Redox reactions at electrodes

Many redox reactions at electrodes involve transfer of more than one electron. It is agreed that such processes usually involve several consecutive one-electron steps rather than a simultaneous multi-electron transfer. The kinetics of the overall reaction (and hence the current flowing) are complicated by such factors as the lifetimes of the transient intermediate species. 

Redox electrode reactions on metal electrodes constitute the simpler case for a theoretical approach to the problem. In particular, outer sphere redox electrode reactions not involving specific adsorption interactions have been treated successfully in analogy with homogeneous redox reactions in solution [54, 56], Approximate extension of the theoretical approach to the case of inner sphere redox reactions at electrodes has been done [56, 57b]. 

The transmission coefficient k — 1 for weak overlap of electronic states of reactants and products in the transition state. It is strong enough to be adiabatic but yet weak enough for the free energy of activation not to have an appreciable contribution from the resonance energy. This condition is almost fulfilled by outer sphere redox reactions at electrodes. 

Recent preliminary measurements of the electrokinetics of simple redox reactions at electrodes at low temperatures (— 40 to — 120°C) have shown a decrease of the energy of activation with decreasing temperature, which may indicate the occurrence of nuclear tunnelling [77]. 

The behavior of b as f T) for ionic redox reactions at electrodes, especially those processes that involve only outer-sphere changes of state, and both red and ox species which are not specifically adsorbed, is of great interest. From some of Weaver s work information on the dependence of 6 on T for such reactions is available and some attempts have been made, e.g., by Parsons and Passeron, to establish if the potential-dependent factor in the electrochemical rate equation in fact includes a quadratic term in 77 as well as the usual linear one however, this is a different question (cf. Ref. 8) related to the harmonicity or, otherwise, of the fluctuations involved in the activation process. 

Further measurements need to be made on the temperature and potential dependence of the rates of simple ionic redox reactions at electrodes with proper corrections for double-layer effects at various temperatures, so that the temperature dependence of (3 for an elementary electron transfer reaction, without chemisorption and coupled atom transfer, would become better known. This is an essential requirement for progress in understanding the true significance of the temperature effects on electrode-kinetic behavior reliable experiments will not, however, be easy to accomplish and will require parallel double-layer studies over a range of temperatures. 

Kinetic problems can also affect redox reactions at electrodes when covalent substances are involved. For example, a practical hydrogen electrode uses specially prepared platinum with a high surface area to act as a catalyst for the dissociation of dihydrogen into atoms (see Topic J5). On other metals a high overpotential may be experienced, as a cell potential considerably larger than the equilibrium value is necessary for a reaction to occur at an appreciable rate. 

The question of whether the oscillator model is adequate to the real conditions of electron transfer in solution is still open for discussion. This concerns, in particular, the assumption of a fixed distance between the two ions during the electron transfer, which means neglecting the kinetic energy of relative motion of the two ions. There is as yet no experimental evidence for the reliability of the oscillator model of electron transfer in polar liquids. Some experiments concerning redox reactions at electrodes will be discussed later. A consideration of the oscillator model for proton transfer and its experimental proof will be given in the next section. 

The adiabatic redox reactions at electrodes were first considered by MARCUS /40a,145/ in a classical (semiclassical) framework. lEVICH, DOGONADZE and KUSNETSOV /146,147/, SGHMICKLER and VIELSTICH /169/ a.o. have developed a quantum theory for non-adiabatic electron transfer electrode reactions based on the oscillator-model. The complete quantum-mechanical treatment of the same model by CHRISTOV /37d,e/ comprises adiabatic and non-adiabatic redox reactions at electrodes. 

Four processes are usually involved in co-reactant ECL systems (as presented in Table 2.1) such as (a) redox reactions at electrode, (b) homogeneous chemical reactions, (c) excited-state species formation, and (d) light emission. Two types of redox reactions, namely heterogeneous and homogeneous redox reactions of co-reactants, are possible, which depend on the redox potential of the co-reactant and nature of the working electrode [2]. 

Interfacial processes redox reaction at electrodes, adsorption and electrosorption, kinetics of homogeneous reactions in solution combined with redox processes, forced mass transfer... 

The potential of a metallic electrode is determined by the position of a redox reaction at the electrode-solution interface. Three types of metallic electrodes are commonly used in potentiometry, each of which is considered in the following discussion. 

Nonfaradaic Currents Faradaic currents result from a redox reaction at the electrode surface. Other currents may also exist in an electrochemical cell that are unrelated to any redox reaction. These currents are called nonfaradaic currents and must be accounted for if the faradaic component of the measured current is to be determined. 

Determining Concentration bet s assume that the redox reaction at the working electrode is... 

For an interfering redox reaction at an ion-selective membrane, the overpotential t B can be easily determined experimentally. It is the potential difference between the ion-selective membrane and an inert redox electrode in the same solution containing the measured ion and an interfering redox system. 

