Phosphoric acid fuel cells carbon monoxide

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... 

Phosphoric acid fuel cell (PAFC) was the first fuel cell to be commercialized. PAFC uses liquid phosphoric acid as an electrolyte, which is soaked in silicon carbide particle matrix using capillary action. PAFC is tolerant to CO2 feed stream because the electrolyte is an acid. However, carbon monoxide poisons the Pt electrodes so that its concentration should be below 3-5 volume % in the feed stocks. 

Classical phosphoric add fuel cells use phosphoric add as the electrolyte, which is immobilized in a Teflon bonded silicon carbide matrix. Phosphoric acid fuel cells usually work at temperatures around 200 °C and are able to tolerate carbon monoxide levels of up to 2 vol.% [1]. Platinum/ruthenium as the anode catalyst may improve the performance in presence of carbon monoxide, similar to PEM fuel cells [33]. 

The phosphoric acid fuel cell (PAFC) was the first fuel cell to be commercialized and shares some technologies with the PEMFC, such as the porous electrodes and the platinum catalysts. The liquid phosphoric acid allows high operating temperatures, around 200 C. Fuels must be free of carbon monoxide, as with the PEMFCs. With rated power over 50 kW, PAFC systems are used for stationary applications. 

Key words fuel, hydrogen, methane, methanol, biogas, alkaline fuel cell (AFC), polymer electrolyte fuel cell (PEFC), phosphoric acid fuel cell (PAFC), platinum, catalyst, degradation, sulphur, carbon monoxide, poisoning, particulates. 

Phosphoric acid fuel cell - PAFC is the first fuel cell to have reached commercialization. Phosphoric acid is used as the electrolyte. More than 75 MW PAFC systems are in operation in 85 cities in 19 countries so far. Similar to the high temperature PEMFC, the operating temperature of a PAFC is higher than 200°C. This helps increase the tolerance of the Pt catalyst against carbon monoxide. Hydrocarbon reforming gas is a promising fuel source for PAFC. The overall efficiency of PAFC can reach 80% if the cogenerated heat is harnessed. Due to these characteristics of the PAFC, its main apphcation focuses on stationary power source. 

A diagram of a fuel-processing system that would be needed for a natural-gas-powered phosphoric acid fuel cell is shown in Figure 8.5. This needs a fuel gas at about 220°C, with carbon monoxide levels down to about 0.5%. The system may look complicated enough, but even this has some simplifications. The following is an explanation of the process. 

Hydrogen gas fuel and air (O2) are fed to anode and cathode Pt catalyst powder layers, respectively. The Pt catalysts is Teflon-bonded to porous carbon sheets to form gas-diffusion electrodes, with a catalyst loading of about 1.0 mg/cm. The Pt anode and cathode are separated by a thin inert porous matrix that is filled with concentrated phosphoric acid. The cell operates at 200°C (to improve the electrode kinetics), with a cell voltage of about 0.67 V at a current density of 0.150 A/cm. Most voltage losses occur at the air cathode. The hydrogen gas must be pure because sulfur and carbon monoxide poison the Pt anode catalyst. This type of fuel cell is commercially available today, with more than 200 systems installed all over the world in hospitals, hotels, office buildings, and utility power plants. 

Second, molten carbonate fuel cells have electric efficiencies of 47 to 50 percent or more, which significantly reduces their fuel costs for stationary applications compared with both phosphoric acid and pem fuel cells, whose overall efficiency when running on natural gas might not exceed 35 to 40 percent. Third, high temperatures allow relatively inexpensive nickel to be used as a catalyst rather than pricey platinum, which is required by the lower-temperature fuel cells. Fourth, these fuel cells are far more tolerant of carbon monoxide, which can poison the electrochemical reaction of pem... 

A combination of 98% H3P04 and 2% water provides a liquid that can be heated to > 200 °C at atmospheric pressures. A high temperature of 150 °C is required to polymerize phosphoric acid to pyrophosporic acid (H4P207), which has a considerably higher ionic conductivity than the parent acid. It was necessary to raise the operating temperature of the fuel cell to 200 °C in order to tolerate a carbon monoxide level of... 

For fuel-cell operation, most often technical hydrogen obtained by the conversion of primary fuels such as methanol or petroleum products is used, rather than pure hydrogen obtained by electrolysis. Technical hydrogen always contains carbon monoxide and a number of other impurities, even after an initial purification. In the first experiments conducted in the mid-1980s it was shown that traces of CO in hydrogen used for the operation of fuel cells with phosphoric acid electrolyte lead to a marked increase in the hydrogen electrodepolarization. 

Raising the temperature, the adsorption equilibrium between hydrogen and carbon monoxide, jointly adsorbing on platinum, shifts in favor of hydrogen adsorption. This raises the highest admissible threshold concentration of carbon monoxide. The effect could be seen in fuel cells with phosphoric acid electrolyte, which work at temperatures of about 180-200°C and admit carbon monoxide concentrations in hydrogen as high as 100 ppm, despite the fact that platinum catalysts are used. 

Poly(2,2 -( 1,4-phenylene)5,5 -bibenzimidazole) can be obtained under certain conditions of polymerization as a high-molecular-weight species [29]. The polymer solutions can be used directly for phosphoric acid doped PBI membranes. Such membranes show high phosphoric acid doping levels. At 160 °C a high carbon monoxide tolerance for fuel cells is observed. 

These cells operate only with hydrogen as the anode fuel and, moreover, the hydrogen must be pure since sulphur compounds and carbon monoxide adversely affect the performance of the Pt catalyst. Each cell consists of two teflon-bonded gas diffusion electrodes on a porous conducting support (see Fig. 10.21). At both anode and cathode the catalyst is platinum particles dispersed on carbon and a recent success has been a reduction in Pt loading from 10 mg cm to 0.75 mg cm ". The electrolyte is concentrated phosphoric acid absorbed onto a solid matrix and the cell operates at 200°C to improve the electrode kinetics. The cells are then mounted in stacks to increase the power output. 

Jiao K, Alaefour IE, Li X (2011) Three-dimensional non-isothermal modeling of carbon monoxide poisoning in high temperature proton exchange membrane fuel cells with phosphoric acid doped polybenzimidazole membranes. Fuel 90 568-582... 

PAFCs are very efficient fuel cells, generating electricity at more than 50 % efficiency [13], About 85 % of the steam produced by the PAFC is used for cogeneration. This efficiency may be compared to about 35 % for the utility power grid in the United States. As with the PEMFC Pt or Pt alloys are used as catalysts at both electrodes [76]. The electrolyte is inorganic acid, concentrated phosphoric acid (100 %) which will conduct protons [77-79]. Operating temperatures are in the range of 150-220 °C. At lower temperatures, PAFC is a poor ionic conductor, and carbon monoxide (CO) poisoning of the platinum catalyst in the anode can become severe [76, 80,81]. 

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