Porous Structure Of Aerogel Research Articles

Development of MoS2/cellulose aerogels nanocomposite with superior application capability for selective lead(II) capture

The development of highly efficient and practical adsorbents for selective lead(II) capture remains an urgent challenge to be overcome. Herein, a new nanocomposite (W-MoS2@CA) was prepared by confining MoS2 nanosheets with widened interlayer spacing in the cross-linked cellulose aerogels. W-MoS2@CA combines the excellent handling, flow characteristics, and porous structure of aerogels with the specific affinity and accessible interior spaces of MoS2 toward lead cation. The adsorbent shows outstanding adsorption capacity and fast kinetics for lead removal. Benefiting from the high affinity between MoS2 and lead, W-MoS2@CA exhibits excellent adsorption selectivity even in the solution containing multiple coexisting heavy metal ions and high concentrations of competing mineral cations. The centimeter/millimeter-sized of the 3D cellulose mesh skeleton significantly improves the hydraulics property of nanocomposite, enabling it to be conveniently used in the flow system. The considerable treatment volume in fixed-bed indicated that W-MoS2@CA is of great potential for lead removal in practice. In addition, W-MoS2@CA can be readily regenerated with negligible adsorption capacity loss after 5 adsorption − desorption cycles. These results confirm that W-MoS2@CA has a great prospect in the water remediation of lead-removing, and this work may provide a new avenue for the application of MoS2 as an effective adsorbent in practical water/wastewater treatment.

Synergistic adsorption‑photocatalytic degradation of tetracycline by microcrystalline cellulose composite aerogel dopped with montmorillonite hosted methylene blue

A flexible and recyclable metal-freephotodynamic aerogel composite was synthesized by simple entrapping montmorillonite particles hosting methylene blue (MT-MB) into microcrystalline cellulose aerogel (MCC/MT-MB) without the use of chemical modifiers. The MT-MB particles act as adsorbent and photocatalyst for tetracycline for the first time. The mechanism of tetracycline adsorption and photodegradation by MCC/MT-MB aerogel was investigated by confocal imaging and radical scavenger experiment. Singlet oxygen was found to play a main role in the degradation confirming Type II photoexitation mechanism. The MCC/MT-MB aerogel exhibited 97% tetracycline removal efficiency within 120 min. This enhanced performance of the aerogels could be an integrated result of interconnected open porous aerogel structure, multi-adsorption mechanisms of the pollutant on the aerogel, and degradation and direct contact of methylene blue with tetracycline. Furthermore, MCC/MT-MB aerogel showed high stability and photocatalytic degradation recyclability upto 10 cycles of uses, and the recycling process was environmentally friendly. The degradation intermediate products were analyzed by HPLC-MS, and two photodegradation pathways were proposed. In addition, MCC/MT-MB aerogel showed tetracycline removal rate of 90% under red-light irradiation and > 90% under UVA or daylight irradiation from dairy manure samples collected from Central Valley California, USA, respectively.These results confirmed the advantageous functions and practicality of the photoactive porous aerogel in wastewater treatments.

Hydrophobic Modification of Biopolymer Aerogels by Cold Plasma Coating.

The aim of this work was to evaluate the potential of cold plasma polymerization as a simple, fast and versatile technique for deposition of protective hydrophobic and oleophobic polymer layers on hydrophilic biopolymer aerogels. Polymerization of different fluorinated monomers (octafluorocyclobutane C4F8 and perfluoro-acrylates PFAC-6 and PFAC-8) on aerogel monoliths derived from alginate, cellulose, whey protein isolate (WPI) and potato protein isolate (PPI) resulted in fast and significant surface hydrophobization after short process times of 5 min and led to superhydrophobic surfaces with static water contact angles up to 154° after application of poly-C4F8 coatings. Simultaneous introduction of hydro- and oleophobicity was possible by deposition of perfluoro-acrylates. While the porous structure of aerogels stayed intact during the process, polymerization inside the aerogels pores led to the generation of new porous moieties and resulted therefore in significant increase in the specific surface area. The magnitude of the effect depended on the individual process settings and on the overall porosity of the substrates. A maximization of specific surface area increase (+179 m2/g) was obtained by applying a pulsed wave mode in the C4F8-coating of alginate aerogels.

