Self-rising synthesis of Ni-SDC cermets as anodes for solid oxide fuel cells

Ni-SDC cermets have been obtained using a self-rising approach by two different ways, one-step direct synthesis (OS) and ball milling the separately prepared NiO and SDC powders (BM). The results showed that self-rising approach was an efficient way for the synthesis of porous materials composed of evenly distributed uniform size nanocrystals. The as-synthesized powders have been applied as anodes for solid oxide fuel cells, whose electrochemical properties have been systematically studied. Cells with anodes from the BM method showed better performance compared with those of the OS method, achieving a maximum power density of 400 mW cm⁻² at 600 °C.     

Advanced perovskite anodes for solid oxide fuel cells: A review

Solid oxide fuel cells (SOFCs) are at the frontline of clean energy generation technologies to convert chemical energy to electricity with high efficiency. In recent years, because of their fuel flexibility, multiple fuels are fed in anode, e.g., hydrogen, ammonia, hydrocarbons, solid carbon, etc.; in addition, these fuels are always mixed with a certain amount of H₂S. Perovskite is one of the most important classes of anode materials being investigated in laboratories, these materials to some extent are immune to coke formation and sulfur poisoning when using hydrocarbon fuels, and retain inherent stability upon reduction and oxidation cycling. In this review, recent developments in perovskite anode materials are summarized and future prospects are discussed.     

Nickel-Zirconia cermet processing by mechanical alloying for solid oxide fuel cell anodes

This paper describes the development of a process based on high energy milling (or mechanical alloying—MA) of metallic Ni and YSZ at 40 vol% Ni composition for the preparation of solid oxide fuel cell anode material. The cermet powder is consolidated using the surface activated sintering (SAS) method. The cermet pellets possess microstructural characteristics that can potentially lead to higher electrocatalytic activity and fuel reforming capability. In addition to the development of a new processing method for this purpose, a further differential of this work is the addition of Cu in partial substitution of Ni as a means to prevent the formation of carbon on its surface and, hence, the anode’s degradation during service. The prepared powder samples are well dispersed and structured at the nanometric level, showing thin lamellar constituents. Suitable sintered pellets can be obtained from the powders with the required porosity and microstructure. The higher the energy delivered by MA the lower the initial sintering temperature. Activation energies are determined by stepwise isothermal dilatometry (SID) for Ni-YSZ and Ni/Cu-YSZ pellets, involving a 2-step sintering process. The Cu additive promotes sintering and leads to a refined microstructure.     

Thermodynamic analysis of interactions between Ni-based solid oxide fuel cells (SOFC) anodes and trace species in a survey of coal syngas

A thermodynamic analysis was conducted to characterize the effects of trace contaminants in syngas derived from coal gasification on solid oxide fuel cell (SOFC) anode material. The effluents from 15 different gasification facilities were considered to assess the impact of fuel composition on anode susceptibility to contamination. For each syngas case, the study considers the magnitude of contaminant exposure resulting from operation of a warm gas cleanup unit at two different temperatures and operation of a nickel-based SOFC at three different temperatures. Contaminant elements arsenic (As), phosphorous (P), and antimony (Sb) are predicted to be present in warm gas cleanup effluent and will interact with the nickel (Ni) components of a SOFC anode. Phosphorous is the trace element found in the largest concentration of the three contaminants and is potentially the most detrimental. Poisoning was found to depend on the composition of the syngas as well as system operating conditions. Results for all trace elements tended to show invariance with cleanup operating temperature, but results were sensitive to syngas bulk composition. Synthesis gas with high steam content tended to resist poisoning.     

