Co-based Zeolitic Imidazolate Framework Research Articles

Nitrogen-Doped Graphitic Carbon-Supported Ultrafine Co Nanoparticles as an Efficient Multifunctional Electrocatalyst for HER and Rechargeable Zn-Air Batteries.

The construction of high-efficiency electrocatalysts for hydrogen evolution, oxygen reduction, and oxygen evolution reactions (HER/ORR/OER) is critical for the overall water splitting system, fuel cells, and rechargeable metal-air batteries. Here, we report a viable strategy for tuning the size of a Co-based zeolitic imidazolate framework (ZIF). As a result, a nitrogen-doped graphitic carbon-supported ultrafine Co nanoparticle electrocatalyst (Co/NGC-3) with multifunctional activity was developed. Owing to the smaller ZIF-67 polyhedrons with relatively uniform distribution, more effective active sites, and a strong coupling effect of Co-pyridinic-N, the proposed Co/NGC-3 catalyst exhibited an impressive HER activity. It also showed brilliant catalytic activity in both the ORR and OER, delivering a more positive half-wave potential and a lower overpotential than that of the Pt/C catalyst, respectively. Moreover, the Co/NGC-3 involved the Zn-air battery displayed satisfactory power density, excellent energy density, and superior stability. This approach provides an efficient strategy for the preparation of high-performance multifunctional electrocatalysts for energy-related applications.

Fabrication of 2D/2D nanosheet heterostructures of ZIF-derived Co3S4 and g-C3N4 for asymmetric supercapacitors with superior cycling stability.

Metal sulfides with high activity are favorable electrode materials for supercapacitors. However, their relatively inferior electronic conductivity and poor stability in alkaline electrolyte solutions impede their applications. To overcome these drawbacks, herein, 2D/2D nanosheet heterostructures of Co3S4 and g-C3N4 have been successfully fabricated by a facile method that involves the in situ growth of 2D Co-based zeolitic imidazolate framework (Co-ZIF-L) crystals on g-C3N4 nanosheets followed by subsequent sulfurization. The as-prepared Co3S4/g-C3N4-10 exhibits a largely enhanced specific capacity (415.0 C g-1 at 0.5 A g-1) in comparison with solitary g-C3N4 (18.9 C g-1) and Co3S4 (194.4 C g-1) derived from Co-ZIF-L. Furthermore, it also displays good rate capability (54.5% retention at 10 A g-1). The asymmetric supercapacitor fabricated from Co3S4/g-C3N4-10 and activated carbon electrodes exhibits an outstanding energy density of 35.7 W h kg-1 at a high power density of 850.2 W kg-1. Most importantly, the asymmetric supercapacitor demonstrates an ultrahigh cycling durability with only 1.9% capacitance loss after 10 000 cycles at 10 A g-1. This superior electrochemical performance can be attributed to the unique 2D/2D nanosheet heterostructures providing rich active sites, short ion diffusion pathways, fast charge transfer as well as improved conductivity and mechanic stability. This work may pave the way for a rational design of the heterostructures of metal sulfides and g-C3N4 for electrochemical energy storage devices with a long cycling lifespan.

Confined growth of porous nitrogen-doped cobalt oxide nanoarrays as bifunctional oxygen electrocatalysts for rechargeable zinc–air batteries

Cobalt oxides are promising bifunctional oxygen electrocatalysts due to their intrinsic activities toward both dissociation and formation of oxygen molecules. Unfortunately, their practical utilization is seriously hindered by the limited accessible active sites and inferior activity. Herein, vertically-aligned porous nanoarrays composed of ultrafine nitrogen-doped cobalt oxide (NP-Co3O4) nanoparticles (4-5 ​nm) are in-situ fabricated on carbon cloth (CC) via mild oxidation of Co-based zeolitic-imidazolate-framework (Co-ZIF) nanoarrays. The oxidation decomposition of 2-methylimidazolate ligands between Co nodes not only confines the growth of cobalt oxides, but also provides the nitrogen sources for realizing low-temperature (200 ​°C) nitrogen doping. In alkaline electrolyte, the as-built NP-Co3O4/CC manifests an extremely high oxygen reduction reaction (ORR) half-wave potential of ~0.9 ​V and a decreased oxygen evolution reaction (OER) overpotential of 330 ​mV ​at 10 ​mA ​cm−2, displaying a record low potential gap of 0.66 ​V. As bifunctional oxygen electrodes in rechargeable aqueous and all-solid-state flexible zinc-air batteries, the NP-Co3O4/CC-based batteries exhibit ultrahigh power densities up to 200 ​mW ​cm−2 and 99.8 ​mW ​cm−3, respectively, which are much higher than those of reported bifunctional oxygen electrocatalysts. This work paves a new avenue for the fabrication of porous nitrogen doped metal oxides towards high-performance zinc-air batteries.

