Stretchable energy storage E-skin supercapacitors and body movement sensors

Journal Pre-proof Stretchable Energy Storage E-skin Supercapacitors and Body Movement Sensors Jian Wang, Hengyi Lou, Junjing Meng, Zhiqin Peng, Bing Wang, Junmin Wan

PII:

S0925-4005(19)31728-9

DOI:

https://doi.org/10.1016/j.snb.2019.127529

Reference:

SNB 127529

To appear in:

Sensors and Actuators: B. Chemical

Received Date:

1 September 2019

Revised Date:

30 November 2019

Accepted Date:

2 December 2019

Please cite this article as: Wang J, Lou H, Meng J, Peng Z, Wang B, Wan J, Stretchable Energy Storage E-skin Supercapacitors and Body Movement Sensors, Sensors and Actuators: B. Chemical (2019), doi: https://doi.org/10.1016/j.snb.2019.127529

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Stretchable Energy Storage E-skin Supercapacitors and Body Movement Sensors

Jian Wanga, Hengyi Loua, Junjing Menga, Zhiqin Penga, Bing Wanga, Junmin Wana,b*

a

National Engineering Lab of Textile Fiber Materials & Processing Technology, Zhejiang Sci-Tech

ro of

University, Hangzhou 310018, PRChina. b

State Key Laboratory of advanced Textiles Materials and Manufacture Technology, MOE, Zhejiang

-p

Sci-Tech University, Hangzhou 310018, PR China.

Corresponding address: Name: Junmin Wan, Email:wanjunmin@zstu. edu. cn; wwjm2001@126.

Jo

ur

na

Graphical abstract

lP

re

com Phone: +86-571-86843867, FAX: +86-571-86843867

1

Highlights 

Stretchable multi-functional energy storage e-skin supercapacitors and sensors were fabricated by low cost and simple process.



2. The e-skin supercapacitors showed a larger areal specific capacitance and a high energy density. 3. The e-skin sensors showed superior sensitivity including stretching and bending.



4. The e-skin has broad prospects of robots, prosthetics and stretchable wearable devices.

ro of



Abstract

Electronic skin (e-skin) with natural stimuli is a key factor in prosthetics, robotics, and wearable

-p

electronics. At present, most traditional stretchable e-skins fabricated by predesigning complex 3D

re

structures are unsuitable in commercial applications. Energy storage devices with stretchable and arbitrary shapes can widely adapt to wearable electronics. Stretchable supercapacitors and sensors

lP

have been used as the primary energy supplies in prosthetics, robotics, and wearable electronics in

na

recent years, because of their superior power density and long calendar life. Here, stretchable energy storage e-skin supercapacitors and sensors were fabricated using two-sublayered silver nanowire

ur

(AgNW)/MnO2NW (MNW) hybrid conductive networks fixed into the polydimethylsiloxane (PDMS) layer and sandwiched using AgNW/MNW film electrodes and PVA–KOH solid electrolyte.

Jo

The obtained e-skin device showed large areal specific capacitance, excellent capacity retention, coulombic efficiency of 2000 cycles, and high energy density. The multifunctional e-skin sensors showed an areal specific capacitance of 371 mF cm−2 at a current density of 1 mA cm−2, extended to 160%, and had superior sensitivity including stretching and bending by hand. Therefore, the

2

multifunctional e-skin sensors are suitable in robotics, prosthetics, and other stretchable wearable devices. Keywords: AgNW; MnO2 nanowire; electronic skin; supercapacitor; sensor.

1. Introduction The demand for stretchable electronic devices has continuously increased in flexible all-solid-state

ro of

supercapacitors and the other wearable electronic products in recent years [1-3]. Wearable electronics have attracted considerable attention because of their good flexibility, outstanding

electrical conductivity, high intensity and light weight, long cycle stability, and their suitability in

-p

intelligence terminals, such as medical monitoring equipment and intelligent robots [4-6].

re

Stretchable supercapacitors, which are stretchable and wearable electronics, have received increasing attention because of their excellent energy storage performance and recyclability [7, 8]. Electronic

lP

skin (e-skin) is another wearable electronics and skin-conformal material that can transduce external

na

pressures into quantifiable electrical signals based on piezoresistive, piezocapacitive, piezoelectric, and triboelectric mechanisms [9, 10]. Some scientific studies have focused on improving some

ur

functions of wearable electronics but ignored the overall performance and reliability. Furthermore, the requirements of electrode structures and complicated material fabrication have limited the

Jo

versatility of today’s wearable electronics, which results in the limitation of supercapacitors, such as the ability to turn into any shape and arbitrary scalable [11]. Thus, new structure design and fabrication of flexible multifunctional electrodes with high capacitance and excellent sensitivity should be developed for future applications [12, 13].

3

State-of-the-art developments have revealed that three obstacles hinder the successful fabrication of multifunctional stretchable electronics [14, 15]. The first issue arises from the detection sensitivity of the forces in 3D directions [16, 17]. The resistance changes of different external forces in most cases are difficult to realize [18]. Therefore, new materials, structure designs, and sensing mechanisms are utilized to easily identify the output of the related signals. New structure designs should be simple and can be economically manufactured at large scale[19]. The second issue arises from the conflict

ro of

between the capacitance and intrinsic rigidity of electrode materials [20]. Conventional carbon-based material electrodes with electrochemical performance cannot withstand the outside mechanical

forces during folding, cutting, and stretching. The addition of high capacitance pseudo capacitive

-p

materials (such as MnO2), which are used to improve the capacitance, reduces the flexibility and

re

conductivity of the obtained electrodes because of the poor electrical performance and activity utilization [21]. The third issue arises from the balance of signal sensitivity and capacitance [22].

