New Type of Electrolyte Can Boost Efficiency of Supercapacitors Used for Energy Storage

Researchers at the Massachusetts Institute of Technology (MIT) and other institutions have come out with a new class of liquids that will enhance the efficiency and stability of supercapacitors. They have also developed a method to reduce the flammability of these devices.

“This proof-of-concept work represents a new paradigm for electrochemical energy storage,” said the researchers in their paper.

Supercapacitors are electrical devices that are used to store a large amount of electrical charge. They are known as double-layer capacitors or ultracapacitors. They store energy and release energy and need a layer of electrolyte.

Supercapacitors, electrical devices that store and release energy, need a layer of electrolyte — an electrically conductive material that can be solid, liquid, or somewhere in between. Electrolytes are very crucial components in the performance of electrochemical supercapacitors.

Ionic liquids are salts in which the ions are not very coordinated. This results in the solvents being liquid below 100°C or at room temperature. This research team has identified and added a compound to these ionic liquids which act as a surfactant, like the materials used to dissolve oil spills. Adding these compounds to ionic liquids results in a material with new properties like a higher viscosity.

Xianwen Mao, the lead author of the paper, says, “It’s hard to imagine that this viscous liquid could be used for energy storage, but what we find is that once we raise the temperature, it can store more energy, and more than many other electrolytes.”

With a regular ionic liquid, the rise in temperatures decreases viscosity. This, in turn, increases the energy storage capacity. In this instance, when the ionic liquid is combined with the surfactant like compound, the viscosity is higher than other electrolytes, the energy storage capacity increases with increasing temperature. Therefore, the energy density of the new material is higher than most conventional electrolytes. The stability and safety of the material are also higher. Energy density is the amount of energy stored in a system per unit volume.

The molecules in this new material behave in an anomalous fashion, thus rendering these useful properties to the liquid. The molecules have a tail on one end, and they line up with head facing toward or away from the electrode. The tails all come together in the center, and this formation is called a self-assembled nanostructure.

“The reason why it’s behaving so differently from conventional electrolytes is because of the way the molecules intrinsically assemble themselves into an ordered, layered structure where they come in contact with another material, such as the electrode inside a supercapacitor,” said T. Alan Hatton, a professor of chemical engineering at MIT and the paper’s senior author.

Overscreening is a process where the surface layer of the electrode (ionic liquid) has more ions than charged atoms resulting in a loss of storage efficiency. In the newly discovered substance, there are more charged particles on the surface layer, thus increasing efficiency.

The new class of materials has been termed Surface-active Ionic Liquids (SAILS) by the researchers. These materials can be applied in high-temperature energy storage; it could be used in lithium-ion batteries; they can improve the performance of supercapacitors. Supercapacitors are sometimes used in place of battery systems in electric vehicles to give an extra boost of power.

“Using the new material instead of a conventional electrolyte could increase its energy density by four or five,” said Mao. These supercapacitors using SAILS electrolytes can replace batteries in electric vehicles, grid-level energy storage, and personal electronics. The material can be used in separation processes lie carbon dioxide capture, resource recovery from waste streams, and so on. SAILS are high conductors of electricity, increasing their range of application potential.

“It is a very exciting result that surface-active ionic liquids (SAILs) with amphiphilic structures can self-assemble on electrode surfaces and enhance charge storage performance at electrified surfaces,” says Yi Cui, a professor of materials science and engineering at Stanford University, who was not associated with this research.

Last year, in a paper titled “Evaluating the Causes of Cost Reduction in Photovoltaic Modules” published the journal Energy Policy, Massachusetts Institute of Technology’s (MIT) associate professor Jessika Trancik, postdoctoral Goksin Kavlak, and research scientist James McNerney tried to ascertain the reasons behind the rapid cost decline recorded over a span of forty years.

The team studied multiple areas analyzing what caused the savings, such as policies and technology changes. It found that government policies played a critical role in the reduction of technological cost across markets.

Image credit: MIT