Activity and active sites of nitrogen-doped carbon nanotubes for oxygen reduction reaction

Nitrogen-doped carbon (CNx) nanotubes were synthesized by thermal decomposition of ferrocene/ethy-lenediamine mixture at 600-900 °C. The effect of the temperature on the growth and structure of CNx nanotubes was studied by transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. With increasing growth temperature, the total nitrogen content of CNx nanotubes was decreased from 8.93 to 6.01 at.%. The N configurations were changed from pyr-rolic-N to quaternary-N when increasing the temperature. Examination of the catalytic activities of the nanotubes for oxygen reduction reaction by rotating disk electrode measurements and single-cell tests shows that the onset potential for oxygen reduction in 0.5 M H₂SO₄ of the most effective catalyst (CNx nanotubes synthesized at 900 °C) was 0.83 V versus the normal hydrogen electrode. A current density of 0.07 A cm^-2 at 0.6 V was obtained in an H ₂/O₂ proton-exchange membrane fuel cell at a cathode catalyst loading of 2 mg cm^-2.