Simple conversion of earth-abundant coal to high-performance bifunctional catalysts for reversible oxygen electrodes

Efficient bifunctional electrocatalysts for reversible oxygen electrodes are vitally important for the realization of important future generation electrochemical energy conversion devices, such as regenerative fuel cells and rechargeable metal-air batteries. Importantly, the electrocatalysts need not only be efficient but also inexpensive and source-abundant from the viewpoint of sustainability. Herein, we report active and stable bifunctional catalysts for oxygen electrodes obtained by pyrolysis of pristine coal in ammonia. The pyrolysis yielded nitrogen-rich mesoporous graphitic carbon containing pyrrolic, pyridinic and graphitic groups, which demonstrated very good electrocatalysis of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in 0.1 M NaOH. The optimized catalyst afforded a stable round-trip over-voltage of 0.97 V when cycling between the ORR at a current density of -1.0 mA cm(-2) and the OER at 10 mA cm(-2), lower than those of RuO2 and IrO2 in the same solution, thus unveiling a unique opportunity to directly transform coal into a valuable catalyst without costly processing.