Environmentally friendly synthesis of Co-based zeolitic imidazolate framework and its application as H2O2 sensor

• A facile and environmentally friendly method for synthesizing Co-based imidazolate framework compounds was provided. • The Co-based imidazolate framework compounds with five different morphologies were synthesized. • A novel H 2 O 2 sensor was constructed. The work presented that by regulating the proportion between cobalt and 2-methylimidazole in distilled water, a series of Co-based imidazolate framework compounds with diverse shapes were synthesized at ambient temperature. The morphology and structural characteristics of the compounds were surveyed by scanning electron microscope, energy disperse spectra, powder X-ray diffraction and N 2 absorption-desorption. It was found by cyclic voltammetry that the zeolitic imidazolate framework compound synthesized at a ratio of 1:20 had the best electrocatalytic activity to H 2 O 2 reduction, therefore, the compound was chosen to fabricate a H 2 O 2 electrochemical sensor. Electrochemical studies indicated that the sensor had a determination limit of 0.7 μM and short response time within 3 s. The catalytic current was linear with H 2 O 2 concentration from 2.5 to 212.5, 212.5 to 1662.5 and 1662.5 to 6662.5 μM and the sensitivity were 12.2, 5.3 and 2.4 μA mM −1 cm −2 , respectively. Furthermore, the proposed sensor showed a good anti-interference capacity, repeatability and stability.

Co-based zeolitic imidazolate framework ZIF-9 membranes prepared on α-Al2O3 tubes through covalent modification for hydrogen separation

Hydrogen has been regarded as the most promising clean and renewable energy. Beside the production of the hydrogen, the separation of hydrogen is also an import issue before it can be used in fuel cells. Membrane-based separation technologies have gained considerable attentions due to its high efficiency and low energy consumption. Zeolite imidazolate framework (ZIF) membranes have drawn intense interest due to their zeolite-like properties such as permanent porosity, uniform pore size and exceptional thermal and chemical stability. It is rather challenged to prepare well-intergrown Co-based zeolitic imidazolate frameworks (ZIFs) membranes on porous α-Al2O3 tubes since Co-based ZIFs prefer to form crystals in the synthesis solution rather than grow as membrane layer on the support surface. In this work, we report the preparation of high-quality ZIF-9 membrane with high H2/CO2 selectivity and excellent thermal stability by using 3-aminopropyltriethoxysilane (APTES) as a covalent linker to modify the α-Al2O3 tube. Due to the formation of covalent bonds between APTES and ZIF-9, ZIF-9 nutrients are bound to the support surface, thus promoting the growth of dense and phase-pure ZIF-9 membrane with a thin thickness of about 4.0 μm. The gas separation performances of the ZIF-9 membrane were evaluated by single gas permeation and mixture gas separation of H2/CO2, H2/N2 and H2/CH4, respectively. The mixture separation factors of H2/CO2, H2/CH4, and H2/N2 of the ZIF-9 membrane are 21.5, 8.2 and 14.7, respectively, which by far exceeds corresponding Knudsen coefficients. Moreover, the as-prepared ZIF-9 membrane exhibits excellent stability at a relatively broad range of operating temperature, which is beneficial for the industrial application of hydrogen separation or further membrane reactor.

