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Contents lists available at ScienceDirect
Nano Energy
journal homepage: www.elsevier.com/locate/nanoen
Full paper
Inkjet printing of δ-MnO2 nanosheets for flexible solid-state
micro-supercapacitor
Yang Wanga, Yi-Zhou Zhangb, David Dubbinka, Johan E. ten
Elshofa,⁎
aUniversity of Twente, MESA+ Institute for Nanotechnology, P.O.
Box 217 7500AE, Enschede, The Netherlandsb Key Laboratory of
Flexible Electronics (KLOFE) & Institute of Advanced Materials
(IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road,
211800Nanjing, China
A R T I C L E I N F O
Keywords:Inkjet printingTwo-dimensional
materialsMicrosupercapacitorFlexible electronicsManganese oxide
A B S T R A C T
Inkjet printing is considered as a promising technique for
flexible electronics fabrication owing to its simple,versatile,
environmental-friendly and low-cost features. The key to inkjet
printing is ink formulation. In thiswork a highly concentrated ink
containing two-dimensional δ-MnO2 nanosheets with an average
lateral size of89 nm and around 1 nm thickness was used. By
engineering the formulation of the δ-MnO2 ink, it could be
inkjetprinted on O2 plasma treated glass and polyimide film
substrates to form δ-MnO2 patterns without undesired“coffee-ring”
effect. As a proof-of-concept application, all-solid-state
symmetrical micro-supercapacitors (MSCs)based on δ-MnO2 nanosheet
ink were fabricated. The fabricated MSCs showed excellent
mechanical flexibilityand good cycling stability with a capacitance
retention of 88% after 3600 charge-discharge cycles. The
MSCsattained the highest volumetric capacitance of 2.4 F cm−3, and
an energy density of 1.8·10−4 Wh cm−3 at apower density of 0.018W
cm−3, which is comparable with other similar devices and show great
potential asenergy storage units for low-cost flexible and wearable
electronics applications.
1. Introduction
Printed electronics is an emerging technology for flexible
electronicdevice fabrication [1–3]. Printed devices, including
organic transistors[4,5], organic light-emitting diodes [6] and
energy storage devices[7–10], can be built by printing liquid
functional materials such asorganic [11] and inorganic
nanomaterials [12], as well as two dimen-sional materials [13] on
arbitrary substrates at relatively low tem-peratures. Inkjet
printing is an ideal method for deposition of nano-materials for
flexible device fabrication because it is a non-contact,precisely
controlled deposition and additive printing process.
Owing to their atomically thin layers, high theoretical specific
ca-pacitance, environmental compatibility and low cost, birnessite
man-ganese dioxide (δ-MnO2) nanosheets are regarded as an
attractiveelectrode material for portable energy storage devices
like super-capacitors (SCs). In addition, the layered structure of
δ-MnO2 enablesthe SCs to be much thinner and flexible than
conventional devices.Their fabrication by inkjet printing shows
great potential in this respect,since it allows the fabrication of
integrated micro-supercapacitors(MSCs) for small size portable,
flexible and wearable electronic devices.Alternative methods such
as spray coating [14] vacuum filtration [15]and spin coating [16]
have been used to construct MSC devices, but
they lack the same degree of control over the roughness of the
elec-trodes and they have limitations in terms of pattern
design.
However, a number of challenges still needs to be addressed in
orderfor inkjet printing to become practically feasible. Firstly,
ink formula-tion involving liquid exfoliation processes is far from
ideal as it requiresmultistep processes and is time-consuming [17].
Secondly, printableink formulations should have proper fluidic
properties, as inkjetprinting imposes specific requirements on the
physical properties of theink such as surface tension and viscosity
[13]. Thirdly, the ink shouldhave a high solids concentration and
high stability in order to improvethe efficiency of the inkjet
printing process [18].
