Hierarchically Superstructured Anisotropic Carbon Particles by Multiscale Assembly Driven by Spinodal Decomposition.

Hierarchical superstructures have novel shape-dependent properties, but well-defined anisotropic carbon superstructures with controllable size, shape, and building block dimensionality have rarely been accomplished thus far. Here, a hierarchical assembly technique is presented that uses spinodal decomposition (SD) to synthesize anisotropic oblate particles of mesoporous carbon superstructure (o-MCS) with nanorod arrays by integrating block-copolymer (BCP) self-assembly and polymer-polymer interface behaviors in binary blends. The interaction of major and minor phases in binary polymer blends leads to the formation of an anisotropic oblate particle, and the BCP-rich phase enables ordered packing and unidirectional alignment of carbon nanorods. Consequently, this approach enables precise control over particles' size, shape, and over the dimensionality of their components. Exploiting this functional superstructure, o-MCS are used as an anode material in potassium-ion batteries, and achieve a notable specific capacity of 156 mA h g-1 at a current density of 2 A g-1, and long-term stability for 3000 cycles. This work presents a significant advancement in the field of hierarchical superstructures, providing a promising strategy for the design and synthesis of anisotropic carbon materials with controlled properties, offering promising applications in energy storage and beyond.

Microfluidic Shape Analysis of Non-spherical Graphite for Li-Ion Batteries via Viscoelastic Particle Focusing.

The size and shape of graphite, which is a popular active anode material for lithium-ion batteries (LIBs), significantly affect the electrochemical performance of LIBs and the rheological properties of the electrode slurries used in battery manufacturing. However, the accurate characterization of its size and shape remains challenging. In this study, the edge plane of graphite in a cross-slot microchannel via viscoelastic particle focusing is characterized. It is reported that the graphite particles are aligned in a direction that shows the edge plane by a planar extensional flow field at the stagnation point of the cross-slot region. Accurate quantification of the edge size and shape for both spheroidized natural and ball-milled graphite is achieved when aligned in this manner. Ball-milled graphite has a smaller circularity and higher aspect ratio than natural graphite, indicating a more plate-like shape. The effects of these differences in graphite shape and size on the rheological properties of the electrode slurry, the structure of the coated electrodes, and electrochemical performance are investigated. This method can contribute to the quality control of graphite for the mass production of LIBs and enhance the electrochemical performance of LIBs.

Spinodal Decomposition-Driven Structural Hierarchy of Mesoporous Inorganic Materials for Energy Applications.

ConspectusMesoporous inorganic materials (MIMs) directed by block copolymers (BCPs) have attracted tremendous attention due to their high surface area, large pore volume, and tunable pore size. The structural hierarchy of inorganic materials with designed meso- and macrostructures combines the benefits of mesoporosity and tailored macrostructures in which macropores have increased ion/mass transfer and large capacity to carry guest material and have a macroscale particle morphology that permits close packing and a low surface energy. Existing methods for hierarchically structured MIMs require complicated multistep procedures including preparation of sacrificial macrotemplates (e.g., foams and colloidal spheres). Despite considerable efforts to control the macrostructures of mesoporous materials, major challenges remain in the formation of a structural hierarchy with ordered mesoporosity.In polymer science, spinodal decomposition (SD) is a physical phenomenon that spontaneously produces a wide variety of macroscale heterostructures from interconnected networks to isolated droplets. Exploitation of SD is a promising method to achieve precise control of the macrostructure (e.g., macropore, particle morphology) and mesostructure (e.g., pore size and structure, composition) of inorganic materials. However, this approach for tailoring the structural hierarchy of MIMs is unexplored due to the lack of effective systems that can control the complex thermodynamic interactions of inorganic precursor/polymer blends and the phase-separation kinetics.In this Account, we present our recent research progress on the development of synthesis systems that combine unique SD behaviors and BCP self-assembly in polymer blends. To generate macropores in MIMs, we have exploited interconnected macrostructures of SD induced by designed quench conditions of multicomponent blends containing BCP. These strategies enable control of the size of the macropores of the nanostructures independently and can be extended to various compositions (e.g., carbon, SiO2, TiO2, WO3, TiNb2O7, TiN). We also control the macroscopic morphology of the MIMs into spherical particles (e.g., solid and hollow mesoporous spheres) by using SD induced by increasing the mixing entropy penalty of polymer blends that consist BCP, homopolymer(s), and inorganic precursors. Furthermore, interfacial tension between polymers determines the macroscopic morphology of MIMs, from isotropic to anisotropic mesoporous particles (e.g., oblate, bowl, 2D nanosheet). The interfacial states of the homopolymer determine the pore orientation and particle morphology of BCP-directed MIMs.We also highlight the application of the hierarchically structured MIMs in energy storage devices. Generated macropores facilitate ion/mass transfer in lithium-ion batteries and stable accommodation of a large amount of sulfur in lithium-sulfur batteries. Designed morphologies of MIMs are beneficial to achieve high packing density as electrode materials in potassium-ion batteries and thereby achieve high volumetric capacities.Recent advances in SD-driven synthesis for the structural hierarchy of MIMs will inspire how polymer science can be used as a platform for preparing the designed inorganic materials. Additionally, broadening the polymer and composition repertoire will guide in novel frontiers in the design and applications of MIMs in various fields.

