The synthesis of soft and machinable, yet orderly porous materials has long been considered unfeasible. This is the case since in nature ordered porosities can be found in long-range extended crystalline networks, which are generally rigid and brittle, while softness and processability commonly originate from network defects and disorder in amorphous materials.
Researchers from Japan and Spain now report to have found a way to unite these properties once considered irreconcilable, paving the way for ultralight and flexible materials, which could find application in energy storage devices, building insulation and aerospace technology.
The very basis for the synthesis of this intriguing material are its small scale building blocks — so called metal-organic polyhedra (MOP) consisting of metal ions and ligands (see figure on the right). The
soluble, stable, permanently porous MOPs used by the researches consist of rhodium ions, oxygen and the so-called H2bdc-C12 ligand.
To achieve the coordination of the MOPs the researchers deployed a cross- linker molecule called 1,4-bis(imidazol-1-ylmethyl)benzene (for short: bix). The addition of this cross-linker resulted in the formation of coordination polymer particles (CPP) via a mechanism consisting of nucleation, elongation and cross-linking (see figure below). Remarkably, it was shown that an exact control of the resulting CPP size was possible through the adjustment of the deployed reaction conditions (e.g. amount of cross-linker added, rate of cross-linker addition). Moreover, the researchers found that the addition of excess bix followed by heating induces the formation of supramolecular colloidal gels (SCG). This means that, the intrinsic porosity of the MOPs was successfully integrated into two different large-scale amorphous geometries.
Through supercritical drying of the resulting SCGs with supercritical CO2, supramolecular aerogels (SAG) were obtained. These aerogels were found to exhibit a hierarchical macro-porous structure built up from fused polymer particles, while the intrinsic porosity of the MOP monomers was retained throughout the entire processing sequence. Consequently, aerogels possessing both micro and macro pores were synthesized (see figure below), leading to extraordinary adsorption properties of the final material (superior to blank MOPs and CPPs).
By fabricating two different macroscopic amorphous materials from monomeric MOPs (CPP and SCG/SAG), the authors found a way to better understand the process of synthesizing macroscopic shapes from molecular building blocks. On the basis of their remarkable findings, they assume that through further research on this topic, this “relationship between molecular scale geometries and resulting macroscopic shapes“ can be further investigated and eventually understood entirely, leading to groundbreaking advances “towards the development of soft matter that is both permanently porous and amenable to materials processing”.
More details: Carne-Sanchez et al. “Self-assembly of metal–organic polyhedra into supramolecular polymers with intrinsic microporosity ” Nature Communications Volume 9, https://www.nature.com/articles/s41467-018-04834-0
Read more at: https://phys.org/news/2018-07-stable-aerogels.html
With both global oil consumption and environmental awareness steadily increasing, both economical as well as environmental aspects require an efficient, reliable, and cheap method to remove spilled oil from water. Selective oil absorption is an auspicious technique for this purpose since it is low cost, generally achieves high absorption capacities and offers the opportunity to recycle the working material. Yet, commercial absorption materials still suffer from several shortcomings (e.g. poor oil water selectivity, complicated fabricating process), which is why further advances in material engineering are required in order to obtain applicable oil absorbents.
With the aim of finding an efficient, environmental compatible and economical sorbent material, researchers from the Dalian Polytechnic University (China) have now successfully synthesized a low-cost, organic aerogel based on corn straw and filter paper, which exhibits good performance as an oil sorbent from aqueous media.
The outstanding feature of the synthesized aerogel material is that it originates from corn straw, which is generally considered as a waste product and thus nowadays is still being burned, positively contributing to global greenhouse gas emissions. However, its abundance, low-cost and biodegradability make it an potent raw material for large scale applications. Its main disadvantage — the inherent brittleness of materials originating from it — was overcome by the addition of filter paper pieces to the precursor material leading to the required flexibility of the final aerogel material.
For the preparation of the aerogels, the corn straw was first ground then washed with sodium hydroxide before hydrochloric acid was added. Subsequent filtration and drying led to corn straw particles (P-CS), which were then dispersed in water together with small pieces of commercial filter paper via vigorous stirring. Thereafter, the dispersion was frozen at -25 °C for 12 hours before freeze drying at -55 °C for 36 hours. The final step of the production process, which is shown in the figure below, was the hydrophobization of the corn straw aerogel (A-CS) through chemical vapor deposition of methyltrimethoxysilane (MTMS).
Analysis of the final freeze-dried aerogel structures unveiled that the material exhibited a porous 3D-structure and a good thermal stability up to 250 °C. Dependent on the solids content and the P-CS:filter paper ratio densities ranging from 14 to 58 mg/cm3 were attained, while porosities between 96 and 99 % were achieved. Water contact angle measurements showed a successful hydrophobization, with measured contact angles reaching values up to 152°.
