Carbon nanotube-encapsulated Co/CoFe 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/CoFe heterojunction in nitrogen-doped carbon nanotubes (Co/CoFe@NCNTs). Hollow CoFe layered-double-hydroxide prisms act as sacrifices for in-situ growth of Co/CoFe nanoparticles, which also catalyze the growth of bamboo-like NCNTs. Tubular structure not only accelerates the charge transfer through the interactions between Co and CoFe, 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/CoFe@NCNTs-800 (pyrolyzed at 800 °C) shows exceptional ORR activity (half-wave potential of 0.89 V) and methanol tolerance. Meanwhile, Co/CoFe@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/CoFe heterointerface. Notably, it also has a high performance as an air-cathode for rechargeable zinc-air battery, showing high power density (165 mW cm) and specific capacity (770.5 m Ah kg). This work provides a new reference for promoting the development of alloy catalysts with heterogeneous interfaces.