Harvesting Energy from Water Flow over Graphene?

It is reported excitingly in a previous letter (<i>Nano Lett.</i> <b>2011</b>, <i>11</i>, 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