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Solid Oxide Fuel Cells
(Released April 2003)

 
  by Eileen J. De Guire  

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Engineers and environmentalists have long dreamed of being able to obtain the benefits of clean electric power without pollution-producing engines or heavy batteries. Solar panels and wind farms are familiar images of alternative energy technologies. While they are effective sources of electrical energy, there are problems with the stability of their energy source as, for example, on a cloudy or windless day. Their applications are somewhat limited due to lack of portability; a windmill is not much help to the power plant of a diesel truck, a solar panel cannot provide power at night, etc.

In 1962 a revolution in energy research occurred. Scientists at Westinghouse Electric Corporation (now Siemens Westinghouse) demonstrated for the first time the feasibility of extracting electricity from a device they called a "solid electrolyte fuel cell" [Weissbart]. Since then there has been an intense research and development effort to develop the alternative energy technology known as fuel cells. Now, as energy issues are at the forefront of current events, fuel cell technology is ripening and on the verge of being ready for large scale commercial implementation.

What is a Fuel Cell?

A fuel cell is an electrochemical device that converts the chemical energy in fuels (such as hydrogen, methane, butane or even gasoline and diesel) into electrical energy by exploiting the natural tendency of oxygen and hydrogen to react. By controlling the means by which such a reaction occurs and directing the reaction through a device, it is possible to harvest the energy given off by the reaction. Fuel cells are simple devices, containing no moving parts and only four functional component elements: cathode, electrolyte, anode and interconnect.

Why Fuel Cells?

Much development has focused on solid oxide fuel cells (SOFC), both because they are able to convert a wide variety of fuels and because they do so with such high efficiency (40-60% unassisted, up to 70% in pressurized hybrid system) compared to engines and modern thermal power plants (30-40% efficient) [Singhal]. SOFC technology dominates competing fuel cell technologies because of the ability of SOFCs to use currently available fossil fuels, thus reducing operating costs. Other fuel cell technologies (e.g. molten carbonate, polymer electrolyte, phosphoric acid and alkali) require hydrogen as their fuel. Widespread use of such fuel cells would require a network of hydrogen suppliers, similar to our familiar gas stations.

High efficiency and fuel adaptability are not the only advantages of solid oxide fuel cells. SOFCs are attractive as energy sources because they are clean, reliable, and almost entirely nonpolluting. Because there are no moving parts and the cells are therefore vibration-free, the noise pollution associated with power generation is also eliminated.

How Does a Fuel Cell Work?

Figure 1 shows schematically how a solid oxide fuel cell works. The cell is constructed with two porous electrodes which sandwich an electrolyte. Air flows along the cathode (which is therefore also called the "air electrode"). When an oxygen molecule contacts the cathode/electrolyte interface, it catalytically acquires four electrons from the cathode and splits into two oxygen ions. The oxygen ions diffuse into the electrolyte material and migrate to the other side of the cell where they encounter the anode (also called the "fuel electrode"). The oxygen ions encounter the fuel at the anode/electrolyte interface and react catalytically, giving off water, carbon dioxide, heat, and -- most importantly -- electrons. The electrons transport through the anode to the external circuit and back to the cathode, providing a source of useful electrical energy in an external circuit.

Figure 1. Operating concept of a SOFC (from: http://www.seca.doe.gov)

Two possible design configurations for SOFCs have emerged: a planar design (Figure 2) and a tubular design (Figure 3). In the planar design, the components are assembled in flat stacks, with air and fuel flowing through channels built into the cathode and anode. In the tubular design, components are assembled in the form of a hollow tube, with the cell constructed in layers around a tubular cathode; air flows through the inside of the tube and fuel flows around the exterior.

