Recently, the three-dimensional hydrogel membrane designed by the team of Associate Professor Wang Jian of Chengdu University of Technology has broken the limitations of traditional hydrogels in terms of pore size and thickness, and effectively improved its energy conversion efficiency in practical applications.
Figure | Wang Jian (Source: Wang Jian)
First, they used the abundant surface charge in sodium alginate and sulfonic acid salts to effectively improve the power density of the hydrogel.
However, the prepared hydrogel membrane is soft and lacks mechanical properties. To enhance its mechanical properties, the researchers soaked it in a solution of different metal ions.
They found that the hydrogel membrane soaked in a zirconium chloride solution had a significant effect, and the color of the hydrogel changed from transparent to white and became tougher.
Surprisingly, the soaked hydrogels showed significant improvements in performance tests, as well as improvements in ion selectivity, energy conversion efficiency, and ion conductivity.
The hydrogel membranes designed by the research team are expected to play a role in many fields.
First, thanks to its excellent cation selectivity and permeability, it can be used to develop efficient osmotic power generation systems to provide renewable electricity to remote areas and islands.
Secondly, as a high-efficiency separation membrane, it can improve the utilization rate of water resources in the process of seawater desalination and wastewater treatment.
In addition, the membrane can be used to create ion-selective sensors to monitor changes in cation concentration in water in real time, due to the membrane's selective permeability of specific ions.
In the biomedical field, it is expected to be used as a drug delivery system to improve drug release efficiency and control, using its biocompatibility and excellent texture.
In addition, the application of membrane materials in environmental monitoring is also promising, which can effectively detect pollutants in water bodies and provide real-time monitoring solutions based on their high ionic conductivity and stability.
These potential applications may drive the practical use of the material and promote the development of related industries.
日前，相关论文以《三维水凝胶膜提升渗透能转化效率：锆离子交联引起的空间限制和电荷调节》（Three-dimensional hydrogel membranes for boosting osmotic energy conversion: Spatial confinement and charge regulation induced by zirconium ion crosslinking）为题发表在 Nano Today[1]。
Wu Caiqin, a master's student at Chengdu University of Technology, is the first author, and Associate Professor Wang Jian, Lecturer Chen Xianfei, and Researcher Kong Xiangyu of the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences serve as co-corresponding authors.
图丨相关论文（来源：Nano Today）
Break the contradiction between ion permeability and selectivity
In recent years, materials science and nanotechnology have evolved, and salinity gradient energy has gradually gained a lot of attention based on the advantages of cleanliness, sustainability, and renewability.
It obtains energy from the difference in salt concentration between seawater and freshwater, and is a green and environmentally friendly form of energy.
The first demonstration of extracting the energy of the salinity gradient from the difference in salt concentration between seawater and river water, reverse electrodialysis, originated in the 1970s and was studied in depth by researchers in the decades that followed.
This technology, which relies on differences in salinity between different water bodies, offers new avenues for energy development and advances in the field of renewable energy.
Reverse electrodialysis technology typically relies on ion exchange membranes, but in practice, it often faces challenges such as low mass transfer efficiency, limited membrane porosity, concentration polarization, and high membrane resistance.
These factors limit the efficiency of energy extraction and become a bottleneck for the further development of the technology.
In recent years, researchers have developed biomimetic nanochannel membranes with one-, two-, and three-dimensional pore structures to improve the efficiency of osmotic energy collection.
These biomimetic membranes optimize the ion transport path by mimicking the ion channel structure in nature, overcoming the limitations of traditional ion exchange membranes, thereby significantly improving the conversion efficiency of salinity gradient energy.
Although one-dimensional and two-dimensional materials have potential in salt differential energy power generation, their practical application effects are limited due to a series of problems such as insufficient ion selectivity, poor structural stability, easy clogging of pores and difficulty in scale.
Fortunately, materials with a three-dimensional interconnected network structure are relatively more cost-effective, and their pore density, ion transport paths, and surface charge can be broadly tuned to optimize performance and improve the efficiency of power generation with salt differential energy.
Hydrogels have a unique three-dimensional interconnected porous network that enables rapid charge and mass transport, resulting in improved electrical conductivity.
However, the limitations of traditional hydrogels in terms of pore size and thickness, coupled with poor interface effects, often result in low output power density, which limits their energy conversion efficiency in practical applications.
（来源：Nano Today）
To a certain extent, this study solves the above problems and successfully breaks the contradiction between ion permeability and selectivity.
The reviewers commented on the study: "This manuscript demonstrates a zirconium modified sodium alginate/potassium 3-sulfacrylate hydrogel membrane with remarkable cation selectivity and efficient osmotic energy harvesting capacity. The theme is very appealing and the results obtained are reasonable......"
