Compressible Supercapacitor with Residual Stress Effect for Sensitive

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Compressible supercapacitor with residual stress effect for sensitive elastic-electrochemical stress sensor Ning Wei, Limin Ruan, Wei Zeng, Dong Liang, Chao Xu, Linsheng Huang, and Jinling Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12745 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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ACS Applied Materials & Interfaces

Compressible Supercapacitor with Residual Stress Effect for Sensitive Elastic-electrochemical Stress Sensor Ning Wei,a,bLimin Ruan,a,b Wei Zeng,a,b,*Dong Liang,a,b,*Chao Xu,a,b Linsheng Huanga,b and Jinling Zhaoa,b a

Key Laboratory of Intelligent Computing & Signal Processing, Ministry of Education,

Anhui University, No. 3 Feixi Road, Hefei 230039, Anhui Province, People's Republic of China b

Anhui Engineering Laboratory of Agro-Ecological Big Data, School of Electronics and

Information Engineering, Anhui University, No. 111 Jiulong Road, Hefei 230601, Anhui Province, People's Republic of China

* Corresponding author. E-mail address:

Wei Zeng

[email protected]

Dong Liang

[email protected]

Abstract In this work, we have synthesized graphene aerogels using natural-drying method and fabricated a compressible all-solid-state supercapacitor (ASC), which offers outstanding energy density of 23.08 Wh kg-1 at 240 W kg-1. We further demonstrate that the device is deformable in squeezed cases with residual stress effect. Taking advantage of the compressibility and excellent electrochemical performance of the graphene aerogel, we

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offer a new type of stress sensor called elastic-electrochemical stress sensor. Served as elastic-electrochemical stress device, the cell demonstrates steady responses current towards the external mechanical force by transforming mechanical energy to electrochemical energy. The high sensitive stress sensor will help us comprehend the interaction principle between electrochemistry and external stress well.

Keywords Graphene aerogels, Natural drying, Residual stress effect, Elastic-electrochemical, Stress sensor

1. Introduction Serving as a new kind of ultralight and porous material, graphene aerogels have drew a great attention due to shortened transport distances between electrolyte and electrode, multidimensional electron transport pathways as well as facile access to the electrolyte 1-3.

Because of those prominent merits, aerogels have acted as potential candidates in a

number of areas, such as sensors, high performance electronics, energy-storage supercapacitor device, catalysis, and adsorption

4-11.

Instead of a traditional

supercapacitor device, a new idea of self-charging supercapacitor power cell has been widely studied recently

12-13.

Many researchers have devoted themselves to assembling

the supercapacitor unit with an energy harvester, such as piezoelectric and triboelectric nanogenerators to make the mechanical energy transform to electrochemical energy to


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realize self-charging

14-17.

Sun et al. have reported a flexible and ultralight self-charging

cell by electrospun paper based supercapacitors as storage device

18.

Maitra et al. have

proposed a fast self-charging flexible and wearable energy power device with fish swim bladder as the bio-piezoelectric separator

19.

Many researchers focus on their attention

and efforts to the self-charging investigation of the supercapacitor with the help of medium

(piezoelectric

and

triboelectric

nanogenerators),

while

they

ignore

self-discharging feature of supercapacitor. Besides, Eric Jacques et al. have demonstrated that the electrochemical and mechanical property can interact with each other by a piezo-electrochemical effect

20.

This cell is short of the elasticity and compressibility in

practical application despite they can serve as an energy harvesting from external force work. Significantly, those researches provide a new idea for us to associate the force with electrochemistry in surpercapacitor field. Consequently, it is of great importance to explore the interaction between the force with electrochemistry under self-discharging process in the energy storage or energy conversion application without the use of the medium. On the other hand, many efforts have been made with all-solid-state supercapacitor (ASC) to explore other novel functions, such as making ASC into desired formats (fiber-shaped, thin-film), compressive, stretchable, self-healing, and shape-memory 5-6, 13, 21-24.

