Nitrogen doped CNTs for oxygen reduction reaction in acidic media
Carbon nanotubes have recently emerged as a support for non precious metal catalyst for oxygen reduction reaction. By comparison, CNTs possess much higher electronic conductivity and a good thermal and chemical stability than the commonly used commercial carbon black, Vulcan XC-72. Nitrogen-doped CNTs have been intensely investigated and they are found to be highly active in alkaline media.
The objective of this study is to develop an understanding of the roles of iron (Fe) and nitrogen (N) in the formation of active sites for the oxygen reduction reaction (ORR) in acidic media. Iron catalysts (5 wt%) were synthesized via wet impregnation with two methods of subsequent nitrogen doping. The nitrogen-containing carbon was produced either by pyrolyzing 5 wt% Fe/C composite in gaseous NH3 or by addition of the nitrogen complex, 2, 4, 6-tripyridyl-s-triazine (TPTZ) which was later decomposed in N2. The pyrolysis temperature for both methods was 700oC. Electrochemical evaluations of the ORR catalysts were carried out in a three-electrode electrochemical cell using a potentiostat EG&G Model 273A. Physical characterisations by XPS and XRD were performed for surface analysis and catalyst morphology. Enhanced ORR activity was obtained from the 5 wt% Fe-N/CNT catalyst where the TPTZ was the source of nitrogen doping.
It was found that only small amount of nitrogen was successfully doped using NH3 but a significant amount was obtained from the complex, TPTZ. A high content of iron was found to have an insignificant on the catalyst activity, but its presence was important.