Stretchable Supercapacitor at -30 oC

At relatively low temperature (e.g, -30 o C), most exible supercapacitors that work well at room temperature will lose their stretchability due to the poor cold intolerance of conventional electrolytes and the absence of intrinsically stretchable electrodes. Herein, an anti-freezing and highly stretchable supercapacitor (AF-SSC) was fabricated for the rst time by one-step in-situ growth of polyaniline onto an organohydrogel polyelectrolyte from cross-linked polyacrylamide networks soaked by ethylene glycol/water/H 2 SO 4 . As no extra stretchable substrates (e.g. elastic ber, polydimethylsiloxane, rubber, etc.) or predening stretchable structures (e.g. helical, spring, wrinkle, honeycombed structures etc.) are needed in such a design, the prepared AF-SSC shows a high mechanical stretchability of 200% under -30 o C and can be repeatedly stretched for 100 cycles without signicant capacitance loss. Besides, an impressive capacitance retention of 73.1% at -30 o C is achieved when the current density increases by 20 times, which is much higher than those of stretchable supercapacitors based on pseudocapacitive materials that operated at room temperature. Furthermore, the device shows an ultralong lifespan with 91.7% capacitance retention after 100000 cycles at -30 o C, outperforming all stretchable supercapacitors reported previously. With other merits of high compressibility, strong adhesion, good processability and excellent anti-drying ability, the current device is an ideal power source to drive multi-functional electronic components in real-life scenarios.


Introduction
Stretchable supercapacitors (SSCs) have been intensively investigated to meet the urgent demand in powering stretchable electronics such as electronic textiles 1 , electronic skins 2 , and wearable health monitors 3 due to their high power density, fast rate of charge-discharge and long cycling lifetime 4,5 . To date, most SSCs work on electrodes with prede ned stretchable structures, such as wave-like 6,7 , bridgeisland 8 , cellular 9 , helical 10,11 , and spring-like 12,13 geometries. Unfortunately, they usually suffer from the mechanical mismatches between predesigned electrodes and gel electrolytes or the volume/mass increase caused by the addition of extra stretchable substrates without capacitance contribution 14,15 .
Alternatively, geometrically editable supercapacitors (SCs) have been developed to overcome the poor stretchability of conventional SCs based on polyvinyl alcohol (PVA) electrolytes and common electrodes 15 , yet these SCs are incapable of enduring large and complex deformations perpendicular to the direction of strain 16 . Meanwhile, they still adopt the conventional sandwich-like device structure of laminating separately prepared electrolyte and electrodes, thus vulnerably suffering from sliding and delamination under large deformations. To address these issues, it is critical to construct integrated SSCs based on intrinsically stretchable electrodes and electrolytes. On the other hand, there are few studies reporting SSCs tested under sub-zero temperatures when the freezing of water content in conventional hydrogel electrolytes would signi cantly restrict ionic transport 17,18 . And even fewer reliable approaches have been proposed to solve the poor stretchability of SSCs under these circumstances.

Results
As illustrated in Fig. 1a and 1b, a chemically cross-linked polyacrylamide (C-PAM) hydrogel was synthesized by polymerizing acrylamide (AM) monomers in the presence of methylene-bis-acrylamide (BIS) as the cross-linking moiety, N, N, N′, N′-tetramethylethylenediamine (TMED) as the catalyst and ammonium persulfate (APS) as the initiator. Then, a simple solvent displacement strategy was adopted to prepare an anti-freezing organohydrogel polyelectrolyte (AF-OHP) by immersing the C-PAM hydrogel in a mixed EG/W/H 2 SO 4 solution. In supplementary Fig. 1, AF-OHP shows outstanding compression and excellent stretchability even when the temperature drops to -30 °C since EG can combine W molecules into stable molecular clusters and compete with the formation of hydrogen bonds between water molecules, resulting in the decrease of the saturated vapor pressure of W in EG/W binary solvents 23,24 .
Subsequently, an anti-freezing, intrinsically stretchable and truly integrated SC (AF-SSC) was fabricated by one-step in-situ growth of PANI electrodes onto both sides of the AF-OHP (Fig. 1a) with the transparent AF-OHP (6 cm in diameter) turing dark green (Fig. 1c). Unless otherwise speci ed, the fabricated SSC was based on the polyelectrolyte with an EG/W volume ratio of 1:1 due to the optimal anti-freezing property (see supplementary Fig. 2). Owing to the superior mechanical property of PANI electrodes and the AF-OHP, the AF-SSC could be arbitrarily deformed in forms of compressing, stretching, bending, and twisting ( Fig.   1d). Meanwhile, it was also convenient to cut the AF-SSC into smaller SSCs of different shapes ( Supplementary Fig. 3) or even shape it into numerical, cartoonish and alphabetic patterns (Figure 1e, 1f and 1g). Overall, the high stretchability, excellent anti-freezing performance and outstanding processibility of the AF-SSC make it suitable for powering multifunctional electronic devices with different dimensions.
