Conformal dispersed cobalt nanoparticles in hollow carbon nanotube arrays for flexible Zn-air and Al-air batteries

The development of both flexible solid-state Zn-air and Al-air batteries are challenged by the efficient and stable air cathodes with high catalytic activities in both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). In this work, we report the rational design of hollow N-doped carbon nanotube arrays embedded with confined Co nanoparticles (HCA-Co) through a facile solution-reaction and annealing process. Due to the unique integration of hollow carbon nanoarray with tiny cobalt nanoparticles, the obtained flexible HCA-Co electrode shows promising catalytic properties toward both ORR and OER that achieves a current density of 10 mA cm⁻² at small overpotential of 290 mV in OER, and demonstrates an onset potential of 0.92 V in ORR. The HCA-Co can be applied as a binder-free air-cathode for flexible all-solid-state zinc-air batteries, which presents a relatively high open circuit potential (1.40 V) with better cycling stability than Pt/C based battery. The HCA-Co is also utilised as cathode for solid-state Al battery, which shows a high open circuit potential (1.966 V) with better mechanical flexibility than that of Pt/C-based battery. Such flexible electrode with excellent bifunctional catalytic properties hold great promise for the application in flexible electronics.