Wenhan Guo

High-Performance Energy Storage and Conversion Materials Derived from a Single Metal–Organic Framework/Graphene Aerogel Composite

Metal oxides and carbon-based materials are the most promising electrode materials for a wide range of low-cost and highly efficient energy storage and conversion devices. Creating unique nanostructures of metal oxides and carbon materials is imperative to the development of a new generation of electrodes with high energy and power density. Here we report our findings in the development of a novel graphene aerogel assisted method for preparation of metal oxide nanoparticles (NPs) derived from bulk MOFs (Co-based MOF, Co(mIM)2 (mIM = 2-methylimidazole). The presence of cobalt oxide (CoOx) hollow NPs with a uniform size of 35 nm monodispersed in N-doped graphene aerogels (NG-A) was confirmed by microscopic analyses. The evolved structure (denoted as CoOx/NG-A) served as a robust Pt-free electrocatalyst with excellent activity for the oxygen reduction reaction (ORR) in an alkaline electrolyte solution. In addition, when Co was removed, the resulting nitrogen-rich porous carbon-graphene composite electrode (denoted as C/NG-A) displayed exceptional capacitance and rate capability in a supercapacitor. Further, this method is readily applicable to creation of functional metal oxide hollow nanoparticles on the surface of other carbon materials such as graphene and carbon nanotubes, providing a good opportunity to tune their physical or chemical activities.

Functional Zeolitic-Imidazolate-Framework-Templated Porous Carbon Materials for CO2 Capture and Enhanced Capacitors

Three types of zeolitic imidazolate frameworks (ZIFs) with different topological structures and functional imidazolate-derived ligands, namely, ZIF-8, ZIF-68, and ZIF69, have been directly carbonized to prepare porous carbon materials at 1000 °C. These as-synthesized porous carbon materials were activated with fused KOH to increase their surface areas and pore volumes for use in gas storage and supercapacitors. The relationship between the local structure of the products and the composition of the precursors has been investigated in detail. The BET surface areas of the resultant activated carbon materials are 2437 (CZIF8a), 1861 (CZIF68a), and 2264 m(2) g(-1) (CZIF69a). CZIF8a exhibits the highest H2 -storage capacities of 2.59 wt.% at 1 atm and 77 K, whereas CZIF69a has the highest CO2 uptake of 4.76 mmol g(-1) at 1 atm and 273 K, owing to its local structure and pore chemical environment. The specific capacities are calculated from the CV curves. CZIF69a exhibits the highest supercapacitor performance of 168 F g(-1) at a scan speed of 5 mV s(-1). These results indicate that the functional chloride group on the benzimidazolate ligand plays a very important role in improving the surface area, pore volume, and, therefore, CO2-capture and supercapacitor properties of the corresponding porous carbon materials.

Pristine Metal–Organic Frameworks and their Composites for Energy Storage and Conversion

Metal-organic frameworks (MOFs), a new class of crystalline porous organic-inorganic hybrid materials, have recently attracted increasing interest in the field of energy storage and conversion. Herein, recent progress of MOFs and MOF composites for energy storage and conversion applications, including photochemical and electrochemical fuel production (hydrogen production and CO2 reduction), water oxidation, supercapacitors, and Li-based batteries (Li-ion, Li-S, and Li-O2 batteries), is summarized. Typical development strategies (e.g., incorporation of active components, design of smart morphologies, and judicious selection of organic linkers and metal nodes) of MOFs and MOF composites for particular energy storage and conversion applications are highlighted. A broad overview of recent progress is provided, which will hopefully promote the future development of MOFs and MOF composites for advanced energy storage and conversion applications.

A Universal Strategy for Hollow Metal Oxide Nanoparticles Encapsulated into B/N Co‐Doped Graphitic Nanotubes as High‐Performance Lithium‐Ion Battery Anodes

Yolk-shell nanostructures have received great attention for boosting the performance of lithium-ion batteries because of their obvious advantages in solving the problems associated with large volume change, low conductivity, and short diffusion path for Li+ ion transport. A universal strategy for making hollow transition metal oxide (TMO) nanoparticles (NPs) encapsulated into B, N co-doped graphitic nanotubes (TMO@BNG (TMO = CoO, Ni2 O3 , Mn3 O4 ) through combining pyrolysis with an oxidation method is reported herein. The as-made TMO@BNG exhibits the TMO-dependent lithium-ion storage ability, in which CoO@BNG nanotubes exhibit highest lithium-ion storage capacity of 1554 mA h g-1 at the current density of 96 mA g-1 , good rate ability (410 mA h g-1 at 1.75 A g-1 ), and high stability (almost 96% storage capacity retention after 480 cycles). The present work highlights the importance of introducing hollow TMO NPs with thin wall into BNG with large surface area for boosting LIBs in the terms of storage capacity, rate capability, and cycling stability.

