Fragility and brittleness have always been flaws of delicate three-dimensional aerogel structures. Especially when exposed to harsh conditions (e.g. in aerospace applications), their frailty has limited the broad application of aerogels in such fields. A Chinese team of researchers from the Beijing Jiaotong University has now attended to this matter. By equipping a ZrO₂–SiO₂ aerogel with Polycrystalline ZrO₂ fibers (ZrO_2f), aerogels possessing high thermal and mechanical resilience have been devised. Astonishingly, these monolithic aerogels do not only excel in terms of stability, but also show low bulk densities and thermal conductivities.

The idea of adding a fibrous agent to the aerogel matrix is that, when dispersed evenly throughout the matrix, the fibers act as an additional mechanical backbone. This means that the fibers hinder fracturing and irreversible deformation through fiber-bridging and crack-deflection (see Figure below) when the monolithic structure is being strained. Furthermore, due to the even dispersion of the fibers in the aerogel matrix, the contribution of heat conduction through the fibers is minimized and hence the overall heat conductivity increases only marginally upon addition of the fibrous ZrO₂.

a) Image of ZrO2f/ZrO2-SiO2 aerogel monolith; b)–d) SEM images of fractures of the aerogel composite.

In summary, these effects lead to highly insulating aerogels possessing compressive strengths 3-10 times higher than previously reported. Therefore, applications in very demanding environments are facilitated, which might prove to be significant in aerospace engineering and other fields in which stable and highly insulating materials are essential.

More details: Xianbo Hou, Rubing Zhang and Daining Fang; An ultralight silica-modified ZrO₂–SiO₂ aerogel composite with ultra-low thermal conductivity and enhanced mechanical strength, Scripta Materialia Volume 143, 15 January 2018, Pages 113-116. http://doi.org/10.1016/j.scriptamat.2017.09.028

The enhancing effects of polyaniline (PANi) coatings on ion and electron conductivity are well known (see also here), which is why researchers from the Sun Yat-sen University (China) devised PANi-coated activated carbon/sulfur aerogels (ACA-500-S) in order to improve the electrochemical properties of the un-coated composite. By synthesizing this novel electrode material, the research team hopes to address drawbacks of Li-S batteries (e.g. low ionic conductivity, dissolution of polysulfides in electrolytes).

Carbon/sulfur composite aerogels were produced by heat-melting state-of-the-art activated carbon aerogels (ACA-500) in the presence of elemental sulfur. As shown in the Figure below, the PANi coating was subsequently applied by in-situ chemical oxidative polymerization at low temperatures, yielding the final ACA-500-S@PANi electrode material. Investigation of the sample morphology via FESEM and TEM revealed that the manufacturing technique led to a homogeneous distribution of sulfur and PANi on the aerogel surface. Because of the even dispersion of the active agents on the aerogel matrix, enhanced electrochemical performance was expected from the ACA-500-S@PANi composite.

Schematic of synthesizing process of ACA-500-S@PANi aerogels from state-of-the-art activated carbon aerogels (ACA-500)

This expectation was confirmed in ensuing experiments, which compared the specific capacity, capacity discharge, and capacity retention of the ACA-500-S and ACA-500-S@PANi materials. It was shown that the latter material outperformed the carbon/sulfur aerogel in all regards, which was attributed to the synergistic effect of the carbon aerogel matrix and the PANi coating on ion and electron conductivity as well as electrode stability. More specifically, the researchers discovered that the ACA-500-S@PANi structure presented high reversible capacities at low (at 0.2 C: 1208 mAh g⁻¹) and high (at 3.0 C: 542 mAh g⁻¹) discharge rates. Furthermore, long term cycling at 1.0 C demonstrated that it exhibits a notable initial discharge capacity of 926 mAh g^-1 and outstanding capacity retention of over 65 % at a low capacity decay rate of 0.48 ‰ per cycle.

Because of these findings, the researchers are confident that the novel aerogel composite will be able to boost the performance of lithium-sulfur batteries, meeting the increasing demand for batteries exhibiting a high energy density and long life span at low cost.

More details: Tang et al.; Polyaniline-Coated Activated Carbon Aerogel/Sulfur Composite for High- performance Lithium-Sulfur Battery, Nanoscale Research Letters 2017. https://doi.org/10.1186/s11671-017-2372-6

Although we might not recognize it, electronic sensors have developed into essential components in our everyday life. They provide our smartphones with necessary information, guarantee safe travels in our cars and ensure that our homes are adequately air conditioned. With increasing automatization in the industrial and private sector (e.g. internet of things, smart home, autonomous driving, etc.), the importance of those tiny helpers for our prosperity, well-being and safety will only increase. Therefore, it will be essential to develop more efficient, powerful and complex sensors, which are able to process information in a reliable fashion.