In this study, we developed a highly concentrated, stable,
water-based δ-MnO2 nanosheet ink. No toxic solvents, solvent
exchange pro-cesses or harsh preparation conditions were required.
The δ-MnO2 inkformulation was optimized for an all-solid-state
flexible MSC applica-tion.
2. Experimental section
2.1. Ink preparation
Colloidal δ-MnO2 nanosheets were prepared similar to a
previously
https://doi.org/10.1016/j.nanoen.2018.05.002Received 17 February
2018; Received in revised form 11 April 2018; Accepted 1 May
2018
⁎ Corresponding author.E-mail address: [email protected]
(J.E. ten Elshof).
Nano Energy 49 (2018) 481–488
Available online 02 May 20182211-2855/ © 2018 The Author(s).
Published by Elsevier Ltd. This is an open access article under the
CC BY-NC-ND license
(http://creativecommons.org/licenses/BY-NC-ND/4.0/).
T
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reported method [19]. Typically, 20mL of a mixed aqueous
solution of0.6 M tetrabutylammonium hydroxide (TBA•OH, 40wt% H2O,
AlfaAesar) and 3wt% H2O2 (30 wt% H2O, Aldrich) was added to 10mL
of0.3 M MnCl2•4H2O (Sigma-Aldrich) aqueous solution within 15 s.
Theresulting dark brown solution was stirred vigorously overnight
in theambient at room temperature. The solution obtained was
centrifugedusing a Sigma 1–14 centrifuge at 1000 g for 20min before
collecting theupper 2/3 of the volume. The lower 1/3 was washed by
water andmethanol at 295 g for 20min, after which the precipitate
was dried atroom temperature. The collected supernatant was
centrifuged at15,000 g for 1 h and the precipitate was re-dispersed
in the printingsolvent. The printing solvent consisted of 1:10
propylene glycol (Sigma-Aldrich): water by mass, 0.06mg/mL Triton
X-100 (Sigma-Aldrich).Then the re-dispersion solution was filtered
through a 0.2 µm syringefilter to remove large flakes which might
block the ink jet printernozzles.
In order to estimate the final concentration of δ-MnO2 in the
aboveink, 100 µL ink was diluted in water by 500 times on volume.
The op-tical absorbance was measured using a Shimadzu UV-1800
UV–Visspectrophotometer at 800–300 nm wavelength. According to
theLambert-Beer law A/l= αC, where A is the absorbance, l the cell
length(here l=1 cm), C the concentration of dispersed δ-MnO2 and
the ab-sorption coefficient α=1.13×104 mol−1 dm3 cm−1 for δ-MnO2
na-nosheets at around 374 nm [20], the δ-MnO2 concentrations C in
theink was estimated to be 8.8 mgmL-1.
PEDOT: PSS (3.0–4.0%, Sigma Aldrich) solution was
filteredthrough a 0.45 µm syringe filter followed by addition of 2
vol% TritonX-100 and 6 vol% ethylene glycol (Merck).
2.2. Printing
All patterns and devices were inkjet printed by a Dimatix
DMP-2800inkjet printer (Fujifilm Dimatix) which equipped with a 10
pL cartridge(DMC-11610). Our formulated δ-MnO2 ink was printed on
differentsubstrates, including glass and 120 µm thick polyimide
film substrates.PEDOT: PSS ink was inkjet printed on top of printed
δ-MnO2 film at adrop spacing of 20 µm at room temperature. The
substrates, includingglass and polyimide film, were cleaned by
ethanol, acetone, isopropanoland water followed by O2 plasma
treatment for 5min.
