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Nano Energy

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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: j.e.tenelshof@utwente.nl (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

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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).

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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.

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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.

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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].

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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 of20mV s−1. c) Schematic circuits of two singleMSC devices connected in series and in parallelconfigurations. d) CV curves of single MSC, andtwo MSCs connected in series or in parallel. e)Cyclability test of MSC at a current density of0.2 A cm−3.

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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.

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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