The design of highly efficient electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) plays a key role in the development of various renewable energy storage and conversion devices. Recently, metal–organic framework (MOF) derived materials have shown great potential in ORR and OER; however, they may suffer from poor conductivity with limited electrochemical performance especially towards OER. In this work, nickel–cobalt oxides supported on Co/N decorated graphene were prepared by introducing nickel in a zeolitic imidazolate framework-67 (ZIF-67) as the precursor, which showed improved OER performance due to the as-derived NixCoyO4, and maintained the ORR performance of Co/N doped carbon at the same time. Meanwhile, graphene was utilized to further enhance the electrochemical surface area and charge transfer efficiency. The resulting composites show a potential of 0.796 V at 3 mA cm−2 and superior stability to Pt/C towards ORR, as well as a potential of 1.629 V to achieve 10 mA cm−2 current density for OER, which is much better than that of IrO2. In all, the overvoltage between ORR and OER was just 0.833 V, demonstrating the great potential of the composite in metal–air batteries as a bifunctional oxygen catalyst.

Photocatalysts with desirable selectivity to transformation and purification of targeted pollutants are of great importance in water purification. Here, we demonstrate that selective photocatalysis can be realized by the assistance of gold-enhanced selective adsorption onto carbon-coated Au/TiO2 mesoporous microspheres (Au/TiO2@C-MM), which were prepared via a surfactant-assisted two-step method that involved the assembly of oleic acid-stabilized titania and gold nanoparticles into colloidal spheres in an emulsion using sodium dodecyl sulfate as a surfactant and the conversion of the surfactants into carbon under annealing in Ar. Due to the negatively charged amorphous carbon, the mesoporous structure, and the surface plasmon resonance absorption of the Au components, the Au/TiO2@C-MM shows enhanced charge- and size-selective adsorption properties, which enables the materials to have high selectivity in the photocatalytic process.选择性光催化剂的开发对于污水处理等环境问题具有重要意义. 本文利用十二烷基硫酸钠作为表面活性剂, 油相Au和TiO2纳米颗粒为前驱体, 经微乳液自组装-表面活性剂碳化两步法, 成功合成了碳包覆Au/TiO2介孔微球(Au/TiO2@C-MM)复合光催化剂材料. 得益于负电荷无定形碳、介孔结构和Au表面等离子体共振特性, Au/TiO2@C-MM在光催化过程中表现出优异的尺寸和电荷选择性.

AbstractHollow metal–organic frameworks (MOFs) are promising materials with sophisticated structures, such as multiple shells, that cannot only enhance the properties of MOFs but also endow them with new functions. Herein, we show a rational strategy to fabricate multi-shelled hollow chromium (III) terephthalate MOFs (MIL-101) with single-crystalline shells through step-by-step crystal growth and subsequent etching processes. This strategy relies on the creation of inhomogeneous MOF crystals in which the outer layer is chemically more robust than the inner layer and can be selectively etched by acetic acid. The regulation of MOF nucleation and crystallization allows the tailoring of the cavity size and shell thickness of each layer. The resultant multi-shelled hollow MIL-101 crystals show significantly enhanced catalytic activity during styrene oxidation. The insight gained from this systematic study will aid in the rational design and synthesis of other multi-shelled hollow structures and the further expansion of their applications.

Hollow metal–organic frameworks (MOFs) with sophisticated structures, such as multiple shells, are promising materials that can not only enhance the properties of MOFs but also endow them with new functions. In their Communication on page 5512 ff., J. Liu, F. Huo et al. report a strategy to obtain multi-shelled hollow MOF crystals that show significantly enhanced catalytic activity during styrene oxidation.

AbstractHollow metal–organic frameworks (MOFs) are promising materials with sophisticated structures, such as multiple shells, that cannot only enhance the properties of MOFs but also endow them with new functions. Herein, we show a rational strategy to fabricate multi-shelled hollow chromium (III) terephthalate MOFs (MIL-101) with single-crystalline shells through step-by-step crystal growth and subsequent etching processes. This strategy relies on the creation of inhomogeneous MOF crystals in which the outer layer is chemically more robust than the inner layer and can be selectively etched by acetic acid. The regulation of MOF nucleation and crystallization allows the tailoring of the cavity size and shell thickness of each layer. The resultant multi-shelled hollow MIL-101 crystals show significantly enhanced catalytic activity during styrene oxidation. The insight gained from this systematic study will aid in the rational design and synthesis of other multi-shelled hollow structures and the further expansion of their applications.

