Heat-treated iron(III) tetramethoxyphenyl porphyrin supported on high-area carbon as an electrocatalyst for oxygen reduction - I. Characterization of the electrocatalyst

Iron(III) tetramethoxyphenyl porphyrin chloride adsorbed on the Black Pearls carbon (FeTMPP-Cl/BP) was heat-treated at temperatures from 200 to 1000 degrees C in Ar atmosphere and characterized by using cyclic voltammetry, Xray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) techniques. Cyclic voltammetry experiments were carried out in acidic and alkaline solutions. Redox peaks due to the Fe(III)/Fe(II) reaction were present in an unheat-treated sample and in the sample heat-treated at 200 degrees C but were significantly reduced in height for the sample heat-treated at 400 degrees C. At higher temperatures they disappeared, indicating decomposition of FeTMPP-Cl. After heat-treatment at 700 degrees C or higher, an anodic peak in the first cathodic sweep was observed and it was attributed to the dissolution of metallic Fe. XPS measurements did not show unambiguously the valence state of the Fe in the electrocatalyst heat-treated at high temperatures, but comparison of cyclic voltammograms of electrodeposited Fe and a bulk Fe electrode with cyclic voltammograms of FeTMPP-Cl/BP indicated that it was metallic Fe. Fe particles were observed on TEM images. Many more iron particles were present in the sample heat-treated at 1000 degrees C than in the sample heat-treated at 800 degrees C. Some of the particles remained in both samples after electrochemical dissolution, and they were completely surrounded by carbon particles or wrapped in the graphite shell. A well-defined graphite shell surrounding iron particles was observed only in the sample heat-treated at 1000 degrees C.