Nitrogen-doped nanoporous carbon/graphene nano-sandwiches: Synthesis and application for efficient oxygen reduction

A zeolitic-imidazolate-framework (ZIF) nanocrystal layer-protected carbonization route has been developed to prepare N-doped nanoporous carbon/graphene nano-sandwiches. The
ZIF/Graphene Oxide/ZIF sandwich-like structure with ultra-small ZIF nanocrystals (i.e. ~ 20 nm) fully covered the graphene oxide (GO) is prepared via a homogenous nucleation followed
by a uniform deposition and confined growth process. The uniform coating of ZIF nanocrystals on the GO layer can effectively inhibit the agglomeration of GO during high-temperature treatment (800 oC). After carbonization and acid etching, N-doped nanoporous carbon/graphene nanosheets are formed, with a high specific surface area (1170 m2/g). These N-doped nanoporous carbon/graphene nanosheets are used as the non-precious metal electrocatalysts for oxygen reduction and exhibit a high onset potential (0.92 V), and a large limiting current density (5.2 mA cm-2 at 0.60 V vs RHE). To further increase the oxygen
reduction performance, nanoporous Co-Nx/carbon nanosheets are also prepared by using cobalt nitrate and zinc nitrate as co-metal sources, which reveal higher onset potential (0.96
V) than both commercial Pt/C (0.94 V) and N-doped nanoporous carbon/graphene nanosheets. Such nanoporous Co-Nx/carbon nanosheets also exhibit good performance such as high activity, stability and methanol tolerance in acidic media.