From the point of the energy-storage and energy-conversion, those still limit the

development of ASC. Inspired by the previous works of self-charging combined with the energy harvester, we attempt to look for a new combination to design the novel idea of



self-discharging supercapacitor power cell and maximize utilization of energy-storage function. As we know that supercapacitor served as stress sensor only presents the traditional variable resistor sensor in the past. Up to now, few scholars unite those two feathers together to broaden ASC in the practical application fields. Hence, the entire performance of integrated ASC device should be improved further and there are rare reports about applications of the ASC in mechanical-electrochemical field. Here, we have synthesized ternary N, B, S atoms co-doping graphene aerogel (NBSGA) by natural-drying method and fabricate the ASC. As energy storage device, ASC endows the high energy density with 23.08 Wh kg-1 at 240 W kg-1 and highly compression-tolerant supercapacitors. We further demonstrate that the ASC can be used as the deformable supercapacitors under three squeezed cases, demonstrating the enhanced specific capacitances and one kind of residual stress effect. After that, the ASC cell acts as a sensor to detect current change, delivering fast current responses toward external force which realizes the mechanics-electricity energy conversion. The proposed self-discharging stress sensor has potential to be the elastic-electrochemical portable electronics in practical applications. 2. Experimental 2.1 Materials Ethanol, ammonia solution, thiourea, and boric acid are bought from Sinopharm Chemical Reagent Co., Ltd., which are used without further purification.


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2.2 Preparation of NBSGA The schematic of the graphene aerogel synthesis process is shown in the Figure 1(a). The preparation of natural-drying graphene aerogel refers to our previous reported method

25.

Figure 1(b) shows that a part of oxygen-containing groups are reduced with

the help of reducing agent and ternary N, B, S atoms are doped in the graphene in the hydrothermal reaction process. The resulted ternary N, B, S atoms co-doping graphene aerogel is named as NBSGA. Moreover, borate and thiourea molecules can promote their self-assembly to form tough aerogel structure by cross-linking adjacent GO sheets.

2.3 Characterization Scanning electron microscopy (SEM, S-4800), and transmission electron microscopy (TEM, JEM-2010) are used to characterize the morphologies and microstructures of samples. To gain the Raman spectras, a Confocal Laser MicroRaman Spectrometer with 532 nm laser (inVia-Reflex) is used. The structure information of the samples is characterized by X-ray diffraction (XRD, SmartLab 9KW). The chemical states of electrode materials are measured using a photoelectron spectrometer (XPS, Escalab250Xi). 2.4 Preparation of ASC and electrochemical measurement The ASC device is fabricated using the monoblock NBSGA aerogels as both the negative electrode and positive electrode with gel electrolyte. The electrolyte is made as follows: 6 g polyvinyl alcohol (PVA) dissolving into 60 mL deionized water is heated to 90 0C with



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strong stirring until the solution turns clear. Then, 6 g H2SO4 is added drop by drop in PVA solution under vigorous stirring after cooling down to room temperature. After that, we immerse two pieces of NBSGA/Ti electrode in PVA–H2SO4 gel for an hour to assemble the symmetric NBSGA//NBSGA supercapacitor device with a separator. An electrochemical workstation (CHI 660E, Chenhua Instruments Co., Shanghai, China) with a two electrode system is used to offer cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. A battery workstation (CT2001A, jinnuo electronic Co., Ltd., Land, Wuhan) is used to test the galvanostatic charge-discharge (GCD). The specific capacitance is calculated as:26-28 Cs 

4 I  t S  V

(1)

where I (A) represents the discharge current, Δt (s) represents the corresponding discharge time, S (cm-2) is the total area of the monoblock NBSGA aerogels electrodes, and ΔV (V) represents the potential change. The energy density E is calculated as:29-30 1 1 E  C (V ) 2 8 3.6

(2)

The calculations of power density P for device is given as:30 P

E  3600 t

(3)

3. Results 3.1. Morphological and structural characterizations The resulted graphene aerogel exhibits with the thickness of about 1 mm and the diameter of about 13 mm in Figure 2(a). The mass of graphene aerogel is about 4.4 mg. SEM in


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Figure 2(b) shows that the interior structure of aerogel displays large porous cross-linking framework, arising from the outside-in solvent evaporation under natural conditions

31.

Large pores are assembled by the dense graphene cell walls with an average thickness of ≈1.5 µm, expecting to exhibit toughness to adapt to the structure change in Figure 2(c). The dense graphene cell wall is attributed to the use of thiourea and boric acid. A part of thiourea is decomposed into ammonia, hydrogen sulfide, as well as other compounds to separate the GO sheets in the hydrothermal process. Besides, the thiourea not only reduces the GO sheets but can also act as roughness cross-lingking the adjacent graphene sheet via the oxygen-containing functional group

32-33.

