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• Co-N polymer is pyrolyzed with dicyandiamide to synthesize a Co-N-C catalyst. • Carbon nanosheet/nanotube-interweaved structure favors 3D electrical conduct. • Balanced micro/mesopore distribution facilitates mass diffusion. • Atomic and metal Co separately enable excellent ORR and OER performances. • Promising liquid and flexible rechargeable Zn-air batteries are achieved. Developing active and stable non-precious metal bifunctional electrocatalysts towards the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) still remains a crucial challenge. Herein, we propose a simple strategy of integrating atomically dispersed and nanoscaled Co species embedded in micro-/mesoporous carbon nanosheet/nanotube architecture. Revealed by control and SCN- poisoning experiments, N species and atomically dispersed Co species are the main active centers for ORR; whereas the metal Co nanocrystals are the chief active species for OER. Additionally, the balanced micro/mesopore distribution offers fast mass diffusion, and carbon nanosheet/nanotube architecture supplies favorable 3D electrical conduct. As a result, the as-prepared CoNC (1:4) exhibits a positive half-wave potential of 0.87 V (vs RHE) for ORR, and a low potential of 1.55 V (vs RHE) at 10 mA cm−2 for the OER. Importantly, the flexible Zn-air battery using the catalyst exhibits a peak power density of 117 mW cm−2, a higher round-trip efficiency of 68% cycling at 2 mA/cm−2, and an excellent cycling stability with 3% decrease after 126 cycles test. This work provides a general strategy to design bifunctional catalysts by integrating atomically dispersed and nanoscaled transitional metal species together with N-doped micro-/mesoporous 3D carbon. [ABSTRACT FROM AUTHOR]

Development of economical, stable and high-efficient electrocatalysts gives vital incentives to promote the large-scale implementation of clean energy devices. Transition metal sulfides and carbon materials provide broad platforms for design of highly active electrocatalysts thanks to their flexibility, high electrical conductivity, and possessing various catalytically active sites towards different reaction systems. Herein, a ternary-component MoS 2 –Co 9 S 8 -NC nanohybrid is synthesized, in which Co 9 S 8 , MoS 2 , and N-doped carbon (NC) are integrated with each other. Owing to forming abundant varied active centers including Co 9 S 8 –MoS 2 heterojunctions, and Co–N, Mo–N coupling centers locating at the contact interfaces between transition metal sulfides and N-doped carbon, the hybrid demonstrates greatly enhanced performances towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Such hybrid represents low overpotentials of 95 mV and 230 mV to achieve 10 mA cm−2 current density for HER and OER in 1.0 M KOH, respectively, and demonstrates favorable prospects for large-scale applications in overall water splitting. Image 1 • MoS 2 –Co 9 S 8 -NC is prepared via a facile precursor-solution-based process. • There are abundant MoS 2 /Co 9 S 8 , Co 9 S 8 /NC and MoS 2 /NC hetero-interfaces. • Abundant interfaces in MoS 2 –Co 9 S 8 -NC facilitate HER and OER performances. • Metal-N bonds are formed at Co 9 S 8 /NC and MoS 2 /NC interfaces with N doping. [ABSTRACT FROM AUTHOR]

[Display omitted] • A series of atomically dispersed Sb-N-C catalysts are prepared via a facile adsorption-pyrolysis strategy. • Atomically dispersed Sb are stabilized by N atoms in terms of Sb-N 5 configuration. • Self-evaporation of Sb during synthesis could create abundant micro/meso-pores. • Sb-N 5 sites enable Sb-N-C catalysts outstanding oxygen reduction reaction activity. • Abundant micro/ meso -pores enable Sb-N-C catalysts enhanced mass transport. Main-group metal based single-atom catalysts are attracting increasing research attention in electrochemical catalysis owing to their partially occupied valence p-orbitals. Herein, we report a series of atomically dispersed Sb-N-C catalysts (including Sb-N-C NP , Sb-N-C NL and Sb-N-C NT) synthesized by a facile adsorption-pyrolysis strategy for oxygen reduction reaction (ORR). Apart from generating atomically dispersed Sb-N 5 sites as active centers, self-evaporation of Sb during pyrolysis process has created abundant micro/meso-pores in Sb-N-C catalysts. Benefitting from these advantageous features, Sb-N-C presents outstanding ORR activity (Sb-N 5 sites) and efficient mass transport (micro/meso-pores). By rotating disk electrode (RDE) test in alkaline media, Sb-N-C exhibits a most positive half-wave potential of 0.90 V vs. RHE and a highest kinetic current density up to 43.8 mA cm−2 at 0.85 V vs. RHE. As gas diffusion electrode (GDE), Sb-N-C NP0.2 demonstrates fast O 2 diffusion and transport that enables smaller mass transport overpotential at high current densities up to 800 mA cm−2. Finally, Zn-air battery that uses the Sb-N-C NP catalyst as air electrode achieves a maximum power density of 180 mW cm−2 and more than 1000 hs of continuous operation. This work further demonstrates the excellent performance of main-group Sb single-atom catalyst toward ORR and applications in practical energy conversion devices. [ABSTRACT FROM AUTHOR]

