Piezoelectric cellulose/poly(vinylidene fluoride) glycerogels with synergistically enhanced energy output for wide temperature range
Abstract
Gel-based piezoelectric materials are stretchable, wearable, and environmentally friendly, unlike their conventional solid counterparts. However, designing environment-tolerant, high-performance piezoelectric gels is challenging. Herein, we develop a piezoresponsive stretchable glycerogel (GG), leveraging the cooperative structure-forming effect of cellulose, poly(vinylidene fluoride) (PVDF) and glycerol (a green extremotolerant solvent). The facile inter- and intramolecular cellulose/PVDF interactions within the hydrogen-bonded network of glycerol generate a highly electroactive crystalline β-phase while retaining mechanical integrity. Therefore, the synergy-driven GG is more piezoresponsive than gels fabricated using individual polymers. Despite having a low polymer density
Keywords
INTRODUCTION
The use of nonrenewable energy sources has resulted in environmental pollution and exacerbated the energy crisis. Consequently, research has focused on reliable, renewable, and environmentally friendly energy sources as alternatives to mitigate pollution and meet growing energy demands[1-3]. Mechanical energy is abundant and can be converted into usable energy using piezoelectric nanogenerators to power micro- to nanoscale devices[4,5]. Commonly used piezoelectric materials, such as lithium niobate [LiNbO3][6], barium titanate [BaTiO3][7], and lead zirconate titanate [Pb(Zr,Ti)O3][8], exhibit excellent piezoelectric energy conversion capabilities. However, existing piezoelectric materials are inherently brittle owing to their highly crystalline structure. They are also toxic, expensive, and environmentally unfriendly[9,10]. Therefore, researchers are actively attempting to identify new eco-friendly materials that are biocompatible, electroactive, flexible, and lightweight[11,12]. Soft biocompatible piezoelectric materials are of particular interest in the rapidly expanding fields of flexible electronics, soft robotics, and wearable devices.
The use of gels (a class of semisolid materials featuring swollen polymer networks) is being increasingly explored in soft robotics, biomedicine, sensing/actuation, and flexible electronics. They also show promise for energy generation and energy storage owing to their unique combination of functions, which are unmatched by any existing solid or liquid material. Gels demonstrate various desirable physical properties, such as stretchability, flexibility, electrical conductivity, biocompatibility, biodegradability, and even piezoelectricity[13-23]. They have recently been used extensively for the fabrication of various stretchable and wearable devices. Proper design strategies can harness the large amount of wasted mechanical energy of gels to generate piezoelectric energy, facilitating the development of sustainable stretchable and wearable devices. Among the many piezoelectric and triboelectric materials that have been developed to harvest this wasted energy[24-26], gel-based piezo/triboelectric materials are most suitable for wearable devices owing to their inherent stretchability, elasticity, and flexibility. Numerous piezocapacitive or piezoresistive gels and gel-based devices/sensors have recently been developed[27-29]; however, the development of piezoelectric gels or gel-based devices is particularly challenging because most piezoelectric materials are unsuitable for incorporation into gel systems. Therefore, the development of soft gel-based piezoelectric materials is still in its nascent stages and represents an emerging research area for energy-harvesting, energy-storage, and wearable devices. For example, a supramolecular strategy was implemented to align dipoles to synergistically enhance the piezoelectric response. This was achieved through hydrogen bonding and charge transfer complexation in organogel-based nanogenerators following a complex fabrication procedure[20]. Moreover, a hydrogel fabricated from graphene-oxide-nanoparticle-doped poly(2-hydroxyethyl methacrylate) was used to generate an electrical current in aqueous media[21]. Additionally, a polyacrylonitrile/poly(vinylidene fluoride) (PAN/PVDF) hydrogel was reported to exhibit enhanced piezoelectric performance[22]. Furthermore, the piezoelectric output of a conductive hydrogel was enhanced by introducing β-phase PVDF-trifluorethylene[23]. However, the piezoelectric-type hydrogels developed to date exhibit poor energy output performances along with weak mechanical and environmental stability.
