Co-Embedded Carbon Nano-Polyhedron Supported on Functionalized Graphene Oxide for Efficient Oxygen Reduction Reaction
Authors: Liu Ziqi , Macedo Andrade Angela , Grewal Simranjit , Lee Min Hwan 
Published: February 2020

Recently, Metal Organic Frameworks (MOFs) have been used as precursors to synthesis nanomaterials because of their crystalline and porous structure. Cobalt (Co) based zeolitic imidazolate frameworks (ZIFs) are one kind of 3D MOF structures with Co sitting in the center of well-organized organic linkers. Metal doped carbon-based nanomaterials obtained by pyrolysis of Co based ZIFs (Co-ZIFs) have been considered as promising candidates for improving oxygen reduction reaction (ORR) due to abundant Co-C/N bonds, which have been proved to be excellent catalytic structure. In addition, the polyhedron structure of the Co-ZIFs could be maintained even after pyrolysis at elevated temperatures resulting in a highly porous and stable support for further enhancement. However, Co-ZIFs tend to aggregate together during pyrolysis process because of their porous structure, which would destroy the polyhedron structure and decrease surface area of the product and further reducing their electrochemical performance.

To prevent agglomeration of the Co-ZIFs usually a supplemental template is incorporated to maintain the well-defined polyhedron structure even after pyrolysis. A potential candidate to use as a template is graphene oxide. Graphene oxide (GO) based materials have been used for electrodes and catalysts due to their excellent electrical conductivity and highly surface area. GO is a bifunctional structure that contains epoxy and hydroxyl functional groups on both side of its surface. These groups could be used as the connectors for MOF forming unique structure with enhanced properties. In our work, we demonstrate a facial synthesis of Co doped porous carbon nanomaterials supported on functionalized GO by combing ZIF-67 with functionalized GO. Several different types of functionalized graphene oxides were synthesized, which include: graphene oxide treated by HBr (HG), HG treated by oxalic acid (HOG) , nitrogen doped graphene oxide (NGO). These functionalized GOs are used to potentially increase the interactions between GO and ZIF-67. Thus, more ZIF-67 will be anchored onto surface of graphene oxides due to increased active sites provided by those functional groups.

Afterwards, a post synthesis pyrolysis is included to get highly ordered porous polyhedron carbon structure supported on GO with high surface area. Pyrolysis of ZIF-67/GO showed comparable onset potential under alkaline atmosphere (1mol/L KOH), which is 0.81mV vs. RHE. We assume that pyrolysis of ZIF-67/HG will exhibit higher ORR properties with more stability than the others due to more epoxy groups created on the surface. All the as-prepared product exhibits comparable or even enhanced electrocatalytic properties than commercially used Pt/C catalyst.

Moreover, graphene oxides could also act as templates to fix ZIF-67 in a certain position, which would prevent ZIFs from aggregating together during pyrolysis process. With the combination of graphene oxide, the electron transfer pathways and active sites will be increased, enhancing the catalytic properties for ORR.

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