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Phiri, Josphat; Gane, Patrick; Maloney, Thad C.Multidimensional Co-Exfoliated Activated Graphene-Based Carbon Hybrid for SupercapacitorElectrode
Published in:Energy Technology
DOI:10.1002/ente.201900578
Published: 01/01/2019
Document VersionPeer reviewed version
Published under the following license:CC BY-NC
Please cite the original version:Phiri, J., Gane, P., & Maloney, T. C. (2019). Multidimensional Co-Exfoliated Activated Graphene-Based CarbonHybrid for Supercapacitor Electrode. Energy Technology, [1900578]. https://doi.org/10.1002/ente.201900578
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/ente.201900578
This article is protected by copyright. All rights reserved
Multidimensional co-exfoliated activated graphene-based carbon composite for supercapacitor electrode
Josphat Phiri; Patrick Gane; Thad C. Maloney
Josphat Phiri; Prof. Patrick Gane; Prof. Thad C. Maloney School of Chemical Engineering, Department of Bioproducts and Biosystems, Aalto University, P.O. Box 16300, 00076 Aalto, Finland Email: [email protected]; [email protected]; [email protected]
Keywords: graphene; nano- microfibrillated cellulose; supercapacitors; electrode; activated carbon
Abstract
Herein, we report a simple route for fabrication of highly porous activated few-layer graphene
for application in supercapacitors as electrode material. The process makes use of natural and
renewable materials which is an essential prerequisite, especially for large scale application.
Few-layer graphene is exfoliated in aqueous suspension with the aid of microfibrillated
cellulose, an environmentally benign eco-friendly medium that is low-cost, biodegradable and
sustainable. The exfoliated product is subsequently activated to increase the surface area and
to form the desired pore structure. The prepared electrode materials exhibit a high surface area
of up to 720 m2 g-1. We also use microfibrillated cellulose as a non-toxic environmentally
friendly binder in the electrode application. The electrochemical performance was evaluated
in a three-electrode system and the prepared samples showed a high specific capacitance of up
to 120 F g-1 at a current density of 1 A g-1. The samples also exhibit a high capacity retention
rate of about 99 % after 5 000 cycles and 97 % after 10 000 cycles. The proposed method for
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eJosphat Phiri; Prof. Patrick Gane; Prof.
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eJosphat Phiri; Prof. Patrick Gane; Prof.School of Chemical Engineering, Department of Bioproducts and Biosystems, Aalto
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eSchool of Chemical Engineering, Department of Bioproducts and Biosystems, Aalto University, P.O. Box 16300, 00076 Aalto, Finland
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eUniversity, P.O. Box 16300, 00076 Aalto, Finland
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Keywords:
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eKeywords: graphene; nano
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egraphene; nano
we report a
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we report a simple route for fabrication of
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simple route for fabrication of
for application
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for application in supercapacitors as electrode material
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in supercapacitors as electrode material
renewable materials
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renewable materials which
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which
layer
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layer graphene
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graphene is
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is
, an
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, an environmentally benign
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environmentally benign
sustainable. The exfoliated product
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sustainable. The exfoliated product
the
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the desired
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desired pore structure. The
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pore structure. The
up to 72
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up to 720 m
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0 m2
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2 g
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g-
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-1
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1.
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. We also use
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We also use
friendly binder in the electrode application.
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friendly binder in the electrode application.
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-electrode system and Acc
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electrode system and
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1
This article is protected by copyright. All rights reserved
fabrication of graphene-based supercapacitor electrode material, based largely on renewable
and sustainable materials, offers potential for commercially viable application.
1. Introduction
The need for renewable and sustainable energy has become of even greater
importance in recent years due to the growing energy demand and ecological concerns such as
climate change. Supercapacitors, also called electric double layer capacitors or
ultracapacitors, are energy storage devices that store electrical energy in an electrochemical
double layer that is formed at the interface between the electrode and electrolyte, and is able
to deliver that stored energy at an extremely high rate. In comparison to conventional
capacitors, supercapacitors are generally more efficient and have high capacitance values,
longer lasting cyclability, low maintenance, wide temperature working range and extremely
high charge-discharge cycles [1]. Due to their ability to release energy quickly, supercapacitors
are ideal for application where quick energy bursts are required, such as in electronic devices,
fuel cells, hybrid vehicles, backup power systems etc. [2].
The electrode is one of the most important parts of supercapacitors. The
performance of supercapacitors is hugely affected by the type of electrode material [3]. Some
electrode materials that are commonly used include conductive polymers (polyaniline, polypyrrole)
[4, 5], metal oxides (MnO2, NiO) [6-8] and carbon-based materials (activated carbon, graphene,
carbon nanotubes etc.) [9-13]. Metal oxides are used due to low resistance and high specific
capacitance and whilst, as with conducting polymers, reduction and oxidation processes are
responsible for the storage of charge [14]. In commercially available supercapacitors, activated
carbon (AC) is typically used due to low cost and high surface area. The main limiting factor for
achieving the highest specific capacitance for activated carbon electrodes is the limited
accessibility of the electrolyte ions to all the carbon atoms [13]. Moreover, low electrical
conductivity of activated carbon limits the application in high power density devices [15]. In the last
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eThe need for r
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eThe need for r
importance in recent years due to the growing
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eimportance in recent years due to the growing
climate change
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eclimate change. Supercapacitors
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e. Supercapacitors
ultracapacitors
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eultracapacitors,
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e, are energy storage devices that store
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eare energy storage devices that store
double layer that is formed at the interf
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double layer that is formed at the interf
that stored
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that stored energy at an extremely high rate
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energy at an extremely high rate
capacitors
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capacitors, supercapacitors
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, supercapacitors
lasting
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lasting cyclabil
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cyclability
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ity
high charge
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high charge-
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-discharge cycles
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discharge cycles
ideal for application
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ideal for application
hybrid vehicles,
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hybrid vehicles,
The e
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The electrode
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lectrode
performance of supercapacitors
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performance of supercapacitors
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electrode materials that
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electrode materials that
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, metal oxides (MnO
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, metal oxides (MnO
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carbon nanotubes etcAcc
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carbon nanotubes etc.Acc
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.)Acc
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) [9
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[9
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capacitance and whilstAcc
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capacitance and whilst,Acc
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,
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two decades or so, carbon nanotubes (CNT) have also been extensively investigated for application
in supercapacitors [16, 17]. However, CNT have not reached the promised potential in terms of
applications and performance due to high production cost and observed high resistance between
the electrode and current collector [15, 16, 18].
