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).

Schematic of hydrophobic corn straw aerogel production process

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).

Soybean oil removal from water with the MTA-CS for different stages. a) Soybean oil water mixture, b) addition of MTA-CS, c) oil absorption by MTA-CS d) removal of oil soaked MTA-CS

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.

Schematic of AC-graphene aerogel hybrid synthesis route (Black quadrangles represent graphene oxide nanosheets, orange triangles represent activated carbon).

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.

SEM images showing a comparison between the microstructure of plain AC (a) and AC-graphene hybrid aerogel (b)

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

Ablaze Grenfell Tower in West London on June 14 2017. Both the exterior cladding and the polyisocyanurate insulation are now considered as the main reasons for the rapid spread of the fire.

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.

Schematic of the structural composition of the PFR/SiO2 composite aerogel and its interpenetrating binary network.

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.

Flame retardant properties of PSi-70 aerogel during exposure to a propane/butane flame. a) Illustration of the measurement set-up b) Pseudo-color thermal image of the sample front side c)–e) Pseudo-color thermal images of the back side of the PSi-70 aerogel at different times. f) The time- dependent temperature profile of the three reference points (P1, P2, P3) on the sample back side. Photography of the back side (g) and the front side (h) after the fire resistance test. i) Corresponding SEM image of the remaining SiO2 network on the front side.

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/