Researchers from the Linköping University (Sweden), were now able to manufacture a type of thermoelectric aerogel, which is able to simultaneously sense pressure and temperature. Moreover, the pressure and temperature signals are decoupled, allowing for a reliable synchronous measuring of both parameters.

The dual-parameter PNG aerogels sensors were synthesized from a mixture of poly3,4-ethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS), nano-fibrillated cellulose (NFC) and glycidoxypropyl trimethoxysilane (GOPS). This precursor selection led to an elastic, yet robust thermoelectric PNG aerogel, being able to sense pressure changes (NFC & GOPS) and temperature variations (PEDOT:PSS). Post-treatment in dimethylsulfoxide (DMSO) vapor not only increased the pressure sensitivity of the aerogel, but also resulted in a decoupling of the temperature and pressure signal by changing the internal charge carrier transport through the material.

Testing of the aerogel composite in an experimental setup, schematically depicted below, showed that pressure and temperature gradient could be obtained from two independent parameters of the current-voltage curve. The pressure signal is indicated by the slope, whereas the y-intercept denotes the temperature gradient across the aerogel (see also Figure below).

a) Schematic of experimental setup to test the dual-parameter sensor made of PNG aerogel. b) I–V curves obtained for a constant pressure (230 Pa) and different temperatures after 10 min DMSO treatment.

These revolutionary findings could pave the way for further research on multi parameter sensing aerogels, facilitating innovations which rely on powerful sensors. The Swedish research team already anticipates the utilization of their novel dual scale PNG aerogel in e-skin applications.

More details: Han et al.; Thermoelectric Polymer Aerogels for Pressure–Temperature Sensing Applications, Advanced Functional Materials, Volume 27, Issue 44 November 24, 2017. https://doi.org/10.1002/adfm.201703549,

Finding non-cytotoxic carrier materials that ensure a controlled release of an active hydrophobic drug over a long period of time is considered to be the “holy grail” in wound dressing applications. A team of researchers from Brazil and the USA report to have found a material that combines all of those desired characteristics.
The novel silica-polymer hybrid (SPH) aerogels, synthesized from silica nanoparticles, polyvinyl alcohol (PVA), polyacrylic acid (PAA), and water, were manufactured via freeze-drying and subsequent thermal treatment (see Figure below). While the freeze-drying ensured a complete removal of water, the ensuing thermal treatment at 160 °C facilitated the cross-linking between PVA and PAA, guaranteeing aerogel stability in aqueous media.

Schematic of manufacturing process of silica-polymer hybrid (SPH) aerogels.

Experiments conducted with dexamethasone (DEX), an agent used for the treatment of skin diseases, allergies, and rheumatic problems, showed that the SPH aerogels exhibit high drug encapsulation efficiencies, taking up around 75 % of DEX from an ethanol/water mixture within 24 hours. This feature was ascribed to the trapping of DEX molecules within the mesoporous silica nanoparticles.
The release rate of DEX from the aerogels was investigated by placing the loaded samples into a stirred phosphate buffered saline (PBS) solution at 37 °C and measuring the progression of the DEX concentration with time. These measurements showed that after an initial rapid discharge of DEX, the release rate leveled off, so that even after two months a steady drug discharge was obtained. This slow and prolonged release of DEX molecules was attributed to the polymer pore structure, limiting the mass transfer from the drug encapsulation site (silica nanoparticles) to the solution. Supporting this finding were results for SPH aerogels of a different constitution, which showed that a change in PVA:PAA ratio, yielding morphological modifications of the polymer pore network, leads to a significant change in drug release behavior.
In order to assess the biocompatibility and cytotoxicity of the synthesized material, the cell viability of vero cells and L929 fibroblasts was investigated in the presence of the SPH aerogels. This line of experiment showed that there is virtually no decrease in cell viability for either cell type after 72 hours, regardless of precursor selection.
The authors see potential applications of SPH aerogels in the treatment of skin burns or melanoma, which require wound dressing over extended periods of time. Furthermore, they see potential in tailoring the aerogel structures (e.g. with antibodies) to further extend their potential biological and medical applications.