Hohle Metall-organische Gerüste (MOFS) mit komplexen Strukturen, z. B. mehreren Schalen, sind vielversprechende Materialien, die nicht nur die Eigenschaften von MOFs verbessern, sondern diese auch mit neuen Funktionen ausstatten können. In der Zuschrift auf S. 5604 ff. berichten J. Liu, F. Huo et al. über eine Strategie, mit der mehrschalige, hohle MOF-Kristalle hergestellt werden können, die eine signifikant höhere katalytische Aktivität bei der Oxidation von Styrol aufweisen.

Carbon nanotube arrays (CNTAs) embedded with transition metal oxides (TMOs) have been considered as effective bifunctional catalysts. However, the hydrophobic nature of CNTAs usually limits the wetting of aqueous solutions containing inorganic precursors of TMOs, which results in poor decoration of TMOs on it. Herein, we report a facile strategy to tune the wettability of 3D CNTAs by controlling the nitrogen-doping degree. Particularly, superhydrophilic nitrogen-doped CNTAs (N-CNTAs) were obtained by using pyridine as the reactant, which allows uniform anchoring of cobalt oxide nanoparticles (NPs) on their surface through a simple self-priming freeze-drying assisted immobilization method. The heteroatom-doping and uniformly distributed cobalt oxide NPs endow the obtained hybrid material (Co3O4/N-CNTAs) with excellent OER and ORR performances, which also demonstrated their high efficiency as bifunctional catalysts for Zn–air batteries.

The rational structure and composition manipulation for the efficient, affordable bifunctional electrocatalysts of the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is critical for renewable-energy technologies including fuel cells and metal–air batteries. Metal oxide nanoarray electrodes are considered to be alternative options for this issue, although they always suffer from poor conductivity and limited ORR performance in terms of onset potential and diffusion current. In this study, we develop an alternative strategy to prepare a novel bifunctional catalyst by coating three-dimensional CoOx nanoarrays with a porous nitrogen-doped carbon layer (NC). The porous NC not only provides a conductive coating to benefit the charge transfer and retains the channels for electrolyte diffusion, but also greatly enhances the electrochemical surface area, which endows the electrode with higher activity for both the OER and ORR in terms of onset potential and diffusion current. When employed as an air cathode in rechargeable zinc–air batteries over 100 cycles, the electrode exhibits durable performance superior to those of Pt/C and IrO2/C. The results fully demonstrate the great potential of the strategy in electrode construction.

Hybrid metal oxide architectures have attracted much attention in recent years due to their great potential to meet the ever-increasing requirements of high energy density and power density in energy storage applications. Here, we report a facile hydrothermal synthesis of a binder-free hierarchical NiCo2O4@NiO nanowire array (HNW) with robust adhesion, for use in electrochemical capacitors (ECs). The resulting hybrid array electrode exhibits superior pseudocapacitive performance with high specific capacitance (2220 F g−1), remarkable rate capability, and excellent cycling performance (93.1% retention after 3000 cycles). Furthermore, a NiCo2O4@NiO//AC asymmetric supercapacitor was prepared and found to exhibit a high energy density (31.5 W h kg−1) at a power density of 215.2 W kg−1 and superior cycling stability (89% of the initial capacity retention at 50 A g−1 over 3000 cycles). This outstanding electrochemical performance benefits from the synergistic contribution of the composite and unique hierarchical architecture. Such highly integrated hybrid array electrodes will be extremely helpful towards the fabrication of high-performance nanoenergy systems.

The design and fabrication of efficient and inexpensive electrodes for use in the oxygen evolution reaction (OER) is essential for energy-conversion technologies. In this study, high OER performance is achieved using novel hierarchical ZnxCo3–xO4 nanostructures constructed with small secondary nanoneedles grown on primary rhombus-shaped pillar arrays. The nanostructures have large roughness factor, high porosity, and high active-site density. Only a small overpotential of ∼0.32 V is needed for a current density of 10 mA/cm2 with a Tafel slope of 51 mV/decade. The nanostructures are also found to perform significantly better than pure Co3O4 and a commercial Ir/C catalyst and to perform similarly to the best OER catalysts that have been reported for alkaline media. These merits combined with the satisfactory stability of the nanostructures indicate that they are promising electrodes for water oxidation.

