Heat Treatment of Carbonized Hemoglobin with Ammonia for Enhancement of Pore Development and Oxygen Reduction Activity

Abstract

A highly porous carbonaceous material was formed from hemoglobin, which is a natural compound that could be abundantly and inexpensively obtained. Porous carbonized hemoglobin was formed using MgO nanoparticles as a template; in addition, heat treatment of the carbonized hemoglobin with ammonia was performed. Two series of ammonia treatments were examined: long-time heat treatments at various temperatures and high-temperature heat treatments for various times. The specific surface area and, in particular, the pore volume were increased by the ammonia treatment most efficiently at a high temperature for a short period of time. X-ray photoelectron spectroscopy showed a significant decrease in the surface nitrogen concentration and a slight decrease in the surface Fe concentration by the ammonia treatment. The extended X-ray absorption fine structure at the Fe K-edge indicated that Fe was coordinated with four nitrogen atoms (Fe–N4 moiety) in the ammonia-treated carbonized hemoglobin. The oxygen reduction activity was evaluated using rotating disk electrodes. The enhancement was observed in accordance with the pore development. A polymer electrolyte fuel cell formed using the ammonia-treated carbonized hemoglobin in the cathode showed an improved performance compared to that formed using the carbonized hemoglobin before the treatment.