Carbon nanofoam supercapacitor electrodes with enhanced
performance using a water-transfer process
Article (Published Version)
Nufer, Sebastian, Lynch, Peter, Cann, Mariah, Large, Matthew J, Salvage, Jonathan P, VíctorRomán, Sandra, Hernández-Ferrer, Javier, Benito, Ana M, Maser, Wolfgang K, Brunton, Adam
and Dalton, Alan (2018) Carbon nanofoam supercapacitor electrodes with enhanced performance
using a water-transfer process. ACS Omega, 3 (11). pp. 15134-15139. ISSN 2470-1343
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Article
Cite This: ACS Omega 2018, 3, 15134−15139
http://pubs.acs.org/journal/acsodf
Carbon Nanofoam Supercapacitor Electrodes with Enhanced
Performance Using a Water-Transfer Process
Sebastian Nufer,*,†,‡ Peter Lynch,‡ Maria Cann,† Matthew J. Large,‡ Jonathan P. Salvage,§
Sandra Víctor-Romań ,∥ Javier Hernań dez-Ferrer,∥ Ana M. Benito,∥ Wolfgang K. Maser,∥
Adam Brunton,† and Alan B. Dalton*,‡
†
M-Solv Ltd, Oxonian Park, Langford Locks, Kidlington, Oxford OX5 1FP, U.K.
Department of Physics and Astronomy, University of Sussex, Brighton BN1 9RH, U.K.
§
School of Pharmacy and Biomolecular Science, University of Brighton, Brighton BN2 4GJ, U.K.
∥
Instituto de Carboquimica (ICB-CSIC), Zaragoza E-50018, Spain
Downloaded via UNIV OF SUSSEX on November 14, 2018 at 15:22:19 (UTC).
See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
ABSTRACT: Carbon nanofoam (CNF) is a highly porous,
amorphous carbon nanomaterial that can be produced
through the interaction of a high-fluence laser and a carbonbased target material. The morphology and electrical
properties of CNF make it an ideal candidate for supercapacitor applications. In this paper, we prepare and
characterize CNF supercapacitor electrodes through two different processes, namely, a direct process and a water-transfer
process. We elucidate the influence of the production process on the microstructural properties of the CNF, as well as the final
electrochemical performance. We show that a change in morphology due to capillary forces doubles the specific capacitance of
the wet-transferred CNF electrodes.
Carbon nanofoams (CNFs) are a further subclass of carbon
foam formed during the interaction of laser radiation and a
carbon-based target.11−14 The surface morphology of CNFs
varies significantly compared to other carbon foams as it is
formed in a diffusion-limited aggregation manner rather than
by increasing the volume of a bulk precursor.14 The CNF
consists of a significant amount of sp2 bonds, making the
material conductive.11−13 Among the drawbacks of the CNF
are poor mechanical stability and poor substrate adhesion.
Although both factors prevent the use of CNF electrodes in
aqueous electrolytes, they are stable in many organic
electrolyte systems. This has advantages as while aqueous
electrolytes offer low cost and ease of processing, organic
electrolytes allow for a wider potential window and
consequently a higher energy density.15
During the preparation of CNFs, the substrate adhesion is
low enough that the contact with water is able to delaminate
the materials. Interestingly, this phenomenon can be used as a
route to transfer the electrode material from an initial substrate
to a target by a water-transfer process. In this way, the CNF
floats on the top of the water subphase and can subsequently
be picked up by withdrawing a submerged substrate from
below. While the material is trapped at the air−water interface,
morphological changes can occur because of capillary forces.
This effect is particularly apparent in materials with a small
INTRODUCTION
Supercapacitors are electrochemical-energy-storage devices
that have gained significant attraction in recent years. They
have much promise owing to fast charge times typically
associated with dielectric capacitors (in the seconds), coupled
with high energy densities normally associated with conventional electrochemical batteries. This is achieved through the
formation of an electrical double layer at the interface between
the high-surface-area electrodes and an interacting electrolyte.