Direct Synthesis of Mixed-Metal Paddle-Wheel Metal-Organic Frameworks with Controlled Metal Ratios under Ambient Conditions.

Currently, synthesizing mixed-metal metal-organic frameworks (MM-MOFs) in a single step remains a challenge due to the varying reactivities of different metal cations. This often results in the formation of mixtures of monometallic MOFs or MM-MOFs with nonstoichiometric metal ratios. A promising approach to overcoming this issue is the controlled precursor method, which uses prebuilt polynuclear complexes with structures similar to the secondary building units (SBUs) of the desired MOFs. In this study, we report that metal acetates can serve as natural prebuilt SBUs, enabling the controlled synthesis of MBDs ([M2(BDC)2DABCO]n, M = metal, BDC = 1,4-benzenedicarboxylic acid, DABCO = 1,4-diazabicyclo[2,2,2]octane) under ambient conditions. By exploiting the fact that metal acetates readily form soluble paddle-wheel dimers similar to the SBUs of MBDs, we achieve the direct synthesis of mixed-metal MBDs at room temperature. The metal ratios (Zn, Co, and Ni) in the resulting MBDs are controllable, and the production yields exceed 90%. The use of metal acetates facilitates the fast and uniform nucleation of MBDs, regardless of the metal cations involved. This similarity in nucleation rates leads to the formation of bimetallic and trimetallic MBDs with predefined metal ratios and homogeneous metal distribution while maintaining the quality of the MOFs. Importantly, this strategy offers an efficient pathway for synthesizing mixed metal MBDs using stoichiometric amounts of metal salts without toxic additives, high energy consumption, and complex synthesis steps.

Polymer Interface-Dependent Morphological Transition toward Two-Dimensional Porous Inorganic Nanocoins as an Ultrathin Multifunctional Layer for Stable Lithium-Sulfur Batteries.

Two-dimensional (2D) porous inorganic nanomaterials have intriguing properties as a result of dimensional features and high porosity, but controlled production of circular 2D shapes is still challenging. Here, we designed a simple approach to produce 2D porous inorganic nanocoins (NCs) by integrating block copolymer (BCP) self-assembly and orientation control of microdomains at polymer-polymer interfaces. Multicomponent blends containing BCP and homopoly(methyl methacrylate) (hPMMA) are designed to undergo macrophase separation followed by microphase separation. The balanced interfacial compatibility of BCP allows perpendicularly oriented lamellar-assembly at the interfaces between BCP-rich phase and hPMMA matrix. Disassembly of lamellar structures and calcination yield ultrathin 2D inorganic NCs that are perforated by micropores. This approach enables control of the thickness, size, and chemical composition of the NCs. 2D porous and acidic aluminosilicate NC (AS-NC) is used to fabricate an ultrathin and lightweight functional separator for lithium-sulfur batteries. The AS-NC layer acts as an ionic sieve to selectively block lithium polysulfides. Abundant acid sites chemically capture polysulfides, and micropores physically exclude them, so sulfur utilization and cycle stability are increased.

Controlling the morphology of metal-organic frameworks and porous carbon materials: metal oxides as primary architecture-directing agents.