Design of house centipede-like MoC–Mo2C nanorods grafted with N-doped carbon nanotubes as bifunctional catalysts for high-performance Li–O2 batteries

Developing effective bifunctional catalysts to promote Li2O2 oxidation and to realize high-performance Li–O2 batteries with long cycle life is a key challenge. Herein, a house centipede-like bifunctional catalyst consisting of a molybdenum carbide nanorod grafted with N-doped carbon nanotubes (denoted as MoC–Mo2C/NCNTs) is developed for enhancing the performance of Li–O2 batteries. This nanocatalyst is successfully fabricated by the post-treatment of MoO3 nanorods coated with a Co-based zeolitic imidazolate framework (ZIF-67). The obtained MoC–Mo2C/NCNT nanorods have a unique structure with a body comprising small MoC–Mo2C nanocrystals and Co@NCNTs extending like legs from it. This structural architecture not only provides numerous catalytic active sites and sufficient space for accommodating Li2O2 but also facilitates electron transport through the catalytically active NCNT conductive pathways. Consequently, the MoC–Mo2C/NCNTs show substantially improved electrocatalytic performance in Li–O2 batteries with a significantly reduced discharge–charge overpotential (0.50 V), high specific discharge capacity (34,862 mA h g−1 at 200 mA g−1), and long cycle stability (up to 162 cycles at current density of 200 mA g−1) and cut-off capacity of 500 mA h g−1. Preliminary results suggest that the MoC–Mo2C/NCNT nanorod is an efficient, stable bifunctional catalyst for Li–O2 batteries.

Construction of sugar gourd-like yolk-shell Ni–Mo–Co–S nanocage arrays for high-performance alkaline battery

Mixed transition metal sulfides (TMSs) with hollow and complex hollow structures have attracted intensive attentions as high-performance cathode materials for rechargeable alkaline batteries. However, these powder-based electrode materials should be further mixed with insulative polymer binders and suffered from complicate electrode fabrication process. Herein, for the first time, we demonstrate the delicate design and synthesis of the sugar gourd-like multi-component yolk-shell Ni–Mo–Co–S nanocage arrays (NCAs) on Ni foam (NF) through a facile metal-organic framework (MOF)-engaged strategy. The synthetic process includes the growth of Co-based zeolitic imidazolate framework (ZIF-67) polyhedra onto the NiMoO4·xH2O nanorod arrays (NRAs) at room temperature and followed by a sufficient sulfidation reaction. Benefiting from the intriguing structural and compositional advantages, the yolk-shell Ni–Mo–Co–S NCAs/NF binder-free electrode exhibits an extremely high areal capacity of 1.96 mAh cm−2 at a current density of 5 mA cm−2 and excellent cycling stability. The electrode kinetics analysis confirms the diffusion-controlled battery-type behavior of the yolk-shell Ni–Mo–Co–S NCAs/NF electrode. The corresponding full cell delivers an high energy density of 92.6 Wh kg−1 at the power density of 1029.1 W kg−1 with the yolk-shell Ni–Mo–Co–S NCAs/NF as cathode electrode and Bi2O3 as anode electrode, indicating the potential for real applications. This work would make contribution to the realization of directly growing multi-component hollow and complex hollow structures on conductive substrate for high-performance electrochemical energy storage.

Self-Template Synthesis of a MnCeOδ/Co3O4 Polyhedral Nanocage Catalyst for Toluene Oxidation

Herein, a novel nanocage catalyst of a porous MnCeOδ/Co3O4-NC polyhedral derived from a Co-based zeolitic imidazolate framework (ZIF-67) was synthesized and its catalytic performance for toluene ox...

Water Splitting-Assisted Electrocatalytic Oxidation of Glucose with a Metal-Organic Framework for Wearable Nonenzymatic Perspiration Sensing.

