Oxygen-limiting current density

Consider cell operation at the oxygen-limiting current density. Formally, operation at the limiting current is equivalent to infinite voltage loss E. The expression under the last logarithm on the left side of (4.207) should... 

Figure 5.31b shows that the carbon corrosion current density in the R-domain is equal to the oxygen-limiting current density (2 A cm in this simulation. Table 5.10). Thus, during the start-stop cycle, carbon corrosion runs very fast and even short transients can severely damage the catalyst. The solution to this problem is in lowering the cell potential during the transient (Takeuchi and Fuller, 2(X)8). 

Eigure 5.32 shows the same as in Eigure 5.31 curves for the hundred times smaller oxygen-limiting current density on the anode side (yjj = 0.02 A cm ). This mimics the situation when in the R-domain oxygen penetrates to the anode side through the membrane (crossover). 

Oxygen-limiting current density on the anode side (A cm ) Oxygen-limiting current density on the cathode side (A cm ) Butler-Volmer ORR current density (A cm ). Equation 5.212 Reference (characteristic) current density (A cm ). Equation 4.52 Reference current density (A cm ). Equation 5.199 Vapor flux density (A cm )... 

In oxygen-free seawater, the J(U) curves, together with the Tafel straight lines for hydrogen evolution, correspond to Eq. (2-19) (see Fig. 2-2lb). A limiting current density occurs with COj flushing for which the reaction ... 

On the other hand, it can be assumed for the oxygen corrosion of steel in aqueous solutions and soils that there is a constant minimum protection current density, 4, in the protective range, U limiting current density for oxygen reduction according to Eq. (4-5) (see Section 2.2.3.2). Then it follows, with V = +1,1 = 2nr, S = 27crs and d = dU from Eq. (24-54), instead of Eq. (24-58) [12-14] ... 

It follows from this that the limiting current density / l is the most significant parameter in a corrosion reaction in which oxygen is the cathodic reactant, and that any factor that increases / l will increase the corrosion rate, since at E ... 

In Section 1.4 see Fig. 1.31h) it has been shown that when a corrosion reaction is controlled by the rate of oxygen diffusion, both the rate of corrosion and the corrosion potential increase with / l. the limiting current density, i.e. 

Turning now to the acidic situation, a report on the electrochemical behaviour of platinum exposed to 0-1m sodium bicarbonate containing oxygen up to 3970 kPa and at temperatures of 162 and 238°C is available. Anodic and cathodic polarisation curves and Tafel slopes are presented whilst limiting current densities, exchange current densities and reversible electrode potentials are tabulated. In weak acid and neutral solutions containing chloride ions, the passivity of platinum is always associated with the presence of adsorbed oxygen or oxide layer on the surface In concentrated hydrochloric acid solutions, the possible retardation of dissolution is more likely because of an adsorbed layer of atomic chlorine ... 

The last part of the polarization curve is dominated by mass-transfer limitations (i.e., concentration overpotential). These limitations arise from conditions wherein the necessary reactants (products) cannot reach (leave) the electrocatalytic site. Thus, for fuel cells, these limitations arise either from diffusive resistances that do not allow hydrogen and oxygen to reach the sites or from conductive resistances that do not allow protons or electrons to reach or leave the sites. For general models, a limiting current density can be used to describe the mass-transport limitations. For this review, the limiting current density is defined as the current density at which a reactant concentration becomes zero at the diffusion medium/catalyst layer interface. 

Equation 18 can be used to write the ORR rate in eq 13 or 15 in terms of the limiting current density, an experimentally measurable quantity, rather than the oxygen partial pressure. 

The final 0-D equation presented here stems from incorporating the gas-pressure dependences directly instead of through a limiting current density, which normally only considers oxygen effects. This equation was proposed by Newmanfor phosphoric-acid fuel cells and predates the above polymer—electrolyte fuel-cell expressions. It has the form... 

The concept of a limiting current density is of great practical importance. Table 7.11 shows some typical experimental limiting current densities for four energy producers employing oxygen (where the calculation of limiting current density is more complicated than that of (7.206). 

Figure 24 Schematic polarization data for oxygen reduction reaction (ORR) in neutral (pH 7.2) solution. Diffusion-limited current density (i L) is present due to mass transport limitations on dissolved oxygen.

Many corrosion systems are controlled by diffusion limitations on oxygen because of its low solubility in aqueous solution (0.25 mM at room temperature). The diffusion-limited current density, iL, can be described mathematically by... 

Figure 9 Polarization curve of carbon steel in deaerated, pH 13.5 solution at 65°C. Sample was initially held potentiostatically at —1.2 V(SCE) for 30 min before initiation of the potentiodynamic scan in the anodic direction at 0.5 m V/s. The cathodic loop results from the fact that the passive current density is only 1 pA/cm2, which is less than the diffusion-limited current density for oxygen reduction for the 0.5 ppm of dissolved oxygen present. (From Ref. 8.)...

Figure 7 (a) Cathodic polarization data for a low carbon steel rotating disk electrode in 0.6 M NaCl with ambient aeration. Oxygen reduction limiting current densities are shown for the indicated rotation rates, (b) Plot of experimental limiting current density versus square root of the rotation rate, showing the experimental and predicted linear behavior. 

Figure 16 shows the steady-state limiting current density, ilim, for the oxygen reduction reaction (ORR) on pure Al, pure Cu, and an intermetallic compound phase in Al alloy 2024-T3 whose stoichiometry is Al20Cu2(Mn,Fe)3 after exposure to a sulfate-chloride solution for 2 hours (43). The steady-state values for the Cu-bearing materials match the predictions of the Levich equation, while those for Al do not. Reactions that are controlled by mass transport in the solution phase should be independent of electrode material type. Clearly, this is not the case for Al, which suggests that some other process is rate controlling. 

Figure 16 Limiting current densities for oxygen reduction on rotating disks for pure Cu, Al2oCu2(Mn,Fe)3, and pure A1 in 0.1 M Na2S04 + 0.005 M NaCl under ambient aeration. The Levich prediction assumes a dissolved oxygen concentration of 6 ppm (1.8 X 10 4 M). (From G. O. Ilevbare, J. R. Scully. Corrosion 57, 134 (2001).)...

Calculate the diffusion-limited current density that is expected for each rotation rate assuming a dissolved oxygen concentration of 6-8 ppm (see experimental data sheet for calculation procedure). 

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