Recently, Professor Zhang Qianqian's team from Beijing University of Technology has selected natural clay as raw material to prepare a fully natural two-dimensional nanofluidic membrane, with an area of up to 700 cm².

Under the simulated conditions of seawater and river water, the permeation power output of the fully natural two-dimensional nanofluidic membrane reaches 8.61 W/m².

Compared with the previous similar achievements of the research group, it has been improved by 1.7 times, and it is at the leading level in the field of two-dimensional membrane salinity gradient energy generation. At the same time, it has also achieved stable and continuous long-term salinity gradient energy generation for up to 30 days.

Compared with the mainstream two-dimensional membrane materials, the layered clay membrane constructed this time has reduced resource consumption throughout the entire life cycle to 1/14, greenhouse gas emissions to 1/9, and production costs to 1/13, demonstrating significant economic benefits, resource benefits, and environmental benefits.

This not only provides a reliable membrane material foundation and new strategy for large-scale salinity gradient energy generation, but also provides new ideas for the development of scalable two-dimensional membrane materials, and is expected to promote the development and application of membrane-based new energy technologies.Specifically:

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Firstly, the outcomes of this research can be applied to "zero-carbon" salinity gradient energy generation.

The vast reserves of ionic salinity electrochemical potential energy present in natural seawater/river water and industrial waste brine can be efficiently captured using the outcomes of this research. By constructing salinity difference battery components, energy storage stations can achieve large-scale capture and utilization of salinity energy.

Secondly, the outcomes of this research are also universally applicable to battery systems such as lithium batteries and flow batteries. They can serve as functional separators to optimize ion transfer, thereby enhancing the energy density and lifespan of the batteries.

Lastly, the ion selective sieving function of the outcomes provides significant application potential in the fields of environmental resources and water resources. Specific applications include wastewater treatment, lithium extraction from salt lakes, brine refining, and seawater desalination.The "Fish and Bear's Paw" of Ion Selectivity and Ion Permeability

It is well known that nature harbors vast reserves of clean energy sources, such as solar and wind energy. How to efficiently collect and utilize these low-density energies poses a significant challenge.

The ocean covers 71% of the Earth's surface, providing not only a rich source of aquatic products and minerals for humanity but also harboring immense energy potential.

Salinity gradient energy is a widely available, environmentally stable clean "zero-carbon" energy source.Salt gradient energy harnesses the chemical potential difference between seawater of varying salinities and river water to generate electricity. The entire power generation process is free of pollutants and carbon dioxide emissions, and it is not dependent on seasonal or weather conditions, offering the advantages of being clean, environmentally friendly, and capable of providing stable and continuous power around the clock.

Theoretically, the total amount of salt gradient energy from global river runoff into the sea could reach 2.6 TW, which is equivalent to 17% of the world's electricity consumption. However, efficiently utilizing salt gradient energy is a challenging task.

At present, the Reverse Electrodialysis (RED) technology, which is based on the ion-selective membrane, is considered to be the most industrially promising salt gradient energy generation technology, with the ion-selective permeable membrane as its core component.

These ion-selective permeable membranes require selective transmission of single-charge ions.

Only in this way can two salt solutions of different concentrations be separated, allowing the salt difference to drive the directional movement of single-charge ions, thereby creating a chemical potential difference on both sides of the membrane. Ultimately, the electrodes placed in the solutions on either side undergo redox reactions, converting chemical potential energy directly into electrical energy.To achieve efficient salinity-gradient energy conversion, an ideal ion-selective membrane should possess both high ion selectivity and high ion permeability simultaneously.

However, these two properties often exist in a competitive relationship where one cannot have the best of both worlds, presenting a significant challenge in constructing high-performance ion-selective membranes.

With the development of nanoscience and membrane science, the emerging nanofluidic technology in recent years has provided a new platform for the design of high-performance ion-selective membranes.

As a type of nanoporous thin film, nanofluidic membranes typically have a charged surface. The strong electrostatic interaction within the confined channels allows ions with opposite charges to selectively pass through.