Investigation of the selectivity and absorptivity for a range of different solvents showed that while water absorptivities were below 1 g/g for the hydrophobized corn straw aerogels (MTA-CS), absoprtivities of organic solvents such as oil or DMF were in the range of 40 g/g. Hence selectivities towards organic solvents were extremely high. Moreover, the MTA-CS did not only absorb the organic phase with a high selectivity, but also in a rapid fashion, leading to fast oil removal from aqueous solutions (see figure below).
With the novel low cost absorbent material synthesized by the Chinese research team disadvantages of conventional oil absorption were overcome, which might pave the way for the widespread utilization of biodegradable oil sorbents originating from corn straw. Certainly, it will be interesting to see whether novel bio-based oil absorbing materials can outperform their synthetic counterparts (e.g. poly(melamine- formaldehyde), polyurethane and polystyren) in the future.
More details: Yuan Li et al. “Preparation of corn straw based spongy aerogel for spillage oil capture” Korean Journal of Chemical Engineering May 2018, Volume 35, Issue 5, pp 1119–1127, https://link.springer.com/article/10.1007/s11814-018-0010-3
Read more at: http://www.chemengonline.com/inexpensive-renewable-aerogel-shows-promise-handling-oil-spills/?printmode=1
The search for alternative energy sources and novel means of (decentralized) power generation have become one of the central modern research topics. Due to their high efficiency and robustness fuel cells are considered to be an integral part in the envisaged future energy supply. Yet, conventional designs still require platinum-based catalysts to promote the oxygen reduction reaction (ORR). This has become a major obstacle in fuel cell technology, prohibiting their cost-efficient and widespread application. In light of this problem, the search for alternative catalyst materials has become a key aspect in fuel cell research.
Recent findings revealed that activated carbon (AC) is one such auspicious material for the catalysis of the ORR, which could replace conventional platinum (Pt) catalysts. Yet, AC exhibits significant shortfalls regarding its electrical conductivity as well as the number of active catalytic sites. Moreover, its application in fuel cells requires extremely large mass loadings to achieve decent performance.
To overcome these limitations, researchers from the Northwestern Polytechnical University, Xi’an (China) have suggested the production of AC-graphene hybrid aerogels as ORR catalyst materials.
The composite aerogel materials were produced via the process schematically shown in the figure below, which includes the addition of AC to a graphene oxide dispersion, followed by hydrogel formation via hydrothermal processing and freeze drying.
Characterization of the synthesized samples showed that through the addition of graphene to the aerogel matrix, the aerogel hybrids exhibited lower densities than pure AC (0.050-0.096 g/cm3), larger specific surface areas (500-750 m2/g) and a meso-porous structure consisting of more micro and meso pores than common activated carbon (see figure below).
Owing to these superior morphological characteristics, the aerogel samples outperformed conventional AC in terms of the ORR catalytic performance (e.g. larger onset potential, limiting current density and exchange current density), which was shown in the course of multiple electrochemical measurements. These superior properties were validated by initial experiments in a electrolytic testing device, in which the AC-graphene aerogel electrode outperformed its plain AC counterpart at 20 times smaller mass loadings.
The authors conclude that the enhanced ORR performance at lower mass loadings can be attributed to the larger surface area and more micro-porous structure of AC-graphene aerogels when compared to pure AC. Since the suggested synthesis route can be scaled-up easily and does not include any expensive techniques or precursors, they see great potential in their new composite material, as it offers a cheap and scalable alternative for applications requiring light weight ORR catalysts (e.g. fuel cells or metal air batteries). Moreover, further enhancements in the hybrid’s electrochemical properties might be attained through doping the aerogel matrix with other atoms (e.g. N, S, P).
With fuel cells being one auspicious alternative to conventional power sources, technical progress in this field is required to reach a more sustainable future. Amongst others, this study shows that due to the extraordinary properties aerogel based materials offer they can help us to reach present or future political and societal milestones.
More details: Yang Yang and Honglong Chang “Multi-scale porous graphene/activated carbon aerogel enables lightweight carbonaceous catalysts for oxygen reduction reaction” Mater. Res. Vol. 33 No. 9 May 14 2018, https://doi.org/10.1557/jmr.2017.372
Both economical as well as environmental considerations demand for high-performance building insulation materials to reduce global energy requirements for space heating and cooling. At the same time, devastating events like the Grenfell Tower fire, which broke out in 2017 in West London causing 72 deaths, highlight that besides low thermal conductivities, insulation materials must be fire retardant and robust even under extreme conditions.