2.3. Fabrication of MSC
First, δ-MnO2 ink was inkjet printed in 5 layers at 20 µm
dropspacing on a 120 µm thick polyimide substrate, followed by
annealingat 350 °C for 1 h under nitrogen atmosphere. Then, 2
layers of PEDOT:PSS were inkjet printed at 20 µm drop spacing on
top of the thermallytreated δ-MnO2 thin films, followed by thermal
annealing at 120 °C for15min. The prepared PEDOT: PSS/δ-MnO2 films
were used as elec-trodes to for a symmetrical MSC. The PVA/LiCl gel
electrolyte wasprepared by mixing 1 g PVA (MW 85,000–124,000,
Aldrich), 2.13 g LiCl(Alfa Aesar) and 10mL DI water thoroughly at
85 °C under vigorousstirring. To complete the MSC, the electrolyte
was deposited on theelectrodes area of MSC, and was dried at room
temperature overnight.
2.4. Electrochemical testing
All electrochemical characterization was done by an
Autolabworkstation (PGSTAT128N). The prepared PEDOT: PSS/δ-MnO2
elec-trode was tested using a three-electrode configuration in 0.5M
Na2SO4(ABCR GmbH) solution. A platinum wire and an Ag/AgCl (3M
KCl)electrode (Metrohm) were used as the counter and reference
electrodes,respectively. The electrochemical performance of the
all-solid-stateMSC was measured in a two-electrode configuration.
CV curves wereobtained at a scan rate of 5–100mV s−1, galvanostatic
charge-dischargecurves were measured at current densities from 0.05
to 0.2 A cm−3.Electrochemical impedance spectroscopy was performed
by applying an
AC voltage of 10mV amplitude in the frequency range from 0.01
to10 kHz.
2.5. Characterization
X-ray diffraction (XRD) was conducted on a PANalytical X′Pert
Prowith Cu Kα radiation (λ=0.15405 nm). High resolution
scanningelectron microscopy (HRSEM; Zeiss MERLIN) was used to
acquire in-formation on the morphology of printed δ-MnO2 nanosheets
films. AFM(Veeco Dimension Icon) was performed in standard tapping
mode. TheAFM data were analyzed by Gwyddion (version 2.47)
software. X-rayphotoelectron spectroscopy (XPS) was measured by an
OmicronNanotechnology GmbH (Oxford Instruments) surface analysis
systemwith a photon energy of 1486.7 eV (Al Kα X-ray source) with a
scanningstep size of 0.1 eV. The pass energy was set to 20 eV. The
spectra werecorrected using the binding energy of C 1s of the
adventitious carbon asa reference. TEM was performed by FEI Titan
80–300 ST (300 kV) withenergy dispersive X-ray spectroscopy (EDS)
capabilities. UV–vis spectrawere measured by a UV-1800 Shimadzu.
The surface tension of the inkwas measured by contact angle system
OCA (Data Physics Corporation).The viscosity was determined by an
Automated Microviscometer AMVn(Anton Paar GmbH). The specific
volumetric capacitance (CV) of filmelectrodes was calculated from
the GCD curves by using Eq. (1):
=C I dV dt V[ /( / )]/V electrode (1)
where I is the discharge current, dV/dt is the slope of
discharge curve,and Velectrode refers to the volume of the film
electrode.
The specific areal capacitance (CA,device) and volumetric
capacitance(CV,device) of the MSC devices were also calculated from
the GCD curveaccording to Eqs. (2) and (3), respectively:
=C I dV dt A[ /( / )]/A, device device (2)
=C I dV dt V[ /( / )]/V , device device (3)
Here Adevice refers to the total area of the device including
theelectrodes and the gap between the electrodes. Vdevice refers to
the totalvolume of the device, including the volume of the
electrodes and thegap between the electrodes.
The volumetric energy densities (EV, Wh cm−3) and power
densities(PV, W cm−3) were calculated from Eqs. (4) and (5)
= ×E C V /(2 3600)V V, device 2 (4)
= × ∆P E t3600 /V V (5)
Where Δt refers to discharge time.
3. Results and discussion
Powder X-ray diffraction (XRD) of a dried sample of a
colloidalsuspension after centrifuging and washing with distilled
water andmethanol, was used to verify the crystal structure and
phase informa-tion of the δ-MnO2 nanosheets as shown in Fig. 1a.