Cost-Effective Additive Manufacturing of Ambient Pressure-Dried Silica Aerogel

Abstract The conventional manufacturing processes for aerogel insulation materials mostly rely on the supercritical drying, which suffers from issues of massive energy consumption, high-cost equipment, and prolonged processing time. Considering the large market demand for the aerogel insulation material in the next decade, a cost-effective and scalable fabrication technique is highly desired. In this paper, a direct ink writing (DIW) method is used to three-dimensionally fabricate the silica aerogel insulation material, followed by room-temperature and ambient pressure drying. Compared with the supercritical drying and freeze-drying techniques, the reported method significantly reduces the fabrication time and costs. The cost-effective DIW technique offers the capability to print complex hollow internal structures, coupled with the porous aerogel structure, it is found to be beneficial for the thermal insulation property. The addition of fiber to the ink assures the durability of the fabricated product, without sacrificing the thermal insulation performance. The foam ink preparation methods and the printability are demonstrated in this paper, along with the printing of complex three-dimensional geometries. The thermal insulation performance of the printed objects is characterized, and the mechanical properties were also examined. The proposed approach is found to have a 56% reduction in the processing time. The printed silica aerogels exhibit a low thermal conductivity of 0.053 W m−1 K−1.

Conductive and superhydrophobic F-rGO@CNTs/chitosan aerogel for piezoresistive pressure sensor

Piezoresistive pressure sensors with high sensitivity, fast response, and simplified signal collection play an important role in a wide variety of fields. However, most of them are water-sensitive and easily attacked by water, leading to serious signal distortion in practical application. Herein, we fabricated a conductive and superhydrophobic 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FAS) modified reduced graphene oxide@carbon nanotubes/chitosan (F-rGO@CNTs/CS) aerogel for piezoresistive pressure sensor. Benefiting from the porous structure of aerogel and the synergy of CNTs and rGO, the aerogel sensor achieved high sensitivity and fast response. Moreover, the sensor maintained a stable electrical resistance response after 1000 loading-unloading cycles. Importantly, owing to the rough structure constructed by CNTs and multi-pores and the low surface energy of FAS, the sensor possessed superhydrophobic property with a high water contact angle of 154°, and exhibited remarkable water repellency even during compression process. In addition, the sensor was successfully applied for detecting human behaviors from small-scale muscle movements to large-scale body motions. Our findings provide a new direction to fabricate functional and high-performance piezoresistive pressure sensor for various applications even under water or wet environment.

Modelling of Mechanical Behavior of Biopolymer Alginate Aerogels Using the Bonded-Particle Model.

A novel mesoscale modelling approach for the investigation of mechanical properties of alginate aerogels is proposed. This method is based on the discrete element method and bonded-particle model. The nanostructure of aerogel is not directly considered, instead the highly porous structure of aerogels is represented on the mesoscale as a set of solid particles connected by solid bonds. To describe the rheological material behavior, a new elastic-plastic functional model for the solids bonds has been developed. This model has been derived based on the self-similarity principle for the material behavior on the macro and mesoscales. To analyze the effectiveness of the proposed method, the behavior of alginate aerogels with different crosslinking degrees (calcium content) was analyzed. The comparison between experimental and numerical results has shown that the proposed approach can be effectively used to predict the mechanical behavior of aerogels on the macroscale.

3D Sulfur and Nitrogen Codoped Carbon Nanofiber Aerogels with Optimized Electronic Structure and Enlarged Interlayer Spacing Boost Potassium-Ion Storage.

Carbonaceous materials are promising anodes for potassium-ion batteries (PIBs). However, it is hard for large K ions (1.38 Å) to achieve long-distance diffusion in pristine carbonaceous materials. In this work, the following are synthesized: S/N codoped carbon nanofiber aerogels (S/N-CNFAs) with optimized electronic structure by S/N codoping, enhanced interlayer spacing by S doping, and a 3D interconnected porous structure of aerogel, through a pyrolysis sustainable seaweed (Fe-alginate) aerogel strategy. Specifically, the S/N-CNFAs electrode delivers high reversible capacities of 356 and 112 mA h g-1 at 100 and 5000 mA g-1 , respectively. The capacity reaches 168 mA h g-1 at 2000 mA g-1 after 1000 cycles. A full cell with a S/N-CNFAs anode and potassium prussian blue cathode displays a specific capacity of 198 mA h g-1 at 200 mA g-1 . Density functional theory calculations indicate that S/N codoping is beneficial to synergistically improve K ions storage of S/N-CNFAs by enhancing the adsorption of K ions and reducing the diffusion barrier of K ions. This work offers a facile heteroatom doping paradigm for designing new carbonaceous anodes for high-performance PIBs.

