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).

Depiction of the process steps for the novel drying technique in the CO₂ phase diagram: (1a) CO₂ cylinder in vapor-liquid equilibrium, (1b) vessel filled with CO₂, (2) CO₂ critical point, (3) drying operating point

 
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.

Schematic of the bi-directional freeze casting technique to synthesize radially aligned graphene oxide aerogels

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.

Schematic illustration of the graphene aerogel top view showing the decreasing width (λ) of the channels with increasing distance from the aerogel center and corresponding SEM images of the channels in the three marked regions (scale bar: 25 μm)

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).

a) Absorption capacities of BDF aerogel and two reference graphene aerogels for different solvents. b) Progression of ethanol absorption capacity of BDF aerogel during cyclic absorption−combustion processes

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.

Images of the new ArmaGel HT insulation blanket by Armacell, highlighting three of its characteristics properties — flexibility, high temperature resistance and hydrophobicity

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).

Thermal conductivity of aerogel blankets as a function of the mean temperature measured according ASTM C177.^*

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.

In search for a straight-forward and economical drying technique for aerogel materials, removing the main obstacle for the wide-spread application of aerogel materials — their tremendous costs — researchers from the University of Newcastle have been inspired by the way dragonflies and damselflies dry their lightweight and porous wings under ambient conditions.
In the course of their research the scientist stumbled upon the fact, that dragonflies dry their highly porous wings — which make up only 2 % of their entire bodyweight despite their large size — in a matter of a handful of hours (sometimes even as little as one hour) during their final metamorphosis into the adult. They hypothesized that this rapid but at the same time gentle drying process of the aerogel-like wings at ambient conditions can be attributed to the involvement of just one simple chemical compound — bicarbonate.

Schematic of novel APD drying technique deploying sodium bicarbonate and TMCS

The formation of sodium chloride (NaCl) and CO₂ from sodium bicarbonate, which takes place upon the addition of trimethylchlorosilane (TMCS), was found to be a straight forward technique to produce CO₂ inside an aqueous porous medium, with the formed gas preventing pore collapse in the course of successive ambient pressure drying (APD), mimicking this process (supposedly) occurring in nature. As schematically shown in the figure on the right, this means that CO₂ formed in situ works as a stabilizer
Schematic of novel APD drying technique deploying sodium bicarbonate and TMCS. of the gel pores, acting against the capillary pressure arising when the solvent is removed from the gel network via APD.

The suggested drying technique was tested using silica gels synthesized from tetraethoxysilane, which were placed into a sodium bicarbonate solution for 24 h after aging. Before drying the gels at ambient pressure (60 °C), small quantities of TMCS were poured upon them to initiate the CO₂-forming reaction and hence prevent pore collapse. To further enhance the aerogel properties by removing the by-product NaCl from the gel pores an additional washing step was added to the procedure either after or during the drying step.

The resulting aerogels were found to exhibit similar properties as samples prepared via supercritical CO₂ drying, exhibiting bulk densities as low as 0.06 g/cm³, porosities exceeding 98 % and specific surface areas of up to 700 m²/g, highlighting the great suitability of this drying technique for the manufacturing of silica aerogels. Further testing showed that the same process can also be applied to dawsonite (NaAlCO₃(OH)₂) aerogels synthesized with aluminum sec-butoxide and therefore is not limited to silica aerogels.

a) and b) SEM images of damselfly wing, c) Silica aerogel produced via novel bio-inspired APD technique, d) SEM image of silica aerogel structure.

Certainly, the most outstanding characteristics of the suggested aerogel synthesis route are its simplicity, cost-efficiency, scalability and wide applicability, making it a strong alternative to conventional drying techniques. Especially its ultra low material cost, estimated at $4 per kilogram of aerogel, and the absence of any specialized or hazardous processing conditions (such as elevated temperatures or pressures and toxic materials) could propel the presented approach to market maturity and hence make aerogel materials accessible for commercial sectors, such as the building or clothing industry.

