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″.     

Anode-supported microtubular cells fabricated with gadolinia-doped ceria nanopowders

Anode-supported microtubular SOFCs based on ceria 3 ± 0.2 mm diameter and about 100 mm in length have been prepared using gadolinia-doped ceria (GDC) nanopowders. Nanometric Ce_0.9Gd_0.1O_1.95 (GDC) powders were deposited on NiO-Ce_0.9Gd_0.1O_1.95 (NiO-GDC) anode supports by dip-coating technique. Fabrication conditions to obtain dense and gas tight electrolyte layers on porous microtubular supports were studied. Three different dispersing agents: commercial Beycostat C213 (CECA, France) and short chain monomer (≤4 carbon atoms) with alcohol or carboxylic acid functional groups were evaluated. By optimizing colloidal dispersion parameters and sintering process, gas tight and dense GDC layers were obtained. Significantly lower sintering temperatures than reported previously (≤1300 °C) were employed to reach ≥98% values of theoretical density within electrolyte layers of ∼10 μm in thickness. A composite cathode, LSCF-GDC 50 wt.% with about 50 μm thickness was dip coated on the co-fired half-cell and then sintered at 1050 °C for 1 h. The electrochemical performance of these cells has been tested. In spite of electronic conduction due to partial reduction of the thin-electrolyte layer, the I-V measurements show power densities of 66 mW cm⁻² at 0.45 V at temperatures as low as 450 °C (using 100% H₂ as fuel in the anodic compartment and air in the cathodic chamber).     

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.     

Development of co-sintering process for anode-supported solid oxide fuel cells with gadolinia-doped ceria/lanthanum silicate bi-layer electrolyte

Gadolinia-doped ceria (GDC) and lanthanum silicate (LS) are expected to be promising materials for electrolytes of solid oxide fuel cells (SOFCs) because of their high ionic conductivities at intermediate temperatures. However, performance degradation of SOFCs is caused by current leakage through GDC and poor densification of LS. In the present study, LS was used as a blocking layer for preventing the current leakage of GDC electrolyte. Thermal shrinkage measurements and scanning electron microscopy (SEM) observation suggested that the addition of Bi₂O₃ in LS electrolyte (LSB) contributed to the decrease in the sintering temperature of the LS and improved densification of the GDC/LS bi-layer electrolyte. Consequently, the open-circuit voltage for the cell with GDC/LS and GDC/LSB bi-layer electrolytes increased effectively in comparison with that of the cell with GDC single-layer electrolyte. The electrical conductivity and fuel cell characteristics were compared among the cells with GDC, GDC/LS, and GDC/LSB electrolytes.     

Effect of addition method of gadolinia-doped ceria-added FeCr gas diffusion layer on performance of direct-methane solid oxide fuel cells

A solid oxide fuel cell (SOFC) was set up with a porous disk of gadolinia-doped ceria (GDC)-added FeCr as a gas diffusion layer under direct-methane feeding. The addition of GDC was done by mixing GDC powder with FeCr powder before disk fabrication, or by coating GDC powder or impregnating GDC precursor to the surface of the porous FeCr disk. When GDC was added by mixing, the direct-methane SOFC (DM-SOFC) performance degraded very rapidly. When GDC was added by either powder coating or impregnating, the DM-SOFC performance can be relatively stable. Both the current density and the CO₂ selectivity with GDC addition by impregnating are larger than those by powder coating.     

The effect of co-dopant addition on the properties of Ln0.2Ce0.8O2−δ (Ln = Gd,Sm, La) solid-state electrolyte

The present work aims at the investigation of Ln_0.2Ce_0.8O_2−δ (where Ln = Sm, La, Gd) structural and electrical properties when in the Ln sub-lattice, Ba²⁺ and Sr²⁺ with ionic radii 1.42 and 1.26 Å, respectively, are introduced. The conductivity measurements were held both in air and in H₂ + 3%H₂O atmosphere using the 4-probe dc technique at the temperature range of 600–900 °C. Among all the samples, the highest value of electrical conductivity is obtained in the case of (Sm_0.75Sr_0.2Ba_0.05)_0.2Ce_0.8O_2−δ, both in air and in hydrogen atmosphere. In the case of H₂ + 3%H₂O the conductivity of the co-doped compounds increases in comparison with air. Moreover, the dependence of conductivity on the oxygen partial pressure, measured at the PO₂$P_{{\text{O}}_2}$ range of 0.21–10⁻²² atm, showed that the electrolytic area of alkaline-earth metals doped Ln_0.2Ce_0.8O_2−δ is considerably enhanced, as predicted by the theory. Finally, by comparing the thermal expansion coefficients of the different materials (TEC), the thermo-mechanical compatibility between the co-doped and the other cell components was also investigated.     

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.     

CuxCo3-xO4-δ (0≤x≤1) coatings for solid oxide fuel cell interconnect applications

In order to obtain the solid oxide fuel cell (SOFC) interconnect coatings with high electrical conductivity, satisfactory protectiveness, and well-fitting thermal expansion, a series of Cu_xCo_3-xO_4-δ (x = 0, 0.5, 0.8, and 1.0) coatings are prepared by supersonic spraying via subsequent sintering. The chemical composition, lattice and morphological structures, electrical properties, and thermal expansion are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), area-specific resistance (ASR), and coefficient of thermal expansion (CTE) measurements. The experimental results show that the formation of CuCo₂O₄ is a reversible and incomplete reaction at the elevated temperature, and the coexistence of CuO, Co₃O₄, and CuCo₂O₄ is inevitable in the coatings. The concentration of the chemicals mentioned above is highly related to the coatings’ Cu:Co molar ratio. The correlation between the chemical composition and the properties is comprehensively studied in this research. The Cu_xCo_3-xO_4-δ coatings exhibit good electrical conductivity when 0 ≤ x ≤ 0.8, satisfactory protectiveness when 0.5 ≤ x ≤ 1.0, and fitting CTE with remarkable robustness through the quick heating-cooling cycles when 0.8 ≤ x ≤ 1.0. In general, Cu_0.8Co_2.2O_4-δ can be an appropriate candidate to meet the advancing interconnect coating demands with high electrical conductivity, satisfactory protectiveness, and well-fitting thermal expansion properties.     

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.     

Synthesis and properties of samaria-doped ceria electrolyte for IT-SOFCs by EDTA-citrate complexing method

An ultra-fine samaria-doped ceria (Ce_0.8Sm_0.2O_1.9, SDC) electrolyte prepared by a non-ion selective EDTA-citric complexing method is developed herein for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The rigid agglomerates due to organic compounds that exist in the SDC precursors during the EDTA-citrate complexing synthesis process inhibit crystalline growth and grain growth, leading to the generation of ultra-fine grain following the sintering procedure. Calcination is necessary above 500 °C for all precursors. The average grain size of the pellets after sintering at 1400 °C for 2 h is submicron in scale (from 200 nm to 600 nm) with various pH values, and the pellets are smaller than those obtained from other synthesis processes. Dense pellets with pH values of 10 (relative density of 99%) are obtained with precursor powder calcination at 900 °C for 3 h. Electrical conductivity is dependent on the calcination temperature and pH value of the solution, and the maximum electrical conductivity is 0.01 S cm⁻¹ at 700 °C with a pH value of 10.