Type-II electrode

A type II electrode is one in which two equilibria are involved in determining the electrode process. These systems often involve a metal electrode coated with an insoluble salt. A well-known example is the silver silver chloride electrode discussed above. The solid silver chloride ensures that the surrounding solution is saturated with sparingly soluble AgCl. The Ag metal is in equilibrium with Ag" " ion in solution ... 

Another well-known example of a type II electrode is the calomel system ... 

Type II electrodes are two-phase composite media that consist of a nanoporous and electronically conductive medium filled with liquid electrolyte or ionic liquid. The electrochemically active interface forms at the boundary of the two phases. The electrolyte phase must provide pathways for diffusion and permeation of protons, water, and reactants. Flooded two-phase CLs could work well when they are made extremely thin, not significantly exceeding a thickness of Icl — 200 nm. Rates of diffusion of reactant molecules and protons in liquid water are then sufficient to provide uniform reaction rate distributions over the thickness of the layer. 

The importance of proton distribution and transport in water-filled nanopores with charged metal walls is most pronounced in ionomer-free UTCLs (type II electrodes), cf. the main case considered in this section. In either type of CLs, proton and potential distribution at the nanoscale are governed by electrostatic phenomena. 

Fig. 3.7 Shape of the potential-composition curve for type-II electrode...

Finally cells containing a p-type semiconductor electrode should be mentioned. In principle the application of p-type electrodes would be even more favorable because electrons created by light excitation are transferred from the conduction band to the redox system. Stability problems are less severe because most semiconductors do not show cathodic decomposition (see e.g. earlier review article. However, there is only one system, p-InP/(V " /V ), with which a reasonable efficiency was obtained (Table 1) . There are mainly two reasons why p-electrodes were not widely used (i) not many materials are available from which p-type electrodes can be made (ii)... 

In general, the physical state of the electrodes used in electrochemical processes is the solid state (monolithic or particulate). The material of which the electrode is composed may actually participate in the electrochemical reactions, being consumed by or deposited from the solution, or it may be inert and merely provide an interface at which the reactions may occur. There are three properties which all types of electrodes must possess if the power requirements of the process are to be minimized (i) the electrodes should be able to conduct electricity well, i.e., they should be made of good conductors (ii) the overpotentials at the electrodes should be low and (iii) the electrodes should not become passivated, by which it is meant that they should not react to form on their surfaces any compound that inhibits the desired electrochemical reaction. Some additional desirable requirements for a satisfactory performance of the cell are that the electrodes should be amenable to being manufactured or prepared easily that they should be resistant to corrosion by the elements within the cell that they should be mechanically strong and that they should be of low cost. Electrodes are usually mounted vertically, and in some cases horizontally only in some rare special cases are they mounted in an inclined manner. 

Three-dimensional electrode nanoarchitectures exhibit unique structural features, in the guise of amplified surface area and the extensive intermingling of electrode and electrolyte phases over small length scales. The physical consequences of this type of electrode architecture have already been discussed, and the key components include (i) minimized solid-state transport distances (ii) effective mass transport of necessary electroreactants to the large surface-to-volume electrode and (iii) magnified surface—and surface defect—character of the electrochemical behavior. This new terrain demands a more deliberate evaluation of the electrochemical properties inherent therein. 

Fig. 6 Cyclic voltammograms of a platinized platinum electrode (type II) in (a) 0.25 mol dm H2SO4, (b) 0.25 mol dm H2SO4 + 10 mol dm HNO3, and (c) 0.25 mol dm H2SO4 - -10 mol dm HNO3. Sweep rate,...

Mixed crystals of type II have been used in the form of thin films on electrodes as well as in the form of chemically synthesized powders immobilized on electrodes. Depending on the radii of the ions involved in the synthesis, solid solutions can also be formed as single phases. In the case of K CuCo[Fe(CN)(5] films, XRD results indicated that a single phase with a cubic face-centered symmetry was formed [31]. The situation is more complex in the case of K NiPd[Fe(CN)6] deposited as a thin film on electrodes [32]. Kulesza etal. have pointed out that there is a critical concentration of Pd + below which Pd + was taken as the countercation at interstitial position, while above that value a solid solution is formed in which both Ni " " and Pd + are nitrogen coordinated. 

The occurrence and deactivation of excited states of the first type are schematically shown in Fig. 35. Let the minority carriers (holes) be injected into the semiconductor in the course of an electrode reaction (reduction of substance A). The holes recombine with the majority carriers (electrons). The energy, which is released in the direct band-to-band recombination, is equal to the energy gap, so that we have the relation ha> = Eg for the emitted light quantum (case I). More probable, however, is recombination through surface or bulk levels, lying in the forbidden band, which successively trap the electrons and holes. In this case the excess energy of recombined carriers is released in smaller amounts, so that hco < Eg (case II in Fig. 35). Both these types of recombination are revealed in luminescence spectra recorded with n-type semiconductor electrodes under electrochemical generation of holes (Fig. 

Frank, SN and Bard, A3, Semiconductor Electrodes. II. Electrochemistry at n-type TiO Electrodes in Acetonitrile Solutions, 3. Amer. Chem. Soc., 97, 7427, 1975. 

Chl-coated semiconductor (n-type) electrodes have thus far been studied using ZnO, CdS, and Sn02, all of which act as efficient photoanodes for converting visible light. Such Chl-sensi-tized photoanodes could be regarded as in vitro models for the photosystem II (oxygen evolution) function in photosynthesis, p-type semiconductor electrodes have not been utilized successfully to produce cathodic Chl-sensitized photocurrents with satisfactory efficiencies. On the other hand, Chl-coated metal electrode systems seem to overcome this problem. 

As for the reoxidation of reduced heteropoly compounds in the solid state, few reliable studies have been reported. It was reported that the reoxidizability increases with an increase in standard electrode potentials of countercations (108). In the case of reoxidation by O2 of le -reduced CsxHj - PMo 12O40, the rates divided by the surface area show a monotonic variation (Fig. 53e) as in Figs. 53c and d, indicating a surface reaction. A similar variation was observed for the Na and K salts. The presence of water vapor sometimes accelerates the migration of oxide ion, probably in the form of OH- or H20, and makes surface-type reactions more like bulk type II reactions (266). 

Fig. 3. Different cell geometries. Type I For a uniform potential distribution at the electrode electroyte interface, the current density is uniform. Type II For a uniform potential distribution at the electro-de electroyte interface, the current density is not uniform.

Electrochromic materials are of three basic types [i]. In a given -> electrolyte solution, type I materials are soluble in both the reduced and oxidized (redox) states, an example being l,l -di-methyl-4,4 -bipyridylium ( methyl viologen ), which, on reduction, switches from the colorless di-cation to the blue radical cation. Type II materials are soluble in one redox state, but form a solid film on the surface of an electrode following electron transfer. An example here is l,l -di-heptyl-4,4 -bipyridylium ( heptyl viologen ). In type III materials, such as -> tungsten oxide, - Prussian blue, and electroactive conjugated polymers, both... 

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