Carbon as Structure-Forming Element in Porous Fuel Cell Electrodes

A critical issue concerning the aforementioned studies on virtually metal-free catalysts is that they were conducted on nanostructures grown over metal/support catalysts. Thus, doubts remained whether these materials still contain (minute) amounts of active metal species from the production process. However, recently it was shown that N-doped carbons synthesized without any metal are active for electrocatalytic oxygen reduction. Xia et al. prepared ORR catalysts by heat treatment at temperatures ranging from 400 to 800 C of mixtures of polypyridine and CB, and observed reasonable catalytic activity for the ORR [126]. 

Even though this issue has not been completely solved yet, research is ongoing in this field. It is likely that various active sites exist, some of them with a central metal ion, some of them metal free, consisting of nitrogen embedded into the carbon structure. With detailed evidence still missing, it is not unlikely that the metal-containing centers are of high activity in both alkaline and acid media, whereas the metal-free ones seem to be mainly active in alkaline media. 

As an example, heat treatment of oxidized CNTs in ammonia has been reported to yield active electrocatalysts with the activity depending on heat treatment temperature in the range of 400-800 °C. Considerably high activity was found in alkaline solution [127]. 

Carbon as Structure-Forming Element In Porous Fuel Cell Electrodes 

Fuel cell electrodes are built up from porous layers of the electrocatalysts and have to fulfill many functions. Above all, they form the porous structure, in which the 

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Carbon as structure-forming element in porous fuel cell electrodes. 

Carbon as Structure-Forming Element in Porous Fuel Cell Electrodes 269 7.4.5.1 Horizontal Structuring by LbL... 

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