Oxygen reduction reaction, polarization

If, however, it is assumed from Eq. (2-40) that the protection current density corresponds to the cathodic partial current density for the oxygen reduction reaction, where oxygen diffusion and polarization current have the same spatial distribution, it follows from Eq. (2-47) with = A0/7 ... 

The impedance polarization performance of LSM electrode is closely related to the mechanism and kinetics of the oxygen reduction reactions. 02 reduction at SOFC cathodes is the most heavily studied subject, and this subject is sufficiently broad and complex to warrant its own review. Interested readers should consult the recent excellent articles by Adler [1] and Fleig [55], Here, only the polarization performance and its influencing factors are discussed. 

Fig. 43. Double-logarithmic plot of the electrode polarization resistance versus the microelectrode diameter measured with impedance spectroscopy (ca. 800 °C) at (a) a cathodic dc bias of -300 mV, and (b) at an anodic dc bias of +300 mV. In (b) the first data point of the 20-pm microelectrode is not included in the fit. (c) Sketch illustrating the path of the oxygen reduction reaction for cathodic bias, (d) Path of the electrochemical reaction under anodic bias the rate-determining step occurs close to the three-phase boundary.

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.

Therefore let us instead consider the more practical case of the tertiary current distribution. Based on the dependency of the Wagner number on polarization slope, we would predict that a pipe cathodically protected to a current density near its mass transport limited cathodic current density would have a more uniform current distribution than a pipe operating under charge transfer control. Of course the cathodic current density cannot exceed the mass transport limited value at any location on the pipe, as said in Chapter 4. Consider a tube that is cathodically protected at its entrance with a zinc anode in neutral seawater (4). Since the oxygen reduction reaction is mass transport limited, the Wagner number is large for the cathodically protected pipe (Fig. 12a), and a relatively uniform current distribution is predicted. However, if the solution conductivity is lowered, the current distribution will become less uniform. Finite element calculations and experimental confirmations (Fig. 12b) confirm the qualitative results of the Wagner number (4). 

Polarization resistance (R,) due to the oxygen reduction reaction (ORR), which is in parallel with the double-layer capacitance (Cdi)... 

Correlation of the effect of particle size on the oxygen reduction reaction has been reviewed extensively by Kinoshita [4,5], Stonehart [8], and Mukerjee [9]. The general consensus, based on a large number of steady-state polarization measurements in several electrolytes, is that ORR exhibits a strong particle size effect in the... 

Figure 1 Correlation of specific activity for oxygen reduction reaction with particle size measured as steady-state polarization values with different electrolytes, (a) 98% H3PO4, 180 °C, (b) 0.5 M H2SO4, 25 °C, and (c) 97% H3PO4, 177 °C. Also superimposed is the surface-averaged distribution (SAD) of (100) sites (solid line). (From Ref. 4.)...

Development of supported Pt electrocatalysts came as a result of intensive research on fundamental and applied aspects of electrocatalysis [especially for kinetically difficult oxygen reduction reaction (ORR)] fueled by attempts at commercialization of medium-temperature phosphoric acid fuel cells (PAFCs) in the late 1960s and early 1970s. Dispersion of metal crystallites in a conductive carbon support resulted in significant improvements in all three polarization zones (activation, ohmic, and... 

Fig. 4.23 Schematic representation of polarization curves for the analysis of galvanic coupling when diffusion control of the oxygen reduction reaction is the dominant factor governing the corrosion rate...

The second step in the procedure requires the working electrode to be anodicaUy polarized, yielding one of the dashed Hnes shown in Fig. 3.6. The electrode is then cathodicaUy polarized, and the other dashed Hne from Fig. 3.6 is obtained. The anodic polarization usuaUy results in the oxidation of the metal species, whUe the reaction resulting from cathodic polarization depends on the medium. In an aerated solution, the oxygen reduction reaction may be the prime cathodic reaction, while, in the case of deaerated aqueous solutions, hydrogen reduction could be the dominant reaction. In Fig. 3.6, the redox reaction is represented by a general reaction... 

The cathodic polarization curve is constructed using the oxygen electrode equilibrium potential and the cathodic slope, 6<- = —0.05V/decade. The equilibrium cathode potential, geq,c is calculated by applying the Nemst equation to the oxygen reduction reaction, Eq. (4.13), for an oxygen concentration of 1.0x10 mol/1 at pH= 11. 

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