Efficient and low-cost electrocatalysts for the oxygen evolution reaction (OER) are essential components of renewable energy technologies, such as solar fuel synthesis and water splitting processes for powering fuel cells. Here, ultrathin NiCoFe layered double hydroxide (LDH) nanoplates, which directly grow on a cobalt-based nanowire array, forming a hierarchical nanoarray structure, are constructed as efficient oxygen evolution electrodes. In alkaline media, the ordered ultrathin hierarchical LDH nanoarray electrode shows dramatically increased catalytic activity compared to that of LDH nanoparticles and pure nanowire arrays due to the small size, large surface area, and high porosity of the NiCoFe LDH nanoarray. Only a small water oxidation overpotential (η) of 257 mV is needed for a current density of 80 mA cm−2 with a Tafel slope of 53 mV per decade. The hierarchical LDH nanoarray also shows excellent structural stability in alkaline media. After continuous testing under a high OER current density (∼300 mA cm−2) for 10 h, the sample maintains the ordered hierarchical structure with no significant deactivation of the catalytic properties.

Thin bimetallic (Ni, Co) carbonate hydroxide nanosheet arrays (NiCoCH-NSAs) were successfully synthesized by a two-step hydrothermal method, and used as supports of Au nanoparticles to achieve a structured catalyst. The unique architecture of such bimetallic carbonate hydroxide nanosheets with an average thickness of about 50 nm and length of 1.5 μm is beneficial for the deposition of small Au nanoparticles without aggregation. HRTEM results clearly showed that the gold nanoparticles with size of about 2.6 nm ± 0.7 nm were uniformly distributed across the surface of the NiCoCH-NSAs over a large area. As a model reaction, the reduction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH4) was used to evaluate the performance of Au/NiCoCH-NSA catalysts. The obtained results demonstrated that the as-prepared Au/NiCoCH-NSAs exhibited excellent catalytic activity and stability for the reduction of 4-NP. Notably, the catalytic conversion of 4-nitrophenol on a relatively large scale showed the potential of Au/NiCoCH-NSAs as a practical catalyst for industrial applications. In addition, Au/NiCoCH-NSAs could be easily recycled due to the unique architectures of nanoarrays.

This work focuses on the construction of three-dimensional (3D) hierarchical MoO3 nanostructures using a wet chemistry method of sacrificing VO2 nanoarray templates. Results suggest that the whole process involves the templates' gradual dissolution and the corresponding hierarchical MoO3 nanorods' oriented growth. This strategy of fabricating hierarchical nanostructures through in situ conversion is potentially scalable and could be applicable for controllable syntheses of other related hierarchical nanostructures.

Immobilization of catalysts is a trend in future industrial catalysis and environmental protection. Here, NiCoFe spinel-type oxide nanosheet arrays were fabricated by using layered double hydroxides (LDH) as precursors and investigated as structural catalysts for alkenes oxidation. The NiCoFe spinel-type oxide nanosheet arrays exhibited high catalytic activity (72% conversion) and selectivity (64% for benzaldehyde) at 12 h towards the oxidation of styrene by tert-butyl hydroperoxide (TBHP). These good catalytic performances were mainly attributed to the unique nanosheet array structure and the substitution of cobalt. This strategy can be extended to other spinel-type oxide nanoarrays on metallic substrates and offers new opportunities for the design of new types of highly efficient structured catalysts.

A general strategy for constructing graphene-based nanocomposites is achieved by emulsion-based bottom-up self-assembly of hydrophobic nanocrystals (NCs) to positively charged colloidal spheres, followed by the electrostatic assembly of NC colloidal spheres with negatively charged graphene oxide in an acidulous aqueous solution. With a simple heat treatment, 3D mesoporous NC spheres/graphene composites are obtained. TiO2/graphene composites typically exhibit a better rate capability and cycle performance than do the corresponding isolated TiO2 spheres.

•Hierarchical Co3O4@NiO nanoarray is constructed by two-step hydrothermal method.•The hierarchical nanoarrays endow an ultrahigh areal capacitance of 39.6 F cm−2.•The electrodes also exhibit excellent cycling stability.•The unique structure and synergistic effect are keys to the superior performance.High areal capacitance of electrodes is highly desirable for practical supercapacitor applications, which requires a combination of high mass-loading and high utilization of electrochemically active material. In this work, we report the fabrication of hierarchical core–shell Co3O4@NiO nanowire@nanorod arrays where ultrathin NiO nanorods (~5 nm) were directly grown on the Co3O4 nanowire arrays via a two-step hydrothermal reaction followed by a calcination process. The hybrid nanoarrays exhibited a high specific capacitance of 2033 F g−1 at the current density of 5 mA cm−2 along with a mass loading as high as 19.5 mg cm−1, leading to an ultrahigh areal capacitance of 39.6 F cm−2 as a supercapacitor electrode, much higher than that of pure Co3O4 nanowire arrays (6.7 F cm−2). In addition, a remarkable rate capability (21.4 F cm−2 at the current density of 30 mA cm−2) and excellent cycling stability (100% after 1000 cycles) were observed. Compared with the pure Co3O4 nanowire arrays, the greatly enhanced capacitive performance is mainly attributed to the unique hierarchical porous architecture and the synergistic effect of the individual components.Hierarchical core–shell Co3O4@NiO nanowire@nanorod arrays were prepared as supercapacitor electrode with high areal capacitance (39.6 F cm−2 at the current density of 5 mA cm−2).

