Monodisperse Co9S8 nanoparticles in situ embedded within N, S-codoped honeycomb-structured porous carbon for bifunctional oxygen electrocatalyst in a rechargeable Zn–air battery

The exploitation of cost-effective, highly active, and robust non-precious metal bifunctional oxygen electrocatalysts for both the oxygen reduction reaction (ORR) and oxygen revolution reaction (OER) is the key to promoting the application of regenerative fuel cells and metal–air batteries. Co9S8 is considered a promising non-precious metal bifunctional oxygen electrocatalyst. However, the electrocatalytic performance of cobalt sulfide-based nanocatalysts is still far from satisfactory because of their poor conductivity, insufficient exposed active sites, and aggregation-prone during continuous work. Herein, based on the inspiration from honeycombs in nature, we synthesized Co9S8/N, S-codoped honeycomb-structured porous carbon (Co9S8/NSC) in situ composites via a simple method. Benefiting from the unique honeycomb composite structure composed of monodisperse Co9S8 nanoparticles in situ embedded within the three-dimensional interconnected network carbon matrix with high conductivity, which facilitates not only the electron transport and charge transfer across the interface but also the exposure of active sites and rapid transport of ORR/OER-related species, the obtained Co9S8/NSC in situ composites exhibit high stability and activity in both the ORR in terms of more a positive half-wave potential (0.88 V vs. RHE) than that (0.86 V vs. RHE) of commercial 20% Pt/C and the OER in terms of a small overpotential (0.41 V vs. RHE) approaching that of commercial IrO2 (0.39 V vs. RHE) in alkaline electrolytes at 10 mA cm−2. Thus, as expected, the assembled rechargeable Zn–air batteries based on the bifunctional electrocatalysts exhibit a small discharge/charge overpotential (0.96 V) with a high voltaic efficiency of 55.1% at 10 mA cm−2 and a long-term cycling stability of over 4000 min. Embedding nanoparticle catalysts inside custom-designed porous carbon materials can improve the efficiency and economics of ultra-lightweight batteries. Zhi-qian Cao from Anhui University in Hefei, China, and coworkers report a protocol to transform the inexpensive mineral cobalt pentlandite (Co9S8) into a catalyst suitable for both charging and discharging metal–air batteries. These devices generate power as a result of oxygen gas in the atmosphere reacting with elements such as zinc. In the team’s approach, Co9S8 nanoparticles are incorporated into a carbon film along with inert silica nanospheres. The silica is then dissolved leaving behind a conductive carbon matrix with honeycomb-structured pores that expose active Co9S8 sites and ensure speedy transport of reactants during battery cycling. Tests with the new materials revealed they could outperform commercial catalysts for zinc-air batteries made from precious metals. N, S-codoped honeycomb-structured porous carbon with monodispersed in situ embedded Co9S8 nanoparticles were designedly fabricated as supported bifunctional electrocatalysts for OER and ORR in rechargeable Zn–air battery, which exhibited a small discharge–charge voltage gap (0.96 V) with a high voltaic efficiency of 55.1% at 10 mA cm−2 comparable to those of assembled battery based on commercial Pt/C–IrO2 composite catalyst (0.85 V, 57.5%), and a better cycling stability over nearly 4000 min.