Polypyrrole-modified flax fiber sponge impregnated with fatty acids as bio-based form-stable phase change materials for enhanced thermal energy storage and conversion

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Introduction
A substantial amount of energy is consumed annually for various purposes, primarily relying on fossil fuel resources.However, fossil energy sources are becoming increasingly scarce, and safeguarding the environment has become a top priority.Hence, it is crucial to explore viable alternatives to traditional fossil fuels and develop efficient energy storage solutions for emerging energy needs.Energy storage technology plays a vital role in alleviating the challenges posed by the spatial and temporal disparity between energy supply and demand, ultimately enhancing energy utilization efficiency.Consequently, it has become a significant area of research in energy management.One highly promising avenue for improving energy efficiency and optimizing the utilization of existing heat sources is thermal energy storage.This technology finds potential applications in various sectors, including renewable energy, industrial waste heat utilization, building energy management, and thermal regulation for electronic devices [1,2].
Among various renewable energy sources, there has been a growing fascination and extensive focus on latent thermal storage in recent years.This heightened interest is primarily attributed to the distinctive characteristics of phase change materials (PCMs).These materials undergo a remarkable and reversible phase transition at a critical temperature, such as melting or crystallizing, leading to substantial storage or release of latent heat into the surrounding environment.PCMs are exceptionally efficient in storing thermal energy under isothermal conditions, resulting in minimal temperature differences between the stored and released energy.Thus, they are considered ideal candidates for storing and releasing thermal energy through latent heat during phase changes.A growing focus has been on developing composite PCMs with energy conversion properties, such as solar photothermal conversion and electrothermal harvesting techniques.These composite materials are gaining popularity due to their ability to combine a large energy storage capacity with high energy charging efficiency [3][4][5][6].
Fatty acids (FAs) belong to the family of solid-liquid organic PCMs and are distinguished by their considerable latent heat storage capacity and well-adjusted phase change temperature, which can be easily modified by blending different types.Furthermore, FAs possess an array of desirable characteristics, including biodegradability, chemical and thermal stability, excellent corrosion resistance, non-toxicity, and costeffectiveness, among others.However, the practical application of solid-liquid organic PCMs faces certain challenges.One such obstacle is the issue of shape instability during the phase transition process, which causes the fluidity and leakage of the molten PCM.The other major concern is their relatively low intrinsic conductivity, which affects the rate at which they can absorb and release energy and influences the PCM's sensitivity to thermal changes [7][8][9].
To address the issue of liquid leakage, several innovative supporting structures have been introduced to ensure shape stabilization during phase transition.These include spinning technology, microencapsulation, physical absorption, and the use of porous supports.Porous materials, known for their high specific surface area, low density, and porosity, have found widespread application in energy storage, heat insulation, adsorption, and various other fields.Aerogels, in particular, have emerged as a preferred choice to prevent PCMs from leaking during their transitions and to simplify the packaging process.Compared to other porous materials used for shaping stabilized PCM composites, aerogels offer superior support for PCMs over a broader range [2,4,[10][11][12].Moreover, scientists have increasingly explored environmentally friendly substrates such as chitosan, cellulose, and sodium alginate to create 3D foam for PCM encapsulation.These green substrates are preferred due to their biocompatibility, non-toxicity, and ecofriendly properties [13][14][15][16][17][18].
The challenge of low conductivity can be effectively addressed by incorporating thermally conductive and photon-capture materials.Having a high thermal conductivity is of utmost importance in facilitating rapid heat transfer within PCM composites, thereby reducing the phase transition time when exposed to sunlight irradiation.To achieve this objective, scientists have investigated the utilization of black porous carbon network structures, renowned for their exceptional light absorption capabilities in the visible and near-infrared light regions abundant in solar energy.These black porous carbon-based materials exhibit great potential in photothermal or electric-thermal energy conversion and storage.Notable examples include carbon nanofibers, carbon nanotubes, graphite nanoplatelets, graphene, expanded graphite, and other porous carbon-based materials obtained from polymer foams or biomass through a carbonization treatment [19][20][21].Conductive polymers, such as polypyrrole (PPy), are also well-known as potential candidates for light/electric-to-thermal energy conversion.Not only do these polymers efficiently absorb light and conduct electricity, but they also facilitate the transfer of heat generated during the phase transition process [4,[22][23][24].By incorporating such materials, the thermal conductivity of PCM composites can be significantly enhanced, enabling efficient energy utilization and capture.
This study presents a bio-based approach for creating leak-free PCM composites with immense potential in energy storage and conversion.The proposed solution combines a biopolymer, such as sodium alginate, reinforced with natural fibers like flax fibers.To enhance thermal and electrical conductivity, crucial parameters for light/electric-to-thermal energy conversion applications, polypyrrole was in situ polymerized.Moreover, the incorporation of two fatty acids, i.e., palmitic acid and decanoic acid, with significantly different phase change temperatures into the structure further improved thermal inertia.The resulting conductive PCM composites demonstrated a relatively high fusion enthalpy, showcasing an exceptional ability to convert electrical energy into thermal energy along the conductive path.Additionally, they exhibited a remarkable capability to effectively capture and store solar energy.These properties open up new possibilities for widespread utilization in thermal energy harvesting, storage, and thermal management applications across various sectors, including electronics, buildings, transportation, and other relevant fields.

Fabrication of cross-linked sponge
To fabricate the cross-linked sponge, the following procedure was followed.Flax fiber (FF) was finely ground using a coffee grinder to obtain a suitable particle size.The ground FF was dispersed in distilled water inside a glass bottle.The bottle was then placed in an oil bath set to a temperature of 80 • C. The mixture of FF and water was stirred continuously for 1 h to ensure thorough mixing and dispersion.Sodium alginate (SA) was introduced into the system, and stirring was continued for an additional 2 h to allow for proper incorporation of SA.After that, the well-mixed solution was carefully poured into a Teflon mold, taking care to maintain its uniformity.The mold containing the mixture was frozen at a temperature of −20 • C and then subjected to vacuum drying for a duration of 48 h to remove the ice and water content, resulting in a freeze-dried sponge.To cross-link the sponge, the freeze-dried specimen was immersed in a 1 M calcium chloride solution for 5 min.The crosslinked sponge was then washed several times with distilled water to eliminate any unreacted calcium chloride molecules.After the washing process, the sponge was dried at room temperature, allowing it to reach its final state.
It is important to note that the weight percentage of SA and FF in the mixture was 3 %, and the proportion of each component was identical.Additionally, a control sponge composed solely of neat sodium alginate (3 wt%) was prepared to serve as a basis for comparison.

