Anodic potential versus current density

FIGURE 11.6 Anodic potential versus current density polarization curve (from Ref. 20). 

Figure 9 Potential versus current density (a) and power density versus current density (b) curves for two different anodes (PAni/Pt-Ru and PAni/Pt-Ru-Mo) in a direct methanol fuel cell (cell temperature 110°C 2 bar O2 pressure, and lOOmLmn gas flow 2M methanol aqueous solution at 2mLmn liquid flow).

Figure 7.56 Determination of the anodic (pa) and cathodic (Pc) Tafel slopes from the potential versus current density plot obtained by potentiodynamic polarisation testing.

The curve shown in Fig. 3 cannot proceed indefinitely in either direction. In the cathodic direction, the deposition of copper ions proceeds from solution until the rate at which the ions are supplied to the electrode becomes limited by mass-transfer processes. In the anodic direction, copper atoms are oxidized to form soluble copper ions. While the supply of copper atoms from the surface is essentially unlimited, the solubility of product salts is finite. Local mass-transport conditions control the supply rate so a current is reached at which the solution supersaturates, and an insulating salt-film barrier is created. At that point the current drops to a low level further increase in the potential does not significantly increase the current density. A plot of the current density as a function of the potential is shown in Fig. 5 for the zinc electrode in alkaline electrolyte. The sharp drop in potential is clearly observed at -0.9 V versus the standard hydrogen electrode (SHE). At more positive potentials the current density remains at a low level, and the electrode is said to be passivated. 

The proton conductive polymer electrolyte used to separate the anode and cathode compartments of fuel cells. The membrane replaces the liquid electrolytes used in some fuel cells. The voltage produced by a fuel ceU stack at a defined current density. A performance or polarization curve refers to a plot of the cell potential (V) versus current density (1) under specified conditions of pressure, temperature, humidity, and reactant stoichiometry. 

Figure 3-1. Anodic polarization curve (current density versus potential) of a metallic material exhibiting passivity. For semiconducting films, the passive region extends over a Volt or less. For the insulating films formed on valve metals, the passive region extends over a few Volts before dielectric breakdown.

Current-voltage relationships are commonly used as measurements of corrosion and are consequently of value in evaluating inhibitors. The first determines the potential versus current curves for both the anodic and cathodic reactions. Data are plotted on a semi-logarithniic current scale and are extrapolated backward toward the low current direction until the anodic and cathodic curves intersect, the current density at that point representing the rate of corrosion. 

Fig. 10.17 Performance of commercially-cast anodes in field trials (free-running) over wide current density range in ambient Gulf of Mexico mud. Potentials negative volts versus Ag/AgCl. Zn 310-350 Ah/lb at 40 and 80mA/ft (after Schreiber and Murray )...

In the study of the anode/cathode polarization behavior of a metal/ environment system, the potentiostat provides a plot of the relationship of current changes resulting from changes in potential most often presented as a plot of log current density versus potential, or Evans... 

Figure 1 shows typical current density-potential curves of an electroorganic reaction. In this example, the thin line represents the anodic oxidation of the electrolyte without reactants at a higher potential, here at more than 0.8 V versus NHE. If the reactant 1 is present, it can be converted according to the thick compact lines at lower potentials above 0.2 V versus NHE, and this selectively can occur up to 0.5 V versus NHE. Over 0.5 V versus NHE also, an additional reactant... 

Fig. 5.17 Real time observation of anodization behavior of a 400 nm Ti thin film anodized at lOV in the HF - aqueous electrolyte (acetic acid and 0.5 vol.% HF mixed in ratio of 1 7). Inset shows a typical current density versus time response observed for a titanium foil (with one face protected with polymer coating) anodized at the same potential and electrolyte.

Figure 3.3.8 Schematic illustration of the origin of activation overpotentials in a hydrogen-oxygen fuel cell. The solid curves represent exponential analytic current densities versus electrode potential of the hydrogen electrode (standard potential 0 V) and the oxygen electrode (standard potential 1.23 V). Relevant for a PEMFC fuel cell are the HOR (anode) and the ORR (cathode) branches. To satisfy a cell current (yceii), the anode potential moves more positive by riact,HOR> while the cathode potential moves more negative by iiact.oRR- As a result of this, the observed cell potential is V, which is smaller than 1.23 V. The shape of the individual characteristics is such that the cathode overpotentials are larger than those at the anode.

Fig. 28. (a) Plots of C o versus potential for the OER on the anodically formed oxide film at Ni based on potential relaxation from a series of six initial potentials and corresponding anodic current densities (298 K). (b) As in (a) but for the chemically formed NiOOH film based on data from five initial potentials (298 K). (From Ref. 2S5.)... 

From the slope of the polarization curve and its variation with time (exposure time of the iron electrode), information on the kind of inhibition can be gained. An inhibition of anodic processes decreases the ia versus E current density and increases the corrosion potential correspondingly, an increase in cathodic inhibition causes a decrease in the i. and lowers the corrosion potential. 

In the case of the DMFC, a typical design point of Vceu = 0.45 V, in a state-of-the-art cell, would allow a current density of 0.2 A cm-2 at cell temperature of 80 °C. Under these conditions, the anode potential would typically be 0.4 V versus... 

For n-Si, anodic reactions in the dark at current densities higher than the limiting current is a conduction band process but can involve both the conduction band and the valence band under illumination. The relative participation of conduction band versus valence band is a function of light intensity the conduction band dominates at a low light intensity and the valence band at a high light intensity [34-36]. Participation of holes and electrons in the dissolution reactions varies with the type of silicon, illumination conditions, and electrode potential. 

Fig. 5 Plots of the anodic potential, a, at which Cr, Ni, and their alloys dissolve at a current density of 8 A cm-2, the current efficiency J], and a Cr(VI) fraction of the total Cr amount in the solution versus the chromium content in the alloy ncr- The experiments were performed in (a) 2 M NaCl solution and (b) 1 M Na2SC>4 (the results obtained in Na2SC>4 and NaN03 solutions are similar) on the rotating (500 rpm) disk electrodes.

Tafel Extrapolation. The most fundamental procedure for experimentally evaluating Icorr is by Tafel extrapolation. This method requires the presence of a linear or Tafel section in the E versus log Iex curve. A potential scan of 300 mV about Ecorr is generally required to determine whether a linear section of at least one decade of current is present such that a reasonably accurate extrapolation can be made to the Ecorr potential. Such linear sections are illustrated for the cathodic polarization curves in Fig. 6.2 to 6.5. The current value at the Ecorr intersection is the corrosion current, Icorr, as shown in Fig. 6.10. Assuming uniform corrosion, the corrosion current density is obtained by dividing Icorr by the specimen area (i.e., icorr = Icorr/A). Anodic polarization curves are not often used in this method because of the absence of linear regions over... 

The sign holds for anodic and cathodic overpotentials respectively. A plot of electrode potential versus the logarithm of current density is called the Tafel plot and the resulting straight line is the Tafel line" The linear part (5=2.3 RT/anF) is the Tafel slope that provides information about the mechanism of the reaction, and "a" provides information about the rate constant of the reaction. The intercept at r =0 gives the exchange current density... 

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