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.     

Ceria-carbonate composite for low temperature solid oxide fuel cell: Sintering aid and composite effect

In this study, the effect of carbonate content on microstructure, relative density, ionic conductivity and fuel cell performance of Ce_0.8Sm_0.2O_1.9-(Li/Na)₂CO₃ (SDC-carbonate, abbr. SCC) composites is systematically investigated. With the addition of carbonate, the nano-particles of ceria are well preserved after heat-treatment. The relative densities of SCC pellets increase as the carbonate content increases or sintering temperature rises. Especially, the relative density of SCC2 sintered at 900 °C is higher than that of pure SDC sintered at 1350 °C. Both the AC conductivity and DC oxygen ionic conductivity are visibly improved compared with the single phase SDC electrolyte. Among the composites, SDC-20 wt% (Li/Na)₂CO₃ (SCC20) presents high dispersion, relative small particle size, and the dense microstructure. The optimized microstructure brings the best ionic conductivity and fuel cell performance. It is hoped that the results can contribute the understanding of the role of carbonate in the composite materials and highlight their prospective application.     

Electrical properties of nanocube CeO2 in advanced solid oxide fuel cells

Search for electrolyte materials with a high ionic conductivity at low temperatures has always been a key challenge for the development of solid oxide fuel cells (SOFCs). In present work, we found un-doped CeO₂ nanocubes used as an electrolyte for advanced fuel cell showed remarkable performances. The CeO₂ nanocubes were synthesized by a simple hydrothermal approach. The synthesized CeO₂ nanocubes were used as an electrolyte sandwiched between two layers of semiconducting Ni_0.8Co_0.15Al_0.05LiO_2-δ to fabricate the fuel cell. Such device has achieved an excellent maximum power density of 406 mW cm^?2 at 600 °C. These results demonstrate CeO₂/CeO_2-δ heterogeneous interfaces could provide a high ionic conductive path conductor for the electrolyte in SOFCs, which widen the selecting range of the electrolyte candidates for advanced SOFCs.     

CeO2 nanoparticles improved Pt-based catalysts for direct alcohol fuel cells

We developed an ultrasonic co-deposition technique to enhance the activity of Pt/C catalyst (and Pt/CNT, PtRu/C catalysts) for direct alcohol fuel cells (DAFCs) by CeO₂ nanoparticles. The composite catalyst architecture is obtained by an ultrasonically mixing commercial Pt/C catalyst and CeO₂ nanoparticles. Both Pt and CeO₂ are dispersed uniformly in the electrodes resulting in a great deal of CeO₂–Pt–C triple junction interfaces. Unlike traditional preparation of metal oxide supported Pt catalysts, CeO₂ will not cut the connection between Pt and C in this composite catalyst structure. Electrochemical measurements confirm that CeO₂ can improve almost all Pt based catalysts (Pt/C, Pt/CNT, and PtRu/C) for almost all small molecular alcohols (methanol, ethanol, ethylene glycol, and glycerol) electro-oxidation. EIS measurement shows that reaction resistance between Pt and alcohols is decreased much by adding small CeO₂ nanoparticles. Besides, these composite catalysts have high stability. It proves CeO₂ a very promising co-catalyst of Pt based catalysts for DAFCs.     

Fabrication and performance of a carbon dioxide-tolerant proton-conducting solid oxide fuel cells with a dual-layer electrolyte

A proton-conducting solid oxide fuel cells with a dual-layer electrolyte, constructed of a highly protonic conductive BaCe_0.8Y_0.2O_3−δ (BCY) electrolyte and chemically stable BaZr_0.4Ce_0.4Y_0.2O_3−δ (BZCY4) electrolyte, was easily fabricated by dry pressing the electrolyte powders onto an NiO + BZCY4 anode substrate, followed by co-sintering at a high temperature. The performance of the as-fabricated cell with the BCY and BZCY4 dual-layer electrolyte was studied. Peak power densities of 249 and 101 mW cm⁻² were achieved at 700 and 500 °C, respectively. Zinc was applied as a sintering promoter to increase the relative density of the BZCY4 electrolyte. Cross-sectional micrographs of the as-fabricated, dual-layer electrolyte cells were obtained by scanning electron microscopy. The results showed that the sintering ability of BZCY4 was improved by using zinc as sintering aid. A cell with BCY and zinc-modified BZCY4 dual-layer electrolyte delivered peak power densities of 276 and 247 mW cm⁻² and OCVs of 1.03 and 1.02 V at 700 °C under humidified hydrogen and 15% CO₂-containing hydrogen atmospheres, respectively. The operation stability of the dual-layer electrolyte cell under a 15% CO₂-containing hydrogen atmosphere was also investigated.     

