Calcined cobalt-chelated, N-containing poly(methylenediphenyl urea) as an ORR cathode catalyst

Abstract

A Co(II)-chelated poly(methylenediphenyl urea) (Co-PMDUr) is successfully prepared from the condensation reaction between 4,4′-methylene diphenyl diamine (MDA) and 4,4′-methylene diphenyl diisocyanate (MDI) in the presence of cobalt chloride (CoCl2). After polymerization, the Co(II) ions chelate to the polymer backbones by coordinating with the urea units' carbonyl or amine groups, confirmed by the FTIR spectra. The Co-PMDUr converts to Co, N-codoped carbon network (Co-N-Cs) via calcination in nitrogen and ammonia gases. X-ray diffraction and Raman spectra of Co-N-Cs reveal the formation of highly conjugated carbon structure, similar to that of carbon nanotube (CNT) or carbon nano-ring (CNR) seen in SEM and TEM micropictures. Except for cobalt nitride (CoN), other nitrogen-related groups like pyridinic, graphitic, and pyrrolic-Ns are present in the Co-N-Cs, and higher temperature calcination creates more pyridinic-Ns and Co-Ns. The BET analysis reveals high surface area and micro-pores for Co-N-Cs. The current-voltage curves of Co-N-Cs show a significant oxygen reduction reaction (ORR) at ca. 0.8 V and a higher limited reduction current density (LRCD) than commercial platinum carbon (Pt/C) catalyst in the O2-saturated KOH(aq). The average number of electrons (n) transferred during ORR is close to 4. The Tafel slope of the Co-N-C catalyst is similar to Pt/C. The max. Current density (Imax) and max. Power density (Pmax) of the single cell with Co-N-C cathodes is comparable to that of Pt/C with less decadency during long-term redox reaction testing.