Porous YFe0.5Co0.5O3 thin sheets as cathode for intermediate-temperature solid oxide fuel cells

In this work, porous YFe_0.5Co_0.5O₃ (YFC) thin sheets were synthesized by citric acid method. The crystal structure, morphology, thermal expansion, electrical conductivity, and electrochemical properties of YFC were investigated to evaluate it as a possible cathode on BaZr_0.1Ce_0.7Y_0.2O₃ (BZCY) electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs). An orthorhombic perovskite structure was observed in YFC. The conductivity of YFC is 183 S cm ^?1 at 750 °C in air. The coefficient of thermal expansion of composite cathode YFC-BZCY is closer to BZCY electrolyte than YFC. The composite cathode represents a relatively low polarization resistance (R_p) of 0.07 Ω cm² at 750 °C in air due to the porous thin sheet-like cathode. The oxygen reduction reaction process and the reaction activation energy of cathode were also analyzed. An anode-supported cell of NiO-BZCY∣BZCY∣YFC-BZCY is fabricated by a simple method of co-pressing. The power density of the cell is 303 mW cm^?2 at 750 °C as the thickness of electrolyte is 400 μm. The results suggest that YFC is a promising cathode candidate for IT-SOFC.     

Addition effects of erbia-stabilized bismuth oxide on ceria-based carbonate composite electrolytes for intermediate temperature-−solid oxide fuel cells

Highly conductive Er_0.2Bi_0.8O_1.5 (ESB) and rare-earth doped ceria solid oxide electrolytes (SOEs) at intermediate temperature (IT) continue to suffer disadvantages in terms of thermodynamic instability and significant electronic conduction, respectively, at low oxygen partial pressure for solid oxide fuel cell (SOFC) operations. It is therefore necessary to improve the low-temperature ionic conductivity in order to enhance the electrolytic domain of these materials and thereby mitigate cell efficiency dissipation by electronic conduction. In this work, an advanced multiphase carbonate composite material based on ceria has been developed to overcome this IT-SOE challenge. This advanced electrolyte is comprise of nanostructured neodymium-doped ceria (NDC) and 38 wt% (Li–0.5Na)₂CO₃ carbonate with a small amount of ESB phase. The addition of 2 wt% ESB in ceria-based materials decreases the grain boundary resistance of the SOEs in the IT range. Further, a small amount of highly conducting ESB phase in the NDC/[(Li–0.5Na)₂CO₃] composite electrolyte increases the overall conductivity of the composite SOEs. The NDC electrolyte containing 38 wt% carbonate shows the highest conductivity of 0.104 Scm⁻¹ at 600 °C, while the conductivity is increased to 0.165 Scm⁻¹ by the addition of 2 wt% ESB. In addition, the activation energy of the multiphase composite electrolytes (0.52 eV) is lower than that of the NDC/carbonates (0.65 eV) in the IT range. This is attributed to the effect of the physical properties of the NDC sample, induced by the light ESB doping, on the ionic conductivity, and this effect is closely associated with the grain boundary property. Furthermore, the interfacial effects of the multiphase materials also contribute to the improved conductivity of this advanced composite electrolyte.     

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.     

Steam reforming on Ni-samaria-doped ceria cermet anode for practical size solid oxide fuel cell at intermediate temperatures

Direct internal and external reforming operations on Ni-samaria-doped ceria (SDC) anode with the practical size solid oxide fuel cell (SOFC) at intermediate temperatures from 600 to 750 °C are carried out to reveal the reforming activities and the electrochemical activities, being compared with the hydrogen-fueled power generation. The cell performance with direct internal and external steam reforming of methane and their limiting current densities were almost the same irrespective of the progress of reaction in the methane reformate at 700 and 750 °C. The durability test for 5.5 h at 750 °C with direct internal reforming operation confirmed that the cell performance did not deteriorate. The operation temperature of the cell controlled the reforming activities on the anode, and the large size electrode gave rise to high conversion due to the slow space velocity of the steam reforming. Direct internal steam reforming attained sufficient level of conversion for SOFC power generation with methane at 700 and 750 °C on the large Ni-SDC cermet anode.     

ALD CeO2-Coated Pt anode for thin-film solid oxide fuel cells

Surface modification of electrodes for realizing high electrochemical reactivity and thermal stability is an attractive strategy for high-performance low temperature solid oxide fuel cells (LT-SOFCs). Herein, the atomic-layer-deposited (ALD) CeO₂-coated Pt anode structure is fabricated and applied to anodized aluminum oxide (AAO)-based thin-film LT-SOFC. The effect of Pt anode morphology on the infiltration of ALD CeO₂ is elucidated. Anode kinetics are improved in the ALD CeO₂-coated porous Pt anode cell possibly due to the larger Pt–CeO₂ interface density, leading to a decrease in activation resistance by 86%. The maximum power density of the cell with the ALD CeO₂-coated porous Pt anode shows 478 mW/cm²; a dramatic improvement by a factor of two compared to the bare porous Pt anode.     