Preparation of polypyrrole-coated sponge, PPy@sponge
In this study, the surface polymerization of pyrrole was utilized to coat polypyrrole (PPy) on the surface of the sponge.The fabrication process involved the following steps.First, the dried cross-linked sponge was accurately weighed to determine its initial mass.After that, the sponge was impregnated with pyrrole monomer.The amount of pyrrole used was three times the weight of the sponge, a ratio determined through experimental observation to achieve complete coverage.The pyrrole-impregnated sponge was placed in a nitrogen atmosphere at a temperature of 5 • C and left undisturbed for 2 h.Subsequently, the sponge was immersed in an iron (III) chloride solution to allow for effective polymerization.The stoichiometric ratio of pyrrole to iron (III) chloride was chosen as 1/2.3 based on literature data, indicating that an initial mole ratio of approximately 1/2.3 yields the highest efficiency [25].The chemical polymerization process was continued for 4 h to promote the formation of the desired polypyrrole coating.
Once the polymerization was complete, the conductive sponge, now referred to as PPy@Sponge, was carefully removed from the solution.To eliminate any residual unreacted iron (III) chloride molecules and uncoated pyrrole, the PPy@Sponge was thoroughly washed multiple times with distilled water.The washed PPy@Sponge was then dried at room temperature for 48 h, allowing it to achieve complete dryness and stability.The change in mass of the sponge before and after PPy polymerization was used to determine the amount of PPy coating.Five measurements were taken, and the mean value was calculated.

Preparation of PCM-incorporated sponge, PCM/PPy@sponge
A vacuum infiltration method was used to integrate PCM into the fabricated sponge.An excessive amount of PCM, such as palmitic acid (PA) or decanoic acid (DA), was accurately weighed and placed in a beaker.The beaker containing the PCM was then positioned inside a vacuum oven pre-heated to 80 • C. Once the PCM was in a molten state, the PPy@Sponge was carefully immersed in the liquid PCM.After that, a vacuum was applied for a duration of 4 h to facilitate the infiltration of PCM into the sample.Following the infiltration process, the PCMincorporated sponge composite, referred to as PCM/PPy@Sponge, was subjected to leaking tests.For DA, the tests were conducted in an oven set at 50 • C, while for PA, the tests were conducted at 70 • C. The duration of the leaking tests was 24 h.After that, the sample was weighted (m f ), and the amount of loaded PCM (L%) was calculated using the below equation, in which m 0 is the initial weight of the sample.
As a control, a PCM-incorporated sponge, denoted as PCM/Sponge, was also prepared.Table 1 provides a summary of the samples' names and compositions.Furthermore, the digital photographs of some specimens are represented in Fig. 1.
Moreover, both pure DA and DA-incorporated samples were subjected to an oven maintained at a temperature significantly higher than the melting point of the PCM (50 • C) to visually demonstrate the absence of any leakage in the developed PCM composites.The digital photographs of the samples are demonstrated in Fig. 1.As the images demonstrate, the un-trapped DA completely melted and saturated the white paper on which the sample was placed.In contrast, the DA/ Sponge sample exhibited only minimal leakage of the trapped DA, confirming the successful encapsulation of the PCM within the porous 3D structure of the Sponge.Remarkably, this slight DA leakage became imperceptible after coating the Sponge with PPy.This phenomenon may be attributed to the reduced quantity of loaded DA, as indicated in Table 1, or to the provision of a more suitable porous structure for retaining DA following PPy polymerization.In essence, the PPy layer exhibited greater compatibility with DA than the pure Sponge.

Characterizations
The functional group present on the sample's surface was investigated by FTIR (Fourier-transform infrared).The analysis was conducted on a PerkinElmer spectrophotometer equipped with an ATR attachment.The FTIR spectra were recorded in transmittance mode within a wavenumber range of 4000 cm −1 to 500 cm −1 .A total of 16 scans were performed with a resolution of 4 cm −1 .For imaging the surface and cross-section of the sample, scanning electron microscope (SEM) micrographs were captured using a Zeiss instrument model Sigma VP.Prior to imaging, a thin layer of gold was applied to the sample to enhance conductivity.SEM imaging was conducted at an applied voltage of 2 kV, and various magnifications were employed to obtain detailed SEM images.The porosity of the sample was determined by measuring the mass of the specimen immersed in an ethanol solution [26].Initially, the sample was weighed (m 0 in g) and then immersed in ethanol for a duration of 8 h.Subsequently, it was carefully removed and immediately reweighed (m s in g).The porosity was calculated using the following equation, with ρ representing the density of ethanol (0.789 g/cm 3 ) and V (cm 3 ) indicating the apparent volume of the sample.
The compressive strength of the sample was evaluated using a compression test conducted on a Universal Tester Instron model 4204.A cylindrical sample was subjected to compression using a 5 kN load cell with a compression rate of 1 mm/min.Prior to the compression test, the sample was conditioned for 72 h at a temperature of 24 • C and a relative humidity of 55 %.The compressive modulus at 1 % strain and the compressive strength at 30 % and 70 % strain were measured and analyzed.Thermal stability analysis of the sample was conducted using thermogravimetric analysis (TGA) on a TA Instruments TGA Q500.Approximately 5 mg of the sample was heated from room temperature to 800 • C at a heating rate of 10 • C/min under a nitrogen flow rate of 60 ml/min.To measure the electrical conductivity of the sponge, a setup developed in our laboratory was utilized.Two silver electrodes were inserted into the sponge, with a fixed distance of 10 mm between them.The electrodes were carefully connected to a power supply.An ammeter was included in the circuit to measure the current, while a diode lamp was used to illustrate conductivity visually.The voltage was incrementally increased from zero, and the corresponding voltage readings were noted when the ammeter indicated the flow of current.The resistance (ohms) of the sample was calculated using the eq.R = V/I, and this value was used to calculate the electrical conductivity (σ in S/m) using Eq. 3.
Where L (m) is the distance between the electrodes, and A is the crosssection area (m 2 ) of the sample.The heat conductivity of the sample was assessed using a TCi Thermal Conductivity Analyzer (C-Therm) employing a modified transient plane source (MTPS) technique.A disk-shaped sample with a diameter of 30 mm was used for the analysis.The phase change performance of the sample was examined using differential scanning calorimetry (DSC) with a TA Instruments Discovery DSC 250 Auto.Around 5 mg of the sample was placed inside an aluminum pan and heated.For DA-loaded samples, the final temperature was set to 60 • C, while for PA-loaded ones, it was 90 • C. The sample was cooled down to 0 • C. Both heating and cooling cycles were conducted at a rate of 5 • C/min under a nitrogen flow rate of 40 ml/min.The heating and cooling cycles were repeated twice, and the phase change enthalpies, as well as the melting point, were determined from the second cycle.For DA/PPy@Sponge, a cycling test involving 100 heating and cooling cycles was performed to investigate the durability of the sample.The amount of loaded PCM was determined using the DSC results and calculated using Eq. 4. In this equation, ΔH m,pcm and ΔH c,pcm represent the melting and crystallization enthalpies of the pure PCM (e.g., DA or FA), while ΔH m,Sponge and ΔH c,

