Presentation on theme: "Chapter 08b Overview of Fuel Cell Types Lecture Notes Dr. Sammia Shahid."— Presentation transcript:
Chapter 08b Overview of Fuel Cell Types Lecture Notes Dr. Sammia Shahid
Five primary types of fuel cells They are based on the electrolyte employed: Phosphoric Acid Fuel Cell Phosphoric Acid Fuel Cell Proton Exchange Membrane Fuel Cell Alkaline Fuel Cell Molten Carbonate Fuel Cell Solid Oxide Fuel Cell
Alkaline Fuel Cell Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the U.S. space program to produce electrical energy and water onboard spacecraft. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs operate at temperatures between 100°C and 250°C (212°F and 482°F). However, newer AFC designs operate at lower temperatures of roughly 23°C to 70°C (74°F to 158°F)
Alkaline Fuel Cell
Some technologists have replaced costly platinum with inexpensive materials as the AFC catalyst and introduced new scrubbing technology to minimize CO 2 effects and thereby extend the alkaline fuel cell life beyond that of other fuel cells. AFC stacks have been shown to maintain sufficiently stable operation for more than 8,000 operating hours. To be economically viable in large-scale utility applications, these fuel cells need to reach operating times exceeding 40,000 hours NASA and the Russian space program had chosen AFCs over PEMs to provide electricity, heat and pure water for all manned space vehicles, due to the AFCs greater fuel efficiency. Alkaline Fuel Cell
Cathode Performance: Improved Low material costs – plastics, carbon, base metals and metal oxides; no platinum. Long life span – 2000-plus hours currently. Superior electrochemical conversion efficiency to other fuel cells and the internal combustion engine. Quick start, even in sub-freezing temperatures down to minus 40 degrees C. Simpler heat and water management when compared to other fuel cell technologies. Like other fuel cells, it is odorless and quiet for enclosed applications. Advantages
Disadvantages Must use pure H 2 -O 2 KOH electrolyte may need occasional replenishment. Must remove water from anode. The disadvantage of this fuel cell type is that it is easily poisoned by carbon dioxide (CO 2 ). In fact, even the small amount of CO 2 in the air can affect this cell's operation, making it necessary to purify both the hydrogen and oxygen used in the cell. This purification process is costly. Susceptibility to poisoning also affects the cell's lifetime (the amount of time before it must be replaced), further adding to cost. Durability is possibly the most significant obstacle in commercializing this fuel cell technology.
Molten Carbonate Fuel Cells - MCFC A molten carbonate salt mixture is used as its electrolyte. They evolved from work in the 1960's aimed at producing a fuel cell which would operated directly on coal. While direct operation on coal seems less likely today, The operation on coal-derived fuel gases or natural gas is viable.
Molten Carbonate Salt used as Electrolyte in MCFC A molten carbonate salt mixture is used as its electrolyte. The composition of the electrolyte (molten carbonate salt mixture) varies, but usually consists of lithium carbonate and potassium carbonate. At the operating temperature of about 650 o C (1200 o F), the salt mixture is liquid and a good ionic conductor. The electrolyte is suspended in a porous, insulating and chemically inert ceramic (LiAlO 3 ) matrix.
Reactions in MCFC Anode Reactions The anode process involves a reaction between hydrogen and carbonate ions (CO 3 = ) from the electrolyte. The reaction produces water and carbon dioxide (CO 2 ) while releasing electrons to the anode. Cathode Reactions The cathode process combines oxygen and CO 2 from the oxidant stream with electrons from the cathode to produce carbonate ions which enter the electrolyte. The need for CO 2 in the oxidant stream requires a system for collecting CO 2 from the anode exhaust and mixing it with the cathode feed stream.
Reactions in MCFC
Description of reactions in MCFCs The anode process involves a reaction between hydrogen and carbonate ions (CO 3 = ) from the electrolyte. The reaction produces water and carbon dioxide (CO 2 ) while releasing electrons to the anode. The cathode process combines oxygen and CO 2 from the oxidant stream with electrons from the cathode to produce carbonate ions which enter the electrolyte. The need for CO 2 in the oxidant stream requires a system for collecting CO 2 from the anode exhaust and mixing it with the cathode feed stream.
