Highly conductive and tough double‐network hydrogels for smart electronics

Development and understanding of highly mechanically robust and electronically conducting hydrogels are extremely important for ever‐increasing energy‐based applications. Conventional mixing/blending of conductive additives with hydrophilic polymer network prevents both high mechanical strength and electronic conductivity to be presented in polymer hydrogels. Here, we proposed a double‐network (DN) engineering strategy to fabricate PVA/PPy DN hydrogels, consisting of a conductive PPy‐PA network via in‐situ ultrafast gelation and a tough PVA network via a subsequent freezing/thawing process. The resultant PVA/PPy hydrogels exhibited superior mechanical and electrochemical properties, including electrical conductivity of ~6.8 S/m, mechanical strength of ~0.39 MPa, and elastic moduli of ~0.1 MPa. Upon further transformation of PVA/PPy hydrogels into supercapacitors, they demonstrated a high capacitance of ~280.7 F/g and a cycle life of 2000 galvanostatic charge/discharge cycles with over 94.3% capacity retention at the current density of 2 mA/cm2 and even subzero temperatures of −20 °C. Such enhanced mechanical performance and electronic conductivity of hydrogels are mainly stemmed from a synergistic combination of continuous electrically conductive PPy‐PA network and the two interpenetrating DN structure. This in‐situ gelation strategy is applicable to the integration of ionic‐/electrical‐conductive materials into DN hydrogels for smart‐soft electronics, beyond the most commonly used PEDOT:PSS‐based hydrogels.


| INTRODUCTION
Conductive hydrogels are considered as emerging materials and substitutions for flexible electronics, soft machines, and human-machine interfaces, 1,2 because of their unique structural integration of both liquid and solid phases in a three-dimensional (3D) hierarchical porous-conducting structure.4][5] These conductive nanofillers and polymers usually contain π-conjugated backbone structures for facilitating the transportation of electrons through the hydrophilic porous microstructure.The nanofiller-based conducting hydrogels are readily prepared by physical mixing/blending or chemical doping methods.Despite preparation simplicity, simple doping and mixing of nanofiller additives into polymer network usually does not necessarily enhance mechanical strength, instead, make the hydrogels become more brittle due to the over-crosslinking effect or network inhomogeneity effect. 6,7Also, the loss of mechanical properties will simultaneously cause unstable electrical conductivity and electrochemical cycle.As a result, nanofiller-based conducting hydrogels usually suffer from poor mechanical robustness and stretchability (tensile strength of 0.02-1.3MPa, tensile strains of 80%-2000%, fracture energy of 500-4500 J/ m2 ) and low electrical conductivity of <3 S/cm. 8lternatively, conductive polymers can be incorporated with another nonconductive polymers (e.g., polyacrylamides, polysaccharides) to form the chemically/ionically crosslinked network by in-situ polymerization. 9,10Due to intrinsic builtin charged or zwitterionic groups in the polymer networks, PANI-, PPy-, and PEDOT:PSS-based hydrogels usually present high conductivity of 10-4000 S/m, but they also suffer from mechanical weakness, because these conducting polymers contain highly rigid, conjugated ring structures that cannot effectively dissipate strain energy. 11,12As opposite to rigid nanofiller additives in polymer hydrogels to empower electronic conductivity, conductive polymer chains are more flexible and compatible with other polymers in the same hydrogel systems for empowering their high stretchability.Moreover, nonconductive polymers in both nanofiller-and conductive polymer-based hydrogels could also act as an electrical insulator to potentially compromise electrical conductivity and electrochemical performances. 13,14Also, given strong hydrophobic nature of most of conducting additives and polymers, it adds additional challenge to incorporate them into highly hydrophilic network without sacrificing either electronic conductivity or mechanical properties.Lastly, structural incompatibility between rigid conductive network and the ductile nonconductive network requires meticulous consideration for the selection and design of polymer systems, polymerization strategies, and/or post-polymerization treatments for achieving desirable or balanced functions.
[17][18][19][20][21][22][23][24][25][26] Intuitively, the design principle of DN hydrogels appears to be a potential solution for a dilemma for the structural disparity issue in inhomogeneous networks, that is, DN structure enables to incorporate a rigid conducting (charged) polymer network into a ductile neutral polymer network, which could reduce the impact of network inhomogeneity on mechanical and electrical properties.DN structure in hydrogels allows to explore different combinations of conducting polymers, neutral polymers, and crosslinkers for achieving both high electronic conductivity and mechanical toughness or minimizing trade-offs between mechanical and conductive properties via in-situ polymerization or in-growth polymerization.Currently, conducting polymerincorporated hydrogels are mostly limited to either single-network (SN) hydrogels or PEDOT:PSS-based hydrogels.Particularly, only few PEDOT:PSS-based DN hydrogels were reported, including PEDOT:PSS/PDMAAm DN hydrogel, 27 PEDOT: PSS/PAAc DN hydrogel, 9 and PEDOT:PSS/PVA DN hydrogel, 28 all of which showed the physically-crosslinked PEDOT:PSS network filled in the voids of covalent networks and entangles with the secondary crosslinked polymers, effectively enhancing the loss modulus of the hydrogels and promoting the local crack-energy dissipation properties.Thus, it is highly desirable to develop new conducting DN hydrogels beyond PEDOT:PSS-based hydrogels with SN, DN, and nanocomposite structures.