In recent years, graphene has emerged as a unique 2-dimensional technological material
that has huge technical potential for application as electrode material in supercapacitors due its
superlative properties, such as extremely high electrical conductivity, large surface area and
excellent chemical stability and mechanical properties [19-21]. Unlike CNT and AC, the effective
surface area of graphene does not depend on the pore distribution in the solid state, but rather on
the number of graphene layers. A single graphene layer has a theoretical surface area of 2 600 m2 g-
1 and if the entire area is utilised, graphene based supercapacitors are capable of reaching specific
capacitance values as high as 550 F g-1 [22]. However, despite all the excellent properties, graphene
fabrication especially for large scale application is still the biggest challenge. Graphene is
commonly produced by chemical oxidation of graphite. Large scale application, however, is still
problematic due to the negative ecological effect from the traditional synthesis processes and toxic
raw materials used in these process. Moreover, the harsh chemical treatment also introduces
defects in graphene structure that leads to deterioration of the excellent graphene properties.
For high performance energy storage devices, electrodes are required to have excellent
electrical and thermal conductivity, interconnected micro- and mesopores and good mechanical
properties.[23] Unfortunately, none of the individual carbon-based electrode materials can meet
these requirements. In order to overcome some of the limitations from these individual
components, sp2 hybrid carbon structures have been synthesized. For example, graphene and CNT
can provide excellent electrical and mechanical properties but limited surface area and accessible
pores mostly due to aggregation whilst activated carbons offer high surface area and abundant pore
volume with a well-structured micro- and mesoporous network but poor electrical conductivity.
These hybrid structures can be tailored during synthesis to supplement the limitations of each other
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eIn recent years, graphene has emerged as a unique 2
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ethat has huge
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ethat has huge technical
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etechnical potential for application as electrode material in supercapacitors
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epotential for application as electrode material in supercapacitors
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esuperlative properties,
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esuperlative properties, such as extremely high electrical conductivity,
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esuch as extremely high electrical conductivity,
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echemical stability
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echemical stability
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surface area of graphene does not depend on the pore distribution
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surface area of graphene does not depend on the pore distribution
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the number of
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the number of graphene
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graphene
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and if the entire area is utili
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and if the entire area is utili
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capacitance
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capacitance values as high as 550 F g
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values as high as 550 F g
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fabrication especially for large scale
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fabrication especially for large scale
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commonly produced by chemical oxidation of graphite.
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commonly produced by chemical oxidation of graphite.
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problematic due to the negative ecological effect from the
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problematic due to the negative ecological effect from the
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materials
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materials used in the
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used in the
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defects in graphene structure that leads to deterioration of the
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defects in graphene structure that leads to deterioration of the
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For high
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For high performance
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performance
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electrical and thermal conductivity, interconnected micro
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electrical and thermal conductivity, interconnected micro
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properties. Acc
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properties.[23]
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[23]
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Unfortunately, Acc
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Unfortunately, Acc
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these requirements. Acc
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these requirements. In order to overcome some ofAcc
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In order to overcome some of
This article is protected by copyright. All rights reserved
[24, 25]. For instance, hydrothermal prepared CNT/carbon composites [26], CNT/activated carbon
blends [27], 3D graphene based framework [28] etc., have been used to fabricate high performance
electrode materials for energy storage devices with superior electrochemical performance. The
homogeneous distribution of sp2 nanocarbons in the carbon matrix is essential in achieving the
desired properties and generally requires one carbon phase to self-organize into the other carbon
matrix or scaffold. The synthesis of hybrid structures with dispersed nanocarbons into a well–
designed and structured carbon matrix is vital in achieving the excellent electrochemical properties
required to meet the growing high performance demand. However, the construction of these high
performance carbon hybrid based electrodes is still a big challenge.
In the present study, we aimed to develop a novel and simple route for fabrication of
few-layer graphene based composites for supercapacitor electrode application based largely on
renewable materials. We showed in our earlier study that few-layer graphene sheets can be directly
exfoliated from graphite in microfibrillated cellulose (MFC) suspension by high shear exfoliation
[29]. MFC is renewable, biodegrade and sustainable and has a high potential for large scale
application. MFC in aqueous suspension was also used as a binder for the electrode fabrication,
which is more ecologically friendly in comparison to commonly used highly toxic binders such as
poly(vinylidene fluoride) or poly(tetrafluoroethylene) and solvents such as N-Methyl-2-
pyrrolidone. We show in this study that this simple and yet ´green` approach for the synthesis of
graphene/activated carbon after co-exfoliation of graphene in MFC suspension, can be used to
synthesise high performance composites for supercapacitor electrode with high scalability
potential.
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edesired properties and generally requires one carbon phase to self
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eand generally requires one carbon phase to self
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ematrix or scaffold.
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ematrix or scaffold. The synthesis of hybrid structures
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eThe synthesis of hybrid structures
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eand s
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eand structured
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etructured
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erequired to meet the growing
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erequired to meet the growing
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performance
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performance carbon
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carbon hybrid based electrodes is still a big challenge.
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hybrid based electrodes is still a big challenge.
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In the
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In the present study
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present study
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graphene
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graphene based composites for
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based composites for
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renewable materials
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renewable materials.
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. We showed in our earlier study that few
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We showed in our earlier study that few
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exfoliated from graphite in microfibrillated cellulose (MFC) suspension by high shear exfoliation
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exfoliated from graphite in microfibrillated cellulose (MFC) suspension by high shear exfoliation
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. MFC is renewable, biodegrade an
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. MFC is renewable, biodegrade an
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application. MFC
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application. MFC in aqueous
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in aqueous
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which is more ecological
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which is more ecological
poly(vinylidene fluoride) or poly(tetrafluoroethylene)
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poly(vinylidene fluoride) or poly(tetrafluoroethylene)
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pyrrolidone
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pyrrolidone.