Table 1
The composition and physical properties of the fabricated samples.

M.R. Yazdani McCord et al.
Sponge refer to the melting and crystallization enthalpies of the sponge incorporated with PCM.
To evaluate the electricity-to-heat capability of the sample, a voltage of 5 V was applied to the material through the silver electrodes, with a distance of 40 mm (L) between them.The temperature change in the middle of the electrodes was measured using a thermocouple connected to a data logger.Furthermore, the light-to-heat capability of the sample was evaluated by exposing its surface to a studio lamp (1 kW) for a duration of 10 min.The angle was 45 • C, and the vertical and horizontal distances were fixed at 48 cm.The sample was situated within a thermally insulated vessel, ensuring that only its upper surface was in direct contact with the surrounding environment.Consequently, heat from the lamp was radiated to the sample, and the heat escaping from the sample was channeled exclusively through its upper surface.Following the exposure, the lamp was switched off, and the subsequent temperature variation at the midpoint of the sample on both sides was recorded for 10 min.Additionally, a thermal infrared camera was utilized to monitor the temperature changes on the surface of the sample.

Chemical structure
The chemical structure and functional groups of the developed samples were examined using FTIR spectroscopy.Fig. 2a and Fig. 2b illustrate the spectra of the plain materials, including SA, FF, DA, and PA.Additionally, Fig. 2c presents the spectra of Sponge, PPy@Sponge, DA/PPy@Sponge, and PA/PPy@Sponge.To facilitate comparison, Fig. 2d shows the FTIR spectra of the individual components with that of PA/PPy@Sponge.The Sponge exhibited characteristic peaks corresponding to both SA and FF.Specifically, the typical peaks of cellulose, observed at 3400, 2900, and 1100 cm −1 , corresponding to -OH, -CH, and -CO stretching vibrations, respectively [27], indicated the presence of FF in the sample.Similarly, the bands at 3345 cm −1 and 2922 cm −1 were attributed to -OH stretching and -CH stretching in the alginate structure.
Peaks around 1618 cm −1 and 1419 cm −1 corresponded to the symmetric and asymmetric vibrations of -COO groups in sodium alginate.Furthermore, the peak at 1025 cm −1 corresponded to CO stretching [28].
In addition to the aforementioned bands for FF and SA, the PPy@-Sponge exhibited characteristic peaks reported for PPy [29].Notably, a broad peak at 3400 cm −1 was associated with stretching modes of C -N from PPy, while a peak at 2850 cm −1 corresponded to the aromatic C-H stretching vibrations.The bands at 1587 cm −1 , 1555 cm −1 , and cm −1 were associated with the fundamental vibrations of C --C and C-C of the protonated pyrrole ring.Furthermore, a wide and low-intensity band at 1285 cm −1 corresponded to the in-plane vibration of C=C-N.Peaks at 1167 cm −1 and 1124 cm −1 originated from the vibrations of C-N and C-H in-plane aromatic stretching vibrations of PPy.
Moreover, the PCM-incorporated samples exhibited characteristic peaks for FF, SA, and PPy and also confirmed the presence of PCM (DA or FA).Common peaks associated with DA and PA, which are typical fatty acids [30,31], included bands at 2920 cm −1 , 2853 cm −1 , and cm −1 , assigned to the vibrations of methyl (-CH 3 ), methylene (-CH 2 ), and carbonyl (-CO) bonds.Additionally, in-plane and out-plane bending vibrations of hydroxyl (-OH) were observed at 1290 cm −1 and 930 cm −1 .In conclusion, the FTIR spectra confirmed the presence of components within the developed samples, as well as the formation of a PPy coating layer on their surfaces through the employed chemical polymerization.