Construction of novel nanocrystal assemblies has attracted increasing interest in recent years. In this work, a general and facile ionic chelator-mediated hydrothermal method was developed to prepare one-dimensional (1D) rare earth orthovanadate (LnVO4, Ln = lanthanides) nanorod (NR) assemblies in an aqueous solution. The structure and morphology of the samples were characterized with XRD, TEM, and HRTEM. Particularly, the morphological evolution of CeVO4 nanocrystals involving the ionic chelator EDTA (ethylenediaminetetraacetic acid) in a hydrothermal reaction was investigated in detail. The effects of varying the EDTA/Ce3+ ratio, concentration of the reactants, pH value, growth time and reaction temperature were examined, which indicated that the self-assembly of LnVO4 NRs was realized in strictly confined synthetic conditions. The size of the nanorods in the array is controllable by tuning reaction parameters, which may benefit future applications. The formation mechanism was also investigated. Photoluminescence characterization showed that there is strong red emission from LaVO4:Eu and yellowish-green light from LaVO4:Dy NR arrays under UV excitation, which broadens the range of possible applications for these materials.

A novel biosensor was fabricated by the anodic electrochemical deposition of α-Fe2O3 nanorod (NR) arrays onto an iron substrate in an aqueous (NH4)2Fe(SO4)2 solution. The NR array electrode exhibits superior electroactivity for the oxidation of nitrite and the reduction of hydrogen peroxide. A wide linear response range (2 × 10−7–5 × 10−3 M), high sensitivity (135.36 μA mM−1 cm−2) and low detection limit (1 × 10−7 M) were obtained for the oxidation of nitrite. This α-Fe2O3 NR array electrode was further utilized in the detection of H2O2 with a detection limit of 2 × 10−7 M, linearity of 5 × 10−7–3 × 10−3 M and a sensitivity of 77.30 μA mM−1 cm−2. The stability and selectivity of the biosensor were also evaluated. The results indicate that the α-Fe2O3 NR arrays are efficient bifunctional sensors, with potential applications in biological and medical systems.

The development of nanotechnology in recent decades has brought new opportunities in the exploration of new materials for solving the issues of fossil fuel consumption and environment pollution. Materials with nano-array architecture are emerging as the key due to their structure advantages, which offer the possibility to fabricate high-performance electrochemical electrodes and catalysts for both energy storage and efficient use of energy. The main challenges in this field remain as rational structure design and corresponding controllable synthesis. This article reviews recent progress in our laboratory related to the hydrothermal synthesis of metal oxide and hydroxide nanoarrays, whose structures are designed aiming to the application on supercapacitors and catalysts. The strategies for developing advanced materials of metal oxide and hydroxide nanoarrays, including NiO, Ni(OH)2, Co3O4, Co3O4@Ni–Co–O, cobalt carbonate hydroxide array, and mixed metal oxide arrays like Co3−xFexO4 and ZnxCo3−xO4, are discussed. The different kinds of structure designs such as 1D nanorod, 2D nanowall and hierarchical arrays were involved to meet the needs of the high performance materials. Finally, the future trends and perspectives in the development of advanced nanoarrays materials are highlighted.

Sea urchin-like Ag–α-Fe2O3 nanocomposite microspheres are developed for gas sensing applications. Such hierarchical nanostructures were synthesized by a two step approach including a hydrothermal reaction and a subsequent incipient wetness impregnation process. The large Brunauer–Emmett–Teller (BET) area and highly porous structure of the hierarchical architecture and the chemical and electronic sensitization effect of Ag nanoparticles endow the Ag–α-Fe2O3 nanocomposite microspheres with enhanced gas-sensing properties in comparison to pristine α-Fe2O3 sensors. The possible formation mechanism of the sea urchin-like nanostructures, and their sensing mechanism are discussed.

