatures of 796–860 °C, depending on the heating rate (5–20 K/min). Significant differences in the temperatures related to the maxima on TG curves for various heating rates follow from DRM kinetics. This work reveals that the coking rate is lower at higher temperatures. After 50 min, the weight gains amount to about 20% and 40% at 800 °C and 600 °C, respectively. Lower sample weight gains were observed at higher temperatures for a methane decomposition reaction over the Ni/CaO catalyst, unlike for the second tested catalyst – activated carbon. For the nickel catalyst, the reaction order for methane decomposition is 0.6 in the temperature range 640–800 °C, while the sign of the activation energy changes at 700 °C. The elaborated kinetic equation predicts the initial CH₄ decomposition rate with 15% accuracy.     

Phase evolution and reactivity of Pr2NiO4+δ and Ce0.9Gd0.1O2-δ composites under solid oxide cell sintering and operation temperatures

In developing a new compositae air electrode for Solid Oxide Cells (SOCs) it is essential to fully understand the phase chemistry of all components. Ruddlesden-Popper type electrodes such as Pr₂NiO_4+δ have previously been proposed as attractive alternatives to conventional La_0·6Sr_0·4Fe_0·8Co_0·2O_3-δ/Ce_1-xGd_xO_2-δ compositae air electrodes for both fuel cell and electrolyser modes of operation. However, Pr₂NiO_4+δ have been shown to have limited stability, reacting with a Ce_1-xGd_xO_2-δ interlayer to form a Ce_1-x-yGd_xPr_yO_2-δ (CGPO) phase of unknown stoichiometry. Additionally, Pr₂NiO_4+δ are known to decompose to Pr₄Ni₃O_10 ± δ under certain conditions.In this work detailed understanding of the chemical reaction between Pr₂NiO_4+δ and Ce_0.9Gd_0.1O_2-δ (CGO10) under normal solid oxide cell fabrication and operating temperatures was obtained, identifying the composition of the resulting CGPO phase reaction products. It is shown that, in addition to the unreacted CGO10 present after sintering the compositae at 1100 °C for up to 12 h, a series of CGPO chemical compositions were formed with various Ce, Gd and Pr ratios depending on the relative distance of the doped ceria phases from the Pr₂NiO_4+δ phases. The extent of the chemical reaction was found to depend on the sintering time and the contact area of the two phases. Further thermal treatment of the resulting products under SOC air electrode operating temperature (800 °C) resulted in the initiation of Pr₂NiO_4+δ decomposition, forming Pr₄Ni₃O_10 ± δ and Pr₆O₁₁ with no detectable change in the composition of previously formed Pr-substituted ceria phases. It is apparent that the Pr₂NiO_4+δ/CGO10 compositae is unsuitable as an air electrode, but there is evidence that the decomposition products, Pr₄Ni₃O_10 ± δ and Ce_1-x-yGd_xPr_yO_2-δ are stable and suitable candidates for SOC electrodes.     

Investigation on properties of BaZr0.6Hf0.2Y0.2O3-δ with sintering aids (ZnO,NiO, Li2O) and its application for hydrogen permeation

BaZr_0.8Y_0.2O_3-δ proton conductor has the characteristics of excellent chemical stability, but its impoverished sinterability and low conductivity hinder its applications in fuel cell and hydrogen separation. Hf doping in Zr site improves BaZr_0.6Hf_0.2Y_0.2O_3-δ sinterability and conductivity. To further enhance BaZr_0.6Hf_0.2Y_0.2O_3-δ properties, three kinds of sintering aids ZnO, NiO or Li₂O were introduced and their effect on the sinterability, microstructure and conductivity of BaZr_0.6Hf_0.2Y_0.2O_3-δ were studied. The experimental results display that 4 mol% ZnO can enhance the sinterability and conductivity of BaZr_0.6Hf_0.2Y_0.2O_3-δ sample sintered at 1400 °C. Compared with BaZr_0.6Hf_0.2Y_0.2O_3-δ sintered at 1600 °C, BaZr_0.6Hf_0.2Y_0.2O_3-δ with 4 mol% ZnO is of larger grain size, higher relative density (95.5%) and lower sintering temperature (reducing by 200 °C). Meanwhile, the conductivity of BaZr_0.6Hf_0.2Y_0.2O_3-δ with 4 mol% ZnO reaches 4.17 × 10^?3 S cm^?1 in wet 5% H₂/Ar at 700 °C, due to the reduction of the grain boundary resistance of sample. BaZr_0.6Hf_0.2Y_0.2O_3-δ with 4 mol% ZnO membrane for hydrogen separation via external short circuit was developed. The membrane with a thickness of 1.08 mm gives a hydrogen permeation flux of 0.098 mL min^?1cm^?2 at 800 °C with 50% H₂/He as feed gas. The presence of water vapor significantly promotes the hydrogen permeability of the membrane. In addition, introduction of 3% CO₂ or 100 ppm H₂S into feed gas does not decrease the hydrogen permeation flux of the membrane.     

Solar sintering of alumina ceramics: Microstructural development

Alumina powders were lab-synthesized and then sintered on a solar furnace (SF) in order to test the capability of these solar devices to produce dense ceramic bodies. The special configuration of the SF at Plataforma Solar de Almería (PSA-CIEMAT) in Spain, allowed to perform several experiments using high temperatures (up to 1780 °C), fast heating rates (50 and 100 °C min^?1) and different atmospheres (air, Ar and 95N₂:5H₂). For comparison, similar alumina samples were sintered in an electric furnace (EF) using standard conditions (5 °C min^?1 at 1600 °C during 240 min in air). An exhaustive microstructural characterization by scanning (SEM) and transmission (TEM) electron microscopies were performed on the sintered materials. Results for SF-samples showed a well-sintered alumina matrix of polyhedral grains even using shorter dwell times and higher heat-up rates than the conventional sintering. Obtained microstructures are in agreement with the presence of some impurities (mainly SiO₂, CaO, ZrO₂ and MgO) which are distributed at grain boundaries, triple points and matrix voids. For solar treatments, the variations of sintering parameters produced significant changes on matrix grain size, porosity and distribution of second phases. An important grain growth and density increase was observed after solar sintering on those tests performed at 1780 °C and under N₂:H₂ sintering atmosphere. The gathered data point out once more the convenience of SFs as sintering reactors to obtain ceramic materials with improved grain sizes.     

Cold sintering-assisted densification of GDC electrolytes for SOFC applications

Cold sintering is a novel technique that promotes the densification of ceramics at a much reduced temperature as compared to conventionally sintered ones. With the aid of this technique, gas-tight GDC electrolytes were sintered up to 96% of their theoretical densities at 1100 °C, a remarkable 300 °C below the conventional sintering temperature. It is one of the rare cases in which the cold sintering process was carried out at room temperature. The activation energy for electrical conduction and OCV values at 600 and 800 °C were found to be 0.69 eV and 0.97 and 0.88 V, respectively. The achieved OCV values were slightly higher than the ones produced by the conventional sintering densification process.