Microstructure and porosity
The microstructure and porosity of the developed samples were analyzed using SEM images.Fig. 3 presents the surface and fractured cross-section images of the fabricated samples.The surface of the supporting material, Sponge, exhibited a microstructure consisting of intertwined flax fibers covered by a sodium alginate matrix, with no sign of phase separation observed.The surface structure displayed a rough texture with a combination of porous and compact film-like morphology, similar to previously reported freeze-dried bio-based foams [32][33][34].On the other hand, the internal skeleton of the sample appeared mainly porous, with an irregular interconnected 3D structure and relatively smooth surfaces resulting from the sublimation of ice crystals during lyophilization.Additionally, the flax fibers were evenly distributed in the sodium alginate matrix without any significant phase separation, indicating good compatibility and interfacial bonding between the two materials.However, the pores were relatively large, had poorly defined internal walls, and experienced partial collapse during freeze-drying.This morphology suggested high porosity and relatively low bulk density.As shown in Table 2, this sample exhibited a porosity of 90.95 ± 1.12 % and a bulk density of 13.00 ± 0.71 kg/m 3 , values consistent with those reported for bio-based insulations developed using the freeze-templating method [18].These values indicate that a relatively high amount of molten PCMs could be trapped inside the 3D structure.Table 1 provides the loading percentages of 79.31 ± 2.12 % and 89.43 ± 3.02 % for DA and PA, respectively, which were infiltrated into the Sponge.Consequently, the surface of the Sponge was impregnated with fatty acids (Fig. 3 b1 and Fig. 3 c1), and most of the pores were filled with molten PCMs.Higher magnification images (the circular images) clearly illustrate the morphology of the infiltrated DA, uniformly coating all the surfaces.
Following in situ polymerization, as depicted in Fig. 3 a2 and d2, PPy particles were evenly deposited on the surface of the sponge, specifically on the flax fibers.The homogeneous coating of the flax fibers is advantageous for imparting conductivity to the sponge, as electrons can traverse a continuous conductive path throughout the sample.As a result, this sample exhibited a conductivity of 1.52 ± 0.07 S/m, as will be discussed further.Importantly, it can be observed that the in situ deposition of PPy did not significantly affect the original macropore channel structure of the sponge.In other words, similar to the plain sponge, the PPy@Sponge was expected to provide relatively high porosity and low bulk density.As summarized in Table 2, the porosity and bulk density of the PPy@Sponge were determined as 85.21 ± 0.98 % and 24.97 ± 1.88 kg/m 3 , respectively.The changes in density and porosity compared to the plain sponge can be attributed to the loading of approximately 90.75 ± 3.21 % PPy after in situ polymerization (Table 1), which increased the bulk density and reduced the porosity.Similar to the plain sponge, these values suggest a high loading of molten PCMs due to capillary forces and physical adsorption [35].Thus, 59.75 ± 2.36 % and 73.84 ± 2.55 % DA and PA, respectively, were captured within the pores of PPy@Sponge through vacuum infiltration.As depicted in Fig. 3 b2 and c2, after the integration of PCMs (DA or PA), both the surface and cross-section exhibited a smooth and highly compacted structure, with most of the pores filled with PCMs, indicating effective inclusion of PCMs inside the sample during the infiltration process [36].Consequently, the bulk density increased from 24.97 ± 1.88 kg/m 3 in PPy@Sponge to 71.30 ± 3.87 kg/m 3 and 98.02 ± 4.02 kg/m 3 in DA/PPy@Sponge and PA/PPy@Sponge, respectively.The higher bulk density of PA/PPy@Sponge can be attributed to the higher loading of PA compared to DA, as provided in Table 1.
All in all, the morphological study confirmed the presence of a 3D porous structure obtained through freeze-drying of the sponge.Furthermore, a uniform coating of PPy was achieved throughout the sponge via in situ polymerization.After polymerization, the morphology and structure did not undergo significant changes, indicating the ability   3. SEM micrographs: a1) and d1) Sponge, b1) and e1) DA/Sponge, c1) and f1) PA/Sponge, a2) and d2) PPy@Sponge, b2) and e2) DA/PPy@Sponge, and c2) and f2) PA/PPy@Sponge. of the fabricated conductive sponge to accommodate a high amount of molten PCMs.Following vacuum infiltration of fatty acids (DA or PA), the morphology transformed from porous to compacted, with most of the pores filled with fatty acids, indicating a high loading of PCMs.These characteristics make the conductive PCM samples particularly interesting for energy storage and management applications.

Mechanical strength
The mechanical properties of foams, particularly their compressive characteristics, play a crucial role in determining their practical applications and long-term durability [37].To investigate the compressive mechanical performance of the developed samples, a widely adopted approach was employed.This involved subjecting the samples to constant compression using a testing platform.The obtained results are illustrated in Fig. 4a, while Fig. 4b to d compare the corresponding data, including the compressive modulus and compressive strength at 30 % and 70 % strain.These properties, extracted from the stress-strain curves, are representative of practical applications.For comparison, the data for plain SA sponge is also included.All samples exhibited typical stress-strain curves, displaying an elastic stage, plateau region, and strengthening stage or bulk compression region.During the elastic stage, stress and strain demonstrated a linear correlation.However, in the strengthening stage, stress changed rapidly as the sample underwent further compaction.This behavior aligns with the compression characteristics of open-cell porous materials and suggests a flexible deformation pattern for the developed samples [17].
The plain SA sponge exhibited relatively low mechanical properties, with a compressive modulus of 1.21 ± 0.07 MPa and a compressive strength of 0.03 ± 0.00 MPa at 30 % strain.Under applied force, the sample experienced approximately 65 % strain and was subsequently crushed between the grips.It is worth noting that the plain SA sponge without crosslinking displayed very poor compressive properties, quickly collapsing upon applying force.However, blending SA with FF significantly enhanced the mechanical stability of the sample, i.e., Sponge, during compression.The compressive modulus and compressive strength at 30 % strain increased to 4.17 ± 0.18 MPa and 0.07 ± 0.00 MPa, respectively.In other words, an improvement of approximately 344 % and 233 % was achieved for the compressive modulus and compressive strength at 30 % strain, respectively, by substituting half of the SA with FF.This improvement indicates good compatibility between FF and SA, likely attributed to the entanglement and physical interaction between uniformly distributed high aspect ratio flax fibers and SA polymer chains.Furthermore, the Sponge maintained its integrity during compression and provided up to 95 % strain when subjected to force.Notably, its ultimate strength, measuring 2.07 MPa, surpassed that of the reported compressive strength of the bio-polyol-based RPU foam (290 kPa to 340 kPa) [38].Moreover, it exhibits notably greater mechanical stability when compared to natural-based insulation foams.For instance, the compressive strength of shape-stable cellulose-based foam was reported as high as 80 kPa [39].Similarly, carboxymethyl cellulosebased foam demonstrated an ultimate strength of up to 120 kPa [40].In a separate relevant study, chitosan-based thermal insulation achieved a maximum compression strength of 100 kPa [41].The relatively high compressive mechanical properties of the developed Sponge make it a compelling alternative for a wide range of applications, including its potential as a support for the integration of PCMs, for example.
After the surface polymerization of PPy, the mechanical properties of the material showed limited changes in the linear region.However, a significant improvement in compressive strength was observed in the strengthening stage.For instance, the compressive strength at 70 % strain increased from 0.19 ± 0.01 MPa in Sponge to 0.55 ± 0.03 MPa in PPy@Sponge, representing an approximate 290 % enhancement.This improvement can be attributed to the coating of pore walls and FF surfaces with PPy molecules, which possess rigid aromatic backbones.This coating helps preserve the structure against deformation and prevents the collapse of structural cells.The hydrogen bonding interaction between PPy, SA, and FF also contributes to improved network stability under load [22].Additionally, the higher apparent density of the PPycoated sample compared to pure Sponge provides another reason for its increased mechanical stability.
Samples incorporating PCMs exhibited significantly higher compressive properties.This improvement can be attributed to the infiltration of PCMs such as DA and PA into the pores of PPy@Sponge, thereby increasing the supporting strength under compression.As mentioned earlier, the higher density of the samples after PCM incorporation also contributes to the enhanced mechanical properties.Furthermore, PCM-incorporated samples demonstrated a steeper slope in the linear region, indicating stronger rigidity under low-pressure stress [42].Importantly, it should be noted that the ultimate strength of all the developed samples falls within acceptable values for non-loadbearing applications [43].