The heterogeneous electrolyte of CuFeO2 nano-flakes composited with flower-shaped ZnO for advanced solid oxide fuel cells

The flower-shaped ZnO was synthesized to form composite with the delafossite structure CuFeO₂. The composite heterojunction formed for the ZnO-CuFeO₂ composite material demonstrates a profound significance for exploring novel materials in solid oxide fuel cell (SOFC) field. At 550 °C, power outputs of 300 mW cm^?2 and 468 mW cm^?2 were achieved for SOFC devices using pure ZnO and composite with CuFeO₂ as the electrolytes, respectively. The composite showed a good performance at low temperatures, for instance, it showed a power output of 148 mW cm^?2 at 430 °C. The studies on photocurrent-time curves with visible light on/off irradiation provided an evidence for electron-hole separation. The heterojunctions separate holes and electrons, preventing short-circuiting while used in the SOFC device. These results demonstrate that introducing the heterojunctions in the electrolyte is an innovative approach for advanced SOFCs.     

An analytical model for solid oxide fuel cells with bi-layer electrolyte

A theoretical model for a solid oxide fuel cell (SOFC) with a bi-layer electrolyte is developed and analytical solutions of various important relationships, such as I–V relationship, distribution of oxygen partial pressure in the bi-layer electrolyte, leakage current density etc. are obtained. Based on the assumptions of constant ionic conductivity and reversible electrodes, the model takes into considerations of transports of both ions and electrons in the electrolyte. The modeling results are compared with both experimental data and results from other models in the literature and very good agreements are obtained.     

Stable glass-ceramic sealants for solid oxide fuel cells: Influence of Bi2O3 doping

Diopside (CaMgSi₂O₆) based glass-ceramics in the system SrO–CaO–MgO–Al₂O₃–B₂O₃–La₂O₃–Bi₂O₃–SiO₂ have been synthesized for sealing applications in solid oxide fuel cells (SOFC). The parent glass composition in the primary crystallization field of diopside has been doped with different amounts of Bi₂O₃ (1, 3, 5 wt.%). The sintering behavior by hot-stage microscopy (HSM) reveals that all the investigated glass compositions exhibit a two-stage shrinkage behavior. The crystallization kinetics of the glasses has been studied by differential thermal analysis (DTA) while X-ray diffraction adjoined with Rietveld-R.I.R. analysis have been employed to quantify the amount of crystalline and amorphous phases in the glass-ceramics. Diopside and augite crystallized as the primary crystalline phases in all the glass-ceramics. The coefficient of thermal expansion (CTE) of the investigated glass-ceramics varied between (9.06–10.14) × 10⁻⁶ K⁻¹ after heat treatment at SOFC operating temperature for a duration varying between 1 h and 200 h. Further, low electrical conductivity, good joining behavior and negligible reactivity with metallic interconnects (Crofer22 APU and Sanergy HT) in air indicate that the investigated glass-ceramics are suitable candidates for further experimentation as sealants in SOFC.     

Textured cermets of CeO2 (or GDC) with Co for solid oxide fuel cells anodes

Cermets composed of submicron size alternating lamellae of CeO₂, or 10% Gadolinia doped Ceria (GDC), and porous-metallic Cobalt have been prepared from eutectic oxide mixtures. A fine eutectic structure was obtained by fast directional solidification of the cobalt oxide–ceria oxide eutectic composite using the Laser Floating Zone (LFZ) technique. The resulting microstructure, with an interphase spacing down to 0.5 μm, was obtained for solidification rates of 750 mm/h. Textured cermets were obtained by subsequent reduction under H₂ containing atmosphere of the eutectic oxide composite. The reduction kinetics was studied in the 550–750 °C temperature range and effective diffusion coefficients were obtained. The reduction process does not correspond to a typical thermally activated process. The cermets are composed of ceria lamellae of about 200 nm thickness alternated with porous-metallic cobalt lamellae of ≤400 nm. The lamellar microstructure of the cermets favours oxygen ion mobility through ceria and its size can be controlled by solidification rate of the eutectic precursor. These materials are proposed as SOFC anodes.     