Novel copper-based anodes for solid oxide fuel cells with samaria-doped ceria electrolyte

A novel copper-based anode for low-temperature solid oxide fuel cells was prepared through the conventional ceramic technology and using CuO and SDC (Ce_0.8Sm_0.2O_1.9) powders with controlled particle size. The new Cu–SDC anode also contained highly dispersed CeO₂ and Ni particles to increase its surface area and fuel cell performance. The specific surface area of the Cu–SDC bare anode, CeO₂ and Ni-dispersed phases were estimated to be 1.53, 39.4 and 86.4 m² g⁻¹, respectively. Solid oxide fuel cells having the new anode were tested for both humid hydrogen and methane. Power densities of ca. 250 mW cm⁻² were achieved in H₂ at 600 °C and in CH₄ 700 °C, even if the SDC–electrolyte supporting membrane was 250-μm thick. Short term stability tests (maximum 64 h) showed an initial impairment, but not dramatic, of the new anode performance and the formation of carbon deposits. The addition of MoO_x to the new anode did not prevent the formation of carbon deposits.     

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.     

Properties of bias-assisted sputtered gadolinia-doped ceria interlayers for solid oxide fuel cells

The growth and electrochemical properties of gadolinia-doped ceria (GDC) interlayers deposited by bias-assisted magnetron sputtering in solid oxide fuel cells have been investigated. Such interlayers act as diffusion barriers to protect the yttria-stabilized zirconia electrolyte, preventing possible degradation when mixed ionic-electronic conductor (La,Sr)(Co,Fe)O_3−δ is used as the cathode. The dependence of the applied bias during the sputtering deposition on both the interlayer microstructure and fuel cell performance has been studied in anode-supported single cells. The main experimental results showed that bias-assisted sputtering of GDC interlayers produced microstructures denser than those of unbiased depositions, which resulted in increased electrochemical properties of fuel cells.     

High performance electrolyte-coated anodes for low-temperature solid oxide fuel cells: Model and Experiments

A geometric micro-model and experiment development are presented for electrolyte-coated anodes with high performance in solid oxide fuel cells. The anodes are based on electron conducting frameworks, where fine, oxygen-ion conducting inclusions are introduced via an ion impregnation process. The model shows that the length of triple-phase-boundary (TPB) increases with the loading of the coated electrolyte, and is dependent only on the loading before a maximum loading for monolayer coverage is obtained. The maximum loading increases with the porosity of the framework. As a result, the prolonged TPB length can be achieved by increasing the porosity and the loading. In the experimental study, Ni was used as the electron conductor, and samaria-doped ceria (SDC) was employed as the electrolyte to form anode-supported single cells. The cell performance was evaluated using humidified hydrogen as the fuel. The peak power density increased with SDC loading to a maximum value and decreased when the loading was further increased. The highest peak power density of the cells whose anodes were prepared with 10, 20 and 30 wt.% pore former was 571, 631 and 723 mW cm⁻², corresponding to 508, 564 and 648 mg cm⁻³ of SDC loading, respectively. The experimental results are in good agreement with the model prediction. Therefore, this work demonstrates theoretically and experimentally that optimization of the porosity and electrolyte loading is critical for further improving the performance of electrolyte-coated anodes.     

Ceria coated Ni as anodes for direct utilization of methane in low-temperature solid oxide fuel cells

A new type of anode, a Ni framework coated with Sm-doped ceria (SDC), was developed for direct utilization of methane fuel in low-temperature solid oxide fuel cells (SOFCs) with thin-film SDC electrolytes. The coated SDC was prepared with an ion impregnating method and the electrolyte films were fabricated with a co-pressing and co-firing technique. The impregnating process produced an ideal anode microstructure where nickel particles were effectively connected and uniformly covered with nanosized SDC. This anode microstructure was believed to enlarge the triple-phase boundaries and therefore enhance the anode performance. The cell performance was much higher than that of a conventional fuel cell with a Ni-SDC composite anode. In addition, the performance increased with impregnated SDC loading up to a maximum at 20 mg cm⁻², indicating that the coated SDC is the contributing factor for the enhanced fuel cell performance. Power density as high as 571 and 353 mW cm⁻² were obtained at 600 °C when humidified hydrogen and methane were used as fuels, respectively. The stability of the cell also increased with the SDC loading. No significant degradation was observed for anodes coated with 20 and 25 mg cm⁻² SDC. This verifies that the coated SDC electrodes are very effective in suppressing catalytic carbon formation by blocking methane from approaching the Ni, which is catalytically active towards methane pyrolysis. The high performance of this anode shows high promise in the developing field of direct hydrocarbon SOFCs.     

