Output density quantification of electricity
generation by flowing deionized water ongraphene
Cite as: Appl. Phys. Lett. 117, 123905 (2020); https://doi.org/10.1063/5.0018862
Submitted: 18 June 2020 . Accepted: 10 August 2020 . Published Online: 23 September 2020
Kei Kuriya, Kotaro Ochiai,

Golap Kalita,

Ohara, Ichiro Yamashita, Fumio S. Ohuchi,
Okada

Masaki Tanemura,
M. Meyyappan,

Atsuki Komiya,

Gota Kikugawa,

Taku

Seiji Samukawa, Katsuyoshi Washio, andTakeruCOLLECTIONSThis paper was selected as Featured
This paper was selected as Scilight

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Appl. Phys. Lett. 117, 123905 (2020); https://doi.org/10.1063/5.0018862© 2020 Author(s).

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Output density quantification of electricity
generation by flowing deionized wateron graphene
Cite as: Appl. Phys. Lett. 117, 123905 (2020); doi: 10.1063/5.0018862
Submitted: 18 June 2020 . Accepted: 10 August 2020 .
Published Online: 23 September 2020
Kei Kuriya,1 Kotaro Ochiai,1 Golap Kalita,2Masaki Tanemura,2Atsuki Komiya,3
Gota Kikugawa,33
4
56
Taku Ohara,
Ichiro Yamashita, Fumio S. Ohuchi, M. Meyyappan,
Seiji Samukawa,3,7
Katsuyoshi Washio,1and Takeru Okada1,a)AFFILIATIONS
1

Department of Electronic Engineering, Tohoku University, Aoba 6-6-05, Aramaki, Sendai 980-8579, Japan2

Department of Physical Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho,Showa-ku, Nagoya 466-8555, Japan

3

Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Sendai 980-8579, Japan

4

Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Department of Materials Science and Engineering, University of Washington, Box 352120, Seattle, Washington 98195, USA

56NASA Ames Research Center, Moffett Field, California 94035, USA

7

AIMR, Tohoku University, 2-1-1 Katahira, Sendai 980-8579, Japana)

Author to whom correspondence should be addressed: takeru.okada@tohoku.ac.jp

ABSTRACTEnergy conversion by water flowing over graphene is a promising mode of energy harvesting. However, the mechanism, energy-conversion
efficiency, and quantification of power density for this mode of interfacial electricity generation remain unknown. Quantification of the output performance for the flow-induced electricity generation using graphene is presented in this work. The output performance per unit contact
water and graphene is found to be proportional to the flow speed, with an electromotive force density of 0.0025! area between"½ lV=mm2 =ðmm=sÞ] for the conditions used in this work. The ability to quantify output density will help to construct guidelines for futureapplications of this form of electricity generation.
Published under license by AIP Publishing. https://doi.org/10.1063/5.0018862

The ever-increasing global energy demand requires the development of alternative modes of energy harvesting. Thus, next generationenergy-conversion systems aim to combine off-the-grid devices with
nano- and micro-based technologies to create feasible solutions.
Among these, an energy-conversion system that utilizes carbon nanomaterials and flowing liquid has great potential because of the abundance of natural bodies of water on the earth. Examples of interfacial
electricity generation by liquid flow over carbon materials have beenreported by several groups.1–7 However, the mechanisms proposed todescribe the phenomena including momentum transfer,8 coulombic
field-based theories,9–12 capacitive charge and discharge,13 and streaming potential14–17 are still under debate. Some studies consider thisform of electricity generation a type of triboelectric nanogenerator,18–22 while others consider the role of specific solute ions in theAppl. Phys. Lett. 117, 123905 (2020); doi: 10.1063/5.0018862
Published under license by AIP Publishingprimary mechanism despite the fact that electricity can be generated
on carbon materials by both de-ionized (DI) water and electrolyte
solutions.
The lack of a definite mechanism makes the conversion efficiencyof liquid flow over carbon materials difficult to define at the nano- andmicro-scale. In turn, the lack of clear conversion efficiency makes
decision-making regarding the practical utilization impossible, thus
hindering the application potential. The total amount of energy generation is not a crucial problem in this system; once devices are deployedin an environment, the amount of generated energy is free from thedevice volume because the environment is regarded as an infinite
energy reservoir. While the need for quantification of the energy generation is generally acknowledged as important, it has not been measured to date. Here, we report the results of our effort on the117, 123905-1

Applied Physics Letters

quantification of electricity generation at the interface between graphene and flowing water. The electromotive force from water flow ongraphene is defined by the average flow speed in a micro-channel andthe contact area between graphene and water.
A glass plate used as a substrate has access ports for water andelectrical connections. The plate was washed successively with acetone,
ethanol, and DI water in a sonication bath. Pristine graphene was synthesized by low pressure chemical vapor deposition on copper foilusing polystyrene as a carbon source.23,24 Graphene was transferred
onto the substrate using poly-methyl-methacrylate (PMMA) as a sacrificial film. The PMMA film was removed by dipping in acetone overnight at room temperature. PMMA residue was then etched byannealing in a 3% hydrogen/argon mixture at 300 $ C for 3 h. In theschematic in Fig. 1(a), a silicone spacer was sandwiched between thegraphene on the substrate and the top plate. The height and width of
the spacer, which give the cross section of the fluidic channel, are 300
and 3400 lm, respectively. Poly-tetra-fluoro-ethylene tubes were usedfor DI water supply, and the water flow was driven by suction using asyringe pump (YMC Corporation, YSP-201) to avoid leakage byapplied pressure. Gold electrodes were used with an arbitrary distanced on graphene and connected to a digital multimeter (Keithley,
DM7510) by coaxial cables through the electrical connecting ports atthe top plate. The electrodes were located outside the fluidic channel
to prevent contact with water. A tunable external load resistance wasconnected in series to the graphene, and a digital multimeter recorded
the generated voltage between both ends of the load. The time interval
between voltage measurements was every 20 ms. All measurements
were made at room temperature in a space that was electrically

FIG. 1. (a) Schematic illustration of a micro-flow-chip with graphene. The flow chip
consists of (1) glass top plate, (2) glass substrate, (3) graphene, (4) electrodes, (5)silicone spacer, (6) electrical connection ports, (7) channel width, (8) channel
length: d, (9) load resistance, RL, and (10) digital multimeter. (b) Typical Raman
spectrum of graphene. The spot size of the laser is approximately 1 lm.

Appl. Phys. Lett. 117, 123905 (2020); doi: 10.1063/5.0018862
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shielded. We used undoped pristine graphene in this study and no
doping effects were considered.
The Raman spectrum measurement was carried out using a commercial micro-Raman system (JASCO, NRS-5100). A typical Raman
spectrum of graphene on the substrate shown in Fig. 1(b) was takenusing a 532-nm laser through a 100% objective lens. Three major
peaks were observed at 1350, 1580, and 2700 cm&1, which areassigned to the D-, G-, and G0 -bands, respectively. The G0 -band consists of a single component, and the intensity ratio of the G-band andG0 -band indicates monolayer graphene.25 The peak of the D-band
originates from the transfer process. The typical electrical resistance ofthe graphene varied from 0.1 kX to several tens of kX.
The electrical property of graphene during water flow was investigated first. The electrical signals were measured at both ends of thegraphene along the water-flow direction. A single pulse-like flow wasapplied for voltage generation. The generated voltage was observed
just after flow started and followed the on/off switching [Fig. 2(a)].
Figure 2(b) shows that the resistance of graphene stays constant during
the water-flow, indicating that the load resistance governs the output
like a typical battery. The direction of the electrical current is always
the same as that of the water flow.
Next, the cyclic response of the voltage generation was investigated. Figure 3(a) shows that the response is repeatable and reproducible, and the magnitude of the generated voltage depends on the flow
speed. A delay in the signal response is observed under higher flow
speed conditions, originating from the experimental setup. This delay
may be caused by the shape change of the connecting components at
higher water pressures or by the inertia of water in the microchannel.
The external load resistance was tuned to maximize the output performance. Figure 3(b) indicates that the generated voltage approaches saturation at high load resistance, as expected. The electromotive force isobtained using the following equation:

FIG. 2. (a) Generated voltage and (b) resistance of graphene. The channel lengthand flow speed are 3 mm and 245 mm/s, respectively.

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FIG. 3. Measurements of typical (a) voltage signal, (b) generated voltage, and (c) electromotive force. (d) Measured output power and calculation curve. The internal electric
resistance of graphene is 26 kX in this case.

E¼

r þ RLV;
RL(1)

where E, V, r, and RL are the electromotive force, generated voltage,electrical resistance of graphene, and external load resistance, respectively. Figure 3(c) indicates that the electromotive force remains nearly
constant throughout the measurements with different load resistancevalues. Furthermore, the electromotive force is also affected by flow
speed as seen in Fig. 3(a). Based on these results, a theoretical curve for
output power P is calculated using the following equation:
P¼

RL
E2 :
ðr þ RL Þ2(2)

Figure 3(d) compares the calculated P with measured values showing
clear agreement. The maximum output under each flow speed is determined using this optimization.
The effect of the channel length on electricity generation wasinvestigated next using several devices. The external load resistance foreach device was optimized in advance for this investigation. Figure4(a) shows the dependence of the electromotive force on flow speed.
The electromotive forces from all devices with different channel
lengths increase linearly with flow speed. The electromotive force perunit channel area! is obtained" after normalization [Fig. 4(b)] with a gradient of 0.0025 [ lV=mm2 =ðmm=sÞ], which is the maximum output
performance of this experimental design. Similarly, the maximum output power is obtained by fitting a quadratic function using Ohm’s law.
When DI water is used, the maximum performance of the flow-

induced electricity per unit area is determined by the flow speed. The
results above clearly show that the generated electromotive force is
proportional to both the channel area (contact area between graphene
and water) and flow speed of water, indicating that the electric-field
gradient in graphene changes linearly along the flow direction.
Several reports on electricity generation by liquid flow on carbon
have focused on the role of electrolytes,3,26 explaining this phenomenon using a coulombic transfer model. However, our results here demonstrate that DI water flow can also generate electricity over graphene,
indicating that a strong contribution of specific ions from the electrolyte solution is not the correct mechanism. This form of electricity
generation probably requires either a polar liquid or an ionized solution. Likely, the ions in electrolyte solutions from previous experiments merely enhanced the output performance. Furthermore, otherreports have proposed momentum transfer theory as a possible mechanism,8 where molecules from the flowing liquid medium transfer
their momentum to the graphene phonons, inducing carrier motion
via phonon-carrier interactions. In this case, the polarity of the generated voltage should depend on the type of charge carrier. Metallic
materials generate high potential on the downstream side, while the ptype semiconductor does the opposite.27 The results from this experiment including other reports3 show that the electrical current and liquid flow are always in the same direction when bulk graphene is used.
Since this form of electricity generation is observed only from bulk
graphene, further investigations are required to explain this phenomenon by momentum transfer theory. This suggests that both the coulombic transfer model and the momentum transfer theory are limited

FIG.
! 4. (a) "Dependence of the generated voltage on the channel length. (b) Normalized electromotive force. The solid line is linear fitting. The gradient of the fitting is 0.0025[ lV=mm2 =ðmm=sÞ]. (c) Normalized output power. The solid curve is fitted by the quadratic function.

Appl. Phys. Lett. 117, 123905 (2020); doi: 10.1063/5.0018862
Published under license by AIP Publishing

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Applied Physics Lettersto specific conditions of electricity generation from the graphene–water interface.
The formation of a continuous potential gradient in parallel tothe flow direction still needs to be explained. This condition implies
that there should be a difference between upstream and downstreamof water flow.28–30 Streaming potential may not be appropriate to
explain this phenomenon. This theory explains potential formation ina solution under the precondition that a fixed layer as a part of theelectrical double layer exists. The properties of the channel walls except
surface charge are not considered. Here, the capacitance charge/discharge model is applicable in terms of imbalance along the flow direction. However, while water flow is laminar in a typical macroscopic
channel, the flow state is poorly understood at the microscopic scaleby the experimental study, especially near the channel walls, where theelectric effects of graphene cannot be accounted for using classicalmodels. Therefore, future work should consider experimental differentiation between different states of the upstream and downstream flowin microchannels. This step will provide the key parameters to help
solve the mechanism of electricity generation by liquid flow on carbon
materials.
In conclusion, this work has quantified the output performanceof electricity generation from the interface between graphene andflowing water. The results reveal that the generation of electromotive
force is proportional to both the flow speed of water and the contact
area, indicating the formation of a continuous potential gradient. In
addition, the maximum performance of! the electricity
generation per"unit area in the current setup is 0.0025 [ lV=mm2 =ðmm=sÞ].

This work was supported by KAKENHI Grant No. 18K04880
and the Nanotechnology Platform Japan Program. The authors
thank M. Nemoto for providing technical support for the
spectroscopy measurement. They also acknowledge vitally
important encouragement and support made through the
University of Washington—Tohoku University: Academic OpenSpace (UW-TU: AOS).
DATA AVAILABILITY
The data that support the findings of this study are available
from the corresponding author upon reasonable request.
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