Moreover, the bridging effect of

borate also plays important roles in connecting the graphene sheets 31. Consequently, our method prevents effectively the graphene sheet restacking and collapse of NBSGA under naturally drying condition. The TEM of the pure GO presents the nearly transparent smooth surfaces in Figure 2(d), indicating a few layers of GO sheets. While after the hydrothermal reaction, transparent graphene sheets with many wrinkled and folded features which derive from cross-link framework structures of NBSGA are easily observed in Figure 2(e). The high-resolution TEM image clearly displays the fringes with an inter-planar distance of 0.34 nm in Figure 2(f), which is ascribed to the (002) plane of NBSGA. This larger pores skeleton with dense graphene cell walls wrapped with PVA-H2SO4 gel electrolyte not only offers ions diffusion and transport channel but also further provides the ASC with excellent mechanical tolerance under compression. To further explore the structure information for the as-synthesized NBSGA, XRD is



performed in Figure 2(g). We note a sharp diffraction peak at 2θ =10.47º with a d-spacing of 0.844 nm for GO sample

34-35.

NBSGA presents the diffraction peak appearing at

25.9° corresponding to the (002) reflection with a d-spacing of 0.34 nm, revealing the reduction of GO as well as the removal of oxygen functional groups. Raman spectra is obtained to analyze the defect information about the NBSGA in Figure 2(h). Two obvious peaks around 1600 and 1350 cm-1 derive from the G and D peaks of carbon, respectively. Generally, the ratio value of the D and G peak intensity (ID/IG) represents the disorder in the carbon structure, the higher ID/IG value illustrates larger defects. The increased ID/IG value (1.05) of NBSGA comparing with GO sample (0.95) suggests the introduction of heteroatoms (N, B, S), defects as well as the reducing of GO in the hydrothermal reaction process 36. To further analyze the chemical bonding for prepared NBSGA, the XPS is applied. The XPS results of pure GO sample show the presence of only carbon (65.00 atomic %) and oxygen atoms (35.00 atomic %) in Figure 3(a). After hydrothermal reaction, we can easily note the C (74.66 atomic %), N (7.91 atomic %), and O (16.85 atomic %) signals, nevertheless B (0.34 atomic %) and S (0.24 atomic %) signals are difficult to be noted because of the small percentage. The raised C atomic ratios as well as the decreased content of the O after hydrothermal reaction suggest that the GO turns to the reduced graphene oxide. Figure 3(b) presents four deconvoluted peaks for C 1s, which ascribes to the C=O, epoxy/alkoxy, C-OH and C-C species, respectively 25, 31, 37. After doping, the C 1s spectrum of NBSGA in Figure 3(c) can be curve-fitted into several peaks


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corresponding to C-C (284.7 eV), C-S-C (283.9 eV), C-N-C (287.2 eV), C-O-B (285.0 eV), C-NH (288.7 eV), and C-OH (285.7 eV),38-40. Figure 3(d) shows the formed B-N, -BC3, and B–C–O bonds at 191.0 and 191.9 eV in the B 1s spectrum, indicating the successful B doping into NBSGA and certain amounts of boron are bonded to nitrogen in the form of B-N

1, 25, 41-43.

N1s spectra of NBSGA in Figure 3(e) shows the graphitic-,

pyrrolic-, and pyridinic-N, which locate at 401.3, 399.73, and 398.2 eV, respectively 37, 44. The fitted S 2p spectrum of NBSGA in Figure 3(f) establishes two peaks at the binding energies of 167.4, and 168.7 eV, in good agreement with S 2p3/2, and S 2p1/2, respectively 45-46.

3.2. Compressible ASC with residual stress effect Figure 4(a) offers a schematic presentation of the fabricated ASC device. The CV curves of the ASC device at low scan rates presents symmetric rectangle shapes with no obvious redox peaks in Figure 4(b), illustrating double-layer capacitive behavior 47. While at high scan rates, the slight deviation from rectangle may be due to the double layer behavior accompanies with the pseudocapacitive behavior

48-49.

