Potential electrochemical energy converters

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. 

It has been shown (Section 13.3.1), that in an electrochemical energy converter, the maximum cell potential is the value Vg obtainable when the reaction in the cell is electrically balanced out to equilibrium, i.e., when no current is being drawn from the cell. As soon as the cell drives a current through the external circuit, the cellpotential falls from the equilibrium value Ve to V. The value of the actual potential V at which the cell works when delivering a current i is always less than the equilibrium potential Vg. Hence, one has from Eq. (13.8)... 

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... 

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,...

Fig. 13.6. Cell potential vs. current density relations for an electrochemical energy converter for negligible values of IR, showing the influence of a limiting current. Curve 1, kinetic parameters correspond to curve 1 in Fig. 13.5 curve 2, the same as curve 1 except /LS = 10-1 A cm-2 and /l so = 1CT1 A cm-2.

Fig. 13.7. Graphical representation of the influence of the internal resistance of an electrochemical energy converter on the cell potential when mass-transfer polarization is negligible. The early nonlinear part of the curve represents the effect of the activation overpotential on the cell potential before ohmic polarization has become important.

This process leads to the development of a voltage difference between anode and cathode and when the external circuit is open , this potential difference is the electromotive force (the e.m.f.) of the cell. When there is an electrical load in the external circuit, for example a motor, then a current will flow and electrochemical energy is converted into mechanical energy. 

Electrochemical energy proportional to the potential difference E — Ep which is consumed according to this formula to overoome the resistance R of both the electrolyte and the external oircuit, is finally converted into heat which increases the temperature of the electrolyte and current conductors. 

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). 

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. 

Another type of thallium compounds which have potential applications in converting light into electrochemical energy are the hi- or oligometallics of the type shown in Fig. 19. Similar compounds, such as (CN)5-Fe -(/u-CN)-Pt (NH3)4-(/L-NC)-Fe -(CN)5, have recently been shown to undergo redox reaction when irradiated by visible light (356). Research in this field has been active during the last few years, and... 

Fuel Cells are electrochemical devices that convert the chemical energy of a fuel directly into electricity, without the necessity of an intermediate. Fuel cells are considered an emerging technology that can deliver clean, quiet, and potentially renewable energy for primary, base-load and back-up power this technology now has to be commercialised. As such, this ARW provided an interactive forum for recent advances in the development and commercialization of SOFC and PEM fuel cell systems, as well as technology improvements in the materials and systems. 

Skulachev, V. P., 1977, Transmembrane electrochemical potential as a convertible energy source for the living cell, FEBS Lett. 74 1. 

Consider the case of a junction between two different metals a and p. Generally, they will have different values of the Fermi energy and work function. Between the two metals, a certain Volta potential will be set up. This implies that the outer potentials at points a and b, which are just outside the two metals, are different. However, it will be preferable to count the Fermi levels or electrochemical potentials from a common point of reference. This can be either point a or point b. Since these two points are located in the same phase, the potential difference between them (the Vofta potential) can be measured. Hence, values counted from one of the points of reference are readily converted to the other point of reference when required. 

Environmental hazards of batteries can be briefly summarized as follows. A battery is an electrochemical device with the ability to convert chemical energy to electrical energy to provide power to electronic devices. Batteries may contain lead, cadmium, mercury, copper, zinc, lead, manganese, nickel, and lithium, which can be hazardous when incorrectly disposed. Batteries may produce the following potential problems or hazards (a) they pollute the lakes and streams as the metals... 

In this section, you learned about electrolytic cells, which convert electrical energy into chemical energy. You compared the spontaneous reactions in galvanic cells, which have positive cell potentials, with the non-spontaneous reactions in electrolytic cells, which have negative cell potentials. You then considered cells that act as both galvanic cells and electrolytic cells in some common rechargeable batteries. These batteries are an important application of electrochemistry. In the next two sections, you will learn about many more electrochemical applications. 

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