Large-scale arrays of hierarchical cobalt iron oxide nanowires with preferentially exposed reactive crystal planes have been fabricated for use as structured catalysts, which showed high catalytic activity and excellent cycling stability.

Parameters such as solution concentrations and composition of the ambient atmosphere are known to be important in phase and morphology control in the solvothermal synthesis of CdS semiconductor nanorods (NRs), but a clear understanding of the underlying mechanisms involved is lacking. In this work, a series of experiments were performed to demonstrate that the key factor affecting the phase and morphology of CdS NRs is the amount of O2 in the space above the reaction solution in the sealed vessel relative to the amount of precursors in solution: O2-depleted conditions resulted in more cubic phase CdS and thick polycrystalline NRs with an aspect ratio usually less than 3, which have small blue shifts in band-edge emission and little surface trap emission, while O2-rich conditions resulted in more hexagonal-phase CdS and slim single-crystal NRs, which have significantly blue shifted band-edge emission and relatively strong surface trap emission. Thus, increasing the amount of solution in the vessel, changing the ambient atmosphere from air to N2, and increasing the reagent concentration all lower the molar ratio of O2 to reagents and lead to more cubic phase and thicker NRs. The results indicate that the composition of the “empty” section of the reaction vessel plays as important a role as the composition of the liquid in determining the phase and morphology, something that has been overlooked in earlier work. A mechanism to explain the effect of oxygen on the nucleation and growth stages has been proposed on the basis of those results and further supported by shaking experiments and ZnS NR synthesis manipulation. The CdS NRs synthesized under different conditions showed obvious differences in photocatalytic activity, which indicated that controlling the synthetic process can lead to materials with tailored photocatalytic activity.

Electrodes with hierarchical nanoarchitectures could offer many opportunities for improved performance in energy storage. Herein, we report the synthesis of a hierarchical Co3O4 nanosheet@nanowire arrays (NSWAs) by a facile hydrothermal and annealing treatment. The synthesis of Co3O4 NSWAs, composed of Co3O4 nanowires standing on nanosheet arrays, was achieved in two steps. The formation of nanosheet arrays on nickel foam and the growth of the nanowires around the sheets in the form of nanowires was achieved by studying the morphology evolution process upon reaction time. This novel structure of the material provides a high specific capacitance of 715 F g−1 and remarkable rate capability (at least 69% can be maintained when the current density increased 6 times). Furthermore, the excellent cycling performance (100% after 1000 cycles) is another essential factor in making the Co3O4 hierarchical NSWAs an advanced supercapacitor material.

High quality gold nanorods (NRs) with a monodisperse size and aspect ratio are essential for many applications. Here, we describe how nearly monodisperse gold NRs can be separated from polydisperse samples using density gradient ultracentrifugation. Size and dimension analysis by transmission electron microscopy (TEM) and absorption spectroscopy revealed that the Au NRs were separated mainly as a function of their aspect ratio. The surface-enhanced Raman scattering (SERS) activity of Au NRs with lower aspect ratio is notably stronger than that of NRs with higher aspect ratio under 633 nm laser excitation, due to the size-dependent absorption of the longitudinal plasmon band. The separation approach provides a method to improve the quality of NRs produced by large scale synthetic methods. Open image in new window

Reduced graphene oxide (RGO) is an intriguing nanomaterial with tremendous potential for many applications. Although considerable efforts have been devoted to develop the reduction methods, it still needs further improvement, and how to choose an appropriate one for a specific application is a troublesome problem. In this study, RGOs were prepared by six typical reduction methods: N2H4·H2O, NaOH, NaBH4, solvothermal, high-temperature, and two-step. The samples were systematic compared by four aspects: dispersibility, reduction degree, defect repair degree, and electrical conductivity. On the basis of the comparison, a simple evaluation criterion was proposed for qualitatively judging the quality of RGO. This evaluation criterion would be helpful to understand the mechanism of reduction and design more ideal reduction methods.

Sea urchin-like Ag–α-Fe2O3 nanocomposite microspheres are developed for gas sensing applications. Such hierarchical nanostructures were synthesized by a two step approach including a hydrothermal reaction and a subsequent incipient wetness impregnation process. The large Brunauer–Emmett–Teller (BET) area and highly porous structure of the hierarchical architecture and the chemical and electronic sensitization effect of Ag nanoparticles endow the Ag–α-Fe2O3 nanocomposite microspheres with enhanced gas-sensing properties in comparison to pristine α-Fe2O3 sensors. The possible formation mechanism of the sea urchin-like nanostructures, and their sensing mechanism are discussed.

