Improving Thermal Processing of Solid-Oxide Fuel Cells
Solid-oxide fuel cells (SOFC) are precision electrochemical reactors that require uniformity, stability and reliability across all cells in a stack to achieve strong market success. Their performance largely depends on the manufacturing process of the individual ceramic cells. As in all ceramic processes, the thermal uniformity in the firing process strongly affects the resulting product properties. SOFC production steps including drying, firing and brazing challenge the process engineer to attain exceptional control to maintain flatness, achieve required density and limit interfacial reactions. To improve the potential for success, SOFC manufacturers must improve the control of the firing process. The steps necessary to achieve control of the SOFC firing process, and recommendations to overcome the challenges to yield improved process and product uniformity are discussed in this article.
SOFC cells are made of three layers of different ceramic materials that are sintered at various temperatures. The methods of applying the layers also can change depending on cell design. High-temperature sintering control of temperature and uniformity for each layer makes for a challenging manufacturing process. The performance of each layer in the cell and the uniformity of the cells within a stack are affected by the temperature control and thermal uniformity across the product load .
Some key thermal processing requirements are:
- Binder removal and sintering during air-atmosphere firing of SOFC planar or tubular cells
- Having a controlled temperature ramp and a continuous supply of clean air for proper organics removal during binder removal (typically between 200 and 450 C, or 390 and 840 F)
- Firing the anode and electrolyte typically between 1350 and 1500 C (2460 and 2730 F) depending on materials properties; the peak temperature determines reaction rates and final densification
- Maintaining temperature uniformity across the furnace to +/-5 C (~+/-10 F) to develop uniform performance in all cells in a stack
- Having controlled cooling after firing to avoid thermal shock and minimize overall process time
Planar and tubular SOFC manufacture
Planar SOFCs can be manufactured with the anode or the electrolyte being the supporting material, just as in tubular SOFC products. If the zirconia ceramic electrolyte is the supporting material, the anode and cathode are applied as thin layers and post-fired to achieve proper adhesion.
Tape casting is used to form the ceramic powder preform for planar fuel-cell bodies. Tape casting involves the use of high binder content to make the flexible green sheet. The high binder content of the green tape is needed for subsequent processes like lamination and screen printing. Because of the large surface area of the planar SOFC (typically 30 to 60 in.2 (193.5 to 387 cm2), temperature uniformity in firing is very important.
A tubular fuel-cell body is formed by extruding ceramic material containing about 5 to 10-wt% binder. The tube is dried in a convection oven at about 100 C (210 F), depending on the need for further processing like green machining or coating. Due to the length of the tube (3.25 to 6.5 ft, or 1 to 2 m), temperature uniformity in firing is critical to achieve proper reactions and maintain tube straightness.
Figure 1 shows a flow diagram for manufacturing SOFCs using a typical green tape method, which requires three firing and two drying steps, and one brazing step. The cast material is either the electrolyte or anode material with the subsequent layers being screen printed onto the fired surfaces . The first firing step is typically at the highest temperature to aid in the control of needed reactions. The cathode material is typically fired last to minimize interactions with the electrolyte layer as discussed below.
Ceramic processes typically involve having a binder present to hold the materials together prior to firing. The binder is removed during firing to obtain the correct physical and electrical properties. Tubular cells typically contain poly vinyl alcohol (PVA) or cellulose-based binders. Poly vinyl butyral (PVB) is a common binder used in the green tape process .
Regardless of the binder material used to manufacture the ceramic part, it must be removed entirely in a rather slow process (referred to as the binder-burnout cycle) prior to achieving full-density temperatures. Removal of binder materials has to be carefully controlled to avoid ruptures and cracks from the build up of gas pressure. Therefore, a slow binder removal process is critical; typically at a rate of one 1 C per minute or slower, depending on the product size and binder material involved. The larger the component, the slower the binder removal process must be, because extra time has to be provided for diffusion of the byproducts out of the component body without causing pressure build-up and rupturing of the ceramic body.
Binder removal occurs at a temperature between 200 and 450 C (Fig. 2). Furnaces of an earlier design often used numerous holding times at different temperatures to improve thermal uniformity at these lower temperatures, which added greatly to the time needed for binder removal. A typical binder removal cycle can take between 5 and 15 hours, depending on material and product dimensions.
Low-temperature forced convection ovens are sometimes used to remove the binder because of the poor heat transfer in a traditional batch system at the lower temperatures. The convection ovens improve heat transfer at the lower temperatures and provide a clean, fresh atmosphere in the furnace. However, this adds additional time to the process because of handling, plus the time needed to cool the product. The ability to provide good convection in a high temperature batch system reduces overall process time and reduces "fallout" due to unnecessary handling of the very fragile parts after binder removal.
Sintering of the three major elements of the SOFC (anode, electrolyte and cathode) achieves densification of all ceramic particles. The anode and cathode contain controlled porosity for proper gas transfer and reactions at the anode and cathode layers. The electrolyte needs to be 100% dense and void free so that no air is allowed to pass through to the fuel causing a chemical short in the operating fuel cell.
Considerable ongoing research is aimed at achieving a reliable single-step or cofired SOFC cell. However, at this stage in the technology, most manufacturers use multiple steps to fire the three layers of the cell. The anode is a mixture of NiO and zirconia, and is fired at a temperature similar to that of the electrolyte. The electrolyte typically is yttria-stabilized zirconia (YSZ) configured to achieve near-zero porosity, and is typically fired at a temperature between 1350 and 1500 C (2460 and 2730 F) depending on particle size distribution and composition .
