One-Pot Synthesis of Hierarchically Macro- and Mesoporous Carbon Materials with Graded Porosity

Non-Equilibrium Block Copolymer Self-Assembly Based Porous Membrane Formation Processes Employing Multicomponent Systems

Porous polymer-derived membranes are useful for applications ranging from filtration and separation technologies to energy storage and conversion. Combining block copolymer (BCP) self-assembly with the industrially scalable, non-equilibrium phase inversion technique (SNIPS) yields membranes comprising periodically ordered top surface structures supported by asymmetric, hierarchical substructures that together overcome performance tradeoffs typically faced by materials derived from equilibrium approaches. This review first reports on recent advances in understanding the top surface structural evolution of a model SNIPS-derived system during standard membrane formation. Subsequently, the application of SNIPS to multicomponent systems is described, enabling pore size modulation, chemical modification, and transformation to non-polymeric materials classes without compromising the structural features that define SNIPS membranes. Perspectives on future directions of both single-component and multicomponent membrane materials are provided. This points to a rich and fertile ground for the study of fundamental as well as applied problems using non-equilibrium-derived asymmetric porous materials with tunable chemistry, composition, and structure.

Soft Template-Assisted Fabrication of Mesoporous Graphenes for High-Performance Energy Storage Systems

Graphene is a promising active material for electric double layer supercapacitors (EDLCs) due to its high electric conductivity and lightweight nature. However, for practical uses as a power source of electronic devices, a porous structure is advantageous to maximize specific energy density. Here, we propose a facile fabrication approach of mesoporous graphene (m-G), in which self-assembled mesoporous structures of poly(styrene)-block-poly(2-vinylpyridine) copolymer (PS-b-P2VP) are exploited as both mesostructured catalytic template and a carbon source. Notably, the mesostructured catalytic template is sufficient to act as a rigid support without structural collapse, while PS-b-P2VP converts to graphene, generating m-G with a pore diameter of ca. 3.5 nm and high specific surface area of 186 m²/g. When the EDLCs were prepared using the obtained m-G and ionic liquids, excellent electrochemical behaviors were achieved even at high operation voltages (0 ∼ 3.5 V), including a large specific capacitance (130.2 F/g at 0.2 A/g), high-energy density of 55.4 W h/kg at power density of 350 W/kg, and excellent cycle stability (>10,000 cycles). This study demonstrates that m-G is a promising material for high-performance energy storage devices.

Lens-Shaped Carbon Particles with Perpendicularly-Oriented Channels for High-Performance Proton Exchange Membrane Fuel Cells

Two-dimensional sheet-like mesoporous carbon particles are promising for maximizing the number of active sites and the mass transport efficiency of proton exchange membrane fuel cells (PEMFCs). Herein, we develop a series of lens-shaped mesoporous carbon (LMC) particles with perpendicularly oriented channels (diameter = 60 nm) and aspect ratios (ARs) varying from 2.1 to 6.2 and apply them for the fabrication of highly efficient PEMFCs. The membrane emulsification affords uniform-sized, lens-shaped block copolymer particles, which are successfully converted into the LMC particles with well-ordered vertical channels through hyper-cross-linking and carbonization steps. Then, an ultralow amount (1 wt %) of platinum (Pt) is loaded into the particles. The LMC particles with higher ARs are packed with a higher density in the cathode and are better aligned on the cathode surface compared to the LMC particles with lower ARs. Thus, the well-ordered channels in the particles facilitate the mass transport of the reactants and products, significantly increasing the PEMFC performance. For example, the LMC particles with the AR of 6.2 show the highest initial single cell performance of 1135 mW cm^-2, and the cell exhibits high durability with 1039 mW cm^-2 even after 30 000 cycles. This cell performance surpasses that of commercial Pt/C catalysts, even at 1/20 of the Pt loading.

