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.     

Characterization of the 75% Gd0.8Sr0.2CoO3−δ/25% Ce0.9Gd0.1O2−δ composite cathode system for use in intermediate temperature solid oxide fuel cells

The composite cathode system is examined for suitability on a Ce_0.9Gd_0.1O_2−δ electrolyte based solid oxide fuel cell at intermediate temperatures (500–700 °C). The cathode is characterized for electronic conductivity and area specific charge transfer resistance. This cathode system is chosen for its excellent thermal expansion match to the electrolyte, its relatively high conductivity (115 S cm⁻¹ at 700 °C), and its low activation energy for oxygen reduction (99 kJ mol⁻¹). It is found that the decrease of sintering temperature of the composite cathode system produces a significant decrease in charge transfer resistances to as low as 0.25 Ω cm². The conductivity of the cathode systems is between 40 and 88 S cm⁻¹ for open porosities of 30–40%.     

Structural, thermal and electrochemical properties of layered perovskite SmBaCo2O5+d, a potential cathode material for intermediate-temperature solid oxide fuel cells

The synthesis, conductivity properties, area specific resistance (ASR) and thermal expansion behaviour of the layered perovskite SmBaCo₂O_5+d (SBCO) are investigated for use as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The SBCO is prepared and shows the expected orthorhombic pattern. The electrical conductivity of SBCO exhibits a metal–insulator transition at about 200 °C. The maximum conductivity is 570 S cm⁻¹ at 200 °C and its value is higher than 170 S cm⁻¹ over the whole temperature range investigated. Under variable oxygen partial pressure SBCO is found to be a p-type conductor. The ASR of a composite cathode (50 wt% SBCO and 50 wt% Ce_0.9Gd_0.1O_2−d, SBCO:50) on a Ce_0.9Gd_0.1O_2−d (CGO91) electrolyte is 0.05 Ω cm² at 700 °C. An abrupt increase in thermal expansion is observed in the vicinity of 320 °C and is ascribed to the generation of oxygen vacancies. The coefficients of thermal expansion (CTE) of SBCO is 19.7 and 20.0 × 10⁻⁶ K⁻¹ at 600 and 700 °C, respectively. By contrast, CTE values for SBCO:50 are 12.3, 12.5 and 12.7 × 10⁻⁶ K⁻¹ at 500, 600 and 700 °C, that is, very similar to the value of the CGO91 electrolyte.     

Electrophoretic deposition of dense BaCe0.9Y0.1O3−x electrolyte thick-films on Ni-based anodes for intermediate temperature solid oxide fuel cells

Proton conducting BaCe_0.9Y_0.1O_3−x (BCY10) thick films are deposited on cermet anodes made of nickel–yttrium doped barium cerate using electrophoretic deposition (EPD) technique. BCY10 powders are prepared by the citrate–nitrate auto-combustion method and the cermet anodes are prepared by the evaporation and decomposition solution and suspension method. The EPD parameters are optimized and the deposition time is varied between 1 and 5 min to obtain films with different thicknesses. The anode substrates and electrolyte films are co-sintered at 1550 °C for 2 h to obtain a dense electrolyte film keeping a suitable porosity in the anode, with a single heating treatment. The samples are characterized by field emission scanning electron microscopy (FE-SEM) and energy dispersion spectroscopy (EDS). A prototype fuel cell is prepared depositing a composite La_0.8Sr_0.2Co_0.8Fe_0.2O₃ (LSCF)–BaCe_0.9Yb_0.1O_3−δ (10YbBC) cathode on the co-sintered half cell. Fuel cell tests that are performed at 650 °C on the prototype single cells show a maximum power density of 174 mW cm⁻².     

Characterization of Pr1−xSrxCo0.8Fe0.2O3−δ (0.2 ≤ x ≤ 0.6) cathode materials for intermediate-temperature solid oxide fuel cells

Cathode materials consisting of Pr_1−xSr_xCo_0.8Fe_0.2O_3−δ (x = 0.2–0.6) were prepared by the sol–gel process for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The samples had an orthorhombic perovskite structure. The electrical conductivities were all higher than 279 S cm⁻¹. The highest conductivity, 1040 S cm⁻¹, was found at 300 °C for the composition x = 0.4. Symmetrical cathodes made of Pr_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (PSCF)–Ce_0.85Gd_0.15O_1.925 (50:50 by weight) composite powders were screen-printed on GDC electrolyte pellets. The area specific resistance value for the PSCF–GDC cathode was as low as 0.046 Ω cm² at 800 °C. The maximum power densities of a cell using the PSCF–GDC cathode were 520 mW cm⁻², 435 mW cm⁻² and 303 mW cm⁻² at 800 °C, 750 °C and 700 °C, respectively.     