Building on our previous research on different DN hydrogels, [29][30][31] here we presented a DN strategy for fabricating a series of conducting polymer hydrogels, consisting of the 1st brittle, rigid chemically crosslinked PPy-PA network and the 2nd soft, elastic physically crosslinked PVA network.Our two-step fabrication strategy begins with an in-situ ultrafast gelation of PPy-PA as the 1st network, followed by the subsequent diffusion and freezing/thawing process of the 2nd PVA network, thus forming PVA/PPy-PA hydrogels with two independent yet interpenetrating networks.The resultant DN hydrogels achieved high conductivity of ~6.8 S/m as contributed by 3D continuous conductive pathways for electronic conduction, as well as excellent mechanical strength of ~0.39 MPa and elastic moduli of ~0.1 MPa as raised from DN structure.Upon assembling PVA/PPy-PA DN hydrogels and two activated PANI/carbon cloth electrode (PANI-CC) electrodes into a symmetric supercapacitor, it demonstrated a high capacitance of ~280.7 F/g at the current density of 2 mA/cm 2 and stable cycling performance with ~94.3% of capacity retention after 2000 galvanostatic charge/discharge (GCD) cycles.Simultaneously, the specific capacitance and cycling stability can be well maintained even at subzero temperatures of −20 °C.Such excellent mechanical and electrochemical performance of this conducting hydrogel is mainly stemmed from well-designed DN structure by integrating continuous conductive channels for electrolyte ion transportation with scarified bonds for energy dissipation.The concept of our design and hydrogel system can be extended to other conducting polymers and hydrogels for fabricating next-generation smart-soft electronics, alternative to PEDOT:PSS-based DN hydrogels and beyond.

| Synthesis of PPy single-network hydrogels
The chemically crosslinked PPy hydrogels were fabricated as follows: 250 μL of purified pyrrole was pre-dissolved into 1.5 mL isopropyl alcohol/water (1:1; V%) and denoted it as solution I. Solution II was then prepared by mixing a certain amount of acid (for instance, 0.5 mL phytic acid), zwitterionic VPES (10 mg), DI-water (0.5 mL) and APS (200 mg).Both solutions were cooled under −20 °C for 10 min.After that, solution II was thoroughly mixed with the solution I, and immediately poured into a predesigned glass mold/plastic syringe and allowed to react for 3 h.To remove excess free ions, acid, and other reactants, the PPy (e.g., PPy-CA, PPy-PA, and PPy-PPA) hydrogels were further sequentially purified by ethanol (12 h) and DI water (24 h).

| Synthesis of PVA/PPy-PA double-network hydrogels
First, the PPy hydrogels were dehydrated via lyophilization under reduced pressure, thus presenting a 3D porous structure.The PVA solution (8 wt%) was prepared by dissolving PVA in DI water and heated to 65 °C.Subsequently, the PPy-PA aerogels was immersed into hot PVA solutions and last for different time (i.e., 12, 24, 48, and 72 h).The fully PVA-diffused PPy hydrogels underwent frozen at −20 °C for 6 h, followed by thawing at room temperature for another 6 h.After three freezing/thawing cycles, the PVA/ PPy-PA DN hydrogels were obtained due to the formation of the secondary crystallization domains among the PVA chains with abundant hydrogen bonding.PVA SN and PVA@PPy SN hydrogels (control groups) were synthesized via the similar synthetic route.

| Preparation of PANI-carbon cloth electrode
PANI-carbon cloth(CC) was prepared via in-situ polymerization as follows.In brief, the carbon cloth pieces were activated in a mixed solution of H 2 SO 4 and HNO 3 , followed by adding 20 mL aniline/HCl solution (0.1 mol/ L) in an ice-water bath.After full infiltration, a mixture containing 0.4 mmol APS and 20 mL HCl solution was added dropwise for another 4 h.The resulting PANI-CC electrode was successively washed with ethanol and DI water for three times, then dried overnight in the vacuum at 50 °C.

| Preparation of supercapacitors
All as-fabricated hydrogels (e.g., PVA/PPy-PA DN hydrogels) were soaked into 1.0 mol/L LiCl solution for 12 h, and then two pieces of PANI-CC electrodes were symmetrically assembled in a button CR2032 battery case to form a symmetrical supercapacitor.The diameters of the gel electrolyte and PANI-CC electrode were 15 mm and 12 mm, respectively.All supercapacitors were placed overnight before being electrochemically tested.

