Development of solid–oxide fuel cell for reduced operating temperatures

A technique for formation of electrolyte thin films with the thickness of 6–10 μm of zirconia stabilized by yttria (YSZ) is developed on the basis of the method of chemical deposition from the vapor phase of organometallic compounds (MOCVD). Planar electrochemical cells based on film electrolyte with a supporting anode with the working surface area of 12 cm² were manufactured. A solid-oxide fuel cell (SOFC) based on two fuel cells was developed and its life cycle tests at reduced operating temperatures (<800°C) were carried out for 400 h. The maximum power density reached in the SOFC tests was 316 mW/cm².     

Improvement of the stability of NiO–YSZ anode material for solid oxide fuel cell

Improvement of long-term stability of 40vol.%NiO–60vol.% yttria-stabilized zirconia (YSZ) anode material in reducing atmosphere and under exposure to thermal shock through the modification of vacancy concentration and pore shape has been investigated for a solid oxide fuel cell. We varied the amount of Y₂O₃ additives from 8 to 10 mol% in YSZ and the type of carbon pore former, from plated activated carbon to spherical carbon black, to improve the strength and the stability of porous NiO–YSZ anode materials. Modifications by varying the amount of Y₂O₃ additives and carbon pore former result in a highly stable anode, even upon exposure to a reducing atmosphere for 1,200 h. In particular, the strengths of the new anode materials are markedly improved at the same porosity level. Higher strengths do not degrade during a longtime durability test in a reducing atmosphere or upon thermal shock testing. The relatively smaller degradation of electrical conductivity of the new anode material is discussed in terms of the possibility of suppression of the disconnectivity of Ni phases during operation of a solid oxide fuel cell.     

Reduction kinetics of NiO–YSZ composite for application in solid oxide fuel cell

A porous nickel–8 mol% yttria stabilized zirconia (Ni–8YSZ) composite, used as anode for solid oxide fuel cell, was obtained by reduction of NiO–8YSZ cermet. The first goal was the evaluation of the temperature effect of powder processing by thermogravimetry. In addition, the influence of porosity in the reduction kinetic of the sample sintered at 1450 °C was evaluated. The final porosity produced in NiO–8YSZ composite by pore former was 30.4 and 37.9 vol.%, respectively, for 10 and 15 mass% of corn starch. The sample with 15 mass% of corn starch promotes a reduction rate almost twice higher than sample with 10 mass% of corn starch. The porosity introduced by the reduction of NiO was 23 vol.%.     

Thermodynamic analysis and multi-objective optimization performance of solid oxide fuel cell–Ericsson heat engine–reverse osmosis desalination

Journal of Thermal Analysis and Calorimetry - This paper targets to consider a hybrid cycle consisting of a solid oxide fuel cell and an Ericsson thermal engine that provides drinking water by...     

Recent developments in Ruddlesden–Popper nickelate systems for solid oxide fuel cell cathodes

Motivated by recent work on the Ruddlesden–Popper material, which was shown to be a superior oxide-ion conductor than conventional solid-oxide fuel cell cathode perovskite materials, we undertook A- and B-site doping studies of the Ruddlesden–Popper nickelate series in an attempt to identify other candidates for cathode application. In this paper, we summarize our most significant results for the and systems and more recently, the higher-order Ruddlesden–Popper phases La _n+1Ni _n O_3n+1 (n=2 and 3), which show greater promise as cathode materials than the n=1 compositions.     

Influence of cerium oxide on properties of glass–ceramic sealants for solid oxide fuel cells

The influence of the cerium oxide concentration on the properties of glasses and glass ceramics of the SiO₂–Al₂O₃–CaO–Na₂O–MgO–K₂O–B₂O₃–CeO₂ system as potential adhesive and sealing materials for solid oxide fuel cells was studied. According to the data of differential scanning calorimetry, variation of the CeO₂ concentration does not appreciably influence the glass transition and crystallization temperatures of glasses. As the cerium oxide concentration is increased, the linear thermal expansion coefficient increases for the glasses but decreases for the partially crystalline samples. The gluing temperature of the glass sealants prepared allows their use for joining YSZ solid electrolytes with interconnectors of Crofer22APU type in solid oxide fuel cells..     