Carbon nanomaterials, in particular, are well suited to
supercapacitor applications.1,2 Many allotropes have high
electrical conductivities, as well as high specific surface areas,
making them ideal electrode materials. Moreover, their high
porosity allows greater ion displacement from the electrolyte,
greatly enhancing the electrical double-layer capacitance
responsible for the overall performance of the device.3 Multiple
carbon nanomaterials have been investigated for use in
supercapacitor electrodes, including carbon nanotubes, pristine
graphene, and reduced graphene oxide (GO). These materials
are highly structured and require significant processing to
achieve porous structures that maximize the available electrode
surface area. An alternative class of materials are carbon foams,
which are volumetric, amorphous, and highly porous.4,5 The
morphologies vary depending on the synthesis method, but all
varieties of carbon foam have a very high specific surface area
up to 1500 m2/g.4 In previous studies, supercapacitor
electrodes based on carbon foams have achieved specific
capacitance up to 330 F/g in aqueous electrolytes, which is
highly competitive with other carbon nanomaterials (typically
between 50 and 370 F/g).1,6−10
■
© 2018 American Chemical Society
Received: August 21, 2018
Accepted: October 29, 2018
Published: November 8, 2018
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Imaging by scanning electron microscopy (SEM) (Figure 3)
shows a significant difference in morphology between the two
deposition methods. The direct-deposited CNF in Figure 3a
has a low-density, high-porosity structure with a weblike
microstructure. This morphology is typical for a diffusionlimited aggregation formation process.17 The magnified image
in Figure 3b reveals the porous microstructure formed by the
clusters, which are then spun into a “web” like appearance. The
volumetric character of the CNF did not allow further
magnification without charging effects.
The water-transferred sample (Figure 3c) appears to be
much denser, where the very fine microstructure of the asformed CNF has seemingly collapsed, even though the largerscale porosity appears to be preserved (Figure 3d). The water
transfer increases the CNF’s conductivity by more than 3
orders of magnitude to 740 μS/m from 360 nS/m.
Sets of electrodes prepared by the two methods described in
Figure 2 were used to perform electrochemical measurements.
Figure 4a shows cyclic voltammetry (CV) analysis of the CNF
prepared by direct deposition onto ITO, as well as of watertransferred films. In both cases, the scan rate was 50 mV/s. All
samples exhibit a near-ideal box like CV character. The two
visible peaks at around −0.3 V originate from the ITO
substrate. Figure 4b shows the scan rate dependence of a
water-transferred sample. It is evident that the capacitance falls
with the increased scan rate; however, it remains constant up
to 100 mV/s scan rate. The box like shape is maintained at all
of the scan rates.
Figure 4c illustrates the scan rate dependence of the specific
capacitance for the samples shown in Figure 4a. The directdeposited CNF shows the lower specific capacitance of around
17.5 F/g, while the water-transferred electrode gives a value of
42 F/g. We see immediately that the specific capacitance of the
water-transferred CNF is more than double the value of the
direct-deposited material. The impedance plot in Figure 4d
shows the characteristic steep slope for all of the electrodes at
lower frequencies. The ITO substrate is plotted as a reference
showing the active material behaving as a supercapacitor.
The cyclic voltammetry measurements of Figure 4a,b show
rectangular behavior for the different samples, suggesting an
ideal propagation of charges within the electrodes.18
The rectangular shape of Figure 4b is maintained up to 100
mV/s scan rate with little variation of the capacitance. The
specific capacitance decreases at higher scan rates because ion
mobility and the substrate’s resistance limit charge separation
in the electrolyte.19 The carbon maintains the rectangular
shape at high scan rates, showing good capacitive behavior.
The specific capacitance of the direct-deposited values
measured for the materials presented are comparable with
the work done on CNF systems created in a pulsed laser
deposition process.15
The results of spectroscopic impedance measurements
plotted in Figure 4d highlight the quality of the CNF
supercapacitor electrodes prepared here. The steeper the
angle at low frequencies, indicating non-diffusion-limited
accumulation of electrode surface charge, the closer the system
behavior to that of an ideal supercapacitor.20 The region
exhibiting a 45° angle to the axes is characteristic of Warburg
behavior (see the inset in Figure 4d), which represents the
frequency range where ion diffusion within the electrodes
limits the capacitance achievable. A greater Warburg region
indicates that the electrode structure is inhibiting the diffusion
of electrolyte ions.18 The Warburg region in these devices is
characteristic pore size because of the enhancement of the
capillary interaction.16
In this paper, we demonstrate the preparation and
characterization of a CNF-based supercapacitor electrode
material through laser processing of a graphene oxide (GO)
target. The prepared CNF is transferred onto an appropriate
substrate by use of a water-transfer process resulting in a
change in the morphology of the foam, which enhances the
specific capacitance of the resulting electrodes by a factor of 2.
Using Raman spectroscopy, we relate the electrochemical
performance enhancement to compression from the capillary
forces during the film transfer.
RESULTS AND DISCUSSION
Figure 1 shows the GO characterization used as a precursor for
the CNF. Raman spectroscopy shows well-defined D and G
■
Figure 1. Characterization of prepared GO: (a) Raman peaks, (b)
scanning electron microscopy (SEM) image to determine the flake
size, (c) atomic force microscopy (AFM) topography, and (d)
thickness measurement of the prepared flake.
peaks of the prepared graphene oxide. The flake size of the
synthesized GO was between 500 nm and several μm. The
flake thickness was 0.8 nm.