PANI has been widely used in SCs because of its relatively high pseudocapacitance and conductivity 25 . In current work, the in-situ growth of PANI and the presence of plentiful micropores in the AF-OHP contribute to the free moving of protons within the PANI/AF-OHP/PANI layers during charging and discharging ( Fig.   2a and Fig. 2c), leading to higher ionic conductivity (5.33 mS cm -1 ) than previously reported stretchable polyelectrolytes 7,18,26,27 and conventional PVA-based gel electrolytes [28][29][30][31] (Supplementary Fig. 4). The cross-section view of the AF-SSC showed the penetration of PANI into the AF-OHP (Fig. 2b) and the growth of PANI nano bers was con rmed by the roughened surface morphology of the oven-dried AF-SSC as compared to the pure AF-OHP ( Fig. 2d and Supplementary Fig. 5). Meanwhile, the presence of the C-PAM and PANI components within the AF-SSC was further identi ed by their characteristic peaks in Fourier transform infrared (FTIR) and Raman spectra. In the FTIR spectrum of the C-PAM (Fig. 2e) 36 . In addition, compared to the amorphous feature of the C-PAM, the X-ray diffraction (XRD) pattern of the AF-SSC features characteristic peaks of crystallized PANI at 2θ of 18.3-30.9°, implying its high electrical conductivity ( Fig. 2g) 37,38 . All above characterizations indicate that PANI was successfully grown on both sides of the AF-OHP electrolyte, thus directly building a self-integrated SSC.
Interestingly, the AF-SSC maintained the good intrinsic tackiness of organohydrogel polyelectrolyte, making the device easily adhered to different substrates, including glass, polytetra uoroethylene (PTFE), rubber gloves and polyethylene terephthalate (PET). In particular, the adhesive strength for rubber gloves could reach a maximum value of 10.4 KPa (Fig. 2i). On one hand, this enabled the direct attachment of current collectors (e.g. highly conductive carbon nanotube paper) without using additional conductive glues and an AF-SSC (~22 g, ~55 cm 2 ) could be easily lifted by a small piece of carbon nanotube paper (9.2 mg, 2390 times lighter in weight) on a small contact area of ~2.2 cm 2 ( Fig. 2h and Supplementary Movie 1). On the other hand, the good tackiness and large deformability ensured a rm attaching of the AF-SSC on rubber gloves even under bending and stretching conditions for wearable electronics applications ( Supplementary Fig. 6). The electrochemical performance of the AF-SSC was further assessed by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) in a two-electrode system. CV curves showed obvious redox peaks at scan rates ranging from 5 to 100 mV s -1 , originating from the characteristic pseudocapacitance behavior of PANI (Fig. 2j). GCD pro les were then measured at current densities ranging from 0.03 to 0.6 mA cm -2 to evaluate the speci c capacitance of the device (Fig.   2k). Without any current collectors, the maximum speci c areal capacitance of the AF-SSC still reached 14.4 mF cm -2 at 0.03 mA cm -2 and the corresponding IR drop was fairly small (Fig. 2l). Even when the current density increased by 20 times to 0.6 mA cm -2 , an impressive capacitance retention of 74.3% can be obtained (Fig. 2l). Alternatively, the speci c capacitance of the device was also calculated by CV curves and its maximum areal capacitance can reach 12.8 mF cm -2 at 5 mV s -1 ( Supplementary Fig. 7). Meanwhile, in the low frequency region of the electrochemical impedance spectrum (EIS), the close-tovertical line demonstrated the good capacitive behavior of the AF-SSC without notable diffusion restrictions ( Supplementary Fig. 8) 39 . The electrochemical stability of the AF-SSC was further investigated based on long-term CV tests at the scan rate of 100 mV s -1 . After 50000 cycles, 84.7 % of its initial capacitance was retained ( Fig. 2m and Supplementary Fig. 9). Apart from PANI, poly (3, 4ethylenedioxythiophene) (PEDOT) could also be in-situ grown on the AF-OHP by the same strategy to fabricate a control device. However, compared to PEDOT-based devices, the AF-SSC showed higher CV integral area, longer discharge time and better speci c capacity, presumably owing to better pseudocapacitive behaviors of PANI ( Supplementary Fig. 10).
