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Nanoporous metals to be used for energy storage

E&T on how nanoporous materials, notable metals and metal oxides, can be used to store energy.

The world today faces an energy challenge. Even if we leave aside the global warming debate and the call for a cleaner environment, increasing worldwide demands for reliable, highly efficient electricity sources and alternatives to fossil fuels remain with us.

Within the electricity sector, the race is on to develop both new sources of energy from wind, sea and solar and to improve the efficiency and usage of electrical storage devices, such as batteries and supercapacitors. While there are a number of technologies to choose from, several start-up companies and university spin-outs are developing new and unique material technologies to improve energy storage products for commercial use.

It was a gooey mess made by a bar of soap in a wet dish that marked the starting point for innovative scientists from the University of Southampton; they saw the potential in this soapy residue to develop novel nanotechnology to meet the growing demand.

The soapy goo is actually the result of a harmless chemical reaction of water with wetting agents added to make the soap lather and stick to skin. The wetting agents are known as surfactants (surface active agents) and the soft, opaque semi-solid substance is called the liquid crystal phase, a distinct state of matter between a solid and a liquid. Just like liquid crystal displays (LCD) seen in televisions and computer screens, the soap feels like a viscous liquid but, in fact, has a regular crystalline structure within it.

These same commonly available surfactants, with their predictable behaviour, can be used to make 'nanoporous' metals and metal compounds that have a nano-scale internal 'honeycomb' structure and therefore a surface area several orders of magnitude higher than the same non-porous material.

The performance of many electrical storage devices is governed by either the surface area of the electrode or the rate at which charge can penetrate the electrode material.

One solution to performance improvement is to make the electrode material from nano-scale particles, which have high surface area in a given volume and a small diameter, resulting in easy access to the bulk material. While this approach has performance advantages, the production and handling costs of nanomaterials is typically high in relation to larger scale particles. Therefore, the alternative nanoporous approach, which also gives easy access to the bulk material, is a huge advantage.

The nanoporous material, produced using surfactants, has a variety of applications including efficient energy storage and delivery in supercapacitors, high surface area coatings, catalysts or sensors in chemical reactions and drug delivery systems.

A crystal scaffold

Based on original work on liquid crystal templating (LCT) carried out at the University of Southampton in the 1990s (see panel at the bottom of p26), Nanotecture has developed a unique process for constructing nanoporous materials, notably metals and metal oxides.

LCT relies upon the inherent behaviour of surfactants in water to organise into highly geometric structures. Each molecule of surfactant has a hydrophilic end which is attracted to water and a hydrophobic end that repels it. At low concentrations, surfactants orient themselves at an interface between water and another substance. For example, in washing detergents, the hydrophilic end attaches to the water and to dirt particles.

Nanotecture's founder, Professor Phil Bartlett, says: 'From the start, it was realised that the ability to use the lyotropic liquid crystal template to control the structure of the deposited metal at the nanometre scale (as determined by the length of the individual surfactant molecules used to make the template) gave unique opportunities to produce new and interesting materials.'

As the concentration of surfactant increases, the molecules start to form simple spherical structures, called micelles. By increasing the concentration further, these micelles interact with each other to self-assemble into regular crystalline structures. This self-assembly is very predictable and can be controlled by the choice of surfactants, concentrations and processing temperature. By specifying these parameters, the pore sizes, wall thickness and surface area of the materials can be precisely controlled.

Of particular interest to Nanotecture is a hexagonal liquid crystal phase where the micelles are arranged like scaffolding poles or pencils packed together in long cylindrical rods. The rods are just 2-20nm in diameter - a thousand times smaller than the width of a human hair.

Nanotecture's process creates these rods and then 'builds' the chosen material around them in a water-based solution. When the materials have formed by precipitation, the surfactant rods are washed away leaving the nanopores behind.

One of the attractions of using this process is it's low cost, using commonly available bulk surfactants and water, both of which remain unchanged and can be recovered and reused.

The equipment, including mixers, stirrers and centrifuges for separation and recovery of the target material, is mainstream and readily available.

Nanotecture's product is a powder made of micron-sized particles with nanopores, which are easy and safe to handle. To-date, Nanotecture's scientists have focused primarily on materials for energy storage. However, the LCT process is compatible with over 30 metals and their oxides, which can have many other uses.

Less is more

The need to find more efficient ways to store and deliver energy is greater than ever; whether in transportation (hybrid electric vehicles or rail systems), or in static applications like uninterruptible power supplies (UPS) for emergency back-up.

While there are a number of energy storage technologies to choose from, the ideal characteristics include reliability, high power and energy densities and rapid response. Nanotecture has applied its material science expertise to the development of a uniquely configured high-performance supercapacitor (also commonly known as an ultracapacitor), which meets every one of these demanding requirements.

Fully carbon-based supercapacitors store charge electrostatically and have excellent power density. 'Asymmetric' supercapacitors, which incorporate a high-speed carbon electrode paired with a battery type electrode increasing the energy density of the device (although power density can be compromised) have also been developed. Improving the power density of the battery electrode would mark a step change in asymmetric supercapacitors, and Nanotecture has proved that nanoporous material can meet the challenge.

