Ionic conductivity of Sm,Gd, Dy and Er-doped ceria

Ceria-based materials are prospective electrolytes for intermediate-temperature solid oxide fuel cells. The ionic conductivities of ceria-doped with Sm, Gd, Dy and Er are investigated as a function of temperature by using a.c. impedance. The results show that conductivity depends on the rare earth dopant, its amount, an appearance of second phase, and the microstructure. With 10 mol% dopant, Sm exhibits higher conductivity than Gd, Dy and Er, respectively. With an increase in Dy content, the total conductivity increases, which is attributed to an increase in grain boundary conductivity. By contrast, an increasing amount of Er from 10 to 20 mol% reduces the conductivity of ceria and results in a separated phase of Er₂O₃ as detected by X-ray diffraction and scanning electron microscopy. In addition, the grain size corresponding to grain boundary density affects the conductivity due to the contributions from the grain interior and grain boundary conductivities.     

Performance of anode-supported solid oxide fuel cell using novel ceria electrolyte

The potential of a novel co-doped ceria material Sm_0.075Nd_0.075Ce_0.85O_2−δ as an electrolyte was investigated under fuel cell operating conditions. Conventional colloidal processing was used to deposit a dense layer of Sm_0.075Nd_0.075Ce_0.85O_2−δ (thickness 10 μm) over a porous Ni-gadolinia doped ceria anode. The current-voltage performance of the cell was measured at intermediate temperatures with 90 cm³ min⁻¹ of air and wet hydrogen flowing on cathode and anode sides, respectively. At 650 °C, the maximum power density of the cell reached an exceptionally high value of 1.43 W cm⁻², with an area specific resistance of 0.105 Ω cm². Impedance measurements show that the power density decrease with decrease in temperature is mainly due to the increase in electrode resistance. The results confirm that Sm_0.075Nd_0.075Ce_0.85O_2−δ is a promising alternative electrolyte for intermediate temperature solid oxide fuel cells.     

Preparation and property-performance relationships in samarium-doped ceria nanopowders for solid oxide fuel cell electrolytes

In a systematic study, Samarium doped ceria (SDC) nanopowders, Sm_xCe_1−xO_2−x/2 (x = 0.1, 0.2 or 0.3), were prepared by a low temperature citrate complexation route. The synthesis and crystallisation of the SDC powders were followed by thermochemical techniques (TGA/DTA), X-ray diffraction, elemental analysis, specific surface area determination (BET) and electron microscopy (SEM and TEM). Mean crystallite sizes were found to be around 10 nm for all compositions calcined at 500 °C. Dense electrolyte bodies were prepared at 1300 °C, 1400 °C and 1450 °C using two sintering times, 4 h or 6 h. Densities of 91-97% of theoretical were obtained, with a marked improvement in density on going from 1300 °C to higher sintering temperatures. Grain size analysis was conducted using SEM. Grain size distributions were related to %Sm and sintering conditions. Impedance spectroscopy was used to determine the total, bulk and grain boundary conductivities, the related activation energies and enthalpies of defect association and ion migration. Sintering at 1400 °C/6 h or 1450 °C/4 h gave superior grain structure and conductivity, with oversintering occurring after more severe treatments. At 600 °C the highest total ionic conductivity was 1.81 × 10⁻² S cm⁻¹ for Sm_0.2Ce_0.8O_1.9. The relationships between chemical composition, sintering parameters, grain structure and electrochemical performance are discussed.     

Effect of Sr and Al or Fe co-doping on the sinterability and conductivity of lanthanum silicate oxyapatite electrolytes for solid oxide fuel cells

The effect of co-doping of Sr and Al or Fe on the microstructure, sinterability and oxide-ion conductivity of lanthanum silicate oxyapatites is investigated in detail at 300–800 °C by the electrochemical impedance spectroscopy. The oxide-ion conductivity is 1.46 × 10⁻² S cm⁻¹ for La_9.5Sr_0.5Si_5.5Fe_0.5O_26.5 (LSSFO) and 1.34 × 10⁻² S cm⁻¹ at 800 °C for La_9.5Sr_0.5Si_5.5Al_0.5O_26.5 (LSSAO), respectively, which is one order of magnitude higher than 6.16 × 10⁻³ S cm⁻¹ measured on La_9.67Si₆O_26.5 (LSO) oxyapatite under the identical test conditions. The grain bulk and grain boundary resistances of co-doped oxyapatite are significantly smaller than that of LSO oxyapatite, and decrease significantly with the increase of the sintering temperature. LSSFO and LSSAO also show significantly higher density as compared to that of LSO. The results indicate that co-doping of Sr and Al or Fe significantly improves the densification, sinterability and oxide-ion conductivity of lanthanum silicate oxyapatites.     