Moreover, the CVs with a slight

curvature can be attributed that electrolyte-ions have difficulty in entering into electrode framework and cannot return back timely at higher current density. The slower migration speed of electrolyte ions illustrates the increased electric resistance. The GCD curves of ASC at different current densities exhibit nearly an ideal symmetric triangle behavior as well as the low IR drop in Figure 4(c) 50. To evaluate the electrochemical performance of



Page 10 of 35

ASC, the calculated area specific capacitance Cs based on the GCD curves is illustrated in Figure 4(d). ASC presents the maximum Cs with 834mF cm-2 at 0.5 mA cm-2 (259 F g-1 at 0.15 A g-1).

The decreasement in specific capacitance with the current density comes

from the difficulty to access into electrode for electrolyte ions under higher current densities. Besides, the decrease of area specific capacitance can be ascribed to the transport

limitations

with

the

pores

at

higher

current

densities

51.

The

charging/discharging process involves not only charge transport in solid phase but also ion transport in electrolyte. While, the rate capability of device is limited by the slow conduction/diffusion of ions and slow conduction of charges, relative to high current density in test process

52.

In other words, only parts of the charges/ions are involved in

the electrochemical process at the higher current density. The as-fabricated ASC delivers a maximum energy density of 23.08 Wh kg-1 at 240 W kg-1 in Figure 4(e), exhibiting larger than many recently reported symmetric supercapacitors, such as N/B doped graphene aerogels (1.6 Wh kg-1 at 1650 Wkg-1)1, activated reduced graphene oxide (4.0 Wh kg-1 at 1989W kg-1)53, N/S co-doped porous carbon (12.4 Wh kg−1 at 400 W Kg-1)54, 3D graphene hydrogel film (0.61 Wh kg−1 at 670 W Kg-1)55, porous carbons and graphene aerogel (12.4 Wh Kg-1 at 2432 W Kg-1)56. The cycle life performance of ASC exhibits bi-activation processes in Figure 4(f). First activation of capacitance is due to the partial exposure of the active materials to the PVA/H2SO4 gel electrolyte during the initial 300 cycles

57.

The second activation process around 700 cycles puts down to the increased

electrochemically active surface area and new opening up of channels related to


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electrolyte ion insertion-deinsertion process

58.

After 5000 cycles, the ASC device

delivers the outstanding electrochemical stability of 94.24% and excellent Coulombic efficiency of 99.65%. The superior electrochemical performance makes the prepared ASC as a suitable candidate for the application of the electrochemical energy device. To investigate the variation of charge storage ability for ASC in the deformation process, the static capacitances of ASC are examined with CV curves when the device is compressed under fixed stress. The CVs of ASC at 30 mV s-1 under the stress of 0, 0.5, 1, 2, 5, and 10 N are shown in Figure 5(a), respectively. Obviously, the shapes of CVs curves are expanded and the current response increases along with the increasing of stress, implying the enlargement of the capacitance of ASC under stress. This can be attributed that the compressed porous interconnected framework provides more efficient highway or electrochemically active site for ions diffusion and transportation under stress. To probe the interfacial contact between the electrode material and electrolyte in ASC under compression process, the EIS is employed. Because, we can estimate this kind of interface contact by the charge transfer Rct and internal resistance Rs. Figure 5(b) presents ASC at initial state exhibits large internal resistance Rs with 16.4 Ω and the charge transfers Rct with 8.7 Ω. While increasing the loading force, the Rs reduces to 5.24 Ω, illustrating the smaller ionic transport resistance of the electrolyte and the contact resistance at the active material/current collector interface

25, 59.

EIS spectrum exhibits a

decreased semicircle with the increase of the stress, which should be related to the low Rct at the interface between the electrolyte and electrode. The results of decreased internal



resistance Rs and Rct demonstrate sufficient contact between the electrolyte and electrode material under stress because gel electrolyte PVA-H2SO4 covers the surface of graphene more in a more compact fashion. To explore the relationship between the capacitance and stress, we discover that the capacitance change (△Cs/Cs) exhibits direct proportionality with the stress as well as Rs in Figure 5(c). The CV measurement is employed to evaluate the resilience of ASC under 5 N for 50 compression cycles in Figure 5(d). No significant change is observed, demonstrating that the as-formed ASC is durably tolerant to the large compressive stress without sacrificing the loss of springiness, electrochemical performance, and any structural collapse. This is attributed to the high physical flexibility and outstanding mechanical integrity of perfect combination between the porous NBSGA structure and gel electrolytes 59-60. To further explore the stress sensor application of ASC under continuous compression process, we squeeze continuously the ASC for 10 times in three different processes, respectively. As seen in Figure 6(a), A0 process represents the GCD curves of ASC at current densities of 7.0 mA cm-2 at initial stage with the charge time of 45.58 s and the discharge time of 44.38 s. In contrast, A1 process represents that the ASC is squeezed continuously for 10 times before initial charging. The GCD results show that the discharging time of ASC in first cycle is prolonged to 46.31 s after being compressed compared to the initial discharging time. A2 case presents that the ASC is squeezed continuously for 10 times in the charging process. Obviously, the charging time is increased to 61.00 s and discharging time is prolonged to 54.91 s for the first GCD cycle,


