Carbon Voltaic Energy

June 27, 2012

We live in a world of constant flow, much of which is useful. Hydroelectricity is actually a form of solar energy. It's created from the flow of water, and it presently provides a little more than six percent of the electricity in the United States. The flow of ocean water in the tides is another source of electrical energy, as I described in a recent article (Tidal Forces, June 20, 2012). You could claim a small fraction of that as being solar energy, but that flow is mostly the work of the Moon.

Flow on smaller scales yields electrical power in some unusual ways. Rubbing certain materials together generates static electricity by the triboelectric effect, and this trick will work also with fluid flow. In one demonstration, a voltage of 300 millivolts (mV) was obtained with a flow of pure water at 45 cm³/sec in a millimeter diameter pipe.[1] This is also a cautionary tale of what materials you can use in gasoline filling pipes.

Carbon nanotubes will generate a few millivolts when water flows over them because of coupling between ions in the water and charge carriers in the nanotubes. Since graphene is the wonder material of the new millennium, it's only reasonable that scientists would try it in similar triboelectric experiments. Engineers at Rensselaer Polytechnic Institute (Troy, New York) and Rice University (Houston, Texas) found that adding ions, in the form of hydrochloric acid (HCl), to the water gives an order of magnitude higher voltage when the flow is over a graphene film (see figure).

Flow of chlorine ions generating a current in a graphene film. The moving chlorine ions might drag associated electron holes in the graphene sheet. (Drawing rendered by the author using Inkscape).

The RPI-Rice team was able to generate 85 nanowatts of power for a 0.01 m/s flow of 0.6 M HCl over a 30 x 16 μm graphene film. This scales to a phenomenal 175 W/m². Computer simulations indicate that the currents in the graphene are a result of the flow of chlorine ions across the film.

When doing experiments, I often used common household materials that I had on hand if they served a purpose. I'm certain that many experiments of today are held together by duct tape, just as in Ernest Rutherford's time they were held together with string and sealing wax. When one of Rutherford's students needed a metal tube for an experiment, Rutherford cut a piece from the handlebar of an old bicycle.[3]

When your funding is good, you reach for the Kapton tape (right), but most of the time, it's the duct tape (left). (Photo by author, via Wikimedia Commons).

One common item found in all laboratories is the pencil. Although these were called "lead pencils" when I was young, the writing material is not lead. It's a composite material of graphite powder in a clay binder. It's conductive, and traces on paper are also conductive; and also piezoresistive, as I demonstrated in a previous article (Paper Accelerometer, March 10, 2011). Pencil lead is not exactly graphene, but it might share some of its properties.

That must be what some scientists were thinking when they made an energy cell from pencils in various metal chloride solutions. These are claimed to be no ordinary type of energy harvester, but a device that converts ambient thermal energy into small amounts of electrical power. We're not talking batteries, or thermopower devices, but a fundamental way to convert the energy of a constant temperature bath into electricity.[4]

Parallel pencil leads in three molar potassium chloride (KCl), sodium chloride (NaCl), nickel(II) chloride (NiCl₂) and copper(II) chloride (CuCl₂) solutions at room temperature produced 0.655, 1.023, 1.023 and 1.828 nanowatts, respectively. This was also the case for graphene oxide and graphene films. To squelch possible criticism of their methods, the authors state that in no case were any connecting wires in contact with the solutions, a condition that would give an electrochemical voltage. As an understatement, the authors write that the mechanism is still unclear.[4]

Graphite thermal energy device built from pencils. The green color of the solution indicates that it's a nickel(II) chloride solution. (Via arXiv Preprint Server).[4]

This work can be considered to be a follow-up to experiments published in March that showed this effect for graphene in a saturated CuCl₂ solution.[5] In those experiments, 0.35 volts were generated for twenty days, and there was a positive correlation between the open-circuit voltage and the temperature, and the open-circuit voltage and the cation concentration. Both of these correlations would be obtained from such a supposed thermal energy harvester.[5]

The research team, composed of scientists from the Hong Kong Polytechnic University, Pacific Northwest National Laboratory, and Nanjing University of Aeronautics and Astronautics, were able to light a light-emitting diode using a series combination of six of these devices.

Since the amount of graphene is small, the calculated power density is about 70 KW/Kg, but this ignores the solution. The claimed power source is the thermal motion of ions in the solution, which have speeds of hundreds of meters per second at room temperature, and energies of about 4 kJ⋅kg^-1⋅K^-1.

All this sounds too good to be true, so there's quite a bit of skepticism among scientists.[6] Unexpected results like these are interesting, because they can lead to new lines of scientific inquiry. At the same time, science says that extraordinary claims require extraordinary evidence. Wanlin Guo, the graduate supervisor of one of the team members, Guoan Tai, while he was at Nanjing University, has never seen an output voltage greater than about 0.1 millivolts in his own experiments; then again, he did see a voltage.[6]

Zihan Xu, an author of both thermal device papers, says that he is "100% confident" that his experiments are true and that he can "repeat them anywhere and anytime."[6] Nikhil Koratkar, an author of the study involving the generation of electricity from ions flowing past graphene, thinks that there might be something there.[6] In any case, the device is so simple to prepare that many confirmatory experiments will be attempted in various laboratories.[7] That's the true nature of science.

References:

1. B. Raveloa, F. Duvala, S. Kanea and B. Nsomb, "Demonstration of the triboelectricity effect by the flow of liquid water in the insulating pipe," Journal of Electrostatics, vol. 69, no. 6 (December, 2011), pp. 473-478.
2. Prashant Dhiman, Fazel Yavari, Xi Mi, Hemtej Gullapalli, Yunfeng Shi, Pulickel M. Ajayan and Nikhil Koratkar, "Harvesting Energy from Water Flow over Graphene," Nano Letters, vol. 11, no. 8 (August 10, 2011), pp. 3123-3127.
3. Richard Reeves, "A Force of Nature: The Frontier Genius of Ernest Rutherford" (W. W. Norton, December 3, 2007, ISBN-13:978-0393057508), via Amazon.
4. Zihan Xu, and Guo'an Tai, "Electricity generated from Ambient Heat by Pencils," arXiv Preprint Server, June 17, 2012.
5. Zihan Xu, Guoan Tai, Yungang Zhou, Fei Gao, Kin Hung Wong, "Self-Charged Graphene Battery Harvests Electricity from Thermal Energy of the Environment," arXiv Preprint Server, March 12, 2012.
6. Edwin Cartlidge, "Sparks fly over graphene energy device," Nature Blog, March 15, 2012.
7. Since I'm a thermodynamicist, one experiment I would do is light an external LED while monitoring the device temperature in an insulated vessel, such as a Dewar flask. The device should cool, since conservation of energy is always supposed.