Hierarchical Pore Distribution Research Articles

Optimizing the micropore-to-mesopore ratio of carbon-fiber-cloth creates record-high specific capacitance

Abstract The application of commercial carbon fiber cloth (CFC) in energy storage equipment is limited by its low specific capacitance and energy density. By a simple one-step activation treatment, the specific surface area of CFCs with porous structure can be increased considerably from 3.9 up to 875 m2/g and the electrochemical properties of CFCs can be improved by three orders of magnitude (1324 mF/cm2). Moreover, the hydrophobicity of CFCs can be transformed into superhydrophilicity. However, the electrochemical performance of CFCs does not show a positive correlation with specific surface area but have a strong relationship with the hierarchical pore distribution forged by the annealing treatment. Only moderate micropore and mesoporous ratio enables optimizing the electrochemical performance of CFCs.

Direct Microwave Conversion from Lignin to Micro/Meso/Macroporous Carbon for High‐Performance Symmetric Supercapacitors

AbstractEnzymatic hydrolysis lignin (EHL) is a common biomass that is difficult to use for the synthesis of porous carbon (PC) via direct microwave heating because it absorbs very little microwave energy. In this study, a novel method was developed to synthesize high‐performance PC directly from EHL using only 8 min of microwave heating. The resulting PC exhibits a high specific surface area of 2482 m2 g−1 and a hierarchical pore distribution with a high mesopore content of 35.6 % and oxygen content of 17.1 %. Nitrogen (as a protecting gas) passes through water molecules provided by steam to absorb microwave energy and acts as a physical porogen to form mesopores. Using solid KOH allows one to bypass the time‐consuming drying process and generate sufficient micropores. Additionally, fast self‐pyrolysis and a high heating rate lead to the generation of both mesopores and macropores in a PC framework. Furthermore, multiple active centers can be formed during microwave heating and subsequently stabilized by oxidation once the PC is exposed to an air atmosphere, resulting in the attachment of oxygen functional groups to the PC surface. Based on these desirable physicochemical characteristics, the PC electrodes fabricated in this study exhibit a high specific capacitance of 338 F g−1 at 1 A g−1 and high capacitance retention of 86 % at 10 A g−1. The final PC‐based supercapacitor delivers a maximum energy density of 17.1 Wh kg−1 in a Na2SO4 electrolyte and 8.6 Wh kg−1 in a KOH electrolyte.

Hierarchical porous structured N-doped activated carbon derived from Helianthus Annuus seed as a cathode material for hybrid supercapacitor device

Abstract Biomass derived activated carbon materials are used as an effective adsorbent for heavy metal ions and effectively used as a potential cathode material for supercapacitors and lithium-ion batteries due to its hierarchical pore distribution and high surface area. Activated/N-doped activated carbon is derived from Helianthus Annuus seed using chemical activation process. After introducing the heteroatom into the carbon layers, the porosity nature is enhanced, and it is observed from the SEM and TEM images. The synergistic effect of heteroatoms and the uniform distribution of pores in the N-doped activated carbon (NASC) endows to attain the superior specific capacitance (capacity) of 330 F g−1 (226 C g−1) at a current density of 2 A g−1.

Porous carbon for high-energy density symmetrical supercapacitor and lithium-ion hybrid electrochemical capacitors

The design of carbon material with high capacitance and stability is one of the key issues in the development of supercapacitor and lithium-ion hybrid electrochemical capacitor (Li-HEC). Herein, porous carbon material with high specific surface area, honeycomb-like structure and hierarchical pore distribution ranging from macropore to micropore, was prepared from silkworm excrement by utilizing its highly porous and three-dimensional open structure. Moreover, the porous carbon is self-doped by nitrogen, which is beneficial to increase its pseudocapacitance and wettability to the electrolyte. The symmetrical supercapacitor and Li-HEC made by as-prepared porous carbon exhibited high energy density (138.4 Wh kg−1 for supercapacitor and 242.2 Wh kg−1 for Li-HEC), excellent rate performance and stability. This study offers a facile and cost-effective strategy to prepare advanced carbon material which can be applied in energy storage devices.

