Advances in Low Temperature Coatings for Solid Oxide Fuel Cell Components

For the last few decades, alternative and sustainable energy sources have been approached which deplete the environmental issues and provides the clean energy conversion towards world energy demands. Fuel cells which convert chemical (Fuel) energy directly into electrical energy (electricity) via electro-chemical reaction mechanism are more advantageous when compared with the other conventional energy conversion technologies. Among the various available fuel cells, Solid oxide fuel cell (SOFC) has proven as most promising, efficient, eco-friendly energy conversion device due to its longer life time, variety choice of fuels and less environmental impact. The fundamentals of fuel cell technologies, background, working mechanism, different components of SOFC, brief note on traditional materials, and development of novel materials for the advanced SOFC were addressed. Working/operating temperature plays a key role in the development of SOFC because high operating temperature curbs the commercialization. Enormous efforts have been done in reducing the operating temperature by introducing various doped /co doped /triple-doped ceria based materials with enhanced electrical properties which led to the development of intermediate temperature SOFCs. On the way of reducing the operating temperature, R&D introduced a composite approach using doped ceria and alkali carbonates which pave a way for the development of low operating temperature SOFC. The recent developments regarding ceria-carbonate composites with emphasis on the ionic conductivity have been discussed towards the electrolytes for advanced SOFC applications at relatively lower temperatures

The central components for solid oxide fuel cells (SOFC) are the electrodes-electrolyte multilayer arrangement (PEN) and the separating bipolar plates. The PEN (Positive electrode- Electrolyte-Negative electrode) assembly consists of a dense gastight yttria-stabilized zirconia (YSZ) electrolyte and porous electrodes for which usually Ni-YSZ cermet anode and Sr-doped LaMnO3 cathode layers are used. The various PEN units are connected in a cell stack by bipolar plates which are either metallic or ceramic ones. Furthermore, a protective layer on the metallic bipolar plates consisting of a chromium alloy is required to prevent chromium evaporation leading to a rapid and strong degradation of the SOFC performance. At the DLR Stuttgart both the DC and the RF vacuum plasma spraying technique have been further developed and adapted to meet the requirements for the manufacture of the different SOFC components. The DCVPS process using specially developed Laval-like nozzles is especially appropriate to the production of thin and dense coatings as required for the electrolyte and the protective layers. However, applying special spray parameters and nozzles it is also possible to deposit porous electrode layers. The production of the entire PEN arrangement in one consecutive DC-VPS process is the objective of the actual development. On the other hand, the RF plasma spray technique is suitable for the near net-shape production of bulk components such as the metallic bipolar plate. The development of the deposition processes for the production of SOFC components using DC and RF plasma spray methods and the results obtained concerning PEN fabrication, deposition of protective layers and the near net-shape production of metallic bipolar plates are presented in the paper.

Solid Oxide Fuel Cells (SOFCs) possess various prominent advantages such as high overall energy conversion efficiency, pollution-free, and no need for expensive metal catalysts, which enable them to have broad application prospects in different fields. Currently, the primary research directions in SOFC studies are focused on material selection and modification through doping to enhance fuel cell performance. This article moved beyond the conventional research methods of material selection and doping, and explored the improvement of interfacial contact between various components in SOFCs through acid treatment. This article studied whether there were also issues where the electrochemical performance of the fuel cell failed to reach the theoretical value due to poor contact between the electrode and electrolyte in SDC ceramic pieces sintered at high temperatures. This article discussed the specific influencing factors and the impact of interfacial engineering on fuel cell performance. Press NCAL powder and SDC powder into a three-layer structural symmetrical fuel cell of NCAL/SDC/NCAL, and studied whether the surface of the acid treatment electrode affected the performance of the fuel cell. Considering that the concentrated acid would react with NCAL, and the concentration of the acid was diluted and dropped on the surface of the NCAL electrode to see if the performance of the fuel cell was affected. The results showed that dilute nitric acid treatment can effectively increase the porosity inside the electrode, making the fuel cell show better electrochemical performance. At 550°C, the maximum output power of NCAL/SDC/NCAL structure reaches 281 mW cm^-2. The results showed that moderate acid treatment can effectively improve the electrochemical performance of the fuel cell, indicating that effective interface treatment engineering can effectively help high temperature annealing fuel cells to achieve theoretical power.