lP

Usually, the products with good sensitivity are relatively soft and thin, which indicates that the

na

content of capacitive materials decreases and the capacitance performance is low [23]. Here, a new type of flexible multifunctional e-skins with multi sensitivity and good capacitance

ur

performance was developed. The so-called stretchable multifunctional energy storage e-skin supercapacitors and sensors were composed of the middle layer of PVA-KOH solid electrolyte, two

Jo

sublayers of silver nanowires (AgNWs)/MnO2 nanowires (MNWs) composite films, and two external layers of polydimethylsiloxane (PDMS). The ultralong MNWs with 1D linear morphology have high specific surface area that can provide high capacity and reliability, and the AgNWs have attracted considerable attentions because of good conductive performance [24, 25]. Therefore, a cross-linked film with good electrical conductivity, large capacitance, high energy density, and good 4

sensibility was fabricated by hybridizing 1D morphology MNWs and AgNWs. The PDMS was used to coat on two outlayer of AgNW/MNW composite film to obtain a flexible electrode. The sandwich structure electrode was prepared by coating two base electrodes with a PVA–KOH gel-based solid electrolyte (Figure 1). The sandwich structure provided the wearable electronics with excellent tensile properties and high capacitance. The assembled device showed a specific capacitance of 371 mF cm−2 at a current density of 1 mAcm−2 and a capacitance retention of 94.4% after 2000

ro of

charge–discharge. The proposed stretchable multifunctional energy storage electrode exhibited sensitivity (tensile and bending test) and stability, and extended up to 80% without decrease in capacitance.

-p

2. Results and Discussion

re

Ultra-long MNWs used for stretchable multifunctional energy storage e-skin supercapacitors and sensors were fabricated using an improved hydrothermal method. MnO2, which is a representative

lP

pseudo capacitive material, has been widely used because of its high theoretical capacitance of 1370

na

Fg−1 [10]. The specific preparation of MNWs is described as follows. The formation of flexible multifunctional electrodes is shown in Figure 1. First, MNW homogeneous suspension was obtained

ur

through ultrasonic dispersion in ethanol. Second, AgNW homogeneous suspension was mixed with MNW suspension. After the obtained suspension was stirred, the AgNW/MNW flexible hybrid film

Jo

was prepared by filtrating the mixed suspension. Then, a thin film of PDMS was spin-coated on the AgNW/MNW conductive layer (Figure 1). After deaeration and heat curing, the PVA–KOH gel electrolyte was coated on the conductive overlapped area in two composite films. A flexible energy storage e-skin was assembled using the AgNW/MNW hybrid film as the relative electrode and the PVA–KOH solid electrolyte as barrier. 5

ro of

.

Fig 1. Schematic illustration of the fabrication process of the energy storage e-skin with symmetrical multi-layer

-p

structures.

re

Figure 2 showed the field emission scanning electron microscopy (FESEM) images, crystal characteristics, and schematics of AgNW/MNW-based energy storage e-skin. Figures 2a showed the

lP

AgNWs with an average diameter of 60-70 nm and a length of 35-45 µm, and Figures 2b showed the

na

MNWs with a diameter of 70–200 nm and a length of 10–45 µm. The SEM images of AgNWs/MNWs composite film (Figure 2c) revealed the interlaced network of AgNWs and MNWs,

ur

which maintained the tight mechanical contact deformation between the AgNWs and MNWs. This condition resulted in enhancement of mechanical strength and conductivity and exposure of the

Jo

layered porosity structure and the infiltration network to a large area of electrolyte, which were favorable for the rapid transport of the ions. The cross-sectional SEM images of AgNWs/MNWs electrode film (Figure 2d) showed the AgNWs/MNWs film and PDMS film were combined to form a two-layer structured composite film. The Figure 2e was the cross-sectional SEM images of the e-skin device. The two characteristic peaks at 346.9 and 638.9 cm−1 in the Raman spectrum of 6

MNWs (Figure 2f), the skeleton bent vibration of the Mn-O bond, and the skeleton stretching vibration of MNWs all confirmed the formation of δ-MnO2 ultra-long NWs through a simple hydrothermal method. Figure 2g showed the XRD patterns of MNWs and AgNWs. The diffraction peaks at 12° and 24°, corresponding to the (001) and (002) planes, showed the crystal of δ-MNWs[21]. The diffraction peaks of AgNWs corresponding to the (111), (200), (220), and (311) planes were consistent with those given by the literature [26-28]. The obtained multifunctional

ro of

composite film exhibited a five-layer ordered structure, as shown in Figure 2h. The electrolyte layer was sandwiched between the two layers of AgNW/MNW cross-linking film, and the two outer

layers were the flexible protective PDMS film. AgNWs and MNWs were firmly anchored in the

-p

flexible film after coating and heat curing by PDMS, which protected the conductive layer against

re

oxidation by atmospheric air. After that, PVA-KOH solid electrolyte was added to assemble the

Jo

ur

na

lP

electronic skin which has five-layers ordered structure shown in the figure 2h.

7

Fig. 2 Characterization and schematic illustration of the assembled e-skin. (a) SEM images of the AgNWs with

different magnification. (b) SEM images of the MnO2 nanowires with different magnification. (c) SEM image of the

AgNWs/MNWs hybrid film. (d) cross-sectional SEM image of the AgNWs/MNWs electrode film. (e) cross-sectional

SEM image of the e-skin device. (f) Raman spectrum of MnO2 nanowires. (g)XRD of AgNWs and MNWs. (h)

Schematic illustration of the energy storage e-skin with symmetrical multi-layer structures.

ro of

Different amounts of AgNWs and/or MNWs were added to the prepared different groups of hybrid films to investigate their effects on the performance of the obtained AgNW/MNW hybrid films. We systematically investigated different groups of AgNW/MNW hybrid films through CV, EIS, and

