Anodic processes, kinetics

Liu P, Nprskov JK. 2001a. Kinetics of the anode processes in PEM fuel cells— The promoting effect of Ru in PtRu anodes. Fuel Cells 1 192. 

The basic relationships of electrochemical kinetics are identical with those of chemical kinetics. Electrochemical kinetics involves an additional parameter, the electrode potential, on which the rate of the electrode reaction depends. The rate of the electrode process is proportional to the current density at the studied electrode. As it is assumed that electrode reactions are, in general, reversible, i.e. that both the anodic and the opposite cathodic processes occur simultaneously at a given electrode, the current density depends on the rate of the oxidation (anodic) process, ua, and of the reduction (cathodic) process, vc, according to the relationship... 

Suppression of Photocorrosion of Photoanodes and Manipulation of Kinetics for Anodic Processes... 

Steady-State Kinetics, There are two electrochemical methods for determination of the steady-state rate of an electrochemical reaction at the mixed potential. In the first method (the intercept method) the rate is determined as the current coordinate of the intersection of the high overpotential polarization curves for the partial cathodic and anodic processes, measured from the rest potential. In the second method (the low-overpotential method) the rate is determined from the low-overpotential polarization data for partial cathodic and anodic processes, measured from the mixed potential. The first method was illustrated in Figures 8.3 and 8.4. The second method is discussed briefly here. Typical current—potential curves in the vicinity of the mixed potential for the electroless copper deposition (average of six trials) are shown in Figure 8.13. The rate of deposition may be calculated from these curves using the Le Roy equation (29,30) ... 

A comparison of the E°s would lead us to predict that the reduction (it) would be favored over that of (i). This is certainly the case from a purely energetic standpoint, but as was mentioned in the section on fuel cells, electrode reactions involving 02 are notoriously slow (that is, they are kinetically hindered), so the anodic process here is under kinetic rather than thermodynamic control. The reduction of water (iv) is energetically favored over that of Na+ (iii), so the net result of the electrolysis of brine is the production of Cl2 and NaOH ( caustic ), both of which are of immense industrial importance ... 

To model all the particular processes, kinetics expressions proposed in Sect. 4.4 are used. As an example, Fig. 4.4 shows the application of the model to a particular case (Canizares et al. 2004a) the oxidation of phenol in a cell with nonactive anodes. 

The electroless deposition of metals on a silicon surface in solutions is a corrosion process with a simultaneous metal deposition and oxidation/dissolution of silicon. The rate of deposition is determined by the reduction kinetics of the metals and by the anodic dissolution kinetics of silicon. The deposition process is complicated not only by the coupled anodic and cathodic reactions but also by the fact that as deposition proceeds, the effective surface areas for the anodic and cathodic reactions change. This is due to the gradual coverage of the metal deposits on the surface and may also be due to the formation of a silicon oxide film which passivates the surface. In addition, the metal deposits can act as either a catalyst or an inhibitor for hydrogen evolution. Furthermore, the dissolution of silicon may significantly change the surface morphology. 

The MCFC anodes are made from a porous sintered nickel with a thickness of 0.8-1.0 mm and a porosity of 55-70% with a mean pore diameter of 5pm. This porosity range provides adequate interconnected pores for mass transport of gaseous reactants and adequate surface area for the anodic electrocatalytic reactions. Because the anode kinetics is faster than that of the cathode, less active surface area is sufficient for the anodic process. Partial flooding of the comparatively thick anode is therefore acceptable at the anode interface. 

Fluorine is produced by electrolysis of molten salts on carbon anodes including KF-21TF at about 100°C, potassium bifluoride at about 250°C, and fluoride salts at about 1000°C. The decomposition potential of molten potassium bifluoride is 1.75 V at 250°C, a value close to that estimated thermodynamically [80]. The kinetics of the anodic process is characterized by a Tafel slope of 0.56 V per decade, j), = 1 x 10 A/cm [81], and by a complex reaction mechanism involving the formation of fluorine atoms on carbon. During the electrolysis, C-F surface compounds on the carbon anode are formed via side reactions. Intercalation compounds such as (CF) contribute to the anodic effect in the electrochemical cell, which can be made less harmful by addition of LiF. 

