Cathode reaction types conversion reactions

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

Conversely, in cathodic reactions (reduction processes), both types of polarization have to be taken into account. The relationship between overvoltage and current density will therefore be as follows ... 

To summarize, the semiconduetor/electrolyte interfaee presents two types of currents in the dark this is a current of majority carriers whereas the photocurrent is a current of minority carriers. The same reactions can be monitored at n- and p-type electrodes but under different conditions. Hole accumulation corresponds to corrosion, since holes are trapped in surface bonds. Electron accumulation is generally not destructive for the surface unless cathodic reduction leads to decomposition. The band diagrams of Fig. 5 indicate that a downward shift of the flat band potential is expected at an illuminated n-type electrode. At negative bias, conversely, the shift is upward since electrons are accumulated in a thin surface layer (metallic-like behavior). 

The ideal cell in order to scale up an electrochemical reaction can depend on the reaction, the electroactivity of the substrate to convert, the concentration of the substrate, as well as the current density at the working electrode. The use of a separator is necessary when the electrode can affect the whole process negatively. With anodic oxidations, the reaction at the counter electrode is most frequently the cathodic formation of hydrogen. In these cases, a separator does not seem indispensable a tank cell (kind of Grignard type reactor equipped with cylindrical electrodes) or a capillary-gap cell (piling of bipolar electrodes in a cylinder-shaped vessel connected to an anodes and a cathode located at the top and the bottom of the cell) can be considered as suitable devices for anodic conversions. More generally, the so-called plate-and-frame cells (Fig. 4) are used in a battery. 

There are two main types of PEC cells, the regenerative type cells, and the photoelectrolysis cells. The first type also called liquid-junction solar cells, serve like solid-state cells for solar energy conversion into electricity. The same reaction proceeds at both electrodes of the cell, directed forward at the anode, backward at the cathode. Hence, composition of the solution is preserved unchanged. The other type, the photoelectrolysis cells serve for conversion of solar energy into chemical energy. Here, two different reactions proceed at the electrodes. As a consequence, certain substance in the cell is transformed in the course of its operation. These two types of the PEC cells are described in more detail below. 

While the membrane represents the heart of the fuel cell, determining the type of cell and feasible operating conditions, the two catalyst layers are its pacemakers. They fix the rates of electrochemical conversion of reactants. The anode catalyst layer (ACL) separates hydrogen or hydrocarbon fuels into protons and electrons and directs them onto distinct pathways. The cathode catalyst layer (CCL) rejoins them with oxygen to form liquid water. This spatial separation of reduction and oxidation reactions enables the electrons to do work in external electrical appliances, making the Gibbs free energy of the net reaction, —AG, available to them. 

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