-p

galvanostatic charge/discharge (GCD). The electrolyte used in the electrochemical test was 1.0 M

re

aqueous Na2SO4. Figures 3a and 3b showed the CV curves of different amounts of AgNW-X/MNW-Y hybrid film electrodes at 5 mv s−1, where X was the AgNW concentration and Y

lP

was the MNW concentration in the mixed solution. AgNWs-0/MNWs-4 indicated pure MNW films

na

prepared from 4 mg/mL of the MNW solution. As shown in Figure 3a, the CV curve of the sterling AgNW electrode (AgNWs-2/MNWs-0) showed a relatively ideal capacitive behavior because of its

ur

approximately rectangular shape. The CV curve shape of the sterling MNW film (AgNWs-0/MNWs-4) was close to a triangle, which indicated its low conductivity and poor

Jo

reversibility. All the CV curves of the AgNW/MNW hybrid film electrodes indicated large CV integration areas and strong current response. The AgNW/MNW hybrid film (AgNWs-2/MNWs-4) had the largest specific capacitance because of its CV curve integration area. The results indicated that appropriate amount of AgNWs effectively improved the conductivity of MNWs and largely improved the capacitance of the electrode. GCD curves were consistent with the CV curves in the 8

same potential window. The capacitance of the hybrid electrodes first increased and then decreased with the increase of AgNW (Figure 3c) or MNW (Figure 3d) contents. The AgNW-2/MNW-4 hybrid film electrode achieved a maximum capacitance of 808 mF cm−2 at a current density of 1 mA cm−2, which was higher than many flexible electrodes reported in the literature (Table 1). The good electrochemical performance of AgNW/MNW electrodes was due to their cross-linked network structure and the excellent electrical conductivity of AgNWs. Excessive addition of AgNWs led to a

ro of

decrease with the content of MNWs, which caused the degradation of capacitance. Nyquist plots and sheet resistance curve of some electrodes are shown in Figures 3e and3f. In the high-frequency

region, the Nyquist plots as a semicircle on the real impedance axis represented the charge-transfer

-p

resistance. The results showed that pure AgNW electrode (AgNWs-2/MNWs-0) had the smallest

re

interface charge-transfer resistance, and the resistance of electrodes increased with the increase of MNW content. The sheet resistance curve of the electrodes obtained the same conclusion. The sheet

lP

resistance of the electrodes was controlled by changing the doping amount of MNWs (Figure 3g). When the pure AgNW electrode was close, the sheet resistance of hybrid film electrodes was small,

na

and the minimum was 1.38 Ω/sq. As shown in Figure 3h, the maximum area capacitance was 808

ur

mF cm−2 at a current density of 1 mA cm−2 (AgNWs-2/MNWs-4). The specific areal capacitance decreased with the increase of current density, and 409 mF cm−2 was retained at a high current

Jo

density of 20 mA cm−2 during the experiment. Regardless of the charge–discharge current density, the specific capacity of AgNW-2/MNW-4 hybrid film electrode was higher than that of others, which indicated its better rate performance and charge storage capacity. The above-mentioned experimental results showed that the performance of the combination of MNW and AgNW network aided the AgNW-2/MNW-4 composite film electrode to obtain good performance. We selected the 9

AgNW-2/MNW-4 hybrid film electrode to assemble a multifunctional stretchable electronic device

na

lP

re

-p

ro of

and evaluated its performance.

Fig. 3 Electrochemical characterization of the AgNWs/MNWs hybrid film electrodes. (a, b) CV curves at a scan rate

ur

of 5 mV s-1 with different amounts of AgNWs and MNWs, respectively. (c, d) Galvanostatic charge/discharge curves at a current density of 0.1 mA cm-2 of different amounts of AgNWs and MNWs, respectively. (e, f) Nyquist plots over

Jo

the frequency range of 100 kHz to 0.1 Hz and circuit diagram, (f) the close-up view of the high-frequency. (g) The

sheet resistance of hybrid film electrodes with different amounts of AgNWs and MNWs. (h) Capacitance retention at

different current densities. (i) Optical image of the AgNWs/MNWs hybrid film electrode.

10

Table 1 Areal and volume specific capacitance of different flexible material electrodes and supercapacitors Electrode

Capacitance

Voltage

Capacitance for

Capacitance Electrolyte

References

for electrode

region

supercapacitor

retention

CNTs-MnO2

203 mF cm-2

0-1V

123 mF cm-2

PVA-KOH

20000 cycles 97.8%

8

RGO-MnO2

1200 mF cm-2

0-1V

300 mF cm-2

0.8 M H2SO4

1600 cycles 75%

9

δ-MnO2/CNT

946 mF cm-2

0-0.8V

358 mF cm-2

PVA-KOH

2000 cycles 91%

10

RGO-MnO2

897 mF cm-2

0-0.9V

205 mF cm-2

1 M Na2SO4

3600 cycles 75%

12

PPy-MnO2-CNT

－

0-0.8V

1.49 F cm-2

0.5 M Na2SO4

3000 cycles 96%

13

MnO2/HCS-30

－

0-0.8V

255 F g-1

1 M H2SO4

5000 cycles 93.9%

20

PANI-PCH film

－

0-0.8V

488 mF cm-2

PVA-H2SO4

7000 cycles 93%

25

MoSe2-Ni(OH)2

1175 F/g

0-0.45V

124 F g-1

6 M KOH

5000 cycles 85%

26

CC-Ni(OH)2

709F/g

0-0.5V

108 F g-1

PVA-KOH

10000 cycles 95.2%

30

AgNWs/MNWs

808 mF cm-2

0-0.55V

371 mF cm-1

PVA-KOH

-p

ro of

material

This work

re

2000 cycles 92.3%

lP

A multifunctional stretchable energy storage e-skin was assembled by sandwiching the PVA–KOH solid-state electrolyte between two symmetric AgNW-2/MNW-4 hybrid film electrodes coated using

na

PDMS. The assembled device was soft and light, and its thickness was adjusted by changing the coating amount. Figure 4a showed the stress–strain curves of AgNW-2/MNW-4 PDMS-based

ur

stretchable electronic. The elongation at break of stretchable electronic reached 65%, and the elastic

