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

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 

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

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

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