Moreover, compared to bulk phase transport, the ionic transport conductivity is usually several orders of magnitude higher.Therefore, based on nanofluidic membranes, it is expected to achieve the optimal balance of ion selectivity and ion permeability for efficient energy conversion in salinity gradient power generation.

Classified by structure, nanofluidic membranes mainly include: one-dimensional porous, two-dimensional layered, and three-dimensional network structures.

In recent years, two-dimensional nanofluidic membranes with layered structures have shown great potential in salinity gradient energy conversion. The experimental values of salinity gradient energy output power of various two-dimensional nanofluidic membranes have reached the industrial application level (5Wm-2).

In addition, they also have the characteristics of simple preparation process and easy functional modification, and have important prospects in large-scale preparation and salinity application.

As a self-supporting thin film, two-dimensional nanofluidic membranes are composed of stacked and assembled charged two-dimensional nanosheets, so two-dimensional materials are the main body of its structure.From a commercial and environmental perspective, constructing two-dimensional nanofluidic membranes using natural raw materials is one of the good choices for realizing large-scale osmotic energy collection and utilization.

As an important carrier of two-dimensional materials, natural clay minerals have the advantages of easy exfoliation, abundant reserves, environmental friendliness, and low cost, making them a good choice for constructing two-dimensional nanofluidic membranes.

However, the surface charge of natural clay minerals is limited, and their mechanical strength is also very low. Therefore, clay membranes made from natural clay minerals still find it difficult to output a considerable salt difference electricity for a long time.

Based on this, Zhang Qianqian and the team explored the structural performance optimization and large-scale preparation technology of layered clay membranes.

With the help of natural intercalating agents, they achieved the stable construction of the clay sheet matrix and completed the large-area preparation of all-natural clay-based two-dimensional nanofluidic membranes.Derived from Nature, Applied in Industry

It is understood that this research began in 2020 when the team chose natural layered clay as the raw material to construct a cation-selective membrane, achieving a salinity gradient energy output of 0.15 Wm-2 without the addition of any modifiers[1].

Although the preliminary verification proved that clay can be used for salinity power generation, its energy output is far from meeting the actual application requirements (industrial standard of 5Wm-2).

Moreover, the mechanical strength of the clay membrane is insufficient, which limits the stability of long-term power generation.In response to the aforementioned issues, the research team utilized aramid nanofibers intercalated with clay flakes to construct a "brick-mortar structure" similar to that of a nacreous shell.

Through this method, the mechanical strength and pore charge density of the clay membrane were effectively enhanced, achieving a substantial increase in the salinity gradient energy output power (5.16 W/m^2), and the stability of long-term power generation was also improved [2].

Building upon this research, the team continued to work on the scaled-up preparation of the clay membrane.

However, the aramid they previously used is a synthetic fiber, and to exfoliate it into nanofibers, a large amount of organic solvents is required, which does not have economic and environmental benefits.

Moreover, the dispersion of aramid in aqueous solutions is poor, which is not conducive to the preparation of high-performance membranes on a large scale. Based on this, the research team hopes to find an environmentally friendly, well-dispersed in aqueous solutions, and low-cost nanofiber substitute.Through research, they discovered cellulose nanofibers, a type of natural nanofiber extracted from plant cellulose, which has environmental friendliness and cost advantages ($20/kg), and is suitable for industrial-scale production.

After determining the type of fiber, they continued to optimize the process parameters such as fiber diameter, precursor solution concentration, and the assembly ratio of clay flakes.

After compounding the clay with cellulose nanofibers, the research team successfully constructed this all-natural two-dimensional nanofluidic membrane.

In this regard, Zhang Qianqian said this is because:

On the one hand, the flexible nanofibers and the rigid nanosheets form a spatial interlocking structure that effectively enhances the stability of the two-dimensional nanofluidic membrane, which is the foundation for constructing a large-area, high-flux, high-strength (149MPa) film.On the other hand, the cellulose-rich negatively charged functional groups can significantly enhance the spatial negative charge density of the interlayer nanochannels, promoting the selective and rapid transport of cations in the two-dimensional nanofluidic membrane.

