Electrode-electrolyte interface Faradaic processes

Application of the foregoing relations to the study of adsorption at electrode surfaces requires an understanding of the electrochemical processes at electrode-solution interfaces. Consider an electrode in contact with a solution containing electroactive species along with supporting electrolytes. Two important processes occur at the electrode surface a faradaic process in which electrons are transferred across the electrodesolution interface (oxidation-reduction reaction). As a result of these reactions current flows through the medium. Adsorption-desorption is... 

Improved charge transfer capacity is commonly estimated by using a reversible charge injection process through either double layer capacitive reactions and reversible faradaic charge transfer reactions at the electrode/electrolyte interface as... 

However, if the kinetics of charge transfer at the electrode/ electrolyte interface are so rapid that the electrochemical reactants and products stay in equilibrium at the electrode surface even though a current passes, the Nernst equation still applies to the surface concentrations. Such a process is said to be electrochemically reversible or Nernstian - sometimes written with a lower case n, a mark of distinction also accorded to the adjectives coulombic, ohmic and faradaic. 

The conventional electrical model of an electrochemical cell that represents the electrode-electrolyte interface (EEI) includes the association of resistances with capacitance as shown in Fig. 1. The parallel elements are related to the total current through the working electrode that is the sum of distinct contributions from the faradaic process and double-layer charging. The double layer capacitance resembles a pure capacitance, represented in the equivalent circuit by the element C and the faradaic process represented by a resistance, R2. The parameters E and Ri represent the equilibrium potential and the electrolyte resistance, respectively. 

Fig. 1 The electrode/electrolyte interface, iUustiatmg Faradaic chaige transfer (top) and capacitive redistribution of chaige (bottom) as the electrode is driven negative, (a) Physical representation (b) Two-element electrical circuit model for mechanisms of charge transfer at the interface. The capacitive process involves reversible redistribution of chaige. The Faradtiic process involves transfer of electrons from the metal electrode, reducing hydrated cations in solution (symbolically 0 + e R, where the cation O is the oxidized form of the redox couple O/R). An example reaction is the reduction of silver ions in solution to form a silver plating on the electrode, reaction (8a). Faradaic charge injection may or may not be reversible...

An ideal electrode-electrolyte interface with an electron-transfer process can be described using Randle equivalent circuit shown in Fig. 2.7. The Faradaic electron-transfer reaction is represented by a charge transfer resistance and the mass transfer of the electroactive species is described by Warburg element (W). The electrolyte resistance R is in series with the parallel combination of the double-layer capacitance Cdi and an impedance of a Faradaic reaction. However, in practical application, the impedance results for a solid electrode/electrolyte interface often reveal a frequency dispersion that cannot be described by simple Randle circuit and simple electronic components. The interaction of each component in an electrochemical system contributes to the complexity of final impedance spectroscopy results. The FIS results often consist of resistive, capacitive, and inductive components, and all of them can be influenced by analytes and their local environment, corresponding to solvent, electrolyte, electrode condition, and other possible electrochemically active species. It is important to characterize the electrode/electrolyte interface properties by FIS for their real-world applications in sensors and energy storage applications. 

The formation of 2D Meads phases on a foreign substrate, S, in the underpotential range can be well described considering the substrate-electrolyte interface as an ideally polarizable electrode as shown in Section 8.2. In this case, only sorption processes of electrolyte constituents, but no Faradaic redox reactions or Me-S alloy formation processes are allowed to occur, The electrochemical double layer at the interface can be thermodynamically considered as a separate interphase [3.54, 3.212, 3.213]. This interphase comprises regions of the substrate and of the electrolyte with gradients of intensive system parameters such as chemical potentials of ions and electrons, electric potentials, etc., and contains all adsorbates and all surface energy. Furthermore, it is assumed that the chemical potential //Meads is a definite function of the Meads surface concentration, F, and the electrode potential, E, at constant temperature and pressure Meads (7", ). Such a model system can only be... 

The situation should then be favourable if the ion electric carriers would take part in the Faradaic process at the inner botmdary inside the film. The ions could pass across the film discharging at the metal/film interface or, vice versa, liberating in the Faradaic process at this interface, and passing through the film into the bulk of the electrolyte. The processes at the negative electrode of a lithium battery (Li metal or Li/C intercalate compound) could be an example of such situation. 

To induce this reaction, the kinetic inhibition of the reaction must be overcome by applying an overpotential, which must be minimized. This reaction, in which electrons are transferred across the metal-solution interface with a resulting nitrate reduction, is called a faradaic process. Also, the complexity of the interfacial system is such that other phenomena do occur that can affect the electrode behavior. These processes include adsorption, desorption, and charging of the interface as a result of changing electrode potential these are called non-faradaic processes. Both the efficiency and the selectivity of nitrate electroreduction strongly depend on several parameters such as the electrode composition, physicochemical properties of the electrolyte (pH, coexisting species, temperature, etc.) and the applied potential. 

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