Nanoporous materials bring high energy and high power density to a range of applications

Supercapacitors have been performing this function since 1957, when engineers at General Electric discovered that devices with porous carbon electrodes showed exceptionally high capacitance. Their conclusion was that energy was stored in the pores, although they couldn't prove it. And the theme of exploiting the properties of porous materials is being pursued by a University of Southampton spin out, which has developed technology that allows a range of nanoporous materials to be created. While the main target market for device based on these materials will be transportation, there is the prospect that consumer applications could be developed in the future. Nanotecture was established in 2003 to commercialise the work of three Southampton academics. According to Bill Campbell, Nanotecture's ceo, two remain with the company as technical advisors, with the third sitting on its board. "What we are doing is developing a method of making nanoporous materials. These are typically micro sized, but with an internal nanopore structure. Think of a sponge," he suggested. "It's along those lines, but far more ordered." Officially, the technology is known as liquid crystal templating. This allows the preparation of thin film monolithic materials with the benefit of ordered nanoporosity. In this way, says the company, surface area is 'considerably higher' than standard materials. Campbell gave an analogy. "Think of a bar of soap," he suggested. "When you put it into water, it goes squishy; that's the soap's liquid phase. We're using the same approach." In fact, liquid crystal templating relies on the inherent behaviour of surfactants to organise into highly geometric long range structures. A surfactant molecule contains hydrophilic (water compatible) and hydrophobic (water repellent) regions. At low concentrations in solution, surfactants orient themselves at interfaces, such that the hydrophobic area is located at the surface. At higher concentrations, surfactants start to aggregate to form micelles. Once a critical concentration is reached, micelles start to interact and self assemble into highly ordered structures. When the surfactant is removed, the material retains this highly ordered structure. "Our knowledge is all about the concentrations of surfactants, temperature and so on needed for the compound we want to create," Campbell noted. The approach works with a number of elements, Campbell claimed. "There are some 30 elements which can be made nanoporous," he said, "and some 170 compounds can be derived from them." This allows materials to be tailor made for specific applications, where such variables as pore size, geometry, wall thickness and surface area can be fine tuned. The company says that nanoporous metals can be created with pore sizes ranging from 2.5 to 20nm, providing a surface area of up to 200m2/g. Meanwhile, nanoporous oxides and hydroxides with similar pore sizes can offer surface areas of up to 1200m2/g. In addition to developing the techniques needed to prepare nanoporous materials, Nanotecture has developed a knowledge base addressing the characterisation and manipulation of nanoporous materials. This expertise allows the company to prepare and characterise samples and then tailor them so they are optimised for specific applications, such as batteries, supercapacitors and sensors. Campbell said: "We had to decide whether to be broad based or to focus on particular applications; we decided on the latter course. In particular, we decided to concentrate on supercapacitors. These are the bridge between the high energy density of a battery and the high power density of a capacitor." A supercapacitor generally features an activated carbon electrode on one side and a nickel hydroxide electrode on the other. "Traditionally," said Campbell, "that gives high energy density. But we've been able to use nanporous nickel hydroxide to improve the power density. So our architecture boosts power and boosts energy." Nanotecture calls its approach an asymmetric supercapacitor (see fig 2), so called because the two electrodes differ and there are two different charge storage chemistries in the device. While the nanoporous positive electrode stores charge like a battery, the carbon negative electrode stores charge like an electrochemical capacitor. In fact, the nanoporous electrode is said to be able to discharge 75% of its charge in less than 2s. The company notes that, by using nanoporous nickel hydroxide, it has made the solid state diffusion lengths up to 10,000 times shorter than in a standard material. Because of this, energy is delivered more quickly and at higher power. Nanotecture has developed its technology to the point where it has prepared sample test cells. "In addition to the chemistry," Campbell pointed out, "we've been doing a lot of hardware development and have engaged with a number of potential end users." Numbered amongst these is the transportation sector, where it believes its technology could be applied to cold starting trucks. "Lead acid batteries are prone to failure," Campbell believed. "With more and more functions on a truck, there are typically four to six lead acid batteries. Truck manufacturers are considering moving to a system with two lead acid batteries and two supercapacitors." A further potential application is in electric vehicles. "Lithium ion batteries are too expensive," Campbell claimed. "There are a number of materials being explored for this application, but they don't seem to have the necessary performance. We're working with a couple of companies, putting in some small contracts for feasibility studies." While transportation is the main target, Campbell sees applications in electricity grids, particularly for dynamic sag correction, which attempts to smooth voltage swings when loads are added or removed from the grid. And Nanotecture has demonstrated thumbnail sized supercapacitors for potential application in mobile phones as the power source for the camera flash. Campbell describes the whole area of energy storage as 'interesting', including vehicle applications. "But, with transportation, we're dealing with a conservative industry," he said. "While they are looking for a 5% performance gain, the market needs to move more quickly. But the two major components of the market – auto manufacturers and battery developers – are conservative. It's interesting to see how they are getting themselves moving." Nanotecture is looking to license its technology to interested parties. "We've been engaging in discussion with companies around the world who might take on manufacture of supercapacitors under license," he asserted. The manufacturing process itself is said to be 'not complex'. Once the nanoporous material has been created, it is then made into a paste and laid down on a nickel substrate. "We're actually using food processing equipment for our development work," Campbell concluded.