Fe-Nx/C electrocatalysts synthesized by pyrolysis of Fe(II)-2,3,5,6-tetra(2-pyridyl)pyrazine complex for PEM fuel cell oxygen reduction reaction

Highlights: → 2,3,5,6-Tetra(2-pyridyl)pyrazine (TPPZ) was employed as a ligand to prepare an iron(II) complex (Fe-TPPZ) at the first time. → The effect of both catalyst and Nafion ionomer loadings in the catalyst layer on the corresponding ORR activity was investigated by both rotating disk electrode and rotating ring disk electrode techniques. → It was found that this catalyst has strong ORR activity with an overall 4-electron transfer process through a (2 + 2)-electron transfer mechanism, which was assigned to the catalytic function of the Fe-Nx center. → The results obtained in a real fuel cell environment assembled with such a catalyst as the cathode are encouraging when compared with similar non-noble catalysts. - Abstract: 2,3,5,6-Tetra(2-pyridyl)pyrazine (TPPZ) was employed as a ligand to prepare an iron(II) complex (Fe-TPPZ) that served as a precursor to synthesize carbon-supported catalysts (Fe-Nx/C) through heat-treatment at 600, 700, 800 and 900 deg. C under N2 atmosphere. Both the structure and composition of the synthesized Fe-Nx/C were analyzed by X-ray diffraction and energy-dispersive X-ray microanalysis, respectively. The rotating disk and ring-disk electrode measurements showed that these catalysts have strong ORR activity with an overall 4-electron transfer process through a (2 + 2)-electron transfer mechanism, which was assigned to the catalytic function of the Fe-Nx center. A study on the heat-treatment temperature on the ORR activity showed that 800 deg. C is the optimal temperature for the synthesized catalysts. Furthermore, the effect of both catalyst and Nafion ionomer loadings in the catalyst layer on the corresponding ORR activity was also investigated. The kinetic parameters such as the chemical reaction rate between O2 and Fe-Nx/C (adduct formation reaction), the rate constant for the rate-determining step (RDS), and the electron numbers in the ORR, were obtained. The methanol tolerance of the catalyst was also tested. To validate the ORR activity, a membrane electrode assembly in which the cathode catalyst layer contained Fe-Nx/C was constructed and tested in a real fuel cell. The results obtained are encouraging when compared with similar non-noble catalysts.