Efficiency metrics for energy storage devices need standardization

Solving the mystery of prematurely dead cell phone and laptop batteries may prove to be a vital step toward creating a sustainable energy grid according to Drexel researcher Dr. Yury Gogotsi. In a piece published in the November 18 edition of Science, Gogotsi, who is the head of the A.J. Drexel Nanotechnology Institute, calls for a new, standardized gauge of performance measurement for energy storage devices that are as small as those used in cell phones to as large as those used in the national energy grid.

Gogotsi is one of the featured experts, along with Bill Gates, tapped by Science to address problems that must be solved en route to the widespread use of renewable energy. His piece, co-authored with Dr. Patrice Simon of the Université Paul Sabatier in Toulouse, France, is entitled "True Performance Metrics in Electrochemical Energy Storage."

"A dramatic expansion of research in the area of electrochemical energy storage has occurred over the past due to an ever increasing variety of handheld electronic devices that we all use," Gogotsi said. "This has expanded use of electrical energy in transportation, and the need to store renewable energy efficiently at the grid level. This process has been accompanied by the chase for glory with the arrival of new materials and technologies that leads to unrealistic expectations for batteries and supercapacitors and may hurt the entire energy storage field."

The main type of energy storage device addressed in the article is the supercapacitor. Supercapacators, which are built from relatively inexpensive natural materials such as carbon, aluminum and polymers, are found in devices, ranging from mobile phones and laptop batteries to trams, buses and solar cells. While supercapacitors tend to store less energy compared to standard lithium-ion batteries, they have the ability to charge and discharge energy more quickly than batteries and can be recharged a near infinite number of times, and operate in a wider temperature range with a high efficiency.

Typically, the performance of both, batteries and supercapacitors, is presented using Ragone plots, graphs that show a relation between the energy density and the power density. For example, a Rangone plot for the battery used in an electric car shows both how far it can travel on a single charge -energy density- and how fast the car can travel -power density. An ideal energy storage device is expected to store plenty of energy and do it quickly.

The issue that Gogotsi and Simon bring to light is the idea that current metrics for grading energy storage devices, including the Ragone plot, may not provide a complete picture of the devices' capability. According to the researchers, other metrics, such as a device's cycle lifetime, energy efficiency, self-discharge, temperature range of operation and cost, must also be reported.

"This paper calls upon the community of scientists and engineers who work on supercapacitors to present data on material performance using metrics beyond the traditional Ragone plot," Simon said. "Although such plots are useful for comparing fully packaged commercial devices, they might predict unrealistic performance for packaged cells from extrapolation of small amounts of materials."

Gogotsi and Simon have a longtime research collaboration, investigating materials for supercapacitors. Their joint work has received global coverage and various awards and distinctions. Funding for the collaboration between Gogotsi and Simon is sponsored by the Partner University Fund (PUF) which supports innovative and sustainable partnerships between French and US institutions of research and higher education.

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Journal Reference:

Y. Gogotsi, P. Simon. True Performance Metrics in Electrochemical Energy Storage. Science, 2011; 334 (6058): 917 DOI: 10.1126/science.1213003

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Nano bundles pack a powerful punch: Solid-state energy storage takes a leap forward

ScienceDaily (Aug. 23, 2011) — Rice University researchers have created a solid-state, nanotube-based supercapacitor that promises to combine the best qualities of high-energy batteries and fast-charging capacitors in a device suitable for extreme environments.

A paper from the Rice lab of chemist Robert Hauge, to be published in the journal Carbon, reported the creation of robust, versatile energy storage that can be deeply integrated into the manufacture of devices. Potential uses span on-chip nanocircuitry to entire power plants.

Standard capacitors that regulate flow or supply quick bursts of power can be discharged and recharged hundreds of thousands of times. Electric double-layer capacitors (EDLCs), generally known as supercapacitors, are hybrids that hold hundreds of times more energy than a standard capacitor, like a battery, while retaining their fast charge/discharge capabilities.

But traditional EDLCs rely on liquid or gel-like electrolytes that can break down in very hot or cold conditions. In Rice's supercapacitor, a solid, nanoscale coat of oxide dielectric material replaces electrolytes entirely.

The researchers also took advantage of scale. The key to high capacitance is giving electrons more surface area to inhabit, and nothing on Earth has more potential for packing a lot of surface area into a small space than carbon nanotubes.

When grown, nanotubes self-assemble into dense, aligned structures that resemble microscopic shag carpets. Even after they're turned into self-contained supercapacitors, each bundle of nanotubes is 500 times longer than it is wide. A tiny chip may contain hundreds of thousands of bundles.

For the new device, the Rice team grew an array of 15-20 nanometer bundles of single-walled carbon nanotubes up to 50 microns long. Hauge, a distinguished faculty fellow in chemistry, led the effort with former Rice graduate students Cary Pint, first author of the paper and now a researcher at Intel, and Nolan Nicholas, now a researcher at Matric.

The array was then transferred to a copper electrode with thin layers of gold and titanium to aid adhesion and electrical stability. The nanotube bundles (the primary electrodes) were doped with sulfuric acid to enhance their conductive properties; then they were covered with thin coats of aluminum oxide (the dielectric layer) and aluminum-doped zinc oxide (the counterelectrode) through a process called atomic layer deposition (ALD). A top electrode of silver paint completed the circuit.

"Essentially, you get this metal/insulator/metal structure," said Pint. "No one's ever done this with such a high-aspect-ratio material and utilizing a process like ALD."

Hauge said the new supercapacitor is stable and scaleable. "All solid-state solutions to energy storage will be intimately integrated into many future devices, including flexible displays, bio-implants, many types of sensors and all electronic applications that benefit from fast charge and discharge rates," he said.

Pint said the supercapacitor holds a charge under high-frequency cycling and can be naturally integrated into materials. He envisioned an electric car body that is a battery, or a microrobot with an onboard, nontoxic power supply that can be injected for therapeutic purposes into a patient's bloodstream.

Pint said it would be ideal for use under the kind of extreme conditions experienced by desert-based solar cells or in satellites, where weight is also a critical factor. "The challenge for the future of energy systems is to integrate things more efficiently. This solid-state architecture is at the cutting edge," he said.

Co-authors of the paper include graduate student Zhengzong Sun; James Tour, the T.T. and W.F. Chao Chair in Chemistry as well as a professor of mechanical engineering and materials science and of computer science, and Howard Schmidt, adjunct assistant professor of chemical and biomolecular engineering, all of Rice; Sheng Xu, a former graduate student at Harvard; and Roy Gordon, the Thomas Dudley Cabot Professor of Chemistry at Harvard University, who developed ALD.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Rice University.

Journal Reference:

Cary L. Pint, Nolan W. Nicholas, Sheng Xu, Zhengzong Sun, James M. Tour, Howard K. Schmidt, Roy G. Gordon, Robert H. Hauge. Three dimensional solid-state supercapacitors from aligned single-walled carbon nanotube array templates. Carbon, 2011; 49 (14): 4890 DOI: 10.1016/j.carbon.2011.07.011

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