PAFC

Phosphoric acid fuel cell is mainly used in stationary power plants ranging from dispersed power to in situ generation plants. Power plants based on PAFCs are being installed worldwide with outputs ranging from 5 to 20 MW supplying towns, cities, shopping malls, or hospitals with electricity, heat, and hot water [13]. 

The advantages of PAFC are its simple construction, thermal, chemical, and electrochemical stability, and the low volatility of the electrolyte at operating temperatures (150-200 °C). These factors probably assisted the earlier deployment into commercial systems compared with the other fuel cell types. 

At the beginning of PAFC development, diluted phosphoric acid was used in PAFCs to avoid corrosion of some of the cell components. Nowadays with improved materials available for cell constmction, the concentration of the acid is nearly 100%. The acid is usually stabihzed in a matrix based on SiC. The higher concentration of the acid increases the conductivity of the electrolyte and reduces the corrosion of the carbon-supported electrodes. 

The electrodes used in PAFCs are generally Pt-based catalysts dispersed on a carbon-based support. For the cathode, a relatively high loading of Pt is necessary for the promotion of the O2 reduction reaction. The hydrogen oxidation reaction at the anode occurs readily over a Pt/C catalyst. 

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A 2 factorial design with two factors requires four runs, or sets of experimental conditions, for which the uncoded levels, coded levels, and responses are shown in Table 14.4. The terms Po> Po> Pfc> and Pafc in equation 14.4 account for, respectively, the mean effect (which is the average response), first-order effects due to factors A and B, and the interaction between the two factors. Estimates for these parameters are given by the following equations... 

AFC = all line fuel ceU MCFC = molten carbonate fuel ceU PAFC = phosphoric acid fuel ceU PEFC = polymer electrolyte fuel ceU and SOFC = solid oxide fuel ceU. 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

Westinghouse Electric Corp. initiated a program to develop air-cooled PAFC stacks, containing cooling plates at six-ceU intervals. Full size 100-kW stacks (468 cells, 0.12-m electrode area) were built, and a module containing four of these stacks was tested. An air-cooled stack operated at 0.480 MPa yielded a cell voltage of 0.7 V at 267 m A /cm (187 mW/cm ). Demonstration of this technology is plarmed for a site in Norway. 

From the standpoint of commercialization of fuel ceU technologies, there are two challenges initial cost and reHable life. The initial selling price of the 200-kW PAFC power plant from IFC was about 3500/kW. A competitive price is projected to be about 1500/kW orless for the utiHty and commercial on-site markets. For transportation appHcations, cost is also a critical issue. The fuel ceU must compete with conventional mass-produced propulsion systems. Furthermore, it is not clear if the manufacturing cost per kilowatt of small fuel ceU systems can be lower than the cost of much larger units. The life of a fuel ceU stack must be five years minimum for utiHty appHcations, and reHable, maintenance-free operation must be achieved over this time period. The projection for the PAFC stack is a five year life, but reHable operation has yet to be demonstrated for this period. 

The bipolar plate material of the PAFC is graphite. A portion of it has a carefully controlled porosity that sei ves as a resei voir for phosphoric acid and provides ffow channels for distribution of the fuel and oxidant. The plates are elec tronically conductive but impervious to gas crossover. 

In a typical PAFC system, methane passes through a reformer with steam from the coolant loop of the water-cooled fuel cell. Heat for the reforming reaction is generated by combusting the depleted fuel. The reformed natural gas contains typically 60 percent H9, 20 percent CO, and 20 percent H9O. Because the platinum catalyst in the PAFC can tolerate only about 0.5 percent CO, this fuel mixture is passed through a water gas shift reactor before being fed to the fuel cell. 

PAFC systems are commercially available from the ONSI Corporation as 200-kW stationary power sources operating on natural gas. The stack cross sec tion is 1 m- (10.8 ft"). It is about 2.5 m (8.2 ft) tall and rated for a 40,000-h life. It is cooled with water/steam in a closed loop with secondary heat exchangers. The photograph of a unit is shown in Fig. 27-66. These systems are intended for on-site power and heat generation for hospitals, hotels, and small businesses. Another apphcation, however, is as dispersed 5- to 10-MW power plants in metropolitan areas. Such units would be located at elec tric utihty distribution centers, bypassing the high-voltage transmission system. The market entiy price of the system is 3000/kW. As production volumes increase, the price is projec ted to dechne to 1000 to 1500/kW. 

FIG. 27-66 PC-25 - commercial 200-k V PAFC generator. iLrilcnialiomi] Fuel CcJJ.s.)... 

Because of this extreme sensitivity, attention shifted to an acidic system, the phosphoric acid fuel cell (PAFC), for other applications. Although it is tolerant to CO, the need for liquid water to be present to facilitate proton migration adds complexity to the system. It is now a relatively mature technology, having been developed extensively for stationary power usage, and 200 kW units (designed for co-generation) are currently for sale and have demonstrated 40,000 hours of operation. An 11 MW model has also been tested. 

In contrast with the AFC, the PAFC can demonstrate reliable operation with 40 percent to 50 percent system efficiency even when operating on low quality fuels, such as waste residues. This fuel flexibility is enabled by higher temperature operation (200°C vs. 100°C for the AFC) since this raises electro-catalyst tolerance toward impurities. Flowever, the PAFC is still too heavy and lacks the rapid start-up that is nec-essaiy for vehicle applications because it needs preheating to 100°C before it can draw a current. This is unfortunate because the PAFC s operating temperature would allow it to thermally integrate better with a methanol reformer. 

The PAFC is, however, suitable for stationary power generation, but faces several direct fuel cell competitors. One is the molten carbonate fuel cell (MCFC), which operates at "650°C and uses an electrolyte made from molten potassium and lithium carbonate salts. Fligh-teinperature operation is ideal for stationary applications because the waste heat can enable co-generation it also allows fossil fuels to be reformed directly within the cells, and this reduces system size and complexity. Systems providing up to 2 MW have been demonstrated. 

The most promising fuel cell for transportation purposes was initially developed in the 1960s and is called the proton-exchange membrane fuel cell (PEMFC). Compared with the PAFC, it has much greater power density state-of-the-art PEMFC stacks can produce in excess of 1 kWA. It is also potentially less expensive and, because it uses a thin solid polymer electrolyte sheet, it has relatively few sealing and corrosion issues and no problems associated tvith electrolyte dilution by the product water. 

An attempt has been made in Table 1 to present the status of fuel cell technologies. For terrestrial applications, the PAFC power plant is the most advanced, and a 200-kW system manufactured by ONSI, a division of United Technologies, Inc., has reached commercialization. Its main applications are focused on on-site integrated energy systems that could... 

PAFC CH4,CH30H, oil 200 °C 200-10,000 40-45 200-300 30,000-40,000 200-3000 On-site integrated energy systems, transportation (fleet vehicles), load leveling... 

PAFC, phosphoric acid fuei ceii MCFC, moiten carbonate fuei ceii SOFC, soiid oxide fuei ceii PEMFC, proton exchange membrane fuei ceii DMFC, direct methanoi fuei ceii AFC, alkaiine fuel cell. 

Medium-temperature phosphoric acid fuel cells (PAFCs). The electrolyte is 85 to 95% phosphoric acid the working temperatures are 180 to 200°C. Such systems were used to build numerous autonomous power plants with an output of up to about 250 kW, and even some with an output of up to 4 MW, in the United States, Japan, and other countries. 

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