Controllable Synthesis of Co@CoOx/Helical Nitrogen Doped Carbon Nanotubes towards Oxygen Reduction Reaction as Binder-Free Cathodes for Al-Air Battery.

Efficient and stable electrocatalysts for oxygen reduction reaction and free-standing electrode structure were developed to reduce the use of polymer binder in the cathode of metal-air battery. Considering the unique geometrical configurations of helical carbon nanotubes (HCNTs) and improved properties compared with straight CNTs, we prepared high-purity Co@CoOx/HNCNTs on a carbon fiber paper by hydrothermal and single-step in-situ CVD strategies. Under the optimized growth time (1 h), the synthesized Co@CoOx/HNCNTs provide richer edge defects and active sites, and show prominent electrocatalytic performance towards ORR under alkaline media compared with Co@CoOx/HNCNTs-0.5 h and Co@CoOx/HNCNTs-2 h. The soft X-ray absorption spectroscopy (XAS) technique is used to investigate the influences of different growth time on electronic structure and local chemical configuration of Co@CoOx/HNCNTs. Furthermore, the Al-air coin cell employing Co@CoOx/HNCNTs-1 h as the binder-free cathode exhibits an open circuit voltage of 1.48 V, a specific capacity of 367.31 mAh g-1 at the discharge current density of 1.0 mA cm-2, and a maximum power density (Pmax) of 3.86 mW cm-2, which is superior to Co@CoOx/HNCNTs-0.5 h and Co@CoOx/HNCNTs-2 h electrodes. This work provides valuable insights into the development of scalable binder-free cathodes exploiting HNCNTs composite materials with outstanding electrocatalytic performance for ORR in Al-air systems.