A formidable challenge in developing high-performance piezoelectric gels is that their structural integrity is naturally disrupted in the highly solvated microenvironment of gels. This can make the material brittle and dramatically less piezoresponsive. To date, few approaches have been devised for upgrading the piezoelectric output of gels. Another concern is the poor environmental adaptability of conventional gels, which limits their practical applications. Recently developed water-based piezoelectric hydrogels only function in aqueous environments because water is required to maintain the gel properties. Thus, the lifetime of these hydrogels is constrained in nonaqueous (e.g., air) or extreme environments (such as extremely high or low temperatures). This limitation arises from the drying and freezing properties of water[13,16,30,31]. To overcome these limitations, we have formulated a molecular engineering technique to integrate carefully chosen piezoelectric polymer(s) and an extremotolerant solvent. Subtle chemical interactions among the selected polymer(s) and solvent are leveraged to simultaneously achieve two goals: (i) a structure with a high degree of piezoactivity; and (ii) robust mechanical integrity in the gel state. This ensures the efficient dissipation of the input mechanical signal through the piezoactive structure, maximizing the piezoelectric output.
Based on this strategy, we developed a novel class of extremotolerant piezoelectric glycerogels (GGs) comprising cellulose and PVDF swollen in glycerol, prepared using a facile method [Figure 1A]. PVDF and its copolymers are semicrystalline piezoactive polymer materials that are widely used in the field of energy harvesting because they are biocompatible and exhibit excellent piezoelectric and dielectric properties[32,33]. Among the different crystalline phases of PVDF (α, β, γ, δ, and ε), the polar β-phase has a non-centrosymmetric structure. It exhibits the highest dipolar moment per unit cell (8 × 10-30 C m), indicating a high piezoelectric response[32,33]. Despite the excellent prospects of PVDF, using it to fabricate wearable and stretchable piezoelectric gels is challenging because of its inherent hydrophobicity, brittleness, and structural mismatch with hydrophilic gel components; therefore, mechanical energy cannot be readily transferred to the crystals in PVDF-based gels. Conversely, cellulose - the most abundant natural biopolymer - is extensively used to develop tough semicrystalline gels owing to its nontoxicity, biocompatibility, low cost, and satisfactory piezoactive properties[34,35]. The rigid backbone of cellulose and its ability to easily crystallize through hydrogen bonding can reinforce the composite gel structure; the rigid networks also facilitate the mechanical signal transferability. Recently, we demonstrated that glycerol- as a green solvent - can be used to integrate a wide range of polymers and crosslinkers to develop remarkably tough extremotolerant GGs; this was facilitated by facile and stable polymer-solvent interactions over a wide temperature range[14,36-38]. Considering these aspects, we rationally designed a facile method to integrate both cellulose and PVDF using glycerol as the solvent to develop tough, flexible, and extremotolerant cellulose/PVDF GGs with synergistically amplified piezoelectric properties compared with those of gels fabricated using the individual constituent polymers. The molecular-level integration and formation of inter- and intramolecular hydrogen bonds among cellulose, PVDF and glycerol promoted the crystallization of the electroactive β-phase, and enhanced the gel’s toughness and extreme temperature tolerance [Figure 1B]. The rigid network of cellulose in the developed GGs efficiently transferred the applied mechanical energy to orient the dipoles of crystalline PVDF, resulting in impressive piezoelectric properties across a wide temperature range
Figure 1. Design and fabrication of cellulose/poly(vinylidene fluoride) (PVDF) glycerogels (GGs) for high-performance piezoelectric applications. (A) Schematic of the process employed to prepare cellulose/PVDF GGs. (B) Magnified schematics of neat cellulose, (1/2)% cellulose/PVDF gel, and (0.5/5)% cellulose/PVDF gel; the circular area indicates the type of bonding and crystalline structures. (C) Illustration of the silver-sputtering process, and photograph of the silver-sputtered (1/2)% cellulose/PVDF GG, which was used to generate piezoelectricity. (D) Illustrative mechanism of force-induced polarization in the (1/2)% cellulose/PVDF GG-based device to generate piezoelectricity for an external circuit. DMAc: N,N′-dimethylacetamide.
EXPERIMENTAL
Materials and instrumentation
Glycerol, N,N′-dimethylacetamide (DMAc), lithium chloride (LiCl), and ethanol were purchased from Samchun Pure Chemicals (Republic of Korea). Cellulose filter paper (ADVANTEC) was procured from Toyo Roshi Kaisha (Japan). PVDF (Mw ≈ 275,000) and poly(vinyl alcohol) (PVA, Mw: 89,000-98,000) were obtained from Sigma-Aldrich. Dimethyl sulfoxide (DMSO) was acquired from Daejung Chemicals & Metals (Republic of Korea). All chemicals were used as received without further purification. Ultrapure deionized (DI) water was used to prepare all hydrogels.