As an indispensable component of SCs, the electrolyte plays an important role in determining the electrochemical performance and functionality of the device. Generally, hydrogel electrolytes tend to dry in 24 h due to inevitable evaporation of water under ambient conditions 40,41 , inducing deteriorations in ionic conductivity as well as mechanical properties (i.e. exibility, stretchability, and compressibility). In contrast, this can be well avoided in the AF-SSC. When the AF-SSC and a control SC (CSC, prepared with the C-PAM hydrogel electrolyte) were intentionally stored in vacuum before testing, the AF-SSC still maintained good stretchability and compressive properties even after 20 h ( Supplementary Fig. 11), showing a much smaller decrease in size and weight ( Fig. 2n and Supplementary Fig. 12a). Speci cally, after put in vacuum for 6h, 24h, and 48h, the solvent retention in the AF-SSC was 69.4%, 61.9 and 52.8%, respectively ( Supplementary Fig. 12a). In contrast, only 22.7% of the solvent in the CSC remained after 6 h in vacuum ( Supplementary Fig. 12a), which was even less than that (44.4%) of the AF-SSC after 72 h. Naturally, the AF-SSC maintained 70.6% and 64% of its initial capacitance after 6 h and 72 h under vacuum, while the capacitance of the CSC dropped to 47.2% only after 6 h ( Fig. 2o and Supplementary  Fig. 12b and 12c). This suggests that EG in AF-SSC forms a strong hydrogen bond network between polymer chains and water molecules, effectively preventing water molecules from volatilizing even under vacuum conditions.
The excellent stretchability of the AF-SSC originates from its rational structure. The penetration of PANI into the C-PAM organohydrogel warrants a high structural integrity and stable electrical conductivity under stretched states, since the external force could be evenly dissipated throughout the interconnected polymer network without causing interfacial separation between the electrode and the electrolyte (Fig.  3a). Consequently, the elongation of the AF-SSC could reach ~350% (Fig. 3b). Furthermore, CV and GCD curves of the AF-SSC were barely changed under stretch ratios from 0% to 200% (Fig. 3c and Fig. 3d), while up to 93.9% and 84% of its original capacitance were retained when the device was stretched for 100% and 200% (Fig. 3e), respectively. In addition, the AF-SSC could be repetitively compressed to 80% of its original height and resistant to a maximum external pressure of 55.08 kPa for 100 cycles, demonstrating its stable elasticity. At the same time, the electrochemical performance of the AF-SSC was completely retained at various compressive strains from 0% to 60%, as indicated by CV and GCD curves ( Fig. 3g and Supplementary Fig. 13a). Under 60% compression, its areal capacitance slightly increased by 9% (Fig. 3h), possibly due to pressure-improved interfacial contact between the electrolyte and electrodes 27,42 . When the AF-SSC was repeatedly compressed to 60% and released for 1000 cycles, its CV and GCD curves barely changed ( Fig. 3i and Supplementary Fig. 13b), with 99.9% and 96.3% of capacitance retained after 600 and 1000 compress/release cycles, respectively (Fig. 3j). Moreover, CV and GCD curves of the AF-SSC almost overlapped when the device was bent from 0° to 180° and its electrochemical performance only showed a negligible decay under large twisting (Supplementary Fig.  14).
Assembling individual SCs in series and parallel is an effective way to customize the output voltage and discharge capacity (Fig. 4a). Naturally, three series-connected and two parallel-connected AF-SSCs show tripled output voltage and doubled discharging time ( Fig. 4b and 4c), respectively. A digital timer was su ciently powered by three series-connected AF-SSCs even they were repeatedly stretched (Fig. 4d, Supplementary Movie 2), compressed (Fig. 4e, Supplementary Movie 2), bent or twisted ( Fig. 4f and 4g).
Owing to their good processability (Fig. 1e-1g), AF-SSCs with different alphabetic (Fig. 4h, Supplementary Movie 3) and numerical patterns (Fig. 4i, Supplementary Movie 3) can be facilely prepared to power a timer and a multifunctional displayer. Series-connected AF-SSCs can be directly adhered to rubber gloves and used to drive an electronic watch (Fig. 4j), validating that the AF-SSC features outstanding processability, good stickiness and excellent deformation adaptability for practical applications.
