YIN Jun,LI Xuemei,YU Jin,ZHANG Zhuhua, ZHOU Jianxin, GUO Wanlin. Generating electricity by moving a droplet of ionic liquid along grapheme. Nature Nanotechnology 2014, 9, 378–383.

NATURE NANOTECHNOLOGY | ARTICLE

Generating electricity by moving a droplet of ionic liquid along grapheme

Jun Yin, Xuemei Li, Jin Yu, Zhuhua Zhang, Jianxin Zhou & Wanlin Guo

Affiliations
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, 29 Yudao Street, Nanjing 210016, China

Contributions
W.G. conceived the project and designed the experiments with J.Yin. J.Yin, X.L. and J.Z. performed the experiments. J.Yu and Z.Z. performed the calculations. W.G., J.Yin and J.Yu analysed the data. W.G., J.Yin and Z.Z. wrote the paper. All authors discussed the results and commented on the manuscript.

Abstract
Since the early nineteenth century, it has been known that an electric potential can be generated by driving an ionic liquid through fine channels or holes under a pressure gradient. More recently, it has been reported that carbon nanotubes can generate a voltage when immersed in flowing liquids, but the exact origin of these observations is unclear, and generating electricity without a pressure gradient remains a challenge. Here, we show that a voltage of a few millivolts can be produced by moving a droplet of sea water or ionic solution over a strip of monolayer graphene under ambient conditions. Through experiments and density functional theory calculations, we find that a pseudocapacitor is formed at the droplet/graphene interface, which is driven forward by the moving droplet, charging and discharging at the front and rear of the droplet. This gives rise to an electric potential that is proportional to the velocity and number of droplets. The potential is also found to be dependent on the concentration and ionic species of the droplet, and decreases sharply with an increasing number of graphene layers. We illustrate the potential of this electrokinetic phenomenon by using it to create a handwriting sensor and an energy-harvesting device.

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