29-08-2017, 11:48 AM
A fuel cell is an electrochemical device that converts the chemical energy of the fuel directly into electrical energy. Intermediate conversions of fuel to thermal and mechanical energy are not required. All fuel cells consist of two electrodes (anode and cathode) and an electrolyte (usually held in a matrix). They work as a battery, except that the reactants (and products) are not stored, but are continuously fed to the cell. Fuel cells were first invented in 1839, but the technology remained dormant until the late 1950s. During the 1960s, NASA used precursors of current fuel cell technology as power sources in spacecraft. Figure 1 shows the flows and reactions in a single fuel cell. Unlike ordinary combustion, the fuel (hydrogen-rich) and the oxidant (typically air) are supplied to the fuel cell separately. The fuel and oxidant streams are separated by an electrode-electrolyte system. The fuel is fed to the anode (negative electrode) and an oxidant is fed to the cathode (positive electrode). The electrochemical oxidation and reduction reactions take place at the electrodes to produce electric current. The main product of the reactions of fuel cells is water.
The PEMFC electrolyte is a solid polymer membrane installed between two porous electrodes catalyzed by platinum. PEMFCs typically operate at about 80-85 ° C (185 ° F), a temperature determined both by thermal stability and by the ionic conductivity characteristics of the polymer membrane (Srinivasan et al., 1993). To obtain sufficient ionic conductivity, the proton-conducting polymer electrolyte requires liquid water. Thus, temperatures are limited to less than 100 ° C. The low operating temperature allows the PEMFC to be brought to steady state operation quickly.PEMFC can operate at high air pressures-up to eight atm have been used-enabling Higher energy density of the cell stack. The solid polymer membrane can also withstand substantial differential differential pressures, which provides some flexibility in system design (Penner, 1995; Prater, 1994). As with many low-temperature fuel cells, PEMFCs require a pure source of hydrogen for operation. Since hydrogen is not readily available, it is typically obtained by reforming a hydrocarbon fuel, such as methanol or natural gas. Rebuilt fuel often contains other gases such as carbon monoxide that are detrimental to the operation of the fuel cell. Carbon monoxide levels of 50 ppm or more poison the catalyst, causing severe degradation in cellular performance. Therefore, all carbon-containing fuels (eg, natural gas, methanol and propane) require further processing of the fuel. Fuel processing in general represents a major challenge for the commercialization of fuel cells; this is particularly true for PEMFCs because of their susceptibility to electrocatalyst poisoning from low levels of carbon monoxide. However, given sufficient fuel processing, PEMFCs are expected to operate using hydrogen, methanol, propane and natural gas fuels (and eventually gasoline). CPEMs have an electrical efficiency of close to 50 percent (McClellen, 1998). However, because the fuel-cell residual heat temperature is too low, in the fuel reforming process the overall system efficiencies have been limited to 42 percent (Rastler et al., 1996). Depending on the type of refurbishment process, PEMFC systems may have the lowest electrical efficiency of all fuel cell systems.
PAFCs have phosphoric acid electrolytes. They typically operate at about 200 ° C (400 ° F). Cooling of the fuel cell stack is done with pressurized boiling water. As with all types of fuel cells, PAFCs operate on hydrogen normally supplied from a reformer supplied with natural gas, although International Fuel Cell's PC25 units have run on propane gas, landfill gas and anaerobic digesters. PAFCs can operate at high pressures (up to eight atm); The electrolytic material consists of 100 percent phosphoric acid, which acts as a transport fluid for the migration of hydrogen ions dissolved from the anode to the cathode and conducts the ionic charge between the two electrodes to complete the electrical circuit. Because the electrolyte is a liquid, evaporation and migration must be carefully controlled. Like PEMFCs, PAFCs also employ platinum electrocatalysts on the cell electrodes. This limits the amount of carbon monoxide the cell can tolerate before performance degradation is established. The present limit is about two percent (by volume) before the cell voltage begins to decay. Corrosion (by the acidic liquid electrolyte) of the carbon components, mainly the carbon support for the catalyst layer and the separator (or bipolar) plate, causes a reduction of cell life in PAFC. Other factors that affect performance degradation of PAFC are platinum particle sintering and electrolyte flooding, both due to changes in the properties of the material at elevated temperatures.