Thermal stability
The thermal stability characteristics of the developed samples were determined using TGA.The TGA/DTG graphs of the plain components, including SA, FF, DA, PPy, and PA, are presented in Fig. 5a and Fig. 5b, while the curves of Sponge, PPy@Sponge, and PCM/PPy@Sponge are depicted in Fig. 5c and Fig. 5d.Additionally, the residual material at 800 • C is tabulated in Table 2.
The Sponge exhibited a four-step decomposition behavior.The first step occurred at a temperature below 100 • C, associated with an initial weight loss of approximately 5 %, attributed to the removal of moisture and absorbed water.In the second step, which took place between 130 • C and 250 • C, rapid degradation of the sponge occurred, corresponding to the pyrolysis of SA [44,45].The mass loss at 250 • C was 30.44 %.The third decomposition stage occurred between 250 • C and 500 • C, resulting from the depolymerization of hemicelluloses and a fraction of cellulose and lignin present in the flax fiber component [46,47].The mass loss in this region was 23.90 %.Finally, a minor mass loss above 400 • C was observed, attributed to the decomposition of lignin in the FF structure [48].The mass loss during this decomposition stage was 12.94 %.The last two decomposition steps, related to the degradation of FF, are clearly visible in the TGA thermogram of plain FF shown in Fig. 5a.The remaining 32.72 % of the residue consisted of inorganic salt (calcium chloride) and some carbon.
By comparing the thermal decomposition behavior of SA and Sponge, it can be concluded that the thermal stability significantly improved after blending SA with FF.This improvement could be due to cellulose, hemicellulose, and lignin in FF, which can contribute to the thermal stability of the Sponge.Additionally, it has been reported that the charring effect of FF acts as an impediment, inhibiting the release of volatile substances and thereby enhancing the thermal stability of FFbased composites [49].
The aforementioned thermal decomposition steps can also be observed in PPy@Sponge; however, the mass loss in each step significantly differs from that of Sponge.Specifically, the mass loss below   • C, between 250 • C and 500 • C, and from 500 • C to 800 • C were 20.37 %, 19.40 %, and 23.33 %, respectively.Furthermore, the residual material at 800 • C was 36.95 %.In other words, after PPy polymerization, the thermal stability of the Sponge improved significantly below 500 • C, which could be attributed to the higher thermal stability of PPy compared to SA and FF at this temperature.The higher mass loss observed after 500 • C in the PPy@Sponge can be attributed to the decomposition of the coated PPy, as the major thermal decomposition of PPy is known to occur in the temperature range of 410 to 560 • C [50].However, the mass residue at 800 • C was nearly identical for plain PPy (Fig. 5a) and Sponge, making it impossible to calculate the amount of coated PPy from TGA results.
The PCM-embedded samples displayed distinct thermal decomposition characteristics.The samples exhibited a three-step decomposition behavior.A minimal weight loss of 0.78 % was observed below 100 • C, indicating a very low moisture content, possibly attributed to the presence of hydrophobic fatty acid.The second decomposition stage, which was the primary one, occurred between 150 • C and 300 • C.This step corresponded to a weight loss of 79.12 % and was attributed to the complete decomposition of fatty acid (e.g., PA) and partial degradation of SA, FF, and PPy.Finally, the last thermal decomposition step occurred after 300 • C, resulting in an additional weight loss of 8.66 %.Based on the remaining mass of each component, the loading mass fraction of PCMs in the PPy@Sponge was calculated to be approximately 70.17 % and 61.21 % in PA/PPy@Sponge and DA/PPy@Sponge, respectively.These values were consistent with the actual loading fraction of the corresponding samples obtained through experimental weighing (Table 1).This indicates that the PCMs were uniformly distributed within the sponge using the applied infiltration method.
Notably, the PCM-incorporated samples exhibited relatively high thermal stability below 200 • C, and their thermal decomposition temperatures were higher than the operating temperature range, considering that the melting temperature of these materials was significantly lower.Therefore, the mixture demonstrates satisfactory thermal stability, making it suitable for practical applications as thermal energy storage without any mass loss [51].

