The development of Pt-free catalysts is strongly desired for globalization of proton exchange membrane fuel cells. Our research group has been developing Pt-free cathode catalysts obtained by pyrolyzing Fe containing polyimide nano-particles. While this sort of polyimide derived cathode catalysts shows very good performance, the structure of the Fe center was obscure. Furthermore, the previous synthesis of the catalysts started from a relatively large amount of Fe additives (2 wt%); therefore, extraordinary Fe species require to be washed out using HCl, the process of which was redundant.

In the present study, we tried to avoid the acid washing process by starting from a lower amount of Fe additives to simplify the synthetic procedure and increase the fraction of atomically dispersed Fe centers. Polyimide nano-particles were synthesized by the precipitation polymerization of pyromellitic acid dianhydride and 1,3,5-tris(4-aminophenyl)benzene with 0.3-0.5 wt% of Fe(acac)₃ (acac = acetylacetonate). The Fe-containing polyimide precursor was heated at 600 °C for 5 h in a nitrogen atmosphere, and then heated again to 800 and 1000 °C for 1 h each in an ammonia atmosphere (50% balanced by nitrogen). Any acid washing was not performed during the whole synthetic procedure. The chemical composition of the resultant carbon was determined using a CHN elemental analyzer (PerkinElmer 2400-II) and an electron probe micro analyzer (Jeol JXA-8100): C 88.3 wt%, H trace, N 2.6 wt% and Fe 1.7 wt%. The Brunauer, Emmett and Teller (BET) surface area was determined to be 1360 m² g^-1 (Bel Japan Belsorp mini II).

Thus obtained Pt-free cathode catalyst was tested under H₂-O₂ fuel cell conditions. Figure 1 shows the I-V performance curves for the membrane electrode assembly (MEA) with the obtained catalyst. The measured and IR-free voltages at 2 A cm^-2 reached 0.45 and 0.65 V, respectively; and the maximum power density was 0.89 W cm^-2, suggesting the newly synthesized catalyst exhibits an extremely high performance as a Pt-free cathode catalyst.