Equivalent conversion of metal-free and metal-based (Co1-xS/Co9S8 nanohybrid) catalysts: easy construction of a "highway" shaped porous carbon material as a dual-functional electrocatalyst for high-performance Zn-air batteries

Abstract Metal-free and metal-based electrocatalysts are designed according to different principles, but there should be an intimate connection between them. In this work, a simple yet cost-effective strategy is proposed to etch and disperse graphene oxide (GO) and porous hollow carbon shell loaded with CoS2 (PHCS-CoS2) into a porous skeleton similar to "highway" to effectively "trap" graphene oxide quantum dots (GOQDs) via a one-step process. Due to interconnected networks and size effects, N-S codoped porous graphene carbon sheets along with GOQDs (N,S-PGCS(QDs)) have eminent properties for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Furthermore, the metal-free system can be transformed to a metal-based electrocatalyst (N,S-PGCS(QDs)-Co1-xS/Co9S8) loaded with Co1-xS/Co9S8 nanohybrids, which demonstrates superior electrocatalytic characteristics than Pt/C+RuO2 in terms of half-wave potential (0.85 V), current density, methanol tolerance, the potential difference between EORR1/2 and EOER10 (0.68 V), and long-term stability. The Zn-air battery assembled with N,S-PGCS(QDs)-Co1-xS/Co9S8 shows a large peak power density of 168.2 mW cm−2, a capacity performance up to 817.9 mAh g−1, an ultra-high specific energy density of 973.3 Wh kgZn−1 and considerable durability. The findings above may provide a scalable synthetic guidance and a novel direction for the design of metal-free and metal-based electrocatalysts.