Jo

modulus was 8.8 MPa because of the sandwich structure and high-elastic PDMS. In addition to mechanical flexibility, the electrodes were protected from short circuit in wearable electronic devices. Therefore, we used solid electrolytes, such as PVA–KOH gel, to act as an effective separator for the entire device. Some studies have used a thin layer of nano cellulose fiber film to prevent short circuits of two electrodes. However, their method increased the costs and introduced many complications for preparation of wearable electronic devices [29]. The electronic device 11

assembled by PVA–KOH gel maintained the good flexibility of the PDMS multilayer composite film, endured large angles during folding and twisting tests, and maintained good mechanical properties. Furthermore, the assembled device exhibited good flexibility and non breakage under low-pressure twisting and bending, which implied that the assembled multifunctional stretchable energy storage e-skin had good mechanical properties (Figure 4b). As shown in Figure 4c, the CV curves of multifunctional stretchable energy storage electronic maintained an approximate

ro of

rectangular shape at different scanning rates. The GCD curves exhibited good linear voltage–time charge curve and nearly symmetrical triangle charge/discharge curves at different current densities

(Figure 4d). The capacitance of the assembled device decreased from 371 mF cm−2 with the increase

-p

in scanning speed, which was lower than the average capacitance of AgNW-2/MNW-4 hybrid film

re

electrodes at the same scanning speed. The capacitance of the assembled device was 304 mF cm−2 at a current density of 5 mA cm−2. As shown in Figure 4e, the CV curve slightly varied after 2000

lP

bends at 180°, and the EIS curve was similar to the original one. This condition showed that the

na

effect of bending behavior within a certain range on the electrochemical performance of the assembled device was relatively small. The main reasons of the decline in capacitance of

ur

AgNW/MNW assembled device were described as follows: (1) PVA–KOH gel affected the transmission of electrons between the two hybrid film electrodes, and (2) the encapsulation of

Jo

PDMS improved the resistance of the entire electronic device, although we used thin copper sheets as contact electrodes [30]. The capacitance retention and coulombic efficiency of the assembled multifunctional energy storage e-skin was tested until 2000 cycles to fully evaluate its cycling stability as a supercapacitor for wearable electronic products (Figure4g). Capacitance retention and coulombic efficiency gradually declined during 2000 cycles, and the decline rate of the latter was 12

comparatively faster than that of the former. The calculation formulas of capacitance retention and coulomb efficiency can be found in the experimental section. The energy storage e-skin had good cycle stability, as shown in Figure 4g. Capacitance retention was 92.3%, and coulombic efficiency reached 94.4% after 2000 cycles. In the present work, a red light-emitting diode glowing under direct current electric field was used on the AgNW-2/MNW-4 assembled e-skin on a bionic palmar. The comparison of the energy and power densities of the e-skin and other energy storage systems

ro of

was shown in Figure 4i. As shown in the energy and power density graph, the AgNW-2/MNW-4 stretchable energy storage electronic displayed a large energy density of 56.1 μW h cm−2 at a power density of 0.27 μW cm−2 (Figure 4i). The energy density was 36.3 μW h cm−2 at a power density of

-p

2.75 μW cm−2 (current density was 10 mA cm−2). This performance was evidently superior to the

re

similar symmetrical carbon nanotube/MnO2 hybrid film that exhibited energy density of 25 μW h cm−2 in previous reports. The stretchable AgNW/MNW assembled device was a flexible energy

lP

storage supercapacitor suitable in many applications because of its excellent electrochemical

Jo

ur

na

performance and mechanical durability.

13

ro of -p

re

Fig. 4 Basic mechanical-electric properties of the AgNWs-2/MNWs-4 assembled e-skin. (a) Stress-strain curve of the

lP

e-skin. (b) Optical images of the energy storage e-skin with bending and twisting. (c) CV curves of energy storage e-skin from 5 to 100 mV s-1. (d) galvanostatic charge/discharge curves at different current densities. (e, f) the CV

na

curves and Nyquist plots of the assembled device by bending 2000 times, respectively. (g) Cycling stability and

coulombic efficiency after 2000 cycles. (h) Optical image of a red light-emitting diode (LED) glows by

ur

AgNWs-2/MNWs-4 assembled e-skin on the bionic palmar. (i) Energy and power densities of the assembled device

Jo

compared with other energy-storage systems reported.

The multifunctional stretchable e-skin should imitate various sensing behavior of humans to verify its suitability in wearable electronics [31-33]. The plot of the measured resistance change with the bending angle showed that the bending angle rate was positively correlated with the resistance change (Figure 5a). The same relationship was also reflected in the strain and resistance change of 14

the assembled device (Figure 5b). The two curves of resistance change with different variables were fitted to the exponential curves. The density of the AgNW/MNW elements diluted and reduced when the flexible e-skin bended and expanded. This condition rapidly disrupted the AgNW/MNW conductive networks and reduced the electronic connectivity between the networks. The resistance of the assembled device rapidly increased when the reduced density of AgNW/MNW elements was lower than the conductive percolation threshold. Similarly, twisting produced specific resistance

ro of

changes. Figure 5c showed that a well-cut assembled device attached on the joint of a bionic palmar, Figure 5e output stable periodic signals subjected to cyclic bending. Similar phenomenon extended

to the cycling stability of repeated tensile loading and unloading (Figure 5d). We conducted several

-p

cyclic tests based on the above-mentioned experiments (2000 times) through repeated bending of

re

wooden artificial finger joint. The experimental results showed that the output signals were stable during thousands of bending cycles (Figure 5f), and the curves of the initial and late bending cycles

lP

were nearly identical. This condition was due to that the AgNW/MNW conductive networks were

na

tightly stuck on the flexible and scalable PDMS protection membrane. Bending and stretching in a certain range did not destroy the conductive network structure protected by PDMS flexible layer [34,

ur

35]. The AgNW/MNW networks returned to the original position when the external force disappeared because of the resilience of the polymer. Therefore, the recyclability of the e-skin