In this work, a nonenzymatic electrochemical sensor based on electrochemical water splitting-assisted electrocatalysis is developed for wearable perspiration glucose analysis. Pd nanoparticles (Pd NPs) encapsulated in a Co-based zeolitic imidazolate framework (ZIF-67) is prepared and used as the electrocatalyst. In comparison to previously reported nonenzymatic glucose sensors which detect glucose in alkaline buffers, the proposed sensor analyzes glucose under physiological pH with no additional reagents, which enables wearable, maintenance-free perspiration glucose monitoring for a long time. The nonenzymatic sensor and a flexible printed circuit board (FPCB) is integrated into a sweatband for real-time analysis of perspiration glucose. The test results of perspiration glucose using our sensor are correlated to those of blood glucose tests by a commercial glucose meter. The sensitivity of the sensor remains stable within 2 months when it is stored unpacked under ambient conditions. Therefore, we believe it is promising for wearable nonenzymatic glucose monitoring for noninvasive clinical analysis and sport applications.

Microwave-assisted hydrothermal synthesis of MOFs-derived bimetallic CuCo-N/C electrocatalyst for efficient oxygen reduction reaction

The oxygen reduction reaction (ORR) plays a significant role in energy conversion technologies such as metal-air batteries and fuel cells. The development of efficient ORR electrocatalyst is highly desirable to achieve the fast oxygen reduction. In this work, the bimetallic Cu and Co embedded nitrogen-doped carbon (CuCo-N/C) is fabricated as efficient electrocatalyst for ORR. The incorporation of Cu-precursor into Co-based zeolitic imidazolate frameworks (ZIF) is innovated here to synergistically enhance the activity of Co. Surprisingly, the addition of Cu can also increase the nitrogen content in the CuCo-N/C catalysts, which could generate more active sites and result in the improvement of ORR activity. The optimized CuCo-N/C catalysts exhibit superior ORR performance with a half-wave potential of 0.85 V (vs. RHE), a limiting current density of 5.61 mA cm−2 and enhanced long-term durability comparing with the state-of-the-art Pt/C catalysts. Benefiting from the unique structure, the as-made CuCo-N/C catalysts as electrode for Zn-air battery delivers excellent performance with a peak power density of 66.9 mW cm−2, a high open circuit voltage of 1.468 V, and a voltage degradation of about 1.4% after 12 h discharging, which is much better than that of Pt/C catalysts.

Carbon-Based Nanostructures Vertically Arrayed on Layered Lanthanum Oxycarbonate as Highly Efficient Catalysts for Oxygen Reduction Reactions.

Controllable pyrolysis of collapsible metal-organic frameworks (MOFs) into carbon-based nanostructures without obvious collapse and aggregation is of importance for the fabrication of well catalytic active and durable carbon-based catalysts for the oxygen reduction reaction (ORR). Herein, we fabricate morphology-controlled carbon-based nanostructures derived from the Co-based zeolitic imidazolate framework (ZIF-67) that epitaxially grows on layered lanthanum oxycarbonate (La2O2CO3) as a structure-oriented template, followed by pyrolysis at 800 °C. These synthesized carbon-based nanostructures show a well-defined dodecahedron morphology and vertical array on the template surface. In 0.1 M KOH solution, the ORR activity and durability of the carbon-based nanostructures are not only much higher than those obtained by pyrolytic carbons derived from pure ZIF-67 but also exceed commercial Pt/C (20 wt %, Pt). The significantly improved ORR performance can be ascribed to the increased Co-N x level, high specific surface area, and graphitization of the pyrolytic carbon, caused by the introduction of the La2O2CO3 phase into the composite catalyst. Therefore, using La2O2CO3 as the template may be a smart synthetic strategy for MOF-derived nanocarbons with a controlled morphology and composition for energy storages and conversions.

Ultrathin nanosheets-assembled NiCo2S4 nanocages derived from ZIF-67 for high-performance supercapacitors