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. We show in this study that this simple and yet ´green` approach for
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We show in this study that this simple and yet ´green` approach for
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graphene/activated carbon after co
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graphene/activated carbon after co
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e high performance composites for Acc
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e high performance composites for
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2. Results and discussion
The general synthesis route of graphene/AC composite is schematically illustrated
in Figure 1. Briefly, natural graphite was added into MFC suspension at a predetermined
range of concentrations and subjected to high shear exfoliation [29]. The exfoliated few-layer
graphene sheets together with MFC fibres were then activated with KOH at 800 °C for 1 h.
Scanning electron microscopy (SEM) was employed to study the morphologies of
the samples (Figure 2 and 3). A porous structure with big pores is observed for G0-0% with
quite smooth surfaces. However, a three-dimensional porous hierarchical structure is observed
for samples with 5 % graphene, as shown in Figure 3. A randomly interconnected porous
network with varying pore sizes is revealed in the magnified SEM image in Figure 2b,
showing that the graphene/carbon skeleton is formed around the pores to create a randomly
porous multidimensional structure, very similar to that previously observed in activated
carbon [30]. The addition of 5 % graphene led to a well-developed pore structure and enhanced
surface area which is expected to increase the ionic transfer within the porous carbon network
and, thus, improve the electrochemical performance [31]. At high graphene concentration of 10
% (Figure 2c), a more densely packed structure is observed with less cavities than those
observed in G1-5%. At 15 % (Figure 2d), small clumps and agglomerates are visible
throughout the structure and this is likely attributed to the agglomeration of graphene.
Chemical activation, with KOH for example, is widely implemented for the fabrication of
highly porous activated carbon/graphene-based composites [32]. KOH is believed not only to
digest the graphene/activated carbon composites but also plays a significant role in
reorganization into highly porous hierarchical structure [33].
Raman spectroscopy is the most used non-destructive method for characterization
of the structure and quality of carbon based materials, particularly to determine the degree of
defects and disorder. In Figure 4a, the Raman spectra show that all graphene doped samples
display the three dominant bands, D-band (~1 540 cm-1), G-band (~1 567 cm-1) and 2D-band
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egraphene sheets together with MFC fib
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egraphene sheets together with MFC fib
Scanning electron microscopy (SEM) was employed to study the morphologies of
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eScanning electron microscopy (SEM) was employed to study the morphologies of
samples
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esamples (
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e(Figure 2 and 3
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eFigure 2 and 3
quite smooth surfaces. However, a
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equite smooth surfaces. However, a
for samples with 5
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for samples with 5 % graphene
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% graphene
network with varying pore sizes is revealed in the magnified SEM image in
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network with varying pore sizes is revealed in the magnified SEM image in
showing that the graphene/carbon skeleton is formed around the pores to create a randomly
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showing that the graphene/carbon skeleton is formed around the pores to create a randomly
porous multidimensional structure
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porous multidimensional structure
[30]
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[30].
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. The
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The addition of 5 % graphene led to a well
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addition of 5 % graphene led to a well
surface area
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surface area which
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which is expected to increase the ionic transfer within the porous carbon network
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is expected to increase the ionic transfer within the porous carbon network
thus, improve the ele
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thus, improve the ele
Figure
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Figure 2
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2c
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c)
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),
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, a more densely pa
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a more densely pa
observed in G1
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observed in G1-
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-5%
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5%. At 15
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. At 15
throughout the structure and
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throughout the structure and
Chemical activation, with KOH for example, is widely implemented for the fabrication of
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Chemical activation, with KOH for example, is widely implemented for the fabrication of
highly porous activated carbon/Acc
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highly porous activated carbon/Acc
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digest the graphene/activated carbon composites but also plays a significant Acc
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e
digest the graphene/activated carbon composites but also plays a significant
This article is protected by copyright. All rights reserved
(~2 678 cm-1), commonly present in graphene. However, for the reference sample without
graphene, the 2D is not visible. The G-band is related to the relative vibration of the ordered
sp2 bonded carbon atoms, while the D-band is due to the ring breathing modes and arises due
to defects and disorder in carbon atoms [34]. The 2D band is the second order of the D-band,
and its presence is not dependent on defects and is always present even in pristine graphene.
The shape and position of the 2D-band in all prepared samples is consistent with that of few-
layer graphene sheets [35]. The ID/IG intensity ratio is a used to estimate the quantity or degree
of disorder and defects. The reference sample showed a high degree of disorder as evident by
the high ID/IG ratio, indicating a lower degree of graphitisation than all the graphene doped
samples which showed a very low degree of disorder as shown by low ID/IG ratio in Table 1.
XPS analysis was used to study the surface chemistry of the samples. The XPS
wide scans are shown in Figure 4b. The pronounced peaks with binding energies at around
284.8 and 533 eV are assigned to the elements C and O, respectively. The exact composition
is shown in Table 1, and it is evident that almost no inorganic impurities are detected in any
of the samples. Generally, the amount of graphene added in the samples did not have much
effect on the chemical composition of the samples. The C1s for the pure carbonised MFC
fibres shown in Figure 4c can be fitted by three components corresponding to C-C, C-O and
C=O functional groups [36]. However, with the addition of graphene, the high resolution C1s
spectra (Figure 4d-f) showed that all samples consist of four main groups. The peaks at
binding energies of around 284.8, 285.5, 286.4 and 287.6 eV corresponding to C=C, C-C, C-
O and C=O groups, respectively [37, 38]. It can also be seen that all the samples are dominated
by the sp2 hybridised carbon atoms, evident by the large area occupied as identified C=C
atoms [39]. The oxygen containing functional groups present in all the samples can aid in the
wettability of the electrodes especially in aqueous based electrolyte which can lead to
enhanced capacitive performance [36]. Moreover, heteroatoms such as oxygen can provide
pseudocapacitance which can lead to an overall increase of effective capacitance.