More details: Han et al. “Bioinspired Synthesis of Monolithic and Layered Aerogels” Advances Materials. https://doi.org/10.1002/adma.201706294

Read more at:
http://www.millenniumpost.in/world/dragonfly-wings-inspire-new-generation-of-aerogels-296492 https://phys.org/news/2018-04-world-oldest-insect-aerogels.html

Quantum dot-sensitized solar cells (QDSCs) are the third generation of solar cells, using quantum dots as the absorbing photovoltaic material, which allow for larger theoretical efficiencies than conventional silica based cells. 
In addition to the major problems of this cell type — insufficient light absorption and electron hole recombination on the QD-electrolyte interface — which have been addressed in recent years yielding efficiencies up to 11 %, one great obstacle standing in the way of their large scale implementation is electrolyte loss due to evaporation. To avoid this undesired leakage of electrolyte entailing inferior cell performance, solid electrolyte carrier membranes have been introduced. Generally, synthetic polymer membranes have been deployed for this purpose. However, in light of environmental aspects, requiring a shift towards bio-compatible and renewable components, bio-based membranes are a logical replacement for their synthetic counterparts.

With the aim of devising biocompatible QDSC carrier membranes, scientists from the Aalto University in Finland have tested bio-based aerogel materials in QDSC cells, consisting of a CdS-sensitized photo-anode, a Pt counter electrode and a polysulfide redox electrolyte (see figure below).

Schematic of deployed QDSC architecture equipped with bio-based aerogel electrolyte carrier membrane.

To investigate the effect of different biopolymers, aerogels consisting of bacterial cellulose (BC), cellulose nanofibers (CNF), chitin nanofibers (ChNF) and TEMPO-oxidized CNF (TOCNF) were synthesized via gelation, solvent exchange and freeze-drying. Apart from their different fibrillar structures, the polymers exhibited a variety of different functional groups and surface charges, facilitating an investigation of the impact of these properties on cell performance.

Subsequent to gel drying, the aerogel samples were soaked with the polysulfide electrolyte, yielding free standing, flexible and stable structures in all four cases, allowing for easy sample handling and cell assembly.

Photocurrent-voltage (J-V) curves obtained under irradiation with one sun for reference QDSC and QDSCs equipped with bio-based polymer aerogel membranes.

The resulting membrane equipped QDSCs were then tested under irradiation of one sun (=1000 W/m2) to compare their performance to that of a reference cell filled with the untapped liquid electrolyte. All samples exhibited a similar photocurrent-voltage behavior, which can be seen in the figure on the right. Furthermore, the obtained values for the internal charge transfer resistance were comparable for all five cell type, suggesting that the membranes did not interfere with the internal charge transfer. Lastly, it was determined that neither membrane type had any detrimental effects on the polysulfide redox reaction, despite the difference surface charges of the deployed polymers.
In light of these findings, the authors concluded that cellulose and chitin based aerogels are suitable materials for organic electrolyte carrier membranes in QDSCs, since the performance of all aerogel-equipped samples was on par with that of the membrane-free reference cell. Out of the four different precursor materials, the researchers see the greatest promise in BC aerogels, due to its cost-efficient production technique via microbial fermentation and the resulting high purity of the biopolymer, guaranteeing a cheap and straight-forward production of BC aerogel membranes.

Certainly, the presented study can only be considered as an initial foray into the field of bio-based aerogel electrolyte carrier membranes. Future work will have to investigate the performance of such materials in more elaborate QDSC architectures to assess their potential for large scale implementation. Still, this pioneering work encourages endeavors aiming at the application of aerogel materials in photovoltaics. In case further work on this topic will underpin these findings, aerogels surely have the potential to become integral building blocks of next generation solar cells.

More details: Borghei et al. “Biobased aerogels with different surface charge as electrolyte carrier membranes in quantum dot-sensitized solar cell” Cellulose, 2018. https://doi.org/10.1007/s10570-018-1807-2