Irreversible electrode potentials concentration overpotential

Overpotential Departure from equilibrium (reversible) potential due to passage of a net current. Concentration overpotential results from concentration gradients adjacent to an electrode surface. Surface overpotential results from irreversibilities of electrode kinetics. Supporting (inert or indifferent) electrolyte Compounds that increase the ionic conductivity of the electrolyte but do not participate in the electrode reaction. 

Reoxidation of the cosubstrate at an appropriate electrode surface will lead to the generation of a current that is proportional to the concentration of the substrate, hence the coenzyme can be used as a kind of mediator. The formal potential of the NADH/NAD couple is - 560 mV vs. SCE (KCl-saturated calomel electrode) at pH 7, but for the oxidation of reduced nicotinamide adenine dinucleotide (NADH) at unmodified platinum electrodes potentials >750 mV vs. SCE have to be applied [142] and on carbon electrodes potentials of 550-700 mV vs. SCE [143]. Under these conditions the oxidation proceeds via radical intermediates facilitating dimerization of the coenzyme and forming side-products. In the anodic oxidation of NADH the initial step is an irreversible heterogeneous electron transfer. The resulting cation radical NADH + looses a proton in a first-order reaction to form the neutral radical NAD, which may participate in a second electron transfer (ECE mechanism) or may react with NADH (disproportionation) to yield NAD [144]. The irreversibility of the first electron transfer seems to be the reason for the high overpotential required in comparison with the enzymatically determined oxidation potential. 

SO that the concentration of [Zn ] under the same conditions will be 10 g-molecule/L. With these ionic concentrations, the deposition potentials of copper and zinc in the absence of any polarization can each be calculated from Eq. (11.1) to be about —1.30 V. It should be mentioned here again that in practice, Eq. (11.1) refers to reversible equilibrium, a condition in which no net reaction takes place. In practice, electrode reactions are irreversible to an extent. This makes the potential of the anode more noble and the cathode potential less noble than their static potentials calculated from (11.1). The overvoltage is a measure of the degree of the irreversibility, and the electrode is said to be polarized or to exhibit overpotential hence, Eq. (11.2). 

Because of irreversibilities associated with electrode kinetics and concentration variations, the potential of an electrode is different from the equilibrium potential. This departure from equilibrium, known as the overpotential, can be measured with a reference electrode. So that significant overpotential at the reference electrode can be avoided, the reference electrode is usually connected to the working electrode through a high-impedance voltmeter. With this arrangement the reference electrode draws negligible current, and all of the overpotential can be attributed to the working electrode. 

The overvoltage or overpotential over is inserted in Eq. (3.20) to adjust for other processes that compete in the system and make electrodeposition less than ideally efficient. These processes are irreversible and include the effects of the decomposition of water, other solutes, and imperfections in the electrode surface. Because of these processes, a greater potential difference than calculated from the reference potential and the ionic concentration must be applied in order to achieve deposition. For the same reason, spontaneous deposition, inferred from a positive value of E°, may not occur if the overvoltage exceeds it. Overvoltage effects occur at both the cathode and the anode. 

The operation of the cell is associated with various irreversibilities and leads to various potential losses. In the case of electrodes the total resistance comprises of the internal resistance, contact resistance, activation polarization resistance, and concentration polarization resistance. Internal resistance refers to the resistance for electron transport, which is usually determined by the electronic conductivity and the thickness of the electrode structure. Contact resistance refers to the poor contact between the electrode and the electrolyte structure. All resistive losses are functions of local current density. However, one can minimize the overpotential losses by appropriate choice of electrode material and controlling the micro-structural properties during manufacturing process. 

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