The XRD patternshows the characteristic peaks at 2θ 12.21°, 24.55°,
36.71°, 65.87°,which are attributable to the (001), (002), (100)
and (110) reflections[21]. These peaks indicate a layered
birnessite-type structure. Thethickness of a δ-MnO2 nanosheet
deposited on a silicon substrate byLangmuir–Blodgett (LB)
technology was measured by atomic forcemicroscopy (AFM) and was
around 1 nm (Fig. S1, Supporting in-formation). Based on its atomic
architecture, the crystallographicthickness of monolayer δ-MnO2
nanosheets has been calculated to be0.52 nm [14]. Hydration and the
presence of organic ions, i.e. tetra-butylammonium (TBA+), on both
sides of the δ-MnO2 nanosheets canexplain the difference between
the crystallographic thickness and theobserved thickness [14]. The
lateral sizes of δ-MnO2 nanosheets esti-mated from AFM images (Fig.
1b) indicate that the majority of na-nosheets has lateral sizes
between 50 and 150 nm (Fig. 1c), which meets
Y. Wang et al. Nano Energy 49 (2018) 481–488
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the requirement of the inkjet printer. In principle, a lateral
size of lessthan 1/50 the diameter of nozzle is preferred to avoid
the nozzle frombecoming clogged during printing [22]. Based on this
rule of thumb, themaximum nanosheet lateral size is around 430 nm
for our inkjet printer
with a nozzle diameter of 21.5 µm. Fig. 1d shows a transmission
elec-tron microscopy (TEM) image of δ-MnO2 nanosheets, illustrating
theultrathin nature of the 2D nanostructure. X-ray photoelectron
spectro-scopy (XPS) was used to determine the oxidation state of Mn
in δ-MnO2
Fig. 1. Characterization of δ-MnO2 nanosheets. a) XRD pattern of
δ-MnO2 nanosheets. b) AFM image of δ-MnO2 nanosheets after
deposition on a Si wafer by LBtechnology. c) Lateral size
distribution of δ-MnO2 nanosheets obtained by measuring 100
nanosheet flakes in Fig. 1b). d) TEM image of δ-MnO2 nanosheets. e)
and f)are the Mn 2p and Mn 3s XPS spectra of δ-MnO2 nanosheets,
respectively.
Fig. 2. Optimization of δ-MnO2 ink formulation. a) Photograph of
formulated δ-MnO2 nanosheet ink. b) Optical image of δ-MnO2 ink
droplet formation vs time asobserved from the printer camera. The
scale bar is 50 µm. c, d) Droplet drying process with c) excess
surfactant and d) optimal surfactant concentration. e) AFMimage of
printed single dot on glass substrate with excess surfactant. f)
Cross-sectional profiles along three different directions in e). g)
AFM image of printed singledot on glass substrate with optimized
surfactant concentration; h) cross-sectional profiles along three
different directions in g).
Y. Wang et al. Nano Energy 49 (2018) 481–488
483
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nanosheets. The two peaks at the binding energies of 641.9 eV
and653.7 eV as shown in Fig. 1e, can be assigned to the Mn 2p3/2
and 2p1/2orbitals of Mn4+, respectively. The Mn 3s spectrum
displays doublepeaks that result from parallel spin coupling
between the electrons in 3sand 3d orbitals, with a splitting width
of 4.8 eV, further indicating thatthe Mn cations have an average
valence close to 4 [23].
Water as such is not suitable for inkjet printing due to its
highsurface tension (about 70mN/m) and low viscosity (about 1mPa
s).The inverse Ohnesorge number Z is often used to evaluate ink
print-ability, and is defined as =Z γρα η( ) /1/2 , where γ is the
surface tension, ρthe density, α the nozzle diameter and η the
viscosity of the fluid. Toformulate a printable δ-MnO2 ink (Fig.