Schematics of two fabrication processes to produce CNF on
tin-doped indium oxide (ITO) substrates are shown in Figure
2. Figure 2a illustrates the direct deposition approach, where
the laser is scanned through an ITO-coated substrate above the
GO target. The very first line ablates a small track into the
ITO, through which the laser passes afterwards to hit the
target. The laser focus is kept on the substrate. The carbon
clusters diffuse out of the laser-formed plasma on the target
during the reduction of the GO and deposit on the substrate
above. Multiple passes form a thick layer of CNF. Since this
process involves the formation of a carbon plasma, we are able
to prepare CNF with any high-carbon-content material and the
feedstock is not limited to GO.
The water-transfer deposition approach is shown in Figure
2b. The glass substrate is immersed in a water bath, whereby
the CNF detaches and floats on the water surface as it is less
dense than water. The cohesion of the foam is strong enough
so it does not break apart when immersed. The CNF is then
picked up using an ITO substrate, immersed in the water, and
withdrawn at a shallow angle to the water surface.
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Figure 2. Fabrication schematics of CNF samples: (a1) ITO on glass is held above the GO target, through which the laser is scanned (using a
prepatterned slit in the ITO layer). (a2) A thick layer of CNF is deposited on the ITO by moving the GO target to feed the deposition process.
(b1) and (b2) show the same deposition process, except a glass substrate is used. (b3) Glass substrate is immersed in water to detach the CNF.
(b4) CNF is transferred to the final ITO substrate through a reverse process to (b3).
nm), meso- (2−50 nm), and micropores (0.7−2 nm). Macroand mesopores enable good diffusion of ions into the
micropores where the surface area is greatly enhanced to
accommodate the ions, whereas solvent molecules penetrate
only as far as the mesopores.21 Besides the porosity, the film
becomes denser, therefore increasing the electrical contact
among the carbon clusters, which increases the conductivity
and the specific capacitance.
To investigate the influence of the water transfer on the
CNF microstructure further, we turn to Raman spectroscopy.
This technique is able to highlight differences in the bonding
character as well as doping or strain in carbon nanomaterials.23−27 Raman spectra of the CNF directly deposited on a
glass substrate and a CNF sample water transferred from a
glass substrate onto a glass substrate are shown in Figure 5.
The spectra are stacked and normalized to the G-peak intensity
for comparison.
We note several changes to the Raman spectra of the watertransferred material, when compared to the direct-deposited
CNF. All three peaks show a blue shift of three wavenumbers
after the water transfer. In carbon materials, this shift to lower
wavenumbers is associated with a mechanical deformation in
the material.23,28−30 The I(D)/I(G) ratio increases after the
water transfer. This is an indication that the disorder in the
carbon increases.31 The shift of the D′-peak is another
indication that the disorder in the material is increasing.32
The peak showing up at around 1100 cm−1 originates in the
underlying borosilicate glass substrate.
The increase of specific capacitance is not only from the
change in morphology but the capillary forces also introduce
stress into the material. During the water transfer, the water
enters the porous CNF and capillary forces start compressing
the film. This not only changes the surface structure but also
induces a compressive stress into the carbon. It is known that
compressive forces can enhance the specific capacitance of
carbon films.33 The enhanced specific capacitance therefore
Figure 3. (a) SEM images of CNF directly deposited on ITO. (b)
Magnified image of CNF directly deposited on ITO. (c) CNF on ITO
after the water-transfer process of Figure 1b. (d) High-magnification
image of water-transferred CNF.
smaller for the water-transferred samples than for the directdeposited ones, showing that the pores are easily accessible for
diffusing ions. The intersection with the x-axis presents the
solution and electrode resistance. The differences lie in the fact
that the substrate width was slightly different in all of the
measurements. The sample with a smaller width showed a
higher intersection value.
The increase of specific capacitance between the directdeposited CNF and the water-transferred CNF is due to the
change in morphology, which we believe is induced by
capillary forces during the film transfer.16 The water transfer
creates a different porous structure for ion interaction, which
results in an increased system capacitance.21,22 Porosity is a key
parameter in supercapacitor structures as the electric double
layer is influenced by the distribution between macro- (>50
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Figure 4. (a) CV scans of the two different CNF electrode materials at 50 mV/s scan rate. (b) Scan rate dependence of the water-transferred
electrode material. (c) Specific capacitance of the two samples as a function of scan rate. (d) Nyquist plot of spectroscopic impedance
measurements of the electrode materials, showing the blank ITO substrate as a reference; the inset shows the low-impedance region.
influence of capillary forces during the transfer process. The
outcome is a significant increase in the specific capacitance of
the CNF, from 17.5 F/g to around 42 F/g.