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eand its presence is not dependent on defects
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eand its presence is not dependent on defects
The shape and position of the 2D
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eThe shape and position of the 2D
layer graphene sheets
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elayer graphene sheets [35]
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e[35]
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eof disorder and defec
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eof disorder and defects. The reference sample showed a high degree of disorder as evident by
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ets. The reference sample showed a high degree of disorder as evident by
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e
D
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e
D/
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e
/I
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e
I/I/
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e
/I/ G
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e
GIGI
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e
IGI ratio
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e
ratio, indicating a lower
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e
, indicating a lower
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e
which
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e
which showed a very low degree of disorder as shown by low
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e
showed a very low degree of disorder as shown by low
XPS analysis was used to study the surface chemistry of the samples.
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e
XPS analysis was used to study the surface chemistry of the samples.
wide scans
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e
wide scans are shown
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e
are shown in
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e
in
284.8 and 533 eV are assigned to the
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e
284.8 and 533 eV are assigned to the
shown in
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e
shown in Table
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e
Table 1
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1,
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, and it is evident that
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e
and it is evident that
samples. Generally
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samples. Generally
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effect on the chemical composition of the samples.
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effect on the chemical composition of the samples.
s shown in
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e
s shown in Figure
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Figure
functional
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e
functional groups
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e
groups
Figure
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e
Figure 4
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4d
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e
d-
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-f)
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f) showed that all samples
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showed that all samples
binding energies of Acc
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binding energies of Acc
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e
around Acc
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around
O and C=O groups, respectivelyAcc
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e
O and C=O groups, respectively
This article is protected by copyright. All rights reserved
The capacitance of supercapacitors is greatly dependent on the surface are and pore
structure of the electrode materials. The pore structure of the samples was analysed by N2
sorption measurements at 77 K (Figure 5). The nitrogen isotherm in Figure 5a for all the
samples shows a typical International Union of Pure and Applied Chemistry (IUPAC) type-IV
isotherm curve with a well-pronounced hysteresis in the p/p° region of around 0.4-1.0,
indicating the presence of mesopores. Besides, the steep increase of adsorbed nitrogen at low
relative pressure indicates the presence of microspores. The Brunauer- Emmett- Teller (BET)
specific surface area (SSA) of G1-5% is ~720 m2 g-1 whilst for G2-10% and G3-15% is ~546
and ~424 m2 g-1, respectively. When the amount of graphene is increased, the distances
between the graphene flakes are reduced, which, in turn, leads to a high overall intensity of
the van der Waals forces acting between the graphene particles and, thus, high graphene
aggregation. These results are in agreement with what was observed in SEM and Raman
analyses. The electrochemical performance of carbon-based supercapacitors is greatly
affected by the pore structure profiles. Micropores are considered to have a great effect on the
specific capacitance [1, 40], whilst the mesopores are thought to have a big influence on
capacitance retention especially at high scan rates [41]. The pore size distribution (PSD) and
cumulative pore volume of the samples are shown in Figure 5b and c, and Table 2 shows
some pore size characteristics. The PSD for G1-5% is dominated by both micro- and
mesopores and peaks at 1.58 nm. As the concentration of graphene is increased, the peak is
shifted to bigger pore width, i.e. 1.69 and 2.66 nm for G2-10% and G3-15%, respectively.
Moreover, at high graphene concentration, the composites consist of a wide range of PSD
comprising a high amount of meso- and macrospores.
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esamples shows a typical
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esamples shows a typical
therm curve with a
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etherm curve with a well
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ewell
indicating the presence of
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eindicating the presence of
relative pressure indicates the presence of microspores.
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erelative pressure indicates the presence of microspores.
specific surface area (
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e
specific surface area (SSA
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e
SSA
424 m
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e
424 m2
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e
2 g
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e
g-
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e
-1
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1, respectively.
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e
, respectively.
between the graphene flakes
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e
between the graphene flakes
van der Waals
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e
van der Waals forces
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e
forces
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e
aggregation. These results are in
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e
aggregation. These results are in
The electrochemical performance of carbon
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e
The electrochemical performance of carbon
affected by the pore structure profiles. Micropores are considered to have a great effect on the
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e
affected by the pore structure profiles. Micropores are considered to have a great effect on the
specific capacitance
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e
specific capacitance [1, 40]
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e
[1, 40]
capacitance retention especially at high scan rates
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e
capacitance retention especially at high scan rates
cumulative pore volume of the samples
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e
cumulative pore volume of the samples
some pore size characteristics
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e
some pore size characteristics
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e
mesopores and peaks at 1.58Acc
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e
mesopores and peaks at 1.58
shifted to bigger pore widthAcc
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e
shifted to bigger pore width
This article is protected by copyright. All rights reserved
The electrochemical performance testing of the samples was conducted in a three-
electrode system by using 6 M KOH aqueous solution as the electrolyte. Cyclic voltammetry
(CV), galvanostatic charge-discharge and electrochemical impedance systems were employed
for the analysis. The CV of the samples is shown in Figure 6a, c and e. The measurements
were performed at room temperature within the potential window of 0 to -1 V. The shape for
all three samples showed a typical rectangular form at all different sweep rates indicating
excellent reversible electric double-layer capacitive behaviour [42]. Moreover, the absence of
Faradic peaks also indicate the excellent capacitive charging and discharging behaviour with
small equivalent series resistance and good electrochemical performance [43]. This behaviour
was observed for all samples at various scan rates. It is also important to note that the broader
voltammogram area observed for G1-5% indicates that it has a much more efficient cycle
reversibility and higher electrical double layer capacitance during the charge/discharge cycles
compared to samples G2-10% and G3-15%.
The galvanostatic charge/discharge cycles (GCD) curves for all samples at different
scan rates are shown in Figure 6b, d and f. All the charge-discharge curves show a similar
symmetrical triangular curve which is a typical charge/discharge characteristic of carbon-
based electrical double-layer capacitators. A sharp and sudden initial potential drop (known as
iR drop) is usually seen at the start of current discharge in electro double layer capacitors.
