New proposed chemistry policy sheds light on more efficient use of energy resources

A research team has developed a new strategy to use a type of molecule called zwitterions-polyoxometalates to improve and extend the processes that work in energy devices such as fuel cells and supercapacitors. Their findings were published in Polyoxometalates.

In their strategy, zwitterions are used to improve the use of polymetallic oxygen cluster electrolytes made of different oxygenated acids with high proton conductivity.

“Using a zwitterion to improve the use of polyoxometalate-based electrolytes in solid capacitors can improve proton conductivity, cycling stability and capacity performance,” said Hong-Ying Zang, professor in the Department of Chemistry at Northeast Normal University. The team’s work is a new strategy to develop the use of zwitterion and polyoxometalates in supercapacitors.

Solid electrolytes based on polyoxometalates have high ionic conductivity and excellent redox activity. They have great potential in applications such as solid 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, especially in solid-state supercapacitors. “Zwitterion, which has high chemical stability and unstable properties, can effectively solve this problem,” said Zang.

This is achieved by the electrostatic force between the positive charge of the zwitterion and the anions of the polyoxometalates which promotes the dissociation of the polyoxometalates and improves its solubility. In addition, the negative part can combine with cations, which is suitable for fast ion transport.

A zwitterion is a molecule that has positive and negative electrical charges. They are sometimes called “internal salts.” The zwitterions’ cations and anions are bound together by covalent bonds. Scientists have studied zwitterions used in surfactants, coatings and catalysts, and as building blocks for water crystals and block copolymers.

Zwitterions are also non-volatile, non-charged, and non-migratory, and are widely used as electrolyte additives or added to polymer gels, showing great promise for application. Zwitterionic structures are diverse, and the different ionic groups in their structures affect their chemical, thermal, and electrochemical properties. Therefore, it was important for the team to choose an appropriate zwitterionic structure to optimize the electrolyte system as much as possible.

The team chose a zwitterion called MIMPS as a molecular additive to improve the solubility and separation of polyoxometalates in gel electrolytes. The zwitterion is used to separate, or dissociate, polyoxometalates, depending on the push and pull of positive and negative charges interacting with the anion of the polyoxometalates. This interaction reduces the binding effect on protons and releases more active protons, which improves proton efficiency.

The team then investigated its use as a solid electrolyte in solid state supercapacitors. “The solubility is improved and meanwhile, the proton conductivity of electrolytes based on polyoxometalates is improved about three times by zwitterion modification,” said Zang. With the addition of the zwitterion, the conductivity of the gel electrolyte and the performance of the solid capacitors were both improved.

Zwitterions support the design of new materials as significant alternatives to the ionic materials commonly used in electrolytes. Most of the zwitterions currently used in electrochemical devices are a class of materials that contain sulfonic acid functional groups. When zwitterions are added to solid electrolytes they increase target ion transport, improve electrode stability, and create solid electrolyte interfaces.

Looking ahead, the group sees the potential for the use of electrolytes based on zwitterion polyoxometalate. “We are trying to achieve the development of other types of polyoxometalates in the electrolyte. Finally we want to use polyoxometalates electrolyte in many electrochemical devices, including liquid or solid batteries and capacitors,” said Zang.