2a), Triton X-100, a non-ionicsurfactant, was selected as surface
tension modifier to decrease thesurface tension of water from
around 73–46mNm−1. Triton may alsohelp to avoid disrupting the
electrostatic stabilization of δ-MnO2 na-nosheets. Propylene glycol
was added to modify the viscosity from 1.00to 1.71mPa s in order to
improve printing reliability. The value of thesurface tension,
viscosity and nozzle diameter of 21.5 µm makes that Zis about 19
for the modified water-based ink. This quality of the ink
wasconfirmed by the optical images of ink droplet formation vs time
whereno satellite droplets are present (Fig. 2b). An additional
advantage ofthe addition of propylene glycol is that it can also
suppress weakMarangoni flow which will reduce the undesired
coffee-ring effect [24].
The concentration of Triton X-100 was optimized since an
excesstends to shrink the droplet size. As schematically outlined
in Fig. 2c,excess Triton X-100 unpins the contact line led to a
non-uniform
distribution of solids, which can indeed be clearly observed by
AFM(Fig. 2e). The cross-sectional profile of the AFM image of Fig.
2e inFig. 2f further confirms the pattern non-uniformity. More AFM
imagesand cross-sectional profiles along different directions of
non-uniformprinted line are shown in Fig. S2a and S2b (Supporting
information).The concentration of Triton X-100 was therefore
carefully optimized toensure the pinning of the contact line of the
ink. Under ideal conditionsthe material is uniformly deposited on
the substrate due to recirculatingMarangoni flow, as schematically
shown in Fig. 2d. The AFM image inFig. 2g shows a printed dot
obtained from an ink with an optimizedTriton X-100 concentration.
The corresponding cross-sectional analysisin Fig. 2h reveals
pattern uniformity in all directions, indicating thereliability and
quality of the printing process. The printed patterns alsoshow a
smooth surface and low root mean square roughness at
highermagnification, as shown in Fig. S3 (Supporting
information).
The morphology of printed δ-MnO2 lines on glass substrate at 50
°Cwith variable droplet spacing is shown in Fig. 3a. The line
becamebulged when the droplet spacing was 15 µm, due to the fact
that dro-plets significantly overlap with each other at this
spacing. As the dro-plet spacing increased to 40 µm, the morphology
of the lines becamemore uniform while the line width decreased. Any
further increase ofthe droplet spacing led to isolated droplets as
they were too far fromeach other to merge. The homogeneous
morphology and fidelity ofprinted lines employing a 40 µm droplet
spacing was confirmed byAFM; Fig. 3b shows a uniform distribution
of nanosheet, while thecross-sectional profiles of Fig. 3b in Fig.
3c confirm the uniformity of
Fig. 3. Optimization of δ-MnO2 ink printing parameters. a)
Optical images of printed lines at different droplet spacings. The
scale bar is 100 µm. b) AFM image ofprinted line at 40 µm drop
spacing. c) Cross sectional profiles along three different
directions in b). d) Optical image of printed δ-MnO2 thin films on
glass substrate. e)Top-view SEM images of d) at different
magnifications, in which the δ-MnO2 nanosheets are uniformly
distributed.
Y. Wang et al. Nano Energy 49 (2018) 481–488
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the printed lines in all directions. The morphology of printed
δ-MnO2lines on polyimide substrate at room temperature with
variable dropletspacing was also studied as shown in Fig. S4
(Supporting information).The lines became uniform using a droplet
spacing from 20 to 50 µm. Inorder to reduce printing layers and
improve printing efficiency, a dro-plet spacing at 20 µm was used
for printing δ-MnO2 ink on polyimidesubstrate. The δ-MnO2 ink was
also used to print thin films with uni-formly distributed δ-MnO2
nanosheets, as illustrated in Fig. 3d wherethe optical image of a
printed δ-MnO2 film on a glass substrate isshown, and Fig. 3e where
the top-view SEM images of Fig. 3d at dif-ferent magnifications are
shown.
