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/H2SO4 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 solvents23, 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 specified, 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 conductivity25. 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 polyelectrolytes7, 18, 26, 27 and conventional PVA-based gel electrolytes28-31 (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 nanofibers was confirmed 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 identified by their characteristic peaks in Fourier transform infrared (FTIR) and Raman spectra. In the FTIR spectrum of the C-PAM (Fig. 2e), the N–H stretching vibration peaks appear at 3346 cm-1 and 3189 cm-1 32, 33, and the bands at 1645 cm-1 and 1602 cm-1 are attributed to the stretching of C=O and the bending of NH2 from the amide group, respectively36. Due to the complexation of the C-PAM and PANI components in the AF-SSC, the N–H characteristic peaks at 3346 cm-1 and 3189 cm-1 in the C-PAM shift to 3432 and 3200 cm-1 in the AF-SSC. Besides, typical peaks of PANI at 1589 cm-1 (C=C stretching of the quinoid structure), 1496 cm-1 (C=C stretching of the benzenoid rings), 1165 cm-1 (electronic-like absorption of N=Q=N, where Q denotes the quinoid ring), and 817 cm-1 (aromatic ring deformation and C-H bond vibrations out of ring plane) are well resolved in the AF-SSC34, 35. Meanwhile, in the Raman spectrum of the AF-SSC (Fig. 2f), PANI bands at 1491 cm-1 (C=N stretching of the quinoid diimine units), 1585 cm-1 (C=C stretching of the quinoid rings), 1167 cm-1 (C-H bending of the quinoid rings), 1330 cm-1 (C-N•+ stretching) are also found36. 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, polytetrafluoroethylene (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 cm2) 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 cm2 (Fig. 2h and Supplementary Movie 1). On the other hand, the good tackiness and large deformability ensured a firm 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 profiles were then measured at current densities ranging from 0.03 to 0.6 mA cm-2 to evaluate the specific capacitance of the device (Fig. 2k). Without any current collectors, the maximum specific 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 specific 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-to-vertical 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, 4-ethylenedioxythiophene) (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 specific 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 conditions40, 41, inducing deteriorations in ionic conductivity as well as mechanical properties (i.e. flexibility, 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). Specifically, 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 electrodes27, 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 sufficiently 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 date, most SSCs only maintain their high electrochemical performance at room temperature. Under -30 oC, water in the C-PAM hydrogel electrolyte (Fig. 5a) was frozen to ice and the CSC lost most of its electrochemical performance due to the greatly reduced ionic conductivity of the electrolyte and increased interfacial charge transfer resistance between the electrolyte and electrodes (Supplementary Fig. 15). In sharp contrast, the AF-SSC could work at this temperature (Fig. 5b and Fig. 5c). As shown in Fig. 5d and Supplementary Fig. 16, the electrochemical performance of the CSC significantly dropped under sub-zero temperatures, while the AF-SSC remained working at -30 oC. Such an excellent anti-freezing ability could be rationalized by the significantly higher ion conductivity of the AF-OHP (1.3 S m-1 at -30 oC) over that of the C-PAM hydrogel electrolyte (only 0.29 S m-1 even at -15 oC, Supplementary 17). Furthermore, CV and GCD experiments at 25 oC to -30 oC (Fig. 5g and Supplementary Fig. 18) showed that the AF-SSC maintained ~75% of its room temperature capacitance (Fig. 5h) with an excellent rate performance under -30 oC (Fig. 2k, Fig. 5i, and Supplementary Fig. 19). In details, the areal capacitance of AF-SSC was 14.4, 12.7, 11.5, and 10.4 mF cm-2 at 0.03 mA cm-2 under 25, 0, -15, and -30 oC, respectively (Fig. 5i). When the current density was increased by 20 folds to 0.6 mA cm-2 under 25, 0, -15, and -30 oC (Fig. 5i), the corresponding areal capacitance slightly changed to 10.7, 9.3, 8.4, and 7.6 mF cm-2, respectively, leading to unprecedented capacitance retention values (for instance, over 73% even at -30 oC) for stretchable supercapacitors in rate performance evaluations (Fig. 5j)12, 15, 16, 43, 25, 43-48. After 100000 change/discharge cycles at -30 oC, the AF-SSC maintained 91.7% of its initial capacitance (Fig. 5k, Supplementary Fig. 20), outperforming all available SSCs so far (Fig. 5l) 6, 15, 16, 25, 43, 44, 46-63.
To rationalize the anti-freezing mechanism of the AF-SSC, density functional theory (DFT) calculations with Dmol3/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 molecules23, 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 significantly 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 oC (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 oC, with a high capacitance retention of up to 96.9% and 89.4%, respectively (Fig. 7e). After repeatedly stretched for 100 cycles at -30 oC, CV curves and GCD profiles 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 oC 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 lifespan6, 7, 12, 15, 16, 25, 27, 43-63, 65-68. 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.