[Display omitted] • Novel CoSb/NC catalysts are prepared via a novel Sb-modulated strategy. • The feeding amount of Sb salt is tuned to modulate the particle size and pore structure of CoSb/NC catalysts. • CoSb/NC catalysts exhibit enhanced ORR activity and kinetics outperforming the state-of-the-art Pt/C. • CoSb/NC catalysts are electrocatalytically active toward OER. • CoSb/NC catalysts can drive rechargeable Zn-air battery with a high peak power density of 180 mW cm−2. Rational modulation and optimization of structural properties plays a significant role in advancing the electrocatalytic performance of electrocatalysts. In this study, we report a novel Sb-modulated strategy to synthesize novel CoSb/NC catalysts comprised of CoSb alloy supported on nitrogen-doped carbon (denoted as CoSb/NC). Apart from alloying with Co to generate CoSb alloys, we demonstrated that the evaporation of Sb could modulate the particle size and pore structure of CoSb/NC. Such modulation has facilitated the formation of micro/meso-porous structure to accelerate the mass/charge transport and increase the accessibility of active sites, resulting in a highest kinetic current density of 22.6 mA cm−2 at 0.85 V vs. RHE as well as a low Tafel slope of 38 mV dec−1, which even outperforms the state-of-the-art Pt/C. Additionally, CoSb/NC is demonstrated to be electrocatalytic active toward OER with the overpotential to be 430 mV at 10 mA cm−2. Rechargeable Zn-air battery tests reveal that CoSb/NC possesses competitive performance in terms of charge-discharge ability and stability, suggesting its potential application in renewable energy conversion devices. This work provides a novel approach to modulate the structure of nitrogen-doped carbon based materials as efficient oxygen electrocatalysts. [ABSTRACT FROM AUTHOR]

• Co/N-C Zn is prepared via high-temperature gas-transport approach. • The intrinsic structure of Co/ N-C Zn has been greatly modulated by ZnCl 2 treatment. • Co/N-C Zn exhibits enhanced ORR and OER performance. • ORR and OER active sites are determined by control experiments. • Co/N-C Zn can drive liquid and solid-state rechargeable Zn-air battery efficiently. In this study, a facile surface treatment on polypyrrole (PPy) nanofiber with ZnCl 2 solution is employed to fabricate N-doped carbon anchored with metallic Co (denoted as Co/N-C Zn) via gas-transport approach. Structural characterizations reveal that the intrinsic structure of Co/N-C Zn in terms of its morphology, pore structure and defects have been greatly modulated by such a surface treatment, resulting in a higher loading amount of metallic Co, larger surface area and increased pyridinic N and Co-N species as efficient active sites. Benefiting from the structural modulation, Co/N-C Zn exhibits outstanding ORR activity and kinetics with a positive half-wave potential of 0.88 V vs. RHE, a high kinetic current density (J k) value of 36.6 mA cm−2 at 0.85 V vs. RHE, and a small Tafel slope of 55.9 mV dec−1, which even outperforms commercial Pt/C. Moreover, enhanced OER performance is also achieved in Co/N-C Zn , with an overpotential of 280 mV at 10 mA cm−2, surpasses that of commercial RuO 2 (295 mV). Finally, Co/N-C Zn shows promise for substitution of commercial Pt/C and RuO 2 to drive both liquid and solid-state rechargeable Zn-air batteries with high peak power densities of 196 mW cm−2 and 130 mW cm−2. [ABSTRACT FROM AUTHOR]

The particle size and dispersity of metal particles are key factors affecting the activity of electrocatalysts. Herein, employing N-doped carbon fiber as support, we report a facile gas-phase transport strategy for the synthesis of a series of transition metal nanoparticles (denoted as M/N-CF, M = Co, Cu, Mn, Ni, Fe). The current synthesis strategy not only yields metal nanoparticles with ultra-small size (∼5 nm) and good monodispersity, but also introduces a porous nanostructure with large surface area (730 m2 g−1) and rich M-N species. When used for catalyzing oxygen reduction reaction, all the M/N-CF catalysts adopt a highly efficient 4e− ORR pathway, reaching a most positive half-wave potential of 0.87 V vs. RHE for Mn/N-CF, quite approaching that of commercial Pt/C (0.88 V vs. RHE). Among them, Co/N-CF can also catalyze oxygen evolution reaction in an overpotential of 380 mV at 10 mA cm−2, approaching that of commercial RuO 2 (295 mV). Furthermore, as air electrode for rechargeable Zn-air battery, Co/N-CF based device achieves a high power density (163 mW cm−2) and good long-term cycling stability (over 50 h at 10 mA cm−2), suggesting the potential application of M/N-CF catalysts in energy conversion and storage systems. Image 1 • A facile and universal gas-phase transport strategy is presented to synthesize a series of transition metal based oxygen electrocatalysts (M/N-CF). • M/N-CF catalysts possess a porous nanostructure with large surface area and rich M − N species. • All the M/N-CF catalysts can catalyze ORR in a highly efficient 4e− ORR pathway. • Co/N-CF can simultaneously catalyze ORR and OER, achieving a high power density (163 mW cm−2) and good long-term cycling stability as air electrode in Zn-air battery. [ABSTRACT FROM AUTHOR]