A research team has proposed a new strategy to use a kind of molecules called zwitterions-polyoxometalates to optimize and broaden practical applications in energy devices such as fuel cells and supercapacitorsTheir findings are published in the peer-reviewed journal Polyoxometalates on 02 February, 2023.
In their strategy, the zwitterions are used to optimize the application of polymetallic oxygen cluster electrolytes which are composed of different oxygen-containing acids with high proton conductivity.
“Using a zwitterion to optimize the application of polyoxometalate-based electrolytes in solid-state capacitors can improve the proton conductivity, cycling stability and capacitance performance,” said Hong-Ying Zang, a professor in the Department of Chemistry at Northeast Normal University. The team’s work is an innovative strategy for developing the applications of zwitterion and polyoxometalates in supercapacitors.
Solid-state electrolytes based on polyoxometalates have high ionic conductivity and excellent redox activity. They hold strong potential in applications as solid-state electrolytes for next-generation energy devices or wearable electronics. However, as a new solid-state electrolyte, polyoxometalates still have some problems, such as low solubility in polymers, specifically in solid-state supercapacitors. “Zwitterion, with its high chemical stability and non-volatile properties, can effectively solve this problem,” said Zang. This is achieved by the electrostatic force between the positive charge of zwitterion and the anions of polyoxometalatesthat promotes the dissociation of polyoxometalates and further enhances its solubility. In addition, the negatively charged part can combine with the cations, which is conducive to the rapid ion transport.
A zwitterion is a molecule containingboth positive and negative electrical charges. They are sometimes called “inner salts.” The zwitterions’ cations and anions are bound by covalent bonds. Scientists have studied zwitterions for use in surfactants, surface coatings and catalysts, and as building blocks for liquid crystals and block copolymers. Zwitterions are also non-volatile, charged, and non-migratory, and are mainly used as electrolyte additives or added to polymer gels, showing great potential application prospects. Zwitterionic structures are diverse, and different ionic groups in their structures affect their chemical, thermal, and electrochemical properties. Therefore, so it was crucial for the team to choose a suitable zwitterionic structure to optimize the electrolyte system as much as possible.
The team chose a zwitterion dubbed MIMPS as a molecular additive to improve the solubility and dissociation of polyoxometalates in gel electrolytes. The zwitterion is used to dissociate, or break up, the polyoxometalates, relying on the push and pull of positive and negative charges to interact with the polyoxometalates anion. This interaction reduces the binding effect on protons and releases more active protons, which improves proton conductivity.Next the team investigated its application as a solid electrolyte in solid-state supercapacitors. “The solubility has been improved and meanwhile the proton conductivity of polyoxometalates-based electrolytes has been improved about three times with the zwitterion modification,” said Zang. With the addition of the zwitterion, the conductivity of the gel electrolyte and the performance of the solid-state capacitors were both improved.
Zwitterions support the design of new materials as valuable alternatives to ionic materials traditionally used in electrolytes. The vast majority of zwitterions currently used in electrochemical devices are a class of materials with sulfonic acid functional groups. When zwitterions are added to solid electrolytes they increase the target ion transport, improve the counter electrode stability, and formmore conductive solid electrolyte interfaces.
Looking ahead the team sees the possibility of wider applications for the use of these zwitterion polyoxometalate-based electrolytes. “We are trying to achieve the development of other types of polyoxometalates in electrolytes. We eventually want to use polyoxometalates electrolytes in more electrochemical devices, including liquid or solid-state batteries and capacitors,” said Zang.
The research team includes Dongming Cheng, Zhixin Gao, Wenwen Wang, Siqi Li, Bo Li, Hong-Ying Zang, who are part of the Faculty of Chemistry at Northeast Normal University.
The research is funded by the National Natural Science Foundation of China, Natural Science Foundation of Jilin Province, Natural Science Foundation of Department of Education of Jilin Province, and the Fundamental Research Funds for the Central Universities.