Solid oxide fuel cell

The solid oxide fuel cell (english solid oxide fuel cell, SOFC ) is a high-temperature fuel cell is operated at an operating temperature of 650-1000 ° C. The electrolytic cell of this type is comprised of a solid ceramic material, which is able to conduct oxygen ions, but acts as an insulator for electrons. General to yttrium - stabilized zirconia (YSZ ) is used. The cathode of which is also made of a ceramic material ( strontium -doped lanthanum manganate ), the addition is conductive for electrons but vs. the electrolyte. The anode is made ​​of nickel with yttria -doped zirconia (so-called cermets ), the ions and electrons also passes.

Construction

The real innovation of an SOFC is in the ceramic material. Some of the boundary conditions are: cathode and anode must be gas permeable and conduct electricity well. The thickness of the oxygen-conducting membrane must be as thin as possible in order to transport the oxygen ions in energy through the membrane can. It shall be no voids (holes ) which can be passed through the other gas molecules. The high temperature makes the development of systems complex.

Designs

Essentially, there are due to the design: tubulare ( tubular, see figure) and planar (flat) SOFCs. Tubular SOFCs offer benefits to each other technically in the sealing of the electrode gas space. Planar SOFCs are more suitable for applications requiring high volumetric power densities. Due to the supporting structure will continue to differentiate between electrolyte- supported and anode -supported SOFC.

Technical challenges

Due to the high operating temperature, it is possible noble, i.e. to use less expensive materials as, for example, in the PEMFC, and simultaneously to generate high power densities and high efficiencies. The material temperature combination also allows operating with various reformats (see also steam reforming ), which makes the SOFC versatile than other fuel cell types. The potential of the fuel cell - type is therefore the one hand in the possibility of being able to develop a low cost fuel cell, and then to integrate these on the other hand, in an existing infrastructure. However, the high operating temperature is the reason for almost all technical challenges. The sealing technique of gas chambers to each other is very expensive (high temperature seal). Conventional gaskets fail utterly. The metallurgical compounds, the electrodes shorted. Therefore special sealing materials are currently being developed such as solder glass for SOFC applications. The aging of the materials (for example due to creep or oxidation processes ) is greatly increased at high temperatures, resulting in a rapid decrease in the performance of the SOFC (see also degradation of the solar cell). High temperature corrosion results in embrittlement of the metal components. The strength of the components is decreased at high temperature. All this can result in stress on the materials to failure. Have voltages on its origin mainly in thermal gradients and different thermal expansion coefficients (TEC ) of the materials. An application must meet these challenges.

Reaction equations

The internal charge transport takes place by means of O2 - ions. On the cathode side of the SOFC requires oxygen and produces water at the anode and / or CO2.

Application

Possible applications for SOFCs are areas where a lot of power ( Requires excessive current flow) is needed. SOFC power plants, based on these fuel cells should be able to achieve a system efficiency of 55-66 %. Still, the SOFC technology is at an early stage of development, but long term, this type of cell are used for decentralized energy supply. Some pilot projects reach, according to the device manufacturer's already an efficiency > 50%. Such fuel cells can be directly operated with natural gas or biogas, without a reformer is needed. By the electrochemical reactions occurring electrical energy and heat is produced.

Micro- CHP (micro -CHP) in which SOFCs are used for combined power and heat production should replace the normal heating burner, that is, they are to generate heat and provide additional power. Such fuel cell heating systems are under investigation, for example in the context of Callux project.

In addition to the decentralized power generation SOFC technology is interesting for use in large centralized power plants. The high efficiency, high exhaust temperature and the Restbrennbarkeit of the SOFC exhaust make a prescreening of solid fuel gasifiers and / or a downstream connection of gas turbine cycles and evaporated very attractive. Electrical efficiencies of up to 70 % are considered possible. Carburetor SOFC systems that operate with biomass as fuel, can achieve twice as high efficiency compared to conventional steam cycle based biomass combustion plants.

Benefits

Due to the high operating temperature, the resulting through the process waste heat can be used separately. In addition, a SOFC insensitive to CO in the fuel gas. An electrolyte management is not required, therefore it is a simple system with a high efficiency constitutes a SOFC is not subject to the Carnot cycle and includes the class of conventional heating (<1 ps), a higher electrical efficiency. Where the fuel gas is CO -heavy CO2 emissions are lower than in conventional devices.

Disadvantages

Disadvantages are apparent due to the high operating temperature. The temperatures between 800-1000 ° C require special demands on the material that can withstand these stresses. Likewise, a long warm-up period is required in order to achieve the desired operating temperature.

Recent Developments

In current research, there is a trend to develop SOFC cells whose operating temperature is well below 800 ° C. These developments are referred to as IT - SOFC (intermediate -temperature SOFC).

Here, American scientists have developed a fuel cell that is 6.5 cm long and tubular and 350 mW guaranteed. Since it operates at temperatures of about 500 ° C, can be dispensed with an external reformer. The liquid fuel is reformed it directly in the system to hydrogen and carbon monoxide. This is made possible by the use of particles from acting as a catalyst metals ruthenium and cerium. However, the cell must be brought from about 300 ° C to start with an external heating to temperatures.

As fuel, the liquefied propane is used in the developed by the American researchers cell, as has long been common in many mobile applications such as in gas lighters or camping stoves and lamps. So that the cell could be run with a fuel, for which there is already an infrastructure.