Basic Fuel Cell Operation

The space shuttle uses a fuel cell as a source of energy. This cell depends on the oxidation of hydrogen by oxygen to form water. The fuel cell operates under basic conditions, so it is sometimes referred to as an alkaline fuel cell. Figure 11.31, on the next page, shows the design of the cell. The half-reactions and the overall reaction are as follows. 

An external gas pressure gradient applied between anode and cathode sides of the fuel cell may be superimposed on the internal gradient in liquid pressure. This provides a means to control the water distribution in PEMs under fuel cell operation. This picture forms the basis for the hydraulic permeation model of membrane operation that has been proposed by Eikerling et al. This basic structural approach can be rationalized on the basis of the cluster network model. It can also be adapted to include the pertinent structural pictures of Gebel et and Schmidt-Rohr et al. ... 

Fuel cells operate in a manner reverse to that of electrolysis, discussed in Chapter 2, combining fuel to make electricity. The basic design consists of two electrodes separated by an electrolyte. The oldest type of fuel cell is the alkaline fuel cell where an alkaline electrolyte like potassium hydroxide is used. The hydrogen enters through the anode compartment and oxygen through the cathode compartment. The hydrogen is ionized by the catalytic activity of the anode material and electrons are released into the external circuit. The protons react with the hydroxyl ions in the electrolyte to form water. The reaction can be written as ... 

Smithsonian Institution. Fuel Cell Basics. Available online. URL http //americanhistory.si.edu/fuelcells/basics.htm. Accessed May 28,2009. This Web page presents an overview of fuel cell operation. Alkali, molten carbonate, phosphoric, proton exchange membrane, and soUd oxide fuel cells are discussed. 

The generation of heat always accompanies the operation of a fuel cell. The heat is due to inefficiencies in the basic fuel-cell electrochemical reaction, crossover (residual diffusion through the fuel-cell solid-electrolyte membrane) of fuel, and electrical heating of interconnection resistances. Spatial temperature variation can occur if any of these heat-generating processes occur preferentially in different parts of the fuel cell stack. For example, non-uniform distribution of fuel across the surfaces of electrodes, different resistances between the interconnections in a stack, and variations among... 

The various subsections given above present the basic individual transient physical processes occurring in fuel cell operation. Any of these may be included with the others to develop a complete dynamic model for a given study. In the next section we show several examples of how this can be done, starting form the simplest problem to more detailed problems. Applications of these models are given in Section 9.5. 

Because the basic fuel cell needs no mechanical drive, its operation is quiet and involves no frictional losses (Figure 2.100). These characteristics should make it possible to locate them near the final user, producing a more even distribution of the generation capacity. Auxiliaries, particularly fans and blowers, must be quiet therefore, they should be well supported to prevent their motion and be provided with variable-speed drives. In addition, the feathering of the blade edges and the use of noise-reducing enclosures are recommended. 

Lecture on basic fuel cell technology and operation at the ICBO/SBCCI Conference in Greensboro, NC, November 1, 2001. 

Basic Principles of Single-Chamber Fuel Cell Operation... 

Many cathode catalyst materials have been used. For noble metal catalysts, platinum was mainly used in fuel cells for space applications. For terrestrial use, one has to use less expensive materials, and non-noble metal catalysts are therefore mainly employed. Bacon used lithium-doped nickel oxide as a cathode catalyst for high-temperature AFCs. Lithium-doped nickel oxide has a sufficient electrical conductivity at temperatures above 150 °C. Currendy, mainly Raney silver and pure silver catalysts are favored. Developments of silver-supported materials containing PTFE are sometimes successful. Silver catalysts are usually prepared from silver oxide, Raney silver, and supported silver. Typically, the catalysts on the cathode are supported by PTFE because it is highly stable under basic and acidic conditions. In contrast, carbon is oxidized at the cathode in contact with oxygen, when carbon is used as an inexpensive support material. In the past, the silver catalysts frequentiy contained mercury as part of an amalgam to increase the stability and the lifetime of the cathode. Because mercury is partially dissolved during the activation procedure (see below) and during the fuel-cell operation, some electrolyte contamination can be observed. Because of the environmental hazard of mercury, this metal is currently not used in silver catalysts. 

Larson JM, Hamrock SJ, Haugen GM, Pham P, Lamanna WM, Moss AB (2007) Membranes based on basic polymers and perfluorinated acids for hotter and drier fuel cell operating conditions. J Power Sources 172 108-114... 

This chapter gives an overview of basic concepts in polymer electrolyte fuel cells (PEFCs). The intent is to provide the reader with an intuitive understanding of the processes that underlie fuel cell operation. General and engineering aspects of fuel cell design and operation are treated in greater detail in recently published books (Bagotsky, 2012 Barbir, 2012). Please refer to these books for further discussions of different types of fuel cells and specific aspects of their operation. 

Fuel Cells Principles, Design, and Analysis considers the latest advances in fuel cell system development and deployment and was written with engineering and science students in mind. This book provides readers with the fundamentals of fuel cell operation and design and incorporates techniques and methods designed to analyze different fuel cell systems. It builds on three main themes basic principles, analysis, and design. 

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