Figure 2. Configuration for a planar design SOFC (from: http://www.spice.or.jp/~fisher/sofc.html#descript)
Figure 3. Configuration for a tubular design SOFC (from: http://www.pg.siemens.com/en/fuelcells/sofc/tubular/index.cfm)

Materials Selection and Processing

Although the operating concept of SOFCs is rather simple, the selection of materials for the individual components presents enormous challenges. Each material must have the electrical properties required to perform its function in the cell. There must be enough chemical and structural stability to endure fabrication and operation at high temperatures. The fuel cell needs to run at high temperatures in order to achieve sufficiently high current densities and power output; operation at up to 1000 C is possible using the most common electrolyte material, yttria-stabilized zirconia (YSZ). Reactivity and interdiffusion between the components must be as low as possible. The thermal expansion coefficients of the components must be as close to one another as possible in order to minimize thermal stresses which could lead to cracking and mechanical failure. The air side of the cell must operate in an oxidizing atmosphere and the fuel side must operate in a reducing atmosphere. The temperature and atmosphere requirements drive the materials selection for all the other components.

In order for SOFCs to reach their commercial potential, the materials and processing must also be cost-effective. The first successful SOFC used platinum as both the cathode and anode, but fortunately less expensive alternatives are available today [Weissbart].

Cathode

The cathode must meet all the above requirements and be porous in order to allow oxygen molecules to reach the electrode/electrolyte interface. In some designs (e.g. tubular) the cathode contributes over 90% of the cell's weight and therefore provides structural support for the cell [Park].

Today the most commonly used cathode material is lanthanum manganite (LaMnO3), a p-type perovskite. Typically, it is doped with rare earth elements (eg. Sr, Ce, Pr) to enhance its conductivity. Most often it is doped with strontium and referred to as LSM
(La1-xSrxMnO3). The conductivity of these perovskites is all electronic (no ionic conductivity), a desirable feature since the electrons from the open circuit flow back through the cell via the cathode to reduce the oxygen molecules, forcing the oxygen ions through the electrolyte. In addition to being compatible with YSZ electrolytes, it has the further advantage of having adequate functionality at intermediate fuel cell temperatures (about 700 C), allowing it to be used with alternative electrolyte compositions. Any reduction in operating temperature reduces operating costs and expands the materials selection, creating an opportunity for additional cost savings.

Fabrication of LSM depends on cell design. For example, at Siemens Westinghouse a tubular cell design is being developed [Singhal]. The cell is constructed by extruding a cathode tube and building the rest of the cell around it. At NexTech Materials, where several planar cell designs are being investigated, the cathode is designed as the bottom supporting layer, and fabricated with tape casting techniques using nanoscale particles [NexTech]. In both cases, the challenge is to sinter the cathode adequately, often by co-sintering with the other components, while maintaining sufficient interconnected porosity.

Electrolyte

Once the molecular oxygen has been converted to oxygen ions it must migrate through the electrolyte to the fuel side of the cell. In order for such migration to occur, the electrolyte must possess a high ionic conductivity and no electrical conductivity. It must be fully dense to prevent short circuiting of reacting gases through it and it should also be as thin as possible to minimize resistive losses in the cell. As with the other materials, it must be chemically, thermally, and structurally stable across a wide temperature range.

There are several candidate materials: YSZ, doped cerium oxide, and doped bismuth oxide. Of these, the first two are the most promising. Bismuth oxide-based materials have a high oxygen ion conductivity and lower operating temperature (less than 800 C), but do not offer enough crystalline stability at high temperature to be broadly useful [Liou].

YSZ has emerged as the most suitable electrolyte material. Yttria serves the dual purpose of stabilizing zirconia into the cubic structure at high temperatures and also providing oxygen vacancies at the rate of one vacancy per mole of dopant. A typical dopant level is 10 mol% yttria [Singhal].

A thin, dense film of electrolyte (approximately 40 microns thick) needs to be applied to the cathode substrate. A reliable way to apply the electrolyte is known as electrochemical vapor deposition which offers high purity and a high level of process control. Electrochemical vapor deposition solves the problem of depositing a dense film onto a porous substrate by passing oxygen through the inside of the cathode tube while chlorides of zirconium and yttrium are passed along the outside. They react at the tube surface to form YSZ and, because the reaction comes to the surface from both sides, the porosity is closed off. Once the porosity is closed off, the electrolyte deposition continues, but now the oxygen diffuses through the growing YSZ layer to react with the chlorides, thereby ensuring a highly dense electrolyte layer. The process, while effective, is expensive and capital-intensive [Singhal].