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.

Highly dispersed Co-based Fischer–Tropsch synthesis catalysts from metal–organic frameworks

The influence of pore texture and nitrogen species of the carbon support for the Fischer–Tropsch synthesis was investigated using well-defined catalysts derived from metal–organic frameworks (MOFs). Two typical MOFs were employed in the carbonization process to prepare the target catalysts, i.e. nitrogen-rich ZIF-67 and nitrogen-free Co-MOF-74. The Co-MOF-74-derived nanocomposite (Co@C) showed a carbon monoxide (CO) conversion of 30%, whereas the ZIF-67-derived nanocomposite (Co@NC) exhibited a CO conversion of 10%. The nitrogen-free Co@C composite showed 65% selectivity for long-chain hydrocarbons (C5+) and 10% selectivity for short-chain hydrocarbons (C2–C4) after 100 h on stream; on the other hand, the Co@NC composite showed 31% selectivity for C5+ products and 37% selectivity for short-chain hydrocarbons (C2–C4) after 100 h on stream. The excellent CO conversion was attributed to the large pore size of the carbon support, which facilitates the diffusion of the hydrocarbons. The high C2–C4 selectivity originates from the influence of nitrogen species in the carbon support. This study is expected to open a new avenue for the design of new catalysts for the Fischer–Tropsch synthesis with high activity and superior selectivity via choosing suitable MOFs precursors.

Kinetic-Controlled Formation of Bimetallic Metal–Organic Framework Hybrid Structures

Heterometallic metal-organic frameworks (MOFs) are constructed from two or more kinds of metal ions, while still remaining their original topologies. Due to distinct reaction kinetics during MOF formation, partial distribution of different metals within a single MOF crystal can lead to sophisticated heterogeneous nanostructures. Here, this study reports an investigation of reaction kinetics for different metal ions in a bimetallic MOF system, the ZIF-8/67 (M(2-mIM)2 , M = Zn for ZIF-8, and Co for ZIF-67, 2-mIM = 2-methylimidazole), by in situ optical method. Distinct kinetics of the two metals forming single-component MOFs are revealed, and when both Co and Zn ions are present in the starting solution, homogeneous distributions of the two metals are only achieved at high Co/Zn ratio, while at low Co/Zn ratio concentration gradient from Co-rich cores to Zn-rich shells is observed. Further, by adding the two metals in sequence, more sophisticated structures are achieved. Specifically, when Co2+ is added first, ZIF-67@ZIF-8/67 core-shell nanocrystals are achieved with tunable core/shell thickness ratio depending on the time intervals; while when Zn2+ is added first, only agglomerates of irregular shape form due to the weak nucleation ability of Zn2+ .

Tailoring biomass-derived carbon for high-performance supercapacitors from controllably cultivated algae microspheres

A high-performance “green” carbon-based supercapacitor electrode material is synthesized from selected algae microspheres, which are grown under controlled cultivation conditions. The best-performing sample possesses a high specific surface area of 1337.9 m2 g−1 with a hierarchically porous structure and naturally intrinsic nitrogen dopant. This leads to an excellent specific capacitance of 353 F g−1 at 1 A g−1 and 92% capacitance retention even after 10 000 charge–discharge cycles at 20 A g−1, which makes it superior to many recently reported biomass- and synthetically derived carbon electrode materials. It is found that residual nitrogen and metal contents in algae-derived carbon are highly influenced by biomass components, such as proteins. The content of these components can be controlled by adjusting the concentrations of nutrients in cultivation media. However, nitrogen in algae proteins is not pyrolytically stable, and studies indicate that excessive residual metal content plays the role of “dead mass” and results in a less-developed porous structure. Therefore, this suggests that both biomass selection and cultivation should aim for proteins with a more stable nitrogen content and minimized content of electrochemically inactive metals such as Mg and Ca. Hence, this study does not just demonstrate a green candidate electrode material for high-performance supercapacitors, but also provides an innovative selection and cultivation strategy to improve the capacitive performance of biomass-derived carbon.

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.