Anisotropic MXene Aerogels with a Mechanically Tunable Ratio of Electromagnetic Wave Reflection to Absorption

AbstractLightweight and mechanically flexible materials that can provide efficient electromagnetic interference (EMI) shielding are highly desirable for protecting portable and smart electronic devices against electromagnetic pollution. Here, the authors report a tunable design of a three‐dimensional (3D) porous aerogel structure made of 2D transition metal carbides and a carbonitride (MXene) with a long‐range order of aligned lamellar architecture for EMI shielding. Bidirectional freeze‐casting of MXene colloidal solutions is used to fabricate robust, compressible and lightweight aerogels, and achieve orientational assembly leading to outstanding EMI shielding performance and tunable ratio of reflection to absorption. EMI shielding effectiveness (SE) of three types of MXene aerogels (Ti3C2Tx, Ti2CTx, and Ti3CNTx) reaches 70.5, 69.2, and 54.1 dB, respectively, while keeping the compression thickness at 1 mm and a density of only ≈11.0 mg cm−3. The highest specific SE reaches 8818.2 dB cm3 g−1, which is among the best values reported for EMI shielding materials. More importantly, during the compression process of the MXene aerogels, the ratio of electromagnetic wave reflection to absorption increases without noticeable change of the total EMI SE. Compressible MXene aerogels with aligned layers offer an effective approach to control the electromagnetic wave absorption and reflection in EMI shielding materials.

Composite Aerogels of Carbon Nanocellulose Fibers and Mixed-Valent Manganese Oxides as Renewable Supercapacitor Electrodes.

Bio-waste derived nanocelluloses show excellent mechanical flexibility and self-aggregated capability, which enable them to be good supporting substrates for the synthesis of electroactive materials. Herein, we present a facile route for fabricating composite aerogels consisting of carbonized nanocellulose fibers (CNF) and mixed-valent manganese oxide (MnOx), toward supercapacitor applications. Mixed solutions of nanocellulose and manganese acetate with different ratios were prepared and freeze-dried into hybrid aerogels. The hybrid aerogels were then transformed into CNF/MnOx composites by a calcination process. The CNF membranes served as porous carbon nano-reservoirs for MnOx and electrolyte. The CNF/MnOx composites also kept a 3D porous aerogel structure with hierarchical pores, which enabled stable transport of both electrolyte ions and electrons to the electrode surface, leading to low a charge-transfer impedance and good electrochemical kinetics. The CNF/MnOx-based symmetric supercapacitor showed a satisfied energy density and power density of 37.5 Wh kg−1 and 2.75 kW kg−1, respectively. All the above results demonstrate the feasibility of using sustainable nanocellulose as a nanoscale carbon substrate for the synthesis of hybrid composite electrodes toward renewable supercapacitor applications.

Enhancing the Selectivity of H2O2 Electrogeneration by Steric Hindrance Effect.

Selective hydrogen peroxide (H2O2) electrogeneration by oxygen reduction reaction (ORR) is an efficient and promising synthetic method for H2O2 production. Herein, we build a particular inorganic-organic interface to enhance the electrocatalytic selectivity of reduced graphene oxide (rGO) aerogels for H2O2 electrogeneration by modifying rGO aerogels with polyethyleneimine (PEI). The three-dimensional porous structure of aerogels and the steric hindrance effect of PEI on rGO endow PEI-functionalized rGO (rGO/PEI) aerogels with enhanced selectivity (90.7%), production rate (106.4 mmol gcat-1 h-1), and durability for H2O2 electrogeneration by the two-electron pathway of ORR.

