Hollow carbon nanospheres@graphitic C3N5 heterostructures for enhanced oxygen electroreduction

Improving the electrocatalytic activity of metal-free catalysts for the oxygen reduction reaction (ORR) is of great significance as promising alternatives to the Pt group precious catalysts in fuel cells. This paper proposes a simple template-based strategy to fabricate a catalyst with enhanced ORR activity. The resulting material contained 3D heterojunction and consisted of hollow carbon nanospheres and graphitic C3N5(HCNs@g-C3N5). The fast electron/charge transports between g-C3N5 and HCNs was achieved because of the conductive heterojunction presence. This heterojunction provided the potential difference, which is beneficial for O-2 activation and desorption of the intermediate reaction products. Compared with stand-alone HCNs and g-C3N5, the HCNs@gC(3)N(5) composite demonstrated significantly enhanced ORR activity with a more positive half-wave potential and faster kinetics in the alkaline medium. Furthermore, linear sweep voltammetry demonstrated that the water generation from O-2 occurred through a four-electron transfer reaction. According to calculations performed using density functional theory (DFT), a built-in electronic field (IEF) formed at the heterojunction because of the charge density difference between C and N atoms. This IEF reduced the O-2 activation energy and lowered the charge transfer resistance, which, in turn, enabled the rapid formation and desorption of the intermediates and significantly enhanced ORR activity of HCNs@g-C3N5.