S.3.3 Electrocatalytic Modified Electrodes Often the desired redox reaction at the bare electrode involves slow electron-transfer kinetics and therefore occurs at an appreciable rate only at potentials substantially higher than its thermodynamic redox potential. Such reactions can be catalyzed by attaching to the surface a suitable electron transfer mediator (45,46). Knowledge of homogeneous solution kinetics is often used to select the surface-bound catalyst. The function of the mediator is to facilitate the charge transfer between the analyte and the electrode. In most cases the mediated reaction sequence (e.g., for a reduction process) can be described by... 

In redox reactions, the electrode is not inert in the full meaning of the term. It serves not only to feed current through the electrolyte but also acts as a catalyst (as a catalytic electrode ) determining the rates and special features of electrochemical reactions occurring at its surface. 

It follows from the Franck-Condon principle that in electrochemical redox reactions at metal electrodes, practically only the electrons residing at the highest occupied level of the metal s valence band are involved (i.e., the electrons at the Fermi level). At semiconductor electrodes, the electrons from the bottom of the condnc-tion band or holes from the top of the valence band are involved in the reactions. Under equilibrium conditions, the electrochemical potential of these carriers is eqnal to the electrochemical potential of the electrons in the solution. Hence, mntnal exchange of electrons (an exchange cnrrent) is realized between levels having the same energies. 

Metal/metal oxides are the materials of choice for construction of all-solid-state pH microelectrodes. A further understanding of pH sensing mechanisms for metal/metal oxide electrodes will have a significant impact on sensor development. This will help in understanding which factors control Nemstian responses and how to reduce interference of the potentiometric detection of pH by redox reactions at the metal-metal oxide interface. While glass pH electrodes will remain as a gold standard for many applications, all-solid-state pH sensors, especially those that are metal/metal oxide-based microelectrodes, will continue to make potentiometric in-vivo pH determination an attractive analytical method in the future. 

Reduction to the neutral radical appears as an irreversible wave at -0.9 V. Neither anodic peak exhibits the shape characteristic of stripping a solid coating from the electrode hence precipitation of the radical cation or neutral radical on the electrode is not evident (11-13). The sharp peaks at +0.46 V are tentatively assigned to desorption and adsorption of the CiebpyMe2 there are no anticipated redox reactions at that potential. 

Spectroelectrochemical methods refer to spectroscopically probing nnique chemical species that are generated in situ during redox reactions at or near electrode... 

The polarization curves shown in Fig. 8-27 correspond to Eqns. 8-62 to 8-65 while the electrode is in the state of band edge level pinning. As examples, Fig. 8-28 shows the cathodic polarization curves of several redox reactions at an... 

Equation (2.1) defines current as the rate of charge movement. An electroanalyst could have re-expressed equation (2.1) with, in words, the magnitude of an electrochemical current represents the number of electrons consumed or collected per second . Each electron consumed or collected represents a part of a heterogeneous redox reaction at an electrode (equations (2.3) or (2.4)), so the magnitude of the current also tells us about the amounts of material consumed or formed at the electrode surface per unit time. 

To appreciate that dynamic electrochemistry implies that concentration changes occur in response to redox reactions at the electrode of interest. 

We will now look at UV-vis and EPR (microwave) spectroscopy in the context of redox reactions at the electrode in an analytical investigation. 

Figure 9. Schematic porous electrode structure (A) Electrons from the external circuit flow in the current collector which has contact to the conductive matrix in the electrode structure. The redox reaction at the electrode produces electrons that enter the external circuit and flow through the load to the cathode, where the reduction reaction at the cathode accepts the electron from the external circuit and the reduction reaction. The ions in the electrolyte carry the current through the device. (B) The reaction distribution in the porous electrode is shown for the case where the conductivity of the electrode matrix is higher than the conductivity of the electrolyte.

The electric current is caused by a pair of redox reactions. At the zinc electrode, the metallic zinc is slowly being dissolved by an oxidation reaction ... 

The quantitative laws of electrochemistry were discovered by Michael Faraday of England. His 1834 paper on electrolysis introduced many of the terms that you have seen throughout this book, including ion, cation, anion, electrode, cathode, anode, and electrolyte. He found that the mass of a substance produced by a redox reaction at an electrode is proportional to the quantity of electrical charge that has passed through the electrochemical cell. For elements with different oxidation numbers, the same quantity of electricity produces fewer moles of the element with higher oxidation number. 

We will study two broad classes of indicator electrodes. Metal electrodes described in this section develop an electric potential in response to a redox reaction at the metal surface. Ion-selective electrodes, described later, are not based on redox processes. Instead, selective binding of one type of ion to a membrane generates an electric potential. 

---