A novel design of anode-supported solid oxide fuel cells with Y2O3-doped Bi2O3, LaGaO3 and La-doped CeO2 trilayer electrolyte

Anode-supported solid oxide fuel cells (SOFCs) with a trilayered yttria-doped bismuth oxide (YDB), strontium- and magnesium-doped lanthanum gallate (LSGM) and lanthanum-doped ceria (LDC) composite electrolyte film are developed. The cell with a YDB (18 μm)/LSGM (19 μm)/LDC (13 μm) composite electrolyte film (designated as cell-A) shows the open-circuit voltages (OCVs) slightly higher than that of a cell with an LSGM (31 μm)/LDC (17 μm) electrolyte film (designated as cell-B) in the operating temperature range of 500-700 °C. The cell-A using Ag-YDB composition as cathode exhibits lower polarization resistance and ohmic resistance than those of a cell-B at 700 °C. The results show that the introduction of YDB to an anode-supported SOFC with a LSGM/LDC composite electrolyte film can effectively block electronic transport through the cell and thus increased the OCVs, and can help the cell to achieve higher power output.     

Electrophoretically deposited thin film electrolyte for solid oxide fuel cell

Abstract

Thin films of 8 mol% yttria stabilised zirconia (YSZ) electrolyte have been deposited on non-conducting porous NiO–YSZ anode substrates using electrophoretic deposition (EPD) technique. Deposition of such oxide particulates on non-conducting substrates is made possible by placing a conducting steel plate on the reverse side of the presintered porous substrates. Thickness of the substrates, onto which the deposition has been carried out, varied in the range 0·5–2·0 mm. Dense and uniform YSZ thin films (thickness: 5–20 μm) are obtained after being cofired at 1400°C for 6 h. The thickness of the deposited films is seemed to be increased with increasing porous substrate thickness. Solid oxide fuel cell (SOFC) performance is measured at 800°C using coupon cells with various anode thicknesses. While a peak power density of 1·41 W cm^?2 for the cells with minimum anode thickness of 0·5 mm is achieved, the cell performance decreases with anode thickness.     

Interlayer-free electrodes for IT-SOFCs by applying Co3O4 as sintering aid

The influence of Co₃O₄ as a sintering aid for a series of cobalt-containing perovskite oxides on the microstructure and electrical properties have been investigated. X-ray diffraction and scanning electron microscopic results showed that well connected electrode particles with firm adhesion to the 8 mol% yttria-stabilized zirconia (YSZ) electrolyte surface were realized at a temperature free from interfacial phase reaction. Both ohmic and polarization resistances of symmetric cells by adopting YSZ electrolyte, measured by electrochemical impedance spectroscopy, were much lower than that without adding Co₃O₄. The peak power density of 1176 mW cm⁻² at 750 °C was achieved when La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ + Co₃O₄ was selected as a representative cobaltite cathode, which is much higher than a similar fuel cell with the cathode fabricated by a conventional way. Fabrication of interlayer-free electrodes by applying Co₃O₄ as a sintering aid is very simple and general, applicable for a wide range of cobalt-containing electrode materials.     

Ni-Sm2O3 cermet anodes for intermediate-temperature solid oxide fuel cells with stabilized zirconia electrolytes

Ni-LnO_x cermets (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd), in which LnO_x is not an oxygen ion conductor, have shown high performance as the anodes for low-temperature solid oxide fuel cells (SOFCs) with doped ceria electrolytes. In this work, Ni-Sm₂O₃ cermets are primarily investigated as the anodes for intermediate-temperature SOFCs with scandia stabilized zirconia (ScSZ) electrolytes. The electrochemical performances of the Ni-Sm₂O₃ anodes are characterized using single cells with ScSZ electrolytes and LSM-YSB composite cathodes. The Ni-Sm₂O₃ anodes exhibit relatively lower performance, compared with that reported Ni-SDC (samaria doped ceria) and Ni-YSZ (yttria stabilized zirconia) anodes, the state-of-the-art electrodes for SOFCs based on zirconia electrolytes. The relatively low performance is possibly due to the solid-state reaction between Sm₂O₃ and ScSZ in fuel cell fabrication processes. By depositing a thin interlayer between the Ni-Sm₂O₃ anode and the ScSZ electrolyte, the performance is substantially improved. Single cells with a Ni-SDC interlayer show stable open circuit voltage, generate peak power density of 410 mW cm⁻² at 700 °C, and the interfacial polarization is about 0.7 Ω cm².     

