Introduction
Conclusions
Stationary Fuel Cells
Source: www.siemens.com
Overview
The solid oxide fuel cells are the most tolerant of sulfur in their fuel. It has electric utility and uses a solid zirconium oxide with a small amount of ytrria, which is a ceramic, solid-phase electrolyte. Because the electrolyte is solid, there is reduced corrosion, however, it has to run at extremely high temperatures (1000˚ C) to achieve ionic conductivity and this enhances breakdown of cell components and needs proper air stream volume to manage the temperature.
The high operating temperature does have advantages, nevertheless; as in the MCFC, it can use more types of fuels and inexpensive catalysts. In addition, the waste heat could be used with a turbine to generate thermal electricity. Both simple-cycle and hybrid SOFC systems have demonstrated the highest efficiencies of any power generation system, combined with low greenhouse gas and pollutant emissions. Recently, SOFC systems with high power densities and lower operating temperatures (700 to 850˚ C) have been developed, meaning less-expensive construction materials, which could lead to applications to mobile generators and small scale stationary power.
When an oxygen molecule contacts the cathode/electrolyte interface, it acquires electrons from the cathode, diffuses into the electrolyte and migrates to the anode. The ions encounter the fuel at the anode/electrolyte interface and reacts catalytically, giving of water, carbon dioxide, heat, and electrons. The reaction is below:
Anode: 2 H2 + 2 O2- → 2 H2O + 4 e-
Cathode: O2 + 4 e- → 2 O2-
Design
Source: www.dodfuelcell.com
The electrolyte, yttria-stabilized zirconia (YSZ), easily conducts oxide ions at high temperatures. Processes are available to produce the electrolyte as thin films on porous electrode surfaces, and thinner films improve performance and reduce operating temperatures. Alternative electrolytes are also being developed. Scandium-doped zirconia, for instance, is more conductive than YSZ and allows for further temperature reduction. Lanthanum gallate with strontium doping and magnesium is another option that could be used at temperatures as low as 600 ˚ C however, matching the thermal expansion coefficients, mechanical strength, and chemical compatibilities remain a challenge. Another option is to use proton conductors instead of oxygen conductors, which would prevent all the products from ending up on one side of the cell and diluting the fuel, reducing the potential.
The anode is a nickel/zirconia cermet (Ni-YSZ), which inhibits sintering of the metal particles and provides a thermal expansion coefficient comparable to other fuel cells. It has a porous design to ease mass transport, as does the cathode, a p-type conductor, magnesium-doped lanthanum manganate, with 20-40% porosity. In one design, the Westinghouse cell, the cell is around a porous zirconia tube that air passes through and into the cathode, which is on the outside of the tube. There is a layer of electrolyte on the outside of the cathode and a final layer of anode over the electrolyte.
Cells are then connected together via high-temperature semiconductor contacts. The hydrogen or carbon monoxide in the fuel stream reacts with oxide ions from the electrolyte to produce water or carbon dioxide and deposit electrons on the anode. The electrons then pass outside the fuel cell, though the load, and back into the electrode where oxygen from the air is converted to oxide. These oxide ions are then injected into the electrolyte.
Future
Despite its success, however, the Ni-YSZ anode has drawbacks:
It is sensitive to sulfur and other contaminants, which requires desulfurization of the anode fuel.
- It is sensitive to HCl, which causes irreversible damage at concentrations above 200ppm.
- The thermal expansion coefficient of the anode is higher than the electrolyte and cathode, causing stability problems.
- Poor activity for hydrocarbon oxidation and a propensity for carbon formation when exposed to hydrocarbons (Copper – ceria anodes are being developed)
While many of these drawbacks can be mitigated with system design, better anodes are still needed.
Most cathodes are based on doped lanthanum manganites and use strontium doped lanthanum manganites (LSM) for high temperature SOFC’s. The material used is based on a number of factors:
- Chemical stability and low interactions with the electrolytes
- Adequate electronic and ionic conductivity - LSM has lower ionic conductivity than YSZ and is inadequate for low temperature cells.
- Relatively high activity
- Manageable interactions with ceramic interconnects
- Thermal expansion coefficients that closely match YSZ
Source: www.epa.gov
The interconnects also present a challenge, as they must maintain uniform contact with the electrodes and be able to withstand strongly oxidizing conditions while maintaining low contact resistance with the electrodes (two characteristics often at odds). Consequently, improvements in contact resistance, corrosion resistance, performance, mechanical stability, manufacturing methods, and strength need to be made.
Currently, unpressurized SOFC’s deliver fuel to electric efficiencies of around 45% and Argonne National Laboratories suggest that pressurized cells could yield efficiencies of 60%. Using the high temperature waste heat could increase the efficiencies further.
Sources:
1. Bellona Foundation. www.bellona.no
2. EG&G Technical Services Under Contract No. DE-AM26-99FT40575 for U.S. Department of Energy. Fuel Cell Handbook 7th Edition. November 2004.
3. Department of Defense. www.dodfuelcell.com
4. Fuel Cell Test and Evaluation Center. www.fctec.com
5. Fuel Cell Today. www.fuelcelltoday.com
