Oxygen reduction reaction diffusion-limiting current density

In the work of Alodan and Smyrl," the pH profiles over the particle were simulated by assuming a diffusion-controlled oxygen reduction reaction at the cathode and by setting a limiting diffusion current density of 0.1 itiA cm, calculated for a diffusion layer thickness of 20 pm and O2 concentration of 0.26 mol m. ... 

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.

The objective of the mass transport lab is to explore the effect of controlled hydrodynamics on the rate at which a mass transport controlled electrochemical reaction occurs on a steel electrode in aqueous sodium chloride solution. The experimental results will be compared to those predicted from the Levich equation. The system chosen for this experiment is the cathodic reduction of oxygen at a steel electrode in neutral 0.6 M NaCl solution. The diffusion-limited cathodic current density will be calculated at various rotating disk electrode rotation rates and compared to the cathodic polarization curve generated at the same rotation rate. 

If the oxygen reduction is the counter reaction of metal dissolution, its kinetics is often determined by diffusion due to the small solubility of O2 gas and long diffusion distances especially in unstirred solution. In this case, the cathodic reaction gets potential independent at negative potentials with a diffusion-limited cathodic current density Ido2 the vicinity of the rest potential (Figure 1.42). In this situation, the corrosion current density ic is equal to the limiting cathodic current density ip,02/ and Equation 1.159 simplifies to Equation 1.164, which becomes Equation 1.165 for n -> 0 (E —> Er). Hence ic can be calculated from Rp and the anodic Tafel slope (Equation 1.166). 

The mass-transport-limited current density for oxygen reduction is independent of the kinetic parameters for this reaction rather it depends on factors such as the concentration and the diffusion coefficient of oxygen in the medium. It depends oh the rate of flow of the liquid in a pipe or around a sailing ship or a structure immersed in a river. 

Reaction (4.26) is controlled by concentration polarization (i.e., diffusion of O2 to the siuface limits the reaction rate) where the limiting current density for oxygen reduction is i, = 107 pA/cm ... 

Fie. 3.18 Illustration of the effect of exchange current density on the polarization curve for oxygen reduction in aerated environments of pH = 0.56 and Pq2 = °-2 atm- Curves converge to the same diffusion limit and are identical when the hydrogen ion reduction is the dominant reaction. 

Alkaline Solutions Foreign metal adatoms cause remarkable catalytic effects on oxygen reduction on several electrode surfaces. The adatoms of Pb [155, 156, 162], Bi [163], and T1 [164,165] cause a shift of the half-wave potential of the O2 reduction to more positive values and an approximate doubling of the diffusion-limited current density. The latter is due to the change of the reaction mechanism from a two-electron reduction on Au into a vital four-electron reduction on Au covered... 

Oxygen reduction is a very important reaction for corrosion processes. Its kinetics have a relatively large overpotential, which causes a negative rest potential for most reactive metals. In a saturated aqueous solution of ca. 2 X10" M, oxygen reduction often occurs under diffusion control. The maximum diffusion limited current density is calculated according to Eq. (1-33) with the diffusion coefficient Do=10" cm s a Nemst diffusion layer thickness 5=2x 10 cm, and n = 4 for complete oxygen reduction to H2O or OH"... 

Much as bacteria catalyze the anode reaction in an MFC, bacteria can also catalyze the terminal reduction reaction at the cathode (Rabaey et al., 2003). Biocathodes are both low-cost and sustainable making them an attractive research area. However, biocathodes have not made much of an impact in the MFC field yet as power densities to date have been quite low. These low power densities appear to be mainly a product of oxygen mass transfer limitations associated with diffusion through a biofilm (Clauwaert et al., 2007). Measured current densities for biocathodes were significantly lower than their Pt counterparts but may have other advantages such as being able to reduce several different oxidized contaminates, such as Cr(IV) and Cr(OH)3 (Clauwaert et al., 2007 Ter Heijne, 2010). 

Each reaction may be subdivided into further detailed reaction steps. The overall reaction follows a Butler-Volmer equation. For large cathodic overvoltages, reduction may become diffusion limited and thus might reach the potentially independent maximum current density j d as described in Equation 1.119 and in Figure 1.23 and as discussed again for metal dissolution with oxygen reduction in Section 1.14. 

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