This iR drop is caused by high electrolyte resistance and limited diffusion mobility of
electrolyte ions in the electrode. The iR drop is virtually negligible in the prepared samples,
showing that they have a very high rate of ion transfer inside and between the electrode and
electrolyte. Furthermore, when comparing the three samples, the discharge slope of G1-5% at
all current densities showed a straight line which indicates excellent stability and discharge
properties [44]. Moreover, G1-5% also shows significantly longer charge-discharge times at
the same current density in comparison to G2-10% and G3-15%, indicating higher
capacitance and more efficient charge/discharge cycles caused by the higher number of
Acc
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ewere performed at room temperature within the potential window of 0 to
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ewere performed at room temperature within the potential window of 0 to
all three samples showed a typical rectangular
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eall three samples showed a typical rectangular
excellent reversible
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eexcellent reversible electric double
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eelectric double
Faradic peaks
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eFaradic peaks also
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ealso i
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eindicate
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endicate
small equivalent series
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e
small equivalent series
was observed for all samples at various scan rates.
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e
was observed for all samples at various scan rates.
voltammogram area observed for G1
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e
voltammogram area observed for G1
reversibility and higher electrical
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e
reversibility and higher electrical
compared to samples
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e
compared to samples G2
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e
G2
The
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e
The galvanostatic
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e
galvanostatic
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e
scan rates are shown in
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e
scan rates are shown in
symmetrical triangular curve which is a typical charge/discharge characteristic of
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e
symmetrical triangular curve which is a typical charge/discharge characteristic of
ctrical double
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e
ctrical double
drop) is usually seen at the start of current discharge in electro double layer capacitors.
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e
drop) is usually seen at the start of current discharge in electro double layer capacitors.
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e
drop is caused by high electrolyte resistance and limited diffusio
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e
drop is caused by high electrolyte resistance and limited diffusio
electrolyte ions in the electrode. The Acc
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e
electrolyte ions in the electrode. The Acc
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e
showing that they have a very high rate of ion transfer inside and between the electrode and Acc
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e
showing that they have a very high rate of ion transfer inside and between the electrode and
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electrons and ions rapidly transferring between the electrolyte and electrode. The linear and
symmetrical charge/discharge curves observed even at very high current density also indicate
the excellent electrochemical reversibility and very low voltage drop at the start of the
discharge curve. In contrast, both G2-10% and G3-15% showed non-linear discharge curves
at all current densities, being more pronounced at high current densities. This shows that at
high graphene loading, there is a poor electrochemical reversibility caused by the low specific
surface area and poorly formed pore structure caused by high concentration of graphene
aggregation and, thus, poor electron and ion exchange between the electrolyte and electrode.
Figure 7a shows the specific capacitance of the samples calculated from the
discharge curves obtained at current densities ranging from 0.5 to 10 A g-1. The decrease of
specific capacitance is usually observed with an increase in current density due to the internal
resistance of the electrode. G1-5% samples exhibit a high specific capacitance of 120 F g-1 at
1 A g-1 which decreases to 102 F g-1 at 10 A g-1 (85 % retention). However, for the G2-10%
and G3-15%, 80 % and 69 % retentions are observed, respectively. The high retention of the
G1-5% can be ascribed to low internal resistance, high surface area and well-developed
porosity that leads to superior high-rate charge/discharge capability, which is in agreement
with the BET measurements.
The electrochemical performance of the prepared electrodes was also analysed by
electrochemical impedance spectroscopy (EIS). Figure 7b shows the Nyquist plot measured in
the frequency range of 0.01 Hz to 1 MHz, where the Z´axis displays the real part representing
ohmic resistance while Z´´ the imaginary part represents the presence of non-resistive
elements [45]. The Nyquist plot for all the three samples reveals three main segments. The first
segment is the vertical line in the low frequency region. This represents the dominance of
capacitive behaviour of the electrode [46, 47]. The G1-5% shows more capacitive behaviour
Acc
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eat all current
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eat all current densities
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edensities,
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e ,
high graphene loading, there is a poor electrochemical reversibility caused by the low specific
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ehigh graphene loading, there is a poor electrochemical reversibility caused by the low specific
surface area and poor
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esurface area and poorly formed
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ely formed
aggregation and
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eaggregation and,
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e, thus, poor electron and ion exchange between the electrolyte and electrode.
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ethus, poor electron and ion exchange between the electrolyte and electrode.
Figure
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e
Figure 7
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e
7a
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e
a shows the specific capacitance of the samples calculated from the
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e
shows the specific capacitance of the samples calculated from the
discharge curves obtained at current densities ranging from 0.5
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e
discharge curves obtained at current densities ranging from 0.5
specific capacitance is usually observed with an increase in current density
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e
specific capacitance is usually observed with an increase in current density
resistance of the electrode
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e
resistance of the electrode
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e
which decreases to 102 F
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e
which decreases to 102 F
15%,
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e
15%, 80
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e
80 % and
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e
% and
can be
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e
can be ascribe
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e
ascribed
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e
d
porosity that leads t
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e
porosity that leads t
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e
o superior high
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e
o superior high
with the BET measurements.
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e
with the BET measurements.
The Acc
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e
The electrochemical Acc
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e
electrochemical Acc
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e
electrochemical impedance spectroscopy (EIS). Acc
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e
electrochemical impedance spectroscopy (EIS).
This article is protected by copyright. All rights reserved
than the other two samples, as shown by the vertical line being more parallel to the imaginary
axis. The second segment is the part of the curve having a slope of 45° in the middle
frequency range, also known as Warburg resistance, represents ion diffusion and transport
between the electrolyte and electrode [48]. Interestingly, the G1-5% has a shorter ion diffusion
path than that of G2-10% and G3-15%, which allows easy access and rapid transfer of ions in
and out of the electrode and, thus, leads to a higher capacitance. The third segment consists of
a depressed semicircle or arc in the high frequency region. This is related to the faradic charge
transfer resistance of the electrode and its pore structure. The lack of a more pronounced
semicircle represents the low resistance of the electrodes. However, G1-5% shows the
smallest arc indicating the smallest charge transfer resistance. This indicates a well-developed
pore structure as earlier indicated by SEM and BET analyses. The semicircle radii of G2-10%
and G3-15% are very similar and much larger than G1-5%, which indicates high resistance.