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respectively, indicating the enhanced capacitance behavior of ASC. More importantly, the enhanced capacitance of ASC illustrates a part of electrochemistry energy-storage converted from the mechanical energy in extrusion process

20, 61-62.

Those demonstrate

that the compressible ASC has the ability of realizing mechanics-electricity conversion and energy harvesting 63. While, A3 case illustrates the continuous 10 times squeezing of ASC in the discharging process, the GCD results exhibits the charge time of 45.60 s and the discharge time of 51.94 s for the first GCD cycle. Amazingly, the charge time is less than the discharge time demonstrates the generation of the extra quantity of electric charge (Q)according to the equation (Q = I*t) in the discharge process. Those suggest that the ASC can be charged via continuous squeezing in discharging process. On the one hand, the extra quantity of electric charge stems from the conversion from the mechanical energy; on the other hand it comes from the added double electric layer capacitance induced by the polarized charge with the help of the internal discharging electric field. Those results suggest that the capacitance of compressible ASC can be enhanced by mechanical work in any emergency situation including deformation or extrusion, providing a new idea of improving and designing the ASC to solve the problem of attenuation of lifetime and be destroyed. Figure 6(b) shows the discharging time of ASC with cycle number in three extrusion processes compared with the initial stage, respectively. Further observation shows the discharging time of ASC gradually restores to the initial value at about 10 GCD cycles, demonstrating a residual stress which is kind innerstress to maintain self-equilibrium and still remains in an object after withdrawing



external force 64. This residual stress effect behaves in the following GCD cycles has the great impact on the ASC squeezing in the charging and discharging process. From the energy standpoint, the case squeezing in the charging process is able to store more electrochemical energy and realize much greater mechanics-electricity conversion efficiency due to the greater charge polarization degree with the help of the external charging electric field 65. Figure 6(c) presents the first GCD cycle at 7.0 mA cm-2 of ASC which is squeezed continuously in the discharging process for different numbers. Generally, all the cases exhibits the some charging time of 45.88 s and discharging time with 44.43, 46.66 s, 47.45 s, and 51.94 s for squeezing 0, 3, 5, and 10 times, respectively. The excessive discharge time demonstrates the producing of excessive quantity of electric charge. 3.3 Sensitive Elastic-electrochemical Stress Sensor To verify whether the ASC can transform the mechanical signal into electric signal directly, we design a new type of elastic-electrochemical stress sensor by charging ASC for 5 minutes at 1.6 V. Without any potential input, the stress sensor is connected with an electrochemical device and undergoes stress tests. Figure 7(a) gives the current response variation toward the external mechanical force. We note a puny current response increase from 0.4257 to 0.4274 mA under loading a tiny 0.5 N on the sensor. The response current recovers timely after withdrawing the external stress. The response current increases to 0.8003 mA while loading the maximum force of 10 N, revealing a larger current response with external stress. This rise in response current under stress can be attributed to


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following factors. Arising from the enhanced internal electric field under compression process, the anions and cations in the electrolyte start to separate and gather at interface between the electrolyte and electrode material, giving rise to charge polarization. This process is similar to the charging process. The enhanced specific capacitance of ASC under stress results in the larger discharged energy density and power density, facilitating the escaping of the charge from the ASC. In other words, the new generation of the electric double-layer leads to the increased current response. Apart from those, the electric charge density increasing with the decrease of the discharging ASC volume under stress as well as sufficient interface contact also plays an important role in producing current response due to expedite charge transport. Hence, synergic effect of those factors cause the transforming from the mechanical signal into electric signal, illustrating that the charged ASC can serve as sensitive sensor. In Figure 7(b), we note ASC presents excellent sensor cycle stability under continuous compression process. This can be owed to the superelastic ASC without any structure damage under stress, which can convert mechanical energy into electrochemical energy unremittingly without loss. In the other hand, the continuous engendering of the polarization charges make up for the loss of the charge during the continuous compression. The response current changes (△I/I) exhibits a good linearly proportional to the stress in Figure 7(c). Based on our analysis that the variations in capacitance and sufficient interface contact of ASC have a great impact on the response current. Figure 7(d) shows that △I/I presents a good linearly increasement with the △Cs/Cs as well as Rs. In capacitors, current is given as:



I  Cs

dV dt

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(4)

where, capacitance Cs is inversely proportional to the distance between the two electrodes 1 . Under compression, the capacitance Cs increases with the decrease of d. For d dV a fixed voltage scan rate ( ), under compressive stress with increasing Cs, the output dt

(d) : Cs 

current I should also increase. As a novel sensitive elastic-electrochemical stress sensor, it is further demonstrated by practical application for detecting the human body movement. We attach the ASC sensor chip onto a human finger in Figure 7(e) and (f). As expected in the bending–unbending moving process, the sensor shows fast sensitive response current. This sensor can also undergo and detect the foot step movement in Figure 7(g). Those suggest it as the potential candidate for stress sensors application. While, there are some limitations including lacking the flexibility due to the inflexibility of the titanium foil, large volume, and unpackaged device. The results above demonstrate our device has the potential to work as energy harvesters operating in the low frequency mechanical input regime

14, 61.Then,

we

measure the open circuit voltage for the discharging ASC under applied stress in Figure 8(a). While loading a tiny 0.5 N on our device, there is a weak open circuit voltage increasement from 0.2206 to 0.2248 V. Besides, the open circuit voltage increases with adding the loading stress. Upon unloading the external stress, the open circuit voltage cannot recover to its initial isopotential state in time, stemming from the residual stress effect. The output power and energy from the device during the loading process under different stress is shown in Figure 8(b), with output response time of about 4 s. Without


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any loading, the device offers output power of 93.91 µW and energy of 375.64 µJ. The raised

output

power

and

energy

with

the

stress

demonstrates

that

this

elastic-electrochemical stress sensor can serve as a mechanical energy harvester. Figure 8(c) reveals the work mechanism and principle of the self-discharging ASC in the process of the serving as elastic-electrochemical stress sensor. Generally, the charges present uniformly on the surface of graphene sheet and in the PVA-H2SO4 electrolyte after charging the ASC. While loading the stress to compress the ASC cell, the stress need to work to overcome the repelling of the like charges between the adjacent lateral graphene sheet or PVA-H2SO4 electrolyte, at some time the mechanical energy converts into the electrochemistry energy storing in ASC. To some content, a part of the extra mechanical energy will convert into elastic potential energy, heat energy consumption, and the gravitational potential energy of ASC. Because the electrolyte absorption and desorption exist in compression-recovery process, at some time there will be the charge polarization at the interface between graphene sheet and PVA-H2SO4 electrolyte. Because of the enhanced internal electric field under compression process, the anions and cations in the electrolyte will separate and gather at interface between the electrolyte and electrode material, which gives rise to the charge polarization and adds double electric layer capacitance of ASC. Due to the enhanced rejection of the like charges, the charge on the vertical graphene sheet will transfer to on the lateral graphene and is much easier to get rid of the ASC to bring sensitive responses current. Those factors not only trigger the enlargement of the capacitance of ASC, but also cause rapid current responses toward



external loading stress. While unloading the stress, the electrochemistry energy transforms into mechanical energy, the charge polarization disappears. 4. Conclusion In summary, we have synthesized superelastic and high energy density graphene aerogels ASC with 23.08 Wh kg-1 at 240 W kg-1. Served as the deformable supercapacitors, the ASC exhibits the robust feature and one kind of residual stress effect. Applied as the sensor, the ASC device offers rapid and steady responses current which realizes the mechanics-electricity conversion stemming from the mechanical energy transforming to electrochemistry energy and the charge polarization. This integrated device with elastic-electrochemical functions not only broadens the application for ASC, but also acts as a novel and promising sensor.


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Acknowledgements This work was supported by Anhui Provincial Major Scientific and Technological Special Project (17030701062), the Science & Technology Project of Anhui Province (16030701091), the Support Project of Outstanding Young Talents in Anhui Provincial Universities (gxyqZD2018006), and the National Natural Science Foundation of China (11504001, 11704002).