PAFCs are the only fuel cell that consistently achieves demonstrated lifetimes of 40,000 hours or more under production conditions. Field units have been operated at ambient temperatures of -32 ° C to 49 ° C and one-mile altitudes. In addition, PC25 units operating in California have been exempted from the air pollution permit process because their emissions have been so low. CPFCs achieve 37-42 percent (lower heating value) electrical Platinum lectrocatalysts and require external reformers to produce a hydrogen-rich gas feed from a hydrocarbon feedstock. The operating temperature of the PFCs of about 200 ° C is sufficient to provide low heat output in the form of hot water vapor of 140 ° -250 ° F or low pressure steam (15 psi). The use of thermal output for cogeneration applications, such as hotels, hospitals and schools, is particularly attractive.
The PEMFC electrolyte is a solid polymer membrane installed between two porous electrodes catalyzed by platinum. PEMFCs typically operate at about 80-85 ° C (185 ° F), a temperature determined both by thermal stability and by the ionic conductivity characteristics of the polymer membrane (Srinivasan et al., 1993). To obtain sufficient ionic conductivity, the proton-conducting polymer electrolyte requires liquid water. Thus, temperatures are limited to less than 100 ° C. The low operating temperature allows the PEMFC to be brought to steady state operation quickly.PEMFC can operate at high air pressures-up to eight atm have been used-enabling Higher energy density of the cell stack. The solid polymer membrane can also withstand substantial differential differential pressures, which provides some flexibility in system design (Penner, 1995; Prater, 1994). As with many low-temperature fuel cells, PEMFCs require a pure source of hydrogen for operation. Since hydrogen is not readily available, it is typically obtained by reforming a hydrocarbon fuel, such as methanol or natural gas. Rebuilt fuel often contains other gases such as carbon monoxide that are detrimental to the operation of the fuel cell. Carbon monoxide levels of 50 ppm or more poison the catalyst, causing severe degradation in cellular performance. Therefore, all carbon-containing fuels (eg, natural gas, methanol and propane) require further processing of the fuel. Fuel processing in general represents a major challenge for the commercialization of fuel cells; this is particularly true for PEMFCs because of their susceptibility to electrocatalyst poisoning from low levels of carbon monoxide. However, given sufficient fuel processing, PEMFCs are expected to operate using hydrogen, methanol, propane and natural gas fuels (and eventually gasoline). CPEMs have an electrical efficiency of close to 50 percent (McClellen, 1998). However, because the fuel-cell residual heat temperature is too low, in the fuel reforming process the overall system efficiencies have been limited to 42 percent (Rastler et al., 1996). Depending on the type of refurbishment process, PEMFC systems may have the lowest electrical efficiency of all fuel cell systems.
PAFCs have phosphoric acid electrolytes. They typically operate at about 200 ° C (400 ° F). Cooling of the fuel cell stack is done with pressurized boiling water. As with all types of fuel cells, PAFCs operate on hydrogen normally supplied from a reformer supplied with natural gas, although International Fuel Cell's PC25 units have run on propane gas, landfill gas and anaerobic digesters. PAFCs can operate at high pressures (up to eight atm); The electrolytic material consists of 100 percent phosphoric acid, which acts as a transport fluid for the migration of hydrogen ions dissolved from the anode to the cathode and conducts the ionic charge between the two electrodes to complete the electrical circuit. Because the electrolyte is a liquid, evaporation and migration must be carefully controlled. Like PEMFCs, PAFCs also employ platinum electrocatalysts on the cell electrodes. This limits the amount of carbon monoxide the cell can tolerate before performance degradation is established. The present limit is about two percent (by volume) before the cell voltage begins to decay. Corrosion (by the acidic liquid electrolyte) of the carbon components, mainly the carbon support for the catalyst layer and the separator (or bipolar) plate, causes a reduction of cell life in PAFC. Other factors that affect performance degradation of PAFC are platinum particle sintering and electrolyte flooding, both due to changes in the properties of the material at elevated temperatures.
PAFCs are the only fuel cell that consistently achieves demonstrated lifetimes of 40,000 hours or more under production conditions. Field units have been operated at ambient temperatures of -32 ° C to 49 ° C and one-mile altitudes. In addition, PC25 units operating in California have been exempted from the air pollution permit process because their emissions have been so low. CPFCs achieve 37-42 percent (lower heating value) electrical Platinum lectrocatalysts and require external reformers to produce a hydrogen-rich gas feed from a hydrocarbon feedstock. The operating temperature of the PFCs of about 200 ° C is sufficient to provide low heat output in the form of hot water vapor of 140 ° -250 ° F or low pressure steam (15 psi). The use of thermal output for cogeneration applications, such as hotels, hospitals and schools, is particularly attractive.