Effect of titania concentration on the grain boundary conductivity of Ce0.8Gd0.2O1.9 electrolyte

The effect of titania (TiO₂) addition on the densification, crystal structure and electrical property of Ce_0.8Gd_0.2O_1.9 (GDC) are examined. TiO₂ addition reduces GDC sintering temperature by ∼200 °C and considerably improves grain boundary conduction. The minimum grain boundary resistivity is obtained at 0.8 mol% TiO₂ concentration. XRD analysis suggests that the solubility limit of TiO₂ in GDC is ∼0.6 mol%. The optimum doping level (0.8 mol%) is slightly higher than the solubility limit (0.6 mol%), which implies that a small amounts of TiO₂ (∼0.2 mol%) at the grain boundaries benefits the grain–boundary conduction.     

Ceria-based electrolyte reinforced by sol–gel technique for intermediate-temperature solid oxide fuel cells

High performance solid oxide fuel cells (SOFCs) based on gadolinia-doped ceria (GDC) electrolyte are demonstrated for intermediate temperature operation. The inherent technical limitations of the GDC electrolyte in sinterability and mechanical properties are overcome by applying sol–gel coating technique to the screen-printed film. When the quality of the electrolyte film is enhanced by the additional sol–gel coating, the OCV and maximum power density increase from 0.73 to 0.90 V and from 0.55 to 0.95 W cm⁻², respectively, at 650 °C with humidified hydrogen (3% H₂O) as fuel and air as oxidant. The impedance analysis reveals that the reinforcement of the thin electrolyte with sol–gel coating significantly reduces the polarization resistance. Elementary reaction steps for the anode and cathode are analyzed based on the systematic impedance study, and the relation between the structural integrity of the electrolyte and the electrode polarization is discussed in detail.     

Effect of zinc oxide on yttria doped ceria

Solid electrolyte ceramics consisted of ceria, yttria and zinc oxide has been synthesized through solid state reaction. With the zinc oxide content over 0.4 mol.%, this material is able to achieve a relative density of 96% at 1375 °C, about 200 °C lower than that without zinc oxide. The result of XRD reveals that the lattice parameter increased with the concentration of zinc oxide up to 0.6 mol%, suggesting its solubility limit for fluorite structure of ceria. It is also found that this doping level is coincident with that where it has the highest ionic conductivity. Furthermore, it is detected by EDS that the excess zinc oxide tends to agglomerate and locate on the surface of sintered sample when the addition exceeds the solubility limit.     

On the equation of electrical conductivity relaxation method to measure kinetic parameters of solid oxide fuel cell materials with a three-dimensional rectangular geometry

The electrical conductivity relaxation method has been widely used to measure kinetic properties of species transport process in solid oxide fuel cell materials using rectangular dense samples. It is found that the equation of ECR used in some studies is flawed. An improved equation is presented in this communication.     

Effect of MgO addition and grain size on the electrical properties of Ce0.9Gd0.1O1.95 electrolyte for IT-SOFCs

The aim of this study is to investigate the effect of grain size on the electrical properties of Ce_0.9Gd_0.1O_1.95-x mol% MgO (GDC-xMgO) and to evaluate them as electrolytes for use in intermediate-temperature solid oxide fuel cells (IT-SOFCs). For this purpose, GDC-xMgO (x = 0–15) electrolytes were synthesized by the glycine-nitrate process and sintered at different temperatures. Impedance spectroscopy measurements revealed that for each composition, the grain-boundary resistivity decreased with decreasing grain size for the samples with grain size of >0.4 μm. Much too small grain sizes (0.2 < d_g < 0.3 μm) produced an increase in grain-boundary resistivity. The addition of MgO could weaken the influence of grain sizes on the grain-boundary resistivity. The interfacial polarization resistances could be decreased by adding MgO to GDC. The GDC-1MgO sample sintered at 1200 °C exhibited the highest total conductivity of 8.11 × 10^?2 S cm^?1 at 800 °C. The maximum power density of the GDC-1MgO-based cell was 0.73 W cm^?2 at 800 °C, which was much higher than that of the GDC-based cell. The results indicated that the GDC-1MgO was a potential electrolyte for IT-SOFCs.     

