Despite their intriguing characteristics (e.g. ultralow density, high porosity & electrical conductivity), the application of carbon aerogels is generally limited by their poor mechanical strength and brittleness. Researchers from the Zhejiang University (China) were now able to manufacture highly flexible, binary carbon aerogels (bCAs) consisting of graphene and multi-walled carbon nanotubes (MWNTs), which can resist compressive and tensile stresses. These novel bCAs were successfully used as strain sensors to detect complex three dimensional movements.

The novel aerogels were fabricated by creating an aqueous solution equipped of graphene oxide and MWNTs which was then given shape by additive 3D-printing. Thereafter, the structures were freeze-dried before being chemically or thermally reduced.

Owing to their hierarchical assembly, which is schematically shown in the figure below, the novel bCAs exhibit an extraordinary stretching stability over a wide range of conditions (e.g. temperatures from 93-773 K). Furthermore, they exhibit a noteworthy fatigue resistance, being able to retain their structural shape to great extents for at least 100 cycles at 200 % tensile strain.

Schematic of hierarchical assembly of bCAs, stretching from centimeter to nanometer range. Fourth order: Graphene and MWNT molecular blocks; Third order: graphene laminates; Second Order: Polygon cell; First Order: Macroscopic truss structure

Another key characteristic of the bCAs is their change in resistance in tension (gentle increase) and compression (steep increase). Exploiting this feature, the researchers equipped the joints of a snake-like robot with bCAs to be able to sense the robot’s movements and configurations. As shown in the figure below, a sensor array consisting of three bCAs was sufficient to map the continuously changing configurations and hence accurately identify the robot’s movements.

Illustration of working principle of a three bCAs sensor array to identify the movements of a snake-like robot

 
The authors identify other potential applications of the bCAs in wearable electronic devices, lightweight mechanical devices and fields of application requiring robustness and reliability in the most extreme conditions (e.g. aerospace engineering). Furthermore, the researchers are confident that their assembly method can be deployed for the fabrication of other highly stretchable aerogel materials.

More details: Fan Guo et al. “Highly stretchable carbon aerogels.” Nature Communications. https://www.nature.com/articles/s41467-018-03268-y
Read more: https://phys.org/news/2018-03-rubbery-carbon-aerogels-greatly-applications.html

Recently, we have reported on the potential of aerogel sorbents for CO₂ capture and storage (CCS). Despite their favorable properties, the deployed amine functionalized aerogels (AMAs) were found to require optimization to allow for their successful economical implementation. Increasing the activity and capacity of solid sorbents while decreasing their cost, is therefore an issue which is currently under investigation. Researchers from the US and Sri Lanka now report to have found an efficient, cheap and environmentally benign solid CO₂ sorbent: KOH-activated carbon chitin aerogels.

The novel sorbent material was synthesized from commercial chitin powder from shrimp shells, which was dispersed in a sodium-urea-water solution. Repeated freezing/thawing cycles of this solution resulted in the formation of a stable hydrogel, which subsequently was freeze-dried to obtain a chitin aerogel. Thereafter, carbonization of the aerogel was achieved by heating the sample to 800 °C under nitrogen atmosphere. In the last step, the aerogel was again heated to 850 °C (under N₂ atmosphere) in the presence of potassium hydroxide (KOH) to obtain the activated carbon aerogel. Consequently, the inexpensive manufacturing technique, which does not require any costly or toxic chemicals, and the abundance of the precursor materials facilitate the cheap production of chitin-based CO₂ sorbents.

The final activated carbon aerogels were found to exhibit large specific surface areas (> 500 m²/g), more than 35 times larger than that of their parent chitin aerogels. Additionally, the micro pore volume, which is an important parameter for CO₂ capture, increased by the factor of 95 between the chitin aerogel and the carbonized and KOH-activated sample. These two factors explain why the obtained CO₂ sorptivity value of 0.48 mmol/g (1 atm, 0 °C), obtained for the chitin aerogel, could be vastly increased to 5.02 mmol/g by further processing (i.e. carbonization and activation). As shown in the figure below, similar increase in sample sorptivity was also measured at room temperature (0.28 mmol/g and 3.44 mmol/g, respectively). This means that the morphological changes taking place inside the aerogel structure during carbonization and activation have a significant impact on the final sorbent properties.

CO2 adsorption isotherms at 1 atm and 0 °C (a) and 1 atm and 25 °C (b) for chitin aerogels (1), carbonized chitin aerogels (2) and KOH-activated chitin aerogels

The authors conclude that they have found an environmentally benign and very inexpensive way of manufacturing highly active chitin-based sorbents for CO₂ capture. Additionally, the sorbents are synthesized from a biopolymer, making the final material biodegradable and non-toxic.

More details: Dassanayake, R.S., Gunathilake, C., Abidi, N. et al.; Activated carbon derived from chitin aerogels: preparation and CO2 adsorption, Cellulose (2018). https://doi.org/10.1007/s10570-018-1660-3

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 search for active yet economical water purification strategies is in full swing as increasing industrial activity results in sharp surges in wastewater production, and the ever-growing global population increases demand for clean drinking water.

Commonly, separated, sophisticated absorption processes are deployed to remove either organic or inorganic contaminants from sewage water due to their high efficiency and moderate cost. However, it remains a challenge to devise robust, efficient and economical absorbents for the wide range of trace elements occurring in wastewater. Ideally, novel absorption materials should be able to remove inorganic compounds such as dyes or heavy metals and also be active against harmful viral or bacterial pathogens.

In pursuit of such a material, researchers from Jinan University (China) have synthesized an aerogel structure exhibiting extraordinary dye and heavy metal absorbing properties, by using graphene oxide (GO) and a type of abundant mineral called montmorillonite (MMT). The desired anti-pathogenic activity was realized through equipping the aerogel matrix with a common anti-bacterial agent, resulting in absorbents displaying excellent antibacterial activity against Gram-positive and Gram-negative bacteria.

The aerogel material exhibiting these intriguing properties was manufactured through mixing GO powder, ascorbic acid, and a MMT solution, then inducing gelation through heat treatment at 95 °C. After aging of the hydrogel in a PVA solution for two days, the gel was then freeze dried at -55 °C, resulting in a monolithic aerogel structure, which is shown in the Figure below.

Image of black GO-MMT aerogel placed on top of kapok tree fiber.

Absorption experiments showed that the aerogel absorbents were not only able to remove more than 95 % of methyl orange and methylene blue dyes from aqueous solutions, but also exhibited great properties for the removal of heavy metals from water (e.g. >90 % removal efficiency for chromium ion removal). This activity was found to be stable over numerous absorption/desorption cycles, with sample regeneration being achieved by vigorous shaking. Furthermore, the addition of antibacterial dodecyl dimethyl benzyl ammonium chloride (1227) to the initial precursor solution was found to provide the aerogel with antibacterial activity, which was shown using E. coli and S. aureus bacteria cultures, each losing over 90 % of their cell viability in the presence of the GO-MMT-1227 aerogel material.

Due to these extraordinary findings, the researchers are confident that they have found an efficient, versatile, recyclable, and robust absorbent material, which has the potential to revolutionize water purification. If economical large scale manufacturing and long term stability can be achieved, the novel material might indeed replace state-of-the-art sorbents in wastewater treatment systems.

More details: Yunyun Zhang et al.; The utilization of a three-dimensional reduced graphene oxide and montmorillonite composite aerogel as a multifunctional agent for wastewater treatment, RSC Adv., 2018,8, 4239-4248. https://doi.org/10.1039/C7RA13103H