Preparation of lignin-based porous carbon with hierarchical oxygen-enriched structure for high-performance supercapacitors

The biomass-based porous carbon produced by the conventional two-step method (carbonization followed by chemical activation) has a high production cost and an undeveloped mesopore/macropore structure. In this study, lignin was successfully converted into porous carbon (LPC) in one step by microwave heating combined with the use of humidified nitrogen. The obtained LPC had abundant micropores (0.70 cm−3·g−1), hierarchical pore distribution (mesopore ratio: 65.8%), and an oxygen-enriched chemical structure (surface oxygen content: 16.5%). These characteristics provided a high energy density (23.0 kW·kg−1 at 10 A·g−1) and excellent rate capability of the prepared supercapacitor in a gel electrolyte (polyvinyl alcohol/LiCl), leading to a high specific capacitance of 173 F·g−1 at 0.5 A·g−1, and 71.1% at 10 A·g−1 remains. The prepared supercapacitor could deliver a high power density of 1.1 kW·kg−1 at the maximum energy density. The obtained results demonstrate the feasibility of the proposed energy-saving cost-effective preparation approach to obtain a high-performance supercapacitor with a low production cost.

Facile in situ hydrothermal crystallization synthesis of SAPO-34/ZSM-5 composite catalyst for methanol to olefin reaction

The SAPO-34/ZSM-5 composite catalyst (SZ-TS) was successfully fabricated using a two-step in situ crystallization method. X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, N2 adsorption–desorption, and ammonia temperature-programmed desorption measurements were used to characterize the synthesized catalysts and evaluate their catalytic performances on methanol-to-olefin (MTO) reaction. The synthesis mechanism of two-step in situ crystallization was further investigated. Results showed that the SZ-TS synthesized by two-step in situ crystallization featured an intimately bonded composite structure owing to the synergistic effects of the interface. However, the physical mixture of SAPO-34 and ZSM-5 (SZ-PM) presented two discrete phase structures. By using the composite catalyst in MTO reaction, the SZ-TS exhibited the longest life of 1400 min, which was much higher than that of SZ-PM (720 min). Methanol conversion reached 97.3%, and the selectivity for light olefins totalled 93.0%. The excellent catalytic performance of SZ-TS was attributed to the hierarchical pore structure and suitably weak acid site distribution. Via pre-crystallization, alkalinity can be adjusted in the adjacent ranges of SAPO-34 and ZSM-5. By adding the pre-crystallized alkaline SAPO-34 slurry into the strong alkaline system of the parent ZSM-5 slurry, phase dissolution of SAPO-34 in the secondary crystallization solution was avoided. The external surface of SAPO-34 was etched in the alkaline environment of the secondary crystallization solution, resulting in hierarchical pore distribution, which benefited the MTO reaction. This in situ synthesis method features the advantages of simplicity, low time consumption, and low cost. Therefore, this approach possesses the potential to satisfy the demands of the MTO market.

Sandwich‐Like Reduced Graphene Oxide/Carbon Black/Amorphous Cobalt Borate Nanocomposites as Bifunctional Cathode Electrocatalyst in Rechargeable Zinc‐Air Batteries

AbstractThe inhibitively high cost of the noble‐metal‐containing materials has become a major obstacle for the large‐scale application of rechargeable zinc‐air batteries (ZABs). To solve this problem in a practical way, a green and scalable method to prepare sandwich‐like reduced graphene oxide /carbon black/amorphous cobalt borate nanocomposites (rGO/CB/Co‐Bi) is reported. These composites are shown to be a highly efficient and robust bifunctional electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this system, the spontaneous assembly of the GO sheet and CB nanoparticles is demonstrated by noncovalent interactions to build the sandwich‐like structure with hierarchical pore distribution. The impressive ORR and OER activities of the obtained nanocomposite are attributed to the high conductivity, large surface area, and the hierarchically porous channels. With room‐temperature synthesis and significant activities shown in the demonstrative battery test, the prepared nanocomposite can potentially serve as an alternative for noble‐metal‐based rechargeable ZAB cathode materials.

Freestanding NiFe Oxyfluoride Holey Film with Ultrahigh Volumetric Capacitance for Flexible Asymmetric Supercapacitors.

In this work, a freestanding NiFe oxyfluoride (NiFeOF) holey film is prepared by electrochemical deposition and anodic treatments. With the combination of good electrical conductivity and holey structure, the NiFeOF holey film offers superior electrochemical performance with maximum specific capacitance of 670 F cm-3 (134 mF cm-2 ), due to the following reasons: (i) The residual metal alloy framework can be used as the current collector to improve electrode conductivity. Moreover, the as-prepared freestanding NiFeOF holey film can be used as a supercapacitor electrode without reliance on binders and other additives. The residual metal alloy framework and binder-free electrode effectively reduce electrode resistance, thus improving electron transport. (ii) The highly interconnected holey structure and hierarchical pore distribution provide a high specific surface area to improve electron transport, enhancing rapid ion transport, and mitigating diffusion limitations throughout the holey film. (iii) The excellent mechanical characteristics facilitate flexibility and cyclability related performance. Additionally, the NiFeOF holey film presents exceptional electrochemical performance, showing that it is a promising alternative for small/microsize electronic devices.