Owing to their large ratio of surface area to mass and volume, metal-organic frameworks and porous carbons have revolutionized many applications that rely on chemical and physical interactions at surfaces. However, a major challenge today is to shape these porous materials to translate their enhanced performance from the laboratory into macroscopic real-world applications. In this review, we give a comprehensive overview of how the precise morphology control of metal oxides can be transferred to metal-organic frameworks and porous carbon materials. As such, tailored material structures can be designed in 0D, 1D, 2D, and 3D with considerable implications for applications such as in energy storage, catalysis and nanomedicine. Therefore, we predict that major research advances in morphology control of metal-organic frameworks and porous carbons will facilitate the use of these materials in addressing major needs of the society, especially the grand challenges of energy, health, and environment.

Polymer Interfacial Self-Assembly Guided Two-Dimensional Engineering of Hierarchically Porous Carbon Nanosheets.

Two-dimensional (2D) carbon nanosheets with micro- and/or mesopores have attracted great attention due to unique physical and chemical properties, but well-defined nanoporous carbon nanosheets with tunable thickness and pore size have been rarely realized. Here, we develop a polymer-polymer interfacial self-assembly strategy to achieve hierarchically porous carbon nanosheets (HNCNSs) by integrating the migration behaviors of immiscible ternary polymers with block copolymer (BCP)-directed self-assembly. The balanced interfacial compatibility of BCP allows the migration of a BCP-rich phase to the interface between two immiscible homopolymer major phases (i.e., homopoly(methyl methacrylate) and homopolystyrene), where the BCP-rich phase spreads thinly to a thickness of a few nanometers to decrease the interfacial tension. BCP-directed coassembly with organic-inorganic precursors constructs an ordered mesostructure. Carbonization and chemical etching yield ultrathin HNCNSs with hierarchical micropores and mesopores. This approach enables facile control over the thickness (5.6-75 nm) and mesopore size (25-46 nm). As an anode material in a potassium ion battery, HNCNSs show high specific capacity (178 mA h g-1 at a current density of 1 A g-1) with excellent long-term stability (2000 cycles), by exploiting the advantages of the hierarchical pores and 2D nanosheet morphology (efficient ion/electron diffusion) and of the large interlayer spacing (stable ion insertion).

Micro-Blooming: Hierarchically Porous Nitrogen-Doped Carbon Flowers Derived from Metal-Organic Mesocrystals.

Synthesis of 3D flower-like zinc-nitrilotriacetic acid (ZnNTA) mesocrystals and their conformal transformation to hierarchically porous N-doped carbon superstructures is reported. During the solvothermal reaction, 2D nanosheet primary building blocks undergo oriented attachment and mesoscale assembly forming stacked layers. The secondary nucleation and growth preferentially occurs at the edges and defects of the layers, leading to formation of 3D flower-like mesocrystals comprised of interconnected 2D micropetals. By simply varying the pyrolysis temperature (550-1000 °C) and the removal method of in the situ-generated Zn species, nonporous parent mesocrystals are transformed to hierarchically porous carbon flowers with controllable surface area (970-1605 m2 g-1 ), nitrogen content (3.4-14.1 at%), pore volume (0.95-2.19 cm3 g-1 ), as well as pore diameter and structures. The carbon flowers prepared at 550 °C show high CO2 /N2 selectivity due to the high nitrogen content and the large fraction of (ultra)micropores, which can greatly increase the CO2 affinity. The results show that the physicochemical properties of carbons are highly dependent on the thermal transformation and associated pore formation process, rather than directly inherited from parent precursors. The present strategy demonstrates metal-organic mesocrystals as a facile and versatile means toward 3D hierarchical carbon superstructures that are attractive for a number of potential applications.

Water-in-Water Pickering Emulsion Stabilized by Polydopamine Particles and Cross-Linking.

All aqueous multiphase systems have attracted significant attention recently, in particular water-in-water Pickering emulsions. In here, polydopamine nanoparticles (PDP) are investigated as stabilizers for dextran and poly(ethylene glycol) (PEG)-based aqueous emulsions. Remarkably, stable emulsions are obtained from the all-biocompatible materials that can be broken either via dilution or surfactant addition. Further cross-linking of PDP via poly(acrylic acid) and carbodiimide strengthens the stability of emulsion droplets in a colloidosome-like structure. After cross-linking, demulsification via dilution or surfactant addition was largely hindered. The PDP-mediated formation of all aqueous emulsions is expected to be generalized to different types of water-in-water emulsions with other polymers and offers new opportunities in surface modification as well as microencapsulation.

