RPI researchers show power generation from water flow over graphene-coated surfaces; potential for autonomous microsensors for applications in oil and gas exploration
|Induced voltage as a function of fluid flow velocity for the graphene film (exposed to ~0.6 and ~0.3 M HCl solutions) and for a multiwalled carbon nanotube (MWNT) film exposed to a ~0.6 M HCl solution. Credit: ACS, Dhiman et al. Click to enlarge.|
Researchers at Rensselaer Polytechnic Institute (RPI) and Rice University led by RPI Professor Nikhil Koratkar have shown that flowing water with various molarities of hydrochloric acid (HCl) over graphene generates induced voltages an order of magnitude higher as compared to carbon nanotubes. This discovery could hasten the creation of self-powered microsensors for more accurate and cost-efficient exploration for oil and gas.
In a paper published in the ACS journal Nano Letters, the research team reports generating 85 nanowatts of power from a sheet of graphene measuring .03 millimeters by .015 millimeters, equating to a power per unit area of ~175 W/m2. Molecular dynamics simulations indicated that the power generation is primarily caused by a net drift velocity of adsorbed Cl- ions on the continuous graphene film surface. The goal is to develop tiny self-powered autonomous sensors that can be introduced into water or other fluids and pumped down into a potential well. This is the first research paper to result from a $1 million grant awarded to Koratkar’s group in March 2010 by the Advanced Energy Consortium.
Over the past decade a number of theoretical and experimental studies have demonstrated that flow of water or other polar liquids over one-dimensional structures such as carbon nanotubes generates a net potential difference and associated electric current in the nanotubes along the flow direction...However, a common observation from the experimental studies with carbon nanotubes is that the induced voltages generated by water flow are <10 mV. While a few millivolts signal is sufficient for miniaturized flow sensor type application, it is not enough for harvesting significant amounts of energy from the flow environment.
Here, we report the results of flow transport experiments over few-layered graphene films and observe an order of magnitude increase in induced voltage for graphene compared to carbon nanotubes.—Dhiman et al.
Hydrocarbon exploration is an expensive process that involves drilling deep down in the earth to detect the presence of oil or natural gas. Koratkar said oil and gas companies would like to augment this process by sending out large numbers of microscale or nanoscale sensors into new and existing drill wells. These sensors would travel laterally through the earth, carried by pressurized water pumped into these wells, and into the network of cracks that exist underneath the earth’s surface.
As the injected water moves through subsurface cracks and crevices, the devices could detect the presence of hydrocarbons and can help uncover hidden pockets of oil and natural gas. Oil companies would no longer be limited to vertical exploration, and the data collected from the sensors would provide these firms with more information for deciding the best locations to drill.
The team’s discovery is a potential solution for a key challenge to realizing these autonomous microsensors, which will need to be self-powered. The graphene-covered microsensors can harvest the power necessary to relay collected data and information back to the surface as water flows over the coating.
It’s impossible to power these microsensors with conventional batteries, as the sensors are just too small. So we created a graphene coating that allows us to capture energy from the movement of water over the sensors. While a similar effect has been observed for carbon nanotubes, this is the first such study with graphene. The energy-harvesting capability of graphene was at least an order of magnitude superior to nanotubes. Moreover, the advantage of the flexible graphene sheets is that they can be wrapped around almost any geometry or shape. We’ll wrap the graphene coating around the sensor, and it will act as a “smart skin” that serves as a nanofluidic power generator.—Nikhil Koratkar
For this study, Koratkar’s team used graphene that was grown by chemical vapor deposition on a copper substrate and transferred onto silicon dioxide. The researchers created an experimental water tunnel apparatus to test the generation of power as water flows over the graphene at different velocities.
Along with physically demonstrating the ability to generate 85 nanowatts of power from a small fragment of graphene, the researchers used molecular dynamics simulations to better understand the physics of this phenomenon. They discovered that chloride ions present in the water stick to the surface of graphene. As water flows over the graphene, the friction force between the water flow and the layer of adsorbed chloride ions causes the ions to drift along the flow direction. The motion of these ions drags the free charges present in graphene along the flow direction, creating an internal current.
This means the graphene coating requires ions to be present in water to function properly. Therefore, oil exploration companies would need to add chemicals to the water that is injected into the well. Koratkar said this is an easy, inexpensive solution.
Along with Koratkar, co-authors on the paper include: Yunfeng Shi, assistant professor in the Department of Materials Science and Engineering at Rensselaer; Rensselaer mechanical engineering graduate students Prashant Dhiman and Fazel Yavari; Rensselaer physics graduate student Xi Mi; along with Pulickel Ajayan, the Benjamin M. and Mary Greenwood Anderson Professor of Engineering at Rice University; and Rice graduate student Hemtej Gullapalli.
Prashant Dhiman, Fazel Yavari, Xi Mi, Hemtej Gullapalli, Yunfeng Shi, Pulickel M. Ajayan, Nikhil Koratkar (2011) Harvesting Energy from Water Flow over Graphene. Nano Letters Article ASAP doi: 10.1021/nl2011559