Subsequently, the team continued to explore the large-scale preparation of uniform and stable all-natural clay membranes.

However, when the membrane was scaled up, a series of problems followed, such as the upgrade of manufacturing equipment, optimization of the precursor solution before film-making, selection of the film-making substrate, and adjustment of process parameters.

After a year of equipment improvement and optimization of the preparation process, they finally produced a uniform and stable large-area all-natural layered clay membrane, and achieved high-efficiency salinity difference power generation (>8Wm-2) and long-term power generation stability (>30 days), laying the foundation for large-scale osmotic energy collection and utilization.

So far, the team has finally achieved the large-scale preparation of high-performance all-natural clay membranes and the application of salinity difference power generation.How to evaluate natural resources that are friendly to the environment has become a key issue to consider for the entire chain of green sustainable development, from membrane manufacturing to energy collection.

Based on the research characteristics of the life cycle assessment of materials in the School of Materials Science and Engineering at Beijing University of Technology, where the research team is located, they believe that it is necessary to trace the front end of the synthesis of different materials and conduct a life cycle assessment and technical and economic analysis of the entire manufacturing process of the membrane material.

Subsequently, the team conducted a survey visit for several months, which provided accurate data related to material production and carried out a systematic life cycle assessment, confirming the advancement of the results.

Zhang Qianqian said: "The achievement of these results is the result of the joint efforts of teachers and students. The main person who completed this study is my doctoral student Tang Jiadong, who is also the first master's student I have had since I came to Beijing University of Technology. After graduating from the master's program, he chose to stay and continue to study for a doctorate and is currently a second-year doctoral student."

After enrolling in 2019, in the second semester of the first year of the master's program, Tang Jiadong published an English review paper in Nanoscale and a Chinese review paper in the "Science Bulletin" respectively.Despite having achieved many results, Tang Jiadong did not make smooth progress in the initial experiments of this topic. The repeated failure in synthesizing new materials also made him feel depressed for a while.

"In order to boost his confidence, I suggested that he should stop the experiment first, calm down and review the materials, and consolidate the foundation. Through this, he had a deeper understanding of the topic, and the experiment also became smooth, and finally successfully synthesized the modified clay membrane material," said Zhang Qianqian.

In the end, the relevant paper was published in Nature Communications[3] with the title "All-natural 2D nanofluidics as highly-efficient osmotic energy generators".

Tang Jiadong and Wang Yun are co-first authors, and Zhang Qianqian, as well as Zheng Zilong and Gu Yifan from the same school, served as co-corresponding authors.

At the same time, they found in the research that there are still many problems in the current system, such as how to achieve high-flux design of clay membranes and how to improve the power of large-area membranes.Therefore, they will subsequently construct layered clay membranes with higher ionic fluxes.

In addition, the current clay membranes have a two-dimensional layered structure. For ionic transmembrane transport, it is necessary to sequentially pass through vertical platelet parallel channels and horizontal inter-platelet channels.

The tortuous transmission path leads to a higher ionic transmission resistance, which limits the further enhancement of ionic flux.

In fact, this is also a common problem with two-dimensional nanofluidic membranes and is the main reason why the efficiency of salinity gradient power generation is difficult to substantially improve.

To address this issue, they will focus on strategies such as reducing membrane thickness and increasing the density of vertical channels to reduce internal resistance, striving to construct high-flux clay membranes and truly enhance the power generation capability of salinity gradient energy.At the same time, based on the economic benefits, resource benefits, and environmental benefits of the all-natural clay membrane, they will continue to explore the mass production process of layered clay membranes.

Committed to using natural raw materials, they develop high-performance ion-selective permeable membrane materials to meet the application needs in the field of energy and the environment.