| Characterizations
XPS analysis was determined by employing an X-ray photoelectron spectroscope (XPS, Axis Ultra [DLD]) via a monochromate Al Kα X-ray source, with the emitting area of 300 μm × 700 μm, an incident angle of 45°at 15 kV and 3 mA.All high-resolution spectra of C 1s, O 1s and N 1s were recorded, followed by being corrected by setting the lowest binding energy component of C 1s spectral envelope to 285.0 eV (Avantage 5.52 software).The morphology of all lyophilized hydrogels was observed by scanning electron microscopy (SEM; Tescan Vega 3) equipped with an EDS detector.The accelerating voltage and working distance were controlled at 10 kV and 8-12 mm, respectively.FTIR spectra of soils were characterized by using a Nicolet-6700 with the resolution of 4 cm −1 and 32 scans in the wavenumber range of 4000-650 cm −1 .All cylindrical gels with a height of ~6.5 mm and a diameter of ~8.66 mm were used for compression tests.The compression rate was 50 mm/min, which was carried out with an Instron Model 3345 machine.The cyclic loading-unloading tests of hydrogels were conducted at the compression strain of 30%.Tensile measurements were all performed on a universal tensile machine (Instron 3345) with a 500N transducer at the stretching rate of 100 mm/min.As-prepared hydrogels were cut into dumb-bell shape ith a width of 3.18 mm, a gauge length of 25 mm, and a thickness of 1.0 mm.Electromagnetic interference shielding effectiveness (EMI SE) was recorded by using the vector network analyzer (VNA-AV3672C) in the frequency range of 8.2-12.4GHz.All testing aerogels were cut by mold, with a total length of 22.3 mm and a total width of 10.2 mm, and a gauge thickness of 1 mm.Scattering parameters were recorded to calculate the power coefficients (reflectivity and absorptivity) and the corresponding absorption loss (SE Abs ), reflection loss SE (SE Ref ), and total SE (SE Total , SE Total ≈ SE Abs + SE Ref ).The electrochemical performance of the capacitor device was measured on the CHI-760E electrochemical workstation with a two-electrode system.Cyclic voltammetry (CV) was conducted between 0 and 0.8 V at different scan rates, while the electrochemical impedance spectroscopy (EIS) was performed at 5 mV amplitude (0.01 Hz-100 kHz).In addition, GCD were performed at different current densities of 1 ~10 mA/cm 2 (0-0.8V).All electrochemical tests are performed at room temperature unless otherwise specified.In particular, the area specific capacitance (C s , mF/cm 2 ) of the supercapacitor was calculated from the GCD curves according to the following Equation (1) 34,35 : where I, Δt, S, and ΔV represent as the discharge current (mA), the discharge time (s), the total area of the PANI-CC electrode (cm 2 ), and the voltage window (V), respectively.We also weighed both free-dried the DN hydrogel electrolytes (dried gels) and PANI-CC electrodes (active agents: conducting polymers PANI; 0.00115 g/cm 2 ), and then converted mF/cm 2 to F/g.

| Design, fabrication, and characterization of chemically crosslinked PPy hydrogels
To obtain an integral conductive network, we first designed and synthesized chemically crosslinked PPy-PA hydrogels.Briefly, all reactants consisting of 250 μL of purified pyrrole, 1.5 mL isopropyl alcohol/water (1:1; V%), excess APS (200 mg) as oxidative initiator, phytic acid (0.5 mL) as crosslinker, and zwitterionic VPES (10 mg) as accelerator were mixed in a one-pot aqueous solution at −20 °C (Figure 1A).During this gelation process, conjugation of zwitterionic VPES with styryl and pyridine groups enabled to accelerate the formation of pyrrole clusters via π-π interactions and the decomposition of APS into free radicals, both of which in turn facilitated redox polymerization. 32Since every phytic acid molecule can interact with more than one PPy chain by protonating its nitrogenous group, such a crosslinking effect led to the formation of a fully conducting polymeric network.The entire gelation process only took ~30 s to form PPy-PA network, followed by removing excessive unreacted precursors from PPy-PA hydrogels by rinsing with ethanol and DI water.The completion of the entire gelation process can also be visually confirmed by the changes from a brown solution state to a black gel state (Figure 1B and Suppporting Information: Movie S1).In parallel, our one-pot synthetic approach also allowed to incorporate PPy into PPA and CA network to form PPy-CA and PPy-PPA hydrogels as controls for comparison.As shown in FTIR spectra (Figure 1D and Suppporting Information: Table S1), all of PPy hydrogels (PPy-PA, PPy-CA, and PPy-PPA) presented the almost same major bands at 1547, 1090, and 960 cm −1 with slightly variation in signal intensities.These major bands corresponded to stretching vibrations of C=C and C-N and in-plane deformations of =C-H and C=C, arising from pyrrole rings.Differently, PPy-CA hydrogels had a peak at 1704 cm −1 assigning to C=O due to the presence of the counterions (-COOH) from citric acid dopants, while both PPy-PA and PPy-PPA hydrogels had distinct peak at 1167 cm −1 corresponding for P=O bonds in phosphorus residues (i.e., phosphoric acid and citric acid).Such different and common signals in FTIR-ATR spectra confirm the successful incorporation of different dopants of PA, CA, and PPA into PPy gel network.