Figure Caption Figure 1. (a) Schematic illustration of prepared procedure for NBSGA. (b) Schematic illustration of graphene self-assembly via borate and thiourea crosslinking. Figure 2. (a) The photographs of NBSGA. (b) (c) SEM images of aerogel with different magnifications. (d) TEM of the pure GO. (e)(f) TEM of NBSGA with different magnifications. (g) XRD patterns of NBSGA. (h) Raman spectra of NBSGA. Figure 3. (a) XPS spectra of of GO and NBSGA, respectively. (b) C1s spectrum of GO. (c) C1s spectrum of NBSGA. (d) B1s spectrum of NBSGA. (e) N1s spectrum of NBSGA. (f) S 2p spectrum of NBSGA. Figure 4. (a) Digital photograph of the ASC. (b) CV curves of ASC in the potential window of 0-1.6 V. (c) GCD curves of ASC in the potential window of 0-1.6 V. (d) The calculated specific capacitance of ASC. (e) The calculated energy density and power density comparing with previous reports. (f) Cycle life plot of ASC at 7 mA cm-2 current density Figure 5. (a) CVs of the compressible ASC cells under different stress at 30 mV/s. (b) EIS of ASC cells under different stress. (c) The capacitance change (△Cs/Cs) and Rs as a function of the stress. (d) Cyclic CVs of the compressible ASC under 5 N. Figure 6. (a) The GCD curves of ASC at current densities of 7.0 mA cm-2 in three


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squeezed cases compared with the initial case in the potential window from 0 to 1.6 V. (b) The discharging time of ASC with cycle number after three extrusion processes compared with the initial stage, respectively. (c) The first GCD cycle at 7.0 mA cm-2 of ASC squeezed continuously in the discharging process for different numbers. Figure 7. (a) The rapid current response of the self-discharging sensor under external mechanical stress. (b) The current response cycle stability of sensor under the stress of 1, 2, and 5 N, respectively. (c) The current response changes (△I/I0) as a function of the external stress. (d) The capacitance change (△Cs/Cs) and Rs as a function of the response current changes (△I/I0). The sensor cell is applied to detect various human motion: (e)(f)finger bending, (g) foot step. Figure 8. (a) The open circuit voltage of sensor under different stress. (b) The output power and energy from the device as a function of the stress. (c) The work mechanism and principle the self-discharging ASC served as elastic-electrochemical stress sensor.



Reference (1) Wu, Z. S.; Winter, A.; Chen, L.; Sun, Y.; Turchanin, A.; Feng, X. L.; Mullen, K. Three-Dimensional Nitrogen and Boron Co-Doped Graphene for High-Performance All-Solid-State Supercapacitors. Adv Mater 2012, 24, 5130-5135. (2) Chen, W. F.; Li, S. R.; Chen, C. H.; Yan, L. F. Self-Assembly and Embedding of Nanoparticles by in Situ Reduced Graphene for Preparation of a 3D Graphene/Nanoparticle Aerogel. Adv Mater 2011, 23, 5679-5683. (3) Choi, B. G.; Yang, M.; Hong, W. H.; Choi, J. W.; Huh, Y. S. 3D Macroporous Graphene Frameworks for Supercapacitors with High Energy and Power Densities. Acs Nano 2012, 6, 4020-4028. (4) Jiang, D. G.; Zhu, H. H.; Yang, W. R.; Cui, L.; Liu, J. Q. One-side Non-covalent Modification of CVD Graphene Sheet using Pyrene-terminated PNIPAAm Generated via RAFT Polymerization for the Fabrication of Thermo-Responsive Actuators. Sensor Actuat B-Chem 2017, 239, 193-202. (5) Yan, C. Y.; Wang, J. X.; Kang, W. B.; Cui, M. Q.; Wang, X.; Foo, C. Y.; Chee, K. J.; Lee, P. S. Highly Stretchable Piezoresistive Graphene-Nanocellulose Nanopaper for Strain Sensors. Adv Mater 2014, 26, 2022-2027. (6) Song, Z. Q.; Li, W. Y.; Niu, F. S.; Xu, Y. H.; Niu, L.; Yang, W. R.; Wang, Y.; Liu, J. Q. A Novel Method to Decorate Au Clusters onto Graphene via a Mild Co-Reduction Process for Ultrahigh Catalytic Activity. J Mater Chem A 2017, 5, 230-239. (7) Liu, W. J.; Liu, N. S.; Yue, Y.; Rao, J. Y.; Cheng, F.; Su, J.; Liu, Z. T.; Gao, Y. H. Piezoresistive Pressure Sensor Based on Synergistical Innerconnect Polyvinyl Alcohol Nanowires/Wrinkled Graphene Film. Small 2018, 14, 1704149. (8) Yue, Y.; Liu, N. S.; Ma, Y. A.; Wang, S. L.; Liu, W. J.; Luo, C.; Zhang, H.; Cheng, F.; Rao, J. Y.; Hu, X. K.; Su, J.; Gao, Y. H. Highly Self-Healable 3D Microsupercapacitor with MXene-Graphene Composite Aerogel. Acs Nano 2018, 12, 4224-4232. (9) Kim, S.; Choi, S. J.; Zhao, K. J.; Yang, H.; Gobbi, G.; Zhang, S. L.; Li, J. Electrochemically Driven Mechanical Energy Harvesting. Nat Commun 2016, 7, 10146. (10) Muralidharan, N.; Brock, C. N.; Cohn, A. P.; Schauben, D.; Carter, R. E.; Oakes, L.; Walker, D. G.; Pint, C. L. Tunable Mechanochemistry of Lithium Battery Electrodes. Acs Nano 2017, 11, 6243-6251.


