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 which is simultaneously assembled with polydopamine to form a multifunctional precursor through coordinated interaction. The resulting Co-NTMCs@NSC sample comprises abundant hierarchical porous textures, a high content of active cobalt species including the nanoparticles of Co, CoO and amorphous CoS, and a considerable amount of defective structures. These characteristics lead to remarkable oxygen electrocatalytic activities, with a half-wave potential of +0.833 V vs. RHE, which is comparable to that of commercial Pt/C for the oxygen reduction reaction (ORR), and a lower overpotential of 284 mV than RuO at 10 mA·cm for the oxygen evolution reaction (OER) in alkaline media. Furthermore, its operational stability is also much higher than that of commercial RuO and Pt/C catalysts. When used as a breathing air electrode for Zn-air batteries, Co-NTMCs@NSC shows a higher open-circuit voltage (1.509 V), higher discharge power density (262 mW cm) and better charge-discharge reversibility than commercial Pt/C. The 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.