The cathode layer is a perovskite-like La0.8Sr0.2 MnO3 (LSM), which is fired at a lower temperature to minimize the development of secondary phases with the zirconia electrolyte. These secondary phases increase electrical resistance at the interface
Convection batch processing
A convection batch furnace provides the tightest temperature and atmosphere uniformity possible for SOFC thermal processing, as opposed to a traditional box furnace. Unlike a conventional box furnace that can have a temperature variation of 20 to 50 C (~35 to 90 F) variation inside the product load, a convection batch furnace with eductor driven convection flow achieves +/-5 C temperature uniformity, even in the 200 to 450 C temperature range. The convection technology uses a high-velocity venturi that circulates the bulk furnace atmosphere from left to right. With the use of dual insertion nozzles, the atmosphere can be cycled from left to right in a controlled time cycle. This is done for improved response and thermal uniformity on both sides of a furnace load, as opposed to the one-direction gas flow of a typical batch furnace. The left-to-right cycling of atmosphere flow reduces temperature variations across the load, specifically top-to-bottom and right-to-left thermal variation that results in cold spots. Instead, eductor technology maximizes thermal uniformity, providing precision reactions, improved shrinkage and reduced binder burnout stress-induced camber.
This eductor system uses the force of the incoming atmosphere to induce a flow of the chamber atmosphere. Gas is injected into the annular opening located under the hearth (Fig. 3). As in a venturi system, the gas expands in the opening, and produces a negative pressure on the entry side of the tube. It then draws in the bulk atmosphere of the chamber and exhausts the flow back into the chamber on the exit side of the tube. It travels across the product load in the chamber to achieve the needed convection flow. This provides improved convection heat transfer in the furnace. The direction is reversible to achieve uniformity on both sides of the chamber. The eductor technology amplifies the chamber atmosphere by 10 to 20 times; for example, 5 liters/min gas injected into the furnace results in 50 to 100 liters/min cycled across the product volume.
Unlike traditional systems that inject cold or slightly preheated air onto the product, the firing atmosphere is added into the eductor to achieve convection heat transfer. This eliminates the need for preheaters or blowers, greatly simplifying furnace operation and costs. The educted flow is directed across heating elements in the furnace to preheat it to the furnace temperature, along with mixing with the furnace atmosphere. The combined preheating of the atmosphere to the furnace temperature eliminates issues of cold-gas impingement on the product, which can cause thermal shock to the material and furnace refractories.
The use of convection throughout the firing cycle maintains very tight temperature control; typically less than +/-5 C throughout the product load. Figure 4 shows the benefits of convection heating at higher temperatures. The traditional use of single directional flow convection at lower temperatures was used to augment the slow heat transfer of radiant heating. Heat transfer increases dramatically when a temperature of 900 C (1650 F) is achieved, but a strong delta in temperature still exists due to the thermal mass of the product load (top graph). This delta causes inhomogeniety in the product and will impact cell uniformity .
The use of eductor recycling shows dramatic improvement in thermal uniformity during the ramp to set point (bottom graph). The response to changes in the product load is rapid and follows the desired furnace profile. By achieving rapid response, product uniformity is improved, and the overall process time can be decreased since the need for long hold times for thermal stability is eliminated.
A solid oxide fuel cell is an electrochemical reactor. To provide good electrical and mechanical performance in a stack of cells, each cell has to react in the very same way. The way to get SOFCs to operate properly is to process them in exactly the same way. The high-temperature convection batch furnace provides consistency in firing the cells, which determines the porosity, permeability, density and the electrochemical response of each cell for uniform replication. All these properties are impacted by the furnace operation and uniformity. A convection batch furnace with eductor technology provides the best process for binder burnout, eliminating thermal lag (Fig. 5). In the sintering phase of the process, uniformity of temperature and atmosphere is achieved throughout the entire furnace chamber. The combination of binder removal and sintering in the same system will reduce the overall costs for firing, because it eliminates the need for a secondary bake-out oven and reduces handling fall-out. An additional benefit is the improved uniformity of cells used in the SOFC stack. Electrochemical uniformity reduces the need for sorting of cells to match the stack performance. The combined improvements will aid market acceptance for SOFCs by increasing stack reliability and reducing the time to market for new stack designs. IH
SIDEBAR: Fuel Cell Basics
Fuels cells produce electricity using an electrochemical reaction. Their main operating components are an electrolyte or membrane that separates two electrodes (anode and cathode). The electrolyte in an SOFC is a semipermeable membrane through which ionized atoms (atoms with the numbers of their electrons changed ) can pass. The SOFC membrane allows ionized oxygen (O2¯) to pass through a zirconia ceramic electrolyte. The ions combine with hydrogen at the anode and produce water vapor and complete the reaction by releasing two electrons to the external circuit. Therefore, a fuel cell is an ionic conductor that operates at a temperature of 700 to 900 C (1290 to 1650 F), and oxygen ions instead of electrons pass through it.
The anode-electrolyte-cathode arrangement is called a cell; a series of cells is added together to form a stack. A typical stack of cells will produce 1 to 2 kW of electricity. Two main types of design concepts being explored for SOFC are tubular and planar cells. These systems have specific thermal processing requirements that must be carefully monitored and controlled to achieve product uniformity and required electrical performance.
Electrical conductivity of the SOFC is key in determining its overall fuel-to-electrical power conversion efficiency. Flatness of a planar cell and the straightness of a tubular cell must be controlled to achieve ease of assembly, reliability of seals, and improved life cycling performance.