Electrocatalytic Activity of Hemin-Derived Hollow Non-Precious Metal Catalyst for Oxygen Reduction Reaction

In recent years, increasing efforts have been undertaken to develop non-precious metal (NPM) catalysts with both high activity and stability toward the oxygen reduction reaction (ORR), since they are much less expensive than commercially available Pt-based electrocatalysts. Transition metal macrocyclic compounds contain transition metal, nitrogen, and carbon species, hence becoming promising precursors for the synthesis of NPM catalysts. Hemin, a natural transition-metal-based macrocyclic compound, is widely applied to the synthesis of NPM electrocatalysts. However, the ORR activity of hemin-derived electrocatalysts must be improved considerably as compared with that of state-of-the-art NPM electrocatalysts. Morphology control is an efficient method to increase the exposure of active sites, thus enhancing the ORR activity. Here, we fabricated a hollow NPM electrocatalyst (hemin hollow derivative, Hemin-HD) using hemin as the precursor and NaCl as the template. First, hemin and NaCl were dispersed and mixed in solution. With an increase in the temperature, the solution was vapored and NaCl began to crystallize. Hemin wrapped the outer surface of NaCl because of the ionic interaction between these two compounds. The as-obtained powders were collected and carbonized at high temperature under a nitrogen atmosphere. Then, the NaCl template was removed by washing, and the hollow material Hemin-HD was obtained. Physicochemical characterization by transmission electron microscopy (TEM), X-ray diffraction (XRD), surface area measurements and X-ray photoelectron spectroscopy (XPS) confirmed that the surface area and pore volume of the as-obtained Hemin-HD electrocatalyst increased by a factor of 6.5 and 3.8, respectively, relative to those of the Hemin-D (hemin derivative) sample without the NaCl template. Owing to the hollow structure and increased surface area, the Fe and N content on the Hemin-HD surface were higher than those on the Hemin-D surface. Consequently, Hemin-HD showed better ORR activity in alkali solution than Hemin-D did, this was confirmed by the fact that the half-wave potential of Hemin-HD was greater than that of Hemin-D by 20 mV, and faster kinetics were observed for the former, as calculated by the Tafel slope. The performance of Hemin-HD was comparable to that of commercial Pt/C catalysts for the ORR in alkali solution. It is believed that the hollow structure allows the dispersion of active sites on both the inner and outer surfaces, thus facilitating the exposure of a great number of active sites. Besides, the pore structure of the electrocatalyst is expected to boost mass transfer and improve the contact between the active sites and reactants, thus enhancing the ORR activity.