Electrodes anodes

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

Positive ions formed near the positive electrode (anode) are repelled by it and move into the positive column. Electrons that reach proximity to the anode are accelerated somewhat because... 

Electrodialysis. Electro dialysis processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields. A negative electrode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes. Alternating compartments carry concentrated brine and purified permeate. Typically, 40—60% of dissolved ions are removed or rejected. Further improvement in water quaUty is obtained by staging (operation of stacks in series). ED processes do not remove particulate contaminants or weakly ionized contaminants, such as siUca. 

To exploit the energy produced in this reaction, the half reactions are separated. The oxidation reaction is carried out at a zinc electrode (Zn Zir + 2 electrons) and the reduction reaction is carried out at a copper electrode (Cu"" + 2 electrons Cu metal). Electrons flow through a metal wire from the oxidizing electrode (anode) to the reducing electrode (cathode), creating electric current that can be harnessed, for example, to light a tungsten bulb. 

Salt Concentration Cells. In this type of cell the two electrodes are of the same metal (i.e., copper). These electrodes are immersed completely in electrolytes of the same salt solution (i.e., copper sulfate) but of different concentrations. When the cell is short circuited, the electrodes (anode) exposed to the dilute solution will dissolve into the solution and plate the electrode (cathode) exposed to the more concen-trated solution. These reactions will continue until the solutions are of the same concentration. Figure 4-432 shows a schematic of a salt concentration cell. 

Half-cell one half of an electrochemical cell, comprising one electrode (anode or cathode) and its immediate electrolyte (anolyte or catholyte). 

In principle at least, any spontaneous redox reaction can serve as a source of energy in a voltaic cell. The cell must be designed in such a way that oxidation occurs at one electrode (anode) with reduction at the other electrode (cathode). The electrons produced at the anode must be transferred to the cathode, where they are consumed. To do this, the electrons move through an external circuit, where they do electrical work. 

In an electrochemical cell, electrical work is obtained from an oxidation-reduction reaction. For example, consider the process that occurs during the discharge of the lead storage battery (cell). Figure 9.3 shows a schematic drawing of this cell. One of the electrodes (anode)q is Pb metal and the other (cathode) is Pb02 coated on a conducting metal (Pb is usually used). The two electrodes are immersed in an aqueous sulfuric acid solution. 

The negative electrode (anode) acts as an electrocatalyst for the reaction of O2 with the fuel, e.g. H2 ... 

Some metals are thermodynamically unstable in aqueous solutions because their equilibrium potential is more negative than the potential of the reversible hydrogen electrode in the same solution. At such electrodes, anodic metal dissolution and cathodic hydrogen evolution can occur as coupled reactions, and their open-circuit potential (OCP) will be more positive than the equilibrium potential (see Section 13.7). 

Now let us consider a model for a SC device that comprises two electrodes (anode and cathode), each of them being electrically connected to a current collector fabricated of A1 foil. Let two of such collectors have a certain thickness of SAi- As an electrode material, an activated carbon powder is considered below. Anode and cathode are interposed with a separator of thickness Ss. The electrodes and separator are impregnated with electrolyte. In this paper we mostly focus on the optimization of SC performance by varying the electrode thickness, while some other effects will briefly be considered in the next section. 

The characteristics of a carbon material used as active reagent of the negative electrode (anode) of a Lithium-Ion cell considerably influence the power characteristics of the cell as a whole. Thus, the major parameters are the values of specific capacity per unit weight and volume, and also the... 

Anode (avoSog) ascent positive electrode anode anode, anodic, anodize... 

The tumor killing area around an electrode is circular with a radius of approximately 1 cm. Therefore the distance between the positive and negative electrode should be approximately 2 cm. Both electrodes should be inserted into the tumor, or within the peripheries of the tumor in order to avoid damage to the healthy tissue around the tumor. It is preferable to insert the positive electrode (anode) in the centre of the tumor since, in general, tissue necrosis is more pronounced around the anode rather than the cathode. For large tumors, several electrodes may be inserted in order to cover the entire tumor for the electrochemical treatment since one anode and one cathode are not effective when they are more than two cms apart. 

It is important to emphasize here that during electroosmosis, the water flow is always from anode to cathode, as indicated in the schematic in Figure 7 here. This arises from the electrostatics of the situation in which the water velocity profile follows the direction of the electric field, i.e., from the positive electrode (anode) to the negative electrode (cathode), as depicted in Figure 8. 

Figure 29.5, depicts the diagram of an electron capture detector. The metal block of the detector housing itself serves as a cathode, whereas an electrode polarizing lead suitably positioned in the centre of the detector housing caters for a collector electrode (anode). The radioactive source from a beta-emitter is introduced from either sides of the detector housing below the electrode polarizing lead. 

Let us now suppose that the waveform of figure 16.3 is applied to study the reversible oxidation of a species R to R in a given solvent. The reaction occurs at the working electrode (anode), and /i°(R/R ) is the standard potential of the R/R- couple. Because the standard potential of the reference electrode in our cell is known accurately relative to the standard potential of the SHE (E° = 0 by definition), we can write the cell reaction and the Nernst equation as... 

Fig. 4.6 Di ssolution valence nv as a function of anodic current density for micropore formation on low doped p-type electrodes anodized in ethanoic HF (1 1, ethanol HF 50%).

The quantum efficiency for solid-state devices, e.g. solar cells, is always below unity. For n-type silicon electrodes anodized in aqueous or non-aqueous HF electrolytes, quantum efficiencies above unity are observed because one or more electrons are injected into the electrode when a photogenerated hole enters the electrolyte. Note that energy conservation is not violated, due to the enthalpy of the electrochemical dissolution reaction of the electrode. 

The surprising variety of photoelectrochemical effects observed at silicon electrodes anodized in HF is solely a consequence of the semiconducting nature of the electrode, because the electrolyte is not photoactive. 

Fig. 5.4 Voltage-time curve for a p-type silicon electrode anodized galvanostatically at 0.1 mA cm"2 in 10% acetic acid. Silicon electrodes were removed from the electrolyte after various anodization times (filled circles) and the thickness of the anodic oxide was measured by ellipsometry (open circles). The curvature of the sample was monitored in situ and is plotted as the value of stress times oxide thickness (filled triangles). The bar graph below the V(t) curve shows a proposed formation mechanism. Galvanostatically a...

Fig. 8.3 SEM micrographs of the interface between bulk and meso PS for p-type doped (100) silicon electrodes anodized galvanostatically in ethanoic HF. After [Le23].

Fig. 8.14 The density of avalanche etch pits depends on substrate doping density, as shown by optical micrographs of two n-type electrodes anodized at high bias in 6% aqueous HF in the dark (a) 50 V, 30 s, 1.5 fi cm ...

A specific feature of macropore formation in n-type silicon is the possibility of controlling the pore tip current by illumination and not by applied bias. This adds another degree of freedom that is not available for mesopore or macropore formation on p-type substrates. The dark current density of moderately doped n-type Si electrodes anodized at low bias is negligible, as shown in Fig. 4.11, therefore all macropore structures discussed below are formed using illumination of the electrode to generate the flux of holes needed for the dissolution process. Illumination, however, is not the only possible source of holes for example, hole injection from a p-doped region is expected to produce similar results. 

Fig. 9.7 SEM micrographs of surface and cross-section of an n-type Si electrode anodized for the indicated times under white light illumination of the front side (0.4 D. cm, (100), 2.5% HF, 10 mAcmf2, 14 V). Micro-...

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