Supercapacitor: Conducting future energy storage

Powerful materials

A new generation of power supply materials with a high energy density is being keenly sought by the manufacturers of electric vehicles, notebook computers and other portable devices. A novel supercapacitor material studied with NMR and IR spectroscopy and other techniques could be suitable for replacing current rechargeable battery materials.

Donglin Jiang of the Precursory Research for Embryonic Science and Technology (PRESTO) in Tokyo and the National Institutes of Natural Sciences in Okazaki and colleagues, Yan Kou, Yanhong Xu, Zhaoqi Guo have now introduced a new material, which they suggest has outstanding supercapacitor properties ripe for exploiting in high-energy density devices.

Fundamentally, supercapacitors overcome the problems of long discharge times needed to keep electric vehicles rolling for long distances between recharging stops and to extend the portability of laptops, tablet computers, mobile phones and other gadgets. A supercapacitor combines the benefits of conventional capacitors and batteries but avoids many of the problems of each. Like a capacitor it can deliver high current densities, on demand while storing a large amount of electric energy like a battery.

Energy densities for supercapacitors are typically expected to be two orders of magnitude greater than batteries and also offer significant improvements on fuel cells, another "alternative" to standard electrical storage. One of the biggest advantages over rechargeable batteries, however, is that a supercapacitor can be charged and discharged millions of times, as opposed to the several hundred times of a battery and so could outlive most of the devices for which it would be suitable whereas batteries often have to be replaced after just a few months.

Dense and conjugated

Supercapacitors, also known as supercondensers or electrochemical double layer capacitors work using an alternative charge-storage paradigm different to standard rechargeable batteries such as lithium metal hydride batteries. They consist of a double layer of electrochemical electrodes, with a wet electrolyte. When a voltage is applied, ions of opposite charge collect on both electrodes to form wafer-thin zones of immobilized charge carriers. In other words, there is merely a shift of charge distribution, no chemical change occurs as is the case with a battery. Researchers and technologists have experimented with a multitude of materials - including nanostructured carbon materials such as templated carbon materials, graphenes, carbon nanotubes, aerogels, and heteroatom-hybridized carbon materials - to build supercapacitors with varying degrees of success. The near-perfect supercapactitor material is yet to be found. However, the researchers in Japan believe they have taken a step closer.

The team explains that the class of materials on which they have focused has particularly interesting properties. The materials are special microporous, framework-like, organic polymers, which contain regions of conjugated double bonds. This arrangement, allows electrons to move freely through the framework, making the material a conductor, of course. Importantly, however, the porous nature of the material means that it has a large inner surface area relative to its overall volume which is important for the formation of electrostatic charge-separation layers in the pores.

Jiang and his team have now synthesized a nitrogen-containing framework with a pore size optimal for allowing ions to flow in and out rapidly; this is an essential requirement for rapid charging and discharging of a supercapacitor. The materials known as "Aza-CMPs" were ionothermally synthesized by the condensation reaction of 1,2,4,5-benzenetetramine with triquinoyl hydrate at 300, 350, 400, 450, and 500 Celsius to generate aza-CMP@300, Aza-CMP@350, Aza-CMP@400, Aza-CMP@450, and Aza-CMP@500, respectively, the team explains. The used carbon-13 cross-polarization/magic angle spinning NMR spectroscopy to confirm that the Aza-CMPs consist of three types of carbon atoms.

The nitrogen centres, the team says, interact with the electrolyte ions, thus favouring the accumulation of charge and the movement of ions. These various properties work synergistically to endow the new material with advantageous properties including an unusually high charge storage capacity and high energy density. The team has also demonstrated that the microporous frameworks are robust to repeated charge-discharge cycles.

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