Cathode proton concentration

We consider dehydration-adsorption of hydrated protons (cathodic proton transfer) and desorption-hydration of adsorbed protons (anodic proton transfer) on the interface of semiconductor electrodes. Since these adsorption and desorption of protons are ion transfer processes across the compact layer at the interface of semiconductor electrodes, the adsorption-desorption equilibrium is expressed as a function of the potential of the compact layer in the same way as Eqns. 9-60 and 9-61. In contrast to metal electrodes where changes with the electrode potential, semiconductor electrodes in the state of band edge level pinning maintain the potential d(hi of the compact layer constant and independent of the electrode potential. The concentration of adsorbed protons, Ch , is then determined not by the electrode potential but by the concentration of h3 ated protons in aqueous solutions. 

According to Equation 6.6, the velocity of the EOF is directly proportional to the intensity of the applied electric held. However, in practice, nonlinear dependence of the EOF on the applied electric held is obtained as a result of Joule heat production, which causes the increase of the electrolyte temperature with consequent decrease of viscosity and variation of all other temperature-dependent parameters (protonic equilibrium, ion distribution in the double layer, etc.). The EOF can also be altered during a run by variations of the protonic concentration in the anodic and cathodic electrolyte solutions as a result of electrophoresis. This effect can be minimized by using electrolyte... 

The Birch and Benkeser reactions of some unsaturated organic compounds [318 and references therein], which consist of a reduction by sodium or lithium in amines, can be mimicked electrochemically in the presence of an alkali salt electrolyte. The cathodic reaction is not the deposition of alkali metal on the solid electrode but the formation of solvated electrons. Most of the reactions described were performed in ethylenediamine [319] or methylamine [308,320]. A feature of these studies is variety introduced by the use of a divided or undivided cell. In a divided cell, the product distribution appears to be the same as that in the classic reduction by metal under similar conditions. In contrast, in an undivided cell the corresponding ammonium salt is formed at the anode it plays the role of an in situ generated proton donor. Under such conditions, the proton concentration... 

Sodium sulfate concentration at constant MR. At MR values < 1, the proton concentration, which is a function of the ratio MR to (1-MR), is expected to remain constant at constant molar ratios, even if the concentrations of both sulfuric acid and sodium sulfate are greatly changed. Consequently, the transport number of the protons decreases as the concentration of sodium sulfate (i.e., the concentration of sodium ions) is increased. At the same time, the cathodic current efficiency increases. 

In some cases, it is possible to observe a small amount of ammonia before imposing a potential. It is likely that the reaction occurs due to the difference in chemical potentials of electro-active ionic species (in this case protons) between two electrodes induces some protons to move from the anode (higher concentration) to the cathode (lower concentration). Theoretically, the reactions at the cathode should not be completed because the transportation of electrons through the electrolyte should be negligibly small, and a high-resistance external source connected to the electrodes also forbids the flow of electrons. Therefore, only the chemical potential difference between the electrodes called a potential open-circuit voltage (Voc) should be measured. In some electrolyte materials, the presence of electrons transport will cause the formation of ammonia at the cathode under open circuit conditions, especially in an elevated temperature sohd oxide proton conductor. 

The expression for Teiec embodies two competing trends of the solution phase potential at the reaction plane. An increase in solution phase potential results in a larger driving force for electron transfer in cathodic direction. This effect is proportional to the cathodic transfer coefficient c. At the same time, a more positive value of (y)- (l) - (po) corresponds to lower proton concentration at the reaction plane, following a Boltzmann distribution (Equation 3.77). The magnitude of this effect is determined by the reaction order yh+ K is> therefore, of primary interest to know the difference of kinetic parameters, - yh+ ... 

Again, Eqns (1.49) and (1.50) are fairly complicated and contain all the reaction constants, the concentrations of both oxygen and protons, and the cathode potential. However, these two equations indicate that both pt OjH and Pt can be expressed as functions of the oxygen and proton concentrations, which are practically controllable and measurable. 

In many MFCs, the proton transfer efficiency from the anode to the cathode is the rate-limiting step and a major cause of internal resistance. Although equivalent amounts of protons and electrons are produced at the anode in MFCs, the migration rate of protons to the cathode is much slower than that of the electrons. It arises from the fact that the migration of electrons is forced by the potential difference between the two electrodes, while the migration of protons is caused by diffusion. A proton exchange membrane (PEM), if present, functions as a proton transfer barrier, and further decreases the proton diffusion rate. Since proton transport inside the fuel cell is slower than its production rate in the anode and its consumption rate in the cathode, a pH difference between the two electrodes occurs without buffer. For example, in the absence of any buffer solution, Gil et al. [76] detected a pH difference of 4.1 (9.5 at cathode and 5.4 at anode) after a 5-h operation with an initial pH of 7 in both chambers. Accumulation of protons at the anode will suppress the microbial activity, thus the electricity production, whereas a limited proton concentration at the cathode may reduce the cathodic reduction rate. 