More details: Follmann et al.; Multifunctional Hybrid Aerogels: Hyperbranched Polymer- Trapped Mesoporous Silica Nanoparticles for Sustained and Prolonged Drug Release, Nanoscale, December 2017. http://doi.org/10.1039/C7NR08464A

Coal fired power plants will play a substantial role in energy production for the majority of industrialized nations until at least the middle of this century. This is an issue since the large-scale combustion of carbon-based resources is a major contributor to the rising atmospheric CO₂ levels. In order to achieve the ambitious multilateral goals to lessen the detrimental effects of global warming by stabilizing the global CO₂ levels outlined in the Paris Agreement, the greenhouse gas emissions from coal fired power plants will need to be reduced drastically.

One way to achieve this reduction in emitted greenhouse gases without abandoning coal-based power production is the capturing and sequestration of emitted CO₂ (see graphic below). This can be done by retro-fitting existing power plants with carbon capture and storage (CCS) units, which remove carbon dioxide from flue gas, before storing it in a designated location. Most commonly, temperature-swing adsorption (TSA) processes, utilizing aqueous amine solutions as sorbents, are proposed for CO₂ capturing. However, the regeneration of the amine solution results in an energy penalty which drastically reduces the thermal efficiency of the power plant. Therefore, alternative sorbents requiring less energy for regeneration are under investigation.

Diagram of carbon capture and storage life cycle.
From: Scottish carbon capture and storage

In a joint effort, a project team consisting of Aspen Aerogels, the University of Akron, ADA-ES, and Longtail Consulting have synthesized and tested solid amine functionalized aerogels (AFA) in a bench scale fluidized bed reactor, in order to assess their potential for future application in CCS.

It was found that the AFAs synthesized by Aspen Aerogels showed promising CO₂ adsorption behavior and good stability over numerous adsorption-desorption cycles. On top of that, the novel solid sorbents possess significantly lower heats of reaction with CO₂ than commonly deployed liquid and solid sorbents. In a subsequent step, the AFAs were coated at the University of Akron, yielding pellets possessing a good cyclic stability in the presence of SO₂. The novel AFA pellets were then tested in a bench scale fluidized bed reactor to determine their physical properties (e.g. fluidizing gas velocities, void fraction, etc.) in such a reactor setup.

Based on those findings, Longtail Consulting modeled the hydrodynamic and heat transfer properties of the solid aerogel sorbent in the fluidized bed reactor and finalized the process requirements. Subsequently, a techno-economic analysis (TEA) of the entire CO₂ capturing unit (assuming 90 % capturing efficiency) fitted to a coal fired, supercritical steam cycle power plant producing 550 MW_e was performed. Despite offering a promising behavior from a technological perspective, the TEA revealed that utilizing novel AFA sorbents results in approx. 20 % higher levelized electricity costs. This finding was mainly attributed to the fact that the attrition of the material during fluidization was unknown, yielding high variable costs for the selected scenario. Additionally, a relatively high apparent particle density inside the fluidized be reactor led to a  massive pressure drop, causing a surge in auxiliary electricity input.

In light of these findings, the project team concluded that further testing under practical conditions and simultaneous optimization of the AFAs will be essential to make solid sorbents an economical alternative to state-of-the-art aqueous amines.

As the need for creative solutions to reduce greenhouse gas emissions is only increasing, it is promising to see that the vast potential of aerogels is being explored to address climate change. Whether or not solid sorbents can be successfully utilized in TSA in an economically reasonable way may prove to be one of the key factors determining the success of CCS.

Full Report: https://doi.org/10.2172/1349123

Read more at: https://www.osti.gov/scitech/biblio/1349123

Because of their unique characteristics, graphene aerogels are attractive materials for a wide range of applications. However, due to their micro-fibrous structure, strength and resilience, which are both desired properties, are generally considered to be mutually exclusive. By employing a bidirectional freeze-drying technique, researchers from the Zhejiang University (China) have successfully manufactured monolithic graphene aerogels uniting both properties.

Comparison of lamellar structure of thalia dealbata stem (left) and graphene aerogel (right)

In order to achieve those exceptional characteristics, the micro-scale architecture of the aerogels was based on the structure of a thalia dealbata stem, which is able to provide sufficient strength to support the plants leaves and blossoms while enduring powerful external forces (e.g. strong winds). The mimicking of this special three-dimensional lamellar structure consisting of bridged layers (see Figure above), yielded aerogel structures exhibiting strength and resilience simultaneously (see Figure below).

Images of fresh cubic graphene aerogel before compression (left), graphene monolith compressed by >6000 times its own weight (middle), recovered aerogel after compression (right).

When compared to a graphene aerogel exhibiting a random structure, the biomimetic aerogel showed a significant superiority in recovery behavior after being strained. Furthermore, the authors found that the aerogel architecture and hence the mechanical properties of the graphene structures can be further optimized by tuning the precursor composition.

