Exercising control on coatings
High surface areas of nanostructured films are crucial in many applications that involve surface electrochemical reactions. In fuel cells and supercapacitors, they determine the achievable current and energy storage respectively.
One approach to manufacturing them is to coat a surface with nanoparticles, but this has drawbacks, not least the fact that the particles need to be bound together and supported in some way, and for electrochemical applications there needs to be electrical contact between the particles, which can be a source of resistive losses.
The governing factor is however not pure geometric surface area but the ‘real’ surface area, which takes account of the surface’s roughness and which can be much higher than the geometric area. Clearly then, the higher the real area and the smaller the geometric area, the better.
Researchers at the University of Southampton, UK, have developed a different approach that uses a surfactant as a template to control the formation of a nanoporous coating.
The method – which is now patented – can be readily scaled up for large areas, they say, and the resulting structures consist of a bulk material penetrated by a great many nanoscale holes or channels, removing the resistivity issue.
‘Our approach is a variation on conventional electroplating, where we use the surfactant lyotropic liquid crystalline phase as our electrolyte and then, after deposition, wash out the surfactant to produce the high surface area coating,’ explains Professor Philip Bartlett, a coinvestigator on the project set up by his colleague Professor Frank Walsh.
As the structure of the lyotropic liquid crystalline phase is thermodynamically controlled, it is predictable, Bartlett says. Therefore, the particular phase (hexagonal, cubic and so on) can be selected to determine the pore structure in the final film.
In addition, pore sizes can be controlled by choosing the surfactant – C16EO8 gives bigger pores than C12EO8 (2.5nm rather than 1.7nm) because the C16 alkyl chain is longer than the C12 one.
Bartlett adds, ‘There is not an existing largescale method to produce films of this type’. ‘By developing this electrodeposition approach we can compete with other coating methods such as those using nanoparticles.’
So far, the team has used the method to deposit metals including platinum, palladium, rhodium, cobalt and nickel, but is currently concentrating on nickel because of its energy storage applications. Although the project is still in progress, the technology is already being commercialised by industrial partner Nanotecture, Southampton, UK, for applications in batteries, sensors and supercapacitors.