Summary form only given, as follows. Solid Oxide Fuel Cells (SOFC) has been thought as one of most important energy resource of 21century for its high efficiency and low pollution. However, low-cost, rapid, and batch fabrication technology has not, been established up to now. Plasma spray, which is not limited by pattern's size or spray material, has received much attention. In this paper, air plasma spray, which has high deposition efficiency, was used for the fabrication of SOFC component films. Electrolyte (8 mol% Y/sub 2/O/sub 3/ stabilized ZrO/sub 2/, YSZ), cathode (Sr-doped LaMnO/sub 3/, LSM) was deposited rapidly on anode-supported (Ni-YSZ), which was prepared by tape-casting process. Influence of plasma spray parameters including powder granularity, spray rate, power, and gun-to-work distance, on the crack microstructures of Solid Oxide Fuel cell component films was investigated and formative mechanism of micro-crack in the films observed by SEM was discussed. The results show that cracks began at pore and ending at another pore and their generating depend on spray powder and its size, however, is independent of spray rate and gun-to-work distance. Performance of films which be annealed was better than which not be annealed after deposited by plasma spray.

Glass-ceramic seals play an important role in the performance of electrochemical devices like solid oxide fuel cell (SOFC), high temperature electrolysis (HTE) and ceramic membrane reactors. In addition to gas tightness and electrical resistivity, a seal material must have a set of thermo-mechanical and chemical properties in order to efficiently seal the SOFC cell components and to maintain performance at elevated temperature over extended operating time. The seal must be stable in oxidizing and reducing atmospheres and withstand thermal cycles between room and the cell typical operating temperature (800 to 900°C). Chemical reaction with the SOFC components should be minimal so as to keep integrity of the seal/cell-interface and prevent degradation in seal reliability over time. In this work, the glass-ceramic seal is discussed for which optimal sintering/crystallization behaviors and thermal stability have been demonstrated. The investigation of the sealant was performed from the point of view of sealing ability and crystallization behavior using Differential Scanning Calorimetry (DSC). The evolution of the seal Coefficient of Thermal Expansion (CTE) has been followed by dilatometric analysis, crystalline phase evolution has been investigated by X-Ray Diffraction (XRD) and the chemical interaction between sealant and cell components has been evaluated by Scanning Electron Microscope (SEM) and Electron Probe Micro Analysis (EPMA).

We report a new sealant material for solid oxide fuel cells (SOFCs) using glass-ceramic. A glass-ceramic based on the CaO-Al2O3-SiO2 system was selected as a sealant because it can be easily molded to give a gastight seal and remains in a solid-like state under typical fuel cell operating conditions. Calcium alumina silicate glass powder is placed in the gas-sealing area before crystallization. During heating, the glass softens and bonds with the SOFC components. At higher temperatures, bulk crystallization occurs in the glass and the viscosity of the glass-ceramic sealant increases. We show that this sealing material which is able to seal at higher viscosities compared with the melt sealing method produces a good gas-tight seal and is chemically stable in contact with SOFC components, e.g., yttria-stabilized zirconia electrolytes and lanthanum chromite separators.

Solid oxide fuel cell (SOFC) is one of the most efficient energy converters which can produce electricity from reactions of gaseous chemicals. However, a high operating temperature (T o) of 800–1200 °C is required to achieve the highest peak performance. Such high T o triggers miscellaneous problems, that is, mechanical disintegration with further degradation of its electrical performance for a long‐term application. Despite accomplishing various approaches for the improvement of SOFC performance, the standard level to determine SOFC performance has never been achieved. To date, a quantitative analysis is only pinpointing the power output density (P out). The SOFC cannot be simply determined by a mere P since other parameters that is chemical stability between SOFC components, mismatch of thermal expansion coefficient, and area‐specific resistance also contribute to the mechanical durability and electrochemical stability of the SOFC. Hence, herein, a novel idea is proposed to characterize the overall performance of SOFC through the figure‐of‐merit compatibility (χ cp) of SOFC cells. The χ cp can reflect a level of SOFC performance, thus it can be utilized for future improvements.

The goals of a research program recently completed at the University of California, Irvine were to develop analysis strategy for solid oxide fuel cell (SOFC) based systems, to apply the analysis strategy to tubular SOFC hybrid systems and to identify promising hybrid configurations. A pressurized tubular SOFC combined with an intercooled-reheat gas turbine (SureCell™ cycle) is chosen as the base cycle over which improvements are sought. The humid air turbine (HAT) cycle features are incorporated to the base cycle resulting in the SOFC-HAT hybrid cycle which shows an efficiency of 69.05 percent while the base cycle has an efficiency of 66.23 percent. Exergy analysis identified the superior efficiency performance of the SOFC component. Therefore, an additional cycle variation added a second SOFC component followed by a low pressure combustor in place of the reheat combustor of the gas turbine of the SOFC-HAT hybrid. The resulting dual SOFC-HAT hybrid has a thermal efficiency of 75.98 percent. The single SOFC-HAT hybrid gives the lowest cost of electricity (3.54¢/kW-hr) while the dual SOFC-HAT hybrid has the highest cost of electricity (4.02¢/kW-hr) among the three cycles with natural gas priced at $3/GJ. The dual SOFC-HAT hybrid plant cost is calculated to be significantly higher because the fraction of power produced by the SOFC(s) is significantly higher than that in the other cases on the basis of $1100/kw initial cost for the SOFC. The dual SOFC-HAT hybrid can only be justified in favor of the single SOFC-HAT hybrid when the price of natural gas is greater than $14/GJ or if a severe carbon tax on the order of $180/ton of CO2 is imposed while natural gas price remains at $3/GJ.

