Novel N-doped carbon/Co/Co3O4 ternary composites derived by direct carbonization of ZIF-67: Efficient electrocatalysts for oxygen reduction reaction

Abstract

Cobalt-containing zeolitic imidazole framework ZIF-67 was synthesized in high yield, and directly carbonized by different heating routes at 800 and 900 °C. The products of carbonization, C(ZIF-67)s, were comprehensively characterized in terms of elemental composition (FAAS, EDX, XPS), crystalline (XRD) and molecular structure (FTIR and Raman spectroscopies), morphology (SEM), electrical conductivity, textural (N2 physisorption), and electrochemical properties. It was found that C(ZIF-67)s represent novel meso/microporous ternary composites of the type N-doped carbon/Co/Co3O4, containing metallic Co nanoparticles (NPs) with cubic body-centred crystalline structure, and predominately amorphous Co3O4. They exhibited high electrical conductivity (up to 4.2 S cm−1), notable BET specific surface area (197–265 cm2 g−1), and almost doubled mesopore volume compared to the parent ZIF-67. The effects of carbonization conditions on the structure, physico-chemical properties, and performance of C(ZIF-67)s as electrode materials in electrocatalysis of oxygen reduction reaction (ORR) and charge storage were studied. All C(ZIF-67) composites showed excellent ORR electrocatalytic activity in 0.1 M KOH, with four-electron reduction pathway. The highest ORR activity (the onset potential of −0.13 V vs. SCE) showed the composite produced by gradual heating up to 800 °C followed by holding at that temperature for 3 h. This is attributed to its highest mesopore volume, appropriate meso/micropore structure, high surface content of heteroatom-containing active sites (C–O–C, Co–N, Co–O), high surface Co2+/Co3+ ratio and the presence of Co NPs. The applied direct carbonization of ZIF-67, without additives and post-synthetic modifications, was shown as a simple way to produce meso/microporous electroconducting composites with high potential in energy related applications