A low-cost templating process has been used to create a new class of materials for high-performance batteries.
The development of high-performance batteries and supercapacitors is being driven by the creation of electrode materials with high surface areas. These can increase the power density of such storage devices by enhancing their electrochemical reaction rate while reducing their weight and size.
To that end, many companies are now developing nanoparticle-based materials for such applications. However, while these materials do have a high surface area, the processes involved in manufacturing them can be expensive and the nanomaterials can be problematic to manage in a manufacturing environment.
Taking an alternative approach, Southampton-based Nanotecture has developed a unique liquid-crystal templating process that can be used to make nanoporous materials with high surface areas. While the company believes that such porous material produced by its low-cost process will be very effective at increasing the power density of the electrodes of storage devices, the micron-sized particles created will be easier to handle than nanoparticles.
The liquid-crystal templating process was originally created as the result of work performed at Southampton University by Prof Phil Bartlett in 2003. Having licensed the original intellectual property from the university, Nanotecture has refined the process over the past seven years and is now ready to realise its commercial potential.
According to Bill Campbell, Nanotecture’s chief executive, the liquid-crystal templating process could produce a range of materials for use in any number of applications. But to prove the commercial viability of the process, the company has initially chosen to manufacture a nanoporous nickel hydroxide material, which it plans to market in a series of supercapacitors through strategic partnerships.
The liquid-crystal templating process exploits the behaviour of surfactants in solution to align themselves in regular geometric structures. These structures, or templates, act as a temporary framework, upon which materials such as metals and inorganic metal compounds can be deposited. When the surfactant framework is then removed, the remaining material displays the same ordered porosity as the original template – it becomes ’nanoporous’, consisting of micron-sized particles or thin films with pores penetrating all the way through them.
Kate Amos, senior scientist at Nanotecture, said that the surfactants used in the process are off-the-shelf materials commonly used in the manufacture of detergents. As such, the company is able to source them from numerous suppliers at low cost.
The concentration and temperature at which they are mixed with water allows Nanotecture to control the geometry of the structure of the surfactant template produced. The geometry of the resultant material that is made from it can also be controlled.
The surfactants form into the liquid-crystal templates through the action of a solvent, typically water, and heat. At low surfactant concentrations, a solution simply comprises surfactant molecules randomly distributed in water. But at a higher concentration, the surfactant molecules arrange themselves into hollow spheres. At the surface of the spheres is a layer of hydrophilic (water-loving) heads, while the hydrophobic (water-hating) tails of the surfactant are hidden in the middle.
As the concentration is increased further, the spheres of surfactants move into a liquid-crystalline phase, forming rod-shaped structures that then group together into hexagonal arrays of rods with hydrophilic surfaces and hydrophobic cores.
Having created a surfactant and water mixture at a known concentration and temperature, a soluble source of nickel is added, creating a solution of that material. The metal salts contained within the solution are deposited around the surface of the hexagonal liquid-crystal structure through electrochemical or chemical deposition.
The surfactant is removed from the nanoporous product by washing with warm water. The result is a material with a controlled porosity where the diameter of each of the pores in the material is representative of the dimensions of the surfactant template used to create them.
The hexagonal template is not the only structure that can be formed using Nanotecture’s templating process; it could also be used to create templates with layered or cubic structures by tweaking the process parameters, such as the heat of the solution and the concentration of the surfactants in the water.
’Presently, however, we have chosen to create hexagonal templates because when a metallic structure is formed around such templates and the surfactant subsequently removed, what is left is a material with a high surface area due to the hollow pores that have been formed within it,’ said Amos.
To ensure the purity of the finished product, she added that the company has put a lot of research into qualifying how much washing was needed to remove all of the surfactant.
As a result, it has developed a controlled process that ensures that, for the nickel hydroxide process, more than 95 per cent of the surfactant can be removed from the nanometre-sized pores of the particles during a first wash and the remainder during repeat wash cycles. Once the surfactant has been removed from the pores, the material can be spray dried and is ready for use.
The actual porous nickel hydroxide material produced using the process has a surface area of more than 400m2/g, comparing favourably with a material with a similar particle size without such a porous structure that only sported a surface area of 4m2/g.
To ascertain the power density of the high-surface nickel hydroxide when used as an electrode for a supercapacitor, the company created an electrode from the material and compared it to one produced using a standard nickel hydroxide material with an identical particle size.
In tests, Nanotecture found that supercapacitors with an electrode built from their porous nickel hydroxide sported a power density twice that of the conventional material. Amos said that the shorter diffusion pathways created in the porous material were also responsible for increasing the rate at which supercapacitors built using such electrodes could be charged and discharged.
The company is currently discussing commercial opportunities for the new material with UK-, US-, Japanand Taiwan-based supercapacitor manufacturers. Campbell said he anticipates that Nanotecture will ship its first batches of porous nickel hydroxide by the end of next year and believes that products based on the technology will be announced in 2012. Significant volume will follow one year later.
He added that the company is still deliberating whether to manufacture the nickel hydroxide itself or to outsource the process. Campbell said that, at present, Nanotecture has the capability in-house to make sufficient material to support early requirements but may consider outsourcing in the future if required.