Furthermore, the intersection with the x-axis in the high frequency region represents the
equivalent series resistance (ESR) of the supercapacitors [49]. ESR can be used to determine
the rate capability of the charge-discharge cycles of the supercapacitors. The ESR also shows
electron conduction and ionic transport resistance within the electrode and to and from the
electrolyte [50]. For samples G1-5%, G2-10% and G3-15%, the ESR is determined to be 0.219,
0.393 and 0.481 Ω, respectively, which is relatively low. The range of the values is typical for
carbon-based supercapacitors, as reported earlier [50, 51]. It is important to note that the
resistance is increasing with increase in graphene content. This is probably due to graphene
aggregation which prevents the formation of a continuous network. The results are in
agreement with what was observed in SEM and Raman analyses.
High stability and durability of electrode materials is required to ensure long term
and high efficient operation. GCD was employed to study the stability of the electrode
material (G1-5%) by constant charge-discharge cycles. As shown in Figure 8, the G1-5%
electrode was tested at 5 000 cycles at the constant current density of 5 A g-1. The insert
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ethan that of G2
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ethan that of G2-
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e-10% and G3
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e 10% and G3
and out of the electrode
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eand out of the electrode
a depressed semicircle or arc in the high frequency region. This is related to
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ea depressed semicircle or arc in the high frequency region. This is related to
resistance of the electrode
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eresistance of the electrode
semicircle represents the low resistance of th
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e
semicircle represents the low resistance of th
smallest arc indicating the smallest charge transfer resistance.
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e
smallest arc indicating the smallest charge transfer resistance.
pore structure as earlier indicated by SEM and BET analyses.
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e
pore structure as earlier indicated by SEM and BET analyses.
15%
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e
15% are
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e
are very similar
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e
very similar
Furthermore, the
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e
Furthermore, the intersection with the
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e
intersection with the
equivalent series resistance
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e
equivalent series resistance
the rate capability of the charge
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e
the rate capability of the charge
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e
electron conduction and ionic transport resistance within the electrode and to and from the
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e
electron conduction and ionic transport resistance within the electrode and to and from the
electrolyte
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e
electrolyte [50]
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e
[50].
Acc
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e
. For samples G1
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e
For samples G1
0.393 and 0.481
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e
0.393 and 0.481 Ω, respectively
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e
Ω, respectively
based
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e
based
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e
supercapacitors
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e
supercapacitors
resistance is Acc
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e
resistance is increasing with increase in graphene content. This is probably due to graphene Acc
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e
increasing with increase in graphene content. This is probably due to graphene
aggregation which prevents the formation of a continuous network. The results are in Acc
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e
aggregation which prevents the formation of a continuous network. The results are in
This article is protected by copyright. All rights reserved
shows the charge-discharge cycles at the beginning and end cycles. It is clear that the shape is
not altered, indicating excellent stability of the electrode. After 5 000 cycles, the electrode
retained 99.1 % of the initial capacitance, showing an excellent electrochemical stability rate
and cycling reversibility. It is important to note that the slightly lower capacitance retention
(~98 %) is observed after the initial 1 000 cycles and then started to increase after about 2 000
cycles. The initial capacitance decrease maybe ascribed to the existence of heteroatoms, such
as oxygen, as confirmed by XPS, which can lead to the pseudocapacitance, and, after a few
cycles, fades away after all the heteroatoms have been exhausted [52]. The further increases in
the capacitance might be due to the complete wetting of the electrode with the KOH
electrolyte. Besides, the swelling of the MFC binder used in this study increases with time
and thus leads to a higher amount of ions stored, which, in turn, translates to high capacitance.
3. Conclusions
In summary, a simple and environmentally benign method is proposed for the synthesis of
activated graphene-based carbon composites for supercapacitor electrodes. A natural and
sustainable material, microfibrillated cellulose (MFC) is used for exfoliation of graphene and
as a binder in the electrode fabrication. In comparison to commonly used toxic materials
employed for exfoliation of graphene and as binders in supercapacitors, MFC is also
biodegradable, renewable and readily available. The optimal graphene concentration for the
system was found to be 5 %. At this concentration, a well-developed porosity and a high
surface area of 720 m2 g-1 insured an excellent electrochemical behaviour. Besides, the
prepared showed a superior cycling stability by retaining 99 % capacitance after 5 000 cycles
and 97 % after 10 000 cycles. This type of composite could find significant potential use in
advanced applications such as catalysts, energy storage and adsorption.
Acc
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e%) is observed
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e%) is observed a
Acc
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e after the
Acc
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e fter the
The initial capacitance decrease maybe ascribed to the existence of heteroatoms
Acc
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eThe initial capacitance decrease maybe ascribed to the existence of heteroatoms
as oxygen
Acc
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eas oxygen,
Acc
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e, as confirmed by XPS, which
Acc
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eas confirmed by XPS, which
fades
Acc
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efades away
Acc
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eaway after all the heteroatoms have been exhausted
Acc
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eafter all the heteroatoms have been exhausted
the capacitance might be due to the complete wetting of the electrode with the KOH
Acc
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e
the capacitance might be due to the complete wetting of the electrode with the KOH
electrolyte. Besides, the swelling of the
Acc
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e
electrolyte. Besides, the swelling of the
and thus leads to a higher amount
Acc
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e
and thus leads to a higher amount
Conclusions
Acc
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e
Conclusions
In summary, a simple and environmental
Acc
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e
In summary, a simple and environmental
activated graphene
Acc
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rticl
e
activated graphene
Acc
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rticl
e
-
Acc
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e
-based carbon composites for supercapacitor electrodes.
Acc
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e
based carbon composites for supercapacitor electrodes.
sustainable material,
Acc
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e
sustainable material, microfibrillated cellulose (
Acc
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e
microfibrillated cellulose (
binder in the electrode fabrication.
Acc
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e
binder in the electrode fabrication.
for exfoliation of graphene and
Acc
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e
for exfoliation of graphene and
biodegrad Acc
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e
biodegradable Acc
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e
able Acc
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e
, renewable and readily available.Acc
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e
, renewable and readily available.
system was found to be 5 %. At this concentration, a wellAcc
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e
system was found to be 5 %. At this concentration, a well
This article is protected by copyright. All rights reserved
4. Materials and methods
Materials: Natural graphite flakes were provided by Asbury Carbon. Microfibrillated
cellulose was provided by Suzano Pulp and Paper at 5 wt% solids. The MFC was a relatively
course grade produced by mechanical defibrillation of hardwood Kraft pulp. The length
weighted average fibre length was 0.52 mm measured using a FiberLab laser instrument from
Metso Automation. Other properties of MFC can be found in our earlier published work [53].