To investigate the internal network structure and composition distribution of PPy-PA hydrogels, we further converted PPy-PA hydrogels into PPy-PA aerogels via lyophilization, during which the 3D porous structure was retained intact in a solid state.At a first glance, a cylindrical PPy-PA aerogel with 2.5 cm of diameter and 1.0 cm of length had a mass of 9.1 mg only, which was very light to be easily sit on top of a dandelion without deforming its branches.SEM images in Figure 1C revealed the agglomeration of polydisperse spheres with the average diameter of 1-5 µm being distributed and connected in 3D porous network in PPy aerogels.Surface elemental compositions of PPy aerogels doped by different acids were further investigated by XPS.XPS survey scanning and mapping in Figure 1E showed that all of PPy aerogels (PPy-PA, PPy-CA, and PPy-PPA) displayed similar signals of O 1s (~530.0eV), N 1s (~400.0eV), C 1s (~286.0eV), S 2s (~226.0eV), and S 2p (~162.0eV), which were attributed to PPy particles and sulfate residues from the decomposed APS.Meanwhile, as compared to PPy-CA, both PPy-PA and PPy-PPA samples presented the two new bands at ~189.0 and ~131.0 eV, indicating the presence of phytic acid and phosphoric acid dopants.PPy-PA samples exhibited the much higher peak signal intensity of P 2s and P 2p at ~189.0 and ~131.0 eV than PPy-PPA, indicating the presence of a larger amount of P atoms from the doped counterions.Figure 1F-H   acid-doped PPy samples.Specifically, for the highresolution C 1s spectra, all of PPy aerogels exhibited three similar deconvoluted peaks centered at ~284.6, ~286.9, and ~288.8 eV, which were indexed to C-C, C-O, and C=O bonds.But, only PPy-PPA sample had a new peak with binding energy ~284.0 eV for C-S bonds.In the case of N 1s XPS spectra, the three deconvoluted peaks (i.e., ~401.2, ~399.9, ~397.7 eV), corresponding to the positively charged nitrogen (-N + -), neutral nitrogen (-NH-), and imine (=N-) bonds from the conjugated pyrrole rings, 36 were observed for all PPy aerogels.The lattice doping of phytic acid, citric acid, and phosphoric acid in PPy network significantly shifted the binding energies of C-O-H from ~532.8 to ~533.5 eV and C=O/ S=O from 531.7 to ~531.9 eV, because of the presence of the weakly coordinated tricarboxylic acid counterions in both PPy-PA and PPy-PPA samples.These characterization results revealed that among different PPy-based hydrogels with different amounts of doped acids and counterions in PPy gel network, PA dopants with multiple phosphate groups outperformed other dopants by forming stable ionic crosslinking with PPy network, thus improving the mechanical property of PPy-PA hydrogels, which will be used as one independent network for further synthesizing PVA/PPy-PA double network hydrogels.