All tests were performed under ambient conditions (humidity, 30%-60%; temperature, ~25 °C) unless otherwise specified. The PVDF phases and crystallinity of the gels were investigated using a Fourier-transform infrared (FTIR) spectrometer (PerkinElmer Spectrum IR, USA) and an X-ray diffractometer (PANalytical X'Pert Pro MRD, the Netherlands). In the FTIR measurements, eight scans were accumulated, and four spectral resolutions were examined. X-ray diffraction (XRD) was conducted using a CuK-radiation source (λ = 0.1541874 nm) operating at 40 kV and 30 mA. The scanning rate was 8.5° min-1, with a step size of 0.026°. A field-emission scanning electron microscope (FE-SEM; S-4700, Hitachi, Japan) was used to investigate the gel structure. The samples were freeze-dried at -80 °C for 1 d and sputtered with platinum before observing the gel surfaces by FE-SEM. The surface potential of GGs was determined through Kelvin-probe force microscopy (KPFM), which was conducted using atomic force microscopy (AFM; NX10, Park Systems, Republic of Korea) at the Energy Convergence Core Facility in Chonnam National University. KPFM was performed on a scanning area of 5 µm × 5 µm using a Cr/Au-coated conductive tip with a spring constant of 0.3 N m-1 and a frequency of 65 kHz. A custom-made device equipped with a force sensor (Nidec-Shimpo FGP-2, Japan) was used to tap the prepared gels. The open-circuit voltage density (OCVD), short-circuit current density (SCCD), and piezoelectric charge constant
Fabrication of cellulose/PVDF GGs
Cellulose filter paper with dimensions of ~5 mm × 5 mm was sequentially washed with water (~6 h), ethanol (~6 h), and DMAc (~12 h) and then vacuum dried at 60 °C for ~24 h. The obtained paper was used to prepare cellulose/PVDF hydrogels. Our previous studies revealed that DMAc/LiCl is excellent for dissolving highly crystalline cellulose by breaking its high-density hydrogen bonds; the resulting solutions were used to fabricate various functional gels with excellent mechanical properties[14,18,39,40]. We found that DMAc/LiCl could also dissolve PVDF at high concentrations because PVDF is also a largely crystalline polymer dominated by hydrogen bonds. Therefore, DMAc/LiCl was chosen to dissolve and integrate both cellulose and PVDF into a high-performance composite gel. A DMAc/LiCl (92:8, w/w) solvent (at 25 °C) was used to dissolve x wt% cellulose filter paper (0.5-1.5 wt%) and y wt% PVDF pellets (0.5-5 wt%) to prepare (x/y)% cellulose/PVDF solutions [Supplementary Table 1]. Each of the resulting cellulose/PVDF solutions was poured into clean, flat glass molds (80 mm × 80 mm × 3 mm) and exposed to controlled atmospheric conditions (temperature: ~25 °C; humidity: 30%-60%) for at least 3 d to enable the gelation of cellulose/PVDF through hydrogen bond formation. This cellulose/PVDF organogel was transformed into a hydrogel via sequential equilibration in ethanol (1 d) and DI water (1 d). The resulting hydrogel was converted into a GG by sequentially equilibrating the gel in aqueous solutions of 25%, 50%, 75%, and 100% glycerol for 12, 12, 24, and 24 h, respectively. The prolonged step-wise solvent exchange helped preserve the structural integrity and ensured that water was completely replaced with glycerol in the gel network[14]. Subsequently, the solvent-exchanged GG was dry-annealed in a closed environment at 120 °C for 6 h and then slowly cooled to room temperature to obtain ≈2-mm-thick cellulose/PVDF GGs, which were used for further experiments [Figure 1A and Supplementary Figure 1]. Note that all the gel precursor solutions were prepared in closed glass vials, and the precursor solution consisting of DMAc, LiCl, cellulose and PVDF was kept inside a closed fume hood during gelation to prevent DMAc from leaking into the environment. After purification, gelation, and the solvent exchange process, ethanol, DMAc and LiCl can be retrieved and separated using standard fractional distillation methods because of their highly distinct boiling points; therefore, no solvent or chemicals will be wasted when these materials are produced at an industrial scale.