For the commercialization of solid oxide fuel cell (SOFC), in addition to the electrical reliability, it is also important to ensure the mechanical reliability of SOFC. Thus, the establishment of a suitable mechanical testing method under various temperatures and oxygen partial pressure conditions is a prerequisite for the development of reliable SOFCs. This paper presents the experimental results of the investigation of elastic modulus and fracture strength on the SOFC components under reducing conditions. An in-situ mechanical testing method was developed in order to investigate the elastic modulus and fracture strength of solid oxide fuel cells components under high oxidizing/reducing environments. The elastic modulus was shown to change under reducing condition, and the behavior was most likely due to non-stoichiometry. It was demonstrated that the in-situ mechanical testing method enabled us to estimate the internal stress in the SOFC cell and stack.

A comprehensive review on durability improvement of solid oxide fuel cells for commercial stationary power generation systems

Among the various oxygen conducting materials, Sr and Mg doped lanthanum gallate (LSGM) is attractive for use as an electrolyte in intermediate temperature Solid Oxide Fuel Cells (SOFC). the LSGM compositions show high oxygen conductivity as well as excellent stability over the range of oxygen partial pressures encountered in SOFC operation. Lowering the operating temperature is expected to provide a more expeditious route to commercial realization of Solid Oxide Fuel Cell (SOFC) based power systems. the intermediate operating temperature (600 to 800°C) allows the use of lower cost materials in both the stack and the balance of plant, and significantly slows down potential deleterious materials interaction, thus extending the useful long-term performance and thermal cycle capability. the primary challenge in long-term stability of LSGM-based fuel cell operation is the chemical reaction between the nickel anode and the LSGM electrolyte. A chemical modification to the anode was found to reduce the reaction. the modified anode composition is also shown to perform equivalent to standard NiO based anode. Single cells using thin (30 micron) electrolyte supported on a thicker anode were found to operate at a power density of greater than 500 mW/cm2 at 700°C. Stack test verification using thick electrolyte supported cells show promising performance. Stack tests using thin electrolytes are planned.

Review: 218 refs.

