Nature of the catalytic centers of porphyrin-based electrocatalysts for the ORR:: A correlation of kinetic current density with the site density of Fe-N4 Centers

In this work, it has been shown that structural changes of an as-prepared catalyst enable the assignment of the catalytic centers responsible for the direct and indirect oxygen reduction reaction, respectively, of porphyrin-based electrocatalysts. An iron porphyrin (FeTMPPCl)-based catalyst as well as a catalyst based on H2TMPP were prepared using the so-called foaming agent technique (FAT). The obtained iron catalyst was used as a generic material for the post-treatments. Structural changes were analyzed by Fe-57 Mossbauer spectroscopy. The catalytic activity toward the oxygen reduction reaction (ORR) was determined using rotating (ring) disc electrode (R(R)DE) experiments. The catalysts exhibit a variation in the iron content between 2.9 and 4.5 wt % caused by the post-treatments. It has been found that the Mossbauer spectra of all catalysts can be fitted assuming two different ferrous Fe-N-4 centers, a CFeN2 center (Fe2+, S = 2) and an Fe3C center (Fe-0). After the intensities found in the Mossbauer spectra were normalized relative to the iron content, a linear correlation between the kinetic current density related to the direct oxygen reduction and the amount of in-plane Fe-N-4 centers is found. Beside this, there is evidence for a correlation between the kinetic current density related to the hydrogen peroxide formation and CFeN2 centers. Heat-treated carbon-supported iron porphyrin, prepared as reference material, exhibits the same behavior as our FAT catalysts. The correlation enables us to obtain the turnover frequencies for both the direct and the indirect oxygen reduction reaction and to determine the site densities, in which we reach a third of the target value published by Gasteiger et al.