Bifunctional catalysts of CoNi nanoparticle-embedded nitrogen-doped carbon nanotubes for rechargeable Zn-air batteries.

It is essentially important to improve the performance of Zn-air batteries by studying bifunctional catalysts for oxygen evolution reactions (OER) and oxygen reduction reactions (ORR) with low-cost, high-efficiency and high-stability properties. Here, CoNi nanoparticles embedded in the bamboo-like N-doped carbon tubes (Co x Ni y @NC) were synthesized, where the optimized catalyst of Co2Ni1@NC exhibits superior bifunctional electrocatalytic activity, showing a low overpotential of 300 mV under the current density of 10 mA cm-2 for OER and a large limiting current density of 3.76 mA cm-2 under 0.40 V for ORR in an alkaline solution. In addition, the Co2Ni1@NC also shows excellent electrocatalytic activity in acidic and neutral solutions. Importantly, primary Zn-air batteries based on Co2Ni1@NC affords an excellent specific capacity of 834 mAh/gZn with a discharge potential of 1.25 V at 5 mA cm-2. A rechargeable Zn-air battery assembled with Co2Ni1@NC shows excellent cycling stability, where the first discharge and charge voltages reach 1.21 and 2.00 V under 1 mA cm-2, respectively. This finding provides a simple synthesis approach, which allows one to construct bifunctional catalysts based on metal@NC for future energy conversion and storage devices.