Sub-1.5 nm Ultrathin CoP Nanosheet Aerogel: Efficient Electrocatalyst for Hydrogen Evolution Reaction at All pH Values.

Transition metal phosphides (TMPs) are certified high performance electrocatalysts for the hydrogen evolution reaction (HER). The ultrathin 2D structure of TMPs can offer abundant adsorption sites to boost HER performance. Herein, an ice-templating strategy is developed to prepare CoP aerogels composed of 2D ultrathin CoP nanosheets (<1.5 nm) using sustainable alginate biomass (seaweed extract) as the precursor. The highly porous aerogel structure can not only deliver facile mass transfer, but also prevent aggregation of the nanosheets into layered structures. As expected, the obtained CoP nanosheet aerogels exhibit remarkable stability and excellent electrocatalytic HER performance at all pH values. For instance, the sample CoP-400 presents a low overpotential of 113, 154, and 161 mV versus RHE at a current density of 10 mA cm-2 in 0.5 m H2SO4, 1 m KOH, and 1 m phosphate buffer solution, respectively. In addition, CoP-400 displays low Tafel slopes at all pH values due to the interconnected highly porous structure of the aerogel, indicating that the sample can provide low-resistance channels for mass transport. Density functional theory calculations reveal that P-top and Co bridge on (011) facet of CoP are more favorable sites during the process of HER in acid and alkaline solutions, respectively.

Photocatalytic Properties of Tetraphenylporphyrins Immobilized on Calcium Alginate Aerogels

We have prepared photocatalytic systems based on tetraphenylporphyrins (TPP) immobilized on calcium alginate solid gels in the conditions of thermal drying on air (xerogel), freeze drying in vacuum (cryogel) and supercritical drying in the supercritical carbon dioxide (scCO2) medium (aerogel). As a test reaction to measure the prepared systems’ efficiency, we studied tryptophan photooxidation in the aqueous medium. We have demonstrated that the systems with aerogel as a carrier exhibited the highest photocatalytic efficiency. In that case, the rate constant for the test substrate (tryptophan) oxidation exceeds the corresponding rate constants of similar systems based on xerogel and cryogel by more than 5 times. Moreover, the aerogel-based photocatalytic systems demonstrated enhanced functional stability and a possibility of multiple use of such a catalyst in tryptophan oxidation. Based on the data of small-angle X-ray scattering and thermooxidative destruction, we have made a conclusion about the relationship between the high photocatalytic activity of aerogel-immobilized TPP and formation of a developed porous aerogel structure in the conditions of drying in the scCO2 medium, which is stabilized due to formation of additional complex bonds of calcium ions with fragments of glycoside rings.

Investigation of Porous Structure of Aerogel Prepared from Nanofibrillated Cellulose

Cellulose aerogels were prepared through changing the concentration of TEMPO-oxidized cellulose suspension and applying high-pressure homogenization, followed by regular or rapid freeze drying methods. The prepared cellulose aerogel was studied by scanning electron microscopy, BET analysis (nitrogen gas adsorption), and mercury intrusion porosimetry to investigate the effect of variables on the porous properties. In the present study, the porous structure of cellulose aerogel was well obtained by varying concentration of NFC and applying rapid freeze drying approach. The specific surface area of prepared cellulose aerogels varied linearly with adjusting concentrations of NFC, and the highest surface area of cellulose aerogel was attained at 2.0% concentration of NFC in this study. Compared to regular freezing in the freezer, rapid freezing in the liquid nitrogen prepared cellulose aerogel has a lower BET value but more uniform structure with relatively narrow pore size distribution.

Three-dimensional porous aerogel constructed by Bi2WO6 nanosheets and graphene with excellent visible-light photocatalytic performance

A three-dimensional porous hybrid aerogel assembly of Bi2WO6 nanosheets and graphene has been prepared by a facile solvothermal route. The products are characterized by X-ray diffraction, Scanning electron microscope, Transmission electron microscopy, and so on. The results show the highly interconnected and porous network microstructure stacked by graphene and Bi2WO6 nanosheets in the hybrid aerogel. The specific surface area of hybrid aerogel (39.4m2g−1) is 1.6 times higher than that of Bi2WO6 nanosheets. The Rhodamine B removal ratio over the composite aerogel reaches up to 99.6% within 45min, which is higher than that of pure Bi2WO6 nansosheets (80%). The excellent photocatalytic performance is mainly owing to the porous structure of aerogel and high electrical conductivity of graphene. The study on the photocatalytic mechanism demonstrates that O2−• is the main reactive species for Rhodamine B degradation.

