Posted on behalf of Edwin Cartlidge.
The astonishing claim that graphene can draw on ambient thermal energy to generate electrical current has been attracting scepticism from some materials scientists — not least the graduate supervisor of one of the scientists responsible for the work.
Graphene is a one-atom-thick layer of carbon that has exceptional electrical, thermal and mechanical properties, and has become the ‘buzz material’ du jour.
Now, Zihan Xu of the Hong Kong Polytechnic University and colleagues have made what they describe as a ‘graphene battery’ by placing a 50-square-millimetre sheet of graphene about onto a silicon substrate, attaching gold and silver electrodes to its ends, and then immersing it in a solution of copper chloride. The device generated a voltage of around 0.35 volts (V) for some 25 days; six of them in series could power a light-emitting diode.
So where does the voltage come from? Xu and co-workers believe that the kinetic energy of the copper ions (a consequence of the atomic jiggling that we measure as temperature) is sufficient to knock electrons free from within the graphene. The team argues that the electrons then travel through the graphene sheet, which has an extremely high “electron mobility”, rather than leaking into the solution. Thus, thermal energy is transformed into electrical potential. Their results appear in a paper on the physics preprint server, ArXiv.
Few researchers are convinced. Condensed-matter physicist Eva Andrei of Rutgers University in New Jersey describes the claim as “far-fetched”, arguing that “there are more plausible mechanisms” to explain the observations. She suggests that the kinetic energy of the ions would probably be dissipated as heat within the graphene, and adds that even if electrons are released, they would almost certainly recombine with the positive ‘holes’ that they leave behind before travelling to the electrodes.
Another sceptic is Wanlin Guo of Nanjing University in China, who was the graduate supervisor of one of the team members, Guoan Tai, now a postdoctoral researcher at the Hong Kong Polytechnic University. Guo says he has carried out tests of his own on the system, varying the size of the graphene sheets and using different kinds of substrate and electrodes, and has never recorded an output voltage higher than about 0.1 millivolts.
Robert Dryfe, an electrochemist at Manchester University is also “not convinced by the explanation”. He thinks that the researchers’ estimate of the speed of the copper ions — around 280 metres per second — is too high, and suggests that a standard chemical reaction is probably the cause. The voltage could be caused if the copper ions simply absorbed electrons from the graphene, he says.
Xu, however, is adamant that his group’s interpretation of the results is correct. The voltage generated by the device increases when the solution is heated and when it is exposed to pulses of ultrasound, which he argues supports the idea that the kinetic energy of the ions is responsible for the voltage. He also says that the group carried out extra tests to prove that chemical reactions were not involved. “I am 100% confident that my experiments are true and that I can repeat them anywhere and anytime,” he insists.
And Xu does have supporters. Materials scientist Nikhil Koratkar, of the Rensselaer Polytechnic Institute in New York, argues that Xu may be on to something, and that he should keep working on his device to see if it can be scaled up. Koratkar was part of a US group that last year reported generating a voltage by passing an ionic liquid across samples of graphene.
This result was subsequently rebutted by Guo and colleagues, however, who carried out their own tests showing that the voltage resulted from the interaction of the ions with the electrodes attached to the graphene and not with the graphene itself.
Koratkar, however, is confident that both his group and the Hong Kong team have identified ways of “breaking graphene’s symmetry” to create a voltage across a pair of electrodes.
Although materials scientist Yury Gogotsi of Drexel University in Philadelphia, Pennsylvania, doubts the latest claim, he points out that the mystery should at least be relatively easy to clear up. “The device described by Xu’s group is very simple and numerous labs can easily repeat this experiment to check the validity of the results,” he says.
Image: AlexanderAlUS/Wikimedia Commons