Silicon carbide cathode

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. 

The PAFC is based on an immobilized phosphoric acid electrolyte. The matrix universally used to retain the acid is silicon carbide, and the catalyst for both the anode and cathode is platinum [8], The active layer of platinum catalyst on a carbon-black support and a polymer binder is backed by a carbon paper with 90% porosity, which is reduced to some extent by a Teflon binder [6,9]. 

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. 

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. 

Pt, which is not trivial over the projected lifetime for a PAFC. Migration of the platinum from the cathode towards the anode is due to the platinum being deposited, not on the anode catalyst, but rather on the silicon carbide matrix, adjacent to the anode, as a consequence of the small solubility of hydrogen in solution at the electrode/matrix interface. 

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. 

A phosphoric acid fuel cell (PAFC) consists of an anode and a cathode made of finely dispersed platinum catalyst on a carbon and silicon carbide structure that holds the liquid phosphoric acid electrolyte. Typically, PAFC systems have an operating temperature of around 200°C. When used for the co-generation of electricity and heat, PAFC cells... 

Among new materials suggested for the porous electrolyte matrix in PAFCs, we mention a mixture of silicon carbide (SiC) and PTFE (Mori et ah, 1998). A suspension of the components is mixed in a ball mill for a long time, then spread onto the surfaces of the cathode and anode. This assures good contact between the electrodes and the electrolyte immobilized in the matrix. 

In general, the porosity of traditional fireclay refractories (Table 1.2) is 22-25 %, while modern alumina silica materials reach 13-15 %, silicon carbide refractories have porosity 12-18 %, cathode carbon refractories have porosity 15-22 %, while almost all the pores are open and permeable. The porosity of low-cement and ultralow-cement castables is 12-15 %, while the ratio of permeable pores is very low (the permeability is 0.1-0.4 pm ). 

For different classes of materials, the values of Weibull modulus fluctuate For structural ceramics, the material is considered to be homogeneous if the Weibull modulus is above 15-20 for itride-bonded silicon carbide, it is 15 for alumina brick, it is 6 for porcelain tableware, it is in the range 4-11 baked carbon anodes are considered to be homogeneous if the Weibull modulus is 6-9. We haven t found the values of the Weibull modulus for cathode carbon blocks. 

The side-wall blocks from carbon or silicon carbide are placed on a refractory shoulder side line. This side line is above the refractory layers under the cathode bottom blocks. It may be made of bricks or from castables, usually together with heat insulation boards. This sideline has two purposes. First, side lining is installed in the refractory side line, and, second, it helps to compensate for the mechanical tensions due to the sodium swelling (and thermal expansion) in carbon bottom blocks. Heat insulation materials are easily deformed due to tension, but the construction of the refractory lining remains tmdamaged. 

Similar to a MCFC, in the PAFC, the electrolyte is stored between the electrodes within a thin 50% porosity, 50-200-pm-thick porous matrix. The purpose for the silicon carbide (SiC) porous matrix used in PAFCs is to retain the acid electrolyte by capillary forces. If there were no matrix, the acid could simply drain into the flow channels or bulge according to gravimetric forces. It is critical that the bubble pressure of the matrix exceed 35 kPa, or electrolyte blow-through and excessive crossover from internal pressure differentials would occur. The maximum pressure differential between the anode and cathode is hmited to around 20 kPa to avoid this problem. 

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