The design and synthesis of new microstructure electrodes with large surface area and high electrochemical response play an essential role in advanced supercapacitors. Herein, NiCo2S4 hollow nanocages assembled by ultrathin nanosheets were synthesized through a facile solid-state chemical sulfuration method derived from the sacrificial template of Co-based zeolitic imidazolate frameworks (ZIF-67). The obtained hollow NiCo2S4 possess inner cavities and largely ultrathin nanosheets for both short ion diffusion distance and rich interfacial active sites which can dynamically speed up faradaic reactions. A specific capacitance of 1232 F g−1 is obtained at a current density of 2.0 A g−1 for the NiCo2S4 electrode. Notably, compared with Co–Ni LDH, NiCo2S4 electrode yields impressive rate capability and excellent cycling stability (80% capacitance retention after 8000 charge–discharge cycles), mainly due to its robust porous architecture and good electronic conductivity. The electrochemical reaction mechanism is studied by in situ XRD, indicating that NiCo2S4 exhibits a single-phase OH− insertion/extraction behavior. The aqueous asymmetric supercapacitor assembled with NiCo2S4 as positive electrode also delivers high energy density of 41.4 Wh kg−1 at power density of 689.5 W kg−1. This work demonstrates that the well-designed NiCo2S4 is promising electrode materials for high-performance supercapacitors.

Significantly enhanced activity of ZIF-67-supported nickel phosphate for electrocatalytic glucose oxidation

Nickel-based nanoparticles have been widely used in detection of glucose due to its multiple oxidation states and high stability in alkaline environment. In this work, amorphous nickel phosphate was supported by Co-based zeolitic imidazolate framework (ZIF-67) through ion exchange to prepare Ni3(PO4)2@ZIF-67 composites as the electrode material. In comparison to glassy carbon electrode (GCE), Ni3(PO4)2/GCE and ZIF-67/GCE, the Ni3(PO4)2@ZIF-67/GCE showed significantly enhanced performances in electrocatalytic oxidation of glucose. The great improvement on glucose oxidation current could be ascribed to the unique structure of Ni3(PO4)2@ZIF-67, the mutual promotion between Ni(III)/Ni(II) and Co(IV)/Co(III) interfacial redox reactions and increased surface areas. Under optimum conditions, as-prepared Ni3(PO4)2@ZIF-67/GCE showed an ultra-high sensitivity of 2783 μA mM−1 cm−2 with a wide linear ranged from 1.0 μM to 4.0 mM, and a low limit detection of 0.7 μM at a signal-to-noise ratio of 3. Moreover, Ni3(PO4)2@ZIF-67/GCE were successfully applied to detect glucose of human serum with good recoveries (92–109%), which indicated that Ni3(PO4)2@ZIF-67 can found application in the clinical detection.

Honeycomb-like Co/C composites derived from hierarchically nanoporous ZIF-67 as a lightweight and highly efficient microwave absorber

A simple method synthesizing hierarchically porous Co-based zeolitic imidazolate frameworks (ZIF-67) is presented. In approach, polyvinyl pyrrolidone (PVP) aggregates on graphene oxide (GO) surface through H-bond between oxygen-containing groups (OH and COOH) on GO and CO in PVP and the results GO/PVP composites is used as a template to synthesize hierarchically porous ZIF-67. After in situ postpyrolysis heat treatment in Ar atmosphere, honeycomb-like Co/C composites with abundant hierarchically pores are obtained. The hierarchically porous structure endows the as-obtained Co/C composites with highly efficient microwave absorption performances. The maximum reflection loss can reach −50.7 dB and the effective bandwidth is 4.6 GHz at a filler loading as low as 10 wt%. Notably, the electromagnetic parameters analysis indicates that such excellent microwave absorption properties are mainly ascribed to well-matched impedance combined with strong attenuation property induced by the hierarchically porous structure. More importantly, this strategy could also be used to fabricate other MOF-derived high performance microwave absorbers.

Facile and green approach to the synthesis of zeolitic imidazolate framework nanosheet-derived 2D Co/C composites for a lightweight and highly efficient microwave absorber

The two-dimensional Co/C composites derived from nanosheet-like ZIF-67 (Co-based zeolitic imidazolate framework) precursor was synthesized by a facile and green method. The microstructure, graphitization degree, magnetic properties and microwave absorption properties of two-dimensional Co/C composites varied in sensitivity with the pyrolysis condition. The one prepared at 700 °C exhibited the better microwave absorption performance in the terms its absorbing capability, matching thickness and the bandwidth for effective absorption. Adding only 10 wt% the as-prepared two-dimensional Co/C composites into paraffin, the maximum reflection loss reached −39.3 dB and the corresponding effective bandwidth was observed to broaden to 5.1 GHz (9.7–14.8 GHz) with matching thickness of 2.4 mm, which is superior to that of typical rhombic dodecahedron ZIF-67 derived composites absorber. Theoretical analysis reveals that dielectric loss are responsible for their excellent performance, i.e., the interfacial polarization processes and dipole polarization. Furthermore, the magnetic loss (natural ferromagnetic resonance and exchange resonance) and the wave interference cancellation model also contribute to microwave absorption. It is expected that this work may provide a systematic guideline to design lightweight and highly efficient microwave absorber.