To rationalize the anti-freezing mechanism of the AF-SSC, density functional theory (DFT) calculations with Dmol 3 /GGA-PBE/DNP (3.5) basis set 1 (3) were carried out to calculate the interaction energy of hydrogen bonds between C-PAM chains and EG as well as W molecules 23,64 . As shown in Fig. 6a and 6b, hydrogen bonds form between small molecules such as W/W and W/EG. DFT results reveal that the hydrogen bonding interaction of EG-W mixture (-4.86 kcal mol -1 ) in the AF-OHP is more stable than that of W-W (-3.74 kcal mol -1 ) in the C-PAM hydrogel (Fig. 6c, d, and e, Supplementary Table1), which is crucial to maintain W molecules at low temperatures and thus enhance the cold tolerance of the AF-OHP 64 . Additionally, the EG-W mixture shows signi cantly stronger interactions (-23.15 kcal mol -1 ) with PAM chains than pure W or EG (-9.68 or -4.98 kcal mol -1 , Fig 6f to 6h and Supplementary Table 2), where W molecules bridge to connect hydroxyl groups of the EG and carbonyl groups of PAM chains with an increased binding energy. As a result, water molecules are tightly locked in the AF-OHP network to suppress the formation of crystal lattices, enabling the AF-SSC to maintain good electrochemical performance at low temperatures.
The AF-SSC further exhibited a highly reversible stretchability at -30 o C (Fig. 7a). At -30 °C, its stress-strain curve was still similar to that measured at room temperature ( Fig. 3b and Fig. 7b), proving the outstanding anti-freezing capacity and excellent stretchability of the AF-SSC. Electrochemically, the AF-SSC maintained typical pseudocapacitive CV curves (Fig. 7c) and GCD curves (Fig. 7d) even when stretched for 100% and 200% at -30 o C, with a high capacitance retention of up to 96.9% and 89.4%, respectively (Fig. 7e). After repeatedly stretched for 100 cycles at -30 o C, CV curves and GCD pro les of the AF-SSC showed no notable changes ( Fig. 7f and 7g) and 94.8% of its initial capacitance was retained (Fig. 7h). As a proof-of-concept, three series-connected AF-SSCs could uninterruptedly power a timer at -30 o C even when one of them was repetitively stretched by 200% for three times (Fig. 7i, Fig. 7j and Supplementary Movie 4), highlighting their excellent anti-freezing and stretchable properties. As summarized in Fig. 7k and Supplementary Table 3, without the need of additional stretchable substrates or pre-existing stretchable structures, the AF-SSC achieved an unprecedented low-temperature intrinsic stretchability and an ultralong cycle lifespan 6,7,12,15,16,25,27,[65][66][67][68] . In addition, the device simultaneously demonstrated high compression performance, strong stickiness, good processability and excellent anti-drying ability (Supplementary Table 3), qualifying it as a suitable candidate for wide applications in multifunctional wearable devices.

Discussion
In conclusion, we have developed an integrated supercapacitor with intrinsic stretchability at -30 o C by insitu growth to form polyaniline onto the anti-freezing organohydrogel polyelectrolyte. Based on its truly integrated structure with facilitated electron/ion transport, the device shows an excellent rate performance with a high capacitance retention (>73.1%) under -30 o C when the current density increases by 20 folds. Additionally, at -30 o C, the device achieves a maximum stretch ratio of 200% and possesses an ultralong lifetime of 100000 cycles (91.7% of its initiall capacitance retained afterwards). To the best of our knowledge, the current device is the rst SSC with such an intrinsic and stable stretchability at low temperatures and its other attractive features including high compressibility, strong adhesion, good processability and outstanding anti-drying ability further promote its potential applications in stretchable and wearable electronics towards real-life thermal environments.

Methods
Synthesis of the anti-freezing organohydrogel polyelectrolyte and the cross-linked polyacrylamide hydrogel polyelectrolyte: First, 6 g of AM (Acros Organics) was dispersed in 20 mL deionized water under the ultrasonic condition and then methylene-bis-acrylamide (0.003 g, Energy Chemical) was added into the solution. Sequentially, 15 μL of N, N, N', N'-tetramethyl-ethylenediamine (TMED) catalysts (Energy Chemical) and 0.02 mg of ammonium persulfate (APS) (Sigma-Aldrich) initiators were added to initiate polymerization under stirring. Then, the mixed solution were quickly poured in plastic vessels. After sealing and then reacting at 50 o C for 24 h, the cross-linked polyacrylamide hydrogel was obtained. Then, the anti-freezing organohydrogel polyelectrolyte (AF-OHP) were obtained by soaking the dried gels in 10 wt% of sulfuric acid ethylene glycol/water (v:v=1:1) solution for 72 h to achieve the equilibrated state. The cross-linked polyacrylamide hydrogel polyelectrolyte was prepared by soaking the cross-linked polyacrylamide hydrogel in 10 wt% of sulfuric acid aqueous solution for the same time. Similarly, control polyelectrolytes with different proportion of ethylene glycol/water (v/v=0, 1/3, 3/1) are prepared through the aforementioned method similar to AF-OHP to construct control SCs.