Samarium doped ceria-(Li/Na)2CO3 composite electrolyte and its electrochemical properties in low temperature solid oxide fuel cell

A composite of samarium doped ceria (SDC) and a binary carbonate eutectic (52 mol% Li₂CO₃/48 mol% Na₂CO₃) is investigated with respect to its morphology, conductivity and fuel cell performances. The morphology study shows the composition could prevent SDC particles from agglomeration. The conductivity is measured under air, argon and hydrogen, respectively. A sharp increase in conductivity occurs under all the atmospheres, which relates to the superionic phase transition in the interface phases between SDC and carbonates. Single cells with the composite electrolyte are fabricated by a uniaxial die-press method using NiO/electrolyte as anode and lithiated NiO/electrolyte as cathode. The cell shows a maximum power density of 590 mW cm⁻² at 600 °C, using hydrogen as the fuel and air as the oxidant. Unlike that of cells based on pure oxygen ionic conductor or pure protonic conductor, the open circuit voltage of the SDC-carbonate based fuel cell decreases with an increase in water content of either anodic or cathodic inlet gas, indicating the electrolyte is a co-ionic (H⁺/O²⁻) conductor. The results also exhibit that oxygen ionic conductivity contributes to the major part of the whole conductivity under fuel cell circumstances.     

Ni/YSZ and Ni–CeO2/YSZ anodes prepared by impregnation for solid oxide fuel cells

In this paper, Ni/YSZ and Ni–CeO₂/YSZ anodes for a solid oxide fuel cell (SOFC) were prepared by tape casting and vacuum impregnation. By this method, the Ni content in the anode could be reduced compared to the traditional tape casting method. It was found that adding CeO₂ into the Ni/YSZ anode by a Ni(NO₃)₂ and Ce(NO₃)₃ mixed impregnation could further enhance cell performance. This was investigated in H₂ at 1073 K. XRD patterns indicated that CeO₂ and Ni were separate phases, and the CeO₂ addition could enhance the Ni dispersion on the YSZ framework surface which was observed by SEM images. It was shown that adding CeO₂ into the Ni anodes could decrease the cell polarization resistance. The maximum power density for cells with 25 wt.% Ni, 5 wt.% CeO₂–25 wt.% Ni/YSZ, or 10 wt.% CeO₂–25 wt.% Ni/YSZ anode was 230 mW cm⁻², 420 mW cm⁻² and 530 mW cm⁻², respectively, in H₂ at 1073 K. The OCV for these cells was 1.05–1.09 V, indicating that a dense electrolyte film was obtained by co-firing porous YSZ layer and dense YSZ layer.     

Measurement of oxygen chemical potential in Gd2O3-doped ceria-Y2O3-stabilized zirconia bi-layer electrolyte, anode-supported solid oxide fuel cells

Solid oxide fuel cells (SOFC) were fabricated with gadolinia-doped ceria (GDC)-yttria stabilized zirconia (YSZ), thin bi-layer electrolytes supported on Ni + YSZ anodes. The GDC and YSZ layer thicknesses were 45 μm, and ∼5 μm, respectively. Two types of cells were made; YSZ layer between anode and GDC (GDC/YSZ) and YSZ layer between cathode and GDC (YSZ/GDC). Two platinum reference electrodes were embedded within the GDC layer. Cells were tested at 650 °C with hydrogen as fuel and air as oxidant. Electric potentials between embedded reference electrodes and anode and between cathode and anode were measured at open circuit, short circuit and under load. The electric potential was nearly constant through GDC in the cathode/YSZ/GDC/anode cells. By contrast, it varied monotonically through GDC in the cathode/GDC/YSZ/anode cells. Estimates of oxygen chemical potential, μO₂, variation through GDC were made. μO₂ within the GDC layer in the cathode/GDC/YSZ/anode cell decreased as the current was increased. By contrast, μO₂ within the GDC layer in the cathode/YSZ/GDC/anode cell increased as the current was increased. The cathode/YSZ/GDC/anode cell exhibited maximum power density of ∼0.52 W cm⁻² at 650 °C while the cathode/GDC/YSZ/anode cell exhibited maximum power density of ∼0.14 W cm⁻² for the same total electrolyte thickness.     

Direct ammonia solid oxide fuel cell based on thin proton-conducting electrolyte

Thin proton-conducting electrolyte with composition BaCe_0.8Gd_0.2O_3−δ (BCGO) was prepared over substrates composed of Ce_0.8Gd_0.2O_1.9 (CGO)-Ni by the dry-pressing method. Solid oxide fuel cells (SOFCs) were fabricated with the structure Ni-CGO/BCGO/Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO)-CGO. The performance of a single cell was tested at 600 and 650 °C, with ammonia directly used as fuel. The open circuit voltages (OCVs) were 1.12 and 1.1 V at 600 and 650 °C, respectively. The higher OCV may be due to both the compaction of the BCGO electrolyte (no porosity) and complete decomposition of ammonia. The maximum power density was 147 mW cm⁻² at 600 °C. Comparisons of the cell with hydrogen as fuel indicate that ammonia can be treated as a substitute liquid fuel for SOFCs based on a proton-conducting solid electrolyte.     