Cobalt and molybdenum co-doped Ca2Fe2O5 cathode for solid oxide fuel cell

Recently, Brownmillerite oxides Ca₂Fe_2-xM_xO₅ (0 ≤ x ≤ 0.2) (M = transition metal such as Co, Mo), have been drawing attention as they possess mixed ionic and electronic conductivity. Fe site of parent Ca₂Fe₂O₅ (CFO) structure is partially substituted by Co and/or Mo as well as CoMo co-doping and tested as cathodes in SOFC. Physical characterizations such as X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM), and Brunauer–Emmett–Teller (BET) have been carried out to assess the phase formation, microstructure, presence of constituent elements, particle size, and surface area of the cathode, respectively. The Co doped CFO cathodes have better percolation, large surface area, and extended triple phase boundary. Further, the doped CFO cathodes exhibited chemical compatibility with other cell components during fabrication and cell testing as evident from SEM micrographs. The Ca₂Fe_2-xM_xO₅ (0 ≤ x ≤ 0.2) oxides show a semiconductor behaviour having sufficient electrical conductivity values in the SOFCs operating temperature 600–800 °C range. The best electrical conductivity, 0.47 S/cm at 800 °C and the corresponding activation energy of 0.17 eV is exhibited by Ca₂Fe_1.8Co_0.2O₅ (CFCO), whereas Ca₂Fe_1.8Mo_0.2O₅ (CFMO) and Ca₂Fe_1.8Mo_0.1Co_0.1O₅ (CFMCO) cathode shows electrical conductivity 0.11 S/cm and 0.15 S/cm at 800 °C, respectively. CFMO performed better with SDC than YSZ electrolyte between 600 and 700 °C although the lowest area specific resistance (ASR) of 1.28 Ω cm² at 800 °C is observed for CFMO with YSZ electrolyte. Similarly, CFMCO provided low ASR at lower temperature with SDC than that with YSZ electrolyte but exhibited lowest ASR of 0.41 Ω cm² at 800 °C with YSZ. The CFCO cathode shows lower ASR with YSZ than that with SDC for all the temperature and provided lowest value of ASR 0.21 Ω cm² at 800 °C. CFCO cathode has been tested in 900 μm thick electrolyte (SDC/YSZ) supported solid oxide fuel cell (SOFC) CFCO-SDC/SDC/NiO-SDC and CFCO-YSZ/YSZ/NiO-YSZ provided maximum power densities of 171 and 506 mW/cm² (i-R corrected) at 800 °C, respectively.     

Ni-samaria-doped ceria (Ni-SDC) anode-supported solid oxide fuel cell (SOFC) operating with CO

The performance of nickel-samaria-doped ceria (Ni-SDC) anode-supported cell with CO-CO₂ feed was evaluated. The aim of this work is to examine carbon formation on the Ni-SDC anode when feeding with CO under conditions when carbon deposition is thermodynamically favoured. Electrochemical tests were conducted at intermediate temperatures (550–700 °C) using 20 and 40% CO concentrations. Cell operating with 40% CO at 600–700 °C provided maximum power densities of 239–270 mW cm^?2, 1.5 times smaller than that achieved with humidified H₂. Much lower maximum power densities were attained with 20% CO (50–88 mW cm^?2). Some degradation was observed during the 6 h galvanostatic operation at 0.1 A cm^?2 with 40% CO fuel at 550 °C which is believed due to the accumulation of carbon at the anode. The degradation in cell potential occurred at a rate of 4.5 mV h^?1, but it did not lead to cell collapse. EDX mapping at the cross-section of the anode revealed that carbon formed in the Ni-SDC cell was primarily deposited in the anode section close to the fuel entry point. Carbon was not detected at the electrolyte-anode interface and the middle of the anode, allowing the cell to continue operation with CO fuel without a catastrophic failure.     

Enhanced CO2 stability of oxyanion doped Ba2In2O5 systems co-doped with La, Zr

In the solid oxide fuel cell (SOFC) field, proton conducting perovskite electrolytes offer many potential benefits. However, an issue with these electrolytes is their stability at elevated temperatures in the presence of CO₂. Recently we have reported enhanced oxide ion/proton conductivity in oxyanion (silicate, phosphate) doped Ba₂In₂O₅, and in this paper we extend this work to examine the stability at elevated temperatures towards CO₂. The results show improved CO₂ stability compared to the undoped system, and moreover this can be further improved by co-doping on either the Ba site with La, or the In site with Zr. While this co-doping strategy does reduce the conductivity slightly, the greatly improved CO₂ stability would suggest there is technological potential for these co-doped samples.     