Novel structured gadolinium doped ceria based electrolytes for intermediate temperature solid oxide fuel cells

Novel three-layered intermediate temperature solid oxide fuel cell (SOFC) electrolytes based on gadolinium doped ceria (GDC) are developed to suppress the electronic conductivity of GDC, to improve the mechanical properties of the cell and to minimize power loss due to mixed conductive nature of GDC. Three different electrolytes are fabricated by sandwiching thin YSZ, ScSZ and ScCeSZ between two relatively thick GDC layers. An electrolyte composed of pure GDC is also manufactured for comparison. NiO/GDC and LSCF/GDC electrodes are then coated on the electrolytes by a screen printing route. SEM results show that it is possible to obtain dense and crack free thin layers of YSZ, ScSZ and ScCeSZ between two GDC layers without delamination. Performance measurements indicate that interlayered thin electrolytes act as an electronic conduction barrier and improve open circuit voltages (OCVs) of GDC based cells.     

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.     

3D printed Sm-doped ceria composite electrolyte membrane for low temperature solid oxide fuel cells

Three dimensional (3D) printing has attracted much more interest from the research community due to its ability to make complex structures with high resolution and simplified fabrication process. Here we constructed composited electrolyte with certain thickness for the application of low temperature of solid oxide fuel cells. The fabrication of a thin and dense Sm-doped ceria composite electrolyte layers with the thickness about 1200 μm utilizing paraffin-based slurry were investigated. To optimize the assembled cells, 1.7 wt. % glassfiber was introduced and an amazing electrochemical performance was observed. The maximum power density can reach 448 mW/cm², 20% higher than the one without glassfiber and the open-circuit voltage is approximately 1.0 V at 550 °C. It is of great potential for 3D printing technology to develop low temperature solid fuel cells with designed mini-structures.     

An additional layer in an anode support for internal reforming of methane for solid oxide fuel cells

Anode supported solid oxide fuel cells (SOFCs) have been extensively investigated for their ease of fabrication, robustness, and high electrochemical performance. SOFCs offer a greater flexibility in fuel choice, such as methane, ethanol or hydrocarbon fuels, which may be supplied directly on the anode. In this study, SOFCs with an additional Ni–Fe layer on a Ni–YSZ support are fabricated with process variables and characterized for a methane fuel application. The addition of Ni–Fe onto the anode supports exhibits an increase in performance when methane fuel is supplied. SOFC with a Ni–Fe layer, sintered at 1000 °C and fabricated using a 20 wt% pore former, exhibits the highest value of 0.94 A cm⁻² and 0.85 A cm⁻² at 0.8 V with hydrogen and methane fuel, respectively. An impedance analysis reveals that SOFCs with an additional Ni–Fe layer has a lower charge transfer resistance than SOFCs without Ni–Fe layer. To obtain the higher fuel cell performance with methane fuel, the porosity and sintering temperature of an additional Ni–Fe layer need to be optimized.     

Nickel/gadolinium-doped ceria anode for direct ethanol solid oxide fuel cell

This report investigates the properties of nickel/gadolinium-doped ceria (Ni/GDC) as anode material for bio-ethanol fueled SOFC. The Ni/GDC cermets with 18 and 44 wt.% Ni were prepared by a hydrothermal method. Ethanol decomposition, steam reforming, and partial oxidation of ethanol were studied using a fixed-bed reactor at 1123 K. Carbon was formed only under dry ethanol for both catalysts. The addition of water or oxygen to the feed inhibited the formation of carbon. Ni/GDC was used as the anode current collector layer and as a catalytic layer in single cells tests. No deposits of carbon were detected in single cells with Ni/GDC catalytic layer after 50 h of continuous operation under direct (dry) ethanol. This result was attributed to the catalytic properties of the Ni/GDC layer and the operation mechanism of gradual internal reforming, in which the oxidation of hydrogen provides the steam for ethanol reforming, thus avoiding carbon deposition.     