Jo

largely depended on the creep of the flexible layer. Materials with evident creep recovery led to the poor stability of the final assembled device [36-38]. These observations suggested that the multifunctional e-skin responding to resistance changes by stretching, bending, and twisting was suitable in displacement monitoring, bionic skin, and robotics[39, 40]. The assembled device had cyclic stability and repeatability and was thus beneficial to practical applications [41]. 15

ro of

Fig. 5 Sensing capabilities of the e-skin. (a) Relative resistance change-bending angle plot and bending configuration

-p

(inserted). (b) Relative resistance change-tensile test plot, in which the inset illustrates circuit connection. (c) Relative

resistance change-bending plot of the e-skin attached on a bionic finger knuckle as illustrated in (e). (d) Relative

re

resistance change-stretching plot of the e-skin stretched by hand. (e) Optical image of the e-skin attached on the

lP

bionic finger knuckle for bending test. (f) Relative resistance change of the e-skin as a function of 2000 cyclic

3. Conclusions

na

bending deformation.

ur

We reported an innovative and collaborative AgNW/MNW networks and symmetrical multilayer

Jo

structure for developing stretchable multifunctional energy storage e-skin supercapacitor and sensor. MNWs with high aspect ratio were repeatedly prepared through a simple hydrothermal treatment. Hybrid film electrodes with different content ratios of AgNWs and MNWs were prepared through simple vacuum filtration, and the suitable preparation ratio was determined by electrochemical test. The obtained AgNW-2/MNW-4 assembled device showed large areal specific capacitance, excellent capacity retention, coulombic efficiency of 2000 cycles, and high energy density. The 16

multifunctional energy wearable e-skin with a symmetrical multilayer structure produced sensitive and stable resistance changes for the stretching and bending forces and had high reproducibility over 2000 loading cycles. Such a high sensitive and versatile e-skin consisted of hybrid film conductive layer, PDMS flexible layer, and PVA–KOH gel electrolyte via some simple materials and a reproducible preparation. The proposed e-skin can be used as portable, stretchable, and wearable electronic products and can be widely applied in medical, biomimetic robotics, biomedical devices,

ro of

and other industries.

4. Methods

-p

4.1. Preparation of MNWs

re

MNWs were synthesized through a hydrothermal method with magnetic stirring. In a typical synthesis, a 100 mL mixture solution contained 60 mL4mol/mL NaOH solution and 40 mL30%

lP

H2O2 solution, and the mixed solution was slowly poured into the Mn(NO3)2 solution (0.3 mol/L,

na

200 mL) under stirring. A brown suspension was formed after vigorous stirring for 30 min in a reaction caldron. The synthesized MNWs were collected and washed with 0.1 mol/mL HCl solution

ur

for two times and deionized water (DI water) for three times using a centrifuge to remove the residues. In the hydrothermal treatment, 5g of the synthesized MnO2 and 100mL 1 mol/ml NaOH

Jo

solution were transferred into a 100 mL hydrothermal reactor and reacted at 150 °C for 24 h with magnetic stirring at 500 rpm. MNWs were collected and washed with 0.1 mol/mL HCl solution and DI water for five times through centrifugation and dried at 50 °C overnight. 4.2. Preparation of AgNWs 2 g polyvinylpyrrolidone (PVP) was added to 100 ml ethylene glycol (EG) and stirred until 17

completely dissolved. Then, 1 ml 30 μmol/ml ferric chloride solution was added to the solution and stirred uniformly. Thereafter, the mixed solution was placed in a 200 ml reaction vessel and then heated. When the temperature reached 150℃, 25 ml 0.12 mol/ml silver nitrate solution was added dropwise to the mixed solution at 160 ℃ under stirring for 6 hours. Then the reaction solution was naturally cooled to room temperature, and transferred to a centrifuge tube, and centrifugally washed with ethanol at 3000 rpm for 3-5 times. Finally, the AgNWs ethanol solution was obtained.

ro of

4.3. Fabrication of Flexible MnO2 Composite Films MNW/AgNW flexible composite film was prepared through vacuum filtration. MNWs (120 mg)

were dispersed in 30 mL 2 mg/mL ethanol solution of AgNWs to obtain the MNW/AgNW mixed

-p

suspension through 1h ultrasonic treatment and stirred at 1000 rpm for 4 h at room temperature. The

re

MNW/AgNW suspension was vacuum filtrated using a polyvinylidene fluoride (PVDF) filter membrane with a pore size of 0.2 μm and thoroughly rinsed with ethanol and dried at room

lP

temperature. Furthermore, a thin film of PDMS (ratio of base/curing agent = 10:1, SYLGARD 184)

na

was spin-coated on the conductive layer with a spinning speed of 500 rpm and baked for more than 4 h at 60 °C. The as-prepared flexible composite film electrodes were peeled off from the PVDF filter

ur

membrane. Different amounts (AgNWs-X/MNWs-Y) of flexible hybrid films were obtained through a similar process by changing the amounts of MNWs and AgNWs, in which X was the AgNWs

Jo

concentration and Y was the MNWs concentration in the mixed solution. AgNWs-2/MNWs-4 indicated that the mixed solution consisted of 2 mg/mL AgNWs and 4 mg/mL MNWs. 4.4. Assembly of Stretchable Multifunctional Energy Storage E-skin PVA (6 g) was dissolved in 30 mL DI water in 90 °C water bath with continuous stirring to prepare the PVA transparent solution. KOH (3 g) was dissolved into 30 mL DI water to prepare the KOH 18

solution. Then, the KOH solution was added to the PVA solution and cooled to room temperature, and the clarified gel solution was obtained through stirring. The precut thin copper sheet was used as the part to connect the current collector and protect the AgNW/MNW electrodes by avoiding the coverage of PDMS in this area. Thereafter, the PVA–KOH gel electrolyte was coated on the conductive side in the composite films. The two electrodes were overlapped together to prepare a symmetric supercapacitor, and PVA–KOH gel electrolytes were pasted on the overlapped area. The

ro of

bonded electrodes were left in the vacuum chamber to release the gas in the electrolyte. Finally, the as-fabricated energy storage e-skin was dried and solidified for approximately 12 h at room

temperature. The prepared energy flexible storage e-skin had high-order layered structure and

re

4.5. Material and Mechanical Characterization

-p

homogenized diffusion between the AgNW/MNW network and PDMS macromolecules.