Morphogenesis of Metal-Organic Mesocrystals Mediated by Double Hydrophilic Block Copolymers.

Mesocrystals-superstructures of crystalline nanoparticles that are aligned in a crystallographic fashion-are of increasing interest for formation of inorganic materials with complex and sophisticated morphologies to tailor properties without changing chemical composition. Here we report morphogenesis of a novel mesocrystal consisting of nanoscale metal-organic frameworks (MOF) by using double hydrophilic block copolymer (DHBC) as a crystal modulator. DHBC selectively prefers the metastable hexagonal kinetic polymorph and promotes anisotropic crystal growth to generate hexagonal rod mesocrystals via oriented attachment and mesoscale assembly. The metastable nature of hexagonal mesocrystals enables further hierarchical morphogenesis by a solvent-mediated polymorphic transformation toward stable tetragonal mesocrystals that retain the outer hexagonal particle morphology. Furthermore, synthesis of hybrid MOFs, where hexagonal mesocrystals are vertically aligned on specific surfaces of cubic MOFs, is demonstrated. The present strategy opens a new avenue to create MOF mesocrystals and their hybrids with controlled size and morphology that can be designed for various potential applications.

Designing a Highly Active Metal-Free Oxygen Reduction Catalyst in Membrane Electrode Assemblies for Alkaline Fuel Cells: Effects of Pore Size and Doping-Site Position.

To promote the oxygen reduction reaction of metal-free catalysts, the introduction of porous structure is considered as a desirable approach because the structure can enhance mass transport and host many catalytic active sites. However, most of the previous studies reported only half-cell characterization; therefore, studies on membrane electrode assembly (MEA) are still insufficient. Furthermore, the effect of doping-site position in the structure has not been investigated. Here, we report the synthesis of highly active metal-free catalysts in MEAs by controlling pore size and doping-site position. Both influence the accessibility of reactants to doping sites, which affects utilization of doping sites and mass-transport properties. Finally, an N,P-codoped ordered mesoporous carbon with a large pore size and precisely controlled doping-site position showed a remarkable on-set potential and produced 70% of the maximum power density obtained using Pt/C.

Direct access to hierarchically porous inorganic oxide materials with three-dimensionally interconnected networks.

Hierarchically porous oxide materials have immense potential for applications in catalysis, separation, and energy devices, but the synthesis of these materials is hampered by the need to use multiple templates and the associated complicated steps and uncontrollable mixing behavior. Here we report a simple one-pot strategy for the synthesis of inorganic oxide materials with multiscale porosity. The inorganic precursor and block copolymer are coassembled into an ordered mesostructure (microphase separation), while the in situ-polymerized organic precursor forms organic-rich macrodomains (macrophase separation) around which the mesostructure grows. Calcination generates hierarchical meso/macroporous SiO2 and TiO2 with three-dimensionally interconnected pore networks. The continuous 3D macrostructures were clearly visualized by nanoscale X-ray computed tomography. The resulting TiO2 was used as the anode in a lithium ion battery and showed excellent rate capability compared with mesoporous TiO2. This work is of particular importance because it (i) expands the base of BCP self-assembly from mesostructures to complex porous structures, (ii) shows that the interplay of micro- and macrophase separation can be fully exploited for the design of hierarchically porous inorganic materials, and therefore (iii) provides strategies for researchers in materials science and polymer science.

Efficient removal of dyes from aqueous solutions using short-length bimodal mesoporous carbon adsorbents.

