ELECTROCHEMICAL ENERGY CONVERSION Fuel Cells

The first part of this chapter has been concerned with the fact that some chemical reactions that take place spontaneously can be split into two electrode reactions which, when joined together in an electrochemical cell, give rise to electric power (electrochemical energy conversion—fuel cells). We now turn to processes by which an... 

In the field of electrocatalysis the situation seems to be somewhat better as far as the problems of clean surfaces and the existence of chemical effects are concerned. Unfortunately, so far only a few reactions have been studied. Thus, much more systematic fundamental research has to be done. From the results already available one can extract the hope that ion-bombardment will be of future importance for fields involving electrocatalytical reactions such as, for example, hydrogen technology, energy conversion, fuel cells and electrochemical redox reactions. 

Fuel Cell Catalysts. Euel cells (qv) are electrochemical devices that convert the chemical energy of a fuel direcdy into electrical and thermal energy. The fuel cell, an environmentally clean method of power generation (qv), is more efficient than most other energy conversion systems. The main by-product is pure water. 

Michael Krumpelt, Ph D , Manager, Fuel Cell Technology, Argonne National Laboratory Member, American In stitute of Chemical Engineers, American Chemical Society, Electrochemical Society. (Electrochemical Energy Conversion)... 

Dr. Ralph J. Brodd is President of Broddarp of Nevada. He has over 40 years of experience in the technology and market aspects of the electrochemical energy conversion business. His experience includes all major battery systems, fuel cells, and electrochemical capacitors. He is a Past President of the Electrochemical Society and was elected Honorary Member in 1987. He served as Vice President and National Secretary of the International Society of Electrochemistry as well as on technical advisory committees for the National Research Council, the International Electrotechnic Commission, and NEMA and on program review committees for the Department of Energy and NASA. 

Electrochemical energy conversion devices are pervasive in our daily lives. Batteries, fuel cells and supercapacitors belong to the same family of energy conversion devices. They are all based on the fundamentals of electrochemical thermodynamics and kinetics. All three are needed to service the wide energy requirements of various devices and systems. Neither... 

The good news for fuel cells is that they run on a different process. Fuel cells are not exempt from scientific laws, but their manner of energy conversion is electrochemical rather than thermal. The maximum efficiency for the electrochemical processes in fuel cells is higher than for the internal combustion engines that power many automobiles today. 

In this section, a number of electrocatalytic processes will be discussed where surface chemical bonding plays a central role in the reaction mechanism. The selection of reactions is far from complete and not representative of the wide range of technologically important electrocatalytic processes. The selection is biased towards the areas of electrochemical energy conversion and fuel cell electrochemistry, which have been catalyzing a renewed interest in the field of electrochemistry. 

A number of hybrid schemes for electrochemical energy conversion have been devised. These include the use of a fuel cell to compress air, which would drive air turbines to provide startup and acceleration.17... 

J. O M. Bockris and S. Srinivasan, The Electrochemistry of Fuel Cells, McGraw-Hill, New York (1969). Electrochemistry and electrochemical engineering associated with practically all types of electrochemical energy conversion systems, as well as their applications. An advanced presentation still relevant in the 1990s. 

Debra R. Rolison is head of Advanced Electrochemical Materials at the Naval Research Laboratory (NRL). She received a B.S. in chemistry from Florida Atlantic University in 1975 and a Ph.D. in chemistry from the University of North Carolina at Chapel Hill in 1980 under the direction of Royce W. Murray. Dr. Rolison joined the Naval Research Laboratory as a research chemist in 1980. Her research at NRL focuses on the influence of nanoscale domains on electron- and charge-transfer reactions, with special emphasis on the surface and materials science of aerogels, electrocatalysts, and zeolites. Her program creates new nano structured materials and composites for catalytic chemistries, energy storage and conversion (fuel cells, supercapacitors, batteries, thermoelectric devices), and sensors. 

Figure 3.3.2 Principle of a fuel cell as an electrochemical energy conversion device. Inside a fuel cell, fuel, e.g. hydrogen, and an oxidant, typically oxygen, combine electrochemically to form products, e.g. water, and electricity and some excess heat (not shown). Figure adapted from ref. [4]...

Fuel cells are versatile electrochemical energy conversion devices. They represent an integral component of an emerging strategy toward a future sustainable energy... 

Fuel cell An electrochemical energy conversion device. It produces electricity from various external quantities of fuel (on the anode side) and oxidant (on the cathode side). These react in the presence of an electrolyte. Fuel cells are different from batteries in that they consume reactant, which must be replenished, while batteries store electrical energy chemically in a closed system... 

Methanol is a potentially attractive fuel for electrochemical energy conversion devices (which is just another name for fuel cells) for two reasons first, because it is relatively easily oxidized electrochemically, and secondly because it can be cheaply manufactured from hydrogen and can, in effect, serve as a chemical means of storing... 

Electrocatalysis is receiving increasing levels of attention in the computational community due to the recent interest in fuel cells and electrochemical energy conversion technology. The presence of an electrolyte and electric potential substantially complicates modeling efforts of these systems. Simple models to account for these phenomena were developed by Norskov and co-workers, while more sophisticated approaches were developed by Neurock and co-workers.There are other approaches as well, but these two are the ones most often used in the work reviewed here. Other notable approaches include that by Alavi and co-workers , and the Anderson group s approach." ... 

Of course, since AG and AH are used in the definition (3.16), the theoretical efficiency of a fuel cell depends on the redox reaction on which it is built. In any case the theoretical efficiency, calculated from thermodynamic quantities, corresponds to an operative condition of infinitesimal electronic flow (by definition of reversible process), which practically means no current drawn from the converter. As it is shown in the following sections, also at open-circuit (no current through the external circuit) the voltage of real fuel cells is slightly lower than °, and the main problem of the electrochemical energy conversion is to obtain potentials in practical conditions (when current is drawn) as near as possible the open-circuit voltage, in order to maximize the real efficiency of the device. 

Three-dimensionally ordered porous materials have been applied to electrochemical energy conversion systems, such as Uthium battery, fuel cell, and electrochanical double layer capacitor. Based on this technique, functional materials for other applications can be produced. The advantages of three-dimensionally ordered materials are based on micro or nano size ordered pores. 

Applications in Hydrogen-Oxygen Fuel Cells. A fuel cell is a device which converts the latent chemical energy of fuel directly into electricity. This involves a constant temperature electrochemical energy conversion process, and its efficiency is not limited by Carnot s theorem (25). Fuel cells find many applications in space missions and military power sources. Mast recently, it is considered as an ideal contender for the uses in transportation and utility sectors. For further details see reference 25. 

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