Performance of strontium- and magnesium-doped lanthanum gallate electrolyte with lanthanum-doped ceria as a buffer layer for IT-SOFCs

La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8 (LSGM8080) powder, showing the highest electrical conductivity among LSGMs of various compositions, is synthesized using the glycine nitrate process (GNP) and used as the electrolyte for an intermediate-temperature solid oxide fuel cell (IT-SOFC). The LDC (Ce_0.55La_0.45O_1.775) powder is synthesized by a solid-state reaction and employed as the material for a buffer layer to prevent the reaction between the anode and electrolyte materials. The LDC also serves as the skeleton material for the anode. An anode-supported single cell with an active area of 1 cm² is constructed for performance evaluation. A single-cell test is performed at 750 and 800 °C. The maximum power density of the cell 459 and 664 mW cm⁻² at 750 and 800 °C, respectively.     

Low temperature assisted chemical coprecipitation synthesis of 8YSZ plasma sprayable powder for solid oxide fuel cells

Plasma spraying is one of the potential manufacturing technologies widely used in tubular solid oxide fuel cells (SOFC) fabrication. The plasma spray technology requires powders with good flowability and large particle size (5-200 μm). A simple, low temperature assisted chemical process was used for the preparation of plasma grade yttria stabilized zirconia powder without any agglomeration process. The powder was characterized by X-ray diffractometry, particle size analysis, scanning electron microscopy and Raman spectroscopy. The powder exhibited cubic phase, good flowability and blocky angular shape. The 8YSZ powder was plasma sprayed and the coatings after sintering showed gas tightness (gas leak rate ∼ 1 × 10⁻⁶ mbar l s⁻¹ cm²). This was substantiated by the presence of densely packed grains as seen in the surface FESEM image of the sintered plasma sprayed 8YSZ free form. Conductivity values in par with the values reported in literature were obtained for plasma sprayed 8YSZ coating.     

The impact of etching during microfabrication on the microstructure and the electrical conductivity of gadolinia-doped ceria thin films

Gadolinia-doped ceria, Ce_0.8Gd_0.2O_1.9−x (CGO), thin films deposited by spray pyrolysis and annealed to different degree of crystallinity between 0% and 95% are exposed to different etchants and etching methods. The attack of the etchants on the CGO thin films is analyzed with respect to changes in microstructure and in-plane electrical conductivity. It is found that amorphous CGO films are dissolved in hydrochloric acid after elongated etching times. Hydrofluoric acid severely attacks CGO thin films after already short times of exposure (1 min), more intense the less crystalline the thin film is. Ar ion etching smoothens the surface of the CGO thin films without considerable removal of material. No microstructural attack of NaOH, CHF₃/O₂ and SF₆/Ar is found. The electrical conductivity is in general only affected when microstructural changes are severe. Therefore, it is concluded that CGO thin films can be well used as functional layers in micro-fabricated devices and that micro-fabrication is, with the exception of hydrofluoric, not harmful for the electrical properties of crystalline CGO thin films.     

Improvement of output performance of solid oxide fuel cell by optimizing Ni/samaria-doped ceria anode functional layer

Anode functional layers (AFLs) were fabricated using slurry spin coating method on anode substrates to improve the performance of cells based on samaria-doped ceria (SDC) films. The effects of the chemical compositions of AFL and AFL thickness on the performance of solid oxide fuel cell anodes were investigated by studying their effect on the ohmic loss, electrode overpotential, and output performance of cells in different atmospheres. With humidified hydrogen used as fuel and oxygen as oxidant, the cell with an 8-μm-thick AFL (NiO:SDC = 6:4) exhibited excellent maximum power densities of 3.41, 2.89, 1.46 and 0.80 W cm⁻² at 650, 600, 550 and 500 °C, respectively.     

Solid oxide fuel cell bi-layer anode with gadolinia-doped ceria for utilization of solid carbon fuel

Pyrolytic carbon was used as fuel in a solid oxide fuel cell (SOFC) with a yttria-stabilized zirconia (YSZ) electrolyte and a bi-layer anode composed of nickel oxide gadolinia-doped ceria (NiO-GDC) and NiO-YSZ. The common problems of bulk shrinkage and emergent porosity in the YSZ layer adjacent to the GDC/YSZ interface were avoided by using an interlayer of porous NiO-YSZ as a buffer anode layer between the electrolyte and the NiO-GDC primary anode. Cells were fabricated from commercially available component powders so that unconventional production methods suggested in the literature were avoided, that is, the necessity of glycine-nitrate combustion synthesis, specialty multicomponent oxide powders, sputtering, or chemical vapor deposition. The easily-fabricated cell was successfully utilized with hydrogen and propane fuels as well as carbon deposited on the anode during the cyclic operation with the propane. A cell of similar construction could be used in the exhaust stream of a diesel engine to capture and utilize soot for secondary power generation and decreased particulate pollution without the need for filter regeneration.     

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.