Ordered mesoporous carbons (OMCs) with controlled mesopore lengths and volumes were synthesized and investigated to remove the model dye methylene blue (MB) from aqueous solutions. The pore size, specific surface area, pore volume, and pore length of OMCs (CMK-3, sCMK-3, and sCMK-5) were analyzed and benchmarked against commercial activated carbon (AC). CMK-3 and sCMK-3 had narrow pore size distributions (PSDs) centered at ∼4.4 nm, whereas the PSD of sCMK-5 was bimodal, derived from the same pores as CMK-3 (∼4.4 nm) and the inner diameter of the carbon nanotubes (∼5.8 nm). The pore length decreased from 743 nm for CMK-3 to 173 nm for sCMK-3 and 169 nm for sCMK-5, facilitating the MB accessibility and efficient utilization of internal mesopores. The MB adsorption on the prepared adsorbents was well described by a pseudo-second-order kinetic model (R2 > 0.999), and the initial adsorption rate (h) on sCMK-5 was 34.07-fold faster than that on commercial AC. The Langmuir model adequately explained the equilibrium adsorption data, and the increase in the Langmuir maximal adsorption capacity (qm) of the OMCs was proportional to the specific surface area. The MB adsorption on sCMK-5 was endothermic and spontaneous, and proceeded primarily through physical adsorption as well as chemisorption reacting with oxygen atoms in hydroxyl groups. The prepared adsorbents were also suitable for polishing textile wastewater containing color-causing substances along with the background organic matter. These OMCs are promising for treating wastewater as efficient adsorbents for large molecular pollutants such as dyes.

Graphene-Based Two-Dimensional Mesoporous Materials: Synthesis and Electrochemical Energy Storage Applications.

Graphene (G)-based two dimensional (2D) mesoporous materials combine the advantages of G, ultrathin 2D morphology, and mesoporous structures, greatly contributing to the improvement of power and energy densities of energy storage devices. Despite considerable research progress made in the past decade, a complete overview of G-based 2D mesoporous materials has not yet been provided. In this review, we summarize the synthesis strategies for G-based 2D mesoporous materials and their applications in supercapacitors (SCs) and lithium-ion batteries (LIBs). The general aspect of synthesis procedures and underlying mechanisms are discussed in detail. The structural and compositional advantages of G-based 2D mesoporous materials as electrodes for SCs and LIBs are highlighted. We provide our perspective on the opportunities and challenges for development of G-based 2D mesoporous materials. Therefore, we believe that this review will offer fruitful guidance for fabricating G-based 2D mesoporous materials as well as the other types of 2D heterostructures for electrochemical energy storage applications.

Polymer blend directed anisotropic self-assembly toward mesoporous inorganic bowls and nanosheets.

Anisotropic mesoporous inorganic materials have attracted great interest due to their unique and intriguing properties, yet their controllable synthesis still remains a great challenge. Here, we develop a simple synthesis approach toward mesoporous inorganic bowls and two-dimensional (2D) nanosheets by combining block copolymer (BCP)-directed self-assembly with asymmetric phase migration in ternary-phase blends. The homogeneous blend solution spontaneously self-assembles to anisotropically stacked hybrids as the solvent evaporates. Two minor phases-BCP/inorganic precursor and homopolystyrene (hPS)-form closely stacked, Janus domains that are dispersed/confined in the major homopoly(methyl methacrylate) (hPMMA) matrix. hPS phases are partially covered by BCP-rich phases, where ordered mesostructures develop. With increasing the relative amount of hPS, the anisotropic shape evolves from bowls to 2D nanosheets. Benefiting from the unique bowl-like morphology, the resulting transition metal oxides show promise as high-performance anodes in potassium-ion batteries.

Interaction Mediator Assisted Synthesis of Mesoporous Molybdenum Carbide: Mo-Valence State Adjustment for Optimizing Hydrogen Evolution.

To overcome inherent limitations of molybdenum carbide (MoxC) for hydrogen evolution reaction (HER), i.e., low density of active site and nonideal hydrogen binding strength, we report the synthesis of valence-controlled mesoporous MoxC as a highly efficient HER electrocatalyst. The synthesis procedure uses an interaction mediator (IM), which significantly increases the density of active site by mediating interaction between PEO-b-PS template and Mo source. The valence state of Mo is tuned by systematic control of the environment around Mo by controlled heat treatment under air before thermal treatment at 1100 °C. Theoretical calculations reveal that the hydrogen binding is strongly influenced by Mo valence. Consequently, MoxC achieves a significant increase in HER activity (exceeding that of Pt/C at high current density ∼35 mA/cm2 in alkaline solution). In addition, a volcano-type correlation between HER activity and Mo valence is identified with various experimental indicators. The present strategies can be applied to various carbide and Mo-based catalysts, and the established Mo valence and HER relations can guide development of highly active HER electrocatalysts.

Approaching Ultrastable High-Rate Li-S Batteries through Hierarchically Porous Titanium Nitride Synthesized by Multiscale Phase Separation.

