Electrode reaction order

Parsons, R., Electrode reaction orders, charge transfer coefficients and rate constants. Extension of definitions and recommendations for publication of parameters. Pure Appl. Chem., 52, 233 (1979). 

In the absence of excess of inert electrolyte, the concentration of an ionic reactant in the pre-electrode plane and the potential at this plane are interconnected and depend on the structure of the double layer. Thus, the apparent electrode reaction order will also be influenced by the double layer. 

Parsons, R., Electrode Reaction Orders, Transfer Coefficients and Rate Constants Amplifi-... 

One of the main uses of these wet cells is to investigate surface electrochemistry [94, 95]. In these experiments, a single-crystal surface is prepared by UFIV teclmiqiies and then transferred into an electrochemical cell. An electrochemical reaction is then run and characterized using cyclic voltaimnetry, with the sample itself being one of the electrodes. In order to be sure that the electrochemical measurements all involved the same crystal face, for some experiments a single-crystal cube was actually oriented and polished on all six sides Following surface modification by electrochemistry, the sample is returned to UFIV for... 

This difference is a measure of the free-energy driving force for the development reaction. If the development mechanism is treated as an electrode reaction such that the developing silver center functions as an electrode, then the electron-transfer step is first order in the concentration of D and first order in the surface area of the developing silver center (280) (Fig. 13). Phenomenologically, the rate of formation of metallic silver is given in equation 17,... 

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

In order for this concept to be applicable, the matrix and the reactant phase must be thermodynamically stable in contact with each other. One can evaluate this possibility if one has information about the relevant phase diagram — which typically involves a ternary system — as well as the titration curves of the component binary systems. In a ternary system, the two materials must lie at comers of the same constant-potential tie-triangle in the relevant isothermal ternary phase diagram in order to not interact. The potential of the tie-triangle determines the electrode reaction potential, of course. 

Electrochemical reaction orders in electrode polymerization, 317 Electrochemical relaxation, as a function of cathode potential, 388 Electrochemical responses during polymer formation, 400... 

As in chemical systems, however, the requirement that the reaction is thermodynamically favourable is not sufficient to ensure that it occurs at an appreciable rate. In consequence, since the electrode reactions of most organic compounds are irreversible, i.e. slow at the reversible potential, it is necessary to supply an overpotential, >] = E — E, in order to make the reaction proceed at a conveniently high rate. Thus, secondly, the potential of the working electrode determines the kinetics of the electron transfer process. 

It has been seen from the above simple examples that the concentration of the substrate has a profound effect on the rate of the electrode process. It must be remembered, however, that the process may show different reaction orders in the different potential regions of the i-E curve. Thus, electron transfer is commonly the slow step in the Tafel region and diffusion control in the plateau region and these processes may have different reaction orders. Even at one potential the reaction order may vary with the substrate concentration as, for example, in the case discussed above where the electrode reaction requires adsorption of the starting material. 

It is instructive at the present stage to study the comparison of zinc and copper electrodes against the hydrogen electrode in order to delineate the very basic and centrally important ingredients on which the essence of electrochemistry is based. For this purpose, a galvanic cell is assembled to study the reaction as shown below ... 

In the above process electrons will be consumed and will have to be generated by a net electron flow along the externally used connecting wire from the electrode Zn in the Cu-Zn cell. In order for electrons to be produced at the electrode Zn, the electrode reaction at Zn must be reversed ... 

The above-described theory, which has been extended for the transfer of protons from an oxonium ion to the electrode (see page 353) and some more complicated reactions was applied in only a limited number of cases to interpretation of the experimental data nonetheless, it still represents a basic contribution to the understanding of electrode reactions. More frequently, the empirical values n, k° and a (Eq. 5.2.24) are the final result of the investigation, and still more often only fcconv and cm (cf. Eq. 5.2.49) or the corresponding constant of the Tafel equation (5.2.32) and the reaction order of the electrode reaction with respect to the electroactive substance (Eq. 5.2.4) are determined. 

The overpotential of an electrode process (Eq. (5.1.11)) is given in the case of a simple first-order electrode reaction as... 

Similar to the rotating disk, the RHSE has the ability to determine the reaction order and reaction rate constants of an electrode reaction. Consider an electrochemical reaction of the type... 

The method permits the simultaneous determination of reaction order, m, and reaction rate constant, k, from the slope and the intercept of the straight line. The procedure can be repeated for various potential values below the limiting current plateau to yield k as a function of electrode potential. The exchange current density and the Tafel slope of the electrode reaction can be then evaluated from the k vs. potential curves. 

The mechanism is quite complex. In free electrolyte the reaction order in C02 is actually negative [30] the order in a functioning fuel cell, with gas-diffusion electrodes, rises to near zero [31]. This low order in C02 is essential in the efficient operation at very low C02 pressures as would be encountered in life-support. The MCFC has been... 

In order to determine the electrochemical properties of the solvent, the electrode process in molten carbamide and in carbamide-MeCl (where Me - NH4, K) mixtures on inert electrodes (platinum, glassy carbon) were investigated using cyclic voltammetry. The electrode reaction products were analysed by spectroscopic methods. The adsorbtion of carbamide- NH4CI anodic product was investigated by differential capacity method. 

This is a little more complex than the general scheme because another species, CN, appears on the left-hand side of the electrode reaction. However, this complication is easily dealt with The anodic current is proportional to c lN so by comparison with Eq. (11.21) we can identify m with the reaction order of CN. Prom Eq. (11.31) we have ... 

In order to explain all the salient features of the key experimental results on ECT (viz. listed as 1. to 6. at the beginning of Section II, Phenomenology of ECT), Vijh25 proposed a detailed electrochemical mechanism in which electroosmosis of the tissue (and thence water movement from anode to cathode) and electrode reactions (thence necrosis of the tissue, pH changes etc.) play the dominant roles. In particular, he presented a model and some quantitative considerations that delineate Nordenstrom s idea of electroosmosis through the narrow interstitial channels lined with fixed charges as the mechanism of the electrochemical destruction of the tumor tissue.10 Also he examined the role of electrode reactions and other events as possible contributory factors, as follows25 in Section III.2. 

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