Effect of synthesis method on the oxygen reduction performance of Co–N–C catalyst

Abstract In recent years, Co, N co-doped carbon (Co–N–C) materials as oxygen reduction reaction (ORR) catalysts have attracted great attention because of their good ORR stability as well as decent activity. Co-doped zeolitic imidazolate framework-8 (Co@ZIF-8) as the precursor for synthesizing Co–N–C has attracted great interest recently. Co@ZIF-8 synthesis method may affect the properties of the as-synthesized Co@ZIF-8 precursors, which will surely affect the properties and ORR performance of Co@ZIF-8-derived Co–N–C catalysts. Herein, three methods, viz. room-temperature stirring method, reflux method, and hydrothermal method, were used to synthesize Co@ZIF-8 precursors. Physical characterization shows that the synthesis method has a great influence on the textural properties, composition, and graphitization degree of the as-synthesized Co–N–C catalysts. Electrochemical characterization shows that Co–N–C-R synthesized with reflux method exhibits an onset potential (Eonset) of 0.905 V, a half-wave potential (E1/2) of 0.792 V and a limiting current density (JL) of 5.85 mA cm−2 in acidic media, which are higher than those of Co–N–C–S (Eonset = 0.870 V, E1/2 = 0.770 V, JL = 4.71 mA cm−2) and Co–N–C–H (Eonset = 0.892 V, E1/2 = 0.785 V, JL = 4.68 mA cm−2) synthesized with room-temperature stirring method and hydrothermal method, respectively. The better ORR activity observed on Co–N–C-R can be attributed to its larger graphitization degree and larger mesopore volume. Catalytic stability test shows that Co–N–C-R has negligible activity loss after 5000 potential cycles. This work demonstrates that reflux method is a more suitable method for synthesizing Co–N–C catalysts for ORR.