Introduction For many potential mobile robot applications, current power supply technology (such as the lithium ion battery technology) is in fact a key limiting factor due to its low energy density. Solid oxide fuel cells (SOFCs) are a promising alternative and can overcome limitations of current battery technology. This is because SOFCs can, in principle, use not only hydrogen but also hydrocarbon fuels which have high energy density such as gasoline and so on.Ni-YSZ cermet is the most promising anode material for SOFCs because of its low cost, chemical stability, and closely matched thermalexpansion coefficients with the dense YSZ electrolyte. Ni is an excellent catalyst for hydrocarbon fuel reforming and cracking, which leads to rapid carbon deposition inside the SOFC anode and causes coking and deactivation1). In principle, the carbon deposition on Ni can be depressed by adding steam in the hydrocarbon fuel2,3), whereas few detailed investigations of SOFCs that operate on gasoline fuel have been reported.In this work, we studied the effect of steam content and demonstrated the direct operation of SOFCs by octane which is a main ingredient of gasoline. In the case of the Ni-YSZ anode, the operation temperature must be high in order to avoid sulfur poisoning4 -6 ), because most commercially available fuels contain sulfur. Therefore, in this study, the SOFCs operation was conducted at high temperature of 950 oC. Experimental Surface-polished YSZ pellets of 2 mm in thickness were used for the electrolyte. Pt cathode electrode and Pt reference electrode were prepared on the surface and side of the pellets, respectively, by firing Pt paste at 1300 oC. Then, NiO powder and GDC powder (NiO/YSZ=6/4 (wt)) were thoroughly mixed in a vacuum using the planetary ball mill (200 rpm, 12 h), and made into NiO-YSZ pastes using ethyl-cellulose and organic solvent. The NiO-YSZ paste was screen-printed in diameter of 8 mm on the YSZ pellets and sintered at 1200 oC for 1 h.Octane and steam were supplied to the SOFC cell by bubbling carrier gas (He) at a constant rate. Octane bubbler temperature was 30 oC, and Steam/Carbon ratio (S/C) were controlled by changing the temperature of the water bubbler from 40 to 70 oC. Electrochemical properties (open circuit voltage, anode electrode resistance, I - V and I - P characteristics, stable power generation property) were measured over a temperature range of 950-700 oC. During the constant current generation, exhaust gas was analyzed by gas chromatography. Results and Discussion Figure 1 shows I - V and I - P characteristics for the cells at 950 oC in wet octane. Maximum power density of 35 mW/cm2 was obtained by S/C of 8.9. The power density decreased with S/C quickly.Figure 2 shows the time course of cell voltage at a drawn current density of 5.2 mA/cm2 for S/C of 3.5. The cell voltage was stable. That is, carbon deposit does not arise even in the cell of the Ni-YSZ anode in low S/C of 3.5 when the operation temperature is high of 950 oC. References 1) S. Mclntosh and R.J. Gorte, “Direct Hydrocarbon Solid Oxide Fuel Cells,” Chemical Reviews, 104 (2004) 4845- 4865.2) K. Sasaki and Y. Teraoka, “Equilibria in Fuel Cell Gases: I. Equilibrium Compositions and Reforming Conditions,” Journal of Electrochemical Society, 850 (7) (2003) A878-A884.3) K. Sasaki and Y. Teraoka, “Equilibria in Fuel Cell Gases: II. The C-H-O Ternary Diagrams,” Journal of Electrochemical Society, 850 (7) (2003) A885-A888.4) K. Sasaki, K. Susuki, A. Iyoshi, M. Uchimura, N. Imamura, H. Kusaba, Y. Teraoka, H. Fuchino, K. Tsujimoto, Y. Uchida, N. Jingo, “H2S Poisoning of Solid Oxide Fuel Cells,” J. Electrochem. Soc., 153 (2006) A2023–A2029.5) P. Lohsoontorn, D.J.L. Brett, N.P. Brandon, “Thermodynamic predictions of the impact of fuel composition on the propensity of sulphur to interact with Ni and ceria-based anodes for solid oxide fuel cells,” Journal of Power Sources, 175 (2008) 60–67.6) H. Kishimoto, Y-P Xiong, K. Yamaji, T. Horita, N. Sakai, M.E. Brito, H. Yokokawa, “Stability of Ni base anode for direct hydrocarbon SOFCs, ” Journal Chemical Engineering of Japan, 40 (13) (2007) 1178-1182.

Solid oxide fuel cell (SOFC) stacks were operating with metallic interconnects that separate each cells. Metallic interconnects play an important role in gas flow system and heat transfer mechanism, because its geometry makes gas flow channel and it has a high thermal conductivity. Many researcher design the flow channel for optimizing the cell performance like a ratio of height and width, channel to channel distance, or new type channels. For high conversion efficiency, most solid oxide fuel cell in power generation application operate at high fuel utilization and low current condition. However, high fuel utilization condition makes a large gradient of chemical potential by fuel depletion and it induces a low cell performance and a large chemical stress in flow direction. This phenomena can be observed at high performance cell or high operating temperature condition, because almost fuel is consumed at upstream of fuel channel by high electrochemical reaction kinetics and most cell area has a low chemical potential environment. Therefore, even distribution of electrochemical reaction rate is important. In this study we reduce the gradient of chemical potential by using diffuser-shape fuel channel, and optimize the diffuser-shape by design study. In chemical species conservation law, higher flow velocity at upstream makes higher convective flux in flow direction compared to diffusive flux in cell-thickness direction, and it can reduce the electrochemical reaction rate at reacting electrode. Optimized diffuser-shape fuel flow channel from this study may enhance the performance and the durability of solid oxide fuel cell stacks.

Tubular solid oxide fuel cells (SOFCs) with cells in series assembled on a porous cermet-support tube with self-sealing were designed for easy manufacturing by thermal spraying. Such design has distinct features of high power output, good thermal matching of support tube with different functional layers, and low ohmic loss from two electrode leaders. Tubular SOFCs with different numbers of cells (6, 10, and 20) in series were manufactured on the flame-sprayed NiCr-Al2O3 support tube with an insulating interlayer, YSZ electrolyte, Ni-YSZ anode, and La0.8Sr0.2MnO3-based cathode. All components in the tubular SOFC were produced by thermal spray processes. The performance test showed that tubular SOFC presented an open circuit voltage and maximum output power being proportional to numbers of cells in series. For tubular SOFC with 10 cells with a total length of 200 mm, the test yielded a peak power output over 20 W at 800 °C with a maximum power density over 200 mW/cm2. Moreover, it was also found that the tubular SOFC with 20 cells presented a comparable peak powder density. Therefore, it is clearly demonstrated that the cermet-supported tubular design is suitable for thermal spraying manufacturing with a high output performance.