Phosphoric acid fuel cell electrolyte matrix

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. 

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. 

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. 

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. 

In chapter 4, Stonehart (a major authority in the field of H2 fuelcell technology and its fundamental aspects) writes, with co-author Wheeler, on the topic of Phosphoric Acid Fuel-Cells (PAFCs) for Utilities Electrocatalyst Crystallite Design, Carbon Support, and Matrix Materials Challenges. This contribution reviews, in detail, recent information on the behavior of very small Pt and other alloy electrocatalyst crystallites used as the electrode materials for phosphoric acid electrolyte fuel-cells. 

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. 

The phosphoric acid fuel cell (PAFC) has a quite similar construction and components as the PEMFC the electrolyte is liquid phosphoric acid in an inert matrix. The operation temperature of 200°C avoids formation of liquid water and improves CO tolerance of the electrocatalyst. For the catalyst properties, the same requirements are valid as for the PEMFC - nanoparticles with a high surface area and a good dispersion on the carbon carrier material are required. The application of PAFC typically is the combined heat and power supply in the 200-kW power range. 

Phosphoric Acid Fuel Cells (PAFC) are operating in the range of 160-200 °C. The phosphoric acid electrolyte is soaked in a microporous matrix of corrosion resistant, non-conducting materials. At this high temperature, PAFC can tolerate a substantial... 

Phosphoric Acid Fuel Cells (PAFCs). The electrolyte used is a 85-95 vol. % H3PO4 solution retained in a matrix of silicon carbide. The working temperature is normally in the range 150-220 °C, but it can be increased to about 300 °C, because of the stability of the concentrated acid solution. 

A phosphoric acid fuel cell (PAFC) uses liquid phosphoric acid as the electrolyte with protons as the charge transport species. The acid is normally imbedded in a solid matrix, such as porous silicon carbide (SiC) or a polybenzimidazole (PBI) membrane. The fuel cell operates best between 160°C and 210°C. The output power ranges from a few kilowatts to a few hundred kilowatts. The anode, the cathode, and the overall reactions are the same as Reactions 1.1,1.2, and 1.3, respectively. 

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]. 

Polymers are nsed in fnel cells. Those of particular interest are the polymer electrolyte membrane (PEM) and the phosphoric acid fuel cell (PAFC) designs. The latter design contains the liquid phosphoric acid in a Teflon bonded silicon carbide matrix. In March 2005 Ticona reported that it had bnilt the first fnel cell prototype made solely with engineering thermoplastics. They claimed that this approach rednced the cost of the fuel by at least 50% when compared with fuel cells fabricated from other materials. The 17-cell unit contains injection moulded bipolar plates of Vectra liquid crystal polymer and end plates of Fortron polyphenylene sulfide (PPS). These two materials remain dimensionally stable at temperatures up to 200 "C. The Vectra LCP bipolar plates contain 85% powdered carbon and are made in a cycle time of 30 seconds. 

Inert electrolyte matrix Fig. 11.23 The basic structure of the phosphoric acid fuel cell. 

Phosphoric acid fuel cells (PAFC) use concentrated phosphoric acid (-100%) as the electrolyte. The matrix used to retain the acid is usually SiC, and the electrocatalyst in both the anode and the cathode is platinum. Operating temperature is typically between 150 and 220°C. Phosphoric acid fuel cells are already semicommer-cially available in container packages (200 kW) for stationary electricity generation (UTC Fuel Cells). Hundreds of units have been installed all over the world. 

Phosphoric acid (PAFC). The electrolyte is concentrated H3P04 absorbed on to a solid matrix, and operates at 200°C. The electrodes are carbon loaded with platinum particles the anode fuel is hydrogen and the cathode fuel is air. The cell voltage is usually around 0.67 V. This type of cell has been tested commercially, producing 4.8 MW for several months at 40 per cent efficiency. 

This cell uses stabilized phosphoric acid on a SIC matrix as the electrolyte, operating at 160-210°C. Platinum or alloys of Pt are used as the catalyst at both electrodes supported on carbon black. The fuel must be H2 rich and contain <2% CO. The efficiency is 36-89%. 

The catalysts and electrode materials used in PAFCs are also similar to those in acidic H2/air fuel cells. Carbon-supported Pt is used as the catalyst at both anode and cathode, porous carbon paper serves as the electrode substrate, and graphite carbon forms the bipolar plates. Since a liquid electrolyte is used, an efficient water removal system is extremely important. Otherwise, the liquid electrolyte is easily lost with the removed water. An electrolyte matrix is needed to support the liquid phosphoric acid. In general, a Teflon -bonded silicon carbide is used as the matrix. 

Phosphoric acid is also used as the electrolyte in so called High Temperature PEFC. In this case, the phosphoric acid is imbibed in a polybenzimidazole polymer matrix. While management of the liquid electrolyte in PAFC requires careful differential pressure control, High Temperature PEFCs are more tolerant Furthermore, the basic nature of the polybenzimidazole matrix prevents electrolyte migration. High Temperature PEFCs therefore are an attractive alternative to PAFC. Nevertheless, phosphoric acid is washed out from PAFC and high temperature PEFC once liquid water can form inside the fuel cell. Therefore, a continuous mode of operation is preferred in both cases. 

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