Fuel cells reaction electrochemistry

Fundamental Research that Underlay Development of this Cell. Three U.S. universities were involved in the work that culminated in manufacture of the proton-exchange membrane by Ballard Power Systems. First, Case-Western Reserve University must be recognized because of the sustained investigations there (Yeager et al., 1961-1983) on the mechanism and catalysis of the reduction of02, the reaction that causes most of the energy losses in the fuel cell. The Electrochemistry of... 

The development of new electrocatalytic materials by deposition of foreign atoms on a host metal is currently a very active area of research in electrochemistry. In this regard, most of the studies have been focused on platinum as a substrate, due to the very high reactivity of this metal for fuel-cell reactions. Still, a surface modifier is necessary to improve the catalytic activity and stability of the electrode and to avoid side poisoning reactions. It has been shown that the modification of platiniun surfaces with submonolayer amounts of elements of the p-block of the periodic table leads, in many cases, to electrode materials with greatly improved catalytic properties. ... 

CO adsorption and oxidation have been studied for many years, but a greater understanding was achieved by the development of ex situ and in situ spectroscopic and microscopic methods for application in electrochemistry [9, 143-146], together with the use of well-defined nanocrystalline electrode surfaces [147]. The opportunity to study in situ electrooxidation of carbon monoxide [148-157] under fuel cell reaction conditions has brought significant progress in understanding interfacial electrochemistry on metallic surfaces, hi combination with conventional electrochemical methods these techniques have been used to find connections between the microscopic surface structures and the macroscopic kinetic rates of the reactions. 

This chapter mainly deals with the fundamentals of H2/air PEM fuel cells, including fuel cell reaction thermodynamics and kinetics, as well as a brief introduction to the single fuel cell and the fuel cell stack. The electrochemistry and reaction mechanisms of H2/air fuel cell reactions, including the anode HOR and the cathode ORR, are discussed in depth. Several concepts related to PEM fuel cell performance, such as fuel cell polarization curves, OCV, hydrogen crossover, and fuel cell efficiencies, are also introduced. With respect to fuel cell stmctures and components, the material properties and effects on fuel cell performance are also discussed. In addition, several important conditions for fuel cell operation, including temperature, pressure, RH, and gas stoichiometries and flow rates, and their effects on fuel cell operation, are also briefly presented. This chapter provides the requisite baseline knowledge for the remaining chapters. 

The section on basic principles contains background information on fuel cells, including fundamental principles such as electrochemistry, thermodynamics, and kinetics of fuel cell reactions as well as mass and heat transfer in fuel cells. The section on design explores important characteristics associated with various fuel cell components, electrodes, electrocatalysts, and electrolytes, while the section on analysis examines phenomena characterization and modeling both at the component and system levels. 

Equilibrium electrochemistry allows us to calculate the standard values of open circuit potential (OCP) of a fuel cell and the decomposition potential (DP = -OCP) of an electrolytic cell if thermodynamic properties required for such calculations are available. The equilibrium electrochemical calculations should be done first before any other calculations or even experimental measurements to see any thermodynamic constrains of the electrochemical system. As an example. Figure 4.3 shows results of such calculations for three fuel cell reactions over a wide tanperature range from ambient up to 900°C. 

On the right hand, the first term can be calculated using equilibrium electrochemistry of the fuel cell reaction as described in Chapter 4, the second term is actually a modified Tafel equation (Chapter 6), with a parasitic current density correction described earlier, the third term is related to the mass transport of chemicals when the limiting current is approached (Chapter 6), and the last term is simply Ohms law (Chapter 2). In this equation, Apc and Bpc are the semiempirical positive coefficients in V, and rpc is the fuel cell area-specific resistance in 2 cm. Current density is in A cm , and the fuel cell and equilibrium potentials are in V. 

In this section, we derive a general expression to describe activation polarization losses at a given electrode, known as the Butler-Volmer (BV) kinetic model. The BV model is not the only (or necessarily the most appropriate) model to describe a particular electrochemical reaction process. Nevertheless, it is a classical treatment of electrode kinetics that is widely applied to study and model a majority of the electrode kinetics of fuel cells. The BV model describes an electrochemical process limited by the charge transfer of electrons, which is appropriate for the ORR, and in most cases the HOR with pure hydrogen. The fundamental assumption of the BV kinetic model is that the reaction is rate hmited by a single electron transfer step, which may not actually be true. Some reactions may have two or more intermediate charge transfer reactions that compete in parallel or another intermediate step such as reactant adsorption (Tafel reaction from Chapter 2) may limit the overall reaction rate. Nevertheless, the BV model of an electrochemical reaction is standard fare for a student of electrochemistry and can be used to reasonably fit most fuel cell reaction behavior. 

Why Do We Need to Know This Material The topics described in this chapter may one day unlock a virtually inexhaustible supply of clean energy supplied daily by the Sun. The key is electrochemistry, the study of the interaction of electricity and chemical reactions. The transfer of electrons from one species to another is one of the fundamental processes underlying life, photosynthesis, fuel cells, and the refining of metals. An understanding of how electrons are transferred helps us to design ways to use chemical reactions to generate electricity and to use electricity to bring about chemical reactions. Electrochemical measurements also allow us to determine the values of thermodynamic quantities. 

The first reported electroorganic synthesis of a sizeable amount of material at a modified electrode, in 1982, was the reduction of 1,2-dihaloalkanes at p-nitrostyrene coated platinum electrodes to give alkenes. The preparation of stilbene was conducted on a 20 pmol scale with reported turnover numbers approaching 1 x 10. The idea of mediated electrochemistry has more frequently been pursued for inorganic electrode reactions, notably the reduction of oxygen which is of eminent importance for fuel cell cathodes Almost 20 contributions on oxygen reduction at modified... 

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. 

The electrochemistry model is based on the assumption that the overall chemical reaction occurring in the fuel cell is ... 

The combination of chemistry and electricity is best known in the form of electrochemistry, in which chemical reactions take place in a solution in contact with electrodes that together constitute an electrical circuit. Electrochemistry involves the transfer of electrons between an electrode and the electrolyte or species in solution. It has been in use for the storage of electrical energy (in a galvanic cell or battery), the generation of electrical energy (in fuel cells), the analysis of species in solution (in pH glass electrodes or in ion-selective electrodes), or the synthesis of species from solution (in electrolysis cells). 

Electrochemistry, to distinguish it from the topics discussed in previous sections, is concerned with low-energy charge transfer in solution. The electron transfer typically occurs on the surface of a charged (usually metal) electrode. Possible chemical reactions that may occur, and that may be of importance in chemical synthesis, are the generation or annihilation of gases (in an electrolysis or a fuel cell, respectively) and the generation or neutralization of ions, which may be accompanied with the dissolution or deposition of a solid material. 

As a first example of the use of reaction mechanism graphs, consider the electrochemistry of molten carbonate fuel cell (MCFC) cathodes. These cathodes are typically nickel-oxide porous electrodes with pores partially filled with a molten carbonate electrolyte. Oxygen and carbon dioxide are fed into the cathode through the vacant portions of the pores. The overall cathodic reaction is 02 + 2C02 + 4e / 2C03=. This overall reaction can be achieved through a number of reaction mechanisms two such mechanisms are the peroxide mechanism and the superoxide-peroxide mechanism, and these are considered next. 

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