Targeting this, researchers from the University of Science and Technology of China Hefei have now successfully manufactured a composite aerogel material which excels in both these categories. The novel phenol-formaldehyde-resin (PFR)/SiO2 aerogel which is composed of a three-dimensional, interpenetrating binary network structure, exhibits lower heat conductivities than conventional insulation materials and possesses outstanding fire retardant properties as well as great structural stability when being subjected to high temperature flames.
Synthesis of the gel matrix consisting of an inorganic SiO2 network and a polymeric PFR network (see Figure on the right) was achieved via the so called chitosan templated method. First, the precursors TEOS, acetic acid, and phenol were solubilized in an ethanol water mixture, which was then added to an aqueous chitosan solution, before adding formaldehyde. Thereafter, the resulting mixture was hydrothermally treated at 160 °C for 10 hours resulting in the hydrogel samples. Lastly, the hydrogels were solvent exchanged with acetone before being supercritically dried with CO2, resulting in the final composite aerogel.
To assess the impact of SiO2 contents on the final aerogel properties, samples of different SiO2/PFR ratios were produced. Characterization of the different aerogels showed that densities increase with increasing SiO2 content, but remain below 75 g/cm3 even for 80 % SiO2 (PSi-80). Additionally, both the strength and the elastic modules were found to increase with SiO2 content, while all aerogel samples could be compressed by more then 60 % without significant structural collapse occurring.
In terms of the samples’ thermal stability aerogels of high inorganic contents exhibited superior properties during cone calorimetry evaluation (lower thermal degradation and lower heat release rate), highlighting the positive impact of SiO2.
For the PSi-70 sample, exhibiting a great mechanical and thermal stability, a minimum thermal conductivity of 24 W/Km was obtained at low temperature and low relative humidity (T=-12 °C, RH<10 %). Remarkably, decent thermal conductivities (<45 W/Km) were also obtained at increased temperatures and high relative humidities (T=22 °C, RH>75 %), greatly outperforming commercial insulation materials such as EPS or mineral wool.
To test the aerogels’ flame resistance, slabs of the composite aerogel were subjected to a propane/butane flame (see Figure below, a), which generates a flame temperature of approx. 1300 °C. Astonishingly, the PSi aerogel retained its monolithic structure even after 30 minutes of flame exposure (see Figure below, g and h). Moreover, while only the white SiO2 network was left on the directly exposed front side, the sample maintained its structural features on the backside to great extents. This was attributed to the fact that the low thermal conductivity of the aerogel prevented a stark increase in temperature on the backside (see Figure below, c-e), despite the large temperatures (>1300 °C) on the sample frontside. As the backside temperature was below 310 °C even after 30 minutes of flame exposure (see Figure below, f), the authors concluded that the employment of the composite aerogel insulation guarantees the prevention of the collapse of reinforced concrete structures, which is reported to occur above 350 °C. Similar flame resistance testing of commercial PF foam and a PFR/attapulgite composite aerogel resulted in larger backside temperatures (>400 °C) and sample disintegration, underlining the outstanding characteristics of the investigated PSi aerogels.
In summary, the authors conclude that their novel aerogel is a promising candidate for next-generation insulation materials, as it unites great mechanical stability and fire retardant properties with outstanding characteristics in terms of thermal insulation. While certain challenges regarding economic scalability and rentability are still to be overcome, phenolic-silica aerogels surely seem very auspicious from a technical perspective alone.
More details: Zhi-Long Yu et al. “Fire-Retardant and Thermally Insulating Phenolic-Silica Aerogels” Angew. Chem. Int. Ed. 2018, 57, 4538 –4542, https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201711717
Read more at: https://www.advancedsciencenews.com/fire-retardant-binary-network-aerogel/
Extraction and impregnation techniques employing supercritical CO2 as the working fluid can be considered as the non plus ultra in their fields, due to the extraordinary properties s.c. CO2 offers (e.g. high solubility, good miscibility, bio-compatibility, non-flammability, etc.). Especially in the field of aerogel synthesis supercritical drying techniques present the most favorable characteristics, yielding superior materials possessing large surface areas and porosities.
Yet, one major drawback of these processes are the high pressures needed during operation, necessitating expensive equipment (e.g. high pressure pumps) as well as extensive amounts of compression work, consequently entailing relatively high energy and plant costs.