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Table of Contents



Figure 1. (a) Schematic illustration of prepared procedure for NBSGA. (b) Schematic illustration of graphene self-assembly via borate and thiourea crosslinking. 127x89mm (300 x 300 DPI)


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Figure 2. (a) The photographs of NBSGA. (b) (c) SEM images of aerogel with different magnifications. (d) TEM of the pure GO. (e)(f) TEM of NBSGA with different magnifications. (g) XRD patterns of NBSGA. (h) Raman spectra of NBSGA. 187x206mm (300 x 300 DPI)



Figure 3. (a) XPS spectra of of GO and NBSGA, respectively. (b) C1s spectrum of GO. (c) C1s spectrum of NBSGA. (d) B1s spectrum of NBSGA. (e) N1s spectrum of NBSGA. (f) S 2p spectrum of NBSGA. 117x77mm (300 x 300 DPI)


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Figure 4. (a) Digital photograph of the ASC. (b) CV curves of ASC in the potential window of 0-1.6 V. (c) GCD curves of ASC in the potential window of 0-1.6 V. (d) The calculated specific capacitance of ASC. (e) The calculated energy density and power density comparing with previous reports. (f) Cycle life plot of ASC at 7 mA cm-2 current density 180x81mm (300 x 300 DPI)



Figure 5. (a) CVs of the compressible ASC cells under different stress at 30 mV/s. (b) EIS of ASC cells under different stress. (c) The capacitance change (△Cs/Cs) and Rs as a function of the stress. (d) Cyclic CVs of the compressible ASC under 5 N. 124x97mm (300 x 300 DPI)


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Figure 6. (a) The GCD curves of ASC at current densities of 7.0 mA cm-2 in three squeezed cases compared with the initial case in the potential window from 0 to 1.6 V. (b) The discharging time of ASC with cycle number after three extrusion processes compared with the initial stage, respectively. (c) The first GCD cycle at 7.0 mA cm-2 of ASC squeezed continuously in the discharging process for different numbers. 126x89mm (300 x 300 DPI)



Figure 7. (a) The rapid current response of the self-discharging sensor under external mechanical stress. (b) The current response cycle stability of sensor under the stress of 1, 2, and 5 N, respectively. (c) The current response changes (△I/I0) as a function of the external stress. (d) The capacitance change (△Cs/Cs) and Rs as a function of the response current changes (△I/I0). The sensor cell is applied to detect various human motion: (e)(f)finger bending, (g) foot step. 189x199mm (300 x 300 DPI)


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Figure 8. (a) The open circuit voltage of sensor under different stress. (b) The output power and energy from the device as a function of the stress. (c) The work mechanism and principle the self-discharging ASC served as elastic-electrochemical stress sensor. 160x143mm (300 x 300 DPI)


Compressible Supercapacitor with Residual Stress Effect for Sensitive

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