Aerogel is a kind of highly porous monolithic material. Since it was successfully prepared in 1931, it has attracted the attention of researchers. The unique 3D interconnected porous structure of aerogel endows it with many excellent physical properties, such as extremely low density (<1 kg/m-3), high surface area (>1000 m2/g-1), excellent mass transfer performance and low thermal conductivity (<0.015W/m-1 · K-1). Therefore, aerogels have great potential applications in energy and environmental related fields, such as energy storage, electrocatalysis and water treatment.

Silica based inorganic gel and organic gel represented by resorcinol formaldehyde (RF) are two traditional aerogels, which are prepared by sol-gel method and supercritical CO2 drying method respectively. For these aerogels, the internal framework is composed of cross-linked primary nanoparticles. It is difficult to precisely control the multi-layer microstructure in the process of sol-gel synthesis. Recently, researchers began to try to assemble aerogels from nano building blocks with different sizes, such as 1D nanofibers (tellurium nanowires, ceramic nanocrystalline nanofibers, electrospun SiO2 nanofibers), and 2D nanoplates (graphene, BN), so as to manipulate the structure of aerogels on a micro scale. In particular, when tellurium nanowires are used as templates, good regulation of nano scale can be achieved by hydrothermal carbonization (HTC) of cheap and renewable polysaccharide biomass. However, the synthesis of these new aerogels requires expensive and harmful prepolymers (tellurium nanowires), and the equipment involved in this process is very complex. The surface treatment and processing modification of cellulose have been proved to be an effective strategy for constructing high-performance aerogels. After high-temperature carbonization, biomass gas gel can be converted into carbon gas gel. Recently, some successful examples of realizing functional carbon aerogels with hyperelastic structure are mainly based on the original structure of wood or bacterial cellulose. However, the inherent structure formed during biological growth will limit the further optimization of structure and performance. Therefore, it is still an urgent and arduous challenge to develop a controllable, sustainable and low-cost strategy to produce aerogels with good mechanical properties.

In view of this, Yu Shuhong, an academician team from the University of Science and Technology of China, proposed a general strategy that can control the preparation of carbon nanofiber aerogels (CNFAs) through biomass derived nanofiber template oriented hydrothermal carbonization. The prepared carbon nanofibers have rich functional groups on the surface. Compared with traditional aerogels based on natural biopolymers, CNFA can achieve excellent combination of good recoverability and high strength by adjusting the synthesis parameters. By combining 3D cross-linking structure with different functional groups on the CNF surface, the author has constructed a low-cost self-made water purification device, which can achieve fast dye removal (3183L/h-1 · m-2) and high throughput (more than 90% removal efficiency). The synthesis strategy and sustainability concept proposed in this work will open up a new way to prepare advanced aerogels with unique properties. The research was published in the latest issue of Advanced Functional Materials as a paper entitled "Robust Carbon Nanofiber Aerogels from All Biomass Precursors".

Preparation of CNF aerogel derived from biomass

Biopolymer nanofiber is a kind of nanostructure which widely exists in natural materials. Cellulose nanofibers, chitin nanofibers and protein nanofibers are three typical biopolymer nanofibers. The author first prepared these three types of nanofibers as templates for hydrothermal carbonization (HTC) process. Then, the author carried out HTC process by using glucose as carbon source. The direct hydrothermal treatment of the mixture of amyloid fibril and glucose at 180 ° C produced a solid blocky gel like product (Figure 1a). Finally, the author also obtained uniform carbon nanofibers by using amino functionalized partially deacetylated chitin nanofibers (D-chitin) as a template to guide the HTC process (Fig. 1b – d).

Figure 1. Schematic diagram of the synthesis process of carbon nanofiber aerogel based on biomass prepolymer solution.

