Oxidation-reduction reactions fuel cells

In me previous chapter we discussed acid-base reactions, which are chemical reactions involving the transfer of pro Lons from one reactant to another. In this chapter, we explored oxidation-reduction reactions, which involve the transfer of one or more electrons from one reactant to another. Oxidation-reduction reactions have many applications, such as in photography, batteries, fuel cells, the manufacture and corrosion of metals, and the combustion of non-metallic materials such as wood. 

The form of eation departure influenee on estimation of environment aetion on thermodynamies and kinetics of surface processes in oxidation - reduction reactions. So conclusions may be derived about the conditions of fuel cell exploitation and their efficiency. 

The Fs are volume flow rates, Ps are the water partial pressures, F is Faraday s constant, and z h + is the current. The fuel cell current equals the effective cell voltage divided by the sum of the load resistance and membrane resistance. The effective cell voltage, Vb, is the thermodynamic potential (Voc) reduced by the overpotential (Vop) associated with the oxidation/reduction reactions at the anode and cathode. 

A DMFC (direct methanol fuel cell) converts the chemical energy stored in liquid methanol to usable electrical energy by a direct electrochemical oxidation-reduction reaction. 

The second difficulty was associated with the catalysts. Platinum, a good catalyst for cathodic oxygen reduction in fuel cells, will also promote anodic oxygen evolution in electrolyzers. However, while this anodic reaction takes place, the platinum surface becomes oxidized, a layer of platinum oxide two or three atom layers thick forming on it. This oxidized platinum is no longer active for cathodic oxygen reduction. The oxidation is irreversible, so an electrode with a platinum catalyst will no longer function as a fuel cell electrode. 

For a fuel-oxidant configuration shown in Equation 9.3 at a given temperature and pressure, the theoretical equilibrium open-circuit potential of a given oxidation-reduction reaction within the cell is determined by Equation 9.4 known as Nemst equation ... 

Fuel cells are based on oxidation-reduction reactions (see Section 4.9). The most common type of fuel cell—called the hydrogen-oxygen fuel ceU— is based on the reaction between hydrogen and oxygen ... 

At the anode elecffode, the electrochemical oxidation of the fuel produces electrons that flow through the bipolar plate (also called cell interconnect) to the external circuit, while the ions generated migrate through the electrolyte to complete the circuit. The electrons in the external circuit drive the load (e.g., electric moter or other device) and return to the cathode catalyst where they recombine with the oxidizer in the cathodic oxidizer reduction reaction (ORR). The products of the fuel cell are thus threefold (1) chemical products, (2) waste heat, and (3) electrical power. 

The area of catalysis has recently received renewed attention because of possible applications in solving environmental problems. Precious metals have been found useful for the oxidation of CO to CO2, but are not effective in the eduction of NO. The possibility of using perovskite type catalysts seems promising since it was reported that LaCoOj, LaMnOs and their substituted derivatives have interesting catalytic properties in regard to application in fuel cell electrodes (Meadowcroft, 1970) and in the oxidation reduction reactions involved in the control of automotive exhaust emissions (Libby, 1971 Voor-hoeve et al., 1972, 1973). 

In this chapter, we will see how chemical reactions can be used to produce electricity and how electricity can be used to cause chemical reactions. The practical applications of electrochemistry are countless, ranging from batteries, fuel cells, and biological processes to the manufacture of key chemicals, the refining of metals, and methods for controlling corrosion. Before we can understand such applications, we must first discuss how to carry out an oxidation-reduction reaction in an electrochemical cell and explore how the energy obtained from, or supplied to, an electrochemical cell is related to the conditions under which the cell operates. 

The aluminum-air fuel cell is used as a reserve battery in remote locations. In this cell aluminum reacts with the oxygen in air in basic solution, (a) Write the oxidation and reduction half-reactions for this cell, (b) Calculate the standard cell potential. See Box 12.1. 

A fuel cell consists of an ion-conducting membrane (electrolyte) and two porous catalyst layers (electrodes) in contact with the membrane on either side. The hydrogen oxidation reaction at the anode of the fuel cell yields electrons, which are transported through an external circuit to reach the cathode. At the cathode, electrons are consumed in the oxygen reduction reaction. The circuit is completed by permeation of ions through the membrane. 

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