Advanced Arena: Complex Ceramic Oxides in SOFC Applications
In general terms, a fuel cell is constructed of the following components within a sealed environment: two electrodes (anode and cathode), an electrolyte, and interconnects. The energy of the cell is produced through a chemical reaction in which hydrogen atoms enter the cell at the anode and are ionized by being stripped of their electrons and thus becoming positively charged. The freed electrons create the flow of energy used for power. Oxygen atoms are also introduced into the cell at the cathode and used to create the waste material of the reaction. Once through the cell, the hydrogen and oxygen form water as the waste material, which then drains from the cell.
Between the cathode and the anode is the electrolyte, which allows only the appropriate ions of either hydrogen or oxygen to flow through the cell. Due to its key role in the process, the electrolyte typically determines how a fuel cell is categorized. When considering fuel cell types, however, it should be noted that the electrolyte type is not the only differentiating factor. The main types of electrolytes currently under development for commercial production include: alkali, molten carbonate, phosphoric acid, proton exchange membrane (PEM) and solid oxide.
Materials Under Consideration
The composition of solid oxide fuel cell (SOFC) components has not changed dramatically since their introduction in the late 1980s. However, as the market for alternative energy sources has been reinvigorated over the last several years, there has been a push to develop a design that provides efficient and cost-effective operation. The ultimate goal is to achieve a high power density at an affordable cost without debilitating operation effects, such as prohibitively high temperatures.
The power density values and temperature conditions can change according to the choice of material type for each component, as well as the choice of fuel gas and the cell configuration. This has led to experimentation with a combination of materials for each fuel cell component, as well as the development of multiple designs.
Much attention has been given to the development of mixed oxide ceramics because of their ionic conducting and insulating qualities in both pure and doped forms for fuel cell components. Material choice differs for each component but includes mixed oxide ceramics such as lanthanum manganite (LSM), yttria-stabilized zirconia (YSZ), cerium (Ce) oxides, perovskite structure ceramics, alkali metal niobates, and others.
As mentioned previously, the electrolyte can be termed the most defining component of the fuel cell. The characteristics of the electrolyte material determine many of the operating factors, including temperature, efficiency and power output.
The most commonly used electrolyte continues to be YSZ, but these materials only function adequately in the higher temperature ranges. Materials that are being considered to replace YSZ include doped ceria and lanthanum gallanate, which can effectively operate at lower temperatures. In fact, doped ceria has been shown to exhibit ionic conductivity an order of magnitude greater that YSZ in the higher temperature ranges as well. Thus, it appears that it will be a strong candidate for future development.
The cathode is where the oxygen reduction takes place. One of the key aspects that must be maintained by the cathode is excellent electric conductivity in an oxygen-based environment in high temperature. One of the best performers under these conditions is strontium (SR)-doped lanthanum manganite (LSM) when mixed with YSZ to form a cathode composite.
LSM is not a good ionic conductor on its own. The necessary chemical reaction is limited to a smaller area of the triple-phase boundary (TPB), the area where the reactant gas, electrolyte, and electrode meet each other. Thus, a LSM YSZ composite is used to widen the TPB area and therefore the effective area of chemical reaction.
As an alternative, mixed ionic-electronic conducting (MIEC) ceramics, such as the perovskite lanthanum strontium cobalt ferrite (LSCF), are being researched and developed for use in intermediate temperature solid oxide fuel cells (IT-SOFCs). These materials exhibit similar conductive qualities except at lower temperatures.
The anode acts as the site for the electrochemical oxidation of the fuel gas. As noted previously, the fuel gas can vary. Hydrogen is the most common under development. However, cells that operate on other fuel gases, including hydrocarbons and biofuels, are being developed and could carve the path to not only providing impact-free energy, but also assisting in other environmentally conscious objectives.
The most common material used at the anode is a nickel-yttria-stabilized zirconium (Ni-YSZ) composite. This combination has been shown to give the highest power density at the standard operating temperatures. However, the use of Ni currently does not allow for the use of other fuel sources without the need for reforming. Other materials, such as copper (Cu), have been suggested to replace Ni, and they would allow the use of other fuel sources. However, operating temperature must be increased substantially to achieve the same power density.
SOFCs are currently attracting tremendous interest because of the growing movement to free the world of its reliance on exhaustible fossil fuels. SOFCs offer an attractive alternative due to their huge potential for power generation. Currently, SOFC efficiency averages above 70%, and practicable power output ranges from 300 mW to over 1,000 mW, depending on the materials used.
Another attractive factor in the development and demand for SOFCs is their versatility across applications in stationary, portable, and transportation environments. SOFCs are not limited to using hydrogen as the fuel source. Currently, special development is being done using a SOFC with any combustible fuel, including carbon monoxide (CO), hydrocarbons, biomasses and coal.
Obstacles to Commercialization
Two main obstacles must be overcome before a roadmap to full implementation and commercialization can be drawn; both revolve around operating temperature. The main drawback of current SOFC designs is that the oxides being used as electrolytes must operate at high temperatures (500-1000ºC); below those temperatures, they show very little ionic conductive activity. High temperatures, however, present challenges not only in materials functionality but also in ambient environment maintenance. Thus, the major impediment of commercializing SOFCs is the high cost from high operating temperatures.
By lowering the operating temperature, the advanced intermediate- and low-temperature SOFCs (ILT-SOFCs) have the potential to greatly reduce the cost of other components such as the interconnect, sealing materials, and containment unit. Further, it would improve reliability, portability, and operational life. However, it should be noted that in certain applications, high-temperature SOFCs would be perfectly acceptable, and that higher temperature operation would yield advantages in efficiency because at higher temperatures there are lower over potentials.
The second drawback arises as operating temperature is reduced. As noted previously, a critical part of most fuel cells is the TPB, where the actual electrochemical reactions take place. The density of these regions and the microstructure of these interfaces play a critical role in the electrochemical performance of SOFCs.
Basically, resistance between components at the TPB increases when temperature is lowered, thus decreasing the efficiency of the fuel cell to the point of inactivity. This is obviously a major drawback when considering lower temperature designs that would enter mainstream markets. To overcome this problem, much development is being done on thin films that could be used to decrease resistance, as well as new electrode materials based on a microstructure design.
Historically, SOFCs were targeted only toward the large-scale power generation industry. However, with developments in design focusing on lower operating temperature, portability and ease of use, it has become apparent that their application could far outstretch what was originally thought.
The industry is now focusing on developing SOFCs for applications as diverse as replaceable batteries and even microchip powering. With development progressing in alternative fuel sources such as biofuels and hydrocarbons, SOFCs represent a truly viable solution to the developing energy shortage and associated environmental problems arising from our present system.
Editor’s note: The information within this article is based on data derived from advanced materials reports published by Dedalus Consulting. Part two of this article (expected publication in January 2013) will focus on the market for these products and examine what the future outlook might be for individual ceramic materials.
Any views or opinions expressed in this column are those of the author and do not represent those of Ceramic Industry, its staff, Editorial Advisory Board or BNP Media.