State Key Laboratory of Mechanics and Control of Mechanical Structures, Key Laboratory for Intelligent Nano Materials and Devices of the Ministry of Education, and Institute of Nanoscience, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
Nano Lett., Article ASAP
DOI: 10.1021/nl300636g
Publication Date (Web): March 1, 2012
Copyright © 2012 American Chemical Society
It is reported excitingly in a previous letter (Nano Lett.2011, 11, 3123) that a small piece of graphene sheet about 30 × 16 μm2 immersed in flowing water with 0.6 M hydrochloric acid can produce voltage 
20 mV. Here we find that no measurable voltage can be induced by the flow over mono-, bi- and trilayered graphene samples of 1 × 1.5 cm2 in size in the same solution once the electrodes on graphene are isolated from interacting with the solution, mainly because the H3O+ cations in the water adsorb onto graphene by strong covalent bonds as revealed by our first-principles calculations. When both the graphene and its metal electrodes are exposed to the solution as in the previous work, water flow over the graphene-electrode system can induce voltages from a few to over a hundred millivolts. In this situation, the graphene mainly behaves as a load connecting between the electrodes. Therefore, the harvested energy is not from the immersed carbon nanomaterials themselves in ionic water flow but dominated by the exposed electrodes.
20 mV. Here we find that no measurable voltage can be induced by the flow over mono-, bi- and trilayered graphene samples of 1 × 1.5 cm2 in size in the same solution once the electrodes on graphene are isolated from interacting with the solution, mainly because the H3O+ cations in the water adsorb onto graphene by strong covalent bonds as revealed by our first-principles calculations. When both the graphene and its metal electrodes are exposed to the solution as in the previous work, water flow over the graphene-electrode system can induce voltages from a few to over a hundred millivolts. In this situation, the graphene mainly behaves as a load connecting between the electrodes. Therefore, the harvested energy is not from the immersed carbon nanomaterials themselves in ionic water flow but dominated by the exposed electrodes. 
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