Phosphoric acid, fuel-cell type

Phosphoric Acid Fuel Cell This type of fuel cell was developed in response to the industiy s desire to expand the natural-gas market. The electrolyte is 93 to 98 percent phosphoric acid contained in a matrix of silicon carbide. The electrodes consist of finely divided platinum or platinum alloys supported on carbon black and bonded with PTFE latex. The latter provides enough hydrophobicity to the electrodes to prevent flooding of the structure by the electrolyte. The carbon support of the air elec trode is specially formulated for oxidation resistance at 473 K (392°F) in air and positive potentials. 

There are six different types of fuel cells (Table 1.6) (1) alkaline fuel cell (AFC), (2) direct methanol fuel cell (DMFC), (3) molten carbonate fuel cell (MCFC), (4) phosphoric acid fuel cell (PAFC), (5) proton exchange membrane fuel cell (PEMFC), and (6) the solid oxide fuel cell (SOFC). They all differ in applications, operating temperatures, cost, and efficiency. 

Phosphoric acid fuel cells (PAFC) use liquid phosphoric acid as an electrolyte - the acid is contained in a Teflon-bonded silicon carbide matrix - and porous carbon electrodes containing a platinum catalyst. The PAFC is considered the "first generation" of modern fuel cells. It is one of the most mature cell types, the first to be used commercially, and features the most proven track record in terms of commercial applications with over 200 units currently in use. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses. 

Fuel cells can be broadly classified into two types high temperature fuel cells such as molten carbonate fuel cells (MCFCs) and solid oxide polymer fuel cells (SOFCs), which operate at temperatures above 923 K and low temperature fuel cells such as proton exchange membrane fuel cells (PEMs), alkaline fuel cells (AFCs) and phosphoric acid fuel cells (PAFCs), which operate at temperatures lower than 523 K. Because of their higher operating temperatures, MCFCs and SOFCs have a high tolerance for commonly encountered impurities such as CO and CO2 (CO c)- However, the high temperatures also impose problems in their maintenance and operation and thus, increase the difficulty in their effective utilization in vehicular and small-scale applications. Hence, a major part of the research has been directed towards low temperature fuel cells. The low temperature fuel cells unfortunately, have a very low tolerance for impurities such as CO , PAFCs can tolerate up to 2% CO, PEMs only a few ppm, whereas the AFCs have a stringent (ppm level) CO2 tolerance. 

Figure 8.5 shows a schematic illustration of a phosphoric acid fuel cell (PAFC). This cell type is another fuel cell operating in acidic media, and the oxidation reaction in the anode is given by... 

Several types of fuel cells have been developed and are classified according to the electrolytes used alkaline fuel cells, molten carbonate fuel cells, phosphoric acid fuel cells (PAFCs), PEMFCs, and solid oxide fuel cells (SOFCs). As shown in Figure 1.3, the optimum operation temperatures of these fuel cells are different, and each type has different advantages and disadvantages. 

From a cross-flow point of view it may be of interest to mention the phosphoric acid fuel cell with the so-called DiGas system (Fig. 9), which is an air-cooled cross-flow configuration for use in utility-power stations [39]. The process air stream is diverted into two types of channels into individual cells with relatively small cross-sectional area, and into cooling plates (approximately one for every five cells) with a lai ge cross-section. Bipolar plates were molded from a mixture of graphite and phenolic resin, with a Pt-on-carbon cathode and a Pt anode combined with colloidal PTFE on a graphite-paper backing. 

The efficiencies of the different energy conversion systems are compared in Fig. 3 as a function of the size of power plants. Figure 3 shows that the efficiencies of two types of fuel cell systems (phosphoric acid fuel cell, PAFC, see Sect. 8.1.3.1.3 andsolid oxide fuel cell, SOFC, see Sect. 8.1.3.2.2) are higher than those of engines and conventional power plants of comparable size. 

Phosphoric acid fuel cells (PAFCs) utilize liquid phosphoric acid as the electrolyte. These types of cells can achieve efficiencies up to 80%. They are usually large and heavy and require warm-up time. 

In the phosphoric acid fuel cell (PAFC), the acid may react vigorously with the metals and consequently the main problem in the development of this type of cell is the choice of acid-resistant materials. The electrodes are made of platinum mounted on a carbon carrier. The working temperature is about 200°C. 

Finally we come to the fuel cell itself. We have already mentioned the original Grove fuel cell, and the alkaline and phosphoric acid fuel cells used in space technology. Three other types of cell are the molten carbonate fuel cell (with a molten Li2C03/Na2C03 electrolyte), the solid oxide fuel cell (containing a solid metal oxide electrolyte) and the... 

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