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Graphene network showing thermoelectric generation of electrical energy from difference in temperature between hot and cold sides. Image credit: Raquel Cano Cordero

(Almost) everything under (and including) the sun generates heat: electronic devices, vehicles and all living bodies emit thermal energy as heat.  However, this heat is largely wasted and research into technologies that capture and convert it into electricity has struggled to overcome the obstacles presented by scalability, practicality and cost. 

Researchers from the Materials Physics group at the ÃûæÂÖ±²¥ have now published recent work on thermoelectric capture, using highly conductive graphene sheets, which might just remove those barriers and offer a cheap, sustainable technology for heat capture and conversion – as well as demonstrating new understanding of how conductivity in graphene-based nanomaterials is best exploited.

The group assembled nanomaterial networks of varying density and size, from few to many layers of graphene sheets, then measured electrical conductivity as the different arrays were exposed to heat.  Their expectation was that the assemblies of larger, thicker sheets would exhibit the highest levels of conductivity but in fact, the opposite outcome was observed, where the smaller, thinner sheets spontaneously formed dense-packed arrays, and performed better than the many-layered samples.  This study led the group to conclude that, where densely packed layers of nanosheets might have been expected to correspond to improved electrical transport, fewer layers worked better despite the greater number of inter-nanosheet junctions.

Keiran Clifford, Doctoral Researcher with the group and first author of the paper said: “The results were really surprising.  This is the first time we’ve examined nanomaterial networks specifically from the point of view of how their structure and properties influence electrical conductivity.  Graphene is well-known to be highly conductive, but the idea that films of many small nanosheets with lots of junctions would be more efficient than bulkier, multi-layered systems is new and opens the door to many applications.”

The group has developed a printable graphene-based coating to exploit thermoelectric capture, enabling the recycling of waste heat into electrical energy.  The coating is printed into a patch or a pad which can then be applied to the heating surface. Where the ambient environment is cooler, electrons travel away from the heat source and move into the cold, generating electrical activity which is conducted through the nanosheets.  This thermoelectric transport system could be connected to an external power bank, to charge a battery, or could directly power another device.

Thermoelectric materials which can convert heat to electrical energy already exist but are typically made from expensive synthetic crystals which are challenging to integrate into varied structures.

Sean Ogilvie, Research Fellow with Materials Physics and corresponding author of the paper said: “Potentially we have an easy and highly efficient way to optimise thermoelectric capture in vehicles, devices, homes – even people! It isn’t unreasonable to imagine how a person’s own body heat might be captured and converted to electricity that could power a phone, for example. We are not currently aware of any other commercially viable, scalable, heat capture coatings, which can be printed for direct coating or as flexible layer. We believe our approach is a viable route to practical, scalable, printable thermoelectrics for versatile application.” 

Work on the inks and coatings is advancing and the group have demonstrated pilot-scale production, where a sample is brought into contact with a heat source and a voltage induced, but there is still significant development required to integrate the coatings with the different types of patches, pads or panels suitable for use with devices.  So you might have to hold onto your phone charger for just a little while longer!

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