Alternative electrolyte deposition methods that show promise are spray coating and dip coating followed by sintering. Colloidal suspensions of YSZ are applied in thin layers of at least 20 microns, using nanosize (5-10 nm) particles in order to meet the critical requirement of low porosity. Through careful engineering of the particle size distribution and dispersions, these deposition methods are likely to replace electrochemical deposition [Liou].

Cerium oxide has also been considered as a possible electrolyte. Its advantage is that it has high ionic conductivity in air but can operate effectively at much lower temperatures (under 700 C); this temperature range significantly broadens the choice of materials for the other components, which can be made of much less expensive and more readily available materials. The problem is that this electrolyte is susceptible to reduction on the anode (fuel) side. At low operating temperatures (500-700 C) grain boundary resistance is a significant impediment to ionic conductivity. Efforts are underway to develop compositions which address these problems [Ralph].

Anode

The anode (the fuel electrode) must meet most of the same requirements as the cathode for electrical conductivity, thermal expansion compatibility and porosity, and must function in a reducing atmosphere. The reducing conditions combined with electrical conductivity requirements make metals attractive candidate materials.

Most development has focused on nickel owing to its abundance and affordability. However, its thermal expansion (13.3 x 10-6/C compared with 10 x 10-6/C for YSZ) is too high to pair it in pure form with YSZ; moreover, it tends to sinter and close off its porosity at operation temperatures. These problems have been solved by making the anode out of a Ni-YSZ composite. The YSZ provides structural support for separated Ni particles, preventing them from sintering together while matching the thermal expansions. Adhesion of the anode to the electrolyte is also improved [Singhal].

Anodes are applied to the fuel cell through powder technology processes. Either a slurry of Ni is applied over the cell and then YSZ is deposited by electrochemical vapor deposition, or a Ni-YSZ slurry is applied and sintered. More recently NiO-YSZ slurries have been used, the NiO being reduced to particulate Ni in the firing process.

In order to maintain porosity, pore formers such as starch, carbon, or thermosetting resins are added. These burn out during firing and leave pores behind.

There are problems with this approach. First, the process tends to form tortuous porosity pathways that reduce the transport efficiency of reacting gasses through the anode. Second, there is an increased likelihood of cracking on firing because of the thinness of the interior solid structure left behind. Third, there are environmental issues associated with the burning of the pore formers.

For these reasons, recent research is investigating the possibility of a freeze-drying approach to forming porous structures without the use of fillers. The slurry is applied through a simple dipping process and then freeze-dried; the resulting ice is then sublimed out of the unfired structure. The resulting pore structure -- neatly aligned because of the way water crystallizes -- allows efficient flow of gases to and from the electrolyte/anode interface. The fineness of the pore structure is easily controlled by adjusting the solids content (and therefore water content) of the slurry [Moon].

Other choices of material are under investigation as well. Although Ni-YSZ is currently the anode material of choice and the freeze-drying process solves most of the associated problems, nickel still has a disadvantage: it catalyzes the formation of graphite from hydrocarbons. The deposition of graphite residues on the interior surfaces of the anode reduces its usefulness by destroying one of the main advantages of SOFCs, namely their ability to use unreformed fuel sources.

Cu-cerium oxide anodes are being studied as a possible alternative. Copper is an excellent electrical conductor but a poor catalyst of hydrocarbons; cerium oxide is used as the matrix in part because of its high activity of hydrocarbon oxidation. A composite of the two thus has the advantage of being compatible with cerium oxide electrolyte fuel cells. Initial results using a wide range of hydrocarbon fuels are promising [Park].

Interconnect

Just as an internal combustion engine relies on several cylinders to provide enough power to be useful, so too must fuel cells be used in combination in order to generate enough voltage and current. This means that the cells need to be connected together and a mechanism for collection of electrical current needs to be provided, hence the need for interconnects. The interconnect functions as the electrical contact to the cathode while protecting it from the reducing atmosphere of the anode.

The high operating temperature of the cells combined with the severe environments means that interconnects must meet the most stringent requirements of all the cell components: 100% electrical conductivity, no porosity (to avoid mixing of fuel and oxygen), thermal expansion compatibility, and inertness with respect to the other fuel cell components. It will be exposed simultaneously to the reducing environment of the anode and the oxidizing atmosphere of the cathode.