| Synthesis and characterization of PVA/PPy-PA double-network hydrogels
After obtaining lyophilized PPy-PA aerogels, next we immersed the PPy-PA aerogels into PVA precursors (8 wt%) at 60 °C, which allows hydrophilic PVA chains to gradually diffuse into the conducting PPy-PA matrix to form different mixtures at different time points of 24 h (DN-1), 48 h (DN-2), 72 h (DN-3), respectively.Here, a lyophilization process would shrink or even condense polymer networks when extracting and evaporating water molecules from the networks.The resultant PVA-diffused PPy hydrogels underwent three freezing/ thawing cycles between −20 °C for 6 h and room temperature for another 6 h.The freezing/thawing process allows the formation of the secondary crystallization domains among PVA chains as crosslinked by abundant hydrogen bonding.At a first glance, PVA/PPy-PA DN hydrogels were highly hydrated and soft, in sharp contrast to relatively dry and brittle PPy-PA hydrogels.SEM images further showed that unlike PPy-PA hydrogels containing interconnected spheres and their clusters, DN-1 hydrogels presented highly crosslinked porous microstructures, where bead-like PPy aggregates appeared to be homogeneously dispersed in the entanglement zones of PVA chains (Figure 2A).However, PVA-PPy (diffusion time: 12 h) hydrogels failed to form a typical double-network structure, instead, they underwent an unusual network separation, as evidenced by a distinct interface between a dark PPy layer and a transparent PVA layer (marked by red arrows).Differently, since the formation of independent and interpenetrating DN structure requires the in-depth diffusion and penetration of PVA chains into the PPy network, DN-2 and DN-3 hydrogels enabled to form DN structures, with a mixed displayed network morphology of honeycomb structures with the average pore size of ~5 µm and filamentous structures with the average length of ~10 µm, confirming the dynamic migration process of PVA polymeric chains into PPy-PA network.Such a morphological change is quite different from the conventional PVA nanocomposites hydrogels whose crosslinked network becomes denser from loosely interconnecting porous structure as the nanofiller increases. 37Figure 2B shows the tensile stress-strain curves of PVA@PPy SN, PPy SN, and three PVA/PPy-PA DN-1~3 hydrogels.As controls, PPy SN gel was too weak to stretch, while PVA@PPy SN gel achieved a tensile strength of ~0.59 MPa at a fracture strain of 450%.Three PVA/PPy DN hydrogels achieved the tensile strength and strain of ~0.19 MPa and 282% for DN-1, ~0.39 MPa and 333% for DN-2, and ~0.31 MPa and 359% for DN-3, respectively (Figure 2C).Similar tensile strains of ~300% for three PVA/ PPy DN hydrogels indicates a common fracture process by pulling PVA chains from the PPy network, similar to hybrid crosslinked PAAm/Agar DN hydrogels (Supporting Information: Movie S2).
To evaluate the contribution of PPy and PVA network to mechanical properties of different PPy-based hydrogels, we conducted compression tests on cylindrical PVA@PPy SN, PPy SN, DN-1~3 hydrogels (diameter of ~8.66 mm, height of 6.5 mm) at a compression rate of 50 mm/min.As shown in Figure 2D, PPY SN hydrogel as a negative control group exhibited the lowest compression strength of ~0.77MPa at 95% of compressive strain, while PVA@PPy SN hydrogel as a positive group can achieve ~7.85 MPa (close to the maximum range of 8.0 MPa) at 88% of compressive strain.After the compression force was removed, PVA@PPy SN hydrogel recovered automatically and rapidly to its original cylindrical shape, but PPy SN hydrogel was too weak to withstand a high level of compression.This characteristic also confirms our fundamental design principal that the PVA network is ductile and compressive, in sharp contrast to the PPy offers intrinsic brittle and weak network.For PVA/PPy DN hydrogels, the maximum compression strength was ~3.79 MPa for DN-1, ~2.83MPa for DN-2, and ~1.54 MPa for DN-3, 2-5 times higher than those of pure PPy hydrogels but less than those of PVA@PPy SN hydrogels, indicating that (i) the mechanical strength of the double network can be constrained by the brittle first network (PPy) and (ii) the excessive PVA diffusion tends to disrupt the electrostatic cohesion of conducting polymer networks.
We then conducted successive loading-unloading compressive tests to understand the energy dissipation mode for double-network PVA/PPy-PA (DN-2) hydrogels.During the cyclic compression tests, no resting time was given between any two consecutive loading cycles.As a control, PVA@PPy SN cylindrical hydrogels showed obvious hysteresis loops at λ = 30%, but the hysteresis loops decreased slightly by <5% as loading-unloading cycles (Supporting Information: Figure S1A).Consequently, the dissipated energies were 4.10 kJ/m 3 at the first loading-unloading cycle and maintained at 3.55-4.00kJ/m 3 in the following four cycles, indicating that crystalline region and physical network were partially restored from the continuous load-unloading deformation.In Supporting Information: Figure S1B, PVA/PPy-PA (DN-2) hydrogels showed a considerable hysteresis loop at the first loading-unloading cycle (λ = 30%), with a dissipated energy of ~7.51 kJ/m 3 that was more than twice of PVA@PPy SN hydrogels.The cyclic compression results indicate that chemically crosslinked PPy network is fractured to dissipate energy at the first loading-unloading cycle, thus yielding a large hysteresis loop.Without any resting, the 2nd loadingunloading compression led to a significant drop of hysteresis loops with the limited energy dissipation of 4.35 kJ/m 3 , another indicator of the irreversible fracture of the PPy network.The continuous cyclic compressions resulted in similar hysteresis loops to the second cyclic loop, with the dissipated energy of 3.50, 3.20, and 3.20 kJ/m 3 at cycle 3, cycle 4, and cycle 5, respectively (Figure 2F).Therefore, we conclude that in sharp contrast to brittle PPy hydrogels, the appropriate interpenetration of PVA network is beneficial to homogeneous brittle network and offers sufficient hydrogen-bonding energy dissipation points.By comparing the mechanical and morphological behaviors of their single-network gels, we also found that the brittleductile transition is strongly related to the polymer strand density ratio of the two networks, which indicates that the force balance between the two networks acts as the key structural parameter for this transition.
