Ruqiang Zou

Metal–organic frameworks and their derived nanostructures for electrochemical energy storage and conversion

Metal–organic frameworks (MOFs) have received a lot of attention because of their diverse structures, tunable properties and multiple applications such as gas storage, catalysis and magnetism. Recently, there has been a rapidly growing interest in developing MOF-based materials for electrochemical energy storage. MOFs have proved to be particularly suitable for electrochemical applications because of their tunable chemical composition that can be designed at the molecular level and their highly porous framework in which fast mass transportation of the related species is favorable. In this review, the recent progress in fabricating MOFs and MOF-derived nanostructures for electrochemical applications is presented. The review starts with an introduction of the principles and strategies for designing targeted MOFs followed by a discussion of some novel MOF-derived structures and their potential applications in electrochemical energy storage and conversion. Finally, major challenges in electrochemical energy storage are highlighted and prospective solutions from current progress in MOF-based nanostructure research are given.

Earth-Abundant Nanomaterials for Oxygen Reduction.

Replacing the rare and precious platinum (Pt) electrocatalysts with earth-abundant materials for promoting the oxygen reduction reaction (ORR) at the cathode of fuel cells is of great interest in developing high-performance sustainable energy devices. However, the challenging issues associated with non-Pt materials are still their low intrinsic catalytic activity, limited active sites, and the poor mass transport properties. Recent advances in material sciences and nanotechnology enable rational design of new earth-abundant materials with optimized composition and fine nanostructure, providing new opportunities for enhancing ORR performance at the molecular level. This Review highlights recent breakthroughs in engineering nanocatalysts based on the earth-abundant materials for boosting ORR.

A metal–organic framework route to in situ encapsulation of Co@Co3O4@C core@bishell nanoparticles into a highly ordered porous carbon matrix for oxygen reduction

Rational design of non-noble metal catalysts with an electrocatalytic activity comparable or even superior to Pt is extremely important for future fuel cell-based renewable energy devices. Herein, we demonstrate a new concept that a metal–organic framework (MOF) can be used as a novel precursor to in situ encapsulate Co@Co3O4@C core@bishell nanoparticles (NPs) into a highly ordered porous carbon matrix (CM) (denoted as Co@Co3O4@C–CM). The central cobalt ions from the MOF are used as a metal source to produce Co metal cores, which are later transformed into a fancy Co@Co3O4 nanostructure via a controlled oxidation. The most notable feature of our Co@Co3O4@C–CM is that the highly ordered CM can provide much better transport pathways than the disordered pure MOF derived nanostructure that can facilitate the mass transport of O2 and an electrolyte. As a result, the well-designed Co@Co3O4@C–CM derived from the MOF shows almost identical activity but superior stability and methanol tolerance for the ORR relative to the commercial Pt/C in alkaline medium. Our work reports a novel Co@Co3O4@C nanostructure from a MOF for the first time and also reveals the important role of the introduction of a highly ordered carbon matrix into the MOF derived catalyst in enhancing the ORR activity and stability. To the best of our knowledge, the Co@Co3O4@C–CM is the most efficient non-noble metal nanocatalyst ever reported for the ORR.

Covalent Organic Frameworks for CO2 Capture

As an emerging class of porous crystalline materials, covalent organic frameworks (COFs) are excellent candidates for various applications. In particular, they can serve as ideal platforms for capturing CO2 to mitigate the dilemma caused by the greenhouse effect. Recent research achievements using COFs for CO2 capture are highlighted. A background overview is provided, consisting of a brief statement on the current CO2 issue, a summary of representative materials utilized for CO2 capture, and an introduction to COFs. Research progresses on: i) experimental CO2 capture using different COFs synthesized based on different covalent bond formations, and ii) computational simulation results of such porous materials on CO2 capture are summarized. Based on these experimental and theoretical studies, careful analyses and discussions in terms of the COF stability, low- and high-pressure CO2 uptake, CO2 selectivity, breakthrough performance, and CO2 capture conditions are provided. Finally, a perspective and conclusion section of COFs for CO2 capture is presented. Recent advancements in the field are highlighted and the strategies and principals involved are discussed.