An intermediate temperature solid oxide fuel cell fabricated by one step co-press-sintering

Lithiated NiO was adopted as electrodes for doped-ceria based IT-SOFCs. Li_0.3Ni_0.7O_y electrode is chemically compatible with Ce_0.8Gd_0.05Y_0.15O_1.9 (GYDC) electrolyte in air. Single cell with configuration Li_0.3Ni_0.7O|GYDC|Li_0.3Ni_0.7O was fabricated by one step dry-pressing and sintering at 1200 °C. Maximum power density of 503 mWcm⁻² at 0.5 V was achieved at 600 °C with H₂ as fuel and air as oxidant. Cell stability test was carried out at 575 °C under a constant voltage of 0.3 V and a current output of 380 mAcm⁻² was continually generated after an initial performance decrease. A stable anode is required for symmetrical IT-SOFCs.     

Characterization of a circular 80 mm anode supported solid oxide fuel cell (AS-SOFC) with anode support produced using high-pressure injection molding (HPIM)

The current study was oriented at analyzing the performance of an anode-supported solid oxide fuel cell produced using high-pressure injection molding. The cell with a total thickness of 550 μm was produced in the Ceramic Department (CEREL) of the Institute of Power Engineering in Poland and experimentally analyzed in the Energy Department (DENERG) of Politecnico di Torino in Italy. The high-pressure injection molding technique was applied to produce a 500 μm thick anode support NiO/8YSZ 66/34 wt% with porosity of 25 vol%. The screen printing method was used to print a 3 μm thick NiO anode contact layer, 7 μm thick NiO/8YSZ 50/50 wt% anode functional layer, 4 μm thick 8YSZ dense electrolyte, 1.5 μm thick Gd_0,1Ce_0,9O₂ barrier layer and a 30 μm thick La_0,6Sr_0,4Fe_0,8Co_0,2O_3–δ cathode with porosity 25 vol%.The experimental characterization was done at two temperature levels: 750 and 800 °C under fixed anodic and cathodic flow and compositions. The preliminary studies on the application of high-pressure injection molding are discussed together with the advantages of the technology. The performance of two generations of anode-supported cells is compared with data of reference cells with supports obtained using tape casting.     

Ionic conduction mechanism of a nanostructured BCY electrolyte for low-temperature SOFC

A solid oxide fuel cell with 1.2 mm thick nanocrystalline BCY (BaCe_0.9Y_0.1O₃) electrolyte was prepared by the copressing method, and its electrochemical performance was tested in H₂/air conditions. The maximum power density of the nanocrystalline electrolyte cell prepared with BCY powders calcined at 1000 °C and operated at 550 °C in H₂ is 405.2 mW cm⁻². The carriers of nanocrystalline BCY electrolyte were studied by using double-layer electrolyte cells with pure oxygen ion conductor GDC (Gd_0.1Ce_0.9O₂) and proton conductor SCY (SrCe_0.95Y_0.05O₃), sintered at high temperatures as proton and oxygen ion filters. It is found that nanocrystalline BCY is a mixed conductor of oxygen ions and protons. The results of hydrogen and oxygen concentration cells show that the transference number of oxygen ions in the nanocrystalline BCY prepared in this study is higher than that of protons. XPS and HRTEM results of BCY particles before and after performance testing showed that an amorphous layer containing a high concentration of oxygen vacancies and suspected carbonate was formed on the surface of BCY particles after fuel cell performance testing. We believe that the amorphous layer should be a high-speed ion conduction channel for nanocrystalline BCY electrolyte during performance testing.     

