Positive electrodes mechanisms

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. 

In topochemical reactions all steps, including that of nucleation of the new phase, occur exclusively at the interface between two solid phases, one being the reactant and the other the product. As the reaction proceeds, this interface gradually advances in the direction of the reactant. In electrochemical systems, topochemical reactions are possible only when the reactant or product is porous enough to enable access of reacting species from the solution to each reaction site. The number of examples electrochemical reactions known to follow a truly topochemical mechanism is very limited. One of these examples are the reactions occurring at the silver (positive) electrode of silver-zinc storage batteries (with alkaline electrolyte) ... 

At the end of the 1990s in Japan, large-scale production of rechargeable lithium ion batteries was initiated. These contained lithium compounds intercalated into oxide materials (positive electrodes) as well as into graphitic materials (negative electrode). The development of these batteries initiated a further increase in investigations of the properties of different intercalation compounds and of the mechanism of intercalation and deintercalation processes. 

As the electric current passes through this system, the cathode (negative electrode) grows in thickness while the anode (positive electrode) shrinks. At the cathode, M+ ions are converted to M atoms, which results in growth of the cathode. From this observation, it is clear that the cations are primarily responsible for conductivity, and this is the result of a vacancy type of mechanism. In this case, the positive ion vacancies have higher mobility than do the vacancies that involve negative ions. 

Although it is difficult to generalize the dependence of the peak potential on e, in general, for an oxidative electrode mechanism, the position of the peak shifts to positive potentials by increasing the rate of the preceding chemical reaction. At the same time, the half-peak width is largely insensitive to the chemical reaction. If log( ) < -2, Ap vs. log(e) is a linear function with a slope of about 30 mV. 

For the catalytic electrode mechanism, the total surface concentration of R plus O is conserved throughout the voltammetric experiment. As a consequence, the position and width of the net response are constant over entire range of values of the parameter e. Figure 2.35 shows that the net peak current increases without limit with e. This means that the maximal catalytic effect in particular experiment is obtained at lowest frequencies. Figure 2.36 illustrates the effect of the chemical reaction on the shape of the response. For log(e) < -3, the response is identical as for the simple reversible reaction (curves 1 in Fig. 2.36). Due to the effect of the chemical reaction which consumes the O species and produces the R form, the reverse component decreases and the forward component enhances correspondingly (curves 2 in Fig. 2.36). When the response is controlled exclusively by the rate of the chemical reaction, both components of the response are sigmoidal curves separated by 2i sw on the potential axes. As shown by the inset of Fig. 2.36, it is important to note that the net currents are bell-shaped curves for any observed kinetics of the chemical reaction, with readily measurable peak current and potentials, which is of practical importance in electroanalytical methods based on this electrode mecharusm. 

For each cathodic stripping mechanism, the dimensionless net peak current is proportional to the amount of the deposited salt, which is formed in the course of the deposition step. The amount of the salt is affected by the accumulation time, concentration of the reacting ligand, and accumulation potential. The amount of the deposited salt depends sigmoidally on the deposition potential, with a half-wave potential being sensitive to the accumulation time. If the accumulation potential is significantly more positive than the peak potential, the surface concentration of the insoluble salt is independent on the deposition potential. The formation of the salt is controlled by the diffusion of the ligand, thus the net peak current is proportional to the square root of the accumulation time. If reaction (2.204) is electrochemically reversible, the real net peak current depends linearly on the frequency, which is a common feature of all electrode mechanism of an immobilized reactant (Sect. 2.6.1). The net peak potential for a reversible reaction (2.204) is a hnear function of the log(/) with a slope equal to typical theoretical response... 

The same group have also studied [325] a mechanism of zinc(II) transfer from the negative to positive electrode of a Ni-Zn battery. 

Therefore, passivation of the positive electrode by poorly conducting PbS04 can be reduced [348]. The porosity is important because it enables the expansion during the solid phase volume increase, which accompanies the transformation of Pb02 to PbS04. In the most popular construction, the electrode paste material (mixture of metallic lead with lead oxides) is held in a framework composed of lead alloys with additions of tin, antimony, selenium, and calcium [348]. Antimony improves the mechanical stability however, it increases the resistance and facilitates the selfdischarge of the battery. Better results are obtained for low antimony content and/or for lead-calcium alloys [203]. Methods of positive electrodes improvement, from the point of view of lead oxide technology have been discussed [350]. Influence of different factors on life cycle, nature, and composition of the positive active mass has been studied by Pavlov with coworkers [200, 351, 352]. 

If proper mechanical arrangements are provided, the Na is collected (as a vapor at the temperature of molten NaCI) in the absence of air at the negative electrode, and Cl2 gas is collected at the positive electrode. Different mechanical provisions must be made if the metal is produced as a liquid or a solid, but the principle is the same in every case. If we look at the completed circuit, we see that electrons have come from the power supply to the negative electrode and have gone to the power supply from the positive electrode, with a hi directional flow ofions within the cell. 

While there are a number of methods used for manufacturing the positive electrodes, the two most important processes are the sintering of silver powders and slurry pasting. The former procedure produces electrodes with superior mechanical properties. The silver mass which is formed by... 

This is a general feature in positive electrode research. While many materials tolerate a one-electron transfer, a two-electron transfer usually leads to mechanical instability owing to volume variation or makes the material chemically unstable. 

The aptitude of a given storage mechanism is mainly determined by the cell voltage connected with it (positive electrodes are typically in the range of 2-5 V vs. Li/Li+ negative electrodes in the range of 0-1.5 V), the respective storage capacity connected with it, and its reversibility. 

Now we turn to the situation when the electrode is negatively charged as shown in Figure 1.14. It is important to note that the solvated cations lie at the outer Helmholtz plane, unlike the situation in which the anions adsorb on the positive electrode surface. In terms of interaction forces the cation-water interactions are stronger than the negatively charged electrode - water interactions. In terms of more intimate mechanism the water molecules attached to the cation do not exchange with water molecules adsorbed on the electrode surface. The solvated cation is situated at the outer Helmholtz plane. 

The fundamental separation mechanism of capillary zone electrophoresis (CZE) is based on differences in the mobilities of solutes. Mobility is defined as the charge/mass ratio for each solute. Since the charge is often a function of pH, the pH is the most important adjustable parameter for control of resolution. The order of elution on bare silica at high pH is cations, unseparated neutrals, and anions. At low pH, where the EOF is very low, the anions may migrate toward the positive electrode and may not be seen using normal polarity. 

The dissociation of molecular hydrogen is the main story of what happens at the negative electrode of a fuel cell. The "liberated" hydrogen atoms can move through the (liquid or solid membrane) electrolyte towards the positive electrode, by the mechanism described in Fig. 3.7, and the electrons will enter the electrode metal and take part in an external current, if the cell is cormected to a load, or will just build up a potential across the cell, creating the conditions for the transfer of positive hydrogen ions. 

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