Durable Freestanding Hierarchical Porous Electrode for Rechargeable Zinc-Air Batteries

The development of freestanding bifunctional air cathodes for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is highly desirable for the next generation of flexible rechargeable metal-air batteries. It remains challenging to achieve efficient OER and ORR bifunctionality on a single lightweight and inexpensive electrode. In this article, a metal-free, and freestanding air cathode based on vertically aligned carbon nanotubes (VACNTs) functionalized with N, P heteroatoms doped carbon is first reported. In addition to the high catalytic activity caused by N, P heteroatoms doping, the importance of efficient gas diffusion and electron transfer provided by the VACNT-GF hierarchical structure is highlighted. The carbonization temperature has been identified to have pronounced effect on catalytic activity, and the samples with P-N bonds have smaller ORR and OER overpotentials, while the quantitative atomic ratio of either P or N has little effect on catalytic activity. The resulting air electrode achieved a high peak power density of 56 mW cm^-2 at a current density of 120 mA cm^-2, outperforming Pt/C- and IrO₂-based rechargeable Zn-air batteries. The zinc-air battery assembled with the air electrode also showed good cyclability, which exceeded that of cells with the Pt/C//IrO₂ catalyst. The increase of voltage difference between the charge and discharge platform was 0.2 V for the cell assembled with N,P-doped VACNT-based freestanding air cathode after 75 h of operation at 10 mA cm^-2, which was less than half of that of cells with Pt/C//IrO₂ catalyst. Impedance analysis further reveals the good performance results from the favorable mass transfer of the electrode.