Constant technological advancement has driven an ever-increasing demand for new smart materials, particularly in energy sectors. For this reason, promising materials with superior electrochemical characteristics, such as Polyoxometalates composites, have long been valued by scientists due to their wide range of uses and enormous potential to fulfill the demands of modern industry.
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Polyoxometalates-Soft Matter Composites: An Overview
Polyoxometalates (POMs) are a distinct type of anionic metal-oxygen cluster formed from transition metals with high valence. They are nanoscale-sized molecules with clearly defined molecular structures.
POMs offer unique advantages in various fields, such as biomimetics, molecular electronics, theranostics, energy conversion, and catalysis. However, they have drawbacks such as high crystalline energies (difficult to process), limited solubility in organic media, and poor recyclability in liquid media (low catalytic efficiency), severely restricting their applicability.
In addition, their molecular composition hinders technological implementation due to increased leaching, degradation, and low reactivity, especially when demanding applications like electrolysis, heat catalysis or extremely acidic/basic solutions are targeted. Therefore, POM research has increasingly focused on immobilizing POMs on diverse substrates.
While much early research focused on semiconductor and metal oxide supports, the latest research has shown that the integration of POM in soft matter matrices such as hydrogels, stimuli-responsive matrices, and polymers has led to significant advances in multifunctional composite design.
How are Polyoxometalates Embedded in Soft-Matter?
Two primary strategies for integrating polyoxometalates into soft matter matrices are covalent (Class-I hybrids) and non-covalent bonds (Class-II hybrids).
Integrating polyoxometalates in soft matter organic films is viable for various applications. This provides highly stable systems, and adding functional polymers improves the polyoxometalates' characteristics.
Polyoxometalate soft matter composites’ distinctive qualities, such as oxygen-rich surfaces, strong acidity, chemical adaptability, electron-accepting capacity, and their wide range of sizes, nuclearities, and structures, make them potential building blocks for manufacturing functional materials.
They are highly stable during electrochemical application because of their quick reversible multielectron redox transitions and adjustable redox characteristics. This makes them an attractive material for developing proton exchange membranes for fuel cell systems.
Integrating polyoxometalates into soft matter polymeric building blocks has been a common strategy for producing hybrid materials for a long time. While this technique easily offers access to hybrid materials, there are certain downsides, such as cargo leakage due to anion exchange, polymer composition limitations due to protonation sites, and poor stability in some situations.
Industrial Applications of Polyoxometalates Soft Matter Composites
The persistent growth of bacterial multidrug resistance emphasizes the need to discover new antibacterial substances.
Polyoxometalates soft matter composites have promising antibacterial properties and are thus being explored as potential future medications for treating bacterial infections.
Polyoxometalates have been used in various biological applications, including antiviral, antibacterial, and anticancer treatments. However, for in vivo treatment, reducing their toxicity, enhancing their selectivity, and modifying their mechanism of action are essential. Therefore, polyoxometalates soft matter composites are ideal for achieving these objectives.
Stimuli-Responsive Materials and Sensing Applications
Polyoxometalates are highly sensitive to pH, temperature, electricity, and light changes. By incorporating conductive soft organic polymers into thin sheets, polyoxometalates may also be used in sensors, theranostics, catalysis, and redox.
This enables substrates to directly access redox-active polyoxometalates via the conductive polymers.
In electronics, polyoxometalate soft matter composites have received minimal attention, with most investigations concentrating on the deposition of polyoxometalates on semiconductor or metal surfaces.
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Soft-matter polymers' flexibility, scalability, and processability, combined with polyoxometalates' adjustable redox and spin states, have demonstrated significant promise for developing nano-scale devices and modules for molecular electronics.
Engineering durable and useful soft polymers that contribute to the device's performance, and understanding how to manage the polyoxometalate distribution within such hybrid materials, is crucial for the future development of molecular electronics.
Catalysis and Energy Storage
Catalysis is the most popular application of polyoxometalates. It uses these molecules' capacity to quickly receive electrons and create electrochemically reduced clusters. Due to their unique energy storage and conversion properties, polyoxometalate soft matter composites are used in fuel cells, supercapacitors, batteries, and electro-catalysis.
Summary and Future Outlooks
Polyoxometalates soft matter composites are flexible materials that integrate inorganic and organic chemistry. They have created new opportunities to use the synergies in these domains to create applications in molecular electronics, energy storage, and biomedicine. However, their molecular to macroscopic characterization remains challenging.
Experimental and theoretical efforts are being combined to better understand the properties of a single polyoxometalate site within a particular soft matter matrix, which is essential to gaining insight into the complicated behavior of these materials.
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References and Further Reading
Kruse, J. H., Langer, M., Romanenko, I., Trentin, I., Hernández‐Castillo, D., González, L., ... & Streb, C. (2022). Polyoxometalate‐Soft Matter Composite Materials: Design Strategies, Applications, and Future Directions. Advanced Functional Materials, 2208428. https://doi.org/10.1002/adfm.202208428
Gao, Y., Choudhari, M., Such, G. K., & Ritchie, C. (2021). Polyoxometalates as chemically and structurally versatile components in self-assembled materials. Chemical science, 13(9), 2510–2527. https://doi.org/10.1039/d1sc05879g
Roy, S. (2014). Soft-oxometalates beyond crystalline polyoxometalates: formation, structure and properties. CrystEngComm, 16(22), 4667-4676. https://doi.org/10.1039/C4CE00115J
Wang, D., Liu, L., Jiang, J., Chen, L., & Zhao, J. (2020). Polyoxometalate-based composite materials in electrochemistry: state-of-the-art progress and future outlook. Nanoscale, 12(10), 5705-5718. https://doi.org/10.1039/C9NR10573E
Zhai, L., & Li, H. (2019). Polyoxometalate-Polymer Hybrid Materials as Proton Exchange Membranes for Fuel Cell Applications. Molecules (Basel, Switzerland), 24(19), 3425. https://doi.org/10.3390/molecules24193425
Zhang, B., Yin, P., Haso, F., Hu, L., & Liu, T. (2014). Soft matter approaches for enhancing the catalytic capabilities of polyoxometalate clusters. Journal of Cluster Science, 25(3), 695-710. https://doi.org/10.1007/s10876-013-0643-7