A Triazole-Containing Metal–Organic Framework as a Highly Effective and Substrate Size-Dependent Catalyst for CO2 Conversion

A highly porous metal-organic framework (MOF) incorporating both exposed metal sites and nitrogen-rich triazole groups was successfully constructed via solvothermal assembly of a clicked octcarboxylate ligand and Cu(II) ions, which presents a high affinity toward CO2 molecules clearly verified by gas adsorption and Raman spectral detection. The constructed MOF featuring CO2-adsorbing property and exposed Lewis-acid metal sites could serve as an excellent catalyst for CO2-based chemical fixation. Catalytic activity of the MOF was confirmed by remarkably high efficiency on CO2 cycloaddition with small epoxides. When extending the substrates to larger ones, its activity showed a sharp decrease. These observations reveal that MOF-catalyzed CO2 cycloaddition of small substrates was carried out within the framework, while large ones cannot easily enter into the porous framework for catalytic reactions. Thus, the synthesized MOF exhibits high catalytic selectivity to different substrates on account of the confinement of the pore diameter. The high efficiency and size-dependent selectivity toward small epoxides on catalytic CO2 cycloaddition make this MOF a promising heterogeneous catalyst for carbon fixation.

Metal‐Organic Framework‐Derived Honeycomb‐Like Open Porous Nanostructures as Precious‐Metal‐Free Catalysts for Highly Efficient Oxygen Electroreduction

Honeycomb-like porous carbon nanostructures are rationally constructed from a metal-organic framework composite. The unique architecture with uniformly distributed high-density active sites significantly enhances the electrocatalytic performance by increasing the accessible active sites and enhancing mass transport of the gas and electrolyte, rendering the resulting catalyst adequate in reaching the desired catalytic performance afforded by Pt for the oxygen reduction reaction.

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.

Storage and separation applications of nanoporous metal–organic frameworks

This Highlight explores the progress and perspective in studies of metal–organic frameworks (MOFs), a new class of nanoporous materials, particularly suited for storage and separation applications related to energy utilization and environmental remediation. Since the discovery of the first MOF compound, hundreds of different MOFs have been developed and reported. MOFs are generally synthesized by self-assembly of metal ions/clusters as coordination centers and organic ligands as linkers. They possess intriguing chemical and physical properties and are structurally tunable, thermally stable and mechanically sound. MOFs are increasingly proving to be a superior class of materials for state-of-the-art applications in crystal engineering, chemistry, and materials science. In this Highlight, we present general routes for MOFs synthesis, discuss reticular design of their pore structures, and show some of their remarkable applications, especially in the areas of storage and separation.

Covalent Organic Frameworks Formed with Two Types of Covalent Bonds Based on Orthogonal Reactions

Covalent organic frameworks (COFs) are excellent candidates for various applications. So far, successful methods for the constructions of COFs have been limited to a few condensation reactions based on only one type of covalent bond formation. Thus, the exploration of a new judicious synthetic strategy is a crucial and emergent task for the development of this promising class of porous materials. Here, we report a new orthogonal reaction strategy to construct COFs by reversible formations of two types of covalent bonds. The obtained COFs consisting of multiple components show high surface area and high H2 adsorption capacity. The strategy is a general protocol applicable to construct not only binary COFs but also more complicated systems in which employing regular synthetic methods did not work.

Electro- and Photodriven Phase Change Composites Based on Wax-Infiltrated Carbon Nanotube Sponges

Organic phase change materials are usually insulating in nature, and they are unlikely to directly trigger latent heat storage through an electrical way. Here we report a multifunctional phase change composite in which the energy storage can be driven by small voltages (e.g., 1.5 V) or light illumination with high electro-to-heat or photo-to-thermal storage efficiencies (40% to 60%). The composite is composed of paraffin wax infiltrated into a porous, deformable carbon nanotube sponge; the latter not only acts as a flexible encapsulation scaffold for wax but also maintains a highly conductive network during the phase change process (for both solid and liquid states). Uniform interpenetration between the nanotube network and paraffin wax with high affinity results in enhanced phase change enthalpy and thermal conductivity compared to pure paraffin wax. Our phase change composite can store energy in practical ways such as by sunlight absorption or under voltages applied by conventional lithium-ion batteries.