Non-platinum oxygen reduction electrocatalysts based on pyrolyzed transition metal macrocycles

Abstract

In this work pyrolyzed porphyrins were investigated for oxygen reduction electrocatalysis. Pyrolysis of non-supported cobalt and iron tetraphenylporphyrins in the temperature range of 500–800 °C generates high surface area catalysts with high degree of exposure of active sites to the reacting species. This is achieved through templating porphyrins on fumed amorphous silica that is removed after pyrolysis by etching with concentrated KOH. Detailed material characterization of the pyrolyzed materials is presented here. X-ray photoelectron spectroscopy (XPS) analysis of cobalt and iron porphyrins was used to elucidate the transformations of nitrogen, carbon, cobalt and iron species resulting from the heat treatment. Partial decomposition of the precursor material and formation of polymer-like network decorated by metal oxide particles are identified. Differences in the chemical composition of products of pyrolysis of FeTPP, CoTPP and Co/FeTPP are discussed. Transmission electron microscopy (TEM) imaging revealed the structure of the pyrolyzed porphyrins and was used to gain insight into the size of the metal crystals formed in the bulk. X-ray diffraction spectra (XRD) provided information about the type of crystals formed in the different formulations of the precursor porphyrins. Further, steady state polarization curves were obtained utilizing gas diffusion type electrodes in 0.5 M sulfuric acid and membrane electrode assembly (MEA) configurations under working PEM fuel cell conditions. This work revealed the necessity of the metal phases for the oxygen reduction process.