Electrode kinetics limitation

Overvoltage. Overvoltage (ti. ) arises from kinetic limitations or from the inherent rate (be it slow or fast) of the electrode reaction on a given substrate. The magnitude of this value can be generally expressed in the form of the Tafel equation... 

Electrode kinetics lend themselves to treatment usiag the absolute reaction rate theory or the transition state theory (36,37). In these treatments, the path followed by the reaction proceeds by a route involving an activated complex where the element determining the reaction rate, ie, the rate limiting step, is the dissociation of the activated complex. The general electrode reaction may be described as ... 

The rate of an electrochemical process can be limited by kinetics and mass transfer. Before considering electrode kinetics, however, an examination of the nature of the iaterface between the electrode and the electrolyte, where electron-transfer reactions occur, is ia order. 

In addition to electrode kinetics, the rate of an electrochemical reaction can be limited by the rate of mass transfer of reactants to and from the electrode surface. In dilute solutions, four principal equations are used. The flux of species i is... 

The values of exchange current density observed for different electrodes (or reactions) vary within wide limits. The higher they are (or the more readily charges cross the interface), the more readily will the equilibrium Galvani potential be established and the higher will be the stability of this potential against external effects. Electrode reactions (electrodes) for which equilibrium is readily established are called thermodynamically reversible reactions (electrodes). But low values of the exchange current indicate that the electrode reaction is slow (kinetically limited). 

Each of the intermediate electrochemical or chemical steps is a reaction of its own (i.e., it has its own kinetic pecnliarities and rules. Despite the fact that all steps occur with the same rate in the steady state, it is true that some steps occur readily, without kinetic limitations, and others, to the contrary, occur with limitations. Kinetic limitations that are present in electrochemical steps show up in the form of appreciable electrode polarization. It is a very important task of electrochemical kinetics to establish the nature and kinetic parameters of the intermediate steps as well as the way in which the kinetic parameters of the individual steps correlate with those of the overall reaction. 

The first limitation is related to interference of the anode and the cathode. The finite permeability of the Nation membrane to fuel and oxygen results in crossover of fuel from the anode to the cathode, and oxygen crossover in the opposite direction. This may have a significant influence on electrode kinetics. 

As with previous kinetic applications of SECM, it should be noted that experimental measurements can be tuned to the kinetic region of interest by varying the radius of the electrode [Eq. (33)] and the separation between the tip and interface. In essence, the smaller the UME, and/or tip-interface separation, the higher the diffusion rates that may be generated and, consequently, the greater the tendency for interfacial kinetic limitations. 

It is convenient to distinguish three components of the overpotential, r. Two of these are associated respectively with mass-transfer restrictions in the electrolyte near the electrode (concentration overpotential, f/c), and with kinetic limitations of the reaction taking place at the electrode surface (surface overpotential, rjs) the third one is related to ohmic resistance. 

A number of metal porphyrins have been examined as electrocatalysts for H20 reduction to H2. Cobalt complexes of water soluble masri-tetrakis(7V-methylpyridinium-4-yl)porphyrin chloride, meso-tetrakis(4-pyridyl)porphyrin, and mam-tetrakis(A,A,A-trimethylamlinium-4-yl)porphyrin chloride have been shown to catalyze H2 production via controlled potential electrolysis at relatively low overpotential (—0.95 V vs. SCE at Hg pool in 0.1 M in fluoroacetic acid), with nearly 100% current efficiency.12 Since the electrode kinetics appeared to be dominated by porphyrin adsorption at the electrode surface, H2-evolution catalysts have been examined at Co-porphyrin films on electrode surfaces.13,14 These catalytic systems appeared to be limited by slow electron transfer or poor stability.13 However, CoTPP incorporated into a Nafion membrane coated on a Pt electrode shows high activity for H2 production, and the catalysis takes place at the theoretical potential of H+/H2.14... 

A simple example of the redox behaviour of surface-bound species can be seen in Figure 2.17, which shows the behaviour of a bare platinum electrode in N2-saturated aqueous sulphuric acid when a saw tooth potential is applied. There are two clearly resolved redox processes between 0.0 V and 0.4 V, and these are known to correspond to the formation and removal of weakly and strongly bound hydride, respectively (see section on the platinum CV in chapter 3). The peak currents of the cathodic and anodic reactions for these processes occur at the same potential indicating that the processes are not kinetically limited and are behaving in essentially an ideal Nernstian fashion. The weakly bound hydride is thought to be simply H atoms adsorbed on top of the surface Pt atoms, such that they are still exposed to the... 

