Negative electrodes electrochemical processes

A characteristic feature of an electrochemical cell is that the electronic current, which is the movement of electrons in the external circuit, is generated by the electrochemical processes at the electrodes. In contrast to the electronic current, the charge is transported between the positive and the negative electrode in the electrolyte by ions. Generally the current in the electrolyte consists of the movement of negative and positive ions. 

In the case of an electrochemical cell with a negative electrode consisting of an elemental metal, the process of recharging is apparently very simple, for it merely involves the electrodeposition of the metal. There are problems, however. 

Cases exist, however, where for fundamental reasons aqueous solutions cannot be used. One such case is that of devices in which electrochemical processes take place at elevated temperatures (above 180 to 200°C) for example, the electrowinning of aluminum performed at temperatures close to 1000°C. Another case is that of devices in which electrodes consisting of alkali metals are used, which are unstable in aqueous solutions, such as batteries with a lithium negative electrode. 

Electrochemical Processes The charged positive electrodes of these batteries contain NiOOH, an oxide hydroxide of trivalent nickel, and the negative electrodes contain metallic cadmium or iron (M). As a rule, KOH solution serves as the electrolyte. The main current-producing reactions on the electrodes and in the cell in general can be written as... 

Apart from the work toward practical lithium batteries, two new areas of theoretical electrochemistry research were initiated in this context. The first is the mechanism of passivation of highly active metals (such as lithium) in solutions involving organic solvents and strong inorganic oxidizers (such as thionyl chloride). The creation of lithium power sources has only been possible because of the specific character of lithium passivation. The second area is the thermodynamics, mechanism, and kinetics of electrochemical incorporation (intercalation and deintercalation) of various ions into matrix structures of various solid compounds. In most lithium power sources, such processes occur at the positive electrode, but in some of them they occur at the negative electrode as well. 

The a-aminoalkyl radical intermediates from electrochemical oxidation of amines show a strong tendency to lose a further electron and form an immonium ion. This process shows an anodic polarographic wave at negative electrode poten-... 

A positive standard cell potential tells you that the cathode is at a higher potential than the anode, and the reaction is therefore spontaneous. What do you do with a cell that has a negative " gii Electrochemical cells that rely on such nonspontaneous reactions cire called electrolytic cells. The redox reactions in electroljdic cells rely on a process called electrolysis. These reactions require that a current be passed through the solution, forcing it to split into components that then fuel the redox reaction. Such cells are created by applying a current source, such as a battery, to electrodes placed in a solution of molten salt, or salt heated until it melts. This splits the ions that make up the salt. 

The electrochemical reactions at the negative electrode are the comparatively straightforward processes ... 

As discussed below, there are problems with morphological changes and passivation reactions at lithium metal negative electrodes in secondary cells, which reduce cycle life and the practical energy density of the system, and may in some circumstances introduce safety hazards. A more recent development involves the replacement of the lithium metal anode by another insertion compound, say C Dm. In this cell, the electrochemical process at the negative side, rather than lithium plating and... 

Carbon is used in lithium-ion cells for different functions conductive carbon black and/or graphite additives are applied in both the negative and the positive electrode to improve the electronic conductivity of the electrodes. These conductive additives constitute a fraction of up to about 10% of the total carbon consumption. The major fraction is represented by the active carbon materials which are electrochemically reduced and oxidized in the negative electrode during the battery charge and discharge process, respectively. 

FIGURE 8.32 Typical potential profiles for (a) positive electrode in a conventional asymmetric capacitor built with a non pre-doped Li inter calation carbon for the negative electrode (curve 1), an asymmetric capacitor with a pre-doped Li-ion intercalation carbon material (curve 2), and (b) positive and negative electrodes of an EDLC during the charging process. (From Aida, T., et al., Electrochem. Solid-State Lett., 9, A534, 2006. With permission.)... 

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