Solutions of nitric acid in 100% sulphuric acid have a high electrical conductivity. If nitric acid is converted into a cation in these solutions, then the migration of nitric acid to the cathode should be observed in electrolysis. This has been demonstrated to occur in oleum and, less conclusively, in concentrated acid, observations consistent with the formation of the nitronium ion, or the mono- or di-protonated forms of nitric acid. Conductimetric measurements confirm the quantitative conversion of nitric acid into nitronium ion in sulphuric acid. ... 

It has been found50 that such a multielectron step does not exist with 58, which exhibits a classical two-electron scission. In general, allylic sulphones (59) without an unsaturated system in a suitable position are not reducible. Thus, they do not exhibit a cathodic step in protic solutions. However, in aprotic media the isomerization may be base catalyzed, since small amounts of electrogenerated bases from electroactive impurities, even at low concentration, may contribute to start the isomerization. Figure 10 shows the behaviour of t-butyl allylic sulphone which is readily transformed in the absence of proton donor. On the other hand, 60 is not isomerized but exhibits a specific step (Figure 10, curve a) at very negative potentials. 

Adiponitrile is readily hydrogenated catalytically to hexamethylenediamine, which is an important starting material for the prodnction of nylons and other plastics. The electrochemical production of adiponitrile was started in the United States in 1965 at present its volume is about 200 kilotons per year. The reaction occurs at lead or cadmium cathodes with current densities of np to 200 mA/cm in phosphate buffer solutions of pH 8.5 to 9. Salts of tetrabntylammonium [N(C4H9)4] are added to the solution this cation is specihcally adsorbed on the cathode and displaces water molecules from the first solution layer at the snrface. Therefore, the concentration of proton donors is drastically rednced in the reaction zone, and the reaction follows the scheme of (15.36) rather than that of (15.35), which wonld yield propi-onitrile. 

On the basis of theoretical calculations Chance et al. [203] have interpreted electrochemical measurements using a scheme similar to that of MacDiarmid et al. [181] and Wnek [169] in which the first oxidation peak seen in cyclic voltammetry (at approx. + 0.2 V vs. SCE) represents the oxidation of the leucoemeraldine (1 A)x form of the polymer to produce an increasing number of quinoid repeat units, with the eventual formation of the (1 A-2S")x/2 polyemeraldine form by the end of the first cyclic voltammetric peak. The second peak (attributed by Kobayashi to degradation of the material) is attributed to the conversion of the (1 A-2S")x/2 form to the pernigraniline form (2A)X and the cathodic peaks to the reverse processes. The first process involves only electron transfer, whereas the second also involves the loss of protons and thus might be expected to show pH dependence (whereas the first should not), and this is apparently the case. Thus the second peak would represent the production of the diprotonated (2S )X form at low pH and the (2A)X form at higher pH with these two forms effectively in equilibrium mediated by the H+ concentration. This model is in conflict with the results of Kobayashi et al. [196] who found pH dependence of the position of the first peak. 

A schematic diagram of the cation flow method for generating N-acyliminium ion 2 is shown in Fig. 5. A solution of carbamate 1 is introduced into the anodic compartment of electrochemical microflow cell, where oxidation takes place on the surface of a carbon fiber electrode. A solution of trifluoromethanesulfonic acid (TfOH) was introduced in the cathodic compartment, where protons are reduced to generate dihydrogen on the surface of a platinum electrode. A-Acyliminium ion 2 thus generated can be analyzed by an in-line FT-IR analyzer to evaluate the concentration of the cation. The solution of the cation is then allowed to react with a nucleophile such as allyltrimethylsilane in the flow system to obtain the desired product 3. 

Barba and coworkers [84-91] have published a number of papers dealing with novel syntheses based on the reduction of phenacyl bromides. Electrolysis of phenacyl bromide at a mercury cathode leads to an intermediate, which reacts to give 2,4-diphenylfuran [84]. However, when a proton donor (CH3OH) is present, the reduction of phenacyl bromide yields acetophenone and 2-bromo-l,3-diphenyl-3,4-epoxy-butan-l-one. Interestingly, if phenacyl bromide is slowly introduced into an electrolysis cell so that the unreduced starting material is maintained at a low concentration, the products are different [85] (Scheme 7). 

The direct reduction of haloalkynes using either mercury or vitreous carbon as the cathode has been examined in considerable detail [80-84] one example is portrayed in Eq (77). The influence of reduction potential, current consumption, proton donor, electrode, and substrate concentration on the course of the process has been examined. Vitreous carbon electrodes are preferred, though mercury has been used in many instances. Unfortunately, these reactions suffer from the formation of diorganomercurials. While both alkyl iodides and bromides can be used, the former is generally preferred. Because of their higher reduction potential, alkyl chlorides react via a different mechanism, one involving isomerization to an allene followed by cyclization [83]. 

The EOD coefficient, is the ratio of the water flux through the membrane to the proton flux in the absence of a water concentration gradient. As r/d,3g increases with increasing current density during PEMFC operation, the level of dehydration increases at the anode and normally exceeds the ability of the PEM to use back diffusion to the anode to achieve balanced water content in the membrane. In addition, accumulation of water at the cathode leads to flooding and concomitant mass transport losses in the PEMFC due to the reduced diffusion rate of O2 reaching the cathode. 

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