Fabrication of PVA/PVDF GGs
First, a PVA/PVDF precursor solution was prepared by dissolving PVA/PVDF at different weight percentages (10/5, 10/10, 5/10 wt%) in DMSO with continuous stirring at 120 C for 1 d. The solution was poured between rectangular glass molds to a thickness of ~3 mm. Then, gelation was performed by freeze-thawing, in which the molded solution was frozen at -50 C (12 h) and then thawed at room temperature
Device fabrication for piezoelectric response analysis
For the piezoelectric response experiments, Ag was sputtered onto the surface of the developed GG
RESULTS AND DISCUSSION
Physicochemical characterization of cellulose/PVDF GGs
Cellulose/PVDF GGs were fabricated with different compositions. The piezoelectric performances of the gels were anticipated to depend on both their piezoelectric crystal structures and mechanical integrity[21,41,42]. Although gels were successfully obtained for each selected composition, increasing the amount of PVDF resulted in physically weak gels.
The chemical structure of the cellulose/PVDF GG matrix [Supplementary Figure 1] was analyzed by FTIR spectroscopy [Figure 2A]. The bands at 763, 795, and 975 cm-1 for pristine PVDF indicated the presence of the α-phase, whereas those at 841, 1,275, 1,403, and 1,431 cm-1 were attributed to the β-phase
Figure 2. Structural characterization of the cellulose/PVDF GGs. (A) FTIR spectra of the cellulose/PVDF GGs. (B) Crystalline phases of PVDF: (i) α-phase and (ii) β-phase. (C) XRD patterns of cellulose/PVDF GGs and (D) corresponding full width at half maximum (FWHM) values.
In the XRD profile of pristine PVDF [Figure 2C], the characteristic crystalline peaks appeared at 2θ values of 18.3° and 19.9°, which have been ascribed to the α-phase peaks[41,43], whereas that of cellulose appeared at 20.4°. However, the incorporation of PVDF into the cellulose matrix decreased the full width at half maximum (FWHM) of the characteristic cellulose peak and shifted it slightly to 2θ° ≈ 20.6° (β-phase; Figure 2D)[22,41]. These changes indicate a higher degree of crystal alignment in the cellulose/PVDF gel compared with that in the neat cellulose gel and validate its cellulose-mediated cooperative β-phase crystallization, which is particularly important for ensuring the mechanical integrity of the β-phase in the gel network. Furthermore, the surface structure of the gel was comparatively investigated by FE-SEM
Piezoelectric response of cellulose/PVDF GGs
The piezoelectric responses of the cellulose/PVDF GGs were evaluated by their output OCVD and SCCD measurements [Figure 3A]. The pure cellulose gel exhibited a low OCVD of 22.78 ± 3.07 mV cm-2 and SCCD of 25.02 ± 3.06 nA cm-2, potentially because of the inferior piezoelectric properties of cellulose[44,45]. Adding PVDF to the composition of (1/2)% cellulose/PVDF significantly enhanced the voltage and current outputs. Maximum values reached 86.06 ± 6.00 mV cm-2 and 104.93 ± 6.66 nA cm-2, respectively, owing to the synergistic contribution of the piezoelectric β-phase crystals and the mechanical strength of the hybrid gel network. However, further increasing the PVDF content significantly decreased the output. This was because excess PVDF weakened the gel by creating phase-separated structures, which possibly curtailed its mechanical signal transduction ability. Furthermore, GGs with different cellulose concentrations were tested to identify the optimal piezoresponsive gel [Supplementary Figure 4]. For a given cellulose concentration, increasing the PVDF content in the gel produced consistent piezoelectric response behavior patterns. The (1/2)% cellulose/PVDF gel delivers the maximum output values, as discussed previously.
Figure 3. Piezoelectric response properties of the cellulose/PVDF GGs. (A) Piezoelectric output OCVD (mV cm-2) and SCCD (nA cm-2) of ≈2-mm-thick cellulose/PVDF GGs (surface area: 2 cm × 2 cm) measured under a cyclic load of 20 N at a frequency of ~0.7 Hz. (B) Voltage and current outputs of the (1/2)% cellulose/PVDF gel under different magnitudes of cyclic loads ranging from 5 to 25 N. (C) Representative current output of the (1/2)% cellulose/PVDF gel under a cyclic load of 20 N in the forward and reverse connection modes of an external circuit. (D) Variation in the piezoelectric charge constant d33 with the concentration ratio of cellulose/PVDF GGs. Data in (A, B and D) are presented as average values with mean absolute deviation from the measurements of three samples.