Two-Dimensional Layered Materials with Expanded Interlayer Distance for Rechargeable Magnesium Batteries

Mg rechargeable batteries (MgRBs) stand out as a promising candidate beyond lithium ion battery technologies due to high volumetric energy density, resource abundance, and the dendrite-free deposition behavior of Mg, which ensures safe operation. Many of the advantages of MgRBs originate from the divalent nature and small ionic size of Mg ions; however, these properties also render the cation too polarizing to diffuse easily in most ion- intercalation materials. To overcome this challenge, we recently adopted an interlayer expansion approach via insertion of external pillars to transform two representative two-dimensional layered materials (MoS2 and V2O5) into efficient Mg storage host materials. To boost the Mg storage performance of MoS2, we intercalated appropriate amount of poly(ethylene oxide (PEO) to expand MoS2 structure through an exfoliation and restacking process. [1,2] The Mg solid state diffusivity and capacity increased accordingly with increasing amount of PEO. PEO2L-MoS2 (two layers of PEO in between MoS2 layers) showed the best performance with Mg2+ diffusivity increase by two orders of magnitude and capacity increase of about 300% compared with bulk MoS2. The flexible PEO serves as a structure pillar, which can increase the interlayer spacing but not block the diffusion of Mg2+ ions. In the case of V2O5, we turned to insert small water molecules in the layered structure to take advantage of the shielding effect of H2O.[3] We prepared graphene decorated V2O5 aerogel nanowires, and demonstrated it to be an effective cathode material, with specific capacity of 330 mAh g-1 at low rate, stable cyclability and good rate performance. The performance enhancement compared with bulk V2O5 is due to the increased space and charge shielding effect of crystal water in the aerogel to enhance the Mg2+ insertion and the porous structure of aerogel allowing easy access of electrolyte into the active material. Moreover, various interlayer spacing and crystal water amount were also studied and we found that the as-prepared sample with the highest amount of water and interlayer spacing showed the best performance. The merit of the interlayer expansion lies at the enlarged space for enhanced Mg2+ ion mobility and weakened interaction between Mg2+ ions and the electrode frameworks. This approach could be further extended to a wide range of host materials for the storage of Mg2+ ion and other multivalent ions. 1. Yanliang Liang, Hyun Deog Yoo, Yifei Li, Jing Shuai, Hector A. Calderon, Francisco Carlos Robles Hernandez, Lars C. Grabow, and Yan Yao, Nano Lett. 2015, 15, 2194-2202. 2. Yifei Li, Yanliang Liang, Francisco C. Robles Hernandez, Hyun Deog Yoo, Qinyou An, Yan Yao, Nano Energy 2015, 15, 453-461. 3. Qinyou An, Yifei Li, Hyun Deog Yoo, Shuo Chen, Qiang Ru, Liqiang Mai, and Yan Yao, Nano Energy 2015, 18, 265-272.

Graphene decorated vanadium oxide nanowire aerogel for long-cycle-life magnesium battery cathodes

We report graphene decorated hydrated vanadium oxide nanocomposite as an effective cathode material for long cycle-life Mg storage. Excellent electrochemical performance with specific capacity of 330mAhg−1 at low rate and stable cycling of 200 cycles with 81% capacity retention at 1Ag−1 was reported. Furthermore, the nanocomposite cathode shows a broad working temperature window from −30°C to 55°C with over 200mAhg−1 capacity at 55°C (1.0Ag−1). The charge shielding effect of crystal water in the aerogel enhances the Mg2+ insertion kinetics and the porous structure of aerogel allows easy access of electrolyte into the active material. The cycling performance, rate performance and broad temperature adaptability demonstrate that the graphene decorated vanadium oxide nanowire aerogel is a promising and attractive cathode material for practical Mg batteries.