In a detailed rotating-disk electrode study of the characteristic currents were found to be under mixed control, showing kinetic as well as diffusional limitations [Ha3]. While for low HF concentrations (<1 M) kinetic limitations dominate, the regime of high HF concentrations (> 1 M) the currents become mainly diffusion controlled. However, none of the relevant currents (J1 to J4) obeys the Levich equation for any values of cF and pH studied [Etl, Ha3]. According to the Levich equation the electrochemical current at a rotating disk electrode is proportional to the square root of the rotation speed [Le6], Only for HF concentrations below 1 mol 1 1 and a fixed anodic potential of 2.2 V versus SCE the traditional Levich behavior has been reported [Cal 3]. 

In order to prevent such a limitation, it is advisable for the counter electrode to be as large as practicable, and certainly larger than the working electrode. With a large CE, the magnitude of the working-electrode current reflects only the flux of analyte solution, thereby obviating the kinetic limitations due to the counter electrode. 

In general, the electrochemical performance of carbon materials is basically determined by the electronic properties, and given its interfacial character, by the surface structure and surface chemistry (i.e. surface terminal functional groups or adsorption processes) [1,2]. Such features will affect the electrode kinetics, potential limits, background currents and the interaction with molecules in solution [2]. From the point of view of electroanalysis, the remarkable benefits of CNT-modified electrodes have been widely praised, including low detection limits, increased sensitivity, decreased overpotentials and resistance to surface fouling [5, 9, 11, 17]. 

However, as we saw in section 3.3 for platinum on YSZ, the fact that i—rj data fits a Butler—Volmer expression does not necessarily indicate that the electrode is limited by interfacial electrochemical kinetics. Supporting this point is a series of papers published by Svensson et al., who modeled the current—overpotential i—rj) characteristics of porous mixed-conducting electrodes. As shown in Figure 28a, these models take a similar mechanistic approach as the Adler model but consider additional physics (surface adsorption and transport) and forego time dependence (required to predict impedance) in order to solve for the full nonlinear i—rj characteristics at steady state. 

Consideration of these asymptotic limits is useful because it potentially helps us to identify, discuss, and study the various physical processes underlying electrode kinetics, even outside those limits. How-... 

In [119], the hydrogen adsorption and desorption reactions in thin palladium electrodes were studied using the potential step method in order to analyze the mechanism of phase transformation. Transient current responses were recorded at the onset of the potential step for 47 pm thick Pd electrodes in 1 mol dm H2SO4 at ambient temperature. A model based on a moving boundary mechanism was proposed to account for the experimental i-t curves. It was found that the hydrogen adsorption reaction shows interfacial kinetic limitations and only numerical solutions can be obtained. Such kinetic limitations were not found for the desorption reaction and a semianalytical solution that satisfactorily fits the experimental data was proposed. 

What are ways oul of this extreme sensitivity to impurities in electrode kinetics for most electrochemical reactions One way to reduce the effects of trace impurities from the solution in electrode kinetics is to use a liquid electrode because such electrodes can be made to form drops, the lifetime of which is small, so that the impurities from the solution don t have time to adsorb on the drops before they break off from the elechxxle. The electrode material then has to be mercury (the only metal that is liquid at room temperatures), so this approach is limited because mercury is a poor catalyst and one wishes particularly to work with electrode materials that catalyze electrode reactions well.27... 

Use of the lower time would give a big advantage in respect to the upper limits of current density at which an electrode kinetic measurement can be made See of diffusion control. At 0.1 ms, the current density will be free of diffusion effects because it is 100 times higher than that at 1 s, when diffusion will in any case affect the measurement (Fig. 8.4). 

In deriving eqn. (80), limitations due to mass transport at the interface were not considered. Strictly speaking, this is not realistic and as the reaction rate increases with overpotential in each direction a variation of the concentrations of reactant and product at the surface operates and concentration polarization becomes more important. Each exponential expression in eqn. (80) must be multiplied by the ratio of surface to bulk concentrations, ci s/ci b. The effect of mass transfer in electrode kinetics has been discussed in Sect. 2.4. 

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