Phosphoric acid fuel cells pressure

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. 

M. Farooque, "Evaluation of Gas-Cooled Pressurized Phosphoric Acid Fuel Cells for Electric Utility Power Generation," Final Technical Report, NASA CR-168298 prepared by Energy Research Corp. under Contract No. DEN 3-201 for NASA Lewis Research Center, September 1983. 

The final 0-D equation presented here stems from incorporating the gas-pressure dependences directly instead of through a limiting current density, which normally only considers oxygen effects. This equation was proposed by Newmanfor phosphoric-acid fuel cells and predates the above polymer—electrolyte fuel-cell expressions. It has the form... 

Phosphoric Acid Fuel Cell. This cell demonstrates the possibilities of using very concentrated phosphoric acid to allow the temperature of the solution to be raised to about 200-205 °C without the need for high-pressure equipment. The higher temperature makes it possible to produce large amounts of free steam for the re-forming of natural gas to the hydrogen upon which most development has been based. 

Hydrocarbon Technologies, Inc. integrated gasification combined-cycle Kellogg-Rust-Westinghouse process molten carbonate fuel cell methanol-to-gasoline process once-through Fischer-Tropsch process phosphoric acid fuel cell pulverized coal polymer electrolyte fuel cell pressurized fluidized bed combustion 1015 Btu... 

Another feature of concentrated phosphoric acid solutions that is very important for fuel cells is the water vapor pressure, which decreases drastically with increasing concentration of the acid. This feature allows the phosphoric acid solution to be immobilized in a porous solid matrix, greatly simplifying the elimination of water as a reaction product from the fuel cell s cathode space by gas (oxygen or air) circulation. It is safe to adjust this circulation to the maximal current load (maximum rate of water production), without the need to readjust it at lower loads, as there is no risk of excessive drying of the matrix on account of the said feature. In this way, water elimination in phosphoric acid fuel cells has a peculiar self-regulation. No such feature exists in sulfuric acid solution, where for water elimination the acid itself would have to be circulated, which would cause problems of sealing and corrosion. 

Non-repeat components. The components described in the previous section are usually referred to as repeat components since every cell has those components with multiple cells present in a cell stack. Non-repeat components are those that are used only once in a cell stack. The main non-repeat components in a cell stack assembly are pressure plates, coolant inlet and outlet manifolds, reactant inlet and outlet manifolds, and manifold seals. Multiple cells placed between coolers are stacked between two stainless steel pressure plates and loaded axially to arotmd 60 psi with the help of tie rods that run the entire length of the cell stack. Reactant manifolds are then assembled onto the cell stack with manifold seals placed between the cell stack and the manifold. These reactant manifolds are made of stainless steel and coated with PTFE coating to prevent phosphoric acid attack of the manifolds. It is very essential to ensure that there are no pin holes in the PTFE coating for the same reason mentioned above. In phosphoric acid fuel cells, manifold seals made... 

Phosphoric acid is unique with respect to its high thermal stability and proton conductivity at high concentrations. Based on concentrated phosphoric acid (85-100 wt%) as electrolyte, the phosphoric acid fuel cell technology operates at temperatures up to 210 °C. The use of concentrated acid substantially minimizes the water vapor pressure. Significant dehydration of phosphoric acid takes place at above 200 °C under limited atmospheric humidity, resulting in the formation of condensed strong acids, which are relatively stable and possess reasonable conductivity. Consequently, the electrolyte is... 

This agrees well with reported results (Bevc, 1997 and Parsons Inc., 2000) for high-temperature cells. However, for other fuel cells, working at lower temperatures, the agreement is not as good. For example, a phosphoric acid fuel cell working at 200°C should be affected by system pressure by the equation... 

The degree of H2 purification depends on its application. For industrial H2, pressure swing absorption systems or Pd membranes are used to produce H2 at up to 99.999% purity whereas for PEM or phosphoric acid fuel cells... 

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... 

In this problem the objective is the functioning of a hydrogen-oxygen fuel cell in phosphoric acid at 180 °C and a gas pressure of 8 bar. The following are in kJ/mol at 180 °C, for the combustion of hydrogen with oxygen ... 

PBI (see chemical structure above) is a hydrocarbon membrane that has been commercially available for decades. Free PBI has a very low proton conductivity ( 10 S/cm) and is not suitable for PEM fuel cell applications. However, the proton conductivity can be greatly improved by doping PBI with acids such as phosphoric, sulfuric, nitric, hydrochloric, and perchloric acids. The PA-doped PBI membrane is the most popular one in PEM fuel cell applications because H3PO4 is a nonoxidative acid with very low vapor pressure at elevated temperature. Savinell et al. and Wainright et al. first demonstrated the use of PBI-PA for HT fuel cells in 1994.270 272 since then, there has been a significant amount of research on the PBI-based membrane because of its low cost and good thermal and chemical stabil-... 

The boiling point of concentrated phosphoric acid is above 250 C, allowing operation of this type of fuel cell at temperatures up to 225°C without the need to pressurize the cell. High temperature helps the kinetics, particularly at the cathode, where oxygen is reduced. It also diminishes the sensitivity of the electrocatalyst to poisoning. 

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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