Subsequently, the output response of the optimized gel was investigated under loads ranging from 5 to 25 N. The piezoelectric output voltage and current densities increased linearly with the applied force [Figure 3B, Supplementary Figure 5]. The increases occurred at rates of 1.2125 mV cm-2 N-1 and
To further verify the piezoelectric properties of the cellulose/PVDF GGs, their d33 values were measured [Figure 3D]. Owing to the inferior piezoelectric properties of cellulose, the neat 1% cellulose gel exhibited a d33 value of 1.37 ± 0.25 pC N-1. The variation in the d33 values of the gels was consistent with the piezoelectric output behavior of the cellulose/PVDF gels. The (1/2)% gel delivered the highest output (2.4 ± 0.53 pC N-1). A further increase in the PVDF content of the gel matrix drastically reduced the d33 value owing to its weak phase-separated structure. Furthermore, the dielectric properties of the gels were examined over a frequency range of 100-105 Hz at room temperature. Among the gels with 1% cellulose, the optimized piezoresponsive (1/2)% cellulose/PVDF gel exhibited the highest dielectric constant at any given frequency. The gel with less cellulose and the highest PVDF content [(0.5/5)% gel] showed the highest dielectric constant among all the tested gels [Supplementary Figure 7]. The neat cellulose gel exhibited the lowest dielectric loss at the selected frequencies, and adding PVDF slightly increased this loss [Supplementary Figure 7]. Although the (0.5/5)% gel exhibited the most impressive dielectric properties, it had a low d33 value and low energy outputs, whereas the optimized (1/2)% gel exhibited a high d33 value and high energy outputs despite its moderate dielectric properties. This was attributed to the efficient mechanical-signal-transferring ability of the optimized gel, which was unlikely in the case of the gel with the highest PVDF content [(0.5/5)%] owing to its phase-separated structure.
Extremotolerance and mechanical properties of cellulose/PVDF GGs
The (1/2)% cellulose/PVDF gel demonstrated excellent stability under long-term exposure to air (~7 d) at temperatures of -20 to 80 °C. In particular, its weight remained nearly unchanged at 25 °C, increased by ~2.8 wt% at -20 °C, and decreased by ~15.4 wt% at 80 °C [Figure 4A and B]. This small increase and decrease in gel weight were presumably related to the absorption of atmospheric moisture and the loss of glycerol from the GG, respectively.
Figure 4. Extremotolerance of cellulose/PVDF GGs. (A) Evaluation of the effect of low (-20 °C), ambient (25 °C), and high (80 °C) temperatures on the cellulose/PVDF GGs. (B) Variation in the weight fractions of the (1/2)% cellulose/PVDF gel during long-term (7 d) exposure to air at -20, 25, and 80 °C and corresponding (C) tensile stress-strain and (D) compressive stress-strain curves. (E) Cyclic compression of the (1/2)% cellulose/PVDF gel at 25 °C with a strain rate of ~830% min-1 (0.277 mm s-1).
Subsequently, the tensile performance of the cellulose/PVDF gels was evaluated [Figure 4C,
The excellent piezoelectric performance of the cellulose/PVDF gel can be explained by the coupling behavior between the mechanical and electrical properties governed by the following constitutive equations[21,42]:
where S is the strain tensor, T is the stress tensor, D is the electric displacement vector, E is the electric field vector, cE denotes the elastic constant (at constant electric field) matrix, e denotes the piezoelectric constant matrix, and s denotes the dielectric constant (at constant strain) matrix. The superscript T appearing in
Cyclic loading-unloading curves of the (1/2)% gel were acquired for over 50 cycles [Figure 4E]. The presence of significant hysteresis loops qualitatively indicated that the cellulose/PVDF gels were viscoelastic materials and toughened by the efficient energy dissipation of hydrogen bonding [Figure 4E and Supplementary Figure 11]. The energy dissipation of our gel also suggests that the mechanical energy input can be efficiently dissipated to the polymer network[13-16], which would generate a higher piezoelectric output; this phenomenon is usually unlikely to occur in weak gels or hard solid materials. The presence of the double loops suggests that the relaxation rate of the material was lower than the unloading rate, which corresponds to strain-rate-dependent hardening[21].
