Ruo Zhao

Facile Synthesis of Ultrasmall CoS2 Nanoparticles within Thin N‐Doped Porous Carbon Shell for High Performance Lithium‐Ion Batteries

Cobalt sulfide (CoS2) is considered one of the most promising alternative anode materials for high-performance lithium-ion batteries (LIBs) by virtue of its remarkable electrical conductivity, high theoretical capacity, and low cost. However, it suffers from a poor cycling stability and low rate capability because of its volume expansion and dissolution of the polysulfide intermediates in the organic electrolytes during the battery charge/discharge process. In this study, a novel porous carbon/CoS2 composite is prepared by using nano metal-organic framework (MOF) templates for high-preformance LIBs. The as-made ultrasmall CoS2 (15 nm) nanoparticles in N-rich carbon exhibit promising lithium storage properties with negligible loss of capacity at high charge/discharge rate. At a current density of 100 mA g(-1), a capacity of 560 mA h g(-1) is maintained after 50 cycles. Even at a current density as high as 2500 mA g(-1), a reversible capacity of 410 mA h g(-1) is obtained. The excellent and highly stable battery performance should be attributed to the synergism of the ultrasmall CoS2 particles and the thin N-rich porous carbon shells derieved from nanosized MOF precusors.

Nanostructured Electrode Materials Derived from Metal–Organic Framework Xerogels for High-Energy-Density Asymmetric Supercapacitor

This work successfully demonstrates metal-organic framework (MOF) derived strategy to prepare nanoporous carbon (NPC) with or without Fe3O4/Fe nanoparticles by the optimization of calcination temperature as highly active electrode materials for asymmetric supercapacitors (ASC). The nanostructured Fe3O4/Fe/C hybrid shows high specific capacitance of 600 F/g at a current density of 1 A/g and excellent capacitance retention up to 500 F/g at 8 A/g. Furthermore, hierarchically NPC with high surface area also obtained from MOF gels displays excellent electrochemical performance of 272 F/g at 2 mV/s. Considering practical applications, aqueous ASC (aASC) was also assembled, which shows high energy density of 17.496 Wh/kg at the power density of 388.8 W/kg. The high energy density and excellent capacity retention of the developed materials show great promise for the practical utilization of these energy storage devices.

Bimetallic Metal-Organic Frameworks: Probing the Lewis Acid Site for CO2 Conversion.

A highly porous metal-organic framework (MOF) incorporating two kinds of second building units (SBUs), i.e., dimeric paddlewheel (Zn2 (COO)4 ) and tetrameric (Zn4 (O)(CO2 )6 ), is successfully assembled by the reaction of a tricarboxylate ligand with Zn(II) ion. Subsequently, single-crystal-to-single-crystal metal cation exchange using the constructed MOF is investigated, and the results show that Cu(II) and Co(II) ions can selectively be introduced into the MOF without compromising the crystallinity of the pristine framework. This metal cation-exchangeable MOF provides a useful platform for studying the metal effect on both gas adsorption and catalytic activity of the resulted MOFs. While the gas adsorption experiments reveal that Cu(II) and Co(II) exchanged samples exhibit comparable CO2 adsorption capability to the pristine Zn(II) -based MOF under the same conditions, catalytic investigations for the cycloaddition reaction of CO2 with epoxides into related carbonates demonstrate that Zn(II) -based MOF affords the highest catalytic activity as compared with Cu(II) and Co(II) exchanged ones. Molecular dynamic simulations are carried out to further confirm the catalytic performance of these constructed MOFs on chemical fixation of CO2 to carbonates. This research sheds light on how metal exchange can influence intrinsic properties of MOFs.

MOF-derived α-NiS nanorods on graphene as an electrode for high-energy-density supercapacitors

Hierarchically porous electrodes made of electrochemically active materials and conductive additives may display synergistic effects originating from the interactions between the constituent phases, and this approach has been adopted for optimizing the performances of many electrode materials. Here we report our findings in design, fabrication, and characterization of a hierarchically porous hybrid electrode composed of α-NiS nanorods decorated on reduced graphene oxide (rGO) (denoted as R-NiS/rGO), derived from water-refluxed metal–organic frameworks/rGO (Ni-MOF-74/rGO) templates. Microanalyses reveal that the as-synthesized α-NiS nanorods have abundant (101) and (110) surfaces on the edges, which exhibit a strong affinity for OH− in KOH electrolyte, as confirmed by density functional theory-based calculations. The results suggest that the MOF-derived α-NiS nanorods with highly exposed active surfaces are favorable for fast redox reactions in a basic electrolyte. Besides, the presence of rGO in the hybrid electrode greatly enhances the electronic conductivity, providing efficient current collection for fast energy storage. Indeed, when tested in a supercapacitor with a three-electrode configuration in 2 M KOH electrolyte, the R-NiS/rGO hybrid electrode exhibits a capacity of 744 C g−1 at 1 A g−1 and 600 C g−1 at 50 A g−1, indicating remarkable rate performance, while maintaining more than 89% of the initial capacity after 20 000 cycles. Moreover, when coupled with a nitrogen-doped graphene aerogel (C/NG-A) negative electrode, the hybrid supercapacitor (R-NiS/rGO/electrolyte/C/NG-A) achieved an ultra-high energy density of 93 W h kg−1 at a power density of 962 W kg−1, while still retaining an energy density of 54 W h kg−1 at an elevated working power of 46 034 W kg−1.