Construction of hierarchical porous carbon nanosheets from template-assisted assembly of coal-based graphene quantum dots for high performance supercapacitor electrodes

Using coal-based graphene quantum dots (GQDs) as the building blocks, we develop a simple template-assisted assembly strategy to prepare hierarchical porous carbon nanosheets (HPCNs) for supercapacitor electrodes. The coal-based GQDs are prepared by a simple liquid phase oxidation of the bituminous coal. Benefiting from their small size, enriched edge structure, abundant functional groups, and good flexibility and chemical reactivity, these GQDs are suitable to construct complex nanoarchitectures for advanced energy storage materials. As a result, the HPCNs show an interconnected loose-stacking graphene-like structure with the specific surface area of 1332 m2 g−1, hierarchical pore distribution, excellent conductivity, abundant active sites, and sufficient ion migration channels. An in situ chemical activation is further applied to improve its energy storage performance. Under the optimized conditions, the activated HPCN with a specific surface area of 1450 m2 g−1 shows a greatly improved capacitive performance, with a high specific capacitance of 230 F g−1 (1 A g−1) and capacitance retention of 74% at 100 A g−1 (170 F g−1). No obvious capacity fade was found even after cycled at 10 A g−1 for 10,000 times, demonstrating their excellent endurability. Our work may provide new thought for the effective use of abundant coal resource to design and preparation advanced carbon nanoarchitectures for energy storage.

Template-free Synthesis of Large-Pore-Size Porous Magnesium Silicate Hierarchical Nanostructures for High-Efficiency Removal of Heavy Metal Ions

It remains a big challenge to develop high-efficiency and low-cost adsorption materials to remove toxic heavy metal ions in water. Here, we developed a template-free synthesis method to fabricate high surface area and large pore size magnesium silicate hierarchical nanostructures in a mixed solvent of ethanol and water and carefully investigated the corresponding adsorption behavior for Pb2+, Zn2+, and Cu2+ in aqueous solution. The results reveal that the ethanol volume fraction in the solvent plays an important role to optimize the pore structure, which directly determines the adsorption capacity and the adsorption rate for heavy metal ions. When the ethanol volume fraction is beyond 50%, the obtained magnesium silicate presents a flowerlike structure with a hierarchical pore distribution: 0.5–2, 2–30, and 30–200 nm. When the ethanol volume faction is 90%, for example, the sample exhibits a maximum adsorption capacity of 436.68, 78.86, and 52.30 mg/g for Pb2+, Zn2+, and Cu2+ ions, which has a BET surface...

Resolving Electrode Morphology's Impact on Platinum Group Metal-Free Cathode Performance Using Nano-CT of 3D Hierarchical Pore and Ionomer Distribution.

This article reports on the characterization of polymer electrolyte fuel cell (PEFC) cathodes featuring a platinum group metal-free (PGM-free) catalyst using nanoscale resolution X-ray computed tomography (nano-CT) and morphological analysis. PGM-free PEFC cathodes have gained significant interest in the past decade since they have the potential to dramatically reduce PEFC costs by eliminating the large platinum (Pt) raw material cost. However, several challenges remain before they are commercially viable. Since these catalysts have lower volumetric activity, the PGM-free cathodes are thicker and subject to increased gas and proton transport resistances that reduce the performance. To better understand the efficacy of the catalyst and improve electrode performance, a detailed understanding the correlation between electrode fabrication, morphology, and performance is crucial. In this work, the pore/solid structure and the ionomer distribution was resolved in three dimensions (3D) using nano-CT for three PGM-free electrodes of varying Nafion loading. The associated transport properties were evaluated from pore/particle-scale simulations within the nano-CT-imaged structure. These characterizations are then used to elucidate the microstructural origins of the dramatic changes in fuel cell performance with varying Nafion ionomer loading. We show that this is primarily a result of distinct changes in ionomer's spatial distribution. The significant impact of electrode morphology on performance highlights the importance of PGM-free electrode development in concert with efforts to improve catalyst activity and durability.