The mechanical performance of the (1/2)% gel at different temperatures was also evaluated after 7 d of exposure to air. The work of extension values of the gels at all the considered temperatures was similar
Finally, when subjected to finger tapping, the gel’s piezoelectric response after 7 d of exposure to air at
Figure 5. Piezoelectric voltage output in extreme conditions. The piezoelectric voltage output response of the (1/2)% cellulose/PVDF GG subjected to finger tapping: (A) Initial response and response after incubation for 7 d at (B) 25 °C, (C) -20 °C, and (D) 80 °C.
Effects of polymer structure
We anticipated that the mechanical signal transferability of piezoelectric crystals in gel state would be extremely important to achieving a good piezoelectric response. The rigid structure and cooperative crystallization ability of cellulose both help create a robust network channel; this efficiently transfers the input mechanical force and improves the piezoelectric output. To validate our hypothesis we compared our results with another type of PVDF-based hybrid GG, which was fabricated using a relatively flexible polymer, PVA, instead of cellulose. The polymer density and mechanical strength of the best-performing (10/10)% PVA/PVDF GG were similar to that of (1/2)% cellulose/PVDF GG; however, the former GG was significantly less piezoresponsive (4.6 times lower OCVD value) [Supplementary Figure 15]. Both cellulose and PVA chains are rich in hydroxyl groups and easily crystallize through dense hydrogen bond formation[36,37,39,40]. However, the backbone of cellulose is more rigid than that of PVA[40]. In the case of the cellulose/PVDF GG, the input mechanical signal can be efficiently transferred through its rigid polymer network; thus, it exhibits much higher piezoelectricity than its PVA-based counterpart.
CONCLUSIONS
We developed a novel piezoresponsive gel by incorporating cellulose and PVDF, which are biocompatible and inexpensive polymers, into glycerol, a green extremotolerant solvent. Although cellulose exhibits relatively poor piezoelectric performance, it serves as a mediator to create highly piezoactive β-phase PVDF crystals and provides good mechanical integrity for an efficient mechanical energy transduction process. This phenomenon is particularly effective for a relatively rigid polymer, which is more sensitive to a mechanical signal. This effect is aided by the presence of glycerol in the hybrid gel network. Therefore, despite differences in the structure and characteristics, the hybrid gels exhibited excellent piezoelectricity, mechanical properties, and stability over a wide temperature range (-20 to 80 °C). The rationale used in this study could provide insights for developing various high-performance piezoelectric gels through a proper combination of mechanical and piezoactive materials for diverse applications.
DECLARATIONS
Authors’ contributions
Contributed significantly to data curation, formal analysis, investigation, methodology, validation, visualization, and writing of the original draft: Mredha, M. T. I.
Contributed significantly to data curation, formal analysis, investigation, methodology, resources, validation, visualization, and writing of the original draft: Wadu, R. R. M.; Varma, A. V. R.; Gupta, T.
Contributed significantly to data curation, formal analysis, investigation, methodology, resources, and writing of the original draft: Li, S.
Provided resources and validation, contributed to conceptualization, investigation, and methodology, and participated in writing the original draft and reviewing and editing the manuscript: Lee, W.
Contributed to investigation, provided resources and supervision, ensured validation, and participated in conceptualization and writing of the original draft and review and editing of the manuscript: Zhang, C.
Performed formal analysis and investigation, provided supervision and validation, and contributed to conceptualization, writing the original draft, and review and editing: Chen, W.
Contributed significantly to conceptualization, formal analysis, funding acquisition, investigation, project administration, supervision, validation, and writing the original draft, as well as reviewing and editing the manuscript: Jeon, I.
Availability of data and materials
The data supporting the findings of this study are available within this article and its Supplementary Material.
Financial support and sponsorship
This study was supported by grants from the National Research Foundation of Korea (NRF; 2020R1A2C2007974), funded by the Korean government (MSIT), a grant from the “Regional Innovation Strategy (RIS)” implemented by the NRF, funded by the Ministry of Education (MOE) (2021RIS-002), and Brain Pool program funded by the Ministry of Science and ICT through the National Research Foundation of Korea (RS-2024-00408127).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
The Author(s) 2025
Supplementary Materials
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Mredha, M. T. I.; Wadu, R. R. M.; Li, S.; Varma, A. V. R.; Gupta, T.; Lee, W.; Zhang, C.; Chen, W.; Jeon, I. Piezoelectric cellulose/poly(vinylidene fluoride) glycerogels with synergistically enhanced energy output for wide temperature range. Energy Mater. 2025, 5, 500074. http://dx.doi.org/10.20517/energymater.2024.238
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