Fuel cells reactions between

Numerous efforfs have been made to improve existing fhin-film catalysts in order to prepare a CL with low Pt loading and high Pt utilization without sacrificing electiode performance. In fhin-film CL fabrication, fhe most common method is to prepare catalyst ink by mixing the Pt/C agglomerates with a solubilized polymer electrolyte such as Nation ionomer and then to apply this ink on a porous support or membrane using various methods. In this case, the CL always contains some inactive catalyst sites not available for fuel cell reactions because the electrochemical reaction is located only at the interface between the polymer electrolyte and the Pt catalyst where there is reactant access. 

The portion AQ = AH - AG = TAS of AH is transformed into heat. Ideal theoretical efficiencies % determined by the types and amounts of reactants and by the operating temperature. Fuel cells have an efficiency advantage over combustion engines because the latter are subdued to the Carnot limitation. High thermodynamic efficiencies are possible for typical fuel cell reactions (e.g., e,h = 0.83 (at 25°C) for H2 + I/2O2 -> H20(i)). The electrical potential difference between anode and cathode, = -AG/W(f, which is also called the electromotive force or open-circuit voltage, drives electrons through the external... 

Figure 19.18. Data of electrochemical fuel cells, (a) Processes in a fuel cell based on the reaction between hydrogen and oxygen, (b) Voltage-current characteristic of a hydrogen-air fuel cell operating at 125°C with phosphoric acid electrolyte [Adlharl, in Energy Technology Handbook (Considine, Ed.), 1977, p. 4.61). (c) Theoretical voltages of fuel cell reactions over a range of temperatures, (d) Major electrochemical systems for fuel cells (Adlharl, in Considine, loc. cit., 1977, p. 4.62).

The electrolyte in this fuel cell is usually a combination of alkali carbonates, which is retained in a ceramic matrix of LiA102. The fuel cell operates between 600°C and 700°C where the alkali carbonates form a highly conductive molten salt, with carbonate ions providing ionic conduction. At the high operating temperatures in the MCFCs, nickel (anode) and nickel oxide (cathode) are adequate to promote the reaction. Noble metals are not required. 

A fuel cell is in principle a galvanic cell where the reactants are added continuously to the system. In order to illustrate the principle in a fuel cell reaction we will look at the exothermal reaction between methane and oxygen which is a redox reaction ... 

Most fuel cell designs employ flow channels to distribute the reactants across the MEA area and to assist pushing liquid water out of the fuel cell the serpentine flow charmel represented in Figure 3.1 is the most popular flow channel design. Because reactants are consumed and water is produced in the fuel cell reaction, there is a decrease in the reactant concentrations between the flow channel inlet and outlet. There is also an increase in the water... 

A one-dimensional PEM fuel exhibited steady state multiplicity, resulting from positive feedback between proton conduction in the membrane and water production from the fuel cell reaction. The critical membrane water activity necessary for ignition of the fuel cell current was identified. The time for current ignition is 100-1000 s, resulting from the titration of the sulfonic acid residues in the polymer membrane from the water formed by the fuel cell reaction. 

Fuel Cell Reactions. Low temperature fuel cells such as proton exchange membrane fuel cells (PEMFC) or direct methanol fuel cells (DMFC) employ large amounts of noble metals such as Pt and Ru. There has been extensive research to replace these expensive metals with more available materials. A few studies considered transition metal nitrides as a potential candidate. In an anode reaction of DMFC, Pt/TiN displayed the electroactivity for methanol oxidation (53). Pt/TiN deposited on stainless steel substrate showed the high CO tolerance in voltammogram performed with a scan rate of 20 mV/s and 0.5 M CH3OH - - 0.5 M H2SO4 electrolyte. The bifunctional effect of Pt and TiN for CO oxidation was mentioned as observed between Pt and Ru in commercial PtRu/C catalysts. 

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. 

In a fuel cell, the difference in reactant gas compositions at the two electrodes leads to the formation of a difference in Galvani potential between anode and cathode, as discussed in the section Electromotive Force. Thereby, the Gibbs energy AG of the net fuel cell reaction is transformed directly into electrical work. Under ideal operation, with no parasitic heat loss of kinetic and transport processes involved, the reaction Gibbs energy can be converted completely into electrical energy, leading to the theoretical thermodynamic efficiency of the cell. 

Chemical/electrochemical degradation Trace metal contamination (foreign cations, such as Ca " ", Fe " ", Cu " ", Na" ", K", and Mg " ") radical attack (e.g. peroxy and hydroperoxy). Peroxide radical attack causes membrane polymer chain decomposition and fluorine loss this results in membrane thinning, pinholes, and gas crossover. Note these peroxide radicals are generated by both the fuel cell reaction and the chemical reaction between O2 and H2 within the membrane. 

Global versus Elementary Reaction An important distinction between reaction steps, which is needed to understand the material presented in the rest of the book, is the concept of a global and elementary reaction. Consider the overall fuel cell reaction ... 

This carbon deposition can be inhibited by adding steam to the fuel. This reaction (Eq. 9.2) is strongly endothermic in nature, whereas fuel cell reactions in Eqs. (9.3) and (9.4) are slow exothermic reactions. This can create instability in the coupling between fast endothermic and slow exothermic reactions. 

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