Fabrication of Co3O4 nanoparticles in thin porous carbon shells from metal–organic frameworks for enhanced electrochemical performance

Cobalt oxides, typically Co3O4, have received considerable attention due to their high theoretical capacity as anode materials for Li-ion batteries. However, their poor electron conductivity and large volume change upon the insertion/removal of Li+ ions limit their practical application. Carbon coating is widely used to improve the electrochemical performance of materials and release the strain during the lithiation/delithiation processes, in which the thickness of the coating carbon shell has a vital role in determining the performance of the material. In this study, Co3O4 nanoparticles coated with a thin carbon shell are obtained from the metal–organic framework (MOF) precursor Co-MOF-74 via a sequential two-step carbonization process, where carbon oxides, e.g., CO2, are used as the oxidation atmosphere in the second step. The carbon content and shell thickness are controlled by changing the calcination time. The electrode containing a certain carbon content (3.17 wt%) exhibits a capacity of 1137 mA h g−1 after 100 cycles tested at 100 mA g−1 between 0.005 and 3.0 V. This enhanced electrochemical performance is attributed to the well-dispersed nanosized Co3O4 particles and thin carbon shell coating on the electrode surface, which shorten the Li+ ion diffusion length and enhance the electron conductivity of the hybrid.

Hierarchical Cobalt Hydroxide and B/N Co-Doped Graphene Nanohybrids Derived from Metal-Organic Frameworks for High Energy Density Asymmetric Supercapacitors

To cater for the demands of electrochemical energy storage system, the development of cost effective, durable and highly efficient electrode materials is desired. Here, a novel electrode material based on redox active β-Co(OH)2 and B, N co-doped graphene nanohybrid is presented for electrochemical supercapacitor by employing a facile metal-organic frameworks (MOFs) route through pyrolysis and hydrothermal treatment. The Co(OH)2 could be firmly stabilized by dual protection of N-doped carbon polyhedron (CP) and B/N co-doped graphene (BCN) nanosheets. Interestingly, the porous carbon and BCN nanosheets greatly improve the charge storage, wettability, and redox activity of electrodes. Thus the hybrid delivers specific capacitance of 1263 F g−1 at a current density of 1A g−1 with 90% capacitance retention over 5000 cycles. Furthermore, the new aqueous asymmetric supercapacitor (ASC) was also designed by using Co(OH)2@CP@BCN nanohybrid and BCN nanosheets as positive and negative electrodes respectively, which leads to high energy density of 20.25 Whkg−1. This device also exhibits excellent rate capability with energy density of 15.55 Whkg−1 at power density of 9331 Wkg−1 coupled long termed stability up to 6000 cycles.

Metal-Organic Frameworks: Bimetallic Metal-Organic Frameworks: Probing the Lewis Acid Site for CO2 Conversion (Small 17/2016).

On page 2334, R. Q. Zou, Y. L. Zhao, and co-workers present a porous metal-organic framework (MOF) that serves as a platform for studying the metal exchange effect on both CO2 adsorption and catalytic fixation. The effect is demonstrated by catalytic CO2 cycloaddition with propylene oxide to produce propylene carbonate. Molecular dynamic simulations are carried out to further confirm the catalytic performance of these MOFs on chemical fixation of CO2 to carbonates. This research sheds light on how metal exchange could influence the intrinsic properties of MOFs.

Exposing residual catalyst in a carbon nanotube sponge

Carbon nanotubes (CNTs) are grown from metal catalysts; after growth, residual catalyst particles are usually encapsulated within the tube cavities and are difficult to remove. Here, we directly expose controlled amounts of Fe catalyst from a bulk CNT sponge by thermal annealing at mild temperature, and produce uniformly dispersed Fe2O3 nanoparticles grafted onto CNTs throughout the sponge. Those exposed catalyst particles remain active and can be used to synthesize CNTs again. The resulting CNT–Fe2O3 composite sponges, which possess a highly porous and conductive network, also serve as freestanding supercapacitor electrodes with significantly enhanced specific capacitance than original CNT sponges. Our results indicate that residual metal catalysts, widely present in CNT-based materials, can be reactivated and utilized for applications in catalysis and energy areas.

Fe2N/S/N Codecorated Hierarchical Porous Carbon Nanosheets for Trifunctional Electrocatalysis

Construction of multifunctional highly active earth-abundant electrocatalysts on a large scale is a great challenge due to poor control over nanostructural features and limited active sites. Here, a simple methodology to tailor metal-organic frameworks (MOFs) to extract highly active multifunctional electrocatalysts on a large scale for oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution reaction (HER) is presented. The N, S codoped Fe2 N decorated highly porous and defect-rich carbon nanosheets are grown using MOF xerogels, melamine, and polyvinylpyrollidone. The resulting catalyst exhibits excellent activity for ORR with an onset (0.92 V) and half-wave (0.81 V) potential similar to state-of-the-art Pt/C catalysts. The catalyst also shows outstanding OER and HER activities with a small overpotential of 360 mV in 1 m KOH and -123 mV in 0.5 m H2 SO4 at a current density of 10 mA cm-2 , respectively. Excellent catalytic properties are further supported by theoretical calculations where relevant models are built and various possible activation sites are identified by first-principles calculations. The results suggest that the carbon atoms adjacent to heteroatoms as well as Fe2 -N sites present the active sites for improved catalytic response, which is in agreement with the experimental results.