Electrical and thermal conductivity study
Electrical conductivity plays a crucial role in achieving fast electricity-to-heat conversion and effective management.However, both sodium alginate and flax fiber, like many other natural materials, exhibit poor electrical conductivity.Similarly, most PCMs, including fatty acids, possess very low electrical conductivity.To address this issue, the incorporation of conductive materials such as conductive polymers, like PPy, has been proposed.Achieving a high conductivity requires close contact between the conductive particles.However, in porous materials like aerogels and sponges, ensuring appropriate particle contact can be challenging.
In this study, the electrical conductivity of the developed samples was evaluated, and the results are presented in Table 2. Additionally, Fig. 6 showcases a digital photograph demonstrating the light-emitting behavior of diodes connected to DA/PPy@Sponge and PA/PPy@-Sponge.The conductivity of the Sponge was beyond the measurement capabilities of the setup, indicating its electrically insulating property.Likewise, the electrical conductivity of both the PA/Sponge and the DA/ Sponge without any coating fell below the detection limit of our measurement setup (data are not provided here), which was expected given the absence of any electrically conductive components in their structures.However, when PPy was polymerized, the resulting sample, referred to as PPy@Sponge, exhibited a conductivity as high as 1.52 ± 0.07 S/m.This confirms that PPy was in situ coated on the sponge using an oxidative initiation method, forming a conductive polymer path.The SEM images further supported this finding.As suggested by Chen et al. [22], the integration of PPy with the sponge was facilitated by hydrogen bonding interactions between the hydroxyl groups in SA, FF, and the amino groups of the pyrrole ring.It is worth noting that higher electrical conductivity has been reported for PPy-based composites, mostly in nonporous structures [52].After the incorporation of PCMs such as DA and PA, the electrical conductivity decreased to 0.32 ± 0.01 S/m and 0.24 ± 0.02 S/m, respectively.This reduction can be attributed to the loading of relatively high amounts of electrically insulated materials, such as DA and PA, into the conductive sponge.
It's worth noting that a wide range of electrical conductivities has been reported for conductive PCM composites.For example, an electrical conductivity of 178 S/m was documented for a shape-stable phasechange composite comprising cellulose nanofiber and polyethylene glycol, which was coated with PPy [22].Similarly, PPy aerogels prepared using ionic surfactant-oleic acid induction achieved an electrical conductivity of 23.9 S/m [2].Additionally, composite phase-change materials consisting of polyethylene, carbon nanotubes (CNTs), and paraffin wax exhibited an electrical conductivity as high as 6.37 S/m [53].The relatively lower electrical conductivity observed in our study can be attributed to the porous structure of the developed sponge, potentially impeding the efficient flow of electrons through the material.However, it's important to emphasize that the obtained electrical conductivity in our study was still significant for studying electricity-to-heat conversion at relatively low voltage, as will be discussed in the following section.
Thermal conductivity is another crucial property of PCM composites that enables the efficient conversion of solar light into heat and facilitates the timely harvesting and transport of thermal energy [21].However, PCMs often exhibit poor thermal conductivities, which restrict their applications in specific areas such as light-to-thermal energy conversion and storage.Fortunately, the thermal conductivity of PCMs can be improved by incorporating electrically conductive fillers that create compact pathways for both electrical and heat conduction within the composite [4].PPy-coated substrates have been shown to possess high thermal conductivity and excellent light absorption capabilities in the visible and near-infrared light regions, which are concentrated in solar energy [11,12].
To evaluate the impact of PPy incorporation on the thermal transfer performance of the samples, the thermal conductivity of the developed samples was measured, and the results are presented in Table 2.The plain sponge material exhibited a relatively low thermal conductivity of 0.035 W/mK, indicating its thermal insulation properties.This value aligns with those reported for bio-based insulations [17,54].Likewise, both the PA/Sponge and the DA/Sponge without any coating exhibited relatively low thermal conductivity, measuring approximately 0.04 W/ mK for both, given the porous structure of the sponge (data are not provided here).In contrast, the PPy-coated sample demonstrated a significantly higher thermal conductivity of 0.453 W/mK, marking a 13fold increase compared to the pristine sponge.This substantial improvement in thermal conductivity can be attributed to the polymerization of PPy, which formed a thermally conductive network within the sponge.However, after the infiltration of DA and PA, the thermal conductivity decreased to 0.31 ± 0.01 W/mK and 0.28 ± 0.01 W/mK, respectively.This reduction can be attributed to the high loading of intrinsically low thermal conductivity PCMs, such as DA and PA.Nevertheless, these results are consistent with those of conductive PCM composite foams used in photothermal conversion applications.For instance, Tao et al. [11] pioneered the development of a PPy-deposited CNTs heterogeneous porous aerogel as a supporting matrix for phase change materials (PCMs) with paraffin wax (PW).Their research yielded an impressive thermal conductivity of up to 0.64 W/mK, while the PCM composite exhibited outstanding performance in solar-to-thermal energy conversion and storage.Additionally, a composite PCM featuring PPy/Fe3O4-functionalized hollow kapok fiber aerogel supports with paraffin wax achieved a remarkable thermal conductivity of 1.06 W/ mK, specifically tailored for thermal energy conversion and storage [12].Similarly, a high thermal conductivity of 1.0621 W/mK was achieved in a scaphigerum/graphene hybrid aerogel incorporating PEG, designed for energy storage applications [55].Furthermore, Ye et al. [21] contributed to the field with their work on PPy-coated conjugated composite PCMs, which exhibited a substantial thermal conductivity of 0.344 W/mK, demonstrating their suitability for solar/electric energy conversion and storage.
In conclusion, the investigation of electrical and thermal conductivity suggests that the developed conductive PCM composites, exhibiting appropriate thermal and electrical conductivity, hold great potential for applications in electricity/photo-to-heat conversion.This, in turn, opens up new possibilities in thermal storage and management applications [5].

