Significant enhancement of the oxygen reduction activity of self-heteroatom doped coal derived carbon through oxidative pretreatment

Abstract

Functional low-cost catalysts derived from earth abundant materials are vitally important for enabling mass implementation of renewable energy conversion systems, such as fuel cells and metal-air batteries. Herein, we report the activation of earth-abundant coal to realize very active and stable catalysts for the oxygen reduction reaction (ORR) through oxidative pretreatment followed by thermal treatment under ammonia forming self-doped heteroatoms and defects. The coal thermally pretreated in air at 350 °C for 30 min followed by treatment in ammonia for 2 h presented very good electrocatalytic ORR activity with onset potential of 0.978 V, half-wave potential of 0.871 V, Tafel slope of 57 mV dec−1, and predominant 4-electron reduction of O2 to OH−. The influence of the preoxidation step prior to ammonia treatment on the properties of the coal-derived carbon and hence the ORR has been specifically studied. The optimized catalyst also displayed stable performance during the ORR in 0.1 M NaOH for a test period of 35 h and exhibited excellent tolerance to methanol. The preoxidation step facilitates elimination of aliphatic and amorphous carbon, condensation of aromatic structures and the formation of initial pore structures, and subsequent removal of unstable heteroatoms from aromatic carbon rings to form surface and edge defects as potential active sites for promoting the ORR. Using pristine coal as raw material to produce functional ORR catalysts will be of great importance for real applications.