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

Abstract

Iron(III) tetramethoxyphenyl porphyrin chloride adsorbed on the Black Pearls carbon (FeTMPP-Cl/BP) was heat-treated at temperatures from 200 to 1000°C in Ar atmosphere and characterized by using cyclic voltammetry, X-ray photoclectron 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°C but were significantly reduced in height for the sample heat-treated at 400°C. At higher temperatures they disappeared, indicating decomposition of FeTMPP-Cl. After heat-treatment at 700°C or higher, an anodic peak in the first cathodic A weep 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 dectrodeposited 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°C than in the sample heat-treated at 800°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°C.