The serendipity and inevitability of scientific research results
In recent years, many research groups at home and abroad have carried out the application of a variety of materials for osmotic energy conversion.
Prior to this work, the team had also carried out research and published papers on the use of other novel materials for osmotic energy conversion.
However, on the whole, the resulting osmotic energy conversion power density is unsatisfactory.
As a material with a three-dimensional interconnection network structure, hydrogel was first used for osmotic energy conversion by the research group of researcher Wen Liping of the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences in 2020, showing good application potential.
Subsequently, some research groups have successively used hydrogels and other materials to construct composite materials for permeability energy conversion.
Based on the characteristics of various materials, hydrogel is undoubtedly an ideal material for osmotic energy conversion, but its potential has yet to be explored.
Based on this background, the research group decided to focus on the development of novel hydrogel membranes with three-dimensional network structures to improve their osmotic energy conversion efficiency.
Prior to the study, they conducted extensive literature research and gained insight into the properties and applications of existing hydrogel materials.
In the course of their preliminary research, they found that materials rich in charged groups such as sulfonic acid, carboxyl and phosphate groups have great potential for osmotic energy harvesting.
Therefore, they conducted several experiments using materials available in the laboratory and finally identified sodium alginate and potassium 3-sulfopropylacrylate as the main hydrogel monomers.
In order to optimize the performance of the membrane, they adjusted the formulation and preparation conditions of the membrane based on the experimental results. This stage of efforts has enabled the membrane to exhibit better stability and adaptability under different environmental conditions.
The preliminary experimental results showed that the prepared hydrogel membranes had good potential. However, due to the presence of a large number of hydrophilic groups, the mechanical properties of hydrogel membranes are weak.
In the course of this research, there was one thing that impressed the research members, and it can even be said to be an "unexpected discovery".
One day, when Wu Caiqin was reviewing the literature, he learned that the introduction of metal ions may improve the conductivity of the hydrogel film.
Therefore, she chose any concentration of zirconium dichloride (ZrOCl₂) solution for hydrogel immersion experiments.
After about half an hour, when she looked at the soaked hydrogel membrane, she noticed that its color had changed from clear and colorless to white.
This phenomenon aroused her curiosity, and she initially speculated that it might be caused by the chelation of metal ions and charged groups in the hydrogel.
It was out of curiosity about this change that she immediately tested the performance of the hydrogel membrane.
The team was amazed by the results – the output current value was nearly twice as high as the untreated original hydrogel membrane, which made them realize that the introduction of ZrOCl₂ might have brought about some unique change in the hydrogel membrane.
Based on previous experiments, they began to further adjust the concentration of the zirconium ion solution and the reaction time to optimize this gain effect.
At the same time, researchers are also trying to introduce other metal ions, such as calcium, copper, aluminum and other ions. However, the results showed that only the hydrogel membranes soaked in ZrOCl₂ solution showed such significant changes.
To better understand this phenomenon, they reviewed a large number of literature and found that ZrOCl₂ decomposes in water into a [Zr₄(OH)₈(H₂O)₁₆]⁸⁺ cluster with an octahedral structure that forms stable organic ligands with carboxyl and sulfonic acid groups in hydrogels.
Subsequently, the team used density functional theory to delve into the potential mechanisms underlying the improved performance of hydrogel membranes incorporated with Zr⁴⁺.
Theoretical calculations show that the enhanced power density of SA/SPAK/Zr⁴⁺ membranes is closely related to the enrichment of Cl⁻ in the membranes after the introduction of Zr⁴⁺. This discovery went from chance to in-depth theoretical analysis, which was not only an experimental success, but also provided them with more directions to explore the properties of materials.
In further testing, the researchers conducted an in-depth evaluation of the modified hydrogel membrane and confirmed that it had significant improvements in ion selectivity, energy conversion efficiency, and mechanical properties. These results provide a solid foundation for the practical application of membranes.
In subsequent experiments, they also experimented with soaking different hydrogel materials. It was found that ZrOCl₂ solution also significantly improved the performance of other hydrogels containing carboxyl or sulfonic acid groups.
The research group also found that long-chain polymers with carboxyl groups generally bind better to zirconium ions, which significantly improves the performance of hydrogels. This discovery helped them further optimize the performance of their hydrogel membranes.
In the future, they will also focus on discovering materials with similar structures to [Zr₄(OH)₈(H₂O)₁₆]⁸⁺ for further research and exploration in the acquisition of permeable energy for hydrogel membranes.
Resources:
1.Wu,C., Wang,J. et al. Three-dimensional hydrogel membranes for boosting osmotic energy conversion: Spatial confinement and charge regulation induced by zirconium ion crosslinking. Nano Today 58,102468(2024). https://doi.org/10.1016/j.nantod.2024.102468
Typesetting: He Chenlong, Liu Yakun