Active sites of the functionalized coals and carbons for oxygen reduction reaction in a fuel cell

Abstract

The active sites of the coals and carbons functionalized with added nitrogen, oxygen and iron were studied for the oxygen reduction reaction (ORR) in a fuel cell. The catalysts were characterized based on the XPS, Raman, TEM, XRD and N₂ adsorption measurements. The ORR activity was promoted by the addition of iron and aluminum as the inorganic components of the ash to the ash-free brown coal. The ORR activity of the ash-components added to the ash-free brown coal was correlated to the I_D/I_G ratio (deficient carbon degree) and the pyridinic nitrogen based on the Raman and XPS analyses, respectively. The active sites of the brown coal were formed at the pyridinic nitrogen on parts of the defective carbons associated with iron on the alumina. On the other hand, for the nitrogen-doped carbons without iron, the ORR activity was related to the pyrrolic-NH, pyridinic nitrogen species and the defective carbon degree. Based on these results, the active sites of the iron-added and nitrogen-doped coals and carbons were the iron sites coordinated with the pyridinic nitrogen, while the active sites of the iron-free and nitrogen-doped carbons without iron were the pyrrolic-NH and pyridinic-NH⁺ sites of parts of the defective carbons. The difference between the active sites of the nitrogen-doped coals and carbons in the presence of iron and those in the absence of iron was discussed. These results suggested that the pyridinic N as a base site transformed into pyridinic-NH⁺ as an acid site by attack of the proton from the anode.