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Co and Co9S8 nanoparticles uniformly embedded in S, N-doped porous carbons (Co/Co9S8@SNC) were fabricated by pyrolytic treatments of [Co(tdc)(bpy)]n2 (Co2+ is taken as the central ion, 2,5-thiophenedicarboxylic acid (tdc) and 4,4-bipyridine (bpy) are taken as the organic ligands) at 3 temperatures: 800 °C, 900 °C and 1000 °C. Among samples obtained, Co/Co9S8@SNC-900 from pyrolysis at 900 °C shows the highest proportion of Co and N atoms in form of Co nanoparticles and Pyridine-N. Due to the coordination between Pyridine-N and Co atoms, Co/Co9S8@SNC-900 has the best ORR/OER bifunctional electrocatalytic activity, conductivity and stability with the aid of the synergy of Co9S8 nanoparticles and C-S-C. Electrochemical test results show that the ORR limit current density of Co9S8@SNC-900 is 5.2 mA cm−2, which is close to commercial Pt/C (20 wt.%). In the OER tests, the overpotential of Co/Co9S8@SNC-900 at 10 mA cm−2 is 0.31 V, which is lower than that of RuO2. Co/Co9S8@SNC-900 also shows the best conductivity and pore volume. In application of Zn-air batteries (ZABs), Co/Co9S8@SNC-900 shows better maximum power density (106.6 mW cm−2) and stability than Pt/C + RuO2 catalyst.

Efficient and stable bifunctional catalysts, facilitating oxygen reduction and oxygen evolution, play a pivotal role in energy conversion and storage fields. In this paper, four core-shell MnCo 2 O 4 catalyst samples are prepared by controlling different solvent heating time, which are 0.5 h, 2 h, 6 h, and 12 h, respectively. The subsequent tests indicate that the sample with a 6 h not only has an ideal core-shell structure, but also exhibits favorable performance. The unique core-shell structure and favorable electrochemical performance are mainly attributed to the prepared precursor spheres. The microspheres have an inherent core-shell structure. Detailed studies of the morphological variations of MnCo 2 O 4 catalysts with the different solvent heating time are performed using transmission electron microscopy and field emission scanning electron microscopy. Furthermore, we analyze the potential formation mechanism of the core-shell structure MnCo 2 O 4 catalyst. The obtained core-shell structured MnCo 2 O 4 catalysts provide a new way for the preparation of cathode catalyst for zinc-air batteries. • The precursor spheres produced had an inherent core-shell structure. • Effect of precursor structure on the performance of MnCo 2 O 4 after calcination. • The formation mechanism of the core-shell structure MnCo 2 O 4 catalyst. • MnCo 2 O 4 provides a fast kinetic for electrons and reaction intermediates. • Core-shell MnCo 2 O 4 has the potential to replace noble metal catalysts. [ABSTRACT FROM AUTHOR]

This study employed a simple and effective method to synthesize a composite material comprising core-shell NiCo 2 O 4 and GO/C. The combination of core-shell NiCo 2 O 4 and GO effectively improved the poor conductivity of NiCo 2 O 4 and prevented the aggregation of NiCo 2 O 4 microspheres. In electrochemical tests, compared to pure core-shell NiCo 2 O 4 , NiCo 2 O 4 @GO/C exhibits improved catalytic performance in both oxygen reduction (ORR) and oxygen evolution reactions (OER). Its OER catalytic performance is notably superior to that of noble metal catalysts. The NiCo 2 O 4 @GO/C-based liquid-state zinc-air battery exhibited a stable cycling efficiency of 51% over 200 h of charge-discharge cycles, significantly outperforming precious metal-based batteries. Additionally, a flexible quasi-solid-state zinc-air battery, assembled with the NiCo 2 O 4 @GO/C composite material as the cathode catalyst, exhibited an outstanding cycling efficiency of 58% in charge-discharge cycles. These results indicate that the NiCo 2 O 4 @GO/C composite material holds great potential as a bifunctional electrocatalytic catalyst for zinc-air batteries. • A concise and fast method was used to combine the core-shell structure NiCo 2 O 4 composite GO/C. • NiCo 2 O 4 @GO/C prevents the aggregation of NiCo 2 O 4 microspheres and improves the relatively poor conductivity of NiCo 2 O 4. • NiCo 2 O 4 @GO/C provides a fast kinetic for electrons and reaction intermediates. • NiCo 2 O 4 @GO/C Great improvement in electrocatalytic performance relative to pure NiCo 2 O 4. • NiCo 2 O 4 @GO/C-based liquid zinc-air battery operated stably with 51% charge/discharge cycle efficiency for over 200 h. [ABSTRACT FROM AUTHOR]