The fabrication of such firm and robust structures could be play a pivotal role in establishing graphene aerogels in sensing applications. Additionally, the manufacturing technique reported by the authors can potentially be extended to other materials in order to obtain tailored micro-architectures.

More details: Miao Yang et al.; Biomimetic Architectured Graphene Aerogel with Exceptional Strength and Resilience, ACS Nano, 2017, 11 (7), pp 6817–6824. https://doi.org/10.1021/acsnano.7b01815

Read more at: https://www.forbes.com/sites/samlemonick/2017/07/31/plant-inspires-super-strong-aerogel/#2b9b35b96415

Increasing atmospheric CO₂ levels and dwindling fossil fuel resources have motivated the search for sustainable and renewable sources of energy. Hydrogen is considered to be an environmentally friendly alternative to common fossil fuels, as it does not emit environmentally harmful greenhouse gases upon combustion. Therefore, the quest for an efficient and sustainable way of generating hydrogen on a large scale is in full swing.

Supercritical water gasification (SCWG) of biomass is one such renewable hydrogen production technique that is currently being investigated. Due to the unique properties of supercritical water (e.g. high diffusivity, miscibility with gases) the process promises high energy conversion efficiencies. However, a suitable catalyst for the reaction, ensuring high H₂ yields while suppressing coke and tar formation, has remained elusive. Researchers from the University of Western Ontario (Canada) have recently synthesized a Ru-Ni-Al₂O₃ catalyst which has shown promise for the SCWG process.

Synthesis of the catalyst required the formation of a clear solution which was achieved by mixing the aluminum support in isopropanol at 75 °C followed by the addition of nitric acid. Thereafter, the metallic precursors (nickel nitrate and ruthenium acetylacetonate) were added to the sol to initiate the aging process. After completion of the gel formation, the liquids contained in the porous structure were extracted via low temperature supercritical drying with CO₂. In a last step the samples were calcined and reduced at 600 °C to obtain the ready-to-use aerogel catalysts (see Figure below).

Schematic of Ru-Ni-Al₂O₃ aerogel catalyst production technique

The Ru-Ni-Al₂O₃ aerogel, synthesized via this process, exhibited a higher specific surface area and pore volume when compared to impregnated or xerogel Ru-Ni-Al₂O₃ catalysts. These superior structural features significantly enhanced the hydrogen yields during SCWG, due to the increase in available active surface area. Furthermore, the porous aerogel morphology was also shown to decrease unwanted coke formation on the catalyst surface. A comparison of Ni-Al₂O₃ and Ru-Ni-Al₂O₃ aerogel catalysts revealed that the promoting nature of ruthenium (Ru) leads to superior catalytic activities for the bimetallic composite. Additionally, the utilization of Ru further decreased coke formation during the gasification reaction.

In order to assess the cyclic stability of the aerogel structures Ni-Al₂O₃ and Ru-Ni-Al₂O₃ catalysts were employed in three consecutive SCWG reactions. Both structures showed signs of deactivation (e.g. decrease in surface area), however, even during the third experimental run a decent catalytic activity was observed for both structures. For example, the recycled Ru-Ni-Al₂O₃ aerogel exhibited only slightly smaller hydrogen yields than the fresh Ru-Ni-Al₂O₃ xerogel and the fresh impregnated Ni-Al₂O₃ catalysts.

Although numerous projects aiming at the development of renewable energy generation processes are in progress, we are still a long way from a fully formed strategy to replace fossil fuels globally. Therefore, technical advancements paving the way to a more sustainable future are essential. This study has shown that aerogels could play an integral role in propelling alternative processes such as supercritical water gasification to market maturity.

More details: Md. Zakir Hossain, Muhammad B.I. Chowdhury, Anil Kumar Jhawar, Paul A. Charpentier; Supercritical water gasification of glucose using bimetallic aerogel Ru-Ni-Al₂O₃ catalyst for H₂ production, Biomass and Bioenergy Volume 107, December 2017, Pages 39-51. https://doi.org/10.1016/j.biombioe.2017.09.010

As demand for oil increases, so does its extraction and, consequently, the frequency of production-related accidents. This necessitated advancements in oil separation and absorption techniques to make sure that environmental disasters can be prevented.

An efficient way to remove oil and solvents from contaminated waters are absorbents, which directly remove the oil in the deployed surroundings. Additionally, they facilitate the possibility of recycling the absorbed oil after recollection. However, common absorbing materials utilized for the cleaning of oil spills are known to have a low environmental compatibility.

