Preparation of Mn-N-C catalyst and its electrocatalytic activity for the oxygen reduction reaction in alkaline medium

Chitosan salicylaldehyde Schiff-base manganese complex (Mn-CS-sal) was synthesized by a simple chemical method and then supported on graphite carbon; the graphite-supported Mn-CS-sal (Mn-CS-sal/C) was heat-treated at different temperatures (t) to obtain the Mn-N-C catalysts (Mn-N-C-t, t=200, 400, 600, 800, 1 000 ℃). The electrocatalytic activity of Mn-N-C catalysts in the oxygen reduction reaction (ORR) was investigated and their structure and composition were characterized by Fourier Transform Infrared (FT-IR) spectrum, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results indicated that all the Mn-N-C catalysts are highly active in ORR, whereas the Mn-N-C-600 catalyst heat-treated at 600 ℃ exhibits the highest ORR activity. The excellent ORR activity of the Mn-N-C catalysts in alkaline media should be attributed to their Mn-N-C configuration. Two important kinetic parameters, i.e. the overall ORR electron transfer number (n) and electron transfer coefficiency (αn_α), were determined by the cyclic voltammetry method. The Mn-N-C-600 catalyst shows an overall electron transfer number of 3.63 for ORR, suggesting that the catalytic ORR is via a mixture of 2- and 4-electron transfer pathways, but dominated by the 4-electron transfer process. Based on these observations, a possible mechanism for ORR over the surface of Mn-N-C modified electrode is proposed.