Thin bimetallic (Ni, Co) carbonate hydroxide nanosheet arrays (NiCoCH-NSAs) were successfully synthesized by a two-step hydrothermal method, and used as supports of Au nanoparticles to achieve a structured catalyst. The unique architecture of such bimetallic carbonate hydroxide nanosheets with an average thickness of about 50 nm and length of 1.5 μm is beneficial for the deposition of small Au nanoparticles without aggregation. HRTEM results clearly showed that the gold nanoparticles with size of about 2.6 nm ± 0.7 nm were uniformly distributed across the surface of the NiCoCH-NSAs over a large area. As a model reaction, the reduction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH4) was used to evaluate the performance of Au/NiCoCH-NSA catalysts. The obtained results demonstrated that the as-prepared Au/NiCoCH-NSAs exhibited excellent catalytic activity and stability for the reduction of 4-NP. Notably, the catalytic conversion of 4-nitrophenol on a relatively large scale showed the potential of Au/NiCoCH-NSAs as a practical catalyst for industrial applications. In addition, Au/NiCoCH-NSAs could be easily recycled due to the unique architectures of nanoarrays.

Hybrid metal oxide architectures have attracted much attention in recent years due to their great potential to meet the ever-increasing requirements of high energy density and power density in energy storage applications. Here, we report a facile hydrothermal synthesis of a binder-free hierarchical NiCo2O4@NiO nanowire array (HNW) with robust adhesion, for use in electrochemical capacitors (ECs). The resulting hybrid array electrode exhibits superior pseudocapacitive performance with high specific capacitance (2220 F g−1), remarkable rate capability, and excellent cycling performance (93.1% retention after 3000 cycles). Furthermore, a NiCo2O4@NiO//AC asymmetric supercapacitor was prepared and found to exhibit a high energy density (31.5 W h kg−1) at a power density of 215.2 W kg−1 and superior cycling stability (89% of the initial capacity retention at 50 A g−1 over 3000 cycles). This outstanding electrochemical performance benefits from the synergistic contribution of the composite and unique hierarchical architecture. Such highly integrated hybrid array electrodes will be extremely helpful towards the fabrication of high-performance nanoenergy systems.

Large-scale arrays of hierarchical cobalt iron oxide nanowires with preferentially exposed reactive crystal planes have been fabricated for use as structured catalysts, which showed high catalytic activity and excellent cycling stability.

Carbon nanotube arrays (CNTAs) embedded with transition metal oxides (TMOs) have been considered as effective bifunctional catalysts. However, the hydrophobic nature of CNTAs usually limits the wetting of aqueous solutions containing inorganic precursors of TMOs, which results in poor decoration of TMOs on it. Herein, we report a facile strategy to tune the wettability of 3D CNTAs by controlling the nitrogen-doping degree. Particularly, superhydrophilic nitrogen-doped CNTAs (N-CNTAs) were obtained by using pyridine as the reactant, which allows uniform anchoring of cobalt oxide nanoparticles (NPs) on their surface through a simple self-priming freeze-drying assisted immobilization method. The heteroatom-doping and uniformly distributed cobalt oxide NPs endow the obtained hybrid material (Co3O4/N-CNTAs) with excellent OER and ORR performances, which also demonstrated their high efficiency as bifunctional catalysts for Zn–air batteries.

The design of highly efficient electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) plays a key role in the development of various renewable energy storage and conversion devices. Recently, metal–organic framework (MOF) derived materials have shown great potential in ORR and OER; however, they may suffer from poor conductivity with limited electrochemical performance especially towards OER. In this work, nickel–cobalt oxides supported on Co/N decorated graphene were prepared by introducing nickel in a zeolitic imidazolate framework-67 (ZIF-67) as the precursor, which showed improved OER performance due to the as-derived NixCoyO4, and maintained the ORR performance of Co/N doped carbon at the same time. Meanwhile, graphene was utilized to further enhance the electrochemical surface area and charge transfer efficiency. The resulting composites show a potential of 0.796 V at 3 mA cm−2 and superior stability to Pt/C towards ORR, as well as a potential of 1.629 V to achieve 10 mA cm−2 current density for OER, which is much better than that of IrO2. In all, the overvoltage between ORR and OER was just 0.833 V, demonstrating the great potential of the composite in metal–air batteries as a bifunctional oxygen catalyst.