As a proof-of-concept, we further summarized and compared the conducting polymer (CP)-PVA hydrogels among our PVA/PPy DN hydrogels and other literatures in terms of the actual network composition between CP networks (%) and PVA networks (%) in Supporting Information: Table S2.6][47] From the point view of DN hydrogels, different from abovementioned studies, we demonstrated a simple interpenetrating strategy to directly incorporate PVA network into pure PPy hydrogels, leading the resultant PVA/PPy DN hydrogels to achieve both highly conductivity and desirable mechanical properties, which was comparable to the reported PEDOT:PSS/PVA DN hydrogels 28 fabricated by in-situ aggregation and densification method.

| Electrical and EMI shielding performances of conductive PVA/PPy-PA DN hydrogels
Upon demonstrating the enhanced mechanical properties of PVA/PPy-PA DN hydrogels, due to intrinsic conducting nature of conducting polymer chains, we further examined and compared the electrical conductivity of different PPy SN, PVA@PPy SN, and PVA/PPy-PA DN-1~3 hydrogels using a standard four-point-probe method at room temperature.As controls, pure PPy SN hydrogel showed the highest electronic conductivity of 7.8 S/m, while PVA@PPy SN hydrogel had the lowest conductivity of ~0.21 S/m.In between, DN-1, DN-2, and DN-3 hydrogels can still achieve high electronic conductivity of ~6.8, ~6.9, and ~4.3 S/m (Figure 3A).A comparison of electronic conductivity for these hydrogels reveals that the introduction of crosslinked PVA to form DN structure, instead simply doping conducting PPy into PVA network, can significantly improve the ion transport as contributed by massive conductive channels in highly crosslinking network.It is also noted that these conductivity values are much lower than those of PEDOT:PSS-based hydrogels with 100-4000 S/m of conductivity (e.g., pure PEDOT:PSS SN and PEDOT:PSS/ PVA DN hydrogels), 28,47 presumably due to intrinsic difference from conducting polymer backbone and dopants. 48ince high electrical conductivity is a key parameter in determining EMI shielding performance for conducting materials, next it is logic to examine EMI shielding efficiency of PVA/PPy-PA DN hydrogels. 49,50To this end, all of lyophilized hydrogels were cut into rectangular pieces of 22.3 mm × 10.2 mm × 2.0 mm, followed by the recording in the frequency range of 8.2-12.4GHz using the X-band waveguide method.Since PPy SN aerogels were too weak to be tested by EMI, here we only compared the EMI shielding performance of PVA SN, PVA@PPy SN, and PVA/PPy-PA DN (DN-2) aerogels.During the EMI tests, the reflection of electromagnetic radiation from each aerogel surface was denoted with S11 and S22, in which the recorded S12 and S21 describe the electromagnetic wave absorption from the aerogels. 51Supporting Information: Figure S2 shows different scattering parameters (S11, S12, S21, and S22) of PVA SN, PVA@PPy SN, and PVA/PPy-PA DN aerogels.A number of the apparent scattering parameters, including absorption shielding effectiveness (SE Abs ), reflection shielding effectiveness (SE Ref ) and the total EMI shielding effectiveness (SE Total = SE Abs + SE Ref ), were computed using the Schelkunoffs theory. 52,53Figure 3B shows the variation of the EMI shielding efficiency of the three aerogels with the thickness of 2.0 mm as a function of microwave frequency (8.2-12.4GHz).PVA SN aerogels as a negative control showed an extremely low SE Total of 5-6 dB, due to the contribution of microwave reflection from sheet-like porous structures.PVA/PPy-PA DN aerogels exhibited an average EMI shielding efficiency (SE Total ) of ~26.5 dB over the X-band range, even comparable to those promising MXene-based hydrogels, superior to PVA@PPy SN aerogels (~18.0 dB) and a commercial standard requirement of >20 dB for practical applications (Supporting Information: Table S3).To analyze the EMI enhancement of PVA/PPy-PA DN hydrogels, we investigated the contributions of absorption shielding effectiveness (SE Abs ), reflection shielding effectiveness (SE Ref ) among PVA SN, PVA@PPy SN, and PVA/PPy-PA DN aerogels.In Figure 3C, THE statistical average ratio between SE Abs and SE Ref of DN-2 aerogels reached to 5.72, which was 2-4 times higher than those of PVA@PPy SN (2.87) and PVA SN aerogels (1.36).This difference in absorption-based shielding is likely stemmed from the dielectric/magnetic polarization and energy dissipation between the conductive PPy-PA and nonconductive sheet-like PVA networks. 54The formation of integral conductive PPy networks that interact with incident electromagnetic radiation not only enhances the impedances or refractive indexes at the interface between two propagation media (i.e., air and aerogel), but also offers multiple conductive channels to increase ohmic loss and dielectric loss, leading the absorbed energy dissipate in the form of heat energy (Figure 3D).On the other hand, it was also observed in PVA@PPy SN sample that if conducting PPy only serves as additives in PVA networks, the inevitable PPy aggregates occur, rupture the insufficient conductive network inside porous structure, and alter the localized density, all of which led to the reduced interactions between electromagnetic waves and aerogel matrix.Taken together, the resultant DN hydrogels enable both enhanced mechanical properties and EMI shielding by introducing DN structure to change intrinsic brittle nature and conducting PPy-PA network.

