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[Display omitted] • Co/Co 3 Fe 7 heterostructure is constructed by using an in-situ growth strategy. • NCNTs mitigates the corrosion of active sites to stabilize bifunctional activity. • Interfacial interactions activate electron transfer/redistribution to boost activity. • A small potential difference (0.62 V) obtains a superior charge/discharge capacity. • Zn-air battery shows high power density (165 mW cm−2) and cycling stability (300 h). 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/Co 3 Fe 7 heterojunction in nitrogen-doped carbon nanotubes (Co/Co 3 Fe 7 @NCNTs). Hollow CoFe layered-double-hydroxide prisms act as sacrifices for in-situ growth of Co/Co 3 Fe 7 nanoparticles, which also catalyze the growth of bamboo-like NCNTs. Tubular structure not only accelerates the charge transfer through the interactions between Co and Co 3 Fe 7 , 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/Co 3 Fe 7 @NCNTs-800 (pyrolyzed at 800 °C) shows exceptional ORR activity (half-wave potential of 0.89 V) and methanol tolerance. Meanwhile, Co/Co 3 Fe 7 @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/Co 3 Fe 7 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. [ABSTRACT FROM AUTHOR]

To overcome slow kinetics of oxygen reduction/evolution reactions (ORR/OER), the development of bifunctional electrocatalysts remains a huge challenge. FeCo alloy as an active-species usually suffers from its poor catalytic stability by easy corrosion. Here, we synthesize a core (FeCo)-shell (carbon) alloy anchored on nitrogen-doped porous carbon-nanosheets (FeCo/NUCSs) via a self-growth strategy. FeCo/NUCSs (140 m2 g−1) exhibits promising half-wave potential (E 1/2 =0.89 V) and methanol tolerance for ORR. Prussian blue analog-derived carbon shell protects binary active-sites (Fe-N x /Co-N x) on FeCo core to stabilize ORR activity. FeCo/NUCSs also displays a higher OER activity (overpotential of 300 mV) than RuO 2 , due to generation of highly-active FeOOH/CoOOH species on FeCo alloys as indicated by in situ X-ray diffraction. Assembled Zn-air battery (ZAB) exhibits promising open-circuit voltage of 1.51 V, specific capacity of 791.86 mA h g−1, and durability (102 h). This novel bimetallic alloy-based catalyst provides an interesting option for design of durable oxygen-electrocatalysts for ZAB. [Display omitted] • A bifunctional FeCo alloy catalyst is obtained by using in-situ self-growth strategy. • Carbon shell protects the Fe-N x /Co-N x sites on FeCo core to stabilize ORR activity. • β-FeOOH/γ-CoOOH intermediates detected by in situ XRD can promote the OER kinetics. • The potential difference from ORR to OER (ΔE) is as excellent as 0.64 V. • The assembled Zn-air battery exhibits good rechargeability and durability (102 h). [ABSTRACT FROM AUTHOR]

Molybdenum carbide (Mo2C) with a Pt-like d-band electron structure exhibits certain activities for oxygen reduction and evolution reactions (ORR/OER) in alkaline solutions, but it is questioned due to its poor OER stability. Combining Mo2C with transition metals alloy is a feasible way to stabilize its electrochemical activity. Herein, CoFe-Prussian blue analogues are used as a precursor to compound with graphitic carbon nitride and Mo6+ to synthesize FeCo alloy and Mo2C co-encapsulated N-doped carbon (NG-CoFe/Mo2C). The morphology of NG-CoFe/Mo2C (800 °C) shows that CoFe/Mo2C heterojunctions are well wrapped by N-doped graphitic carbon. Carbon coating not only inhibits growth and agglomeration of Mo2C/CoFe, but also enhances corrosion resistance of NG-CoFe/Mo2C. NG-CoFe/Mo2C (800 °C) exhibits an excellent half-wave potential (E1/2 = 0.880 V) for ORR. It also obtains a lower OER overpotential (325 mV) than RuO2 due to the formation of active species (CoOOH/β-FeOOH, as indicated by in-situ X-ray diffraction tests). E1/2 shifts only 6 mV after 5000 ORR cycles, while overpotential for OER increases only 19 mV after 1000 cycles. ORR/OER performances of NG-CoFe/Mo2C (800 °C) are close to or better than those of many recently reported catalysts. It provides an interfacial engineering strategy to enhance the intrinsic activity and stability of carbides modified by transition-metals alloy for oxygen electrocatalysis. [ABSTRACT FROM AUTHOR]

