The ORR activity of nitrogen doped-reduced graphene oxide below decomposition temperature cooperated with cobalt prepared by strong electrostatic adsorption technique

Abstract

Nitrogen doped reduced graphene oxide (NG) was prepared using urea as a nitrogen source. The pyrolysis temperature was carried on at 300, 400, 500 and 800 °C, where it was below and above the urea decomposition temperature (350 °C), respectively. Pyrrolic-N was the main component of NG300, whereas pyridinic-N was the major component of NG800. Moreover, NG300 had greater nitrogen content and performed a bit better in oxygen reduction reaction (ORR) activity. The points of zero charge were determined in order to adsorb cobalt precursor. Afterward, cobalt metal was impregnated onto NGs by strong electrostatic adsorption at pH 12.0. The Co/NG300 had better metal distribution, and the average Co particle size was about 14.0 nm, while Co/NG800 was composed of a lump of metals due to the agglomeration of cobalt. Nitrogen composition decreased after the synergy with Co, but its content on Co/NG300 was still greater than Co/NG800. The Co-Nx structure, where x can be 2 and 4 according to pyrrolic-N and pyridinic-N configuration, was identified for both Co/NGs; nitrogen atoms were chelated to the edges and the defects of the NG moiety. Co/NG300 had superior ORR activity over the other samples, and it had outstanding durability and stability after running within saturated oxygen for over 2000 cycles. This is probably attributed to its lower pyrolysis temperature yielding a high density of nitrogen and Co-Nx active sites.