Generating electricity by moving a droplet of ionic liquid along graphene

Journal name:
Nature Nanotechnology
Year published:
Published online


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.

At a glance


  1. Illustration of the experimental set-up.
    Figure 1: Illustration of the experimental set-up.

    A liquid droplet is sandwiched between graphene and a SiO2/Si wafer, and drawn by the wafer at specific velocities. Inset: a droplet of 0.6 M NaCl solution on a graphene surface with advancing and receding contact angles of ~91.9° and ~60.2°, respectively.

  2. Voltage induced in graphene by drawing one or more droplets of 0.6 M NaCl solution.
    Figure 2: Voltage induced in graphene by drawing one or more droplets of 0.6 M NaCl solution.

    a, Typical voltage signal produced by drawing a droplet on a graphene strip from left (L) to right (R) ends and then back. The inset highlights the voltage signal during the movement of the droplet. b, Voltage induced by moving one, two and three droplets. Dashed lines are curves linearly fitted to the measured data. The error bars represent the amplitudes of fluctuation in the voltage signal as highlighted by the inset in a. c,d, Voltage response to triangular-wave-like (c) and sine-wave-like (d) velocity of movement (red dashed lines) of a droplet.

  3. Mechanism for the drawing potential.
    Figure 3: Mechanism for the drawing potential.

    a, DFT results for the distribution of differential charge near monolayer graphene caused by adsorbing one to three rows of hydrated sodium cations, and the corresponding adsorption energy (Ea). b, Schematic illustration of the pseudocapacitance formed by a static droplet on graphene. c, Schematic illustration of the potential difference induced by a moving droplet. d, Equivalent circuit for c. Solid arrows indicate the flow direction of electrons in graphene and Na+ ions in the droplet. e, Simplified circuit of the system. f, Schematic illustration and equivalent circuit for three moving droplets on graphene. g, Drawing potential and resistance change with number of graphene layers. Inset: Advancing and receding contact angles of the solution on single-, bi- and trilayer graphene. h, Equivalent circuit for a moving droplet on trilayer graphene.

  4. Contact angles and drawing potential for various ionic solutions on monolayer graphene.
    Figure 4: Contact angles and drawing potential for various ionic solutions on monolayer graphene.

    a, Advancing and receding contact angles of deionized water and different 0.6 M solutions on graphene. b, Voltage induced by three droplets of different solutions. c, Fitted slope A = V/v for three droplets of different chloride and sodium salts.

  5. Applications of the drawing potential.
    Figure 5: Applications of the drawing potential.

    a, Photograph of handwriting with a Chinese brush on graphene. b, Sensing the stroke directions (arrows) by the drawing potentials between electrodes E1+– E1 and E2+– E2 as shown in a. c,d, Voltage (c) and power (d) produced by dropping droplets of 0.6 M CuCl2 solution onto graphene at an angle of 70° (inset) from a height of 15 cm. The output power is measured through a connected load  =  17.4 k.


  1. Delgadoa, A. V. et al. Measurement and interpretation of electrokinetic phenomena. J. Colloid Interface Sci. 309, 194224 (2007).
  2. Ghosh, S., Sood, A. K. & Kumar, N. Carbon nanotube flow sensors. Science 299, 10421044 (2003).
  3. Zhao Y. et al. Individual water-filled single-walled carbon nanotubes as hydroelectric power converters. Adv. Mater. 20, 17721776 (2008).
  4. Liu, J., Dai, L. & Baur, J. W. Multiwalled carbon nanotubes for flow-induced voltage generation. J. Appl. Phys. 101, 064312 (2007).
  5. Král, P. & Shapiro, M. Nanotube electron drag in flowing liquids. Phys. Rev. Lett. 86, 131134 (2001).
  6. Yuan, Q. & Zhao, Y. P. Hydroelectric voltage generation based on water-filled single-walled carbon nanotubes. J. Am. Chem. Soc. 131, 63746376 (2009).
  7. Persson, B. N., Tartaglino, J. U., Tosatti, E. & Ueba, H. Electronic friction and liquid-flow-induced voltage in nanotubes. Phys. Rev. B 69, 235410 (2004).
  8. Cohen, A. E. Carbon nanotubes provide a charge. Science 300, 12351236 (2003).
  9. Schedin, F. et al. Detection of individual gas molecules adsorbed on graphene. Nature Mater. 6, 652655 (2007).
  10. Robinson, J. T., Perkins, F. K., Snow, E. S., Wei, Z. & Sheehan, P. E. Reduced graphene oxide molecular sensors. Nano Lett. 8, 31373140 (2008).
  11. Wehling, T. O. et al. Molecular doping of graphene. Nano Lett. 8, 173177 (2008).
  12. Fowler, J. D. et al. Practical chemical sensors from chemically derived graphene. ACS Nano 3, 301306 (2009).
  13. Yin, J., Zhang, Z. H., Li, X. M., Zhou, J. X. & Guo, W. L. Harvesting energy from water flow over graphene? Nano Lett. 12, 17361741 (2012).
  14. Dhiman, P. et al. Harvesting energy from water flow over graphene. Nano Lett. 11, 31233127 (2011).
  15. Newaz, A. K. M., Markov, D. A., Prasai, D. & Bolotin, K. I. Graphene transistor as a probe for streaming potential. Nano Lett. 12, 29312935 (2012).
  16. Nandi, D., Finck, A. D. K., Eisenstein, J. P., Pfeiffer, L. N. & West, K. W. Exciton condensation and perfect Coulomb drag. Nature 488, 481484 (2012).
  17. Gorbachev, R. V. et al. Strong Coulomb drag and broken symmetry in double-layer graphene. Nature Phys. 8, 896901 (2012).
  18. Weber, C. P. et al. Observation of spin Coulomb drag in a two-dimensional electron gas. Nature 437, 13301333 (2005).
  19. Price, A. S., Savchenko, A. K., Narozhny, B. N., Allison, G. & Ritchie, D. A. Giant fluctuations of coulomb drag in a bilayer system. Science 316, 99102 (2007).
  20. Yan, Z. et al. Toward the synthesis of wafer-scale single-crystal graphene on copper foils. ACS Nano 6, 91109118 (2012).
  21. Li, X. et al. Large-area synthesis of high-quality and uniform graphene films on copper foils. Science 324, 13121314 (2009).
  22. Li, X. et al. Transfer of large-area graphene films for high-performance transparent conductive electrodes. Nano Lett. 9, 43594363 (2009).
  23. Lyklema, J. J., de Keizer, A., Bijsterbosch, B. H., Fleer, G. J. & Cohen Stuart, M. A. Fundamentals of Interface and Colloid Science (Academic, 1995).
  24. Kresse, G. & Furthmuller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 54, 1116911186 (1996).
  25. Kresse, G. & Furthmuller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 6, 1550 (1996).
  26. Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 38653868 (1996).
  27. Kresse, G. & Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 59, 17581775 (1999).
  28. Kielland, J. Chemical hydration numbers. J. Chem. Educ. 14, 412413 (1937).
  29. Makov, G. & Payne, M. C. Periodic boundary conditions in ab initio calculations. Phys. Rev. B 51, 40144022 (1995).

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  1. 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

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


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.

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