Phosphoric Acid Fuel Cells PAFC

For PAFC, the electrolyte consists of concentrated phosphoric acid and sihcon carbide matrix is used to retain the acid. The electrodes are made up of platinum, hence serving the purpose of catalysts as well. PAFC may produce power output up to 1 MW. Their operating temperature is also low, i.e., 200 °C. The anode and cathode reactions are given as 

Phosphoric acid (H3PO3) has thermal, electrochemical stability and low volatility. Hydrogen fuel is also produced from a hydrocarbon through steam reformation i.e., 

along with hydrogen, carbon monoxide is produced, which is poisonous and a potential problem for the fuel cell (especially for Pt catalysts). Hence, the CO must be processed via water-gas shift reactions, as done for PEMFC cells i.e.. 

Porous plate Impervious Porous plate Sandwich bipolar plate carbon plate 

Manufacturers in the USA (Electrochem, United Technologies) and Japan (Fuji Electric, Mitsubishi Electric, Toshiba) have developed PAFC plants in 

All chemical compounds have a heat (enthalpy) of formation , AH, which equates to the heat liberated or absorbed when 1 mole of the compound is formed from its constituent elements. An element in its standard state is defined as having zero heat of formation. The standard molar heat of formation, A//f, of a compound is then the change in enthalpy, positive or negative, when 1 mole of the compound is formed at standard conditions (298.15 K and 101.325 kPa) from the elements in their most stable physical forms (gas, liquid or solid). 

For hydrogen, the heat of combustion equates to the heat of formation of the product water, i.e., 

AH[ (for liquid water) =—285.83 kJ per mole of water. The negative sign indicates that heat is evolved in the process. 

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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. 

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. 

Stonehart P. 1990. Development of advanced noble metal-aUoy electrocatalysts for phosphoric-acid fuel cells (PAFC). Ber Bunsenges Phys Chem 94 913-921. 

In applications where Nafion is not suitable, at temperatures above 200 °C with feed gas heavily contaminated with CO and sulfur species, a phosphoric acid fuel cell (PAFC)-based concentrator has been effective [15]. Treating the gas shown in Table 1, a H2 product containing 0.2% CO, 0.5%CO2 and only 6 ppm H2S was produced. The anode electrode was formed from a catalyst consisting basically of Pt-alloy mixed with 50% PTFE on a support of Vulcan XC-72 carbon. The cathode was... 

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 cell (PAFC) Poison <0.5% olefins n.i. n.i. Poison <0.2 mol% (NH4)3P04 in electrolyte Poison <4 ppm73 Poison <4 ppm73... 

Phosphoric acid-based systems, for cellulosics, 11 488 Phosphoric acid esters, 24 159 Phosphoric acid fuel cells (PAFC), 13 858— 860 12 203-204, 216-219 19 626 effects of carbon monoxide and sulfur in, 12 219... 

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

Close to a thousand systems that produce over 10 kilowatts each have been installed worldwide. Most of these are fueled by natural gas. Phosphoric acid fuel cells (PAFCs) have typically been used for large-scale applications, but molten carbonate and solid oxide units also compete with PAFCs. 

Phosphoric Acid Fuel Cell (PAFC) Phosphoric acid concentrated to 100% is used for the electrolyte in this fuel cell, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor, and CO poisoning of the Pt electrocatalyst in the anode becomes severe. The relative stability of concentrated phosphoric acid is high compared to other common acids consequently the PAFC is capable of operating at the high end of the acid temperature range (100 to 220°C). In addition, the use of concentrated acid (100%) minimizes the water vapor pressure so water management in the cell is not difficult. The matrix universally used to retain the acid is silicon carbide (1), and the electrocatalyst in both the anode and cathode is Pt. 

IFC Corp, of South Windsor, Connecticut, has the most commercially advanced fuel cell for electricity generation, the PC25, a 200-kW phosphoric acid fuel cell (PAFC). IFC has over 200 fuel cells delivered around the world. 

Phosphoric acid fuel cell (PAFC) working at 180-200 °C vfith a porous matrix of PTFE-bonded silicon carbide impregnated with phosphoric acid as electrolyte, conducting by the H cation. This medium-temperature fuel cell is now commercialized by ONSI (USA), mainly for stationary applications. 

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. 

Phosphoric acid fuel cell (PAFC) 200 Phosphoric acid Hydrogen, reformed methane 35-45 1 up to 100 MW power plants, 5-500 kW heating power station... 

Phosphoric acid fuel cell (PAFC)—Phosphoric acid electrolyte with platinum catalyst. It can use hydrocarbon fuel and is suited for stationary applications. It can generate both electricity and steam. As many as 200 units in sizes ranging from 200 kW to 1 mW are in operation. 

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... 

Phosphoric acid fuel cells (PAFC) with concentrated H3P04 (in silicon carbide matrices) electrolyte, which transports H+ cations, generated at the anode, to an ambient-air-exposed cathode, where they are electro-oxidised to water at moderate temperatures. 

Phosphoric-acid fuel cell (PAFC) — In PAFCs the -> electrolyte consists of concentrated phosphoric acid (85-100%) retained in a silicon carbide matrix while the -> porous electrodes contain a mixture of Pt electrocatalyst (or its alloys) (-> electrocatalysis) supported on -> carbon black and a polymeric binder forming an integral structure. A porous carbon paper substrate serves as a structural support for the electrocatalyst layer and as the current collector. The operating temperature is maintained between 150 to 220 °C. At lower temperatures, phosphoric acid tends to be a poor ionic conductor and poisoning of the electrocatalyst at the anode by CO becomes severe. 

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