Energy converter, electrochemical electrodes

In considering the behavior of electrochemical systems in action when a current is flowing through them, an expression was developed earlier for the cell potential V as a function of the overall current/. It was shown that for a very simple electrochemical energy converter having electrodes of the same area A and delivering a current I, one has... 

The term electromembrane process is used to describe an entire family of processes that can be quite different in their basic concept and their application. However, they are all based on the same principle, which is the coupling of mass transport with an electrical current through an ion permselective membrane. Electromembrane processes can conveniently be divided into three types (1) Electromembrane separation processes that are used to remove ionic components such as salts or acids and bases from electrolyte solutions due to an externally applied electrical potential gradient. (2) Electromembrane synthesis processes that are used to produce certain compounds such as NaOH, and Cl2 from NaCL due to an externally applied electrical potential and an electrochemical electrode reaction. (3) Eletectromembrane energy conversion processes that are to convert chemical into electrical energy, as in the H2/02 fuel cell. 

It is in this sense it is said that in an electrochemical energy converter, the ideal maximum efficiency is 100% for, as in the above idealized situation, if one could carry out reactions in such a way that the electrode potentials were infinitely near the equilibrium values, the electrical energy one could draw2 from the reaction would be nFVe and this is all of the free-energy change AG, which is the maximum amount of useful work one can obtain from a chemical reaction. 

Fig. 13.5. Cell-potential vs. current-density relations for an idealized electrochemical energy converter with planar, smooth electrodes. Assumed are the following parameters Curve 1, LSI > 1 A cm-2, /0 so = 10"3 A cm-2 /L S) = 1 A cm, /L SO = 1 A cm-2,...

There is a grave disadvantage in this important (and inevitable) electrode reaction. It has an i0 value in the region of 10-10 A cm-2, and hence (from the equation p -RT/aF In i/i0) the reaction usually contributes considerably to the overpotential in the functioning of an air-burning electrochemical converter. Electrocatalysis of this reaction is needed more than any other in electrochemical energy converters. 

Special Configurations of Electrodes in Electrochemical Energy Converters... 

Figure 12.13 illustrates a versatile experimental set-up for microwave conductivity measurements with the microwave source (8 0 GHz), a circulator and a detector, which monitors the microwave energy reflected from the electrochemical or photovoltaic cell. The cell and electrode geometries are designed in such a way that the microwave power can reach the energy-converting interface (losses in metal contacts or aqueous electrolyte should be minimised). Depending on the experimental conditions, time-resolved, space-resolved or potential-dependent measurements are possible as well as combinations (for further details, see Schlichthbrl and Tributsch, 1992 Wiinsch et al., 1996 Chaparro and Tributsch, 1997 Tributsch, 1999). 

Batteries store chemical energy, which is converted into electric energy by electrochemical reactions. Those that cannot be used once the electric energy is totally discharged are called primary cell. Batteries that can induce chemical reaction by charging electric energy to reactivate the electrode material and be used many times are called secondary batteries. 

Fuel cells are power generation devices converting chemical energy into electric energy by electrochemical reactions. A typical fuel cell is comprised of two electrodes separated by an electrolyte, with a provision of reactant supply and product removal. Among various types of fuel cells, Ha-O -based polymer electrolyte membrane (PEM) fuel cells (PEMFC) have attracted special attention due to their high efficiency, low temperature operation and suitability for low to medium power generation. Basic components of a PEMFC are PEM, catalyst layer, gas diffusion layer and... 

The following sections summarise the various factors, including, properties and selection of cell component materials, cell geometry and fabrication, and electrode kinetics, that can influence the performance of electrochemical energy converters. 

Batteries and fuel cells are electrochemical devices which convert chemical energy into electrical energy by electrochemical oxidation and reduction reactions, which occur at the electrodes. A cell consists of an anode where oxidation takes place during discharge, a cathode where reduction takes place, and an electrolyte which conducts the electrons (via ions) within the cell. 

An electrochemical cell is a device by means of which the enthalpy (or heat content) of a spontaneous chemical reaction is converted into electrical energy conversely, an electrolytic cell is a device in which electrical energy is used to bring about a chemical change with a consequent increase in the enthalpy of the system. Both types of cells are characterised by the fact that during their operation charge transfer takes place at one electrode in a direction that leads to the oxidation of either the electrode or of a species in solution, whilst the converse process of reduction occurs at the other electrode. 

This energy is consumed to overcome the overpotentials of the electrode reactions (rja, rjc), the IR drop in the external circuit and electrolyte, and it might partly be converted into the free energy, AG, of stable products (if any) of the endoergic electrochemical reaction ... 

A battery is defined as a device for converting chemical energy into electrical energy. A battery is therefore an electrochemical cell that spontaneously produces a current when the two electrodes are connected externally by a conductor. The conductor will be the sea in the example of the eel above, or will more typically be a conductive... 

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