Bottom-up fabrication of two-dimensional Co-based zeolitic imidazolate framework tubular membranes consisting of nanosheets by vapor phase transformation of Co-based gel for H2/CO2 separation

Two-dimensional (2D) metal-organic framework (MOF) nanosheet membranes demonstrate superhigh separation properties and offer great potential in practical separation applications, but the preparation of such membranes, especially tubular ones, still remains a huge challenge. Here, a vapor phase transformation strategy is presented for the direct synthesis of a 2D Co-based nanosheet MOF membrane on a porous tubular substrate via ligand vapor phase transformation of the Co-based gel layer coated on the substrate. The gel layer acts as a multifunctional role such as providing active metal source, guiding the oriented growth of the nanosheets and controlling the membrane thickness in the formation of the nanosheet membrane. The achieved Co-based nanosheet membrane with a thickness of around 57 nm exhibits excellent gas separation performance with H2/CO2 ideal selectivity of as high as 58.7. This method shows great promise for the scalable fabrication of MOF nanosheet membranes.

Functionalized Carbon Nanofibers Enabling Stable and Flexible Absorbers with Effective Microwave Response at Low Thickness.

Lots of work has been done to develop microwave absorbing materials (MAM) utilized as flexible electronic devices and communication instruments. Conventionally developed powder MAM are often limited in practical applications because of the bad stability and poor durability, which is out of the scope for exploiting flexible and long-term microwave absorbers. To overcome such limitations, a facile and binder-free technique from a Co-based zeolitic imidazolate framework (ZIF-67, a member of metal-organic frameworks)-coated carbon fiber precursor is developed for the in situ horizontal growth of Co3O4 nanoparticles, which embedded nitrogen-doped carbon array (triangular nanoplates) on the surface of carbon fibers in the carbon paper (NC-Co3O4/CP) as low-thickness MAM. The maximum reflection loss (RL) values reaches -16.12 and -34.34 dB when the thickness is 1.1 and 1.5 mm, respectively. As the thickness increases, the absorbing performance at low frequency performs well (RL < -20 dB). The hierarchical architecture is facilely originated from a metal-organic framework precursor. In view of the simple preparation technique, NC-Co3O4/CP exhibit huge potential in large-scale production of portable microwave absorbing electronic devices with strong microwave response at low thickness.

Design of porous Co3O4 nanosheets via one-step synthesis as high-performance anode materials for lithium-ion batteries

The morphology of Co-based zeolitic imidazolate framework is affected by the molar ratio of 2-methylimidazole and Co2+ used during the synthesis. In this study, we found that by further controlling the molar ratio of 2-methylimidazole and Co2+, hexagonal Co(OH)2 nanosheets can by formed. By calcination of such Co(OH)2 nanosheets, the original two-dimensional morphology was maintained and hierarchical pores were formed with 2-methylimidazole as the porogen. Such porous Co3O4 nanosheets exhibited good electrochemical performance and delivered a high specific capacity of 850 mAh g−1 at current density of 300 mA g−1 after 100 cycles when used as anode of lithium-ion batteries.

