Investigation of hollow nitrogen-doped carbon spheres as non-precious Fe-N₄ based oxygen reduction catalysts

The development of inexpensive non-precious oxygen reduction catalysts has become one of the most important efforts in polymer electrolyte membrane fuel cells. In this report, we synthesized a non-precious electrocatalyst from a single precursor, iron(iii) diethylene triaminepentaacetate, using a heat-treatment effect to prepare an active catalyst. A series of catalysts were prepared at different temperatures leading to different degrees of graphitization, heteroatom content and activity. In 0.1 M KOH electrolyte solution, the oxygen reduction reaction (ORR) onset potential of the HNCS71 catalyst was as high as 0.97 V, and half-wave potentials were only 20 mV lower than those for Pt/C. X-ray absorption measurements of the Fe K-edge showed the structure of Fe-N₄ centers, formed in HNCS71, which were responsible for the ORR activity. An alkaline exchange membrane fuel cell fabricated with HNCS71 as the cathode was tested in a H₂-O₂ single cell and showed a maximum power density of ∼68 mW cm^-2. The 100 hour fuel cell durability test of the HNCS71 cathode showed a decay in the current density of about 14% at 0.4 V. Therefore, the HNCS catalyst appears to be a promising new class of non-precious catalysts for fuel cell applications.