the theoretical operating voltage for a fuel cell dAs the operating temperature increases, ecreases and with it the maximum theoretical fuel efficiency. On the other hand, increasing the operating temperature increases the rate of the electrochemical reaction and Thus increases the current which can be obtained at a given voltage. The net effect for the MCFC is that the real operating voltage is higher than the operating voltage for the PAFC at the same current density. Description of reactions in MCFCs
The higher operating voltage of the MCFC means that more power is available at a higher fuel efficiency from a MCFC than from a PAFC of the same electrode area. As size and cost scale roughly with electrode area, this suggests that a MCFC should be smaller and less expensive than a "comparable" PAFC. Description of reactions in MCFCs
The MCFC also produces excess heat at a temperature which is high enough to yield high pressure steam which may be fed to a turbine to generate additional electricity. In combined cycle operation, electrical efficiencies in excess of 60% (HHV) have been suggested for mature MCFC systems. The MCFC operates at between 1110°F (600°C) and 1200°F (650°C) which is necessary to achieve sufficient conductivity of the electrolyte. To maintain this operating temperature, a higher volume of air is passed through the cathode for cooling purposes. Description of reactions in MCFCs
As mentioned above, the high operating temperature of the MCFC offers the possibility that it could operate directly on gaseous hydrocarbon fuels such as natural gas The natural gas would be reformed to produce hydrogen within the fuel cell itself. The need for CO 2 in the oxidant stream requires that CO 2 from the spent anode gas be collected and mixed with the incoming air stream. Description of reactions in MCFCs
Before this can be done, any residual hydrogen in the spent fuel stream must be burned. Future systems may incorporate membrane separators to remove the hydrogen for recirculation back to the fuel stream. At cell operating temperatures of 650 o C (1200 o F) noble metal catalysts are not required. The anode is a highly porous sintered nickel powder, alloyed with chromium to prevent agglomeration and creep at operating temperatures. The cathode is a porous nickel oxide material doped with lithium. Description of reactions in MCFCs
Significant technology has been developed to provide electrode structures which position the electrolyte with respect to the electrodes and maintain that position while allowing for some electrolyte boil-off during operation. The electrolyte boil-off has an insignificant impact on cell stack life. A more significant factor of life expectancy has to do with corrosion of the cathode. The MCFC operating temperature is about 650 o C (1200 o F). Description of reactions in MCFCs
At this temperature the salt mixture is liquid and is a good conductor. The cell performance is sensitive to operating temperature. A change in cell temperature from 650 o C (1200 o F) to 600 o C (1110 o F) results in a drop in cell voltage of almost 15%. The reduction in cell voltage is due to increased ionic and electrical resistance and a reduction in electrode kinetics. Description of reactions in MCFCs
Evolution of Cell Component Technology for MCFC
Advantages Fuel flexibility: Works well with reformed fuels Internal reforming reactions possible Nonprecious metal catalyst High quality waste heat for cogeneration applications.
Durability: The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. Relatively expensive materials Must implement CO 2 recycling Cathode reactions not well understood - anomalous kinetics are hard to control Very sensitive to sulfur Disadvantages
Solid Oxide Fuel Cell (SOFC) Solid oxide fuel cells (SOFCs) use a hard, non- porous ceramic compound as the electrolyte. Because the electrolyte is a solid, the cells do not have to be constructed in the plate-like configuration typical of other fuel cell types. SOFCs are expected to be around 50%–60% efficient at converting fuel to electricity. In applications designed to capture and utilize the system's waste heat (co-generation), overall fuel use efficiencies could top 80%–85%.
Solid Oxide Fuel Cell (SOFC) Solid oxide fuel cells operate at very high temperatures— around 1,000°C (1,830°F). High-temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system. SOFCs are also the most sulfur-resistant fuel cell type; they can tolerate several orders of magnitude more of sulfur than other cell types. In addition, they are not poisoned by carbon monoxide (CO), which can even be used as fuel. This property allows SOFCs to use gases made from coal.
Advantages Fuel flexibility. Nonprecious metal catalyst High-quality waste heat for cogeneration applications. Solid electrolyte Relatively high power density. Sealing issues
Disadvantages High-temperature operation has disadvantages. It results in a slow startup and requires significant thermal shielding to retain heat. The high operating temperatures also place stringent durability requirements on materials. The development of low-cost materials with high durability at cell operating temperatures is the key technical challenge facing this technology. Sealing issues
Direct Liquid-Fueled Fuel Cells Direct formic acid fuel cell Direct sodium borohydride fuel cell
Direct Liquid-Fueled Fuel Cells a) A 20 W DMFC notebook computer charger can directly power a notebook or recharge the battery b) A 2 W prototype DMFC system can change a cell phone in 2 hours using 10cc methanol fuel cartridge.