N,S-Codoped Hierarchical Porous Carbon Spheres Embedded with Cobalt Nanoparticles as Efficient Bifunctional Oxygen Electrocatalysts for Rechargeable Zinc-Air Batteries
Rational design and fabrication of cost-effective, efficient bifunctional electrocatalysts is fundamentally important for the air cathode of metal–air batteries. Herein, a Co(II) ion-driven self-assembly strategy is described for the synthesis of cobalt-based nanostructured transition metal compounds (Co-NTMCs) embedded in nitrogen and sulfur codoped hierarchical porous carbon submicrospheres (Co-NTMCs@NSC), where condensation of thiourea-ethylenediamine-formaldehyde resin (TEFR) is induced by Co(II) ions and simultaneously assembled with polydopamine to form a multifuntional precursor through coordinated interaction. The resulting Co-NTMCs@NSC sample comprises abundant hierachical porous textures, a high content of active cobalt species including the nanoparticles of Co, Co3O4 and amorphous CoSx, and considerable amount of defective structures. These characteristics lead to remarkble oxygen electrocatalytic activities, with a half-wave potential of +0.833 V vs. RHE that is comparable to that of commercial Pt/C for oxygen reduction reaction (ORR), and a lower overpotential of 284 mV than RuO2 at 10 mA·cm−2 for oxygen evolution reaction (OER) in alkaline media. Furthermore, its operation stability is also much higher than commercial RuO2 and Pt/C catalysts. When used as a breathable air electrode for Zn-air batteries, Co-NTMCs@NSC shows a higher open-circuit voltage (1.509 V), higher discharge powerdensity (262 mW cm-2) as well as better charge-discharge reversibility than commercial Pt/C. Results from the present work suggest that controlled assembly of functional polymers may be exploited for the preparation of doped carbon/metal nanoparticle nanocomposites as viable, high-performance catalysts for electrochemical energy technologies.