Fuel cell high temperature, SOFC

Appendix A shows that for equilibrium fuel cells, high-temperature operation offers no advantage, but indeed a theoretical disadvantage. However, in a practical non-equilibrium set-up, where unused fuel and unconsumed oxygen are features and these have to be combusted in a gas turbine, the high-temperature fuel cell is likely to be at an advantage. Further discussion appears on this point in Chapters 4 and 5, in respect of the SOFC and the MCFC. The PEFC (Chapter 6) must, for... 

Carbon Monoxide. Carbon monoxide, a fuel in high-temperature cells (MCFC and SOFC), is preferentially absorbed on noble metal catalysts that are used in low-temperature cells (PAFC and PEFC) in proportion to the hydrogen-to-CO partial pressure ratio. A particular level of carbon monoxide yields a stable performance loss. The coverage percentage is a function of temperature, and that is the sole difference between PEFC and PAFC. PEFC cell limits are < 50 ppm into the anode major U.S. PAFC manufacturers set tolerant limits as < 1.0% into the anode MCFC cell limits for CO and H20 shift to H2 and C02 in the cell as the H2 is consumed by the cell reaction due to a favorable temperature level and catalyst. 

The ideal performance of a fuel cell depends on the electrochemical reactions that occur with different fuels and oxygen as summarized in Table 2-1. Low-temperature fuel cells (PEFC, AFC, and PAFC) require noble metal electrocatalysts to achieve practical reaction rates at the anode and cathode, and H2 is the only acceptable fuel. With high-temperature fuel cells (MCFC, ITSOFC, and SOFC), the requirements for catalysis are relaxed, and the number of potential fuels expands. Carbon monoxide "poisons" a noble metal anode catalyst such as platinum (Pt) in low-temperature... 

Solid oxide fuel cellsoperateatvery high temperatures, around 1,000°C. High temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system. SOFCs are also the most sulphur-resistant fuel cell type they can tolerate several orders of magnitude more sulphur than other cell types. In addition, they are not poisoned by carbon monoxide (CO), which can even be used as fuel. This allows SOFCs to use gases made from coal. 

In the case of the high temperature SOFC discussed below the principles outlined above equally apply. The technical differences are that the cell runs typically on hydrocarbon fuels (e.g. natural or coal-gas) and that the electrolyte is an oxygen ion conductor rather than a proton conductor. The complex fuel molecules, in the presence of the water molecule and at the high operating... 

Figure 3.20 shows a cylindrical layout often used for high-temperature SOFCs. Alternatives are a stack of planar cells or a disk concept with feed tubes in the centre. A consideration of efficient heat exchange is the cormnon design strategy for the high-temperature fuel cell geometry. 

Solid oxide fuel cells (SOFCs) use a nonporous ceramic compound as the electrolyte and operate at very high temperatures (1,800°F). Heat can be recaptured for co-generation, making these fuel cells highly efficient (80-85%). Because of size, heat output, and a long start-up time, these fuel cells are more suitable for stationary applications. 

PEMFC) high-temperature models include molten carbonate fuel cell (MCFC) and SOFC. The wide range of power outputs available make fuel cells suitable for a variety of applications. 

High temperature fuel cells (MCFC and SOFC) allow for the reforming process to take place inside the fuel cell stack which lowers the requirement for cell cooling and reduces cost due to absence of the external reformer vessel. A future SOFC application could also be the production of hydrogen (and electricity) by internal reforming of natural gas where more H2 as a component of the synthesis gas is produced than can be converted electrochemically into electricity the heat losses from the fuel cell operation would be used as the endothermal heat source for the reforming step [65]. 

Solid oxide fuel cells (SOFCs) are one of the most efficient energy conversion devices [1]. The main demand in the current SOFC development is lowering operation temperature to the range of 600-800 C - intermediate temperature SOFC (IT-SOFC). In order to lower operational temperature and increase or at least sustain performance comparable to that at high temperature SOFCs, it is necessary to decrease the resistance of the electrolyte and lower the overpotential of the electrodes. One of the ways to achieve this goal is to decrease the thickness of the electrolyte and optimize the structure of the electrodes. 

The development of high-performance electrode and electrolyte materials for SOFC is an important step towards reducing the fuel cell operation temperature to the low and intermediate range (500 - 700 °C). As the operating temperature is reduced, many cell ports, such as the auxiliary components can be easily and cost-efficiently produced. To meet long operational lifetime, material compatibility and thermomechanical resistance would be less critical as the range of possibilities for lower temperature increases. To that end, recent research at UFRN, Natal, Brazil has successfully focused on novel synthesis processes based on microwave-assisted combustion and modified polymeric precursor methods in order to synthesize high performance cobaltite-based composite cathodes for low-intermediary-temp>erature SOFCs. 

A SER based on low-pressure interconnected bubbling fluidised beds applied to power plants with CO2 capture is proposed by Ref. [54], In this case, the H2-rich gas produced is converted in a high-temperature SOFC and regeneration is carried out by recovering the waste heat from the fuel cell through an internal heat transfer loop. 

For high-temperature fuel cells, such as molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs), where there is no liquid water at the fuel cell operating temperatures, the limiting current density can be much higher than that from a PEMFC, implying again that accumulation of liquid water plays a key role in limiting current. 

Anodes perform electrooxidation of fuel by catalyzing the reaction and facilitating fuel access and product removal [1]. These require sufficient reaction sites for the fuel oxidation and electronic conductivity to transfer electrons from the oxidant to the cell components, i.e., electrol34e and current collectors. In high-temperature SOFCs, porous cermets, made from a percolating metal phase and... 

For many practical catalysts, b is known from experiments. The characteristic values of b for the low- (PEFC and DMFC) and high-temperature (SOFC) fuel cells are listed in Table 1.1. Note that in the electrochemical hterature the dependence of Q on 77 is often represented as a power of 10, rather than the exponent the respective value bw 2.36. 

The type of the catalyst employed in the fuel cell depends on the type of fuel, the solid electrolyte used, and the operating temperature. Here we will consider the recent trends in catalysis for the two major types of the fuel cells, including low-temperature proton exchange membrane fuel cells and high-temperature SOFCs. Depending on the fuel used, low-temperature PEMFCs fall into two major categories hydrogen and direct methanol fuel cells (DMFCs). 

Different types of fuel cells are considered for larger vehicle and locomotive power generation systems. This includes the high-temperature fuel cells such as SOFCs and MCFCs and low-temperature fuel cells such as AFCs and PEMFCs. 

High-temperature fuel cells (MCFC and SOFC) operate at temperatures exceeding 950 K and produce an exuberant amount of heat that can be either given off... 

Fuel cells are usually classified by the electrolyte employed in the cell. An exception to this classification is DMFC (direct methanol fuel cell) that is a fuel cell in which methanol is directly fed to the anode. The electrolyte of this cell does not determine the class. The operating temperature for each of the fuel cells can also determine the class. There are, thus, low- and high-temperature fuel cells. Low-temperature fuel cells are alkaline fuel cells (AFCs), polymer electrolyte membrane fuel cells (PEMFCs), DMFC, and phosphoric acid fuel cells (PAFCs). The high-temperature fuel cells operate at temperatures —600-1000 °C and two different types have been developed, molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFCs). AU types of fuel cells are presented in the following sections in order of increasing operating temperature. An overview of the fuel cell types is given in Table 1.1 [1,5-7]. 

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