Electrodes affect electron transfer

S.2.2 Carbon Electrodes Solid electrodes based on carbon are currently in widespread use in electroanalysis, primarily because of their broad potential window, low background current, rich surface chemistry, low cost, chemical inertness, and suitability for various sensing and detection applications. In contrast, electron-transfer rates observed at carbon surfaces are often slower than those observed at metal electrodes. The electron-transfer reactivity is strongly affected by the origin... 

There are several ways in which the solvent-supporting electrolyte system can influence mass transfer, the electrode reaction (electron transfer), and the chemical reactions that are coupled to the electron transfer. The diffusion of an electroactive species will be affected not only by the viscosity of the medium but also by the strength of the solute-solvent interactions that determine the size of the solvation sphere. The solvent also plays a crucial role in proton mobility water and other protic solvents produce a much higher proton mobility because of fast solvent proton exchange, a phenomenon that does not exist in aprotic organic solvents. 

Third, many pretreatment methods have been developed for activating carbon electrodes for electron transfer. Pretreatments often affect more than one electrode property (e.g. surface chemistry and microstructure) making it difficult to elucidate structure-function relationships. As a consequence of the variable nature of pretreatments, carbon electrode surfaces tend to vary greatly from laboratory to laboratory and from day to day, complicating one s ability to relate the electrode surface structure to the electrochemical response [1,5,12-16],... 

VI. FACTORS AFFECTING ELECTRON TRANSFER AT DIAMOND ELECTRODES... 

Diminution of the diffusion layer aroimd an electrode, which affects electron transfer and allows higher current densities to be achieved. 

Influence of the Kinetics of Electron Transfer on the Faradaic Current The rate of mass transport is one factor influencing the current in a voltammetric experiment. The ease with which electrons are transferred between the electrode and the reactants and products in solution also affects the current. When electron transfer kinetics are fast, the redox reaction is at equilibrium, and the concentrations of reactants and products at the electrode are those specified by the Nernst equation. Such systems are considered electrochemically reversible. In other systems, when electron transfer kinetics are sufficiently slow, the concentration of reactants and products at the electrode surface, and thus the current, differ from that predicted by the Nernst equation. In this case the system is electrochemically irreversible. 

Determining Equilibrium Constants for Coupled Chemical Reactions Another important application of voltammetry is the determination of equilibrium constants for solution reactions that are coupled to a redox reaction occurring at the electrode. The presence of the solution reaction affects the ease of electron transfer, shifting the potential to more negative or more positive potentials. Consider, for example, the reduction of O to R... 

The principles outlined above are, of course, important in electro-synthetic reactions. The pH of the electrolysis medium, however, also affects the occurrence and rate of proton transfers which follow the primary electron transfer and hence determine the stability of electrode intermediates to chemical reactions of further oxidation or reduction. These factors are well illustrated by the reduction at a mercury cathode of aryl alkyl ketones (Zuman et al., 1968). In acidic solution the ketone is protonated and reduces readily to a radical which may be reduced further only at more negative potentials. 

The orientation of the electroactive substance (the material undergoing electron-transfer at the electrode) with respect to the electrode surface can very substantially affect its electrochemical reactivity. This ought not be surprising electron transfer is a heterogeneous process, and ought therefore to be substantially dependent upon the exact nature of the contact between the electroactive species and the electrode. Orientational effects ought to be particularly important when the electroactive species is adsorbed upon, and hence in intimate contact with, the electrode surface. What kinds of effects are associated with orientation of substances at an electrode surface Generally what one observes is... 

Very often, the electrode-solution interface can be represented by an equivalent circuit, as shown in Fig. 5.10, where Rs denotes the ohmic resistance of the electrolyte solution, Cdl, the double layer capacitance, Rct the charge (or electron) transfer resistance that exists if a redox probe is present in the electrolyte solution, and Zw the Warburg impedance arising from the diffusion of redox probe ions from the bulk electrolyte to the electrode interface. Note that both Rs and Zw represent bulk properties and are not expected to be affected by an immunocomplex structure on an electrode surface. On the other hand, Cdl and Rct depend on the dielectric and insulating properties of the electrode-electrolyte solution interface. For example, for an electrode surface immobilized with an immunocomplex, the double layer capacitance would consist of a constant capacitance of the bare electrode (Cbare) and a variable capacitance arising from the immunocomplex structure (Cimmun), expressed as in Eq. (4). 

A first type of reaction that may affect the first electron transfer intermediate is its reduction (or oxidation) at the electrode. In most cases, the second electron transfer is energetically more costly than the first (for a discussion of exceptions to this rule, see Section 1.5). The two processes thus occur at successive values of the electrode potential. There is therefore no difficulty in preventing the occurrence of the second reaction by an appropriate adjustment of the electrode potential. 

For the electron transfer of hydrated redox particles (the outer-sphere electron transfer), the electrode acts merely as a source or sink of electrons transferring across the compact double layer so that the nature of the electrode hardly affects the reaction kinetics this lack of influence by the electrode has been observed for the ferric-ferrous redox reaction. On the other hand, the electron transfer of adsorbed redox particles (the inner-sphere electron transfer) is affected by the state of adsorption so that the nature of the electrode exerts a definite influence on the reaction kinetics, as has been observed with the hydrogen electrode reaction where the reaction rate depends on the property of electrode. 

Several important energy-related applications, including hydrogen production, fuel cells, and CO2 reduction, have thrust electrocatalysis into the forefront of catalysis research recently. Electrocatalysis involves several physiochemical environmental dfects, which poses substantial challenges for the theoreticians. First, there is the electric potential which can aifect the thermodynamics of the system and the kinetics of the electron transfer reactions. The electrolyte, which is usually aqueous, contains water and ions that can interact directly with a surface and charged/polar adsorbates, and indirectly with the charge in the electrode to form the electrochemical double layer, which sets up an electric field at the interface that further affects interfacial reactivity. 

The kineties of eleetron-transfer reactions, which is also affected by the electrode potential and the metal-water interface, is more difficult and complex to treat than the thermodynamic aspects. While the theoretical development for electron transfer kinetics began decades ago, a practical implementation for surface reactions is still unavailable. Popular transition state-searching techniques such as the NEB method are not designed to search for minimum-energy reaction paths subject to a constant potential. Approximations that allow affordable quantum chemistry calculations to get around this limitation have been proposed, ranging from the electron affinity/ionization potential matching method to heuristic arguments based on interpolations. 

Bardeen considers two separate subsystems first. The electronic states of the separated subsystems are obtained by solving the stationary Schrodinger equations. For many practical systems, those solutions are known. The rate of transferring an electron from one electrode to another is calculated using time-dependent perturbation theory. As a result, Bardeen showed that the amplitude of electron transfer, or the tunneling matrix element M, is determined by the overlap of the surface wavefunctions of the two subsystems at a separation surface (the choice of the separation surface does not affect the results appreciably). In other words, Bardeen showed that the tunneling matrix element M is determined by a surface integral on a separation surface between the two electrodes, z = zo. 

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. 

Hydration-dehydration equilibria can affect the heights of polarographic waves when they precede the electron transfer, or are interposed between two electron transfers or shift the values of half-wave potentials when they follow after a reversible electrode process. 

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