■
METHODS
Graphene oxide was prepared using the Hummers method.34,35
The GO was characterized using atomic force microscopy to
determine the flake thickness, SEM to characterize the flake
size, and Raman spectroscopy. For characterization, the GO
was spin-coated onto a silicon substrate. The GO was
deposited by drop-casting onto a borosilicate glass slide. The
glass slide was completely covered and left to dry at 50 °C to
hasten the drying process. A second drop-casting step was
added to create a thick layer of GO on the glass slide; it was
again left to dry at 50 °C. The film thickness was aimed to be
more than 200 μm. The GO target was put in an oven and
heated up to 250° in a ramp process for 1.5 h and then left at
room temperature to cool down.
The exact experimental setup to create the CNF is described
elsewhere.17 A brief summary is as follows: the targets were
irradiated with a Multiwave Nd:YAG nanosecond pulsed
infrared laser (set at 10 ns) in ambient conditions with a set
fluence of 417 mJ/cm2. To form the CNF on the substrate, it
was held stationary over the target with a small (<1 mm) air
gap in between, allowing the aggregation of the carbon clusters
on the substrate.
The weight of the CNF was measured using a Mettler
Toledo microbalance. Six lines were deposited onto the same
substrate to get weight well outside of the error of the balance.
The substrates were measured before and after the deposition.
Figure 5. Raman spectra of direct-deposited and water-transferred
CNFs onto a glass substrate.
originates from two different components, namely, a change in
morphology, making a more dense film as well as the pores
more accessible, and a compressive stress induced by the
capillary forces, which increases the capacitance further.
CONCLUSIONS
We have demonstrated the preparation and characterization of
a carbon-nanofoam-based supercapacitor electrode material.
The material is prepared through infrared laser treatment of a
graphene oxide target, and may be directly deposited onto a
current collector layer (in this case ITO), or may be
transferred from a glass substrate using a water-transfer
process. We observe significant changes to the morphology
of the CNF when transferred in this fashion, including the
development of compressive stress (as characterized using
Raman spectroscopy). This effect arises because of the
■
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The measured weight was then divided by the number of lines
deposited.
Samples were imaged with a Zeiss SIGMA field emission
gun scanning electron microscope (FEG-SEM) using a Zeiss
in-lens secondary electron detector. The FEG-SEM working
conditions used were as follows: 1 kV accelerating voltage, 20
μm aperture, and 2 mm working distance.
Electrochemical measurements were performed with a 3electrode configuration using a Gamry 600+ reference
potentiostat. The counter electrode was a platinum wire, and
the reference electrode was silver/silver chloride (Ag/AgCl).
The electrolyte used was 0.1 M lithium perchlorate (LiClO4)
in acetonitrile. Cyclic voltammetry measurements were
performed in a range of −1 to +1 V versus Ag/AgCl for a
variety of scan rates from 20 to 5000 mV/s. Impedance
spectroscopy applied a perturbation signal of 10 mV around 0
V at frequencies from 100 kHz to 0.1 Hz.
Raman measurements were carried out using a Renishaw
Invia Microscope. A 532 nm 50 mW continuous wave laser
was used at 10% intensity for 10 s to produce the Raman
spectrum. A total of 10 accumulations were used to enhance
the signal.
A Bruker Dimension Icon atomic force microscope (AFM)
was used in peak force mode to measure the thickness of the
GO flakes.
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail: sebastian.nufer@m-solv.com (S.N.).
*E-mail: A.B.Dalton@sussex.ac.uk (A.B.D.).
ORCID
Sebastian Nufer: 0000-0003-2636-8738
Peter Lynch: 0000-0001-8805-0632
Matthew J. Large: 0000-0001-7295-7619
Sandra Víctor-Román: 0000-0003-0924-5840
Ana M. Benito: 0000-0002-8654-7386
Wolfgang K. Maser: 0000-0003-4253-0758
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work has received funding from the European Union’s
Horizon 2020 research and innovation programme under the
Marie Skłodowska-Curie grant agreement no. 642742. A.M.B.,
J.H.-F., and W.K.M. acknowledge Spanish MINEICO (project
ENE2016-79282-C5-1-R), the Gobierno de Aragón (Grupo
Reconocido DGA-T03_17R), and associated EU Regional
Development Funds. S.V.-R. thanks Spanish MINEICO for her
Ph.D. grant (BES2014-068727 and associated EU Social
Funds).
■
■
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