Effects of co-doped barium cerate additive on morphology,conductivity and electrochemical properties of samarium doped ceria electrolyte for intermediate temperature solid oxide fuel cells

The composite of samarium doped ceria (Sm_0.2Ce_0.8O_2-δ, SDC) and co-doped barium cerate (BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ, BZCYYb) is prepared by mechanical mixing and investigated as electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). Coexistence of SDC and BZCYYb are observed for composite electrolyte by X-ray diffraction after sintering at 1500 °C for 5 h, while the slight deviation of the diffraction peak indicating the element diffusion between two phases. The scanning electron microscope and electron probe microanalyzer results demonstrate that small BZCYYb grains disperse uniformly around the grains of SDC, limiting the growth of SDC grains and decreasing the average grain size of composite electrolyte. Impedance spectroscopy measurement reveals that the grain boundary resistance can be significantly reduced by about an order of magnitude through adding 15–30 wt. % BZCYYb to SDC. Single cells based on the composite electrolyte are fabricated using nickel cermet (Ni-SDC) anode and perovskite (La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ, LSCF) cathode.Relatively high open circuit voltage (OCV), much lower polarization resistance and encouraging high power density are obtained for cells with composite electrolyte compared to those with single SDC electrolyte. Among all of the samples, single cell based on 15 wt. % BZCYYb-85 wt. % SDC composite electrolyte exhibits the lowest total resistances of 0.641 Ω·cm² and the highest peak power densities of 0.56 W·cm^?2 at 600 °C.     

Numerical simulation of intermediate-temperature direct-internal-reforming planar solid oxide fuel cell

A numerical model for an anode-supported intermediate-temperature direct-internal-reforming planar solid oxide fuel cell (SOFC) was developed. In this model, the volume-averaging method is applied to the flow passages in the SOFC by assuming that a porous material is inserted in the passages as a current collector. This treatment reduces the computational time and cost by avoiding a full three-dimensional simulation while maintaining the ability to solve the flow and pressure fields in the streamwise and spanwise directions. In this model, quasi-three-dimensional multicomponent gas flow fields, the temperature field, and the electric potential/current fields were simultaneously solved. The steam-reforming reaction using methane, the water-gas shift reaction, and the electrochemical reactions of hydrogen and carbon monoxide were taken into account. It was found that the endothermic steam-reforming reaction led to a reduction in the local temperature near the inlet and limited the electrochemical reaction rates therein. Computational results indicated that the local temperature and current density distributions can be controlled by tuning the pre-reforming rate. It was also found that a small amount of heat loss from the sidewall can cause significant nonuniformity in the flow and thermal fields in the spanwise direction.     

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.     

A high performance composite ionic conducting electrolyte for intermediate temperature fuel cell and evidence for ternary ionic conduction

The performance of a composite electrolyte composed of a samarium doped ceria (SDC) and a ternary eutectic carbonate melt phase was examined. The formation temperature of a continuous carbonate melt phase is crucial to the high conductivity of this material. The electrolyte contains 30 and 50 wt% carbonate exhibited a sharp increase of conductivity at a temperature close to the melting point of the eutectic carbonate, ca 400 °C, which is more than 100 °C lower than those electrolytes using binary carbonate. At around 650 °C, and with CO₂/O₂ used as the cathode gas, the fuel cell gave a power output 720 mW cm⁻² at a current density 1300 mA cm⁻². Water was measured in both the anode and cathode outlet gases and CO₂ was detected in the anode outlet gas. When discharged at 800 mA cm⁻², a stable discharge plateau was obtained. The CO₂ in the cathode gas enhances the power output and the stability of the single cell. Based on these experimental facts, a ternary ionic conducting scheme is proposed and discussed.     

Dopant-induced surface activation of ceria nanorods for electro-oxidation of hydrogen and propane in solid oxide fuel cells

Ceria is an excellent oxide catalyst to break H₂ in the absence of noble metals and has shown great promise for potential applications in diverse technological fields. The catalytic activity of ceria is critically linked to surface composition and structure. Herein, selective doping with moderate lanthanide ions is reported to regulate surface oxygen vacancies and bonded adsorbates of ceria nanorods so as to finely tune their activities toward electro-oxidation of H₂ and C₃H₈ in reduced-temperature solid oxide fuel cells. Lanthanide doped ceria nanorods are hydrothermally synthesized, and electrochemically evaluated as the anode catalysts for reduced-temperature SOFCs. Measurements of anode polarization resistances and fuel cell power densities show a catalytic activity in the order of Ce_0.8Pr_0.2O_2-δ < Ce_0.8Gd_0.2O_2-δ < Ce_0.8Sm_0.2O_2-δ. Probing the surface structure with hydrogen temperature-programmed reduction, UV-Raman and XPS reveals that such catalytic activities are essentially determined by surface reducibility, availability of surface oxygen vacancies and strongly bonded hydroxyls.     

