It’s the dream of practically everyone studying energy storage: the supercapacitor, a device that combines the high energy capacity of batteries with the rapid discharge and recharge times of a capacitor. While some devices could be called "supercapacitors," results out of the Fluid Interface Reactions, Structures and Transports (FIRST) Energy Frontier Research Center are providing a route towards truly high capacity, ultra-fast charging energy storage. “We all want our batteries to be charged quickly, last forever—longer than devices that they power—and store enough energy for practical use,” said Yury Gogotsi, professor at Drexel University and one of the leaders with FIRST.
The kind of supercapacitors on the market today, called electrical double-layer capacitors, are very useful. They’re often found in small wireless devices (which need quick bursts of power) and inside computers and servers (where they act as emergency power). “They can store energy quickly—in seconds—and can survive a million cycles or more,” explained Gogotsi, “but they store much less than 1% of the energy of batteries.”
Researchers with FIRST see the potential for a new generation of supercapacitors that exploit the ultra-fast chemical reactions of a class of sheet-like nanomaterials called MXenes. Combining computer simulations and experiments, scientists have begun to identify the most promising ways to improve energy storage using the special characteristics of MXenes. These newly developed nanomaterials have been synthesized in the lab, but the full breadth of their capabilities is still being explored. By leveraging their knowledge of surface chemical reactions, researchers with FIRST have already built supercapacitors that can supply large amounts of power while still being recharged in just minutes or even seconds.
Building next generation supercapacitors
Within FIRST, a special class of supercapacitor is being explored—a pseudocapacitor. Pseudocapacitors present an interesting middle ground between a true battery and a true capacitor, and may be able to offer the best of both. Like a normal capacitor, pseudocapacitors store charge only on their electrode surfaces. This is in contrast to a battery, in which charges are stored throughout the volume of the electrode. The difference with pseudocapacitors is that rather than letting the charges just stick to the surface anywhere, an electrochemical reaction tightly binds these charges. The binding reaction in pseudocapacitors is very similar to what happens in a battery, only much faster because the reaction only needs to happen at the electrode’s surface instead of deep within it.
Through their fundamental investigations of surface reactions, teams with FIRST are tackling the supercapacitor problem in a new way. For any battery, there are specific reactions that allow electric charges to be transferred between the (solid) electrodes at either end and the (typically liquid) electrolyte in between them. By studying the reactions that occur at the interface where the liquid electrolyte meets the solid electrode, researchers gain enormous insight into the way an energy storage device behaves under highly demanding loads.
Compared to an existing supercapacitor (the electrical double-layer kind), the dramatic increase in energy storage you get with pseudocapacitors has prompted a lot of people to consider whether pseudocapacitors might be able to compete with batteries. Pseudocapacitors retain all the incredibly useful features of capacitors, such as incredibly fast recharge times and a long lifetime. Researchers with FIRST recently reported on a pseudocapacitor based on nanosheets of MXenes that can retain almost 90% of its storage capacity after recharging 20,000 times. That would be like a cell phone battery that was still good after 55 years of use.
The special properties of MXenes that enable ultra-fast energy storage
MXenes were first discovered in 2011 after several years of effort to find the right combination of materials. Researchers with FIRST use specially developed methods to break down common ceramic materials into MXene nanosheets. These nanosheets are then aligned together and built into pseudocapacitor electrodes and other devices. With insight gained from studies of the liquid-solid electrochemical reactions, researchers match the best electrolytes for each MXene material to make better supercapacitors.
The MXenes themselves are simple chemical compounds, like titanium carbide (Ti3C2). Their general chemical formula can be written as MnXn-1 (hence their name), where M denotes a transition metal (like titanium or niobium) and X denotes either carbon or nitrogen. The unique behavior comes from their 2-D, layered crystal structure in which each nanosheet is held together by weak forces. These weak forces leave enough space for charge to be stored between the nanosheets, which means there’s a tremendous amount of surface available. These wide spaces between layers also make it easy for charges to find a resting position on the surface of the MXene nanosheets, making them an obvious choice for energy storage devices.
“We are now exploring a variety of MXene and other 2-D materials for fast electrochemical energy storage, with the goal to understand the factors that limit the amount of charge stored,” explained Gogotsi.
Recent work by researchers with FIRST has shown that it’s not always as simple as finding a new MXene. The choice of electrolyte can dramatically impact the width of the space between the layers, either increasing or reducing it. With potentially millions of combinations of different MXene elements and electrolytes, understanding how the properties change requires a huge, collaborative effort between experimentalists and theorists. A team with FIRST just recently finished a computational study of 24 MXene materials to identify which ones would be promising to make next, a feat that could have taken months in the lab.
Designing the future of energy storage
To approach the lofty goals of delivering high-power and high-energy storage, researchers are exploring a range of possible mechanisms to improve the performance of batteries and their less-considered counterparts, supercapacitors. Batteries can usually store a lot of energy but are slow and cannot supply high currents, while capacitors are extremely fast but hold less charge compared to batteries. While each type of device has already enabled countless technologies on their own, we continue to strive for improvement.
“I think the battery and supercapacitor communities are moving to the same point, but from different directions,” said Gogotsi. As the teams with FIRST continue to study pseudocapacitors, their work has immediate parallels to challenges in battery research. “At the end of the day, we are aiming at creating versatile, high-power, high-energy storage solutions for current and future technologies. Therefore, whatever we learn about the electrochemical behavior of MXenes may be of use by the batteries community.”
In the near term, FIRST’s researchers have a lot of ground to cover exploring the energy storage properties of MXene nanomaterials. “We don’t even know the fundamental limits for those yet,” Gogotsi said. “With such a new material family, there are many open questions about how to control their electrochemical behavior. If we determine what limits the performance and how far we can push those limits for energy storage, we will be able to create faster and better energy storage devices.”