To investigate the electrochemical performance of a printed
δ-MnO2film, printed poly(3,4-ethylenedioxythiophene):
poly(styrenesulfonate)(PEDOT: PSS)/δ-MnO2 electrodes on polyimide
substrates were madeand studied in three-electrode measurements.
The reliable printingprocess allowed us to print multilayered
δ-MnO2 films with different δ-MnO2 film thicknesses. As shown in
Fig. 4a, the thickness of theseprinted δ-MnO2 films was
proportional to the number of printed layers.A series of electrodes
with varying δ-MnO2 films thicknesses between65 and 1245 nm were
made. These electrodes are referred to as Mn-65,Mn-380, Mn-530,
Mn-880, and Mn-1245, respectively, depending ontheir thickness (in
nanometers). All electrodes were characterized in athree-electrode
setup in 0.5M Na2SO4 solution. The cyclic voltammetry(CV) curves of
these electrodes at a scan rate of 10mV s−1 show rec-tangular-like
shapes, which are explained by the redox reaction MnO2+Na+
+e-⇄MnOONa. The galvanostatic charge/discharge (GCD)curves in Fig.
4c were acquired at a current density of 0.5 A cm−3. Thecalculated
volumetric capacitances (CV) are shown in Fig. 4d. As thethickness
of δ-MnO2 films increased to 65 nm, the CV of the Mn-65electrode
reached 78.4 F cm−3, which is higher than the pure PEDOT:PSS
electrode (23.4 F cm−3). The maximum CV of 271.6 F cm−3 wasobtained
with the Mn-530 electrode, and this value is about an order
ofmagnitude higher than the pure PEDOT: PSS film. This value is
alsoclearly higher than the CV of the 65 nm thick film, showing
that the δ-
MnO2 nanosheet layers contribute to the electrode reaction. In
contrast,when the thickness of the δ-MnO2 film was further
increased to 880 nm,the CV of the Mn-880 electrode decreased
dramatically to 156.6 F cm−3.Most likely, electron transfer between
layers becomes limiting in thickδ-MnO2 film, probably to the extent
that the MnO2 nanosheet layers ofthe electrode furthest away from
the external electrode are electricallyisolated and do not
contribute to the capacity of the supercapacitor.Slow electron
transfer kinetics or electrical insulation between
adjacentnanosheet layers has been observed in various studies
involving mul-tilayers of nanosheets [25]. The CV of the even
thicker Mn-1245 elec-trode decreased further to 100 F cm−3.
Possibly, the electrically in-sulating top part of the electrode
acts only as a diffusion barrier forNa+. In any case these results
clearly show that the optimum thicknessof the MSC is in the range
of about 500 nm.
To further investigate the use of δ-MnO2 nanosheets for
practicalapplication, a symmetrical MSC with interdigitated
electrode config-uration was fabricated using inkjet printing
δ-MnO2 on a flexiblepolyimide substrate, as schematically
illustrated in Fig. S5 (Supportinginformation). Functional δ-MnO2
based devices including 10 in-planeinterdigitated patterns were
printed. The δ-MnO2 film was about530 nm thick, as shown in the SEM
image of the cross-section of the filmin Fig. S6 (Supporting
information). After drying the δ-MnO2 pattern,PEDOT: PSS conducting
electrodes were inkjet printed on top of the δ-MnO2 patterns. Then
a poly(vinyl alcohol)/lithium chloride (PVA/LiCl)gel electrolyte
was cast onto the surface of the PEDOT: PSS/δ-MnO2electrode to
complete the fabrication of the MSC (Fig. 5a). In order toevaluate
the electrochemical performance of the MSC, CV and galva-nostatic
charge-discharge measurements were carried out in a potentialwindow
from 0 to 0.8 V. The CV curves of the MSC at different scanrates
showed a rectangular-like shape at low scan rates, which
wasmaintained at high scan rates up to 100mV s−1 (Fig. 5b). The
charge-discharge curves are shown in Fig. 5c. The volumetric
capacitance ofthe MSC was calculated based on the charge-discharge
measurements.As shown in Fig. 5d, the MSC showed a highest
volumetric device
Fig. 4. Electrochemical performance of printedδ-MnO2 films with
varying thickness. a)Relationship between δ-MnO2 film thicknessand
the number of printed layers. b) CV curvesof δ-MnO2 films with
varying thicknesses at ascan rate of 10mV s−1. c) GCD of
δ-MnO2electrodes with varying thicknesses at a cur-rent density of
0.5 A cm−3. d) Volumetric ca-pacitances CV of δ-MnO2 electrodes as
a func-tion of film thickness at 0.5 A cm−3.
