Ceramic technologies and manufacturing processes are key to the success of solid oxide fuel cells.
Solid oxide fuel cells (SOFCs) are the most flexible of all fuel cell technologies in respect to fuel, design, operating parameters and applications. Cells are multilayer ceramics where the structural support element can be the electrolyte, cathode, anode or inert structure. Cell shapes vary from single tubes up to 2 m in length, segmented tubes, flat tubes, flat plates and monolithic structures. Cells are low-voltage/high-current devices and are therefore linked in series and parallel using metallic or ceramic interconnectors to form the fuel cell stack.
Ceramic Fuel Cells has chosen a flat plate configuration and has developed and field-tested both all-ceramic and metal-ceramic stack structure technologies. All-ceramic stacks are potentially easier to seal (seals between components with matched expansion coefficients), but the low thermal conductivity of ceramics can make thermal management more difficult due to large in-plane temperature gradients and the need for large air flows for compensation.
The high-efficiency approach requires steam reforming of methane (a highly endothermic reaction) on the cells, resulting in strong cooling on the fuel inlet side and the need for heat transport from the hot exit side. A ferritic stainless steel (10x thermal conductivity of a ceramic) was therefore chosen as the interconnector option. However, even this "metal structure" includes three key ceramic components: the multilayer ceramic cell, an essential ceramic protection layer on the cathode side, and a glass-ceramic sealing arrangement.
Figure 1. Structure and performance of a solid oxide fuel cell.
Cell and Stack Technology
The high-performance cell is a multilayer structure consisting of a porous, about 170-micron-thick mechanical support made of nickel-3 mol% yttria-stabilized zirconia (3YSZ) material, a 15-micron functional anode, a 10-micron electrolyte of 8 mol% YSZ, a doped ceria barrier layer, a cobalt-iron perovskite cathode (30 microns), and contact layers on both the cathode and anode side. The cell is manufactured by laminating and co-firing the substrate, anode functional layer, electrolyte and barrier layers to form a 200-micron-thick half-cell. The cathode is printed onto the half-cell and fired separately.
The size of the cell is about 70 x70 mm, with an approximate 35 cm2
active area. In order to achieve the robustness and performance required, this multilayer ceramic cell needs to be carefully optimized for strength and porosity. The cell has been designed for the internal steam reforming of methane, and methane slippage at the operating temperature of 750øC is less than 0.5%. The cell delivers approximately 650 mW/cm2
at a potential of 0.8 V and 60% fuel utilization. Figure 1 shows the cell structure and performance.
Figure 2. The stack design is a 2 x 2 window frame (four cells in parallel) design.
The stack design is a 2 x 2 window frame design (four cells in parallel), with 51 layer sets for a 2 kW NET AC stack (dimensions about 200 x 160 x 210 mm), as shown in Figures 2 and 3. The layer set consists of four cells, a 2-3-mm-thick steel interconnect with flow channels and internal gas manifolds, and a 0.3-mm-thick window foil. This stack design has been tested in a test station for over two years under system operating conditions and proved very stable, with degradations of about 0.3% per 1000 hours. The key to achieving low degradation is high-quality protection layers on the cathode side to avoid the poisoning of the cathode with volatile chromium compounds escaping from the steel surface.
On the cathode side, volatile chromium species such as CrO3
(g) and CrO2
(g) can be released from the steel surface, depending on temperature and oxygen and water partial pressures. These CrOx
species react with the cathode or deposit at the cathode/electrolyte interface, leading to performance deterioration.
Figure 3. A 2 kW NET AC stack incorporates 51 layer sets.
This poisoning process can be prevented with the application of a high-quality protective coating on the steel surface. In order to fulfill its function, the coating needs to be chemically stable over all of the operating conditions. It also needs to be a good electrical conductor and capable of forming a dense layer. A spinel layer containing manganese and cobalt has been developed,* and a 20-micron-thick coating has been found to give good protection. The coating can be applied by thermal spray methods or powder spraying followed by thermal treatment.
The resistance increase of the coating has been measured and fulfils the five-year lifetime target. A CrOx
emission test has also been developed to specify the maximum allowable emissions for a > five-year life target, and only high-quality spinel coatings have been found to achieve this objective.
*Developed and patented (U.S. 5942349) by Ceramic Fuel Cells Limited.
Figure 4. Seals are dispensed onto the components and form during stack sintering.
Glass-Ceramic Sealing Technology
Sealing in planar SOFCs is a significant technical challenge, and the high-efficiency stack technology requires effective sealing. The seal material needs to fulfill several requirements:
- thermal expansion match
- chemically inert to surface coatings and stable in both fuel and air environment
- leak-tight at fuel and air side
- strong adhesion to the sealing surfaces
- low volatility
To date, only glass and glass-ceramic seals fulfill these requirements. Alkali-free glass seals have been developed that are compatible with the steel component coatings and that form a gas-tight and well-adhering interface. The seals are dispensed onto the components and form during stack sintering (see Figure 4). This step initiates glass crystallization, which continues during operation over time; the seals become predominantly ceramic after about 500 hours.
These changes can lead to a reduction in thermal expansion over time, making thermal cycling after extended operating times more difficult. Development continues for seal compositions that remain stable over the lifetime target of five years.
Well-characterized powders are the lifeblood of ceramics. Powder properties control the sintering behavior and affect shrinkage, density and micro-structure. Variations (particularly in multilayer ceramics) lead to internal stresses, distortions, cracks and broken parts.
A flexible continuous powder process has been developed based on novel chemistry to significantly reduce batch-to-batch variations. This process was used for a number of years for the production of cathode powders on a laboratory scale and has been expanded to pilot-scale production of about 20 tons per year. The plant currently produces high-quality yttria-stabilized zirconia powders for Ceramic Fuel Cells' fuel cells and other applications.For more information regarding SOFCs, contact Ceramic Fuel Cells Ltd. at 170 Browns Rd., Noble Park, Victoria 3174, Australia; (61) 039554-2300; fax (61) 039790-5600; e-mail firstname.lastname@example.org; or visit www.cfcl.com.au.
CFCL's pilot powder plant in Bromborough, UK.
SIDEBAR: About Ceramic Fuel Cells Limited
Formed in 1992, Ceramic Fuel Cells Limited is a publicly listed Australian company that develops and commercializes SOFC technology. The company has developed all of its own intellectual property in-house, building up a patent portfolio and expertise over the complete value chain-from ceramic powders to complete power systems. The end-to-end skill set allows Ceramic Fuel Cells to optimize SOFCs and maximize their full advantages, including through on-cell steam reforming of methane, high-voltage operation, continuous high-fuel utilization, minimal pressure drop throughout the system, and minimal thermal and parasitic losses.
In early 2009, Ceramic Fuel Cells demonstrated a system electrical efficiency of 60% net AC (LHV) with a 2 kW prototype system fuelled with natural gas. Prior to this development, only a combined cycle gas turbine plant of 100 MW or more could achieve electrical efficiencies in the 50-60% range.
The result was achieved with Ceramic Fuel Cells' Gennex fuel cell module, which is the result of extensive fuel cell stack and system optimization through rigorous field testing (> 150,000 hours). The high electrical efficiency, which delivers a low heat-to-power ratio, enables year-round operation. Unlike engine-based generator systems, fuel cell systems have a relatively flat efficiency vs. power output, and Ceramic Fuel Cells' generator is able to achieve an electrical efficiency of > 40% at quarter-power.