In recent years, due to the continuous emergence of nanofluid systems assembled by various new micro and nano materials, osmotic energy has attracted extensive attention as a clean and reliable renewable energy source with huge reserves. The most common nanofluid systems used for osmotic energy collection are 2D lamellar membranes assembled from 2D (two-dimensional) nanosheets by vacuum filtration, solution casting, and wet or microfluidic spinning. However, in these 2D layered membranes, the layer spacing or nanochannels between two adjacent nanosheets are often at the sub-nanometer level, significantly hindering ion flux and resulting in low output power density. An effective solution is to etch micro-nanopores in the assembled micro-element 2D nanosheet to increase the number of nanochannels and shorten the ion migration path. However, the nanosheets commonly used (e.g., graphene, graphene oxide, boron nitride, carbon nitride, MXene, and 2D metal-organic frameworks) are not conducive to large-scale practical applications, both in the initial preparation of 2D nanosheets and in the additional process of etching holes.
Recently, the team of Associate Professor Wang Huiqing of Hefei University of Technology and the team of Professor Ye Dongdong of Anhui Agricultural University reported a new 2D material obtained by controllable peeling of the intrinsic Bouligand structure of chitin, a two-dimensional multi-scale structure chitin nanosheet (2D H-CNS). The assembled 2D lamellar membrane after vacuum filtration can be used for efficient osmotic energy harvesting. This research has led to the development of biomass materials with high-performance osmotic energy harvesting. The preparation method of this new two-dimensional nanomaterial 2D H-CNS is low-cost and environmentally friendly, so it shows great potential in industrial production and application. Prof. Pan Chen, a researcher at Beijing Institute of Technology, provided important suggestions and simulation support for this work.
Conceptual map of the stripped nanosheets based on the intrinsic structure of chitin and used for osmotic energy collection
2024年6月21日，相关研究成果以“Sustainable chitin-derived 2D nanosheets with hierarchical ion transport for osmotic energy harvesting”为题发表在期刊《Advanced Energy Materials》上。 第一作者为合肥工业大学硕士研究生向忠润。
【Preparation of 2D H-CNS and its Laminar Assembly Membrane for Osmotic Energy Collection】
In this work, the preparation of 2D H-CNS involves crushing and purification of crab shells, deacetylation pretreatment, hydrochloric acid vapor acidolysis, and assisted sonication. It is worth noting that the surface charge introduced by deacetylation pretreatment promotes the loosening of the dense intrinsic structure of chitin, increasing the possibility of further structural detachment. The modified chitin (Fig. 1b2) was transferred to a closed reactor containing hydrochloric acid vapor to obtain fluffy chitin composed of loose chitin sheets and interconnected micro/nanofibers (Fig. 1b3), and then by low-power sonication, the fluffy chitin was stripped into a 2D H-CNS (Fig. 1b4) with abundant micro-nanoporosity on the surface, and its assembled membrane was used for osmotic energy collection.
Figure 1. The preparation process of two-dimensional multi-scale chitin nanosheets (2D H-CNS) and its layered assembly film (2D-HM) were used for efficient osmotic energy collection
【Peeling mechanism, structure regulation and properties of chitin nanosheets】
In addition, the authors characterized the difference in packing density of chitin nanofibers at different spatial locations by monitoring the strength of amide III bonds (1328 cm-1) at different locations of purified chitin by two-dimensional Raman imaging technique (Fig. 2a). The results show that the characteristic peak intensity of the surface is significantly higher than that of the cross section, which proves that the bulk density of nanofibers in the cross-section is low. The difference in bulk density is due to the intrinsic Bouligand structure of chitin, which opens up the possibility of layer-by-layer peeling of chitin.
The introduction of surface charge can affect the peeling process of chitin. After treatment with acid vapor under the same conditions, unmodified chitin was converted to dense chitin (Figure 2b2), while modified chitin was converted to fluffy chitin (Figure 2c2). After sonication, dense chitin is stripped into dense 2D dense chitin nanosheets (2D D-CNS) with high thickness (~151.3 nm), while fluffy chitin is stripped into 2D H-CNS with low thickness (~1.34 nm) and rich micro/nanostructure.
Figure 2. Mechanism of nanosheet stripping from chitin intrinsic Bouligand structure and regulation of nanosheet structure
【Effect and mechanism of 2D H-CNS in-plane micro and nano pores on ion transport】
Subsequent authors evaluated the ion transport characteristics of 2D-HM and 2D-DM lamellar membranes based on 2D H-CNS and 2D D-CNS, respectively. It is confirmed that the abundant micro-nano pores in the 2D H-CNS plane can reduce the tortuosity of ion mobility and improve the ion flux. Notably, at very low concentrations, the conductivity of 2D-HM was increased by a factor of 18.5 compared to 2D-DM. Molecular dynamics simulations show that the migration velocity of Cl- in the in-plane pores (0.7542 nm ps−1) is much greater than that in the pores formed between adjacent nanosheets (0.2885 nm ps−1), which proves that the existence of in-plane pores can accelerate ion migration and increase ion flux. At the 50-fold KCl concentration gradient, the maximum output power density of the 2D-HM system (2.59 W m−2) was 2.51 times that of the 2D-DM system (only 1.03 W m−2).
Figure 3. The abundant micro-nano multi-scale structures in the multi-scale chitin nanosheets (2D H-CNS) can shorten the ion transport path and increase the osmotic energy harvesting power
[Performance evaluation and concept demonstration of RED system based on 2D H-CNS]
Finally, the authors designed a RED system consisting of 20 units in series under artificial seawater-river water conditions (Fig. 4a) to achieve the high output capacity of the 2D-HM embedded RED system. The 20 devices were connected in series with an output voltage of 2.72 V and an output current of 4.63 μA, which successfully drove a small calculator, stopwatch, and LED light, demonstrating the feasibility of replacing other 2D film-based RED systems for enhanced osmotic energy harvesting.
Figure 4. Electrical appliances for the drive of a reverse electrodialysis system (RED) based on a two-dimensional multi-scale structure chitin nanosheet lamellar assembly membrane (2D-HM).
Summary: The authors developed a strategy combining chemical modification, acid vapor, and ultrasound to strip 2D chitin nanosheets with hierarchical porous structures (2D H-CNS) from crab shells. The results verify that the chemical modification of chitin is crucial in the formation of 2D H-CNS. This modification not only facilitates stripping and reduces the thickness of the nanosheets (1.34 nm), but also creates abundant pores for ion transport in the 2D H-CNS. In addition, experimental and simulation results demonstrate that the ion transport performance of the 2D H-CNS-assembled membrane (2D-HM) in the layered ion transport pathway is enhanced, and the ion conductance in the 2D-HM is increased by 18.5 times compared with the chitin membrane without hierarchical porous structure (2D-DM). In addition, at a 50-fold KCl concentration gradient, the 2D-HM embedded in the RED system achieves an output power density of 2.59 W m-2, which is 2.51 times that of the 2D-DM. In simulated artificial seawater and river water, the output power can be further increased to 2.87 W m−2. This work has led to the development of natural biomass materials with high-performance osmotic energy conversion. Biomass-based Sustainable Materials Group Homepage:
https://www.x-mol.com/groups/ydd
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Paper Links:
https://onlinelibrary.wiley.com/doi/abs/10.1002/aenm.202402304 Source: Frontiers of Polymer Science