Fabrication of the supercapacitors
First, 5 mmol of aniline monomer (Macklin) was added into the 30 mL of above-mentioned sulfuric acid ethylene glycol/water solution with the same the same proportion under stirring in 0 o C for 10 min. Then, 3.33 mmol of APS was dissolved in 20 mL of the same sulfuric acid ethylene glycol/water solution under stirring and ultrasonic conditions at 0 °C. Next, the crosslinked AF-OHP with different sizes was immersed into sulfuric acid ethylene glycol/water solution containing 5 mmol of aniline monomer for 0.5h. Then, as-prepared APS solution was added to initiate polymerization under stirring for 12h at the room temperature. Finally, an antifreezed, essentially stretchable and truly integrated SC (AF-SSC) was fabricated by cutting their edges of the obtained sandwich-like block with the deposited PANI onto the two-sided faces to avoid the short circuit. Similarly, the different contrasted SCs (CSC, CSC-1/3 and CSC-3/1) based on the prepared polyelectrolytes with different proportion of ethylene glycol/water (v/v=0, 1/3 and 3/1) were also fabricated through the aforementioned method similar to AF-SSC, respectively. We utilize carbon nanotube paper (Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences, China) as wire to directly xed on both ends of the device for testing due to the strong stickiness of AF-SSC. Similarity, an integrated SC based on other conductive polymer such as poly (3, 4ethylenedioxythiophene) (PEDOT) (Alfa) by using a similar construction strategy.
Characterizations: The morphologies of the samples were observed and investigated by SEM (JSM-7500F). Raman spectra and FT-IR spectra were carried out by a Horiba JY HR-800 Raman spectrometer and a Bruker VERTEX 700 spectrometer, respectively. X-ray diffraction (XRD) were tested by adopting Netherlands 1,710 diffractometer with a Cu-Kα irradiation source (λ=1.54 Å). A material testing system (SHIMADZU AGS-X) and DMA 850 was used to record the mechanical tensile tests. The performance of the device at low temperatures was tested with the aid of a commercial cryogenic refrigerator (Guangzhou Aoxue Refrigeration Equipment Co., Ltd., China). The ionic conductivity of the polyelectrolyte can be calculated by the relationship: σ= L/(RA), where L is the height between the current collectors, A is the contact area (m 2 ) between the polyelectrolyte and the current collectors, and R is the resistance measured by sandwiching the polyelectrolyte between two Au plates as current collectors using a CHI 760E electrochemical workstation. GCD pro les, CV curves, and electrochemical impedance spectroscopy (EIS) measurements of SCs were obtained by using a CHI 760E electrochemical workstation in two electrode con guration. The speci c capacitance (C) can be calculated according to the single electrode data following the formulae: C =I△t/(AU), C= , where △t , I, U, , v, and A is the the discharging time, the discharge current, and the voltage variation (excluding the IR drop), the integrated area of the CV curve, scan rate and the area of SC, respectively. The adhesiveness of the SC was characterized by a tensileadhesion test using different substrate materials on a universal mechanical testing machine (SHIMADZU AGS-X), according to previous literature 64,69 .

Density Functional Theory (DFT) Calculations
The simulation was performed using the density functional theory program DMol 3 in Material Studio (Accelrys, San Diego, CA) 23,64 . The physical wave functions were is extended in terms of numerical basis sets, Dmol 3 /GGA-PBE/DNP(3.5) basis set, which is comparable to 6-31G** basis sets. The core electrons are processed by the DFT semi-core pseudo potential. Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) is used to calculate exchange correlation energy. A Fermi smearing of 0.005 Ha (1 Ha = 27.211 eV) and a global orbital cutoff of 5.2 Å were utilized. The convergence criteria for geometric optimization and energy calculation were set as follows: (a) Self-consistent eld tolerance was 1.0×10 -6 Ha/atom; (b) Energy tolerance was 1.0×10 -5 Ha/atom; (c) The maximum force tolerance was 0.002 Ha/Å; (d) The maximum displacement tolerance was 0.005Å.

Interaction energy calculation
The interaction energy (E int ) is obtained according to the following equation, which represents the intensity of the interaction between the various components in the system: