**Electrochemical actuation of carbon nanotube yarns**

> Tissaphern Mirfakhrai[^1], Jiyoung Oh^2^, Mikhail Kozlov[^2],
>
> Eddie Chi Wah Fok^1,^ Mei Zhang^2^, Shaoli Fang^2^,
>
> Ray H Baughman^2^ and John D W Madden^1^

**Abstract**

We report on actuation in high tensile strength yarns of twist-spun
multiwall carbon nanotubes. Actuation in response to voltage ramps and
potentiostatic pulses is studied to quantify the dependence of the
actuation strain on the applied voltage. Strains of up to 0.5 % are
obtained in response to applied potentials of 2.5 V. The dependence of
strain on applied voltage and charge is found to be quadratic, in
agreement with previous results on the actuation of single walled carbon
nanotubes, with the magnitude of strain also being very similar. The
specific capacitance reaches 26 F/g. The modulus of the yarns was found
to be independent of applied load and voltage within experimental
uncertainty.

# Introduction

Individual carbon nanotubes (CNTs) offer exceptional properties. The
tensile modulus of single walled carbon nanotubes (640 GPa) approaches
that of diamond, while their tensile strength of approximately 20-40 GPa
is about ten times higher than for continuous yarns or fibers of any
type \[1\]. Many research groups have been working to make bulk
assemblies or yarns that can approach the modulus and tensile strength
of individual CNTs. Interest in nanotube assemblies expanded in a new
direction when it was found that sheets of nanotubes (Bucky paper)
actuate when used as an electrode in an electrochemical cell \[2\].
Since then actuation strains of up to 0.4 % \[3\] have been reported at
effective strain rates of as high as 19 %/s
\[4\].![](media/image1.png){width="2.876388888888889in"
height="2.7888888888888888in"}

Electrochemical double layer charge injection in the nanotubes is
believed to induce actuation via quantum mechanical and electrostatic
effects that dominate for small and large degrees of charge injection,
respectively \[2, 5\]. This double layer charge injection, shown in
Figure 1, is a non-faradaic process in which changing the charge on the
carbon atoms results in changes of C-C bond lengths \[6, 7\]. The excess
charge on the nanotubes is compensated at the nanotube-electrolyte
interface by ions (cations or anions). The nanotubes act as double layer
capacitors that are charged and discharged during actuation cycles.

Because the Young\'s modulus of individual CNTs is extremely high, a
relatively small strain of actuation can result in high stress
generation, producing unprecedented work densities per cycle \[8\]. The
actuation process can potentially be fast if charge can be injected
quickly, leading to high power to mass ratios \[4\]. CNTs are stable at
temperature as high as 1000°C if sealed from oxygen, making them
appropriate for high-temperature actuation. However, before any of this
can be realized it is necessary to make macroscopic structures out of
CNTs that preserve the superb mechanical and electronic properties of
the component CNTs. Actuation of sheets of multiwall carbon nanotubes
(MWNTs) has been reported \[9\], showing reasonable strain but having
only moderate tensile strength. Another approach is to spin fibers of
aligned nanotubes. The present paper reports the first actuation
measurements made on MWNT yarns that have been recently developed by the
NanoTech Institute of the University of Texas at Dallas \[10\]. A
scanning electron micrograph of such a yarn is show in Figure 2.

# Methods 

MWNT yarns were prepared by a dry spinning method \[10\], i.e., by
drawing and twisting tubes from a forest. The MWNT forest was
synthesized by catalytic CVD using acetylene gas as the carbon source.
MWNTs, which are about 10 nm in diameter, are simultaneously drawn from
the MWNT forest and twisted. The nanotube length in the forest is about
300 µm, resulting in yarn diameters between 1 and 60 µm, depending upon
the forest width used for spinning. The twist is characterized by the
helix angle (α), which depends directly upon the degree of twist and
inversely on the yarn diameter. The degree of twist is typically 15,000
turns/m. ![](media/image2.png){width="3.00625in"
height="2.259027777777778in"}

# In order to apply forces and record actuation forces and displacements generated by the CNT yarns, an Aurora Scientific ASI 300 muscle analyzer and its LabView-based software (www.aurorascientific.com) have been employed. Figure 3 is a photo of the apparatus used to apply force and of the yarn and clamping mechanism. The fiber is mounted between a lower fixed clamp and an upper movable motor arm. A computer- controlled potentiostat, similar to the one described in \[11\], is used to apply potentials. A data acquisition card (National Instruments 6036E) gathers the force and displacement data from the muscle analyzer and also logs the applied voltage and current. Figure 4 is a block diagram of the measurement setup with the yarn as one of the electrodes in the actuation cell. This configuration is referred to as Setup 1. ![](media/image3.png){width="3.004166666666667in" height="4.0055555555555555in"} {#in-order-to-apply-forces-and-record-actuation-forces-and-displacements-generated-by-the-cnt-yarns-an-aurora-scientific-asi-300-muscle-analyzer-and-its-labview-based-software-www.aurorascientific.com-have-been-employed.-figure-3-is-a-photo-of-the-apparatus-used-to-apply-force-and-of-the-yarn-and-clamping-mechanism.-the-fiber-is-mounted-between-a-lower-fixed-clamp-and-an-upper-movable-motor-arm.-a-computer--controlled-potentiostat-similar-to-the-one-described-in-11-is-used-to-apply-potentials.-a-data-acquisition-card-national-instruments-6036e-gathers-the-force-and-displacement-data-from-the-muscle-analyzer-and-also-logs-the-applied-voltage-and-current.-figure-4-is-a-block-diagram-of-the-measurement-setup-with-the-yarn-as-one-of-the-electrodes-in-the-actuation-cell.-this-configuration-is-referred-to-as-setup-1. .unnumbered}

# For some of the strain measurements a Perkin-Elmer DMA 7e with a custom sample holder and electrochemical cell was employed. The voltage was applied and current recorded via a Gamry Instruments potentiostat. This setup is referred to as Setup 2. All observed dimensional changes were normalized by sample length and are reported in percent.  {#for-some-of-the-strain-measurements-a-perkin-elmer-dma-7e-with-a-custom-sample-holder-and-electrochemical-cell-was-employed.-the-voltage-was-applied-and-current-recorded-via-a-gamry-instruments-potentiostat.-this-setup-is-referred-to-as-setup-2.-all-observed-dimensional-changes-were-normalized-by-sample-length-and-are-reported-in-percent. .unnumbered}

![](media/image4.png){width="5.993055555555555in"
height="4.965277777777778in"}

Figure 4: The experimental setup for measuring actuation of the yarns

# Actuation

A 12 mm length of single-ply yarn with a diameter of 18 µm was loaded to
11 MPa using Setup 1 and was left in the electrolyte to relax for 30
minutes. The electrolyte used is 0.2 M tetrabutylammonium
hexafluorophosphate (TBAP) in acetonitrile (AN). Afterwards a triangular
voltage input was applied at a scan rate of 1000 mV/s, and the resulting
currents and strains were measured. The reference electrode was aqueous
Ag/AgCl and the counter electrode was a sheet of polypyrrole. The charge
transferred (Figure 5d) was calculated by numerically integrating the
current (Figure 5c).

The strain during the CV is plotted vs. nominal charge per C atom
(including atoms on the interior of the tube) in Figure 6 for charging
and discharging half-cycles with a quadratic fit for the charging
half-cycle superimposed. The number of atoms is calculated using the
measured density of the yarns (0.8 g/cm^3^) and the molar weight of the
carbon atom (12 g/mol). The results match those obtained using sheets of
carbon single walled nanotubes (SWNTs) \[5\] over the same range of
nominal charge per atom. The similarity in actuation is consistent with
the fact that the SWNT are bundled, since charge injection occurs
non-uniformly in both cases -- on the outer SWNTs in a SWNT bundle and
the outer wall of MWNTs. Although the MWNTs are also bundled, the
surfaces of the MWNTs are largely exterior to the bundle, where double
layer charge injection can occur.

The bold dots in Figure 6 represent the average strains reported by
Hughes and Spinks \[9\] in MWNT sheets, and the error bars represent the
range of strains. The similarity between the responses for MWNT sheets
and the yarns suggests that the alignment of nanotubes and the degree of
twist is not substantially influencing the magnitude of strain. This
similarity is consistent with the proposed mechanism of CNT actuation in
which strain is the result of charge induced changes in C-C bond lengths
within the nanotube, rather than inter-tube interactions. Although
strain in yarns and sheets is similar in magnitude, the much larger
tensile strengths of the yarns makes them far more suitable for
application.

  -----------------------------------------------------------------------
  ![](media/image5.png){width="4.895138888888889in"
  height="4.374305555555556in"}

  Figure 5: (a) Strain, (b) working electrode voltage, (c) cell current
  and (d) transferred charge during cyclic voltammetry of the MWNT yarn
  in 0.2 M TBAP in acetonitrile for an applied tensile load of 11 MPa.
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image6.png){width="5.4319444444444445in"
  height="4.073611111111111in"}

  **Figure 6: Strain vs. nominal charge per C atom during CV. The origin
  of the horizontal axis is arbitrary since the initial charge of the
  MWNTs is unknown. The dots represent strain to charge observed in MWNT
  sheets \[9\].**
  -----------------------------------------------------------------------

The experiment was repeated at a scan rate of 100 mV/s. The resulting
currents for 1000 mV/s and 100 mV/s are plotted vs. the applied voltage
(Figure 7a). The gravimetric capacitance of the yarn in TBAP in
acetonitrile was estimated using the CVs to be 26±5 F/g over a potential
range of --0.7 V to +0.7 V, as shown by the square response of an ideal
capacitor in Figure 7a. This high capacitance is very similar in
magnitude to the capacitance observed in single wall carbon nanotubes
\[9\], and suggests that the external surfaces of the MWNTs are largely
accessible to electrolyte. The double layer capacitance of the nanotubes
is estimated to be 0.17 F/m^2^. This result is derived from the measured
capacitance and an estimate for the surface area per mass of MWNTs
having an outer diameter of 10 nm and 8 inner layers, where the MWNT
geometry was measured by transmission electron microscopy. It has been
assumed that charge injection is only to outer walls and that all outer
walls are accessible for charge injection. The capacitance drops by
about 20 % as the scan rate is increased from 100 to 1000 mV/s. This can
be because of the existence of fast and slow charging processes in the
yarn similar to those described in \[12\] or may be limited by the mass
transport.

A similar experiment was performed using Setup 2 in 0.5 M
tetrabutylammonium tetrafluoroborate (TBATFB) in acetonitrile (AN) again
vs. Ag/Ag+ and with a Pt mesh/CNT counter electrode. The potential
window was -2.5 to +2.5 V. The response to a triangular input scanned at
10 mV/s is shown in Figure 7.b. The voltammetry results suggest a
capacitive response due to double layer charging of the nanotubes over a
voltage range of approximately −0.7 V to +0.7 V vs. Ag/Ag^+^. The
capacitance at 10 mV/s scan rate in TBATFB in acetonitrile is estimated
at 22±2 F/g. Beyond this potential range a kinetics-limited reaction
appears to be contributing to the current. It may be the result of
electrolyte degradation.

The CV plots are considerably less resistive compared to those of MWNT
sheets reported in \[9\]. Better axial alignment of the nanotubes in the
yarns and fewer defects in the MWNTs used in the yarns are likely causes
of the better conductivity along the yarn axis, resulting in a much less
resistive response. This makes the yarns a much better candidate to be
used as a supercapacitor compared to the MWNT sheets.

  -----------------------------------------------------------------------
  ![](media/image7.png){width="6.973611111111111in"
  height="2.9458333333333333in"}

  **Figure 7: (a) Cyclic voltammogram for a 12mm-long single ply yarn at
  scanned at 1000 and 100 mV/s. The rectangle shows the ideal capacitor
  response for a gravimetric capcacitance of \~ 26 F/g at 1000 mV/s. (b)
  CV of an 18 mm long yarn in 0.5 M tetrabutylammonium tetrafluoroborate
  (TBATFB) in acetonitrile (AN) at a scan rate of 10 mV/s**
  -----------------------------------------------------------------------

Figure 8 shows the actuation strain and the current in response to a
series of voltage pulses with amplitudes between -2.5 V and +2.5 V. The
electrolyte was 0.5 M TBATFB in AN. The maximum actuation strain
achieved was measured to be 0.5 % using Setup 2. A similar experiment
was performed using Setup 1. The voltages were applied using a Solartron
SI 1287 electrochemical interface. The electrolyte was 0.2 M TBAP in AN,
the reference electrode was aqueous Ag/AgCl and the counter electrode
was a sheet of polypyrrole. The potential of the reference electrode
stayed the same before and after the experiment. The pulses in each
train had the same voltage amplitude. The experiment was repeated for
voltage pulses relative to 0 V vs. Ag/AgCl of -2.5V, -2V, -1.5V, -1V,
-0.5V, 0V, 0.5V, 1 V and 2 V. The maximum strain achieved is about 0.2
%.

Some creep is observed during actuation in both cases similar to what
has been reported before \[9, 13\]. The creep seems to depend on the
voltage amplitude and the actuation rate, but the nature of this
dependence has not yet been studied. The creep rate is very close to the
rate in \[9\] but the creep itself is smaller in this case over the same
number of actuation cycles than the creep reported in \[9\]. This
becomes more significant if we consider that actuation in this case
takes place under up to 30 MPa of load as opposed to 200 kPa in \[9\],
demonstrating one of the benefits of employing the yarns.

The amplitude of the resulting actuation strain is plotted vs. the
applied voltage pulse amplitude in Figure 9. For comparison, the
voltages are all plotted versus Ag/Ag+ by adjusting the voltages
measured against Ag/AgCl by 0.335 V \[14\]. The plots from our two
experiments look very similar. This implies that the size difference
between the PF~6~^-^ and BF~4~^-^ ions has little effect on the
actuation. The relationship is very close to a quadratic relationship,
which is expected from previous results in single wall carbon nanotubes
and theory \[2, 6, 7 and 13\]. This supports the suggestions that the
forces producing strain are primarily coulombic in nature when the
amount of charge injection is large. The quadratic fit plotted in Figure
9 is calculated using all data points except the -3 V and -2.5 V. A
different mechanism may be at work for these two points that are at one
voltage extreme, where the actuation strain is much larger than what is
predicted by the quadratic relationship. This mechanism may be related
to the 'pneumatic actuation' due to formation of bubbles at high
voltages as reported in \[5\].

  -----------------------------------------------------------------------
  ![](media/image8.png){width="4.820833333333334in"
  height="3.421527777777778in"}

  Figure 8: Applied voltage pulse trains vs. Ag/Ag+ and resulting
  actuation strains for actuation in 0.5 M TBATFB in AN
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image9.png){width="4.165972222222222in"
  height="3.7493055555555554in"}

  Figure 9: The strain amplitude plotted vs. the applied voltage
  amplitude.
  -----------------------------------------------------------------------

# The Young\'s modulus as a function of the oxidation state

The magnitude of isotonic actuation strain (i.e., for fixed load) can be
influenced by potential- induced changes in modulus, as seen in
polypyrrole \[15\]. The dependence of the Young\'s modulus on the charge
state of the MWNTs was therefore studied.

The ASI 300 muscle analyzer was used to obtain stress/strain curves at a
number of fixed potentials. The yarn was elongated by 0.1 % over 0.5 s
and then brought back to its original length. The applied strain
waveform and the resulting stresses are plotted in Figure 10. Lines were
fit to the resulting stress vs. strain curves for loading and unloading
half-cycles. The relative slopes of these lines provide the effective
Young\'s moduli of the yarn during loading and unloading.
![](media/image10.png){width="3.1430555555555557in"
height="2.698611111111111in"}

The measured Young\'s moduli under the loads of 35, 60 and 70 MPa are
plotted in Figure 11. Within experimental error, no significant change
in modulus is observed as the oxidation state is changed between -1.5 V
to 1.5 V. A. The modulus in that range of voltages is 16±5 GPa, but it
seems to drop at the two extremes of voltage (-2 V and 2 V). This
variation is within the range of experimental uncertainty, however.
Nevertheless the uncertainty in modulus is large enough that it could
induce a strain comparable to the actuation strains in Figure 9. The
present results provide a first bound on the variation. If modulus is
changing at extreme voltages (e.g. due to bubble generation inside the
yarn) it could explain the 0.5 % strain observed in Figure 9 and the
deviation from the quadratic response.

  -----------------------------------------------------------------------
  ![](media/image11.png){width="5.999305555555556in"
  height="4.199305555555555in"}

  **Figure 11: The Young\'s modulus of the CNT yarn as a function of the
  oxidation state under the loads of (a) 35, (b) 60 and (c) 70 MPa. The
  horizontal axes in all the plots indicate a working electrode voltage
  ranging from -2V to 2 V.**
  -----------------------------------------------------------------------

# Conclusions

The actuation behavior of yarns spun from multiwall carbon nanotubes was
studied. The yarns were found to actuate as a result of applied voltage.
The actuation strain changes almost quadratically with applied voltage,
as has been observed in single wall nanotubes, suggesting a coulombic
interaction as the primary mechanism of actuation. A maximum actuation
strain of 0.5 % is reported at an applied voltage of -2.5 V. The
gravimetric capacitance of the yarn is found to reach 23 F/g. The
dependence of the tensile modulus on oxidation state has been studied
and it is concluded that the Young\'s modulus is not strongly dependent
on the oxidation state within experimental uncertainty. Since modulus
does not measurably change with charge injection, actuation strain
should be insensitive to applied load.

**Acknowledgements**

The UBC team is grateful to the Natural Sciences and Engineering
Research Council of Canada for supporting this work through a Discovery
Grant. The NanoTech Institute team was supported by a National Science
Foundation NIRT award, a Texas Advanced Research Program grant, the Air
Force STTR program on topic AF04-TO20, and the Robert A. Welch
Foundation grant AT-0029.

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[^1]: Department of Electrical and Computer Engineering and the Advanced
    Materials & Process Engineering Laboratory (AMPEL), University of
    British Columbia, 2332 Main Mall, Vancouver BC, Canada V6T 1Z4.
    E-mail: tissa@ece.ubc.ca

[^2]: NanoTech Institute, the University of Texas at Dallas, Richardson,
    Texas, USA, 75080.