Slow electrode processes

Rather slow electrode processes (especially in the case of gas electrodes) which have low exchange current densities. At steady state, the overall rates are generally determined by the rates of charge transfer and/or of secondary chemical reactions at the electrode-melt interface. 

After having described one equilibrium method for the measurement of kinetics of rather fast electrode processes and two steady state methods for measurement of slow electrode processes, let us turn now to consider the so called transient or, sometimes, relaxation techniques, where time is a very important factor in the equations. 

The measurement of a from the experimental slope of the Tafel equation may help to decide between rate-determining steps in an electrode process. Thus in the reduction water to evolve H2 gas, if the slow step is the reaction of with the metal M to form surface hydrogen atoms, M—H, a is expected to be about If, on the other hand, the slow step is the surface combination of two hydrogen atoms to form H2, a second-order process, then a should be 2 (see Ref. 150). 

Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

How can such problems be counterbalanced Since a large capacitance of a semiconductor/electrolyte junction will not negatively affect the PMC transient measurement, a large area electrode (nanostructured materials) should be selected to decrease the effective excess charge carrier concentration (excess carriers per surface area) in the interface. PMC transient measurements have been performed at a sensitized nanostructured Ti02 liquidjunction solar cell.40 With a 10-ns laser pulse excitation, only the slow decay processes can be studied. The very fast rise time cannot be resolved, but this should be the aim of picosecond studies. Such experiments are being prepared in our laboratory, but using nanostructured... 

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. 

Cu9ln4 and Cu2Se. They performed electrodeposition potentiostatically at room temperature on Ti or Ni rotating disk electrodes from acidic, citrate-buffered solutions. It was shown that the formation of crystalline definite compounds is correlated with a slow surface process, which induced a plateau on the polarization curves. The use of citrate ions was found to shift the copper deposition potential in the negative direction, lower the plateau current, and slow down the interfacial reactions. 

Polarization is produced by the slow rate of at least one of the partial processes in the overall electrode process. If this rate-controlling step is a transport process, then concentration polarization is involved if it is the charge transfer reaction, then it is termed charge transfer polarization, etc. Electrode processes are often classified on this basis. 

Case (a) If the chemical reaction preceding the electrode reaction, C(a), and the electrode reaction itself, E(a), are sufficiently fast compared to the transport processes, then both of these reactions can be considered as equilibrium processes and the overall electrode process is reversible (see page 290). If reaction C(a) is sufficiently fast and E(a) is slow, then C(a) affects the electrode reaction as an equilibrium process. If C(a) is slow, then it becomes the rate-controlling step (a detailed discussion is given in Section 5.6.3). 

It is often useful to carry out voltammetric measurements at low temperatures in order to evaluate both the stability of an electrogenerated species (the decrease in temperature will slow down the kinetics of any decomposition processes of the species formed in the electrode process) and the variation in formal electrode potential of a redox couple as a function of temperature. The latter point regards thermodynamic considerations of the redox processes, which will be discussed in Chapter 13, Section 3. 

A fundamental improvement in the facilities for studying electrode processes of reactive intermediates was the purification technique of Parker and Hammerich [8, 9]. They used neutral, highly activated alumina suspended in the solvent-electrolyte system as a scavenger of spurious impurities. Thus, it was possible to generate a large number of dianions of aromatic hydrocarbons in common electrolytic solvents containing tetraalkylammonium ions. It was the first time that such dianions were stable in the timescale of slow-sweep voltammetry. As the presence of alumina in the solvent-electrolyte systems may produce adsorption effects at the electrode, or in some cases chemisorption and decomposition of the electroactive species, Kiesele constructed a new electrochemical cell with an integrated alumina column [29]. 

The suitability of boron-doped diamond as anode material for the generation of aggressive reagents, such as bromine, has been investigated. Vinokur et al. reported that the electron transfer at boron-doped diamond electrodes is strongly affected by the nature of the electrode process. Irmer-sphere processes such as the bromine evolution from bromide seem to be Id-netically slow, [73,120] in particular, when occurring positive of a potential of approximately —0.05 V vs. SCE. This is currently limiting possible wider applications of boron-doped diamond electrode materials. 