To overcome this limitation of supercritical impregnation, extraction and drying techniques a team of scientists from the University of Birmingham (UK) have now suggested a novel cost-efficient plant setup
To achieve a plant setup not requiring any pumping or compression equipment, the authors suggest a relatively straight-forward approach — cooling down the pressure vessel (to 253 K/ -20 °C) prior to its filling with CO2. This simple preparation step led to the fact, that CO2 stored at ambient temperature, entered the vessel voluntarily when the inlet valve was being opened, due to the natural pressure gradient arising from the temperature difference (see figure below: 1a → 1b). As soon as the desired amount of CO2 was located inside the autoclave, it was sealed and subsequently heated to the desired final temperature, entailing an increase in system pressure and hence the obtainment of a supercritical fluid inside the vessel (see figure below: 1b → 3).
Consequently, a process not requiring any external pressurization equipment to pump CO2 into the setup or obtain the desired operating pressure was devised successfully.
To estimate the required final temperature and amount of CO2 inside the autoclave, which both have to be selected correctly for the process to work safely and properly, thermodynamic calculations based on a suitable equation of state (EOS) were conducted. Due to its suitability for supercritical fluids, the authors selected the Soave-Redlich-Kwong EOS for this purpose.
For the mapping and setting of both crucial parameters during operation, the setup was equipped with a temperature regulating unit and a high-resolution hanging scale attached to the autoclave.
Using the proposed setup, the authors were able to successfully dry monolithic LA gellan gum gels, impregnate freeze-dried gel structures with Vitamin E and extract caffeine from green coffee beans and black tea leaves. Furthermore, their economical analysis unveiled that the associated plant costs were reduced by a factor of 3 and 5, when compared to conventional batch and semicontinuous drying modes respectively, while the predicted energy costs were more than 30 % lower than for batch drying and 72 % lower than for semicontinuous drying.
Therefore, the authors concluded that their new plant setup is an auspicious alternative to conventional drying, impregnation and extraction configurations due to its flexibility and the lower investment and operation costs associated to it.
While these arguments may be true for small scale lab or tabletop configurations, for which the costs of the process periphery are relatively high and a broad applicability might be desired, it is dubious whether the same is true for larger, specialized configurations, for which the majority of costs generally stem from the autoclave. Especially the batch type mode of operation of the suggested technique can be considered as an exclusion criteria for industrial-scale plants, since it leads to dramatically longer processing times, vastly reducing the plant throughput.
A last concern of the novel process is its robustness and safeness, as small errors during operation (e.g. overfilling the autoclave or exceeding the desired temperature) can have devastating effects, turning the setup into a hazardous system. Since it will require a great deal of work to devise a fail-save plant, it is doubtful whether drying units deploying the suggested technique will ever be sold commercially.
More details: Cassanelli et al. “Design of a Cost-Reduced Flexible Plant for Supercritical Fluid-Assisted Applications” Chem. Eng. Technol. 2018, 41, No. 00, 1–11. https://doi.org/10.1002/ceat.201700487
Due to their outstanding mechanical properties, such as high flexibility and rigidity, graphene aerogels are considered a promising type of aerogel for multiple applications. Recent studies found that by accurately tailoring the three-dimensional alignment of the graphene sheets inside the aerogel, these properties can be further enhanced. Simulations even predicted graphene structures exhibiting only 4.6 % the density of steel but 10 times its strength. Therefore, the ordered alignment of graphene sheets has has become a crucial part in graphene aerogel synthesis.
Because of its cheapness, scalability and environmental compatibility, freeze casting has been shown to be an efficient way to achieve the desired alignment of graphene-oxide (GO) sheets in a frozen monolith. This is the case, since GO sheets are rejected from the ice crystal upon freezing, which means that by determining the freezing direction, the alignment of the resulting GO scaffold can be tailored, as the sheets are “trapped” in between the resulting ice crystals. Through deploying various different directional freezing techniques, multiple GO-alignments (e.g. micro-honeycomb or ordered lamellar structures) have already been achieved.
Motivated by these findings, researchers from the Institute for Basic Science and the National Institute of Science and Technology in Ulsan (Korea) have managed to synthesize vertically and radially align graphene aerogels, resembling the structure of a micro-turbine.
The stated aerogels were manufactured by freeze casting an aqueous GO dispersion in a tailor-made setup in which two perpendicular temperature gradients (one in radial and one in axial direction) could be generated. With this setup, the researchers aimed at a simultaneous axial and radial alignment of the GO-sheets inside the frozen dispersion, resulting in a turbine-like GO scaffold, as shown in the figure below.
Initial tests conducted in this bi-directional freezing (BDF) setup unveiled that interactions between water and the GO sheets (e.g. H-bonding) influenced the ice crystal growth, thereby entailing undesired final crystal shapes and sizes, which is why several additives (e.g. ethanol, cellulose and chitosan) were successfully employed to reduce these unwanted interactions.