The crystal structures of MNWs and AgNWs were investigated using an X-ray diffractometer

lP

(XTRA, Thermo ARL) with a scanning rate of 5°/min. The microscopic morphologies of various

na

nanoparticles and composites were recorded through thermal FESEM (vltra55, Carl Zeiss SMT Pte Ltd). Tensile tests of e-skin were conducted using a universal material experiment machine (AG-I,

ur

SHIMADZU). The electrical conductivity of AgNW/MNW composite films was measured using a four-probe resistivity measurement system. Nine-point sampling was used to measure the average

Jo

conductivity of each film with different specific compositions. The specific areal capacitance Cs (mF cm−2) is calculated by

Cs  I  t /( V  S ) .

Energy density Es and power density Ps of the e-skin are calculated using the equations:

Es  1/ 2  Cs  V 2 , 19

Ps  Es / t , Where I is the applied current (A),∆t is the discharge stage time (s), ∆V is the voltagechange (V) during the discharge process, and S is the active material area of the electrode (cm2). Capacitance retention η (%) is calculated as

η  Δtx/Δt 0 , Where tx is the charge–discharge time (s) after x cycles and t 0 is the charge–discharge time

ro of

(s) of initiation. Coulombic efficiency ηce (%) is calculated by

ηce  tc/td ,

-p

Where tc is the charge stage time (s) and td is the discharge stage time (s).

re

Resistance change rate ηR (%) is calculated by

ηR  Ron  R0  / R0 ,

lP

Where Ron is the instant resistance (Ω) and R0 is the initial resistance (Ω).

ur

na

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Jo

Acknowledgements

We gratefully acknowledge financial support from Zhejiang Top Priority Discipline of Textile Science and Engineering, Science Foundation of Zhejiang Sci-Tech University (ZSTU) (No. 1101820-Y), National Natural Science Foundation of China (No. 21473161 and 21271155).

20

References [1]

J. Zhou, X. Xu, Y. Xin, G. Lubineau, Coaxial Thermoplastic Elastomer-Wrapped Carbon Nanotube Fibers for Deformable and Wearable Strain Sensors, Advanced Functional Materials 28 (2018).

[2]

W. Zhong, C. Liu, C. Xiang, Y. Jin, M. Li, K. Liu, Q. Liu, Y. Wang, G. Sun, D. Wang, Continuously Producible Ultrasensitive Wearable Strain Sensor Assembled with Three-Dimensional Interpenetrating Ag Nanowires/Polyolefin Elastomer Nanofibrous Composite Yarn, ACS Appl Mater Interfaces 9 (2017) 42058-42066.

[3]

J. Wu, H. Wang, Z. Su, M. Zhang, X. Hu, Y. Wang, Z. Wang, B. Zhong, W. Zhou, J. Liu, S.G. Xing, Highly Flexible and Sensitive Wearable E-Skin Based on Graphite Nanoplatelet and Polyurethane Nanocomposite Films in Mass Industry Production Available, ACS Appl Mater Interfaces 9 (2017) 38745-38754.

[4]

Y. Zhao, X. He, R. Chen, Q. Liu, J. Liu, J. Yu, J. Li, H. Zhang, H. Dong, M. Zhang, J. Wang, A flexible

ro of

all-solid-state asymmetric supercapacitors based on hierarchical carbon cloth@CoMoO4NiCo

layered double hydroxide core-shell heterostructures, Chemical Engineering Journal 352 (2018) 29-38. [5]

S. Zhao, L. Guo, J. Li, N. Li, G. Zhang, Y. Gao, J. Li, D. Cao, W. Wang, Y. Jin, R. Sun, C.P. Wong,

Binary Synergistic Sensitivity Strengthening of Bioinspired Hierarchical Architectures based on

-p

Fragmentized Reduced Graphene Oxide Sponge and Silver Nanoparticles for Strain Sensors and Beyond, Small 13 (2017). [6]

S. Wang, Y. Wu, Y. Gu, T. Li, H. Luo, L.H. Li, Y. Bai, L. Li, L. Liu, Y. Cao, H. Ding, T. Zhang, Wearable

re

Sweatband Sensor Platform Based on Gold Nanodendrite Array as Efficient Solid Contact of Ion-Selective Electrode, Anal Chem 89 (2017) 10224-10231. [7]

S. Zheng, H. Xue, H. Pang, Supercapacitors based on metal coordination materials, Coordination

[8]

lP

Chemistry Reviews 373 (2018) 2-21.

L. Dong, C. Xu, Y. Li, Z. Pan, G. Liang, E. Zhou, F. Kang, Q.H. Yang, Breathable and Wearable Energy Storage Based on Highly Flexible Paper Electrodes, Adv Mater 28 (2016) 9313-9319.

