Co-N/C catalysts

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.

As discussed above, the ORR mechanism catalyzed by the Co-N/C catalyst was a mixed process of 2- and 4-electron-transfer pathways, dominated by the 4-electron pathway. An ORR mechanism was proposed in this work as follows ... 

In this mechanism. Reaction (7-XIII) is the chemical reaction discussed above. Reaction (7-XIV) is the ORR RDS on the Co-N/C catalyst surface. Reaction (7-XV) is peroxide formation. This H2O2 will go two ways (1) further 2-electron reduction to H2O through Reaction (7-XVI), or (2) chemical desorption through Reaction (7-XVII) to form free H2O2, and then entrance into the bulk solution, which can be detected by the ring electrode. The relative proportions of Reactions (7-XVI) and (7-XVII) determine the overall electron number and the amount of H2O2 produced. In Reaction (7-XVI), an x is used to express the proportion that is a 4-electron-transfer process, and in Reaction (7-XVII), (1 — x) to express the proportion of 2-electron process. It can be seen that when x=l, the mechanism will be a completely 4-electron-transfer pathway, and when x = 0, the mechanism will be a completely 2-electron pathway. If x is in between, the ORR will be a mixture of 2- and 4-electron-transfer pathways. 

Jaouen F, Dodelet JP (2007) Average ttmi-over frequency of O2 electro-reduction forFe/N/C and Co/N/C catalysts in PEFCs. Electrochim Acta 52 5975-5984... 

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