Anodic potential versus current

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.

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. 

In a detailed rotating-disk electrode study of the characteristic currents were found to be under mixed control, showing kinetic as well as diffusional limitations [Ha3]. While for low HF concentrations (<1 M) kinetic limitations dominate, the regime of high HF concentrations (> 1 M) the currents become mainly diffusion controlled. However, none of the relevant currents (J1 to J4) obeys the Levich equation for any values of cF and pH studied [Etl, Ha3]. According to the Levich equation the electrochemical current at a rotating disk electrode is proportional to the square root of the rotation speed [Le6], Only for HF concentrations below 1 mol 1 1 and a fixed anodic potential of 2.2 V versus SCE the traditional Levich behavior has been reported [Cal 3]. 

Fig. 10-28. Polarization curves for cell reactions of photoelectrolytic decomposition of water at a photoezcited n-type anode and at a metal cathode solid curve M = cathodic polarization curve of hydrogen evolution at metal cathode solid curve n-SC = anodic polarization curve of oxygen evolution at photoezcited n-type anode (Fermi level versus current curve) dashed curve p-SC = quasi-Fermi level of interfadal holes as a ftmction of anodic reaction current at photoezcited n-type anode (anodic polarization curve r re-sented by interfacial hole level) = electrode potential of two operating electrodes in a photoelectrolytic cell p. sc = inverse overvoltage of generation and transport ofphotoezcited holes in an n-type anode.

Again it seems not necessary to discuss the considerations of the chemical versus electrochemical reaction mechanism. It is clear from the extremely negative standard potential of silicon, from Eqs. (2) and (6), that the Si electrode is in all aqueous solutions a dual redox system, characterized by its OCP, which is the resultant of an anodic Si dissolution current and a simultaneous reduction of oxidizing species in solution. The oxidation of silicon gives four electrons that are consumed in the reduction reaction. Experimental results show clearly that the steady value of the OCP is narrowly dependent on the redox potential of the solution components. In solutions containing only HF, alternatively alkaline species, the oxidizing component is simply the proton H+ or the H2O molecule respectively. 

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. 

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.

Figure 23. Cyclic voltammetry, (a) Imposed potential versus time variations, (b) Resulting transient current-potential curve for a simple electron transfer. The concentration profiles of the reactant R and product P are indicated at various characteristic potentials of the voltammogram. Epc and Epa, cathodic and anodic peak potentials, (c) Schematic change of the cyclic voltammogram as a function of the chemical stability of the product.

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... 

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.

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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