Died water droplets on top of hydrophobic cotton cellulose aerogel

In light of these facts, researchers from the National University of Singapore have synthesized novel environmentally benign cotton-cellulose aerogels which exhibit promising oil absorption characteristics. The monolithic pure cotton (PC) and cellulose-cotton (CC) aerogels were manufactured using a freeze-drying technique. In order to ensure hydrophobicity of the materials (see figure on the right), the aerogels were silanized using methyltrimethoxysilane. Thereafter, the absorption capacity of the aerogels was investigated for different solvents (e.g. dichloromethane, motor oil, ethanol).

The synthesized PC and CC aerogels were able to absorb all utilized solvents to large extents with loading capacities of up to 100 g/g being measured. Additionally it was discovered that absorption capacities increase with solvent density. In order to analyze the recyclability of the aerogels, two different recycling techniques were investigated. These experiments revealed that distillation cycling guarantees a superior sustaining of the absorbing performance when compared to squeeze cycling.

The intriguing findings of the authors once again highlight that aerogels can be applied in areas reaching beyond the field of insulation.

More details: Hanlin Cheng et. al; Cotton aerogels and cotton-cellulose aerogels from environmental waste for oil spillage cleanup, Materials & Design Volume 130, 15 September 2017, Pages 452-458. https://doi.org/10.1016/j.matdes.2017.05.082

Although monolithic aerogels of numerous types and forms have been produced using a broad palette of techniques, it remains a challenge to accurately tailor the micro- and macrostructure of the resulting three-dimensional structures. Recently, researchers from Kansas State University (USA) have presented a 3D-printing freeze drying technique promising just that.

Different aerogel geometries produced using the 3DFAP production technique

The so-called 3D freeze assembling printing (3DFAP) technique facilitates the fabrication of monolithic aerogels of various macrostructures (see figure on the right). Using this process, the team of researchers was able to produce silver nanowire aerogels (SNWA), that have ultra-low density (1.3 mg/cm³) and high electrical conductivity (0.24 S/cm), while also exhibiting outstanding mechanical features such as good compressive resistance and tunable Poisson ratios (even negative). Experiments investigating the effect of mechanical stress on material resistance revealed outstanding cyclic stability. Furthermore, the different structures were found to exhibit extremely high mechanical resilience, even under tensile stress.

In light of these promising results, the authors conclude that through facilitating the manipulation of the aerogel macrostructure, the novel production technique offers the possibility to manufacture three-dimensional aerogel structures for applications in the fields of sensing, energy storage or catalysis. They are also convinced that the 3DFAP technique can be applied to produce other 3D nanomaterial architectures.

More details: Pengli Yan et. al; 3D Printing Hierarchical Silver Nanowire Aerogel with Highly Compressive Resilience and Tensile Elongation through Tunable Poisson’s Ratio, Small Volume 13, Issue 38 October 11, 2017. http://onlinelibrary.wiley.com/doi/10.1002/smll.201701756/abstract

Read more at: http://www.advancedsciencenews.com/3d-printing-tunable-poisson-ratio-metallic-aerogels/

Owing to their unique characteristics, graphene aerogels are considered promising materials for a wide range of applications in fields such as energy storage, catalysis, and sensing. A research team from the Tsinghua University (China) has successfully demonstrated that another item can be added to this impressive list — adsorption and pre-concentration of air pollutants. Hierarchical porous graphene aerogels (HPGAs) synthesized via self-assembly, freeze drying and subsequent calcination have been shown to possess outstanding characteristics for extracting chemical warfare agents (CWAs) from ambient air.

Morphological structure images of hierarchical porous graphene aerogel (HPGA) at different magnifications.

The researchers found that the graphene aerogels, composed of a porous three-dimensional pore network (see Figure above), exhibited a good thermal and mechanical stability. Adsorption experiments with sarin, a highly toxic nerve agent, showed that the HPGAs display outstanding adsorption/desorption behavior in a wide range of operation conditions (e.g. desorption temperature, relative humidity). Furthermore, repeated cycling of the graphene aerogels did not result in a drop in adsorption efficiency or a change in material morphology, underlining the high resilience of HPGAs.

Given those intriguing results, the authors hypothesize that graphene aerogels could be efficient materials for the removal of other hazardous gases from air and hence might prove to be a promising alternative in cases of industrial accidents or terrorist attacks.

More details: Qiang Han, Liu Yang, Qionglin Liang and Mingyu Ding; Three-dimensional hierarchical porous graphene aerogel for efficient adsorption and preconcentration of chemical warfare agents, Carbon Volume122, October 2017, pages 556-563. https://doi.org/10.1016/j.carbon.2017.05.031