High activation overpotential

Two limiting cases exist for Eq. 3.17. At very high activation overpotential the first exponent in Eq. 3.17 turns out to be much greater than the second exponent and hence can be neglected, leading to... 

Since electrode potentials are quoted on the hydrogen scale, it might be expected that only metals with positive standard electrode potentials could be deposited from acid solutions, and that hydrogen would be discharged in preference to other cations, such as lead (E = -0.126 V) or nickel (E =-0.25 V). The failure of this prediction is due to the high activation overpotential (q.v.) of hydrogen on many metals, values obtained at a current density of 1 mA cm varying from 0.01 V on a platinised platinum cathode to as much as 0.67 V on lead and 1.04 V on mercury. 

Seconday Current Distribution. When activation overvoltage alone is superimposed on the primary current distribution, the effect of secondary current distribution occurs. High overpotentials would be required for the primary current distribution to be achieved at the edge of the electrode. Because the electrode is essentially unipotential, this requires a redistribution of electrolyte potential. This, ia turn, redistributes the current. Therefore, the result of the influence of the activation overvoltage is that the primary current distribution tends to be evened out. The activation overpotential is exponential with current density. Thus the overall cell voltages are not ohmic, especially at low currents. 

Fuel cell stack voltage varies with external load. During low current operation, the cathode s activation overpotential slows the reaction, and this reduces the voltage. At high power, there is a limitation on how quickly the various fluids can enter and... 

The anode potential is so positive, due principally to the activation overpotential, that the majority of the impurity metals (Fe, Cu, Co, etc.) in the anode dissolve with the nickel sulfide. In addition, some oxygen is evolved (2 H20 = 02 + 4 H+ + 4 e ). The anodic current efficiency reduced to about 95% on account of this reaction. Small amounts of selenium and the precious metals remain undissolved in the anode slime along with sulfur. The anolyte contains impurities (Cu, Fe, Co) and, due to hydrogen ion (H+) liberation, it has a low pH of 1.9. The electrolyte of this type is highly unfit for nickel electrowinning. It is... 

The first two terms on the right-hand side of this equation express the proper overpotential of the electrode reaction rjr (also called the activation overpotential) while the last term, r)c, is the EMF of the concentration cell without transport, if the components of the redox system in one cell compartment have concentrations (cOx)x=0 and (cRed)x=0 and, in the other compartment, Cqx and cRcd. The overpotential given by this expression includes the excess work carried out as a result of concentration changes at the electrode. This type of overpotential was called the concentration overpotential by Nernst. The expression for a concentration cell without transport can be used here under the assumption that a sufficiently high concentration of the indifferent electrolyte suppresses migration. 

A number of metal porphyrins have been examined as electrocatalysts for H20 reduction to H2. Cobalt complexes of water soluble masri-tetrakis(7V-methylpyridinium-4-yl)porphyrin chloride, meso-tetrakis(4-pyridyl)porphyrin, and mam-tetrakis(A,A,A-trimethylamlinium-4-yl)porphyrin chloride have been shown to catalyze H2 production via controlled potential electrolysis at relatively low overpotential (—0.95 V vs. SCE at Hg pool in 0.1 M in fluoroacetic acid), with nearly 100% current efficiency.12 Since the electrode kinetics appeared to be dominated by porphyrin adsorption at the electrode surface, H2-evolution catalysts have been examined at Co-porphyrin films on electrode surfaces.13,14 These catalytic systems appeared to be limited by slow electron transfer or poor stability.13 However, CoTPP incorporated into a Nafion membrane coated on a Pt electrode shows high activity for H2 production, and the catalysis takes place at the theoretical potential of H+/H2.14... 

To obtain a reasonable estimate of the nonadiabatic rate for low overpotential FT reactions (typically high activation energy), one may replace Eq. (28) by... 

In a PEMFC, the power density and efficiency are limited by three major factors (1) the ohmic overpotential mainly due to the membrane resistance, (2) the activation overpotential due to slow oxygen reduchon reaction at the electrode/membrane interface, and (3) the concentration overpotential due to mass-transport limitations of oxygen to the electrode surfaced Studies of the solubility and concentration of oxygen in different perfluorinated membrane materials show that the oxygen solubility is enhanced in the fluorocarbon (hydrophobic)-rich zones and hence increases with the hydrophobicity of the membrane. The diffusion coefficient is directly related to the water content of the membrane and is thereby enhanced in membranes containing high water content the result indicates that the aqueous phase is predominantly involved in the diffusion pathway. ... 

The activation overpotentials for both electrodes are high therefore, the electrochemical kinetics of the both electrodes can be approximated by Tafel kinetics. The concentration dependence of exchange current density was given by Costamagna and Honegger.The open-circuit potential of a SOFC is calculated via the Nernst equation.The conductivity of the electrolyte, i.e., YSZ, is a strong function of temperature and increases with temperature. The temperature dependence of the electrolyte conductivity is expressed by the Arrhenius equation. 

Whereas Pt in an acidic solution saturated with H2 acquires the reversible potential of the hydrogen electrode, this is not the case for the same Pt electrode in an acidic solution saturated with O2. This is related to the high activation energies involved in breaking and forming chemical bonds. Thus the O2 reaction is known to be highly irreversible. In particular, a Pt electrode in 02-saturated solution acquires a potential 0.9V (SHE) rather than 1.23 V. Hence an overpotential of >0.3 V can already be expected from an analysis of the equilibrium conditions. 

Another feature of the spiral tip is that it has an abnormally high step and kink density and perhaps the tip has a higher exchange-current density for deposition than the corresponding planar surface. If this were so, the activation overpotential would be much less at the tip of the spiral than around its base. 

At high positive overpotentials, platinum and gold form an oxide layer, which is reduced back to the metallic phase in the return-scan direction. From the current of these peaks, it is possible to estimate the true active surface, which can be seriously different from the geometrical surface area. 

Activation overpotential — When the activation energy of the - charge transfer reaction is high an -+ overpotential is needed to drive the reaction in the desirable direction with an appreciable rate. It is called activation overpotential (qac). The electric field (the -> inner electric potential, f) in the phase a determines the energy of the charged species this can be expressed by the -> electrochemical potential (pi). 

Overpotential — is the deviation of the - electrode potential from its equilibrium value required to cause a given -> current density to flow through the electrode. This notion is widely applied to the qualitative characteristic of electrode activity in various reactions, namely low overpotential means high activity, and high overpotential means low activity (it is assumed that the values of overpotential are compared for some fixed current density and solution composition). See also - activation overpotential, -> crystallization overpotential, - diffusion overpotential, -> reaction overpotential. 

In order to derive the equation for pure reaction overpotential we assume that j0 is high, i.e., an equilibrium exists at the electrode metal surface in respect of the surface concentrations of O and R. All other steps including diffusion are fast enough, i.e., much faster than reaction (2). (In general, r is a sum of different contributions [- activation overpotential, - diffusion overpoten-tial etc.] which might be interdependent.)... 

Activation overpotential may become important with a number of electrocatalysts however, as Debenedetti and Vayenas (15) have discussed, the actual current-voltage behavior of the unit at moderate and high current densities can be well approximated by subtracting the activation overpotential from E For CO... 

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