Thermal energy storage and conversion
3.1.6.1.Phase change properties.DSC analysis was conducted to determine the phase change temperatures and investigate the heat-releasing and absorbing capacity of pristine PCMs, namely DA and FA, as well as the PCM-incorporated samples.The corresponding DSC curves are presented in Fig. 7, and the relevant data can be found in Table 3.
The plain DA and PA exhibited a prominent endothermic peak at 32.00 • C and 62.75 • C, respectively, indicating their phase change temperature or melting point.These components also displayed relatively high enthalpy values.The latent heat values of the phase change associated with melting were found to be 174.79J/g for DA and 210.23 J/g for FA.During the cooling process, DA crystallized at 26.46 • C, while FA crystallized at 57.99 • C, both accompanied by realizing almost similar enthalpy values absorbed during the heating cycle.The phase change temperatures and enthalpies observed were consistent with those reported in the literature for DA and FA [56,57].In other words, both DA and PA demonstrated typical PCM properties by releasing and absorbing sufficient energy during phase transition.
In contrast to DA and FA, both Sponge and PPy@Sponge did not exhibit any distinct phase change temperatures within the tested temperature range.Their DSC thermograms showed no clear melting or crystallization peaks.However, all PCM-incorporated samples displayed one clear endothermic peak and one clear exothermic peak, similar to the pristine fatty acids (FA and DA).This behavior indicated the phase change properties of the trapped PCMs.In other words, the phase change performance stemmed from the PCMs, while the sponge served as a support to retain the fatty acids.As indicated in Table 3, the loading ratio was relatively high for all prepared samples, indicating efficient retention of the fatty acids within the porous structures through capillary and surface tension forces [58].Additionally, the melting/solidifying temperatures of the fatty acids slightly increased after loading in the sponges.This could be attributed to the porous structure's interference, weak intermolecular hydrogen bonding between the components, and the confinement effect of fatty acids [59,60].
More specifically, the DSC curves of DA/Sponge and DA/PPy@-Sponge exhibited a single melting peak and a single crystallizing peak associated with the fusion and solidification phase change of decanoic acid.Compared to pristine DA, the composite materials showed reduced latent heat.The melting and solidification latent heats of DA/Sponge were determined to be 134.32J/g and 133.29 J/g, respectively, corresponding to 76.85 % and 78.77 % of pure DA.The loading ratio was 77.79 %, which closely matched the experimental loading ratio reported in Table 1 (79.31 ± 2.12).This confirmed that the interactions between DA and the inner surfaces of the Sponge did not significantly affect the phase change properties of DA.The enthalpies were lower in DA/ PPy@Sponge, but the loading ratio remained consistent with the experimental data.In other words, the amount of trapped DA in DA/ PPy@Sponge was slightly less than that in DA/@Sponge, potentially due to the lower porosity of the sample, which provided less area for DA infiltration.
Similar conclusions can be drawn for the PA-incorporated sample.However, some differences are worth noting.Despite using the same substrates and infiltration conditions for both DA and PA, the PA loading ratio in both Sponge and PPy@Sponge was higher than that observed in the DA-incorporated samples, suggesting better compatibility between PA and the substrate.Additionally, in the PA/Sponge sample, a shoulder peak was observed during the heating cycle, indicating the interruption of the melting process of palmitic acid after loading into the sponge.A similar shoulder peak was visible during the cooling cycle, where the imperfect crystals crystallized earlier than normal.This could be attributed to the restriction of crystal arrangement caused by the porous substrate and the orientation of PA molecular chains within the porous substrate, leading to a decrease in the regularity of crystalline regions and an increase in lattice defects [61,62].Consequently, the formed imperfect crystals decomposed at a slightly higher temperature than the normal melting point of PA.It is worth noting that the shoulder peak was not observed in PA/PPy@Sponge, indicating the positive effect of PPy on the crystallization of PA.
It is worth noting that, as shown in Table 3, the temperature difference between the melting temperature and crystallization temperature (ΔT) increased when phase change material (PCM) was integrated into the support matrix.For example, in the case of the DA-incorporated PCM composite, ΔT increased from 5.54 • C to 8.72 • C, while for the PAincorporated PCM composite, it remained within the expected range, consistent with existing literature [63].This phenomenon can be attributed to the entrapment of PCM molecules within the porous structure of the matrix, leading to intermolecular secondary interactions that influence their crystallization behavior and result in a slightly higher degree of supercooling.Such behavior is commonly observed in shape-stabilized PCMs with various support matrices.
The DSC results have confirmed that the composite PCMs, incorporating DA and PA, maintain a high latent heat, consistent with values reported for PCM composites based on fatty acids.For instance, Li et al. [64] achieved a remarkable latent heat value of 129.27 J/g in a novel polyurethane sponge incorporating myristic acid, which also demonstrated simultaneous solar energy conversion and storage capabilities.Similarly, Zhang et al. [65] developed a sebacic acid/CNT sponge phase change material with an impressive latent heat of 131.8 J/g, designed for energy storage and photothermal applications.Moreover, a paraffin wax integrated into porous carbon from a loofah sponge exhibited a melting enthalpy of up to 146.10 J/g, making it suitable for energy storage applications [66].Altogether, the PCM composites developed in our study show significant promise for use in thermal regulation, thermal storage management, and applications within the realm of solar thermal energy storage.
3.1.6.2.Durability test.PCM must demonstrate a considerable level of long-term capability to meet the performance requirements of thermal energy storage [67].Therefore, a durability or cycling test was conducted to assess the ability of the developed PCMs to maintain their thermal properties after multiple melting-solidification cycles.DA/ PPy@Sponge was subjected to 100 melting-solidification cycles.The

Table 3
Phase change properties and PCM loading percentage.ΔT is the temperature difference between melting and crystallization temperatures.complete cycle data, as well as a comparison of DSC curves before and after thermal cycling, is presented in Fig. 7e and f.The corresponding phase change enthalpies were also included in Fig. 7f.The results indicated that the phase transition temperature remained nearly unchanged even after 100 cycles.Likewise, the PCM samples exhibited consistent phase transition behavior with a high latent heat of fusion and solidification throughout multiple melting-solidification cycles.Specifically, the melting and crystallization enthalpies of DA/ PPy@Sponge changed from 99.55 J/g and 98.23 J/g, respectively, to 98.83 J/g and 97.74 J/g after the thermal cycling test, representing only a small reduction of approximately 1 % in phase change enthalpies.This minor change, which could be attributed to factors such as fatty acid leakage, changes in the chemical structure of fatty acids during thermal cycling, confinement of PCMs in the microscopic pores of Sponge, and measurement errors, was within an acceptable range for practical applications [61].These outcomes indicate that the incorporation of the fabricated sponge did not significantly impact the cyclic thermal energy storage effectiveness of the developed conductive PCMs.Additionally, it confirms that the fabricated conductive PCMs exhibited high thermal stability and reliability, with the ability to absorb and release a substantial amount of latent heat.Moreover, they demonstrated a relatively long service life, meeting the requirements for thermal reliability and reusability in thermal energy storage systems.