Fabrication of 10%Gd-doped ceria (GDC)/NiO–GDC half cell for low or intermediate temperature solid oxide fuel cells using spray pyrolysis

Solid oxide fuel cells (SOFCs) with comparably low operating temperature play a critical role in its commercialization and reliability by allowing low-cost fabrication and a promised longer life. Recently, 10%Gd-doped ceria (GDC) has revealed its importance as solid electrolytes for intermediate temperature SOFCs. Additionally, if GDC is employed in thin film form, rather higher ionic conductivity at further lower temperatures can be obtained and thereby allowing its use in low temperature SOFC. In the present investigation, the preparative parameters of spray pyrolysis technique (SPT) were optimized to deposit dense and adherent films of GDC on ceramic substrate. NiO–GDC was used as ceramic substrate, which also acts as a precursor composite anode for GDC-based SOFCs. Prepared half cells (GDC/NiO–GDC) were characterized using XRD, SEM, and electrochemical impedance spectroscopy. The surface and fractal SEM observations of post heat-treated (at 1,000 °C) GDC/NiO–GDC structure revealed that GDC films were uniform in thickness with improved adherence to substrate. The relative density of post heat-treated films was of the order of 96%, which was attributed to the presence of nano-granules in the thin films. Maximum thickness of the GDC film prepared with optimized preparative parameters (in single run) was of the order of 13 μm. Fractal SEM of post heat-treated GDC/NiO–GDC system showed homogenous interface, which was further analyzed by electrochemical impedance spectra and found that it does not affect electrical properties of structure significantly.     

Electrochemical performance of strontium-doped neodymium nickelate mixed ionic–electronic conductor for intermediate temperature solid oxide fuel cells

The Nd_2???x Sr _x NiO_4?+?δ (x?=?0.1–0.5) solid solutions prepared by combustion synthesis are of submicron/superfine crystallite size. The crystal structure estimated by Rietveld analysis reveals increase in space in the rock salt layer on partial replacement of Nd³⁺ by Sr²⁺. The transition from negative temperature coefficient to positive temperature coefficient of conductivity is observed at 913 K. The maximum dc conductivity (σ?=?1.3?±?0.02 S?cm^?1 at 973 K) is obtained for x?=?0.2 in Nd_2???x Sr _x NiO_4?+?δ . The low dc conductivity compared with reported (≈100 S?cm^?1) is due to high porosity (low relative density) resulting from agglomeration of submicron crystallites. The variation in the conductivity with Sr content in Nd_2???x Sr _x NiO_4?+?δ is understood on the basis of defect chemistry. The electrochemical properties of the cathode materials are studied using electrochemical impedance spectroscopy at various temperatures and oxygen partial pressures. Nd_1.8Sr_0.2NiO_4?+?δ cathode exhibits lowest-area-specific resistance?=?0.52?±?0.015 Ohm?cm² at 973 K. At low $ {P_{{{{{\bf O}}_2}}}} $ (<1,000 Pa), oxygen ion transfer from Nd_1.8Sr_0.2NiO_4?+?δ cathode to gadolinium-doped ceria electrolyte is the rate-limiting step, whereas, charge-transfer reaction on the cathode becomes more important at high oxygen partial pressures and temperature (973 K).     

Carbothermal reduction of the YSZ–NiO solid oxide fuel cell anode precursor by carbon-based materials

The thermal behavior of the yttria-stabilized zirconia (YSZ) and nickel oxide (YSZ–NiO) composite mixtures with the addition of graphite, multiwall carbon nanotubes and functionalized multiwall carbon nanotubes was studied. The YSZ–NiO composite is the precursor of the YSZ–Ni anode of solid oxide fuel cells. The anode exhibits a porous structure, which is usually obtained by the addition of carbon containing pore formers. Thermal analysis and X-ray diffraction evidenced that the properties of carbonaceous materials (C) and atmosphere have a strong influence on the thermal evolution of the reactions taking place upon heating the anode precursor. The dependence of both the carbon content and the chemical nature of the ceramic matrix on the thermal behavior of the composite were investigated. The discussed results evidenced important features for optimized processing of the anode.     