[Display omitted] • CoP NPs encapsulated in porous walls of hollow nanobox are wrapped by carbon layer. • Porous hollow structure creates abundant active sites and mass transfer pathways. • N atom elevates electron delocalization degree of Co atom to boost ORR/OER activity. • Coated carbon-layer protects CoP from fast corrosion to gain high ORR/OER stability. • The assembled Zn-air battery exhibits high power density and rechargeability (132 h). Catalysts with high activity and corrosion-resistance are desirable for oxygen reduction and evolution reactions (ORR/OER) in rechargeable zinc-air batteries (ZABs). Herein, via an etching-phosphating-annealing strategy, CoP is in situ formed in hollow carbon-nanobox, both of which are wrapped by a thin nitrogen-doped carbon layer (CoP/HNBs@NCL). Tannic acid etches ZIF-67 nanobox to release Co2+ and 2-methylimidazole to generate hollow structure. As-marked CoP/HNBs@NCL-2 (271.9 m2 g−1) exhibits robust activities towards ORR (half-wave potential of 0.88 V) and OER (overpotential of 0.36 V), which outperforms many non-precious metal catalysts due to the strong interactions between CoP and N-species and the smooth mass transfer via the porous hollow structure. Even at high potentials (OER), HNBs@NCL framework can still relieve the leaching of Co species (leaching ratio of 15.5 % after 6 h) to obtain high corrosion resistance and stability. Theoretical calculations reveal that N atoms enhance the electron delocalization degree of Co sites in CoP to lower energy barriers and balance adsorption free-energies of O-intermediates to enhance oxygen electrocatalysis. Notably, ZAB with CoP/HNBs@NCL-2 air-electrode exhibits high peak power density (139.8 mW cm−2) and charge–discharge cycling stability (132 h). This work presents a reliable protocol to design hollow electrocatalysts with desirable bifunctional activity and stability for ZABs. [ABSTRACT FROM AUTHOR]

[Display omitted] • ZIF-67-derived CoO@GC/WO 3 @CL is used as a bifunctional catalyst for the first time. • Intra-particle maturation of WO 3 from microrod to nanosphere forms more O-vacancies. • PVP-derived CL protects Co2+ (CoOOH) and O-vacancies to stabilize ORR/OER activities. • Cooperation between WO 3 and Co2+ boosts electron transfer to promote 4e− ORR pathway. • Interaction between CoOOH (Co3+) and O-vacancies (WO 3) promotes O 2 generation (OER). Oxygen reduction/evolution reactions (ORR/OER) play the vital roles in energy-conversion devices, especially for Zn-air batteries (ZABs). However, it is always a challenge to explore a stable bifunctional catalyst. Here, WO 3 -anchored ZIF-67-derived CoO@graphitized carbon is wrapped by a thin carbon-layer to obtain CoO@GC/WO 3 @CL catalysts. The well-crystallized WO 3 nanospheres are evolved from WO 3 microrods via an intra-particle maturation. For ORR, the peak-potential of CoO@GC/WO 3 @CL-800 (800 ℃) (0.81 V vs. RHE) is higher than that of Pt/C (0.79 V). Synergies between WO 3 (oxygen vacancies) and CoO (Co2+) improve mass/charge transfer to boost the 4e− pathway. For OER, CoO@GC/WO 3 @CL-800 has a lower overpotential (330 mV) than RuO 2 (490 mV) at 10 mA cm−2. The O 2 evolution rate can reach 0.125 mmol s−1 with a high Faraday efficiency of 96.7 %. Cooperation between CoOOH and oxygen vacancies promotes the H 2 O-oxidation to boost the O 2 -generation. A relatively low ΔE [E j=10 (OER)-E 1/2 (ORR)] of 0.72 V confirms the highly-stable ORR/OER activities of CoO@GC/WO 3 @CL-800, which obtains a higher power density (138 mW cm−2) than Pt/C + RuO 2 cathode in primary ZABs. This study indicates a new direction to build a multilayer-structured ORR/OER catalyst by using ZIFs as templates. [ABSTRACT FROM AUTHOR]