ORR electron transfer number

As discussed in Chapter 5 for RDE technique, this apparent electron-transfer number of ORR or the overall ORR electron-transfer number can be obtained using the slope of the... 

To obtain the overall ORR electron-transfer numbers, the RRDE measurements were also carried out in this work. The obtained overall electron numbers and the percentages of H2O2 production are shown as a function of electrode potential in Figure 7.20(A) and (B), respectively. It can be seen that the ORR electron-transfer number varies in the range of 3.6—3.8,... 

From Figure 7.22, it can be seen that the overall ORR electron-transfer numbers catalyzed by Ti407 in these KOH solutions are in... 

M H2SO4 reported in Ref. 69. The average ORR electron-transfer number obtained was very close to 4.0, suggesting that the 20wt% CoSe2/C catalyst favors a 4-electron ORR process from O2 to H2O. 

The nanostructured Au and AuPt catalysts were found to exhibit electrocatalytic activity for ORR reaction. The cyclic voltammetric (CV) curves at Au/C catalyst reveal an oxidation-reduction wave of gold oxide at +200 mV in the alkaline (0.5 M KOH) electrolyte but little redox current in the acidic (0.5 M H2SO4) electrolyte. Under saturated with O2, the appearance of the cathodic wave is observed at -190 mV in the alkaline electrolyte and at +50 mV in the acidic electrolyte. This finding indicates that the Au catalyst is active toward O2 reduction in both electrolytes. From the Levich plots of the limiting current vs. rotating speed data, one can derive the electron transfer number (w). We obtained n = 3.1 for ORR in 0.5 M KOH electrolyte, and 2.9 for ORR in 0.5 M H2SO4 electrolyte. The intermittent n-value between 2 and 4 indicates that the electrocatalytic ORR at the Au/Ccatalyst likely involved mixed 2e and 4e reduction processes. 

On the basis of the combined weight of the above results, we believe that bifunctional electrocatalytic properties may be operative for both MOR and ORR on the AuPt bimetallic nanoparticle catalysts depending on the nature of the electrolyte. For ORR in acidic electrolyte, the approaching of both the reduction potential and the electron transfer number for the bimetallic catalyst with less than 25%Pt to those for pure Pt catalyst is indicative of a synergistic effect of Au and Pt in the catalyst. For MOR in alkaline electrol)he, the similarity of both the oxidation potential and the current density for the bimetallic catalyst with less than 25%Pt to those for pure Pt catalyst is suggestive of the operation of bifunctional mechanism. Such a bifunctional mechanism may involve the following reactions ... 

Electrode Material/ Catalyst ORR Exchange Current Density, (A cm ) Electron Transfer Coefficiency Electron Transfer Number in the Rate-Determining Step Measurement Conditions Reference... 

ORR Apparent Electron-Transfer Number and the Formed Percentage of Peroxide Measured by RRDE Technique 221... 

Due to the production of H2O2 or HO2 through a 2-electron-transfer pathway, the overall electron-transfer number of the ORR process is always less than 4. This electron-transfer number is normally called the apparent number of electrons transferred per O2 molecule. Actually, this apparent number of electron transfer can be measured by the RRDE technique, from which the percentage of H2O2 formation in the ORR can also be calculated. Generally to say, the apparent number of electron transfer and the percentage of peroxide produced in the ORR process are two important pieces of information in evaluating the ORR catalyst s catalytic activity. 

Koutecky—Levich plot, measured at different electrode rotating rates, if the O2 concentration, the O2 diffusion coefficient, and the solution kinetic viscosity are known. Here, we give the expression of apparent electron-transfer number of ORR as functions of both disk and ring currents, measured using the RRDE technique. 

Substituting Eqn (6.49) into Eqn (6.48), the expression of apparent electron-transfer number of ORR can be obtained ... 

Table 7.6. ORR Kinetic Current Densities and Overall Electron Transfer Numbers. Obtained Based on tbe Data in Figure 7.19, Collected on a Co-N/C Coated Glassy Carbon Electrode Rotating at Various Rotation Rates. Electrolyte 02-Saturated 0.5 M H2SO4 Solution Co Loading in the Coating Layer ...

Table 7.7. Electron-Transfer Coefficiency a (Assuming the Electron-Transfer Number in ORR Rate-Determining Step is 1), the Exchange Current Density i°, the Electron-Transfer Rate Constant Ar, and the Chemical Reaction Rate Constant kc...

Figure 7.20 (A) Electron-transfer numbers for the ORR catalyzed by 5wt% Co-N/C catalyst obtained after heat treating 5wt % Co-TPTZ/C at 700 °C (B) the corresponding %H202 produced. Data obtained using rotating ring (Pt)-disk electrode (glassy carbon coated by Co-N/C) in an 02-saturated 0.5 M H2SO4 solution Co-N/C catalyst loading in the disk coating 0.1 mg cm. Reprinted with permission from Ref. 65.

Chapter 7 reviews the applications of RDE and RRDE techniques in ORR research and its associated catalyst evaluation. Some typical examples for RDE and RRDE analysis in obtaining the ORR kinetic information such as the overall electron transfer number, electron transfer coefficiency, and exchange current density are also given in this chapter. It demonstrates that both RDE and RRDE methods are a powerful tool in ORR study, and using RDE and RRDE methods, ORR has been successfully studied on Pt electrode, carbon electrode, monolayer metal catalyst, Pt-based catalyst, and nonnoble metal-based catalysts. 

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