High performance of proton-conducting solid oxide fuel cell with a layered PrBaCo2O5+δ cathode

A dense BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) electrolyte is fabricated on a porous anode by in situ drop-coating method which can lead to extremely thin electrolyte membrane (10 μm in thickness). The layered perovskite structure oxide PrBaCo₂O_5+δ (PBCO) is synthesized by auto ignition process and initially examined as a cathode for proton-conducting IT-SOFCs. The electrical conductivity of PrBaCo₂O_5+δ (PBCO) reaches the general required value for the electrical conductivity of cathode absolutely. The single cell, consisting of PrBaCo₂O_5+δ (PBCO)/BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY)/NiO-BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) structure, is assembled and tested from 600 to 700 °C with humidified hydrogen (3% H₂O) as the fuel and air as the oxidant. An open-circuit potential of 1.01 V and a maximum power density of 545 mW cm⁻² at 700 °C are obtained for the single cell, and a low polarization resistance of the electrodes of 0.15 Ω cm² is achieved at 700 °C.     

Novel cobalt-free cathode materials BaCexFe1−xO3−δ for proton-conducting solid oxide fuel cells

A series of cobalt-free and low cost BaCe_xFe_1−xO_3−δ (x = 0.15, 0.50, 0.85) materials are successful synthesized and used as the cathode materials for proton-conducting solid oxide fuel cells (SOFCs). The single cell, consisting of a BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7)-NiO anode substrate, a BZCY7 anode functional layer, a BZCY7 electrolyte membrane and a BaCe_xFe_1−xO_3−δ cathode layer, is assembled and tested from 600 to 700 °C with humidified hydrogen (3% H₂O) as the fuel and the static air as the oxidant. Within all the cathode materials above, the cathode BaCe_0.5Fe_0.5O_3−δ shows the highest cell performance which could obtain an open-circuit potential of 0.99 V and a maximum power density of 395 mW cm⁻² at 700 °C. The results indicate that the Fe-doped barium cerates can be promising cathodes for proton-conducting SOFCs.     

Doping strategies towards acceptor-doped barium zirconate compatible with nickel oxide anode substrate subjected to high temperature co-sintering

Protonic ceramic fuel cells (PCFCs) are attractive next-generation fuel cells working at intermediate temperature (450–700 °C). Currently, the most common method to produce PCFCs is based on co-sintering anode-supported half cells composed of thin BaZr_0.8Y_0.2O_3-δ (BZY20) electrolyte layers and thick NiO anode substrates. However, the BZY20 electrolyte reacts with NiO at the high temperature during co-sintering (1400–1600 °C), leading to significant loss of dopants in the electrolyte and thereby degradation in the conductivity. With the aim to improve the chemical compatibility between the BaZrO₃-based electrolyte and NiO, in this work, we annealed the mixture of NiO and BaZrO₃ doped with different dopants, and found that good compatibility was obtained between NiO and BaZr_0.8Yb_0.2O_3-δ or BaZr_0.8Tm_0.2O_3-δ at 1500 °C, and NiO and BaZr_0.8Yb_0.2O_3-δ at 1350 °C. Furthermore, BaZr_0.8Y_0.1Yb_0.1O_3-δ and BaZr_0.8Y_0.12Yb_0.08O_3-δ also showed good chemical compatibility with NiO at 1350 and 1500 °C. Then, the half cells using the BaZrO₃ electrolyte doped with Y and Yb were fabricated, and the ionic conductivities of the electrolytes implemented into the co-sintered half cells were measured directly in wet hydrogen. The results revealed that BaZr_0.8Y_xYb_0.2-xO_3-δ (x = 0.10, 0.12, 0.13) showed higher conductivity than that of BaZr_0.8Y_0.2O_3-δ, and BaZr_0.8Y_0.1Yb_0.1O_3-δ exhibited the highest conductivity among the three Y and Yb co-doped samples studied in this work.     

ZnO-doped BaZr0.85Y0.15O3−δ proton-conducting electrolytes: Characterization and fabrication of thin films

ZnO-doped BaZr_0.85Y_0.15O_3−δ perovskite oxide sintered at 1500 °C has bulk conductivity of the order of 10⁻² S cm⁻¹ above 650 °C, which makes it an attractive proton-conducting electrolyte for intermediate-temperature solid oxide fuel cells. The structure, morphology and electrical conductivity of the electrolyte vary with sintering temperature. Optimal electrochemical performance is achieved when the sintering temperature is about 1500 °C. Cathode-supported electrolyte assemblies were prepared using spin coating technique. Thin film electrolytes were shown to be dense using SEM and EDX analyses.     