NiMo-calcium-doped ceria catalysts for inert-substrate-supported tubular solid oxide fuel cells running on isooctane

A calcium-doped ceria (Ce_1-xCa_xO_2−δ, 0 ≤ x ≤ 0.3) has been applied as a ceramic support in NiMo-based catalysts for an internal reforming tubular solid oxide fuel cell running on isooctane. Introducing calcium into the CeO₂-based ceramic was found to improve conductivity of Ce_1-xCa_xO_2−δ. The Ce_0.9Ca_0.1O_2−δ (x = 0.10) sample exhibited an optimum conductivity of 0.045 S cm⁻¹ at 750 °C. The transport of oxygen ions in Ce_1-xCa_xO_2−δ promoted the catalytic partial oxidation of isooctane in the NiMo–Ce_1-xCa_xO_2−δ catalyst, which increased the fuel conversion as well as H₂ and CO yields. As a result, the NiMo–Ce_0.9Ca_0.1O_2−δ (x = 0.10) catalyst exhibited a high isooctane conversion of 98%, and the H₂ and CO yields achieved 74% and 83%, respectively, for reforming of isooctane and air at the O/C ratio of 1.0 at 750 °C. Furthermore, the NiMo–Ce_0.9Ca_0.1O_2−δ catalyst has been applied as an internal reforming layer for an inert-substrate-supported tubular solid oxide fuel cell running on isooctane/air. Due to its high reforming activity, the single cell presented an initial maximum power density of 355 mW cm⁻² in isooctane/air at 750 °C and displayed stable electrochemical performance during ~30 h operation. These results demonstrated the application feasibility of the NiMo–Ce_0.9Ca_0.1O_2−δ catalyst for direct internal reforming solid oxide fuel cells running on isooctane/air.     

Alternative anode materials for solid oxide fuel cells

The electrolyte of a solid oxide fuel cell (SOFC) is an O²⁻-ion conductor. The anode must oxidize the fuel with O²⁻ ions received from the electrolyte and it must deliver electrons of the fuel chemisorption reaction to a current collector. Cells operating on H₂ and CO generally use a porous Ni/electrolyte cermet that supports a thin, dense electrolyte. Ni acts as both the electronic conductor and the catalyst for splitting the H₂ bond; the oxidation of H₂ to H₂O occurs at the Ni/electrolyte/H₂ triple-phase boundary (TPB). The CO is oxidized at the oxide component of the cermet, which may be the electrolyte, yttria-stabilized zirconia, or a mixed oxide-ion/electron conductor (MIEC). The MIEC is commonly a Gd-doped ceria. The design and fabrication of these anodes are evaluated. Use of natural gas as the fuel requires another strategy, and MIECs are being explored for this application. The several constraints on these MIECs are outlined, and preliminary results of this on-going investigation are reviewed.     

Recent anode advances in solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are electrochemical reactors that can directly convert the chemical energy of a fuel gas into electrical energy with high efficiency and in an environment-friendly way. The recent trends in the research of solid oxide fuel cells concern the use of available hydrocarbon fuels, such as natural gas. The most commonly used anode material Ni/YSZ cermet exhibits some disadvantages when hydrocarbons were used as fuels. Thus it is necessary to develop alternative anode materials which display mixed conductivity under fuel conditions. This article reviews the recent developments of anode in SOFCs with principal emphasis on the material aspects. In addition, the mechanism and kinetics of fuel oxidation reactions are also addressed. Various processes used for the cost-effective fabrication of anode have also been summarized. Finally, this review will be concluded with personal perspectives on the future research directions of this area.     

