By manipulating molecules tens of thousand’s times smaller than the cross section of a human hair, experts in nanoscale research have discovered a ‘green and simple’ way to connect charge storing molecules – potentially paving the way for new sustainable battery materials.
Working at the nanoscale – the stuff we can’t see – they have successfully wired individual polyoxometalate (POM) molecules into an electrode and demonstrated that up to 90% of the molecules in the device can exchange electrons with the macroscopic world – the stuff we can see.
The research team from the UK and Germany was led by Dr Darren Walsh and Dr Graham Newton in the Carbon Neutral Laboratories for Sustainable Chemistry, and Professor Andrei Khlobystov in the School of Chemistry at the University of Nottingham. Their findings – ‘Host-guest hybrid redox materials self-assembled from polyoxometalates and single-walled carbon nanotubes’ – are published today (Wednesday 21 August) in Advanced Materials, a journal focussed on cutting edge developments in materials science.
The future of batteries, computers, fuel cells and many other devices depends on our ability to move electrons to and from molecules – a necessary function for storing charge, information or catalysing chemical reactions. In an ideal device every molecule must be hard-wired into an electrical circuit so that all the molecules perform useful work.
POMs in nanotubes
The team utilised carbon nanotubes – atomically thin hollow cylinders of carbon with diameters at the molecular scale (1-2 nm) and lengths at the macroscale (up to several mm).
Andrei Khlobystov, Professor of Nanomaterials, said: “Effectively, the carbon nanotube serves as a physical bridge between the two worlds – the world of molecules and our macroscopic world. This allows us to control the positions of molecules in space as well as passing electrons to and from the molecules by utilising the exceptionally high electrical conductivity of the carbon nanotubes.”
The scientists selected versatile POM molecules as each of them can reversibly receive up to 6 electrons. However, POMs are delicate molecules which can be easily damaged by various external factors. Jack Jordan, a PhD student in the EPSRC CDT for Sustainable Chemistry carried out this work. He said: ‘We made a real breakthrough in this area because we decided not just to deposit the POM molecules onto the nanotubes, which is a normal approach, but to entrap them inside the nanotube cavity. Once inside the nanotubes, the molecules are held securely in place and are protected by the atomically thin nanotube wall like a suit of armour.”
This allowed the scientists to increase the stability of POMs in electrochemical charge-discharge cycles by a factor of 50 as compared to unprotected molecules.
A sustainable future for electrical energy storage?
By cycling the voltage applied to the nanotubes encapsulating the POMs the scientists were able to emulate the process that happens inside batteries, showing that almost all the molecules actively participate in the charging-discharging facilitated by efficient electron transfer from the nanotube to the hard-wired molecules. Darren Walsh, Associate Professor in Physical Chemistry, said: ‘This new hybrid material harnessing the electrical conductivity of nanotubes and the redox activity of POMs shows promise for electrical energy storage devices that may provide more sustainable alternatives to the batteries we currently use.’
The research team also made a surprising discovery. The POM molecules are attracted to the nanotubes and enter the nanotube cavity spontaneously, irreversibly, and at room temperature, using only water.
Dr Newton, Assistant Professor in Inorganic and Materials Chemistry, said: ‘Initially, the spontaneous and extremely efficient entrapment of the molecules in a tiny nanoscale channel felt like magic, defying the laws of nature; however, eventually we discovered that the nanotube is not just a passive spectator but an active participant that gives up some of its own electrons to the POM molecules before encapsulation. We end up with a chain of negatively charged molecules held tightly inside a positively charged nanotube.”
Green and simple
Once the POMs and nanotubes exchange electrons in solution they become oppositely charged, providing a driving force for encapsulation.
Jack Jordan said: “The process of preparation of this remarkable material is extremely simple and ‘green’ which is becoming an increasingly important criterion for everything we do in chemistry today.”
Dr Newton said: ‘We used tungsten-based POMs in this study as a test system to discover the fundamental principles of nanoscale self-assembly and confinement, and to explore nano-encapsulation as a means to enhance the performance and durability of these redox-active molecules. Our next challenge is to move to molecules based on lighter elements, such as vanadium, that have the potential to make these hybrid materials commercially viable for storing electrical energy and other industrial applications.
