Electrode-electrolyte interface Faradaic charge transfer

Faradaic current — A -> current can flow through the external circuit connecting the -> electrodes of an - electrochemical cell for two reasons. First, electrons or ions cross the electrode-electrolyte -> interfaces, and these charge transfer steps (- charge transfer reaction) are accompanied by oxidation reactions at the... 

Figure 3.1 shows a typical equivalent circuit of an electrochemical cell. Rel represents the electrolyte resistance between the working electrode surface and the point of reference electrode Cd is a pure capacitor of the capacity associated with the double layer of the electrode/electrolyte interface and Zf refers to the Faradaic impedance, which corresponds to the impedance of the charge transfer at the electrode/electrolyte interface. The connection of X, and Cd in Figure 3.1 is in parallel. The impedance X, can be subdivided in two equivalent ways, as seen in Figure 3.1 b ... 

Electrochemical capacitors, also called supercapacitors, are very attractive electricity sources because of their high power, very long durability, and intermediate energy between the classical dielectric capacitors and batteries. The performance of a typical electrochemical capacitor is based on the accumulation of charges in the electrical double layer without faradaic reactions (no electron transfer The electrons involved in double layer charging are the delocalized conduction-band electrons of the electrode material. As shown in Fig. 23.9, an electrochemical capacitor contains one positive electrode with electron deficiency and the second one with electron excess (negative). The capacitance C of one electrode due to a pure electrostatic attraction of ions is proportional to the surface area S of the electrode-electrolyte interface, according to the formula (23.3) ... 

Pseudocapacitors store charge based on reversible (faradaic) charge transfer reactions with ions in the electrolyte. For example, in a metal oxide (such as RUO2 or I1O2) electrode, charge storage results from a sequence of redox reactions. Electrochemical capacitors (ECs) based on such pseudocapacitive materials will have both faradaic and nonfaradaic contributions. The optimization of both EDLCs and pseudocapacitors depends on understanding how features at the nanoscale (e.g. pore size distribution, crystaUite or particle size) affect ion and electron transport and the fundamental properties of electrochemical interfaces. 

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. 

There are two primary mechanisms of charge transfer at the electrode-electrolyte interface, illustrated in Fig. 1. One is a non-Faradaic reaction, where no electrons are... 

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...

If only non-Faradaic redistribution of charge occurs, the electrode/electrolyte interface may be modeled as a simple electrical capacitor called the double-layer capacitor Cdi. This capacitor is formed due to several physical phenomena [2, 3, 4, 5, 6]. First, when a metal electrode is placed in an electrolyte, charge redistribution occurs as metal ions in the electrolyte combine with the electrode. This involves a transient transfer of electrons between the two phases, resulting in a plane of charge at the surface of the metal electrode, opposed by a plane of opposite charge, as counterions, in the electrolyte. The excess charge on the electrode surface, symbolized... 

It is stated in [3] that even completely symmetrical bi-phasic current waveforms would not result in charge balance and will cause a residual voltage and charge build-up on the electrodes. The reason is the presence of a faradaic resistor Rfw parallel to the electrode-electrolyte interface capacitor. This resistor models the electron transfer across the electrode-electrolyte surface. The resulting electrode model which is called Randles model is shown in Fig. 3.2. For example in [3], for sputtered iridium oxide electrodes with 400 p,m diameter in saline solution, Rp f = 17.12 kQ, Rs = 2.1 and Chw = 909nF were extracted using the step response of the electrode voltage to an input current. 

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. 

Faradic impedance (//) is directly related to the rates of charge transfer reactions at and near the electrode/electrode interface. As shown in Figure 3.1, the Faradaic impedance acts in parallel with the double-layer capacitance Cd, and this combination is in series with the electrolyte resistance Rei The parameters Rei and Cd in the equivalent circuit are similar to the idea of electrical elements. However, X/ is different from those normal electrical elements because Faradaic impedance is not purely resistive. It contains a capacitive contribution, and changes with frequency. Faradaic impedance includes both the finite rate of electron transfer and the transport rate of the electroactive reagent to the electrode surface. It is helpful to subdivide Zj into Rs and Cs, and then seek their frequency dependencies in order to obtain useful information on the electrochemical reaction. 

The semiconductor electrode must be ideally polarizable over the potential range of interest. This means that there is no leakage current or Faradaic reaction to allow charge transfer across the semiconductor-electrolyte interface. This restriction is not too important if measurements are taken at sufficiently high frequency that the effects of Faradaic reactions are suppressed. 

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