Earth-abundant coal-derived carbon nanotube/carbon composites as efficient bifunctional oxygen electrocatalysts for rechargeable zinc-air batteries

Abstract The exploration of active and robust electrocatalysts for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is the bottleneck to realize the commercialization of rechargeable metal-air batteries and regenerative fuel cells. Here we report facile synthesis of three-dimensional (3D) carbon nanotube (CNT)/carbon composites using earth-abundant coal as the carbon source, hydrogen reductant and heteroatom dopant to grow CNTs. The prepared composite featuring 3D structural merits and multiple active sites can efficiently catalyze both ORR and OER, affording high activity, fast kinetics, and long-term stability. With the additional incorporation of manganese, the developed catalyst afforded a potential difference of 0.80 V between ORR at the half wave potential and OER at a current density of 10 mA cm−2. The optimized sample has presented excellent OER performance within a constructed solar-powered water splitting system with continuously generating oxygen bubbles at anode. Notably, it can be further used as a durable air-electrode catalyst in constructed Zn-air battery, delivering an initial discharge/charge voltage gap of 0.73 V, a remained voltaic efficiency of 61.2% after 160 cycles and capability to power LED light for at least 80 h. This study provides an efficient approach for converting traditional energy resource i.e. coal to value-added alternative oxygen electrocatalysts in renewable energy conversion systems.