Deionised water was used throughout the experiments.
Preparation of graphene/MFC suspensions: Co-exfoliation of graphene in MFC suspension
was achieved using an IKA Magic Lab micro-plant equipped with a single-walled open 1-
dm3 vessel). The MFC was first diluted to a consistency of 0.8 wt%. Subsequently, natural
graphite was added to the suspension in various proportions to yield a graphite solid content
of 5, 10 and 15 wt% with respect to that of solid MFC. The resulting mixtures were then
subjected to high shear exfoliation for 60 min. The same procedure was repeated with 100 %
MFC suspension alone. The prepared samples are denoted as G0-0%, G1-5%, G2-10% and
G3-15%, where 0, 5, 10 and 15 %, represent the concentration of graphite in the initial
production process. The detailed experimental procedure can be found in our earlier
publication [29].
After co-exfoliation, excess water from the suspensions was removed by vacuum
filtration. The suspensions were then soaked in KOH solution at the mass ratio of 1:1 for 2 h
followed by drying in an oven at 105 °C. The dried samples were then activated at 800 °C for
1 h, at a heating rate of 5 °C min-1 in nitrogen atmosphere. The yield of pure MFC after
carbonization with KOH at a MFC/KOH ratio of 1 was estimated to be around 5 %. The
overall yield was around 9.5 %. After carbonisation, the samples were thoroughly washed
with HCl and water and subsequently dried for at least 24 h at 105 °C.
Acc
epte
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eweighted average fibre length was 0.52 mm measured using a FiberLab laser instrument from
Acc
epte
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eweighted average fibre length was 0.52 mm measured using a FiberLab laser instrument from
Metso Automation. Othe
Acc
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eMetso Automation. Othe
Deionised water was used throughout the experiments.
Acc
epte
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eDeionised water was used throughout the experiments.
Preparation of graphene/MFC suspensions:
Acc
epte
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ePreparation of graphene/MFC suspensions:
was achieved using an IKA Magic Lab micro
Acc
epte
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rticl
e
was achieved using an IKA Magic Lab micro
dm3 vessel). The MFC was first diluted to a consistency of 0.8 wt%. Subsequently, natural
Acc
epte
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rticl
e
dm3 vessel). The MFC was first diluted to a consistency of 0.8 wt%. Subsequently, natural
graphite was added to the suspension in various proportions to yield a graphite solid content
Acc
epte
d A
rticl
e
graphite was added to the suspension in various proportions to yield a graphite solid content
of 5, 10 and 15 wt% with respect
Acc
epte
d A
rticl
e
of 5, 10 and 15 wt% with respect
subjected to high shear exfoliation for 60 min. The same procedure was repeated with 100 %
Acc
epte
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rticl
e
subjected to high shear exfoliation for 60 min. The same procedure was repeated with 100 %
MFC suspension alone. The prepared samples are denoted as G0
Acc
epte
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rticl
e
MFC suspension alone. The prepared samples are denoted as G0
15%, where 0, 5, 10 and
Acc
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rticl
e
15%, where 0, 5, 10 and
production process. The detailed experimental procedure can be found in our earlier
Acc
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e
production process. The detailed experimental procedure can be found in our earlier
publication
Acc
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e
publication [29]
Acc
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e
[29].
Acc
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rticl
e
.
After co
Acc
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e
After co-
Acc
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e
-exfoliation, excess water from the suspensions was removed by vacuum
Acc
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e
exfoliation, excess water from the suspensions was removed by vacuum
filtration. The
Acc
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e
filtration. The suspensions were then soaked in KOH solution
Acc
epte
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rticl
e
suspensions were then soaked in KOH solution
Acc
epte
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rticl
e
followed by drying in an oven at 105 °C. The dried samples were then activated at 800 °C for Acc
epte
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e
followed by drying in an oven at 105 °C. The dried samples were then activated at 800 °C for
1 h, at a heating rate of 5Acc
epte
d A
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e
1 h, at a heating rate of 5
This article is protected by copyright. All rights reserved
Material characterization: The structure of carbonised materials was recorded using a Zeiss
Sigma VP scanning electron microscope at 5 kV acceleration voltage. The powders were first
sputtered with a gold film before SEM measurements. Raman spectra were measured using a
WITec alpha300 R Raman microscope (alpha 300, WITec, Ulm, Germany) equipped with a
piezoelectric scanner using a 532 nm linear polarised excitation laser. Surface area and pore
volume were determined by nitrogen sorption using a Micromeritics Tristar II by Brunauer-
Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) theory, respectively.
Electrochemical measurement: Cyclic voltammetry, galvanostatic charge-discharge, and
electrochemical impedance spectroscopy (EIS) measurements were carried out using a Gamry
600+ Reference potentiostat, using a three-electrode cell configuration system. The
measurements were conducted in 6 M KOH aqueous electrolyte solution at room temperature.
The KOH solution was purged with nitrogen before the experiment. A platinum wire and
Ag/AgCl electrode were used as counter and reference electrodes, respectively. The specific
capacitance was obtained from galvanostatic charge-discharge using the formula:
C = IΔt/(ΔVm), (where, C (F g-1) is specific capacitance, I (A) is discharge current, Δt (s) is
discharge time, ΔV (V) is the potential window, and m (g) mass of the active material).
To prepare the electrode, the carbonised materials were directly mixed with MFC
suspension, diluted to 1 wt%, to form an electrode consisting of 90 wt% active material and
10 wt% MFC binder. No conductive additives were used. The prepared paste was then coated
on a Ni foam and dried in an oven at 105 °C for at least 24 h. The areal mass loading was
between 1.5-2 mg cm-2. Before the measurements, the coated Ni foam was pressed at about 7
MPa for 30 s.
Acc
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epiezoelectric scanner using a 532 nm linear polarised excitation laser.
