Practical Considerations 1 Electrochemical Cells

Because of the assumption of semiinfinite diffusion made by Warburg for the derivation of the diffusion impedance, it predicts that the impedance diverges from the real axis at low frequencies, that is, according to the above analysis, the dc-impedance of the electrochemical cell would be infinitely large. It can be shown that the Warburg impedance is analogous to a semi-infinite transmission line composed of capacitors and resistors (Fig. 8) [3]. However, in many practical cases, a finite diffusion layer thickness has to be taken into consideration. The first case to be considered is that of enforced or natural convection in an... 

It has become useful and necessary to summarize the new advances in both fundamental and practical researches in this field when we want to review the new development in recent years. In this book, parallel efforts have been put on both new advances in fundamental research and recent developments for applications of electrochemical cells, which include electrode design, electrochemical probe, liquid electrol3des, fuel cells, electrochemical detectors for health and environment consideration. Of course, electrochemical cells have been developed into a relatively large research category, which also means this book can only cover a comer of these topics. 

The first consideration is the choice of model system, and this may involve an aqueous electrolyte or nonaqueous media such as an organic electrolyte or molten salt. The selection of electrolyte and scavenger will depend on the energetics of the electrolyte reduction (good stability to reduction is needed) and upon the acceptor reactivity. Apparatus and electrochemical cells for metal/ electrolyte and semiconductor/electrolyte systems are similar generally. Some guidelines are presented below to assist with experimental practice (see also Chapter 3 in Ref. 10). Theses, also, are a good source of practical information, e.g., Refs. 20, 21, and 81. 

In the real non-equilibrium conditions of a present-day MCFC with very successful electrode reform, the cell electrode reaction, voracious for fuel, consumes the reformer product and favourably influences the reform process. The latter turns out to operate well at 600 °C, compared with about 800 °C in a fired reformer coupled, say, to much less voracious hydrogen separation and storage. In the practical SOFC, 1000 °C at the anode promotes excessively vigorous electrode reform, which leads to a local electrode cold spot. There are also stability considerations (Gardiner, 1996). Hence the contemporary movement towards lower SOFC temperatures, via new ceria electrolytes, and interconnect change from ceramic to steel. A PEFC near Tq, must have a combustion-operated 800 °C reformer, since a Tq electrochemical reform process does not exist in practice. 

The major sources of dilute, metal ion liquors are identified within the metals production/processing and chemical industries. Problems associated with traditional methods of metal ion removal are highlighted and the developing role of electrochemical techniques is discussed. Electrode and cell reactions are illustrated via typical examples from laboratory and industrial practice. The need to select an appropriate cell design and to control the reaction conditions is emphasised via consideration of the problems caused by secondary reactions. Important design criteria for electrochemical reactors are summarised. Available reactors are classified according to the nature of the product which may be metal flake or powder, a metal deposited onto a disposable substrate, a metal ion concentrate or an insoluble metal compound. The applications for electrochemical techniques in environmental treatment are illustrated by examples which show features of reactor construction and their typical performance. Current trends are summarised and recommendations are made for further work in critical areas. 

As can be seen from Fig. 8.3 the maximum efficiencies are higher for the electrochemical energy conversion than for heat engines at temperatures below 1150 K. However, this theoretical advantage has to be realized in practical systems and therefore the losses of fuel cells under realistic operation conditions have to be taken into consideration. 

The electrochemical eflBdency gives more information about fuel cells than the thermodynamic efHciency as it is directly related to the performance of the cell. For a complete description of the fuel cell efHciency further factors associated with practical operation have to be considered. These include the faradaic efHciency ep which is deHned as the ratio of the actual current fact and the maximum possible current Imax- This faradaic efficiency considers the possibility of parallel reactions which can lead to a lower current yield than expected theoretically. Furthermore, the total efficiency of the cell requires the consideration of practical aspects concerned with the specific fuel used. In most cases fuel cells are not operated with 100 % fuel utilization in order to avoid fuel depletion in some areas of the electrodes. Therefore the fuel utilization should be included in the total efficiency, given as... 

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