Since the electrocatalytic reaction implies the existence of an adsorbed species as an intermediate, reactant, or product, the direct interaction with the electrode surface has to be considered first. In this sense, the kinetics of the formation and the stability of the adsorbate are of great importance and may be the determining step for the final value of The slow adsorption kinetics in the case of a reactive adsorbate will make the reaction at the electrocatalyst not fast enough to become operative. However, the same situation can occur in the case of an adsorbed product with a slow desorption kinetics. The most problematic situation can arise due to the stability of an adsorbed intermediate on the surface, which is the rate-determining step of the whole process. In the case of an anodic process, the species desorption can be aided by the presence of a metal oxide on the surface. An interesting example of stable and efficient anodes is the dimensionally stable electrodes (DSE) used in brine... 

In this paper, we present a method of fabrication of periodic nanostmctured layers in the thin film system Al/Ta/Al. Main kinetic features of the anodization process and properties of the nanostmctures formed have been studied. 

Ozone is a very strong oxidizer with a high potential of 2.07V. It has a high reactivity and a short stability, and its solubility in water is exceedingly modest. Therefore, it is more practicable to inject ozone as gas into the unsaturated zone of the soil than to try to create a solution with ozone. Transport of dissolved ozone in the soil-water phase is only possible by electro-osmosis, and due to the short lifetimes, only short distances are possible. Ozone is a by-product in many anodic processes, and the amount of its production is directly proportional to the applied current density. The more efficient way to produce ozone uses classical corona discharge ozone generators. Ozone is not the optimal mediator for electro-kinetic remediation, but an alternative or additional method for a remediation concept using aeration techniques. Ozone works as a direct oxidant and as well as a free radical ... 

A possible side reaction to be avoided is the decomposition of the solvent, which in most cases is water. Therefore, it has to be made sure that the pH-dependent potential stability boundaries of water are not exceeded in order to assure 100% current efficiency for the analytical process. For reductions it is often beneficial, as it is in other electrochemical methods, to use mercury electrodes because the reduction of water, i.e., the evolution of hydrogen, is kinetically hindered on this material. Therefore, the reduction of species such as metals with redox potentials more negative than that of hydrogen is enabled. For anodic processes, i.e., oxidations, the... 

Measurements in aqueous electrolyte solutions at elevated temperatures and pressures. By extending kinetic studies to higher temperatures and pressures, the rate constants can be increased to values at which it should be possible to carry out kinetic measurements at potentials near the reversible values, where the anodic and cathodic processes are complementary. From such measurements it should be possible to obtain stoichiometric numbers and complementary reaction orders for the cathodic and anodic processes. Electrode stability and particularly the solubility may prove to be a problem. It also does not necessarily follow that the mechanism and rds will be the same as at lower temperatures. 

Gusev, N. I., and Rakcheev, P. V. (I%8). Kinetics of anodic processes and polarization of a rotating zinc electrode in zinc sulfate and sodium perchlorate. Z. Fiz. Khim., 42(%), 1983-1986 (in Russian). 

At noble metals, the growth of submonolayer and monolayer oxides can be studied in detail by application of electrochemical techniques such as cyclic-voltammetry, CV 11-20) and such measurements allow precise determination of the oxide reduction charge densities. Complementary X-Ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), infra-red (IR) or elUpsommetry experiments lead to elucidation of the oxidation state of the metal cation within the oxide and estimation of the thickness of one oxide monolayer 12,21-23), Coupling of electrochemical and surface-science techniques results in meaningful characterization of the electrified solid/liquid interface and in assessment of the relation between the mechanism and kinetics of the anodic process under scrutiny and the chemical and electronic structure of the electrode s surface 21-23). 

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