Effects of Surfactants on the Synthesis of Silica Aerogels Prepared by Ambient Pressure Drying

In this research, aerogels were synthesized by a two-step sol-gel process without supercritical conditions. During the process, tetraethoxysilane (TEOS) was used as precursor, and different surfactants, i.e. cetyltrimethyl ammonium bromide (CTAB) and polyethylene glycol–600 (PEG–600), were used as dopants respectively. In order to minimize the drying shrinkage and preserve the high porosity structure, the surface of the gels was modified by trimethylchlorosilane (TMCS) before the ambient pressure drying (APD). The effect of surfactent on the structure of the resulting aerogels was investigated. The aerogels which involved surfactants exhibited resistance to cracking during the APD of the alcogels. The modification by TMCS has been confirmed using Infrared (IR) spectroscopy. The porous structure of aerogels was investigated by Brunauer-Emmett- Teller (BET) instrument, differential scanning calorimeter (DSC) and scanning electron micrograph (SEM). The results indicated that surfactants as dopants could significantly affect the structure and optical property of aerogels and be beneficial for obtaining crack-free silica aerogels via ambient pressure drying process.

Effect of dehydration on porous structure of monolithic aerogels based on fibrous aluminum oxyhydroxide

Aluminum oxide monolithic aerogels (PMAOs) with specific surfaces of up to 400 m2/g and densities of up to 0.004 g/cm3 were obtained by the direct oxidation of metallic aluminum by water vapors. Studies of the obtained aerogels using the methods of scanning and transmission electron microscopy showed that a freshly prepared aerogel represents the amorphous structure from alumina fibrous with a diameter of about 5 nm. Parameters of a porous aerogel structure were determined from the isotherms of adsorption of nitrogen vapors at 77 K. It is shown that the specific surface area decreases by 50% upon the dehydration of aerogel by temperature pretreatment at 900°C until the phase transition temperature is reached. At the same time, heat treatment at temperatures above 1200°C results in a phase transition at which significant contraction of structure occurs and the specific surface area decreases to 20 m2/g. The number of primary adsorption centers was obtained from comparative plots of water vapor adsorption. It was found that preliminary dehydration only slightly affects the number of primary centers of water adsorption.

Highly water repellent aerogels based on cellulose stearoyl esters

Herein we combine in a novel way the physical effect of porous structure of a cellulosic aerogel with the chemical effect of long alkyl tails by a well known homogeneous green esterification method, to achieve purely bio-based and highly water repellent cellulosic materials. As an alternative for a traditional fluoro derivatized hydrophobization, here long fatty acid tails are utilized to lower the surface energy. To minimize the process emission, ionic liquid (IL) 1-allyl-3-methylimidazolium chloride is used for the esterification, due to its non-volatility and recyclability. We have shown here that low degree of substitution (DS) of the fatty acid cellulose material enables the spontaneous formation of aerogels. Additionally, the very low content of the long stearoyl tails combined with the porous aerogel structure resulted in significant increase in hydrophobicity from an aqueous contact angle of 0° up to 124°. We foresee that this approach can allow sustainable and completely bio-based coatings and insulators paving the way for a new green application potential for cellulose based materials.

Organic and carbon aerogels derived from poly(vinyl chloride)

Organic aerogels were derived from dimethylformamide solution of poly(vinyl chloride) (PVC) via dehydrochlorination using a strong base, 1,8-diazabicyclo[5,4,0]undec-7-ene, and supercritical drying using carbon dioxide. From these organic aerogels, carbon aerogels were yielded via stabilization and carbonization. Changes in the porous structure of the aerogels during the preparation process and influences of the preparation conditions on the porous structure were investigated. The framework of the aerogels composed the walls of the meso- and macropores. The volume and the size of these pores were reduced during stabilization and carbonization due to the shrinkage of the framework caused by the release of decomposition gases and densification of the material. Simultaneously, the release of decomposition gases produced additional micropores. The extent of dehydrochlorination, the concentration of PVC in the starting solution and the molecular weight of PVC were the factors with which the porous structure of the aerogels could be controlled over a wide range. In addition, the stabilization conditions notably influenced the carbonization behavior of the organic aerogels and the porous structure of the carbon aerogels. The optimum stabilization conditions that minimized the loss of mass and maximized the pore volume of the carbon aerogels were determined.