Nanocrystalline ceria–praseodymia and ceria–zirconia solid solutions for soot oxidation

The relative effects of Zr⁴⁺ and Pr^3+/4+ dopants on the structure, redox properties, and catalytic performance of nanosized ceria was studied. The investigated ceria?Czirconia and ceria?Cpraseodymia (CP) solid solutions were prepared by a modified coprecipitation method, characterized by a variety of techniques, and evaluated for soot oxidation. The characterization results indicate that CP has more surface and bulk oxygen vacancies, redox sites, and lattice oxygen mobility, and better thermal stability. Besides having low specific surface area, CP is more active in soot oxidation. This better activity has been attributed to the presence of more surface and bulk oxygen vacancies, which promote the adsorption of gas-phase oxygen and the formation and mobility of large numbers of active oxygen species.     

Modeling current–voltage relationships of mixed conducting cathode materials for solid oxide fuel cells

In recent decades, high-temperature oxygen reduction reaction on mixed conducting cathodes were investigated intensively by many researchers. Computational approaches as well as electrochemical and spectroscopic studies have been made to elucidate the kinetics. Contribution of oxygen vacancy to the reaction rate was suggested in multiple reports, and plausible reaction pathways were proposed based on density functional theory (DFT) calculations. The picture of oxygen reduction reaction has become clearer in these years. However, there still is a discussion about a credible formula that represents the current–voltage relationships. Discrepancies are found among the reported data on the magnitude of the rate constant and on its dependencies on partial pressure and temperature. The difference is significant between a model electrode and a practical porous electrode. Comparison of the results suggests the existence of series reaction barriers, that is, the surface reaction and subsurface transport, which should be considered for consistent representation of the total electrode process.     

Performance studies of electrolyte-supported solid oxide fuel cell with Ni–YSZ and Ni–TiO2–YSZ as anodes

Redox cycling of Ni-based anode induces cell degradation which limits the cell's lifetime during solid oxide fuel cell operation. In the present study, the redox testing of electrolyte-supported cells has been investigated with TiO₂-added NiO–YSZ anode matrix. Button cells were fabricated by die-pressing YSZ powder as electrolyte, and onto which NiO–YSZ or NiO–TiO₂–YSZ anode and LSM–YSZ composite cathode were painted. The electrochemical performance and stability have been evaluated by measuring current–voltage characteristics followed by impedance spectroscopy after each redox cycling. Anode matrices before and after cell operation have been characterized by X-ray diffraction (XRD), elemental dispersive X-ray (EDX), and scanning electron microscopy (SEM). During cell operation the peak power density decreases from 111 mW cm^?2 (239 mA cm^?2) to 84 mW cm^?2 (188 mA cm^?2) between 23 and 128 h with five redox cycles for cell having NiO–YSZ (40:60) anode. But for cell with NiO–TiO₂–YSZ (30:10:60), the anode peak power density was constant and stable around 85 mW cm^?2 (194 mA cm^?2) throughout the cell run of 130 h and five redox cycles. No loss in the open circuit voltage was observed. SEM and XRD studies of NiO–TiO₂–YSZ (30:10:60) anodes revealed formation of ZrTiO₄, which may be responsible for inhibition of Ni coarsening leading to stable cell performance.     

Carbon monoxide reduction in solid oxide fuel cell–mini gas turbine hybrid power system

In this paper, combined heat and power frameworks employing solid oxide fuel cell power module and a small-scale gas turbine are presented. The offered system is utilized as heat and power supply for residential consumers with a carbon dioxide sorption circulating fluidized bed. As well a favorable solution for the high penalties associated with CO₂ capture and reuse of the CO contents is offered. The combined heat and power system considered by a different arrangement in order to high proficiency, controllability, heat recovery and high capacity of energy. In the proposed system, the unburned product from the solid oxide fuel cell is re-extracted and utilized as a fuel source. The suggested system is analyzed by the first and second law of thermodynamics. During this study, comprehensive calculations of chemistry and thermal within the fuel cell are performed to get accurate results. The impact of various parameters, for example fuel and oxidant rate, carbon dioxide removal, operating pressure, compressor parameter on work and heat output of the cycle as well as the discharge of carbon dioxide contamination, is investigated. The optimal pressure ratio of the compressor to minimize the carbon dioxide production is found.

     

Improvement of Ni/SDC anode by alkaline earth metal oxide addition for direct methane–solid oxide fuel cells

The anodic performances of Ni/CeO₂–Sm₂O₃(Ni/SDC) modified by the addition of alkaline earth metal oxides (MgO, CaO, and SrO) were investigated for direct oxidation of CH₄ in solid oxide fuel cells (SOFCs). Although the initial power density of cell with Ni/SDC anode modified by the addition of CaO was slightly lower than that of cell with Ni/SDC, the former anode exhibited an excellent stability compared to the latter one. Such a high stability of Ni–CaO/SDC anode may come from the inhibition of carbon deposition in addition to the retained ionic conductivity of anode.     