The mechanism of the reduction of cadmium ions at DME in NaCl04 solutions with varied water activity was also studied [26]. In these solutions, the electrode process of the Cd(II)/Cd(Hg) system was described by the mechanism that includes (1) fast loss of 12.5 water molecules in a preceding equihbrium, (2) a slow chemical step, which is not a desolvation, (3) slow transfer of the first electron. 

Pb Lead, and particularly underpotentially deposited Pb, exhibits electrocatalytic properties in numerous electrode processes. The model reaction can be oxygen reduction with slow step of peroxide reduction ]374-376] or reduction of nitrobenzene and other nitrocompounds [377, 378]. In the case of... 

Transport Processes. The velocity of electrode reactions is controlled by the charge-transfer rate of the electrode process, or by the velocity of the approach of the reactants, to the reaction site. The movement or trausport of reactants to and from the reaction site at the electrode interface is a common feature of all electrode reactions. Transport of reactants and products occurs by diffusion, by migration under a potential field, and by convection. The complete description of transport requires a solution to the transport equations. A full account is given in texts and discussions on hydrodynamic flow. Molecular diffusion in electrolytes is relatively slow. Although the process can be accelerated by stirring, enhanced mass transfer... 

Numerous examples could be cited in which two or more suspected products of an electrode process exhibit similar electrochemical behavior. In other instances, the species that is stable during the time required to complete the cyclic voltammetric experiment may undergo a slow chemical reaction to give the product that is isolated. These problems arise sufficiently frequently that the identification of products and the determination of the product distribution are required. 

Because the potential is more positive than E i, the formation of only one wave is observed. In acidic and alkaline media where the rate of the acid-base catalysed reaction (35 b) is fast and during the reaction all the phenylhydroxylamine derivative is transformed to quinoneimine, the height of the single wave corresponds to a transfer of six electrons [(35 a) plus (35 c)]. Because the life-time of the quinoneimine intermediate is short, its hydrolysis to form quinone does not affect the electrode process. In the medium pH range where the rate of dehydration is slow, the wave-height corresponds to a four-electron process. A theoretical... 

In these electrode processes, the use of macroelectrodes is recommended when the homogeneous kinetics is slow in order to achieve a commitment between the diffusive and chemical rates. When the chemical kinetics is very fast with respect to the mass transport and macroelectrodes are employed, the electrochemical response is insensitive to the homogeneous kinetics of the chemical reactions—except for first-order catalytic reactions and irreversible chemical reactions follow up the electron transfer—because the reaction layer becomes negligible compared with the diffusion layer. Under the above conditions, the equilibria behave as fully labile and it can be supposed that they are maintained at any point in the solution at any time and at any applied potential pulse. This means an independent of time (stationary) response cannot be obtained at planar electrodes except in the case of a first-order catalytic mechanism. Under these conditions, the use of microelectrodes is recommended to determine large rate constants. However, there is a range of microelectrode radii with which a kinetic-dependent stationary response is obtained beyond the upper limit, a transient response is recorded, whereas beyond the lower limit, the steady-state response is insensitive to the chemical kinetics because the kinetic contribution is masked by the diffusion mass transport. In the case of spherical microelectrodes, the lower limit corresponds to the situation where the reaction layer thickness does not exceed 80 % of the diffusion layer thickness. 

When the electrode processes given in reaction scheme (6.II) are slow, the Nersntian relationship given by Eq. (6.176) is not fulfilled, and the following relationships can be established for the time variation of the surface coverages ... 

In a series of papers between 1956 and 1965, Marcus solved much of the mystery by outlining a description of the probability of fluctuations in the geometry of reactants and their solvents. These fluctuations lead to changes in the energy barriers that the reactants must surmount before an electron can be transferred from one molecule to another. Marcus extended the theory to other systems, such as electrochemical rate constants at electrodes, and to chemiluminescent electron transfer reactions. The by-now famous inverted effect is a consequence of his theory after a certain point, adding more energy to an electron transfer reaction actually slows the process. Scientists believe photosynthesis can occur because of the inverted effect. 

The slowness of the electrode processes when a stronger current flows through the electrolyzer results in an increase of the back electromotive force Ev, above its theoretical value (i. e. above the EMF of the corresponding galvanic cell). In such an instance Ohm s law (see formula VII-1) will still be valid but the value Ev will no longer be constant but will increase in proportion to the current density and will also depend on the duration of the electrolysis. 

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