Hollow carbon nanospheres@graphitic C3N5 heterostructures for enhanced oxygen electroreduction

Abstract

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 O2 activation and desorption of the intermediate reaction products. Compared with stand-alone HCNs and g-C3N5, the HCNs@g-C3N5 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 O2 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 O2 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.