Carbon nanotube-encapsulated Co/Co3Fe7 heterojunctions as a highly-efficient bifunctional electrocatalyst for rechargeable zinc-air batteries

In oxygen electrocatalysis, how to rationally design low-cost catalysts with reasonable structure and long-term stability is a crucial issue. Here, an in-situ growth strategy is used to construct a shaped structure encapsulating a uniformly-dispersed Co/Co3Fe7 heterojunction in nitrogen-doped carbon nanotubes (Co/Co3Fe7@NCNTs). Hollow CoFe layered-double-hydroxide prisms act as sacrifices for in-situ growth of Co/Co3Fe7 nanoparticles, which also catalyze the growth of bamboo-like NCNTs. Tubular structure not only accelerates the charge transfer through the interactions between Co and Co3Fe7, but also limits the aggregation of the particles, thereby promoting the 4e- oxygen reduction/evolution reactions (ORR/OER) kinetics and stabilizing the bifunctional activity. Co/Co3Fe7@NCNTs-800 (pyrolyzed at 800 °C) shows exceptional ORR activity (half-wave potential of 0.89 V) and methanol tolerance. Meanwhile, Co/Co3Fe7@NCNTs-800 shows a small OER overpotential of 280 mV, which increases by only 9 mV after 1000 cyclic voltammetry (CV) cycles. The outstanding bifunctionality (potential gap of 0.62 V) is ascribed to the electronic structure modulation at the Co/Co3Fe7 heterointerface. Notably, it also has a high performance as an air–cathode for rechargeable zinc-air battery, showing high power density (165 mW cm−2) and specific capacity (770.5 m Ah kg−1). This work provides a new reference for promoting the development of alloy catalysts with heterogeneous interfaces.