Electrode reactions irreversible reaction pathway

If the electrode potential is further increased near the electrolysis potential, it will eventually start to decompose water to form oxygen or hydrogen gas. The gas quickly diffuses away, making the reaction chemically irreversible. The irreversible reaction pathway is then dominated by the electrolysis reaction. 

In simplified electrode models, the irreversible reaction pathway is typically represented by a resistor Ri. The resistor denotes that charge is consumed and not stored. However, the pathway actually behaves more like a diode. The reaction-current equation is described approximately by ... 

Figure 10. Kleitz s reaction pathway model for solid-state gas-diffusion electrodes. Traditionally, losses in reversible work at an electrochemical interface can be described as a series of contiguous drops in electrical state along a current pathway, for example. A—E—B. However, if charge transfer at point E is limited by the availability of a neutral electroactive intermediate (in this case ad (b) sorbed oxygen at the interface), a thermodynamic (Nernstian) step in electrical state [d/j) develops, related to the displacement in concentration of that intermediate from equilibrium. In this way it is possible for irreversibilities along a current-independent pathway (in this case formation and transport of electroactive oxygen) to manifest themselves as electrical resistance. This type of chemical valve , as Kleitz calls it, may also involve a significant reservoir of intermediates that appears as a capacitance in transient measurements such as impedance. Portions of this image are adapted from ref 46. (Adapted with permission from ref 46. Copyright 1993 Rise National Laboratory, Denmark.)...

As illustrated in Fig. 7-2, in dichloromethane solution the complex undergoes a rather complicated redox pathway [18]. The first, irreversible anodic step has been tentatively attributed to the oxidation of one ferrocene ligand, at an electrode potential p of 0.25 V, according to the following reaction ... 

Dissolution of the oxidant or reductant during a redox reaction can greatly reduce the reversibility of the system. The dissolution can alter an electrode material from an undissolved state to a dissolved one. In an imdissolved state, it exhibits a controlled three-dimensional morphology that is closely linked to the conductive transport pathways of the current collector. When a redox reaction moves the material into a dissolved state, the charge is lost. Then the process becomes heavily diffusion limited and if the electrode was designed for charging based on its undissolved state, the diffusion will likely generate an overpotential on the material and the capacitance will be irreversible. 

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