Y. Wang et al. Nano Energy 49 (2018) 481–488
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capacitance of 2.4 F cm−3 at a current density of 0.05 A cm−3.
Thisvalue corresponds with an areal capacitance of 0.26mF cm−2.
Theareal capacitance is comparable to most graphene-based MSCs
fabri-cated by other techniques [26,27], and can be used in many
on-chipintegrated systems which only require areal capacitances of
around1 μF cm−2 [28,29]. Notably, the volume used in the
calculation of thevolumetric capacitance included the volume of the
electrodes and thespatial gap between the electrodes, while the
area used in the calcu-lation of the areal capacitance includes
both the electrode area and thearea of the gap between the
electrodes. Fig. 5e shows Ragone plots ofthe volumetric energy
density and the power density of the MSC, aswell as a comparison
with other recently reported SC systems. The highequivalent series
resistance (ESR) of the MSC indicates a low charge/discharge rate
(see Fig. S7, Supporting information). The energy densityfor the
MSC is evaluated to be 1.8× 10−4 Wh cm−3, with a powerdensity of
0.018W cm−3. Hence, the energy density of the nanosheet-based
inkjet printed MSC is superior to a commercial 3 V/300 μF
Alelectrolyte capacitor [30], as well as to other supercapacitors
such asZnO@MnO2 carbon fiber [31] and graphene [32]. The
performance ofthe nanosheet-based MSC is comparable to other
devices made ofMnO2/carbon fibers [33] and laser-induced graphene
(LIG) MSC [34].
To demonstrate the mechanical flexibility of the MSC, the
devicewas bent at different angles (Fig. 6a). The CV curves
remained nearlyunchanged while the device was highly bent over 120°
with a bendingradius of about 1 cm (Fig. 6b), indicating that the
MSC has potential asenergy storage unit cell for small flexible
electronics applications.Furthermore, the device was also bent for
250 times with a bending
radius of about 1 cm. As shown in Fig. S8 (Supporting
information), theCV curves showed a slight decrease after 100 times
bending and afurther decrease after 250 times bending due to the
occurrence of asmall crack in the electrode (Fig. S9, Supporting
information). How-ever, the devices was still functional, albeit
operating at a lower per-formance. To meet the requirements for
practical application to satisfyspecific energy and power needs,
MSCs can be connected in series orparallel configurations (Fig.
6c). The voltage window was doubled byconnecting two MSCs in
series, while the output current was increasedby a factor of almost
2 when two MSCs were connected in parallel(Fig. 6d), indicating
that these devices can be integrated to scale up thevoltage and
current output. A 22% drop in volumetric capacitance ofMSC over
3600 charge-discharge cycles was observed (Fig. 6e), in-dicating
good cycling stability. It is noted that this work focused
ondemonstrating the efficiency and possibility of inkjet printing
tech-nology for realizing flexible δ-MnO2 nanosheet-based MSC
devices. Wedid not attempt to determine the performance limits of
these devices.Devices performance improvements may be expected by
integratingother fabrication strategies with our inkjet printing
technology, such aschemical doping of δ-MnO2 nanosheets in order to
improve con-ductivity and/or energy density.