Materials Combining Asymmetric Pore Structures with Well-Defined Mesoporosity for Energy Storage and Conversion

Porous materials design often faces a trade-off between the requirements of high internal surface area and high reagent flux. Inorganic materials with asymmetric/hierarchical pore structures or well-defined mesopores have been tested to overcome this trade-off, but success has remained limited when the strategies are employed individually. Here, the attributes of both strategies are combined and a scalable path to porous titanium nitride (TiN) and carbon membranes that are conducting (TiN, carbon) or superconducting (TiN) is demonstrated. These materials exhibit a combination of asymmetric, hierarchical pore structures and well-defined mesoporosity throughout the material. Fast transport through such TiN materials as an electrochemical double-layer capacitor provides a substantial improvement in capacity retention at high scan rates, resulting in state-of-the-art power density (28.2 kW kg^-1) at competitive energy density (7.3 W-h kg^-1). In the case of carbon membranes, a record-setting power density (287.9 kW kg^-1) at 14.5 W-h kg^-1 is reported. Results suggest distinct advantages of such pore architectures for energy storage and conversion applications and provide an advanced avenue for addressing the trade-off between high-surface-area and high-flux requirements.

Polymer-Based Synthetic Routes to Carbon-Based Metal-Free Catalysts

Carbons are increasingly important as possible alternatives to expensive metal catalysts owing to the wide range of chemical properties they can exhibit and the growing set of synthetic routes available to produce them. This progress report discusses the process of making catalytic carbons from polymeric precursors, focusing on mechanisms of carbonization and how the polymer structures and synthetic procedures affect the resulting carbons. In considering what is necessary to move laboratory catalytic carbons to industrial and commercial applications, the cost and complexity to produce them are a considerable challenge to overcome. Industrially produced carbons are typically made from biopolymers such as lignin while many of the catalytic carbons studied in literature are from synthetic polymers. Thus, studying polymer-derived carbons can provide insights into the carbonization process and the properties of catalytic carbons, which can subsequently be translated to improve biopolymer-derived carbons in an economical way. Aspects of polymer carbonization discussed include carbonization mechanisms, effects of crosslinkers, polymer microstructure, heteroatom control, and effects of nanostructuring.

Facile one-pot hydrothermal synthesis of particle-based nitrogen-doped carbon spheres and their supercapacitor performance

Particle-based nitrogen-doped carbon spheres (PNCSs) were prepared via a hydrothermal and carbonization route and PNCSs-1.2 demonstrated an enhanced supercapacitor performance.

Comprehensive studies on phosphoric acid treatment of porous titania toward titanium phosphate and pyrophosphate monoliths with pore hierarchy and a nanostructured pore surface

A reaction of porous titania monoliths in aq. H₃PO₄ provides hierarchically porous titanium phosphates with a variety of nanotextured surfaces.

Templating for hierarchical structure control in carbon materials

Carbon-based materials show a remarkable variety of physical properties. For this reason, they have recently been explored for many advanced applications and emerging technologies. In the absence of actual "chemical" functionalities in these materials, tailoring these physical properties requires control on all levels of the structural hierarchy, from the atomic structure (carbon connectivity, defects, impurities), to the supramolecular level (domain orientations), nanoscopic length scale (domain sizes, porosity), microscopic structure (morphology), and macroscopic aspects (shape, surface chemistry). When preparing carbon materials, all these features can be tailored through the use of hard, soft, or molecular templates. Based on such templating approaches or through their combination, tremendous progress towards hierarchically structured carbon materials has recently been accomplished. Novel carbon nanomaterials such as brick-walled carbon tubes, carbon nanotube forests, coral-like carbon monoliths, or functional carbon nanosheets have become available, some of which exhibit unusual combinations of electronic, mechanical, and chemical properties. This review aims to discuss how the different templating approaches allow the control of structure formation on various length scales, how hierarchical structure formation can be realized, and which challenges remain, such as the detailed control over the carbon connectivity or the surface chemistry.