For a YSZ SOFC operating at about 1000 C, the material of choice is LaCrO3 doped with a rare earth element (Ca, Mg, Sr, etc.) to improve its conductivity. Ca-doped yttrium chromite is also being considered because it has better thermal expansion compatibility, especially in reducing atmospheres [Chou]. Interconnects are applied to the anode by plasma spraying and then the entire cell is co-fired.

Any reduction in component costs (either raw materials or processing) directly translates into improved energy affordability. The strong economic incentive to use traditional metals for the interconnect is driving the development of intermediate and low temperature SOFCs. At operating temperatures in the 900-1000 C range, interconnects made of such nickel base alloys as Inconel 600 are possible [Matsuzaki]. At or below 800 C, ferritic steels can be used. At even lower temperatures (below 700 C), it becomes possible to use stainless steels, which are comparatively inexpensive and readily available [Ralph].

Applications and Markets

The United States government is taking a proactive role in expediting the technology through the Solid State Energy Conversion Alliance (SECA), which is coordinated by the Department of Energy and Pacific Northwest National Laboratory. The technical goal is to develop mass producible, modular SOFC units capable of 3-10 kW at a price of $400/kW. SECAs approach is to develop industrial collaborations and to extend financial support of technical research [SECA].

There seems, therefore, to be little doubt that SOFC technology will be implemented. Analysts expect that the overall market for fuel cell technology could reach $95 billion by the year 2010 [ceramic]. The market share that will belong to SOFCs is unclear but will surely be significant, as SOFCs are targeted for use in three energy applications: stationary energy sources, transportation, and military applications.

Stationary installations would be the primary or auxiliary power sources for such facilities as homes, office buildings, industrial sites, ports, and military installations. They are well suited for mini-power-grid applications at places like universities and military bases. According to the SECA, worldwide demand for electricity is expected to double in the next 20 years. SOFC technology is ideal for such an expansion, since much of the anticipated demand is expected to come from growing economies with minimal infrastructure. SOFCs can be positioned on-site, even in remote areas; on-site location makes it possible to match power generation to the electrical demands of the site.

Stationary SOFC power generation is no longer just a hope for the future. Siemens Westinghouse has tested several prototype tubular systems, with excellent results. A plant in the Netherlands has been operational for two years and an earlier prototype installation has been operating for 8 years. The fuel cells have been through over 100 thermal cycles and the voltage degradation during the test time has been minimal less than 0.1%/thousand hours. Siemens Westinghouse expects to have its first fully operational tubular fuel cell plant in place by October 2003 [Siemens]. Meanwhile, in Australia, Ceramic Fuel Cells, Ltd. has been operating prototype planar fuel cell plants since 2001 and expects to be ready with market-entry products in 2003 [CFCL].

In the transportation sector, SOFCs are likely to find applications in both trucks and automobiles. In diesel trucks, they will probably be used as auxiliary power units to run electrical systems like air conditioning and on-board electronics. Such units would preclude the need to leave diesel trucks running at rest stops, thereby leading to a savings in diesel fuel expenditures and a significant reduction in both diesel exhaust and truck noise. Meanwhile, automobile manufacturers have invested at least $4.5 billion in fuel cell research (not all SOFC) [ceramic]. There are an estimated 600 million vehicles worldwide, 75% of which are personal automobiles, and the number is expected to grow by 30% in the next 10 years [SECA]. With more stringent environmental restrictions in the United States and European Union, automobile manufacturers are under growing time pressure to bring non-polluting cars to the marketplace. SOFCs are attractive prospects because of their ability to use readily available, inexpensive fuels.

Finally, SOFCs are of high interest to the military because they can be established on-site in remote locations, are quiet, and non-polluting. Moreover, the use of fuel cells could significantly reduce deployment costs: 70% by weight of the material that the military moves is nothing but fuel [SECA].

Conclusion

Forty years have passed since the first successful demonstration of a solid oxide fuel cell. Through ingenuity, materials science, extensive research, and commitment to developing alternative energy sources, that seed of an idea has germinated and is about to bloom into a viable, robust energy alternative. Materials development will certainly continue to make SOFCs increasingly affordable, efficient, and reliable.

© Copyright 2003, All Rights Reserved, C SA

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