Following the freeze-casting procedure in the presence of these additives, the resulting monoliths were freeze-dried and reduced chemically using hydrazine vapor, yielding dry and reduced graphene aerogel structures.
The characterization of the obtained graphene aerogel monoliths showed that a highly ordered structure with the desired features (e.g. increasing lamellae channel widths towards the outer edges of the monolith) were obtained, which can be seen in the figure below. Furthermore, the aerogels exhibited a density of 6.9 g/cm3 and a specific surface area of 45.9 m2/g.
To highlight the superiority of the novel ordered structure, the mechanical properties and absorptivities of the BDF aerogel were compared to those of aerogels of an unordered and a honeycomb pore structure. Apart from its outstanding rigidity, being able to carry ∼10 000 times its own weight without deformation, the “turbine” aerogel exhibited neither a substantial loss in strength nor visible plastic deformation upon multiple successive compression cycles. Even after 1000 compression cycles at 50 % strain, the BDF aerogels exhibited a deformation as little as 8 %, while the unordered and honeycomb structures exhibited similar or larger values after just 15 cycles. This was attributed to the fact that the blades of the turbine-like structure did not brake or dislocate when being strained.
To assess the absorptivity of the different samples, the maximum uptake of different liquid organic compounds (e.g. ethanol, IPA, acetone, pump oil, etc.) from an aqueous solution was investigated. Not only did the BDF aerogel outdo its counterparts in terms of the absorption capacity, but it also exhibited an extraordinary cycling stability when repeatedly removing the solvent from the aerogel pores via combustion subsequent to absorption (see figure below).
These outstanding findings once more highlight the unique features graphene aerogels offer and reveal that enhancements in their characteristic properties are readily attainable. Hence, further substantial advancements in the field of graphene aerogels are only a matter of time, making these materials one of the more auspicious candidates for future groundbreaking innovations. Certainly, the aerogel community will be captivated by this remarkable material for many years to come.
More details: Wang et al. “Freeze-Casting Produces a Graphene Oxide Aerogel with a Radial and Centrosymmetric Structure” ACS Nano, March 2018. https://doi.org/10.1021/acsnano.8b01747
Back in November 2016, the US based insulation firm Armacell has established a joint venture with the Silica Aerogel specialist JIOS Aerogel Ltd. (Korea), to develop and manufacture aerogel blankets.
Recently, the first fruits of this partnership were reaped as Armacell has released the ArmaGel HT insulation blanket, a flexible and versatile blanket equipped with aerogel particles. The novelty of this type of aerogel blanket is that, in contrast to conventional commercial manufacturing techniques, Armacell has developed a new type of process during which aerogel particles are injected mechanically into an existing blanket structure, instead of synthesizing aerogel matrices in situ. This does not only reduce processing times vastly (down to just two hours), but also increases the process scalability and efficiency as well as the freedom in design. Hence, Armacell promises a more economical and tailored solution for its customers.
In addition to its flexibility, the ArmaGel HT insulation blanket is thin and light-weight, allowing for easy handling, cheap transportation as well as simple assembly and replacement, which decreases machine down times and labor costs. Furthermore, the absence of any substantial dust formation upon machining allows for a simple and straight forward cutting of the blankets into tailored shapes. Therefore, it is an ideal material for thermal and sound insulation of “pipes, vessels and ducts (including elbows, fittings, flanges etc.) in offshore, industrial (typically oil and gas) and process equipment” for applications up to 650 °C.
Due to the absence of any toxic constituents, the aerogel blankets are environmentally safe and can be disposed on landfills, while their hydrophobicity and breathability significantly diminishes the risk of corrosion under insulation.
Most importantly, ArmaGel HT blankets exhibit great thermal and acoustic insulation properties, their thermal conductivities being comparable to those of commercially available aerogel blankets manufactured by their direct competitor Aspen Aerogels (USA), which can be seen in the figure below. Therefore, the market entry of Armacell could intensify the competition in the aerogel blanket market, which is currently being dominated by Aspen aerogels (especially in the US).
Although insulation blankets are just one of many applications for aerogel materials, they have certainly been one of the most widely used aerogel products so far. Hence, it’s good news that new competitors are entering the market to revive competition and accelerate innovation cycles, making the manufactured goods accessible for a large consumer base. We are confident that such developments will further motivate research and entrepreneurship in the versatile field of aerogel materials and therefore propel aerogels to common everyday materials.
Read more at:
https://corporate.armacell.com/en/armagel/
https://insulatenetwork.com/armagel
Edits:
* The thermal conductivity curve for ArmaGel HT have been updated according to the latest product TDS.