Reinforced composite sealants for solid oxide fuel cell applications

Glass-ceramic sealants are commonly used as joining materials for planar solid oxide fuel cells stacks. Several requirements need to be fulfilled by these materials: beside of electrical insulation and appropriate thermal expansion, a good adhesion on the ceramic and metallic components of a SOFC stack is necessary to form a gas-tight joint. Even though the joining process might have been successful, failures and leaks often occur during the stack operation due to fracture of the brittle material under thermal stresses or during thermal cycling of the components. This study focusses on composite materials consisting of a glass matrix based on the system of BaO-CaO-SiO₂ and various filler materials, e.g. yttria-stabilized zirconia fibres or particles and silver particles. In order to evaluate a possible reinforcing influence of the filler material of the composite, tensile strength tests were carried out on circular butt joints. The highest strength values were found for the composite material with addition of silver particles, followed by the glass matrix itself without any filler addition and the lowest values were measured for the composite with YSZ particles. SEM investigations of cross-sections of the joints elucidated these results by the microstructure of the glass-ceramic sealants.     

Various synthesis methods of aliovalent-doped ceria and their electrical properties for intermediate temperature solid oxide electrolytes

This article investigates the relationship between ionic conductivity and various processing methods for aliovalent-doped, ceria solid solution particles, as an intermediate temperature-solid oxide electrolyte to explain the wide range of conductivity values that have been reported. The effects of doping material and content on the ionic conductivity are investigated comprehensively in the intermediate temperature range. The chemical routes such as coprecipitation, combustion, and hydrothermal methods are chosen for the synthesis of ceria-based nanopowders, including the conventional solid-state method. The ionic conductivity for the ceria-based electrolytes depends strongly on the lattice parameter (by dopant type and content), processing parameters (particle size, sintering temperature and microstructure), and operating temperature (defect formation and transport). Among other doped-ceria systems, the Nd_0.2Ce_0.8O_2−d electrolyte synthesized by the combustion method exhibits the highest ionic conductivity at 600 °C. Further, a novel composite Nd_0.2Ce_0.8O_2−d electrolyte consisting of a combination of powders (50:50) synthesized by coprecipitation and combustion is designed. This electrolyte demonstrates an ionic conductivity two to four times higher than that of any singly processed electrolytes.     

Investigation of Ni2+-doped ceria nanorods as the anode catalysts for reduced-temperature solid oxide fuel cells

Tailoring the surface chemistry of oxides has been widely used to adjust their catalytic behavior in the energy conversion and storage devices. Herein, nanorods of Ni²⁺-doped ceria (Ce_1-xNi_xO_2-δ, x = 0, 0.05, 0.1, 0.15) are synthesized via a modified hydrothermal method, and evaluated as the anode catalysts for reduced-temperature solid oxide fuel cells (SOFCs). X-Ray diffraction patterns of as-synthesized powders in air imply successful incorporation of Ni²⁺ into the fluorite lattice of ceria for x = 0.05 and 0.1, with a secondary phase of NiO observed for x = 0.15. Transmission electron microscopy (TEM) examination confirms a rod-like morphology with a diameter of 10–13 nm and a length of 55–105 nm. Exposure of these powders in H₂ at 600°C results in exsolution of some spherical Ni particles of 11 nm in diameter. Electrochemical measurements on both symmetrical anode fuel cells and functioning cathode-supported fuel cells show an order of the catalytic activity toward hydrogen oxidation - CeO_2-δ < Ce_0·95Ni_0·05O_2-δ < Ce_0·9Ni_0·1O_2-δ. The anode polarization resistances in 97% H₂ – 3% H₂O are 0.24, 0.31 and 0.37 Ω?cm² for Ce_0·9Ni_0·1O_2-δ, Ce_0·95Ni_0·05O_2-δ and CeO_2-δ at 600°C, respectively. Thin (La_0·9Sr_0.1) (Ga_0.8Mg_0.2)O_3-δ-electrolyte fuel cells with nanostructured SmBa_0.5Sr_0·5Co₂O_5+δ cathodes and Ce_0·9Ni_0·1O_2-δ anodes yield the highest power densities among the investigated series of anodes, e.g., 820 mW?cm^?2 in 97% H₂ – 3% H₂O and 598 mW?cm^?2 in 68% CH₃OH - 32% N₂. XPS analyses of reduced nanorods indicate that the highest catalytic activities of Ce_0·9Ni_0·1O_2-δ toward fuel oxidation reactions should be correlated to the presence of highly active Ni nanoparticles and increased surface active oxygen, as confirmed by substantially facilitated extraction of the lattice oxygen on the surface by H₂ in temperature-programmed reduction (TPR) measurements.     