Co-sintering anode and Y2O3 stabilized ZrO2 thin electrolyte film for solid oxide fuel cell fabricated by co-tape casting

Fabricating a large-area unit cell is very important for the development of solid oxide fuel cell (SOFC) stack. In this study, details of sintering process of half cell with NiO-yttria stabilized zirconia (YSZ) anode-supported YSZ thin electrolyte film fabricated by co-tape casting have been discussed. The results demonstrates that the shrinkages and shrinking rates mismatches between the electrolyte and the anode can be controlled by the organic additive content in the anode slurry composition and heating rate. Low heating rate suppresses the cracks formation in the electrolyte films. A warp-free unit cell with size of 100 × 100 mm² and dense electrolyte has been successfully fabricated. A power of 22.2 W, with a power density of 0.27 W cm⁻² has been achieved at 0.7 V and 750 °C in O₂/humidified H₂ atmosphere. The area specific resistance of the cell is 1.20 Ω cm² at 0.7 V and 750 °C.     

Combustion-synthesized Ru-Al2O3 composites as anode catalyst layer of a solid oxide fuel cell operating on methane

Ru-Al₂O₃ composites with varied Ru contents were synthesized by a glycine-nitrate combustion technique. Their potential application as anode catalyst functional layer of a solid-oxide fuel cell operating on methane fuel was investigated. Catalytic tests demonstrated the 3-7 wt.% Ru-Al₂O₃ composites had high catalytic activity for methane partial oxidation and CO₂/H₂O reforming reactions, while 1 wt.% Ru-Al₂O₃ had insufficient activity. The 3 wt.% Ru-Al₂O₃ catalyst also showed excellent operation stability and good thermal-mechanical compatibility with Ni-YSZ anode. H₂-TPR and TEM results indicated there was strong interaction between RuO_x and Al₂O₃ in the as-synthesized catalysts, which may account for the good catalytic stability of 3 wt.% Ru-Al₂O₃ catalyst. O₂-TPO results demonstrated Ru-Al₂O₃ also had excellent coking resistance. Furthermore, the carbon deposited over Ru-Al₂O₃ had lower graphitization degree than that deposited over Ni-Al₂O₃, suggesting the easier elimination of potential carbon deposited over the Ru-Al₂O₃ catalysts. A cell with 3 wt.% Ru-Al₂O₃ catalyst functional layer was prepared, wh-ich delivered peak power densities of 1006, 952 and 929 mW cm⁻² at 850 °C, operating on methane-O₂, methane-H₂O and methane-CO₂ gas mixtures, respectively, comparable to that operating on hydrogen fuel. It highly promised 3 wt.% Ru-Al₂O₃ as a coking resistant catalyst layer for solid-oxide fuel cells.     

Co-extruded dual-layer hollow fiber with different electrolyte structure for a high temperature micro-tubular solid oxide fuel cell

In this study, the phase inversion-based co-extrusion method was employed to fabricate a structural-improved electrolyte/anode dual-layer hollow fiber (HF) precursor, which was then co-sintered at 1450 °C. The electrolyte structures were thoroughly investigated by varying the loading of electrolyte material (i.e. Yttria-stabilized zirconia, YSZ) with differing particle sizes (i.e. micron, sub-micron, and nano-sized) during suspension preparation. The results showed that the most promising electrolyte layer with thin, dense, gas-tight, and defect-free properties was obtained by mixing 70% submicron-YSZ and 30% nano-YSZ in electrolyte suspension (E-0.7sub0.3nano). This electrolyte formulation co-extruded with a thick nickel-oxide-YSZ (NiO-YSZ) anode layer yielded the highest bending strength of 85 MPa, providing major mechanical strength to the HF. Besides that, the nitrogen permeability value at 2.87 × 10^?6 mol m^?2 s^?1 Pa^?1 suggested that the electrolyte was gas-tight, preventing fuel and oxidant transport. The fiber was then reduced to nickel (Ni)-cermet anode. It was developed to be a complete micro-tubular solid oxide fuel cell (MT-SOFC) by depositing the lanthanum strontium cobalt ferrite (LSCF)/YSZ cathode via brush painting on the dual-layer HF. The cell was fed with hydrogen gas and yielded an open-circuit voltage (OCV) as high as 1.06 V with maximum power density of 0.243 W cm^?2, at 875 °C. Based on this test, it was found that the electrolyte structural-modified dual-layer hollow fiber-based MT-SOFC using mixed particle sizes may result in a promising OCV. However, the relatively low value for power density may be due to a less porous anode; thus, improvements in the anode's structure are required in future research.