Abstract
Nitrogen doped graphene hollow microspheres (NGHSs) have been used as the supports for the growth of the CoO nanoparticles. The nitrogen doped structure favors the nucleation and growth of the CoO nanoparticles and the CoO nanoparticles are mostly anchored on the quaternary nitrogen doped sites of the NGHSs with good monodispersity since the higher electron density of the quaternary nitrogen favors the nucleation and growth of the CoO nanoparticles through its coordination and electrostatic interactions with the Co2+ ions. The resulting NGHSs supported CoO nanoparticles (CoO/NGHSs) are highly active for the oxygen reduction reaction (ORR) with activity and stability higher than the Pt/C and for the oxygen evolution reaction (OER) with activity and stability comparable to the most efficient catalysts reported to date. This indicates that the CoO/NGHSs could be used as efficient bi-functional catalysts for ORR and OER. Systematic analysis shows that the superior catalytic activities of the CoO/NGHSs for ORR and OER mainly originate from the nitrogen doped structure of the NGHSs, the small size of the CoO nanoparticles, the higher specific and electroactive surface area of the CoO/NGHSs, the good electric conductivity of the CoO/NGHSs, the strong interaction between the CoO nanoparticles and the NGHSs, etc.
Highlights
Electrocatalytic activities of the TMOs include reducing the particles to nanometer size scale, doping them with electron donor and integrating them with conducting materials23–25
The CoO/nitrogen doped graphene hollow microspheres (NGHSs) reported in this work were synthesized from a procedure involving the preparation of the positively charged polystyrene (PS) spheres, the fabrication of the GO/PS composites by adsorption of the negatively charged GO onto the surface of the PS spheres through the electrostatic interaction, the formation of the NGHSs through the calcination of the GO/PS composites in the presence of melamine, and the subsequent deposition of the CoO nanoparticles
TEM shows the presence of some sheet-like materials (Fig. 1a), which could be attributed to free nitrogen doped graphene (NG) formed from GO unadsorbed on the PS spheres during the fabrication of the GO/PS composites, or NG formed from the damaged NGHSs, since the ultrasonication and mechanical stirring during the washing and sample preparation for TEM imaging may destroy the specific hollow structure of the NGHSs
Summary
Electrocatalytic activities of the TMOs include reducing the particles to nanometer size scale, doping them with electron donor and integrating them with conducting materials. The doping with nitrogen is an alternative way to make graphene available for the deposition of the TMOs, since nitrogen containing groups in nitrogen doped graphene (NG) with high electron density could act as the centers coordinating with transition metal ions in the reaction mixture, facilitating the nucleation of the TMOs and the subsequent growth of the TMO particles with small sizes In such NG supported TMOs (TMOs/NG), the improved electric conductivity of NG due to its nitrogen doped structure could make an additional contribution on improving the electrocatalytic activities of the TMOs. It is recently demonstrated that NG is electrocatalytically active for both the ORR and the OER31–33, which makes the TMOs/NG even more attractive as the bifunctional catalysts, since collective interactions between NG and TMOs may enhance their functionalities for the catalytic ORR and OER. The CoO/NGHSs are highly active for the oxygen reduction reaction (ORR) with activity and stability higher than the commercial Pt/C 20 wt.% and for the oxygen evolution reaction (OER) with activity and stability comparable to the commercial RuO2/C 20 wt.% and the most efficient catalysts reported to date, suggesting a great potential of using the CoO/NGHSs as bifunctional catalysts for the ORR and the OER
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