Ab initio and empirical defect modeling of LaMnO(3±δ) for solid oxide fuel cell cathodes

Sr doped LaMnO(3) is a perovskite widely used for solid oxide fuel cell (SOFC) cathodes. Therefore, there is significant interest in its defect chemistry. However, due to coupling of defect reactions and inadequate constraints of the defect reaction equilibrium constants obtained from thermogravimetry analysis, large discrepancies (up to 4 eV) exist in the literature for defect energetics for Sr doped LaMnO(3). In this work we demonstrate how ab initio energetics and empirical modelling can be combined to develop a defect model for LaMnO(3). Defect formation enthalpies, including concentration dependence due to defect interactions, are extracted from ab initio energies calculated at various defect concentrations. Defect formation entropies for the defect reactions in LaMnO(3) involving O(2-)(solid) ? ?O(2)(gas) + 2e(-) are shown to be accessible through combining the gas phase thermodynamics and simple models for the solid phase vibrational contributions. This simple treatment introduces a useful constraint on fitting defect formation entropies. The predicted defect concentrations from the model show good agreement with experimental oxygen nonstoichiometry vs. P(O(2)) for a wide range of temperatures (T = 873-1473 K), suggesting the effectiveness of the ab initio defect energetics in describing the defect chemistry of LaMnO(3). Further incorporating a temperature dependent charge disproportionation energy within 0.0-0.2 eV, the model is capable of describing both defect chemistry and oxygen tracer diffusivity of LaMnO(3). The model suggests an important role for defect interactions which are typically excluded from LaMnO(3) defect models, and sensitivity of the oxygen defect concentration to the charge disproportionation energy in the high P(O(2)) region. Similar approaches to those used here can be used to model the defect chemistry for other complex oxides.     

Tests and optimization of low–power power plant based on solid–oxide fuel cells

The paper presents the results of experimental development of the fuel processor of natural gas steam–air conversion and power plants with different design layouts based on solid–polymer and solid–oxide fuel cells. The preferability of using solid–oxide fuel cells in stationary power plants with natural gas as fuel is confirmed. The test results confirm the working efficiency and safety of the chosen solutions. Directions for the future activity in the field of design and development of low–power power plants based on solid–oxide fuel cells are formulated.     

Effect of cosintering of anode–electrolyte bilayer on the fabrication of anode-supported solid oxide fuel cells

A comparative study is carried out on the effect of cosintering temperature of anode–electrolyte bilayer on the fabrication and cell performance of anode-supported solid oxide fuel cells from commercially available tape casting materials. It was found that the sintering conditions have profound effects on the anode characteristic and cell performance. For low cosintering temperature as low as 1,250 °C, the electrolyte is unable to sinter fully and forms a porous structure which leads to a reduced open-circuit potential and poor cell performance especially under low current output. For further increasing cosintering temperature to 1,350 °C, the cell performance was lower under low current operation. However, the cell performance turns out to be better than that of high-temperature cosintering under high current output. Although at temperature as high as 1,500 °C the cell performs better than that of low temperature cosintering, the trend turn out to be reverse for high current operating due to less anode surface area resulting from overagglomeration of anode layer. An optimal cosintering temperature of 1,350–1,450 °C is recommended for commercially available anode–electrolyte bilayer of anode-supported solid oxide fuel cells.     

The physical properties and electrochemical performance of Ca-doped Sr2MgMoO6-δ as perspective anode for solid oxide fuel cells

Journal of Solid State Electrochemistry - The double perovskites Sr2-xCaxMgMoO6–δ with x = 0, 0.25, and 0.5 were obtained by combustion of organometallic precursors and were...     

Preparation of graphene oxide–chitosan composite and adsorption performance for uranium

The composite adsorbent graphene oxide–chitosan was prepared using graphite and chitosan as the initial materials. The structures and morphology of the products were characterized by Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy. The products were used to remove uranium from aqueous solution by batch adsorption experiments. The effects of pH, adsorbent dosage, contact time, initial uranium concentration and temperature on the uranium removal were investigated, and the results were fitted by the adsorption isotherm models. The adsorption kinetic and thermodynamic were also studied in detail.