Chitosan modified halloysite nanotubes as emerging porous microspheres for drug carrier

Natural halloysite (Hal) nanotubes were well dispersed and thoroughly emulsified to prepare the porous microspheres Hal-chitosan (Hal-CTS) in a water/oil microemulsion, which was further loaded with aspirin (Asp) as a model drug to investigate the drug release behavior. The charge-mediated adsorption of molecules to the surface of Hal could facilitate the dispersion of Hal in aqueous media and enabled Hal to assemble into microspheres. The microspheres had interconnected pores, large pore volume, high specific surface area and hierarchical pore distribution, which was beneficial to the entrance of Asp molecules. The porous microspheres showed excellent loading capacity for Asp as high as 42.4wt%, around 20 times higher than the pristine Hal (2.1wt%). The release results indicated that Hal-CTS/Asp nanocomposites had the lower release in the simulated gastric fluid, whereas more rapid release rate and higher release amount in the simulated intestinal fluid, which could help to maintain effective concentration of Asp in the body and reduce the maximum extent of the side effects to the stomach. The as-prepared Hal-CTS/Asp nanocomposites could have potential application in the drug carrier systems due to their natural and excellent biocompatibility.

A sustainable freeze-drying route to porous polysaccharides with tailored hierarchical meso- and macroporosity.

Bio-derived polysaccharide aerogels are of interest for a broad range of applications. To date, these aerogels have been obtained through the time- and solvent-intensive procedure of hydrogel fomation, solvent exchange, and scCO2 drying, which offers little control over meso/macropore distribution. A simpler and more versatile route is developed, using freeze drying to produce highly mesoporous polysaccharide aerogels with various degrees of macroporosity. The hierarchical pore distribution is controlled by addition of different quantities of t-butanol (TBA) to hydrogels before drying. Through a systematic study an interesting relationship between the mesoporosity and t-butanol/water phase diagram is found, linking mesoporosity maxima with eutectic points for all polysaccharides studied (pectin, starch, and alginic acid). Moreover, direct gelation of polysaccharides in aqueous TBA offers additional time savings and the potential for solvent reuse. This finding is a doorway to more accessible polysaccharide aerogels for research and industrial scale production, due to the widespread accessibility of the freeze drying technology and the simplicity of the method.

A facile self-template strategy to fabricate three-dimensional nitrogen-doped hierarchical porous carbon/graphene for conductive agent-free supercapacitors with excellent electrochemical performance

Carbon materials with large surface area, high conductivity and suitable pore distribution are highly desirable for high-performance supercapacitors. The paper reported a self-template strategy for the fabrication of three-dimensional nitrogen-doped hierarchical porous carbon/graphene (3D-NHPC/G). The mixture of beer yeast cells, graphite oxide, urea, potassium hydroxide and phytic acid was dispersed in water by ultrasonication to form homogeneous slurry, which was then dried by spray drying and finally heated at 850°C in Ar/H2 environment. The study shows that such a new strategy creates a 3D carbon framework, hierarchical pore distribution, moderate graphitization and appropriate nitrogen doping. The as-prepared 3D-NHPC/G offers excellent dispersion, specific surface area of 3108.7 m2g−1 and electronic conductivity of 3096 S m−1, which is more than six-fold that of 3D-NHPC (494 S m−1). Owing to the greatly enchanced electron transfer and mass transport, the 3D-NHPC/G electrode displays the largely enhanced electrochemical performance. In 6mol l−1 potassium hydroxide aqueous electrolyte, its specific capacitance is 318 F g−1 at the current density of 1 A g−1, 277 F g−1 at the current density of 10 A g−1, and 155 F g−1 at the current density of 100 A g−1 that can remain at least 96.5% after 10000 cycles. In 1mol l−1 tetraethylammonium tetrafluoroborate organic electrolyte, the specific capacitance is 138 F g−1 at the current density of 1 A g−1 and 109 F g−1 at the current density of 10 A g−1. The study also provides an approach for design and synthesis of carbon-based materials with high supercapacitor behaviors.

Encapsulating Sulfur into Hierarchically Ordered Porous Carbon as a High‐Performance Cathode for Lithium–Sulfur Batteries

A three-dimensional (3D) hierarchical carbon-sulfur nanocomposite that is useful as a high-performance cathode for rechargeable lithium-sulfur batteries is reported. The 3D hierarchically ordered porous carbon (HOPC) with mesoporous walls and interconnected macropores was prepared by in situ self-assembly of colloidal polymer and silica spheres with sucrose as the carbon source. The obtained porous carbon possesses a large specific surface area and pore volume with narrow mesopore size distribution, and acts as a host and conducting framework to contain highly dispersed elemental sulfur. Electrochemical tests reveal that the HOPC/S nanocomposite with well-defined nanostructure delivers a high initial specific capacity up to 1193 mAh g(-1) and a stable capacity of 884 mAh g(-1) after 50 cycles at 0.1 C. In addition, the HOPC/S nanocomposite exhibits high reversible capacity at high rates. The excellent electrochemical performance is attributed exclusively to the beneficial integration of the mesopores for the electrochemical reaction and macropores for ion transport. The mesoporous walls of the HOPC act as solvent-restricted reactors for the redox reaction of sulfur and aid in suppressing the diffusion of polysulfide species into the electrolyte. The "open" ordered interconnected macropores and windows facilitate transportation of electrolyte and solvated lithium ions during the charge/discharge process. These results show that nanostructured carbon with hierarchical pore distribution could be a promising scaffold for encapsulating sulfur to approach high specific capacity and energy density with long cycling performance.