| Application of PVA/PPy-PA DN hydrogels as smart supercapacitors
From a practical application viewpoint, we transformed PVA/PPy-PA DN hydrogel into an all-solid-state symmetrical supercapacitor with a sandwich structure by placing a PVA/PPy-PA DN hydrogel pre-soaked with 1.0 mol/L LiCl solution in between two activated PANI-CC electrodes (diameter: 12 mm) in a button CR2032 battery case. 55,56We first evaluated the basic electrochemical performances (e.g., CV, GCD, Nyquist plot) of PVA/PPy-PA DN supercapacitors at room temperature (25 °C). Figure 4A displayed the typical CV curves of PVA/PPy-PA DN supercapacitor at different scanning rates of 5-100 mV/s.Different from a typical rectangularlike CV curve, a combination of both electrostatic doublelayer capacitor (EDLC) and pseudo capacitance was observed.The overall shape of CV curves was well retained as the scanning rates changed, indicating the rapid, cyclic charge/discharge properties of PVA/PPy-PA DN supercapacitors.Particularly, the weak redox peak in the CV curves is mainly attributed to the faradaic redox reaction of PANI.Given a stable potential window of two electrodes in the range of 0-0.8 V, a 0.8 V working voltage window was applied to the assembled supercapacitor.All GCD curves at current density of 1-10 mA/cm 2 showed quasi-triangle shapes, indicating the Coulomb efficiency of the devices (Figure 4B).In the Nyquist plots, DN devices showed a series of low equivalent resistance of 4.7 Ω•cm 2 and charge transfer resistance (R CT ) of 3.0Ω•cm 2 at high-frequency area, benefiting from the interconnected, microporous PPy conductive network that facilitated ion transportation.However, at the low-frequency region, the EDLC of porous network reflected the pseudocapacitance of the PANI-CC electrode formed by the redox reaction (Figure 4C).
We also probed the charge storage process for PVA/PPy-PA DN hydrogel electrolyte by analyzing the electrochemical kinetics (Supporting Information: Figure S3).Based on a linear relationship between current response and scanning rate, the EDLC constant was calculated to be 0.825, demonstrating that the charging process is dominated by dual surface capacitive and diffusion-controlled processes (Supporting Information: Figure S3A,B).Moreover, the cyclic voltammetry curves in Supporting Information: Figure S3C showed that there was only 50.3% of surface capacitive contribution (green region) at a scan rate of 1 mV/s, indicating that the diffusion-controlled effect is dominated during the charge/discharge process.Further detailed investigation on the relationship between surface and diffusion contributions at a scanning rate range of 1-30 mV/s was provided in Supporting Information: Figure S3D.Evidently, surface capacitive contribution increased linearly from 50.3%, 55.7%, 63.5%, 69.4%, to 87.0% as the scan rates increased from 1, 5, 10, 15, to 30 mV/s, which was mainly attributed to the decrease of diffusion-controlled capacitance to some extents as the scan rate increases.On the other hand, the assembled devices not only exhibited a high capacitance of 315.9 F/g at the charging/discharging rate of 1 mA/cm 2 , but also maintained its high value of 280.7-195.0F/g as the current density increased to 2-10 mA/cm 2 (Figure 4D).Since the type and loading contents of electrode materials have a significant impact on the actual performance of supercapacitors, our PVA/PPy-PA DN devices exhibited comparable capacitance to the most commonly used PANI-CC-based supercapacitors.Such PVA/PPy-PA DN supercapacitors also delivered a capacity of ~280.7 F/g with a retention of ~94.3% of its initial capacity after 2000 cycles at a testing current density of 2 mA/cm 2 , confirming the excellent electrochemical/physical stability of conductive PPy network in the doublenetwork structure (Figure 4E).