Low-temperature ceria-electrolyte solid oxide fuel cells for efficient methanol oxidation

Low temperature anode-supported solid oxide fuel cells with thin films of samarium-doped ceria (SDC) as electrolytes, graded porous Ni-SDC anodes and composite La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF)-SDC cathodes are fabricated and tested with both hydrogen and methanol fuels. Power densities achieved with hydrogen are between 0.56 W cm⁻² at 500 °C and 1.09 W cm⁻² at 600 °C, and with methanol between 0.26 W cm⁻² at 500 °C and 0.82 W cm⁻² at 600 °C. The difference in the cell performance can be attributed to variation in the interfacial polarization resistance due to different fuel oxidation kinetics, e.g., 0.21 Ω cm² for methanol versus 0.10 Ω cm² for hydrogen at 600 °C. Further analysis suggests that the leakage current densities as high as 0.80 A cm⁻² at 600 °C and 0.11 A cm⁻² at 500 °C, resulting from the mixed electronic and ionic conductivity in the SDC electrolyte and thus reducing the fuel efficiency, can nonetheless help remove any carbon deposit and thereby ensure stable and coking-free operation of low temperature SOFCs in methanol fuels.     

Thin films for micro solid oxide fuel cells

Thin film deposition as applied to micro solid oxide fuel cell (μSOFC) fabrication is an emerging and highly active field of research that is attracting greater attention. This paper reviews thin film (thickness ≤1 μm) deposition techniques and components relevant to SOFCs including current research on nanocrystalline thin film electrolyte and thin-film-based model electrodes. Calculations showing the geometric limits of μSOFCs and first results towards fabrication of μSOFCs are also discussed.     

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.     

Effect of sintering aids on sintering kinetic behavior of praseodymium doped ceria based electrolyte material for solid oxide cells

The present study investigates the effect of sintering additives (Li, Co, Fe, and Mg) on the sintering kinetic behavior of the praseodymium-doped-ceria (PDC) electrolyte of solid oxide electrolyzer cell. 3Li-PDC, 3Co-PDC, 3Fe-PDC, and 3 Mg-PDC pellets were obtained from the synthesis of PDC nano-powder by microwave-assisted co-precipitation method using isopropyl alcohol as a solvent and followed by sintering additive wetness impregnation method. Linear shrinkage and shrinkage rate data suggest a positive sintering effect for 3Li-PDC and 3Co-PDC pellets and a negative sintering effect for 3 Mg-PDC and 3Fe-PDC pellets than compared to PDC pellets alone. The addition of lithium as a sintering additive (3Li-PDC) had reduced the sintering temperature of PDC from 1100 °C to 850 °C. For PDC, 3Li-PDC, 3Co-PDC, 3Fe-PDC and 3 Mg-PDC pellets sintered at 1100 °C, 850 °C, 1000 °C, 1200 °C, 1100 °C for 2 h resulted in a relative density of 93.6 ± 0.25, 95.8 ± 0.45, 95.0 ± 0.20, 92.7 ± 0.10, and 94.5 ± 0.10%, respectively. The XRD patterns of the sintered PDC pellets suggested a secondary phase formation (PrO₂) in 3Co-PDC, 3Fe-PDC, and 3 Mg-PDC pellets indicating that the addition of these sintering aids results in poor solubility limit of Pr in CeO₂. On the other hand, XRD patterns of PDC and Li-PDC sintered pellets displayed no secondary peak indicating good solid-solution formation. The activation energy of the 3Li-PDC pellet is obtained from CHR and Dorn methods and was found to be 182 kJ/mol and 196 kJ/mol. From the CHR method, for the 3Li-PDC pellet, the initial sintering behavior is by the grain boundary diffusion mechanism (m = ~2).     

Platinum-antimony doped tin oxide nanoparticles supported on carbon black as anode catalysts for direct methanol fuel cells

Antimony doped tin oxide supported on carbon black (ATO/C) has been synthesized using an in situ co-precipitation method, and platinum-ATO/C nanoparticles have been prepared using a consecutive polyol process to enhance the catalyst activity for the methanol oxidation reaction. The Pt-ATO/C electrocatalyst is characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microcopy (SEM), energy dispersive X-ray spectroscopy (EDS) and cyclic voltammetry. The Pt-ATO/C catalyst exhibits a relatively high activity for the methanol oxidation reaction compared to Pt-SnO₂/C or commercial Pt/C catalyst. This activity can be attributed to the high electrical conductivities of the Sb-doped SnO₂, which induces the electronic effects with Pt catalysts. Pt-ATO/C is a promising methanol oxidation catalyst with high activity for the reaction in direct methanol fuel cells.