Stable, easily sintered BaCe0.5Zr0.3Y0.16Zn0.04O3−δ electrolyte-based protonic ceramic membrane fuel cells with Ba0.5Sr0.5Zn0.2Fe0.8O3−δ perovskite cathode

A stable, easily sintered perovskite oxide BaCe_0.5Zr_0.3Y_0.16Zn_0.04O_3−δ (BCZYZn) as an electrolyte for protonic ceramic membrane fuel cells (PCMFCs) with Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3−δ (BSZF) perovskite cathode was investigated. The BCZYZn perovskite electrolyte synthesized by a modified Pechini method exhibited higher sinterability and reached 97.4% relative density at 1200 °C for 5 h in air, which is about 200 °C lower than that without Zn dopant. By fabricating thin membrane BCZYZn electrolyte (about 30 μm in thickness) on NiO–BCZYZn anode support, PCMFCs were assembled and tested by selecting stable BSZF perovskite cathode. An open-circuit potential of 1.00 V, a maximum power density of 236 mW cm⁻², and a low polarization resistance of the electrodes of 0.17 Ω cm² were achieved at 700 °C. This investigation indicated that proton conducting electrolyte BCZYZn with BSZF perovskite cathode is a promising material system for the next generation solid oxide fuel cells.     

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).     

Low-temperature preparation of dense (Gd,Ce)O2−δ–Gd2O3 composite buffer layer by aerosol deposition for YSZ electrolyte-based SOFC

The (Gd_0.1Ce_0.9)O_2−δ (GDC)–Gd₂O₃ composite buffer layer was fabricated on yttria stabilized zirconia (YSZ) electrolyte by aerosol deposition for usage as diffusion barrier layer between YSZ and (La_0.6Sr_0.4)(Co_0.2Fe_0.8)O_3−δ (LSCF)–GDC composite cathode. The deposited composite buffer layer was quite dense in nature and effectively prevented the formation of SrZrO₃ and La₂Zr₂O₇ interlayer with low conductivity at the interfaces. The cell's I–V performance was enhanced with an increase in the GDC content in the composite buffer layer. The cell containing composite buffer layer showed maximum power density of up to 1.74 W/cm² at 750 °C, which was ∼30% higher than that of the cell containing GDC buffer layer prepared using conventional process.     

Intermediate temperature single-chamber methane fed SOFC based on Gd doped ceria electrolyte and La0.5Sr0.5CoO3−δ as cathode

Single-chamber fuel cells with electrodes supported on an electrolyte of gadolinium doped ceria Ce_1−xGd_xO_2−y with x = 0.2 (CGO) 200 μm thickness has been successfully prepared and characterized. The cells were fed directly with a mixture of methane and air. Doped ceria electrolyte supports were prepared from powders obtained by the acetyl-acetonate sol–gel related method. Inks prepared from mixtures of precursor powders of NiO and CGO with different particle sizes and compositions were prepared, analysed and used to obtain optimal porous anodes thick films. Cathodes based on La_0.5Sr_0.5CoO₃ perovskites (LSCO) were also prepared and deposited on the other side of the electrolyte by inks prepared with a mixture of powders of LSCO, CGO and AgO obtained also by sol–gel related techniques. Both electrodes were deposited by dip coating at different thicknesses (20–30 μm) using a commercial resin where the electrode powders were dispersed. Finally, electrical properties were determined in a single-chamber reactor where methane, as fuel, was mixed with synthetic air below the direct combustion limit. Stable density currents were obtained in these experimental conditions. Temperature, composition and flux rate values of the carrier gas were determinants for the optimization of the electrical properties of the fuel cells.     

Effect of CaO concentration on enhancement of grain-boundary conduction in gadolinia-doped ceria

This study examines the effect of calcium oxide (CaO) addition to Ce_0.9Gd_0.1O_1.95 (gadolinia-doped ceria, GDC) containing 500 ppm SiO₂ on grain-interior and grain-boundary conduction. The GDC can be used as a solid electrolyte for intermediate and low-temperature solid oxide fuel cells. Doping with ≥2 mol% CaO results in a decrease in apparent grain-boundary resistivity at 300 °C from 746.7 kΩ cm to 2.8–3.5 kΩ cm. The total resistivity exhibits a minimum at 2 mol% CaO. Further increase in CaO concentration to 10 mol% results in an increase in grain-interior resistivity from 3.1 to 40 kΩ cm. Although most of the CaO is incorporated into the GDC lattice, a small amount of CaO scavenges the intergranular siliceous phase, which leads to a significant increase in grain-boundary conduction. The increase in grain-interior resistivity at high CaO concentration is attributed to defect association between V_O and Ca_Ce″.     