[9]

H. Chen, S. Zeng, M. Chen, Y. Zhang, L. Zheng, Q. Li, Oxygen Evolution Assisted Fabrication of

na

Highly Loaded Carbon Nanotube/MnO2 Hybrid Films for High-Performance Flexible Pseudosupercapacitors, Small 12 (2016) 2035-2045. [10]

L. Wang, M. Huang, S. Chen, L. Kang, X. He, Z. Lei, F. Shi, H. Xu, Z.-H. Liu, δ-MnO2 nanofiber/single-walled carbon nanotube hybrid film for all-solid-state flexible supercapacitors

[11]

ur

with high performance, Journal of Materials Chemistry A 5 (2017) 19107-19115. Y. Zhang, Y. Fang, J. Li, Q. Zhou, Y. Xiao, K. Zhang, B. Luo, J. Zhou, B. Hu, Dual-Mode Electronic

Jo

Skin with Integrated Tactile Sensing and Visualized Injury Warning, ACS Applied Materials & Interfaces 9 (2017) 37493-37500.

[12]

A. Sumboja, C.Y. Foo, X. Wang, P.S. Lee, Large areal mass, flexible and free-standing reduced graphene oxide/manganese dioxide paper for asymmetric supercapacitor device, Adv Mater 25 (2013) 2809-2815.

[13]

N. Liu, W. Ma, J. Tao, X. Zhang, J. Su, L. Li, C. Yang, Y. Gao, D. Golberg, Y. Bando, Cable-Type Supercapacitors of Three-Dimensional Cotton Thread Based Multi-Grade Nanostructures for Wearable Energy Storage, Advanced Materials 25 (2013) 4925-4931.

[14]

J. Xu, S. Wang, G.-J.N. Wang, C. Zhu, S. Luo, L. Jin, X. Gu, S. Chen, V.R. Feig, S. Rondeau-Gagné, J. Park, B. C.Schroeder, C. Lu, J.Y. Oh, Y. Wang, Y.-H. Kim, H. Yan, R. Sinclair, D. Zhou, G. Xue, B. 21

Murmann, C. Linder, W. Cai, Jeffery, Tok, J.W. Chung, Z. Bao, Highly stretchable polymer semiconductor films through the nanoconfinement effect, Science (2017) 355. [15]

Y.L. Rao, A. Chortos, R. Pfattner, F. Lissel, Y.C. Chiu, V. Feig, J. Xu, T. Kurosawa, X. Gu, C. Wang, M. He, J.W. Chung, Z. Bao, Stretchable Self-Healing Polymeric Dielectrics Cross-Linked Through Metal-Ligand Coordination, J Am Chem Soc 138 (2016) 6020-6027.

[16]

W. Xi, F. Kong, J.C. Yeo, L. Yu, S. Sonam, M. Dao, X. Gong, C.T. Lim, Soft tubular microfluidics for 2D and 3D applications, Proc Natl Acad Sci U S A 114 (2017) 10590-10595.

[17]

J.H. Pikul, S. Li, H. Bai, R.T. Hanlon, I. Cohen, R.F. Shepherd, Stretchable surfaces with programmable 3D texture morphing for synthetic camouflaging skins, Science 358 (2017) 210-214.

[18]

J. Bae, Y. Li, J. Zhang, X. Zhou, F. Zhao, Y. Shi, J.B. Goodenough, G. Yu, A 3D Nanostructured Hydrogel-Framework-Derived High-Performance Composite Polymer Lithium-Ion Electrolyte, Angew Chem Int Ed Engl 57 (2018) 2096-2100. Q. Wang, C. Wang, M. Zhang, M. Jian, Y. Zhang, Feeding Single-Walled Carbon Nanotubes or

ro of

[19]

Graphene to Silkworms for Reinforced Silk Fibers, Nano Lett 16 (2016) 6695-6700. [20]

W. Du, X. Wang, J. Zhan, X. Sun, L. Kang, F. Jiang, X. Zhang, Q. Shao, M. Dong, H. Liu, V.

Murugadoss, Z. Guo, Biological cell template synthesis of nitrogen-doped porous hollow carbon spheres/MnO2 composites for high-performance asymmetric supercapacitors, Electrochimica [21]

-p

Acta 296 (2019) 907-915.

Z. Lv, Y. Luo, Y. Tang, J. Wei, Z. Zhu, X. Zhou, W. Li, Y. Zeng, W. Zhang, Y. Zhang, D. Qi, S. Pan, X.J. Loh, X. Chen, Editable Supercapacitors with Customizable Stretchability Based on Mechanically

[22]

re

Strengthened Ultralong MnO2 Nanowire Composite, Adv Mater 30 (2018). L. Miao, D. Zhu, M. Liu, H. Duan, Z. Wang, Y. Lv, W. Xiong, Q. Zhu, L. Li, X. Chai, L. Gan, Cooking carbon with protic salt: Nitrogen and sulfur self-doped porous carbon nanosheets for [23]

lP

supercapacitors, Chemical Engineering Journal 347 (2018) 233-242. L. Kou, T. Huang, B. Zheng, Y. Han, X. Zhao, K. Gopalsamy, H. Sun, C. Gao, Coaxial wet-spun yarn supercapacitors for high-energy density and safe wearable electronics, Nat Commun 5 (2014) 3754.

F. Xu, Y. Zhu, Highly conductive and stretchable silver nanowire conductors, Adv Mater 24 (2012)

na

[24]

5117-5122. [25]

K. Wang, X. Zhang, C. Li, X. Sun, Q. Meng, Y. Ma, Z. Wei, Chemically Crosslinked Hydrogel Film Leads to Integrated Flexible Supercapacitors with Superior Performance, Adv Mater 27 (2015)

[26]

ur

7451-7457.

E.D. Martínez, A.F. García Flores, H. Pastoriza, R.R. Urbano, C. Rettori, Electrothermal silver

Jo

nanowire thin films for In-Situ observation of thermally-driven chemical processes, Sensors and Actuators B: Chemical 259 (2018) 475-483.

[27]

S. Jiang, S. Reich, B. Uch, P. Hu, S. Agarwal, A. Greiner, Exploration of the Electrical Conductivity of Double-Network Silver Nanowires/Polyimide Porous Low-Density Compressible Sponges, ACS Appl Mater Interfaces 9 (2017) 34286-34293.