Nanoscale bismuth oxide impregnated (La,Sr)MnO3 cathodes for intermediate-temperature solid oxide fuel cells

Bismuth oxide based oxygen ion conductors are incorporated into (La,Sr)MnO₃ (LSM), the classical cathode material for solid oxide fuel cells (SOFC), to improve the cathode performance. Yttria-stabilized bismuth oxide (YSB) is taken as an example and is impregnated into a preformed porous LSM frame, forming a highly active cathode for intermediate-temperature SOFCs (IT-SOFCs) with doped ceria electrolytes. X-ray diffraction indicates that YSB is chemically compatible with LSM at intermediate temperatures below 800 °C. The impregnated YSB particles are nanosized and are deposited on the surface of the framework. Significant performance improvement is achieved by introducing nanosized YSB into the LSM electrodes. At 600 °C, the interfacial polarization resistance under open-circuit conditions for electrodes impregnated with 50% YSB is only 1.3% of the original value for a pure LSM electrode. The resistance is further reduced dramatically when current is passed through. In addition, the YSB impregnated LSM electrodes has the highest electrochemical performance among those based on LSM. Single cell with 25% of YSB impregnated LSM cathode generates maximum power density of 300 mW cm⁻² at 600 °C, indicating the promise of using LSM-based electrodes for IT-SOFC.     

Fibrous mixed conducting cathode with embedded ionic conducting particles for solid oxide fuel cells

The Sm_0.5Sr_0.5CoO_3−δ (SSC) fibers with embedded nano-Sm_0.2Ce_0.8O_1.9 (SDC) particles are fabricated by electrospinning process using commercial SDC nanopowders and an SSC precursor gel containing polyvinyl alcohol (PVA) and aqueous metal nitrate. After calcination at 800 °C, fibers with diameters ranged between 300 and 500 nm and well-developed SSC cubic-perovskite structure and SDC fluorite are successfully obtained. The calculated crystallite sizes of SSC and SDC are 20.78 and 45.35 nm, respectively. Over whole measured temperature ranges during the symmetrical cell test, the fiber composite cathode exhibits much lower polarization resistance than conventional powder composite cathodes. The polarization resistances are estimated to 0.06 and 1.23 Ω cm² for the fiber composites and 0.15 and 2.10 Ω cm² for the powder composites at 700 and 550 °C, respectively. The single cell with the fiber composite cathode shows much higher performances; its maximum power density is 380.5 mW cm⁻² at 550 °C and higher than 1278 mW cm⁻² at 700 °C.     

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.     

Effects of coal syngas impurities on anodes of solid oxide fuel cells

A literature review is conducted to summarize the studies on the identification of impurities in coal syngas and their effects on the performance of Ni-yttria stabilized zirconia (Ni-YSZ) anode of solid oxide fuel cells (SOFCs). Coal syngas typically contains major species, CO, H₂, CO₂, H₂O, CH₄, N₂, and H₂S as well as trace impurities. Thermodynamic equilibrium calculations have indicated that trace impurities species such as Be, Cr, K, Na, and V in the coal syngas form condensed phases under warm gas cleanup conditions and can be effectively removed by the cleanup processes. For meaningful data comparison, a practical parameter is formulated to quantify the level of degradation normalized with respect to the relevant experimental parameters. Experimental results show that the existence of Hg, Si, Zn and NH₃ in the coal syngas does not significantly affect the performance of the Ni-YSZ anode. The presence of Cd and Se in the syngas impacts the SOFC anode performance to some extent. Impurity species such as Cl, Sb, As, and P cause severe cell voltage degradation due to attack on the Ni-YSZ anode. Sb, As and P have the potential to react with Ni to form secondary phases in the Ni-YSZ anode, which deteriorate the catalytic activity of the anode.