Co-Embedded Carbon Nano-Polyhedron Supported on Functionalized Graphene Oxide for Efficient Oxygen Reduction Reaction

Recently, Metal Organic Frameworks (MOFs) have been used as precursors to synthesis nanomaterials because of their crystalline and porous structure. Cobalt (Co) based zeolitic imidazolate frameworks (ZIFs) are one kind of 3D MOF structures with Co sitting in the center of well-organized organic linkers. Metal doped carbon-based nanomaterials obtained by pyrolysis of Co based ZIFs (Co-ZIFs) have been considered as promising candidates for improving oxygen reduction reaction (ORR) due to abundant Co-C/N bonds, which have been proved to be excellent catalytic structure. In addition, the polyhedron structure of the Co-ZIFs could be maintained even after pyrolysis at elevated temperatures resulting in a highly porous and stable support for further enhancement. However, Co-ZIFs tend to aggregate together during pyrolysis process because of their porous structure, which would destroy the polyhedron structure and decrease surface area of the product and further reducing their electrochemical performance. To prevent agglomeration of the Co-ZIFs usually a supplemental template is incorporated to maintain the well-defined polyhedron structure even after pyrolysis. A potential candidate to use as a template is graphene oxide. Graphene oxide (GO) based materials have been used for electrodes and catalysts due to their excellent electrical conductivity and highly surface area. GO is a bifunctional structure that contains epoxy and hydroxyl functional groups on both side of its surface. These groups could be used as the connectors for MOF forming unique structure with enhanced properties. In our work, we demonstrate a facial synthesis of Co doped porous carbon nanomaterials supported on functionalized GO by combing ZIF-67 with functionalized GO. Several different types of functionalized graphene oxides were synthesized, which include: graphene oxide treated by HBr (HG), HG treated by oxalic acid (HOG) , nitrogen doped graphene oxide (NGO). These functionalized GOs are used to potentially increase the interactions between GO and ZIF-67. Thus, more ZIF-67 will be anchored onto surface of graphene oxides due to increased active sites provided by those functional groups. Afterwards, a post synthesis pyrolysis is included to get highly ordered porous polyhedron carbon structure supported on GO with high surface area. Pyrolysis of ZIF-67/GO showed comparable onset potential under alkaline atmosphere (1mol/L KOH), which is 0.81mV vs. RHE. We assume that pyrolysis of ZIF-67/HG will exhibit higher ORR properties with more stability than the others due to more epoxy groups created on the surface. All the as-prepared product exhibits comparable or even enhanced electrocatalytic properties than commercially used Pt/C catalyst. Moreover, graphene oxides could also act as templates to fix ZIF-67 in a certain position, which would prevent ZIFs from aggregating together during pyrolysis process. With the combination of graphene oxide, the electron transfer pathways and active sites will be increased, enhancing the catalytic properties for ORR. References Hou, Y., Wen, Z., Cui, S., Ci, S., Mao, S., & Chen, J. (2015). Advanced Functional Materials, 25(6), 872.Wei, J., Hu, Y., Liang, Y., Kong, B., Zheng, Z., Zhang, J., et al. (2017). Journal of Materials Chemistry A, 5(21), 10182.Xia, B. Y., Yan, Y., Li, N., Wu, H. B., Lou, X. W. D., & Wang, X. (2016). Nature Energy, 1(1), 15006.

Yolk–shell structured Co-C/Void/Co9S8 composites with a tunable cavity for ultrabroadband and efficient low-frequency microwave absorption

A yolk–shell structured Co-C/Void/Co9S8 ternary composite composed of a Co nanoparticle-embedded porous carbon core and Co9S8 shell was synthesized by the sulfidation of a Co-based zeolitic imidazolate framework and subsequent pyrolysis. The composition and interior cavity of the Co-C/Void/Co9S8 composite could be precisely modulated by controlling the sulfidation reaction. Due to the abundant heterointerfaces, well-controlled cavity, and magnetic–dielectric synergistic effects, the Co-C/Void/Co9S8 exhibited excellent and tunable microwave-absorbing properties. The optimized Co-C/Void/Co9S8, having a loading of 25 wt.% and thickness only 2.2 mm, displayed an ultrabroad absorption bandwidth of 8.2 GHz at high frequencies. Moreover, the composite could achieve an extremely high reflection loss of–54.02 dB at low frequencies by adjusting its loading to 30 wt.%. This study provides a new insight into promising lightweight microwave-absorbing materials with ultrabroad absorption bandwidths and strong low-frequency absorption.