Mechanical properties of CNFAs

In the HTC process, the dispersed nanofibers in the solution gradually form a complete cross-linked network with the increase of diameter. After gel, the crosslinked nanofiber skeleton structure endows CNF aerogel with unique mechanical properties. Cyclic compression tests were carried out to study its mechanical properties. The results show that after 40% deformation, CNFA almost completely recovered its original volume (Fig. 2a). P-CNFA shows good flexibility and recovers to 95% of the original size after 60% deformation (Fig. 2a). Each load curve shows a linear elastic region under small strain, followed by a yield behavior. Like most flexible elastomers such as rubber and polymer aerogel, CNFA exhibits typical softening behavior. In the second cycle, the aerogel became more flexible than the first cycle, and then the stress-strain curve in the continuous compression cycle tended to converge to the softened mechanical curve (Fig. 2b). After 100 loading/unloading cycles, P-CNFA and C-CNFA showed better mechanical stability, with less deformation, close to 25%. The author further compared the hardness and strength of CNF aerogel with the recently reported lightweight honeycomb materials (Fig. 2c, d).

Fig. 2 Compressibility of CNF aerogel.

Water purification performance of CNFAs

At neutral pH, the negatively charged carboxyl group on the surface of carbon nanofibers can promote the good adsorption of positively charged dyes and metal ions. Therefore, the author carried out batch adsorption experiments on various pollutants (methylene blue (MB), rhodamine B (RB), lead nitrate and copper nitrate) under neutral pH conditions. The amount of pollutants adsorbed is determined by the difference between the initial added concentration and the residual concentration after batch adsorption (Fig. 3a-d). The results showed that P-CNFA had strong adsorbability for cationic pollutants such as methylene blue, rhodamine B, metal cations and anions. The author compared the preparation cost with the adsorption capacity of CNFA and other adsorbents. CNFA showed the best performance in terms of price and adsorbent capacity (Figure 3e).

Fig. 3 Adsorption of dyes and heavy metal ions by P-CNFA.

The authors evaluated the performance of CNFA membranes by manually extruding injection filters (Fig. 4a). First, the author cut a small piece of P-CNFA membrane and put it into the syringe, and then filtered RB and MB aqueous solutions on the CNFA membrane (Fig. 4b, d). The dye removal was evaluated by UV Vis spectroscopy. The results show that almost 100% removal is achieved (Fig. 4c, e).

In order to improve the adsorption capacity, more nanofibers are needed. However, traditional biomass nanofiber membranes and aerogels are usually densified under water pressure during filtration. With the increase of the thickness of the nanofiber membrane, the flux decreases rapidly. It is still a huge challenge to achieve high processing capacity with high processing rate. The CNFA has good recoverability and high strength, which can protect its cross-linked structure from densification under high pressure. The author constructed a large water purification filter under gravity pressure using a filter cartridge and a tank (Fig. 4f, g) containing CNFA particles. The flux decreases linearly with the increase of CNFA charge (Fig. 4h). The balance between processing capacity and processing rate can be achieved by adjusting the amount of CNFA. Using MB as a model, the removal effect of dyes from water was studied. The results show that P-CNFA can achieve fast dye removal (3183L/h-1 · m-2) and high throughput (more than 90% removal efficiency), and has greater advantages than activated carbon in terms of treatment capacity and speed.

Figure 4. Rapid removal of dye from CNFA.

[Summary]

This work proposes a general and sustainable strategy for controllable manufacturing of a series of all biomass derived carbon nanofiber aerogels with rigid 3D cross-linking structure. A variety of biomass derived nanofibers, including starch like nanofibers, aminated cellulose nanofibers, and deacetylated chitin nanofibers, are used for template oriented HTC. By adjusting the synthesis parameters (the type of biomass derived nanofiber template, the ratio of template to glucose and the reaction time), it is easy to adjust the microstructure (diameter, length) of CNF and the physical properties (apparent density, Young's modulus and yield stress) of aerogel. P-CNFAs prepared from amyloid nanofibers and glucose are superior to other aerogels and engineering materials derived from natural biopolymers in terms of recoverability and strength. By combining the 3D cross-linked structure with different functional groups exposed on the surface of carbon nanofibers, the author constructed a low-cost self-made water purification device, and achieved rapid dye removal (3183L/h-1 · m-2) and high throughput (more than 90% removal efficiency). This sustainable method of synthesizing CNFA with unique structure will create more opportunities for the commercial application of biopolymer derived gel, especially in the fields of adsorbent and separator, thermal insulation materials, buffer materials, etc.