Electrochemical cell classification

The pre-existing concentrations in an electrochemical cell prior to the start of the experiment naturally affect the subsequent transport behaviour. Before discussing the various options for such initial conditions, it is useful to provide a classification of electrochemical experiments. 

Concepts developed in nonlinear dynamics facilitated the classification of nonlinear phenomena in electrochemical systems and revealed the origins of the diversity of temporal and spatial patterns in electrochemical systems. The diversity results on the one hand from the fact that the electrode potential might act as a positive or as a negative feedback variable. On the other hand, it is a consequence of the different kinds of spatial coupling present in an electrochemical cell and of the unique property that the extent of the spatial couplings is influenced by parameters that can be easily manipulated in an experiment. 

A systematic classification of homogeneous chemical reactions (her) in electrochemical cells can be read in Chapter 1 in Volume 8 and here, only some examples that impinge on simulations will be mentioned. 

Electrochemical cells and batteries are identified as primary (nonrechargeable) or secondary (rechargeable), depending on their capability of being electrically recharged Within this classification, other classifications are used to identify particular stmctures or designs. The classification used in this handbook for the different types of electrochemical cells and batteries is described in this section. 

Table 2.1 Classification of electrochemical cells according to the nature of energy transformation...

Crystalline solids are built up of regular arrangements of atoms in three dimensions these arrangements can be represented by a repeat unit or motif called a unit cell. A unit cell is defined as the smallest repeating unit that shows the fuU symmetry of the crystal structure. A perfect crystal may be defined as one in which all the atoms are at rest on their correct lattice positions in the crystal structure. Such a perfect crystal can be obtained, hypothetically, only at absolute zero. At all real temperatures, crystalline solids generally depart from perfect order and contain several types of defects, which are responsible for many important solid-state phenomena, such as diffusion, electrical conduction, electrochemical reactions, and so on. Various schemes have been proposed for the classification of defects. Here the size and shape of the defect are used as a basis for classification. 

Factors Involved in Galvanic Corrosion. Emf series and practical nobility of metals and metalloids. The emf. series is a list of half-cell potentials proportional to the free energy changes of the corresponding reversible half-cell reactions for standard state of unit activity with respect to the standard hydrogen electrode (SHE). This is also known as Nernst scale of solution potentials since it allows to classification of the metals in order of nobility according to the value of the equilibrium potential of their reaction of dissolution in the standard state (1 g ion/1). This thermodynamic nobility can differ from practical nobility due to the formation of a passive layer and electrochemical kinetics. 

Electrochemical reactors are heterogeneous by their very nature. They always involve a solid electrode, a liquid electrolyte, and an evolving gas at an electrode. Electrodes come in many forms, from large-sized plates fixed in the cell to fluidizable shapes and sizes. Further, the total reaction system consists of a reaction (or a set of reactions) at one electrode and another reaction (or set of reactions) at the other electrode. The two reactions (or sets of reactions) are necessary to complete the electrical circuit. Thus, although these reactors can, in principle, be treated in the same manner as conventional catalytic reactors, detailed analysis of their behavior is considerably more complex. We adopt the same classification for these reactors as for conventional reactors, batch, plug-flow, mixed-flow (continuous stirred tank), and their extensions. 

Fuel cells are electrochemical devices that convert chemical energy contained in fuel directly into electrical energy through electrochemical reactions. Fuel cells consist of an anode, where the fuel is oxidized, a cathode where the oxidant is reduced and an electrolyte which separates anode from cathode and conducts ions. The general classification of fuel cells is usually based on the type of electrolyte used, and their operation conditions are typically related to the characteristics of the electrolyte. More detailed discussion of fuel cells as stand-alone power sources can be found in the next chapter of this book. 

Two different technological approaches are reflected by this classification. The main problem of low-temperature cells consists in finding efficient electrocatalysts so that the rates of the electrochemical reactions are still satisfactory at low polarization. Since the reaction rates increase with temperature, the role of the electrocatalyst is not so critical in the high-temperature cells. Other problems like corrosion and conductivity of the electrolyte become pertinent for the construction of a practical unit. 

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