Hollow Structured N, S Co-Doped Carbon Nanotubes Encapsulating Co/CoS₂ Nanoparticles as Highly Efficient Electrocatalyst for Zinc-Air Battery

The development of sustainable energy conversion and storage systems is crucial owing to the depletion of fossil fuels and energy crisis. Recently, rechargeable Zn-air batteries (ZABs) are regarded as promising energy devices due to their high specific energy density (1084 Wh kg^-1), reliable safety and cost effectiveness. However, the ORR and OER involving a four-electron reaction process at cathode always suffer from sluggish kinetic behaviors during the charge and discharge process. Although noble metal-based materials (Pt/C for ORR and RuO₂ for OER) are commonly used as the most excellent catalysts for ORR and OER, their high cost, monofunctionality and low electrochemical durability limit the practical application of ZABs. Consequently, it is essential to develop non-noble metal-based electrocatalysts with superior durability and high electrochemical activity for utilization of ZABs. Transition metal-based materials (alloys/ sulfides/ oxides/ nitrides/ phosphides) have attracted considerable attention as electrocatalysts for rechargeable ZABs benefiting from their low cost, high electrochemical activity, and stable operation. Among the various candidates, transition metal sulfides have superior electrical conductivity compared with other chalcogenides, which is mainly attributed to the shallower valence band generated by S 3p orbital, resulting in formation of narrower band gap. Moreover, the formation of heterojunction between <i>n</i>-type metal sulfides and metals, which is known as Mott-Schottky contact, can further improve the charge transfer efficiency of electrocatalysts.<br/>Inspired by above considerations, hollow structured microsphere with N, S co-doped carbon nanotube shell encapsulating Co/CoS₂ nanoparticles (Co/CoS₂/NSCNT) was prepared via facile polymerization and subsequent pyrolysis steps. First, we synthesized polystyrene (PS) spheres by polymerization, which were used as self-sacrificial template to generate uniform hollow structures. And then, Co²⁺ ions and 2-methylimidazole were mixed in PS solution and ZIF-67 was coated on the surface of PS spheres. Subsequently, the obtained core-shell structured PS@ZIF-67 was pyrolyzed with dicyandiamide at 750°C for 1.5 h under Ar flow. During pyrolysis, PS spheres were completely removed and surface of the spheres became rougher with the growth of N-doped carbon nanotubes owing to the catalytic effect of Co nanoparticle, resulting in the formation of hollow structured Co/NCNT. The well-defined hollow structure can not only reduce ion diffusion length but also increase active surface area. Finally, Co/NCNT was annealed with sulfur at 550°C for 2h under Ar atmosphere to simultaneously conduct partial sulfidation of metal species and sulfur doping into carbon matrix (denoted as Co/CoS₂/NSCNT). Generally, the CNT provide an efficient electron pathway for active species and protect the encapsulated nanoparticles from aggregation and dissolution during the electrochemical reaction. Furthermore, introducing heteroatoms (e.g. N, S, and P) into the carbon materials can affect the electron neutrality of the sp² carbon lattice and lead to partial polarization of adjacent carbon, thereby promoting the adsorption of reaction intermediates. As expected, the prepared Co/CoS₂/NSCNT exhibited the excellent ORR activity with half-wave potential of 0.890 V (vs. RHE), outperforming that for the state-of-the-art Pt/C (0.855 V (vs. RHE)). Additionally, it also showed outstanding OER kinetics and low overpotential of 363 mV at a current density of 10 mA cm^-2, which was superior to the other prepared electrocatalysts.