In a battery electrode, the rate of response is determined by the solid-state diffusion length of the material. Simply explained, this is the distance that an ion from the electrolyte has to travel to make its way into the electrode material. In normal, non-porous, material this can be a huge distance - up to several microns.

By introducing nanopores, the electrolyte is in contact with both the outer surface of the electrode and the honeycomb network within the material. The diffusion length is reduced from microns to just a few nanometres, a 1,000 times reduction. This has a dramatic effect on the performance of the electrode; in tests, the nanoporous electrode can deliver twice as much energy as the non-porous equivalent in the first two seconds of discharge.

The inclusion of nanoporous material into this electrode configuration has enabled a unique hybrid supercapacitor, which has key energy and, crucially, power advantages over existing devices.

Case Study: Heavy Vehicles

Applications for supercapacitors are wide-ranging: from regenerative braking in hybrid electric vehicles and uninterruptible power supplies and wind turbine blade pitch control, to hand-held power tools and camera flashes on mobile phones.

One very well-defined application is in engine starting; either static in stand-by generators, or mobile in commercial trucks. The motivation of commercial truck suppliers to consider supercapacitors is an interesting response both to incoming government legislation and increasingly energy-intensive in-cab functions.

Governments are responding to public demand for a cleaner environment and greater fuel efficiency by introducing anti-idle laws for commercial vehicles. A large number of US states, initially led by California, have already introduced legislation restricting the amount of time that large commercial vehicles can continue running while stationary. The primary purpose of extended idling in heavy trucks is to retain battery charge for engine starting while operating in-cab 'hotel' functions such as heating, TV and refrigeration for driver comfort.

As a result of this legislation, truck manufacturers are seeking solutions that ensure sufficient stored energy is available for starting, hence avoiding costly jump starts and maintenance costs in a system where full battery charge can no longer be guaranteed. Not only must there be sufficient energy but the energy must also be delivered at a high enough rate to crank the engine, especially at low temperatures. Traditionally, batteries have had to be oversized to meet this demand, but on a truck, 'real estate' is at a premium, so size and weight are important considerations for truck manufacturers.

To-date, engine starting has primarily been serviced by lead acid batteries. While these have a cost advantage over supercapacitors, they are prone to much shorter life times, are less reliable, heavier and are larger than a supercapacitor-based replacement. Battery life times are measured in thousands of cycles; in comparison, supercapacitors - in hundreds of thousands of cycles.

Similarly, weight and size are significantly reduced, for example a Nanotecture-based alternative to a lead acid battery is just 35 per cent of the volume.

Although the case has been made for the inclusion of supercapacitors for engine starting, initial investment cost has remained a significant barrier to conversion. However, the lifetime cost of ownership can be significantly reduced and vehicle up-time increased, making the initial investment over lead acid less of an obstacle.

Initial cost is being addressed at two levels. Firstly, devices such as Nanotecture's, which are developed around a water-based electrolyte, enable a low-cost manufacturing process. By comparison, the current generation of capacitors use an acid-based electrolyte, which requires a costly drying process to remove moisture from the electrodes prior to insertion in the acid.

Secondly, and most importantly, the use of unique hybrid electrochemistry has combined an asymmetric electrode configuration with patented nanoporous material which allows high discharge rate, but also stores significantly more energy per unit volume that competing products. The benefit for truck manufacturers is that, at the system level, fewer supercapacitor cells are required to meet the specification which ultimately leads to lower cost.

The second barrier to entry is safety and there has recently been increased industry focus on battery safety because of the potentially harmful chemicals used in manufacture. Devices, such as Nanotecture's, can use a water-based electrolyte, which is less hazardous than acid-based systems used in carbon/carbon supercapacitors.

A leading US truck manufacturer confirms that lead acid batteries, used for truck starting, are typically replaced thrice during the lifetime of the vehicle, leading to increased disposal costs, whereas the average supercapacitor, in addition to being smaller, would be expected to last for the lifetime too.

Pores for thought

The development and use of nanomaterials in the field of energy storage is of critical importance to increase the efficiency and performance of batteries and supercapacitors. The increasing demand for clean, secure electricity supply and the electrification of much of the world's transportation systems is placing greater demands on these devices. Nanotechnology-based companies are at the forefront of much of this research and are adopting different strategies to meet the demands.

Nanomaterials are now in use in a wider range of applications such as coatings, fuel and oil additives and tennis rackets. Energy efficiency and storage are high-priority applications which can use new and innovative materials to respond to both consumption and environmental demands. Nanotecture's alternative approach to the creation of high surface area materials brings many of the advantages of nanoparticles but with a simpler and therefore lower cost of manufacturing.

LCT has provided a platform technology that Nanotecture has used to create nanoporous materials. Currently nanoporous Nickel Hydroxide is being used to enable a high-performance supercapacitor and, from the end-users perspective, this performance advantage can either enable new applications or generate considerable cost savings.

New nanoporous materials are already under development at Nanotecture and are focused on Lithium Ion battery chemistry, since basic research has indicated that their inclusion may improve low-temperature performance and increase lifetime.

Large particle-size nanoporous materials offer an interesting contrast to nanoparticle materials. Both offer high surface area. But the approach taken by Nanotecture is based on low-cost raw materials, a low-cost production and dispersion process and, above all, on safe handling.

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