Acc
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epiezoelectric scanner using a 532 nm linear polarised excitation laser.
volume were determined by nitrogen sorption using a Micromeritic
Acc
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evolume were determined by nitrogen sorption using a Micromeritic
Teller (BET) method and Barrett
Acc
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eTeller (BET) method and Barrett
Electrochemical measurement:
Acc
epte
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e
Electrochemical measurement:
electrochemical impedance spectroscopy (EIS) measurements were
Acc
epte
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e
electrochemical impedance spectroscopy (EIS) measurements were
600+ Reference potentiostat, using a three
Acc
epte
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e
600+ Reference potentiostat, using a three
measurements were conducted in 6 M KOH aqueous electrolyte solution at room temperature.
Acc
epte
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e
measurements were conducted in 6 M KOH aqueous electrolyte solution at room temperature.
The KOH solution was purged with nitrogen before the exp
Acc
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rticl
e
The KOH solution was purged with nitrogen before the exp
Ag/AgCl electrode were used as counter and reference electrodes, respectively. The specific
Acc
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e
Ag/AgCl electrode were used as counter and reference electrodes, respectively. The specific
capacitance was obtained from galvanostatic charge
Acc
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e
capacitance was obtained from galvanostatic charge
/(Δ
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e
/(ΔVm
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e
Vm), (where,
Acc
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e
), (where,
discharge time, Δ
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e
discharge time, ΔV
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e
V (V) is the potential window, and
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e
(V) is the potential window, and
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e
To prepare the electrode, the carbonised materials were directly mixed with MFC
Acc
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rticl
e
To prepare the electrode, the carbonised materials were directly mixed with MFC
suspension, diluted to 1 wt%, to form an electr
Acc
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rticl
e
suspension, diluted to 1 wt%, to form an electr
wt% MFC binder. No conductive additives were used. The prepared paste was then coated Acc
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e
wt% MFC binder. No conductive additives were used. The prepared paste was then coated
on a Ni foam and dried in an oven at 105 °C for at least 24 h.Acc
epte
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e
on a Ni foam and dried in an oven at 105 °C for at least 24 h.
This article is protected by copyright. All rights reserved
Acknowledgements
The authors appreciate financial support from Omya International AG. This work
made use of the Aalto University Nanomicroscopy Centre (Aalto-NMC) for SEM
imaging and XPS. The authors also appreciate support from Dr. Jouko Lahtinen for
conducting the XPS experiments. The authors would also like to extend their
appreciation to Asbury Carbons for the supply of free graphite samples.
Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))
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Figure 1 schematic representation of the fabrication process of graphene/AC composites
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schematic representation of the fabrication process of graphene/AC composites
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schematic representation of the fabrication process of graphene/AC composites
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Figure 2 SEM images showing the morphologies of the synthesised samples (a) G0-0%; (b) G1-5%; (c) G2-10%; (d) G3-15%
Figure 3 SEM images of G1-5% at different resolutions showing a more pronounced 3-dimensional structure.
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SEM images showing the morphologies of the synthesised samples (a) G0
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SEM images showing the morphologies of the synthesised samples (a) G05%; (c) G2
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5%; (c) G2-
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-10%; (d) G3
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10%; (d) G3
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Figure 4 (a) Normalised Raman spectra for the prepared samples showing the presence of all typical bands; (b) Wide XPS survey spectra. High resolution C1s XPS spectra of G0-0% (c), G1-5% (d), G2-10% (e) and G3-15% (f).
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(a) Normalised Raman spectra for the prepared samples showing the presence of all Acc
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(a) Normalised Raman spectra for the prepared samples showing the presence of all typical bands; (b) Wide XPS survey spectra. High resolution C1s XPS spectra of G0A
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typical bands; (b) Wide XPS survey spectra. High resolution C1s XPS spectra of G05% (d), G2 A
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Figure 5 Pore structure characterisation of the samples: (a) Nitrogen adsorption-desorption isotherms; (b) pore size distribution
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isotherms; (b) pore size distribution
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Figure 6 Electrochemical evaluation of the samples: (a), (c) and (d) the cyclic voltammetry curves at different scan rates and (b), (d) and (f) the galvanostatic charge/discharge cycles at various current densities ranging from 0.5 to 10 A g-1.
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Electrochemical evaluation of the samples: (a), (c) and (d) the cyclic voltammetry
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Electrochemical evaluation of the samples: (a), (c) and (d) the cyclic voltammetry curves at different scan rates and (b), (d) and (f) the galvanostatic
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curves at different scan rates and (b), (d) and (f) the galvanostaticvarious current densities ranging from 0.5 to 10 A gA
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Figure 7 (a) Specific capacitance as a function of current density and (b) Nyquist plots showing the imaginary part versus the real part of impedance with the insert showing the magnification of the high-frequency region
Figure 8 Cycling stability tests of the best performing sample G1-5% at a current density of 5 A g-1; the insert shows the charge-discharge cycles at different intervals
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showing the imaginary part versus the real part of impedance with the ins
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showing the imaginary part versus the real part of impedance with the insmagnification of the high
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magnification of the high
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Table 1 Average ID/G value and XPS chemical composition of the samples
Samples ID/G
Composition, (at. %)
C O
G0-0% 0.864 88.4 11.0
G1-5% 0.147 81.0 14.7
G2-10% 0.153 96.8 2.9
G3-15% 0.201 97.3 2.2
Table 2 Porosity characteristics of the samples and specific capacitance at the current density of 1 A g-1
Samples BET,
m2 g-1
Pore volume, cm3
g-1
Average pore
width, nm
Specific capacitance,
F g-1
G0-0% 314 0.42 8.2 15
G1-5% 720 0.24 1.8 120
G2-10% 546 0.35 3.1 94
G3-15% 424 0.42 6.5 61
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A simple method for fabrication of graphene based hybrid electrodes for energy
storage application is presented. This method can serve as route for fabrication of high
performance, energy storage devices based largely on renewable and sustainable materials
with a potential for large-scale application.
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A simple method for fabrication of graphene based hybrid electrodes for energy
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A simple method for fabrication of graphene based hybrid electrodes for energy
storage application is presented. This method can serve as route for fabrication of high
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storage application is presented. This method can serve as route for fabrication of high
performance, energy storage devices based largely on renewable and sustainable materials
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e
performance, energy storage devices based largely on renewable and sustainable materials
with a potential for large
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with a potential for large
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