Highly efficient PbS/CdS co-sensitized solar cells based on photoanodes with hierarchical pore distribution

A new TiO2 photoanode with hierarchical pore distribution was first employed to fabricate PbS/CdS co-sensitized quantum dot-sensitized solar cell (QDSC). Under AM 1.5 illumination of 100mW·cm−2, up to 3.82% of light-to-electricity conversion efficiency has been achieved, higher than the reported results of PbS/CdS QDSCs so far. The effects of PbS SILAR cycles, CdS deposition time and film thickness on cell performance were investigated. Furthermore, comparison between this photoanode with large spherical voids and conventional photoanode with large size TiO2 scattering layer was carried out. This kind of photoanode can provide good scattering ability and large surface area, which is suitable for QDs efficient deposition and electrolyte penetration, exhibiting potential application in highly efficient QDSCs especially with high photocurrent.

Synthesis of mesoporous MOR materials by varying temperature crystallizations and combining ternary organic templates

An approach, which involved perturbations of synthesis conditions in crystallization temperature and organic template composition, has been proposed for derivation of mesopores from bulk structural defects formed in mordenite (MOR) phases, based on the character of intrinsic crystal defects and tensional distortions of two compressed 8-membered rings in MOR framework. Consequently, hierarchically porous materials with MOR structures were prepared by varying temperature synthesis and using ternary organic micelles templates. The synthesized materials were characterized by powder X-ray diffraction, nitrogen adsorption at 77 K and transmission electron microscopy. These crystalline materials combined the advantages of both mordenites with the micropore system and mesoporous materials by featuring an additional intracrystalline mesopore system in MOR single crystals, resulting in a hierarchical pore distribution in range of 1.7–80.0 nm together with the significant specific surface areas. In particular, individual crystal morphologies of typical mesoporous MOR materials showed the defect-rich structures responding essentially to the mesoporosity; the mesoporous materials were substantiated matchable well to normal mordenites in acid capacity and acid strength distribution. Furthermore, several factors acting on intracrystalline mesopore formation were elucidated for explicating the perturbation synthesis approach, and whole reaction routes for synthesizing different solid products under related crystallization conditions were summarized coupling with a demonstration of part TEM micrographs. The overall results suggested that the investigative approach of perturbation imposed on crystal growth provides an alternative route for direct hydrothermal synthesis of mesoporous zeolite materials and implied that bulk structural defects within single crystals lead to the formation of intracrystalline mesopores in various sizes.

Flower-like porous hematite nanoarchitectures achieved by complexation–mediated oxidation–hydrolysis reaction

Flower-like porous hematite (α-Fe 2O 3) nanoarchitectures composed of ultra-thin nanoflakes were prepared by annealing the iron oxide precursor formed via the oxidation-hydrolysis reaction between Fe(II) ions and Tris(hydroxymethyl)aminomethane (abbreviated as Tris). The microstructure of the prepared FeOOH and hematite samples were fully characterized by field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis, Fourier-transforming infrared spectra, thermogravimetric analysis, and nitrogen adsorption–desorption isotherm. Based on the influences of reactant concentrations, reaction time and reaction temperature on the morphologies of the resultant samples, a formation mechanism of etching was proposed, Fe(II)–Tris complexes were self-assembled via hydrogen bonds into brick-like building blocks, which then aggregated into rudimentary nanoparticles, and the synergistic effect between the crystallization of FeOOH and dissociation of Fe(II)–Tris complexes make the rudimentary nanoparticles evolve into the flower-like products. The as-prepared flower-like α-Fe 2O 3 nanostructures possessed a Brunauer–Emmett–Teller specific surface area of 191.63 m 2 g −1, hierarchical pore distribution ranging from micropores to macropores, and good crystallinity, and excellent visible photocatalysis in terms of removing chemical oxygen demand of dimethyl sulfoxide industrial wastewater. The current work provides a reliable approach for building functional hierarchical nanoarchitectures and the prepared iron oxide nanomaterials demonstrate an excellent ability to remove toxic pollutants in industrial wastewater.