Wei Chen, a recent Ph.D. graduate student from the group of Dr. Husam Alshareef, Professor of Material Science and Engineering, recently collaborated with KAUST’s Imaging and Characterization Lab scientists to explain the mechanism underpinning the charge storage process in a common supercapacitor material and its behavior during charge/discharge cycling.

Supercapacitors are energy storage devices that fill the gap between batteries and electrostatic capacitors. They have a high power density and yet enough energy density to allow them to be used to power portable devices or to compliment batteries in electric and hybrid electric vehicles. The market size for supercapacitors is growing extremely fast, and they are already appearing in many applications, including portable power tools, cranes, intercity trains, and street lamps.

There are two common types of supercapacitors. The first type, the double-layer capacitor, relies primarily on carbon-based electrodes, which store charge much like a conventional electrostatic capacitor found in electronic circuits. The second type, called an ultracapacitor or pseudocapacitor, utilizes the so-called pseudocapacitive materials, which include transition metal oxides such as MnO₂, to achieve even higher capacitance.

These pseudocapacitive materials undergo Faradic reactions and provide an additional charge storage mechanism. This means that pseudocapacitive electrodes can produce supercapacitors with a much higher energy density. However, a problem with pseudocapacitive materials is their cycling stability: they typically show a drop in capacitance as they are cycled between charge/discharge processes.

Using electron tomography and X-ray photoelectron spectroscopy, Chen and postdoctoral fellow Dr. Rakhi Raghavan Baby collaborated with KAUST Core Labs scientists Qingxiao Wang and Nejib Hedhili, to show how the morphology and crystal phase of manganese oxide electrodes affect their energy storage density and, more importantly, their unique behavior during charge/discharge cycling.

By using 3-D tomography, the team established how the morphological evolution of the electrode increases its surface area, leading to enhanced energy densities. Furthermore, through the use of a combination of tomography and spectroscopy, the team showed that the electrolyte actually etches nanoscale openings in the manganese oxide sheet electrodes, which surprisingly enhanced the electrolyte permeability and increased the energy density of the device during cycling.

“This work improves our understanding of manganese oxide, one of the most promising pseudocapacitive materials for energy storage applications, and acts as a guide for future experiments,” said Prof. Alshareef.

The results from this project were published in Advanced Functional Materials (DOI: 10.1002/adfm.201303508). Prof. Alshareef’s group has been active in the area of energy storage, focusing on electrode material development for supercapacitors, Li-ion batteries, and more recently Na-ion batteries. 

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