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• Co 1-x S@NC is prepared via a facile "CBD deposition-PDA coating-sulfurization" method. • The excellent OER and ORR electrocatalytic activities both mainly derive from Co 1-x S. • The 3D array structure of Co 1-x S@NC facilitates OER/ORR-related charge-and-mass transport. • There is wrapping-protection of NC on Co 1-x S and strong electron coupling between them. • Promising quasi-solid-state rechargeable Zn-air battery based on Co 1-x S@NC is achieved. It is of great significance to develop high-performance bifunctional electrocatalysts for catalyzing two vital reactions in rechargeable zinc-air battery (ZAB). Transitional metal sulfides and N-doped carbon materials are widely reported as effective electrocatalysts of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), respectively. Herein, an 3D-array-structured catalyst composed of Co 1-x S nanoneedles enwrapped by nitrogen-doped carbon (NC) is in-situ grown on carbon fibre paper substrate. It is revealed that the metastable hexagonal Co 1-x S itself possesses attractive electrocatalytic activities toward both OER and ORR. Additionally, owing to the wrapping-protection effect of polydopamine-derived NC on Co 1-x S, the strong electron coupling between Co 1-x S and NC, and the 3D array structure, Co 1-x S@NC presents excellent OER/ORR catalytic activities and long-term stabilities, superior to Co@NC, Co 1-x S, and NC, and comparable to noble metals and other reported state-of-the-art bifunctional oxygen electrocatalysts. A small energy gap (Δ E) of 0.66 V between the ORR half-wave potential (0.86 V vs RHE) and OER potential (1.52 V vs RHE) at a current density of 10 mA cm−2, and a diffusion-limit ORR current density of 11.37 mA cm−2 are achieved. Notably, the flexible ZAB using the catalyst exhibits a markedly higher peak power density of 242 mW cm2, and a small charge/discharge voltage gap of ∼ 0.68 V with a high round-trip efficiency of ∼ 64% when cycling at 5 mA/cm2. [ABSTRACT FROM AUTHOR]

Employing P-doped polypyrrole nanotubes as precursor, we have constructed a structure of N, P-codoped carbon nanotubes coupled with Co 2 P nanoparticles (Co 2 P/NPC). Herein, P element serves as a bridging agent enabling strong electronic coupling between NPC and Co 2 P. While Co 2 P with good electrical conductivity facilitates electron transfer during electrocatalytic process. As a result, Co 2 P/NPC can catalyze oxygen reduction reaction (ORR) in a direct 4e− pathway, reaching a half-wave potential of 0.835 V vs. RHE and a smaller Tafel slope of 45.9 mV dec−1 than commercially available Pt/C. On the other hand, Co 2 P/NPC can also catalyze oxygen evolution reaction (OER) in a low overpotential of 320 mV at 10 mA cm−2 and a smaller Tafel slope of 59.6 mV dec−1 than commercially available RuO 2. Finally, the potential application of Co 2 P/NPC catalyst is estimated in case of rechargeable Zn-air battery. As air electrode, Co 2 P/NPC achieves a power density of 154 mW cm−2 and good long-term cycling stability (over 90 h at 5 mA cm−2). Our work thus provides a facile strategy to achieve bifunctional ORR/OER performance for application in next-generation energy conversion systems. • N, P-codoped carbon nanotubes coupled with Co 2 P nanoparticles is constructed. • Co 2 P/NPC exhibits strong electronic coupling between NPC and Co 2 P. • Co 2 P/NPC can catalyze ORR and OER with good activity and stability. • Co 2 P/NPC demonstrates potential application in rechargeable Zn-air battery. [ABSTRACT FROM AUTHOR]

The development of highly efficient and bifunctional electrocatalysts is highly desirable for commercializing the rechargeable Zn-air battery. Herein, employing graphitic carbon nitride as carbon precursor, N-doped carbon sheets loaded with Ni 3 Fe nanoparticles (denoted as Ni 3 Fe/N-C) have been prepared via a simple pyrolysis strategy. Benefitting from the introduction of triethylene diamine as organic ligand, Ni 3 Fe nanoparticles with a mean size of 29 nm are dispersed on N C sheets uniformly. As a result, Ni 3 Fe/N-C exhibits excellent electrocatalytic activity toward both oxygen reduction and evolution reactions (For oxygen reduction reaction, the half-wave potential of Ni 3 Fe/N-C is 0.81 V; for oxygen evolution reaction, the overpotential at 10 mA cm−2 is 310 mV). Finally, when used as air electrode in rechargeable Zn-air battery, a high power density of 128 mW cm−2 and a long cycling life up to 134 h at 10 mA cm−2 are achieved. Our study thus provides a promising air electrode material for rechargeable Zn-air batteries. [ABSTRACT FROM AUTHOR]