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

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–N x /C) through heat-treatment at 600, 700, 800 and 900 °C under N 2 atmosphere. Both the structure and composition of the synthesized Fe–N x /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–N x center. A study on the heat-treatment temperature on the ORR activity showed that 800 °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 O 2 and Fe–N x /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–N x /C was constructed and tested in a real fuel cell. The results obtained are encouraging when compared with similar non-noble catalysts.