[28]

C. Liu, H. Liu, C. Lu, K. Tang, Y. Zhang, Polyethyleneimine-modified graphene oxide/PNIPAm thermoresponsive hydrogels with rapid swelling/deswelling and improved mechanical properties, Journal of Materials Science 52 (2017) 11715-11724.

[29]

H.J. Xian, C.R. Cao, J.A. Shi, X.S. Zhu, Y.C. Hu, Y.F. Huang, S. Meng, L. Gu, Y.H. Liu, H.Y. Bai, W.H. Wang, Flexible strain sensors with high performance based on metallic glass thin film, Applied 22

Physics Letters 111 (2017) 121906. [30]

Yu Ouyang, Rongjiao Huang, Xifeng Xia, Haitao Ye, Xinyan Jiao, Liang Wang, Wu Lei, Q. Hao., Hierarchical structure electrodes of NiO ultrathin nanosheets anchored to NiCo2O4 on carbon cloth with excellent cycle stability for asymmetricsupercapacitors, Chemical Engineering Journal 355 (2019) 416-427.

[31]

X. Wang, Y. Zhang, X. Zhang, Z. Huo, X. Li, M. Que, Z. Peng, H. Wang, C. Pan, A Highly Stretchable Transparent Self-Powered Triboelectric Tactile Sensor with Metallized Nanofibers for Wearable Electronics, Advanced Materials 30 (2018).

[32]

P. Li, Z. Jin, L. Peng, F. Zhao, D. Xiao, Y. Jin, G. Yu, Stretchable All-Gel-State Fiber-Shaped Supercapacitors Enabled by Macromolecularly Interconnected 3D Graphene/Nanostructured Conductive Polymer Hydrogels, Advanced Materials 30 (2018).

[33]

D. Wang, Y. Zhang, X. Lu, Z. Ma, C. Xie, Z. Zheng, Chemical formation of soft metal electrodes for flexible and wearable electronics, Chemical Society Reviews 47 (2018) 4611-4641. X.-S. Zhang, M. Han, B. Kim, J.-F. Bao, J. Brugger, H. Zhang, All-in-one self-powered flexible

ro of

[34]

microsystems based on triboelectric nanogenerators, Nano Energy 47 (2018) 410-426. [35]

Z. Lou, S. Chen, L. Wang, R. Shi, L. Li, K. Jiang, D. Chen, G. Shen, Ultrasensitive and ultraflexible e-skins with dual functionalities for wearable electronics, Nano Energy 38 (2017) 28-35.

[36]

T. Lei, I. Pochorovski, Z. Bao, Separation of Semiconducting Carbon Nanotubes for Flexible and

[37]

-p

Stretchable Electronics Using Polymer Removable Method, Acc Chem Res 50 (2017) 1096-1104. L. Feng, K. Wang, X. Zhang, X. Sun, C. Li, X. Ge, Y. Ma, Flexible Solid-State Supercapacitors with Enhanced Performance from Hierarchically Graphene Nanocomposite Electrodes and Ionic [38]

re

Liquid Incorporated Gel Polymer Electrolyte, Advanced Functional Materials 28 (2018) 1704463. S. Chen, Z. Lou, D. Chen, G. Shen, An Artificial Flexible Visual Memory System Based on an UV-Motivated Memristor, Adv Mater 30 (2018).

Y. Cai, J. Shen, Z. Dai, X. Zang, Q. Dong, G. Guan, L.J. Li, W. Huang, X. Dong, Extraordinarily

lP

[39]

Stretchable All-Carbon Collaborative Nanoarchitectures for Epidermal Sensors, Adv Mater 29 (2017). [40]

S. Choi, S.I. Han, D. Jung, H.J. Hwang, C. Lim, S. Bae, O.K. Park, C.M. Tschabrunn, M. Lee, S.Y. Bae,

na

J.W. Yu, J.H. Ryu, S.-W. Lee, K. Park, P.M. Kang, W.B. Lee, R. Nezafat, T. Hyeon, D.-H. Kim, Highly conductive, stretchable and biocompatible Ag-Au core-sheath nanowire composite for wearable and implantable bioelectronics, Nature Nanotechnology 13 (2018) 1048-+. [41]

X. Pu, M. Liu, X. Chen, J. Sun, C. Du, Y. Zhang, J. Zhai, W. Hu, Z.L. Wang, Ultrastretchable,

ur

transparent triboelectric nanogenerator as electronic skin for biomechanical energy harvesting

Jo

and tactile sensing, Sci Adv 3 (2017) e1700015.

Author Biographies Jian Wang received her BS degreein Material Engineering from Zhejiang Sci-Tech University. He then joined Dr Junmin Wan'sresearch group and is currently pursuing his MS degree in Material Engineering at Zhejiang Sci-Tech University. His research interests include flexible electronics, wearable biosensors and supercapacitor. 23

Hengyi Lou is currently pursuing a bachelor's degree in Material Engineering at Zhejiang Sci-Tech University. He is a member of Dr. Junmin Wan's research group. His research interests include flexible electronics, wearable biosensors and MOFs. Junjing Meng received her BS degree in Material Science and Engineering from Zhejiang Sci-Tech University. She joined Dr Junmin Wan's research group and is currently pursuing her MS degree in Material Science Engineering at Zhejiang Sci-Tech University. She mainly studies electrocatalytic materials. Zhiqin Peng professor of National Engineering Lab of Textile Fiber Materials &

ro of

Processing Technology, Zhejiang Sci-Tech University. Bing Wang professor of National Engineering Lab of Textile Fiber Materials & Processing Technology, Zhejiang Sci-Tech University.

Junmin Wan professor of National Engineering Lab of Textile Fiber Materials &

-p

Processing Technology, Zhejiang Sci-Tech University. His current research interests

Jo

ur

na

lP

re

include flexible electronics, wearable biosensors, supercapacitor, and MOFs.

24