Surface confinement assisted synthesis of nitrogen-rich hollow carbon cages with Co nanoparticles as breathable electrodes for Zn-air batteries

Abstract

Integrating high electrocatalytic activities for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) into one nonprecious catalyst entity is highly desired for simplifying the fabrication, operation as well as lowering the production cost of rechargeable Zn-air batteries. Herein, zeolite imidazole framework-67 (ZIF-67) polyhedron nanocrystals are used as N-containing carbonaceous precursors to synthesize N-rich hollow carbon cage composites with Co nanoparticles (Co@NHCC) in the presence of silica nanoparticles and graphitic carbon nitride (g-C3N4), an additional high-temperature decomposable nitrogen source. The assembly of dense silica nanoparticles on ZIF-67 surface may serve as hard encapsulation layers, which not only facilitates the incorporation of nitrogen atoms onto graphitized carbon skeletons, but also prevents the ZIF-67 polyhedrons from collapsing and effectively impedes the growth of carbon nanotubes during pyrolysis through surface confinement. Gasification in such an encapsulated environment concurrently generates abundant mesopores in the thin walls of carbonized ZIF-67 polyhedrons, which contributes to a high specific surface area of 910.71 m2 g−1 in the resulting Co@NHCC-800 sample, and hence endows an efficient bifunctional electrocatalyst, with a half-wave potential of +0.837 V for ORR in 0.1 M KOH aqueous solution and a required potential of only +1.512 V to reach 10 mA cm−2 in OER catalysis. When used as a breathable electrode for Zn-air batteries, Co@NHCC-800 shows a much higher open-circuit voltage of 1.490 V, a higher discharge power density of 248 mW cm−2 as well as a smaller increasement of only 0.100 V for the charge-discharge voltage gap after cycling for 12 h, as compared with commercial Pt/C catalyst. The results in present work pave a new avenue to the synthesis of sophisticated carbon nanomaterials for electrocatalysis.