Fabrication of cathode supported solid oxide fuel cell by multi-layer tape casting and co-firing method

La_0.3Sr_0.7FeO_3-δ (LSF)/CeO₂ cathode supported Ce_0.8Sm_0.2O_2-δ (SDC) electrolyte was prepared by a simple multilayer tape casting and co-firing method. SDC electrolyte slurry and LSF/CeO₂ cathode slurry were optimized and the green bi-layer tapes were co-fired at different temperature. Phase characterizations and microstructures of electrolyte and cathode were studied by X-ray diffraction (XRD) and Scan Electronic Microscopy (SEM). No additional phase peak line was observed in electrolyte and cathode support when the sintering temperature lower was than 1400 °C. The electrolytes were extremely dense with the thickness of about 20 μm. The cathode support was porous with electrical conductivity of about 4.21 S/cm at 750 °C. With Ni/SDC as anode, Open Current Voltage and maximum power density reached 0.61 V and 233 mW cm⁻² at 750 °C, respectively.     

Electrochemical properties of micro-tubular intermediate temperature solid oxide fuel cell with novel asymmetric structure based on BaZr0.1Ce0.7Y0.1Yb0.1O3−δ proton conducting electrolyte

This study employed a simple phase-inversion method to achieve anode-supported micro-tubular solid oxide fuel cells on the basis of the BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ proton conducting electrolyte. The typical cell with configuration of Ni–BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ|BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ|La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ-Sm_0.2Ce_0.8O_2-δ. The novel “sponge-like micro-pores electrode | homogeneous porous functional layer” asymmetric pore structure is obtained. Achieved results include: i) the electrodes revealed the single phase collected by the powder X-Ray Diffractometer analysis; ii) observed by Scanning Electron Microscope, the single cell presenting uniform distribution of micro sponge-like pores electrode was well-adhered to the dense and crack-free 12 μm thick electrolyte layer; iii) the cells showed excellent electrochemical performance with the maximum power densities of 1.070, 0.976, 0.815, and 0.700 W cm⁻² at 750, 700, 650 and 600 °C, respectively, characterized by Electrochemical Impedance Spectroscopy; iv) the designed cell clearly indicated a very low concentration polarization value (0.01 and 0.02 Ω cm² at 750 and 700 °C). Our findings provide a promising approach to improve intermediate temperature solid oxide fuel cells performance by optimizing the electrode-electrolyte interface microstructure, based on proton and oxide ion mixed conductor electrolytes.     

In situ screen-printed BaZr0.1Ce0.7Y0.2O3−δ electrolyte-based protonic ceramic membrane fuel cells with layered SmBaCo2O5+x cathode

In order to develop a simple and cost-effective route to fabricate protonic ceramic membrane fuel cells (PCMFCs) with layered SmBaCo₂O_5+x (SBCO) cathode, a dense BaZr_0.1Ce_0.7Y_0.2O_3?δ (BZCY) electrolyte was fabricated on a porous anode by in situ screen printing. The porous NiO–BaZr_0.1Ce_0.7Y_0.2O_3?δ (NiO–BZCY) anode was directly prepared from metal oxide (NiO, BaCO₃, ZrO₂, CeO₂ and Y₂O₃) by a simple gel-casting process. An ink of metal oxide (BaCO₃, ZrO₂, CeO₂ and Y₂O₃) powders was then employed to deposit BaZr_0.1Ce_0.7Y_0.2O_3?δ (BZCY) thin layer by an in situ reaction-sintering screen printing process on NiO–BZCY anode. The bi-layer with 25 μm dense BZCY electrolyte was obtained by co-sintering at 1400 °C for 5 h. With layered SBCO cathode synthesized by gel-casting on the bi-layer, single cells were assembled and tested with H₂ as fuel and the static air as oxidant. A high open-circuit potential of 1.01 V, a maximum power density of 382 mW cm^?2, and a low polarization resistance of the electrodes of 0.15 Ω cm² was achieved at 700 °C.