Covalently bonded graphene/carbon nanotubes as high-performance carbon support for efficient bifunctional oxygen electrocatalysts

The activity and stability of oxygen electrocatalysts in electrochemical energy conversion technologies heavily rely on the structure of carbon support, which faces the challenge of simultaneously improving surface area, conductivity, and graphitization degree. Herein, we demonstrate a new class of covalently bonded graphene/carbon nanotube (G/CNT) hybrid as the carbon support for electrocatalysts. The covalent junctions between carbon nanotubes and graphene facilitate rapid electron transport, and graphene increases the available electrochemically active surface area. By loading two types of active sites - atomic Fe-N-C and Fe3O4 nanoparticles on G/CNT support, the hybrid catalyst exhibits superior bifunctional activity with a potential gap of 0.640 V between oxygen reduction and evolution reactions, surpassing that of noble-metal-based catalysts (0.747 V) and most reported catalysts. Due to the appropriate balance between high graphitization and surface functionalization provided by the G/CNT support, this catalyst exhibits exceptional resistance to corrosion and delivers excellent cycle performance in both liquid- and solid-state zinc-air batteries. Especially, a narrow voltage gap of 0.742 V over 1000 hours at 10 mA cm(-2) is achieved in liquid-state zinc-air batteries. Covalently bonded G/CNT possesses high electrical conductivity, a large surface area, modifiability, and good stability, making it a promising support for designing high-performance electrocatalysts.