Activation overpotential

Oscillatory reactions carbon monoxide oxidation, 388 electrochemical promotion of, 389 Overpotential activation, 124 anodic, 122 cathodic, 122 cell, 123... 

Enzymes are efficient catalysts for cathodic and anodic reactions relevant to fuel cell electrocatalysis in terms of overpotential, active site activity, and substrate/reaction specificity. This means that design constraints (e.g., fuel containment and anode-cathode separation) are relaxed, and very simple devices that may take up ambient fuel or oxidant from their environment are possible. While operation is generally confined to conditions close to ambient temperature, pressure, and pH, and power densities over about 10 mW cm are rarely achieved, enzyme fuel cells may be particularly useM in niche environments, for example scavenging trace H2 released into air, or sugar and O2 from blood. Thus, trace or unusual fuels become viable for energy production. 

The thickness distribution of electrodeposits depends on the current distribution over the cathode, which determines the local current density on the surface. The current distribution is determined by the geometrical characteristics of the electrodes and the cell, the polarization at the electrode surface, and the mass transfer in the electrolyte. The primary current distribution depends only on the current and resistance of the electrolyte on the path from anode to cathode. The reaction overpotential (activation overpotential) and the concentration overpotential (diffusion overpotential) are neglected. The secondary... 

The values in Table 26-1 are called thermodynamic potentials, that is, they are ideal values. Several factors, such as overpotential, activation energy, complexation, and pH, can change these values. These will be discussed later. 

Consider first the primary current distribution, which represents the distribution when the surface overpotentials (activation and concentration) are neglected, and the electrode is taken as an equipotential surface. For a disk electrode of radius ri embedded in a large insulating plane with a counter electrode at infinity, the potential distribution under such conditions is as shown in Figure 9.3.9. The current flows in a direction perpendicular... 

Activation loss (overpotential). Activation overpotential is caused by the kinetics of the electrode reactions. It is associated with both sluggish reaction kinetics and low catalyst activity. The effects of these losses are most pronounced at low current densities (< 0.1 A/cm ). Minimization of the activation overpotential thus requires maximization of the catalyst area available for reactions and improvement of catalyst activity. 

Charge Transport. Side reactions can occur if the current distribution (electrode potential) along an electrode is not uniform. The side reactions can take the form of unwanted by-product formation or localized corrosion of the electrode. The problem of current distribution is addressed by the analysis of charge transport ia cell design. The path of current flow ia a cell is dependent on cell geometry, activation overpotential, concentration overpotential, and conductivity of the electrolyte and electrodes. Three types of current distribution can be described (48) when these factors are analyzed, a nontrivial exercise even for simple geometries (11). 

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 activation overpotential, and hence the activation energy, varies exponentially with the rate of charge transfer per unit area of electrode surface, as defined by the well-known Tafel equation... 

The situation is different, however, in near-neutral or alkaline solutions in which the concentration of HjO will be small (< 10 mol dm ), and in these solutions the water molecule will act as the electron acceptor, and although diffusion occurs rapidly its reduction is kinetically more difficult than that of HjO, and will therefore require a higher activation overpotential. 

Electroplating passive alloys Another application of strike baths reverses the case illustrated in the previous example. The strike is used to promote a small amount of cathode corrosion. When the passivation potential of a substrate lies below the cathode potential of a plating bath, deposition occurs onto the passive oxide film, and the coating is non-adherent. Stainless steel plated with nickel in normal baths retains its passive film and the coating is easily peeled off. A special strike bath is used with a low concentration of nickel and a high current density, so that diffusion polarisation (transport overpotential) depresses the potential into the active region. The bath has a much lower pH than normal. The low pH raises the substrate passivation potential E pa, which theoretically follows a relation... 

Activation Overpotential that part of an overpotential (polarisation) that exists across the electrical double layer at an electrode/solution interface and thus directly influences the rate of the electrode process by altering its activation energy. 

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