Since temperature is another critical parameter affecting the electrochemical stability of supercapacitors, we examined PVA/PPy-PA DN hydrogel electrolytes in a wide temperature range of −20-50 °C.As temperatures decreased, GCD curves maintained isosceles triangle shape with a slightly faster voltage drop/rise at current density of 1 mA/cm 2 , and the time for a complete charging/discharging cycle decreased from ~270.9 s at 50 °C, ~242.1 s at 30 °C, ~210.4 s at 0 °C, to ~160.3 s at −20 °C (Figure 4F).This fact demonstrates that the reversible double-layer capacitance and conductivity are largely retained due to the continuous conducting polymer skeleton structure of PVA/PPy DN hydrogels at different temperatures.In Figure 4G, Nyquist plots of PVA/PPy DN devices at −20-50 °C exhibited small semicircle regions and short 45°W arburg regions, indicating that the devices possess a stable electrical conductivity in a wide temperature range.Similarly, the active PVA/PPy DN supercapacitor enabled to deliver a stable electrode specific capacitance of ~236.6 F/g at −20 °C, ~276.0F/g at 0 °C, ~318.3F/g at 30 °C, ~336.9F/g at 50 °C, showing a temperaturedependent capacity behavior (Figure 4H).Two different mechanisms are proposed to explain the high stability of electrochemical properties at a wide range of temperatures.At subzero temperatures, the stability of PVA/PPy DN hydrogel electrolytes is mainly attributed to the excellent ice recrystallization inhibition activity by PVA chains with precisely arranged hydroxyls, which are paired with the ice foremost to inhibit the expansion of ice crystals, [56][57][58] thus retaining their ionic conductivity/mechanical flexibility.At higher ambient temperatures (30-50 °C), the interlocked secondary PVA networks allow to support the integral conductive channels and reduce the possibility of inevitable degradation, thus resulting in a better thermal stability and/ or thermoelectric properties.PVA/PPy-PA DN hydrogels with the high-temperature resistance outperform other conductive polymers that would degrade exponentially over time above room temperatures. 59

| CONCLUSION
In this study, we proposed a functioning strategy to design conductive PVA/PPy-PA DN hydrogels through an in-situ ultrafast gelation of the 1st PPy-PA network and subsequent infiltration and freezing/thawing process of the 2nd PVA network, independent of but interpenetrating into the PPy-PA network.The resultant PVA/PPy-PA DN hydrogels achieved a high conductivity of ~6.8 S/m and a high mechanical strength of ~0.39 MPa simultaneously, due to the integration of conducting PPy-PA polymers with tough PVA network in a porous DN structure.Benefitting from high electronic conductivity, the lyophilized PVA/PPy-PA DN hydrogels with continuous electrically conductive PPy network demonstrated their excellent electrochemical properties, as evidenced by the increase of impedances, refractive indexes, ohmic loss, dielectric loss, and EMI shielding efficiency.Upon assembling PVA/PPy DN hydrogels into all-solid-state supercapacitors, they showed not only high capacitance of ~315.9F/g at the current density of 1 mA/cm 2 , but also ~94.3% of capacitance retention after 2000 GCD cycles at a wide range of temperatures from −20 °C to 50 °C.Both high mechanical performance and electrochemical conductivity of PVA/PPy-PA DN hydrogels are attributed to the hierarchical DN structure of the hydrogels, which serves as a continuous cthree-dimensional pathway for electronic conduction and energy dissipation.Different from (i) conventional fabrication strategies for conducting hydrogels by directly mixing of conducting polymers into the polymer network and (ii) the most commonly used PEDOT:PSS-based conducting hydrogels, our in-situ polymerization fabrication method and conducting hydrogel system are highly compatible with other conductive materials and DN hydrogels for the design of mechanically robust electronic materials and beyond.
also showed the highresolution C 1s, N 1s, and O 1s XPS spectra of all

F
I G U R E 1 Fabrication and characterization of chemically crosslinked PPy-based hydrogels.(A) Chemical structures of polypyrrole (PPy) as polymer network, VPES as an accelerator, and phytic acid (PA), phosphoric acid (PPA), and citric acid (CA) as doping acids.(B) Digital photographs show an ultrafast gelation process (I-IV) of PPy-PA hydrogels within 30 s after simply hand shaking of the precursor solution (left panel) and the standing of PPy-PA aerogel by lyophilizing hydrogels on the top of a dandelion flower (right panel).(C) SEM images of lyophilized PPy-PA hydrogels.(D) FTIR, (E) XPS, and high-resolution (F) C 1s, (G) N 1s, and (H) O 1s spectra of lyophilized PPy-PA, PPy-PPA, and PPy-CA hydrogels.SEM, scanning electron microscope.

F I G U R E 3
Electrical and EMI shielding performances of conductive PVA/PPy-PA DN hydrogels.(A) Electrical conductivity of PPy SN, PVA@PPy SN, and PVA/PPy-PA DN-1~3 hydrogels.(B) Frequency-dependent absorption shielding effectiveness (SE Abs ), reflection shielding effectiveness (SE Ref ), and total EMI shielding effectiveness (SE Total ) of PVA SN, PVA@PPy SN, and DN-2 aerogels in the X-band frequency range of 8.2-12.4GHz.(C) Average ratios between SE Abs and SE Ref by analyzing (B) results.(D) Schematic illustration of the shielding mechanism via the conducting polymer network of PVA/PPy-PA DN hydrogel.