4. Conclusions
We have developed water-based, inkjet printable and highly
con-centrated δ-MnO2 nanosheets inks for supercapacitor
application. Byink formulation engineering, examining the drop
spacing, we
Fig. 5. Electrochemical performance of inkjet printed MSC. a)
Schematic diagram of MSC with interdigitated electrode
configuration. b) CV curves of MSC at scanrates from 5 to 100mV
s−1. c) Galvanostatic charge-discharge curves of MSC at current
densities from 0.05 to 0.2 A cm−3. d) Volumetric capacitance of MSC
atdifferent current densities. e) Ragone plot of MSC and recent
data from literature [28–33].
Y. Wang et al. Nano Energy 49 (2018) 481–488
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determined the optimal printing conditions to prevent the
undesired“coffee-ring” effect. We have shown that the inkjet
printed MSCs aremechanically flexible and achieve high performance,
which is com-parable with other MSCs fabricated by different
techniques. The inkjetprinting of two-dimensional materials also
shows a high potential forall-solid-state flexible energy storage
devices. Overall, such inkjetprinted flexible energy storage
devices shows great promising as energystorage units for low-cost
flexible and wearable electronics applica-tions.
Acknowledgements
The authors acknowledge the financial support of the
ChinaScholarships Council program (CSC, No. 201608340058). M.
Smithersis acknowledged for performing the HR-SEM experiments.
Appendix A. Supporting information
Supplementary data associated with this article can be found in
theonline version at
http://dx.doi.org/10.1016/j.nanoen.2018.05.002.
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Fig. 6. Flexibility and cycling measurement, aswell as assembly
of two MSC devices in seriesand parallel configurations. a) Optical
imagesof MSC bent under different angles. The scalebar is 1 cm. b)
CV curves of MSC under dif-ferent bending angles at a scan rate
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Yang Wang is now a Ph.D. candidate under Dr. ProfessorJ.E. ten
Elshof’s supervision in the MESA+ Institute forNanotechnology at
the University of Twente, theNetherlands. He is currently focusing
on inkjet printingtwo-dimensional materials for flexible energy
storage de-vices application with specific emphasis on
supercapacitors.
Dr. Yi-Zhou Zhang received his Bachelor from NanjingUniversity,
after which he obtained Ph.D. from NanjingUniversity of Posts &
Telecommunications in 2016 underthe supervision of Prof. Wei Huang
and Prof. Wen-Yong Lai.He has been working in the field of flexible
electronics,especially on printed flexible devices including
super-capacitors, strain sensors, and integrated functional
sys-tems.
Dr. David Dubbink obtained his BS and MS degrees(2013), and his
Ph.D. (2017) from the University of Twente.Currently, he is working
as a postdoctoral researcher at theUniversity of Twente. His work
focuses on Pulsed LaserDeposition and analysis of complex oxide
thin films.
Dr. Johan E. ten Elshof is professor of Inorganic &
HybridNanomaterials Chemistry at the MESA+ Institute
forNanotechnology of the University of Twente in
Enschede,Netherlands. His research is focused on novel
functionalmetal oxide & organic-inorganic nanomaterials,
nano-patterns and nanostructures from colloidal and
chemicalsolutions, with specific emphasis on
low-dimensionalstructures like flexible nanofibers, nanosheets and
nano-wires. The main application areas of these materials are inthe
fields of energy materials and nanoelectronics.
Y. Wang et al. Nano Energy 49 (2018) 481–488
488
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Inkjet printing of δ-MnO2 nanosheets for flexible solid-state
micro-supercapacitorIntroductionExperimental sectionInk
preparationPrintingFabrication of MSCElectrochemical
testingCharacterization
Results and discussionConclusionsAcknowledgementsSupporting
informationReferences