New electrode gives micro-supercapacitor macro storage capacity

© Anaïs Ferris – LAAS-CNRS Image obtained from scanning tunneling microscopy on a porous 3D-gold structure.

Anaïs Ferris – LAAS-CNRS
Image obtained from scanning tunneling microscopy on a porous 3D-gold structure.

Micro-supercapacitors are a promising alternative to micro-batteries because of their high power and long lifetime. They have been in development for about a decade but until now they have stored considerably less energy than micro-batteries, which has limited their application. Now researchers in the Laboratoire d’analyse et d’architecture des systèmes (LAAS-CNRS)1 in Toulouse and the INRS2 in Quebec have developed an electrode material that means electrochemical capacitors produce results similar to batteries, yet retain their particular advantages.

This work was published on September 30, 2015 in Advanced Materials.

With the development of on-board electronic systems3 and wireless technologies, the miniaturization of energy storage devices has become necessary. Micro-batteries are very widespread and store a large quantity of energy due to their chemical properties. However, they are affected by temperature variations and suffer from low electric power and limited lifetime (often around a few hundred charge/discharge cycles). By contrast, micro-supercapacitors have high power and theoretically infinite lifetime, but only store a low amount of energy.

Micro-supercapacitors have been the subject of an increasing amount of research over the last ten years, but no concrete applications have come from it. Their lower energy density, i.e. the amount of energy that they can store in a given volume or surface area, has meant that they were not able to power sensors or microelectronic components. Researchers in the Intégration de systèmes de gestion de l’énergie team at LAAS-CNRS, in collaboration with the INRS of Quebec, have succeeded in removing this limitation by combining the best of micro-supercapacitors and micro-batteries.

They have developed an electrode material whose energy density exceeds all the systems available to date.

The electrode is made of an extremely porous gold structure into which ruthenium oxide has been inserted. It is synthesized using an electrochemical process. These expensive materials can be used here because the components are tiny: of the order of square millimeters. This electrode was used to make a micro-supercapacitor with energy density 0.5 J/cm², which is about 1000 times greater than existing micro-supercapacitors, and very similar to the density characteristics of current Li-ion micro-batteries.

With this new energy density, their long lifetime, high power and tolerance to temperature variations, these micro-supercapacitors could finally be used in wearable, intelligent, on-board microsystems.


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Hydrogen Storage Gets New Hope

Elements of the hydrogen economy
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A new method for “recycling” hydrogen-containing fuel materials could open the door to economically viable

In an article appearing in Angewandte Chemie, Los Alamos National Laboratory and University of Alabama researchers working within the U.S. Department of Energy’s Chemical Hydrogen Storage Center of Excellence describe a significant advance in hydrogen storage science.

Hydrogen is in many ways an ideal fuel for transportation. It is abundant and can be used to run a fuel cell, which is much more efficient than internal combustion engines. Its use in a fuel cell also eliminates the formation of gaseous byproducts that are detrimental to the environment.

For use in transportation, a fuel ideally should be lightweight to maintain overall fuel efficiency and pack a high energy content into a small volume. Unfortunately, under normal conditions, pure hydrogen has a low energy density per unit volume, presenting technical challenges for its use in vehicles capable of travelling 300 miles or more on a single fuel tank—a benchmark target set by DOE.

Consequently, until now, the universe’s lightest element has been considered by some as a lightweight in terms of being a viable transportation fuel.

In order to overcome some of the energy density issues associated with pure hydrogen, work within the Chemical Hydrogen Storage Center of Excellence has focused on using a class of materials known as chemical hydrides. Hydrogen can be released from these materials and potentially used to run a fuel cell. These compounds can be thought of as “chemical fuel tanks” because of their hydrogen storage capacity.

Ammonia borane is an attractive example of a chemical hydride because its hydrogen storage capacity approaches a whopping 20 percent by weight. The chief drawback of ammonia borane, however, has been the lack of energy-efficient methods to reintroduce hydrogen back into the spent fuel once it has been released. In other words, until recently, after hydrogen release, ammonia borane couldn’t be adequately recycled.

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Smoothing Out the Wind

Wind Storage

A cunning plan to store energy underwater may help fulfil the promise of wind power

THE problem with wind power is that is cannot always be relied upon. The wind—and other transient, environmental energy sources such as solar—must either be used when it is harvested or stored expensively in batteries or specially designed hydroelectric schemes that use the resulting energy to pump water uphill. Alternatives would be extremely welcome. Alexander Slocum, of the Massachusetts Institute of Technology, thinks he has one. Observing that the fashion among wind-power fans is to build turbines out at sea, where the wind blows strongest, he proposes a pumped-storage system that uses seawater.

Dr Slocum’s scheme involves anchoring a hexagonal array of hollow, 31-metre-diameter concrete spheres to the ocean floor at a depth of approximately 350 metres. Floating turbines would be tethered to these spheres and surplus power from these turbines, generated during periods of high wind and low electrical demand, would be used to pump water out of the spheres, evacuating the central chamber. When the wind faltered or the lights went back on, water forced into the central chamber by the pressure of the surrounding ocean would pass through a turbine and generate electricity. Each sphere would provide a five megawatt turbine with four hours of storage capacity.

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