Porous architectures are important in determining the performance of lithium-sulfur batteries (LSBs). Among them, multiscale porous architecutures are highly desired to tackle the limitations of single-sized porous architectures, and to combine the advantages of different pore scales. Although a few carbonaceous materials with multiscale porosity are employed in LSBs, their nonpolar surface properties cause the severe dissolution of lithium polysulfides (LiPSs). In this context, multiscale porous structure design of noncarbonaceous materials is highly required, but has not been exploited in LSBs yet because of the absence of a facile method to control the multiscale porous inorganic materials. Here, a hierarchically porous titanium nitride (h-TiN) is reported as a multifunctional sulfur host, integrating the advantages of multiscale porous architectures with intrinsic surface properties of TiN to achieve high-rate and long-life LSBs. The macropores accommodate the high amount of sulfur, facilitate the electrolyte penetration and transportation of Li+ ions, while the mesopores effectively prevent the LiPS dissolution. TiN strongly adsorbs LiPS, mitigates the shuttle effect, and promotes the redox kinetics. Therefore, h-TiN/S shows a reversible capacity of 557 mA h g-1 even after 1000 cycles at 5 C rate with only 0.016% of capacity decay per cycle.

Generalized Access to Mesoporous Inorganic Particles and Hollow Spheres from Multicomponent Polymer Blends.

Mesoporous inorganic particles and hollow spheres are of increasing interest for a broad range of applications, but synthesis approaches are typically material specific, complex, or lack control over desired structures. Here it is reported how combining mesoscale block copolymer (BCP) directed inorganic materials self-assembly and macroscale spinodal decomposition can be employed in multicomponent BCP/hydrophilic inorganic precursor blends with homopolymers to prepare mesoporous inorganic particles with controlled meso- and macrostructures. The homogeneous multicomponent blend solution undergoes dual phase separation upon solvent evaporation. Microphase-separated (BCP/inorganic precursor)-domains are confined within the macrophase-separated majority homopolymer matrix, being self-organized toward particle shapes that minimize the total interfacial area/energy. The pore orientation and particle shape (solid spheres, oblate ellipsoids, hollow spheres) are tailored by changing the kind of homopolymer matrix and associated enthalpic interactions. Furthermore, the sizes of particle and hollow inner cavity are tailored by changing the relative amount of homopolymer matrix and the rates of solvent evaporation. Pyrolysis yields discrete mesoporous inorganic particles and hollow spheres. The present approach enables a high degree of control over pore structure, orientation, and size (15-44 nm), particle shape, particle size (0.6-3 µm), inner cavity size (120-700 nm), and chemical composition (e.g., aluminosilicates, carbon, and metal oxides).

Multiscale Phase Separations for Hierarchically Ordered Macro/Mesostructured Metal Oxides.

Porous architectures play an important role in various applications of inorganic materials. Several attempts to develop mesoporous materials with controlled macrostructures have been reported, but they usually require complicated multiple-step procedures, which limits their versatility and suitability for mass production. Here, a simple approach for controlling the macrostructures of mesoporous materials, without templates for the macropores, is reported. The controlled solvent evaporation induces both macrophase separation via spinodal decomposition and mesophase separation via block copolymer self-assembly, leading to the formation of hierarchically porous metal oxides with periodic macro/mesostructures. In addition, using this method, macrostructures of mesoporous metal oxides are controlled into spheres and mesoporous powders containing isolated macropores. Nanocomputed tomography, focused ion beam milling, and electron microscopy confirm well-defined macrostructures containing mesopores. Among the various porous structures, hierarchically macro/mesoporous metal oxide is employed as an anode material in lithium-ion batteries. The present approach could provide a broad and easily accessible platform for the manufacturing of mesoporous inorganic materials with different macrostructures.

Simple synthesis of multiple length-scale structured Nb2O5 with functional macrodomain-integrated mesoporous frameworks.

We report the simple synthesis of macro- and mesostructured Nb2O5 that have functional submicrometer-sized particles (macrodomain) embedded in mesoporous frameworks (nanodomain). Resol can macrophase-separate by self-polymerization and co-assemble with niobia sol into mesostructured frameworks. The resultant materials increase the power conversion efficiency due to light-scattering capability of submicrometer-sized particles.