03-22 supercapacitors

[Image above] For decades researchers and technologists have regarded batteries and supercapacitors (pictured) as two distinct energy storage devices. But studies on new and emerging energy storage devices suggest that energy storage mechanisms are more fluid than previously believed. Credit: Windell Oskay, Flickr (CC BY 2.0)

Every day is a good day for pizza, but last Monday specifically was a great time to order your favorite pie to honor one of the most recognized science-based celebrations of the year.

My high school math teachers celebrated Pi Day by offering one point of extra credit for every five digits of pi memorized. My peers used various techniques to memorize the most digits, with the most popular method being to memorize the digits in groups rather than as individual numbers.

Grouping or categorizing data is an effective way to track large amounts of information. However, relying too much on established categories can be detrimental when new information arrives that does not fit nicely into an existing box. Thus, for some situations, arranging information on a spectrum instead of as discrete sets allows for better insights.

That logic is the basis of a recent perspective piece published in Nature Energy. In the article, an international team of researchers from the United States, Germany, and France advocate for a more nuanced view of electrical energy storage mechanisms to help advance next-generation energy storage technologies.

Electrical energy storage systems are typically divided into two main types: batteries and electrochemical capacitors (also called supercapacitors). These systems store electrical energy in two distinct ways.

Batteries store energy using chemical mechanisms known as Faradaic processes. Charged particles move between a battery’s electrodes through the electrolyte, leading to redox reactions at the interfaces. These reactions cause a change in the molecular or crystalline structure of the electrode materials.

On the other hand, energy storage in supercapacitors is based on a physical mechanism. An electrostatic attraction temporarily entraps charged particles within and on the surface of the electrodes. When charged particles are transferred between the electrodes, no chemical or phase changes take place.

Each system has pros and cons. Batteries’ use of chemical reactants allows them to have higher energy density but slower charge/discharge speeds. Conversely, the physical mechanism through which supercapacitors operate means they have lower energy density but faster charge/discharge speeds.

For decades, researchers and technologists have regarded these two energy storage mechanisms as discrete categories. The authors of the new perspective piece, however, argue these mechanisms should instead be viewed as two ends of a continuous spectrum.

Their argument is based on the observation that some newer and emerging energy storage technologies appear to display pseudocapacitance behavior, or charge storage based on both chemical and physical mechanisms.

While development of the pseudocapacitance concept is well-documented, “some scientists have attempted to reject pseudocapacitance completely, claiming that there are only these two extreme cases and everything else is a superposition of two mechanisms acting in parallel,” Yury Gogotsi, Distinguished University and Bach professor in Drexel University’s College of Engineering, says in a Drexel press release.

But this insistence on binary categorization can hamper scientists’ approach to powering new technologies, which may be best served by energy storage devices that are neither ideal batteries nor supercapacitors.

“New industries that require flexible, transparent, conformal, wearable energy storage, devices combined with energy harvesting, and other unconventional electrical energy supplies will benefit greatly from the new agile energy storage. … So, it will be very important to acknowledge and work to characterize these new devices as existing within a spectrum, rather than falling somewhere short of either end of it,” Volker Presser, Saarland University professor and former research fellow in Gogotsi’s group at Drexel, says in the press release.

The researchers write that close collaboration between theory and experiment will be required to achieve this goal.

“From the modelling side, ab initio molecular dynamics augmented with machine-learning force fields are promising to gain an understanding of the coupling between electronic structure and ion distribution in confinement. Experimentally, the use of operando techniques to track structural or chemical changes of the electrode materials during electrochemical operation at timescales relevant to the charge storage process is particularly valuable, such as operando X-ray diffraction or X-ray absorption. Techniques capable of probing the composition, structure, and dynamics of the confined electrolytes are also critical, such as in situ NMR,” they suggest.

The paper, published in Nature Energy, is “Continuous transition from double-layer to Faradaic charge storage in confined electrolytes” (DOI: 10.1038/s41560-022-00993-z).