Composite PCMs application 3.2.1. Electricity-to-heat conversion study
The excellent conductivity of the samples enables rapid responsiveness to electrical stimuli.In order to investigate the electricity-to-heat capability of the developed samples, we monitored the temperature variation in the middle of the sample when exposed to a DC current of 5 V.The temperature change in the middle of the surface of the Sponge, PPy@Sponge, and DA/PPy@Sponge samples is plotted in Fig. 8a.As anticipated, no significant temperature change was observed for the plain Sponge after applying voltage.This can be attributed to its electrical insulation property, which has been previously observed.However, upon surface polymerization of PPy, the temperature of the PPy@Sponge sample increased rapidly over time until it reached a balanced temperature of approximately 105 • C. At this point, the input energy equaled the heat dissipation to the environment.Once the voltage ceased, the temperature of the sample quickly dropped to room temperature since the heat produced was not stored by PPy@Sponge [5].These results confirm the uniform coating of the insulated sponge with a conductive layer of PPy, resulting in a fast electrothermal response in the PPy@Sponge sample [22,68].
After incorporating PCM, namely DA, the sample exhibited electrothermal performance; however, significant differences were observed.For instance, in the temperature-time curve, the maximum temperature reached approximately 77 • C. Furthermore, the rate of temperature rise was altered, with the maximum temperature being achieved after 62 s, compared to around 30 s in the plain sponge.More importantly, after removing the applied voltage, the temperature reduction occurred at a remarkably slower rate.These observations indicate that the heat was transferred to DA around the conductive PPy, resulting in an initial rise in temperature followed by phase change accompanied by latent heat storage.During the withdrawal of the applied voltage, thermal energy in DA/PPy@Sponge was gradually lost to the environment.The lost latent heat was marked by the flattened part of the temperature-time curve.In other words, during the phase change period, the electrical energy was stored in DA and subsequently released during the cooling process.The presence of the plateau in the cooling curves indicates that DA/PPy@-Sponge was able to slowly release the heat converted from electrical energy [5].In conclusion, after the incorporation of DA, the conductive sample was able to store the generated heat as both sensible and latent heat, thanks to its relatively high phase change enthalpies.Furthermore, in this sample, the electrical energy was more efficiently converted into thermal energy as Joule heat along the conductive path.

Light-to-heat conversion study
The high light-to-heat conversion rate means that the material can store energy effectively, so the light-to-heat conversion performance is an important characteristic index of PCMs [2].To investigate the lightto-heat capability of the developed samples, the temperature variation in the middle of the sample exposed to a light source was monitored.Fig. 8b to Fig. 8f illustrates the temperature change at the midpoint of the Sponge, PPy@Sponge, DA/Sponge, and DA/PPy@Sponge on both sides.Furthermore, Fig. 8g and Fig. 8h present the thermal camera images captured from Sponge, DA/Sponge, and DA/PPy@Sponge samples after 600 s of exposure to the light source (heating step) and 600 s after switching off the light source (cooling step), respectively.
Upon 10 min of irradiation, the surface temperature of the Sponge reached a maximum of 39 • C. Once the light was turned off, the temperature on the surface quickly returned to the ambient temperature.A similar trend was observed on the backside of the sample, with a maximum achieved temperature of 32.5 • C.This indicates the sample's low thermal conductivity in conducting surface heat, as previously observed.However, after coating the sample with PPy, a sharp increase in temperature was observed due to the instant absorption of sunlight and the simultaneous occurrence of photothermal conversion [21].The maximum surface temperature reached 50 • C, indicating significantly enhanced thermal absorption properties compared to the plain sponge.In other words, this sample absorbed more energy from the light source than the plain sponge and converted it into heat energy through the photothermal effect of PPy, resulting in a higher temperature increase.It's worth mentioning that other researchers have confirmed the black surface of PPy-based aerogels as being more effective in capturing nearinfrared and visible light [2,69].Similar to the plain sponge, the temperature rapidly dropped to room temperature after turning off the light source.The maximum temperature on the backside of the PPy@Sponge reached 45.5 • C after 10 min, which was 91 % of the maximum temperature on the surface, indicating relatively high thermal conductivity of the sample in conducting heat through the specimen, thanks to the highly thermally conductive PPy coating.
In the PCM-incorporated sample, such as DA/Sponge, the maximum surface temperature was significantly lower than that observed for the plain sponge.Specifically, the maximum achieved temperature was approximately 33.5 • C, indicating that under continuous illumination, the DA/Sponge underwent a phase transition at the phase change temperature of 32 • C, effectively storing the energy and preventing further temperature increase.After the light radiation was turned off, the temperature dropped sharply and then slowly reached a steady value, releasing latent heat during the phase change from liquid to solid.This is also the main reason why the temperature did not reach the ambient temperature after removing the light source.During the phase change from liquid to solid, the crystallization enthalpy was released into the environment, increasing the temperature above room temperature.
Upon coating the DA/Sponge with a conductive PPy layer, in the DA/ PPy@Sponge temperature plot, a melting plateau appeared in the photothermal conversion curve within the temperature range of 33.5 to 35.5 • C.This indicated the phase change of DA from solid to liquid, effectively storing heat and stabilizing the temperature.In other words, the temperature rose from room temperature to 33 • C in 35 s, followed by a phase change stage lasting approximately 100 s.This relatively short phase-changing time demonstrates that this sample has a good capacity to capture and convert solar energy into thermal energy [70], thanks to its relatively high thermal conductivity.The temperature quickly ascended after the energy storage process, reaching a maximum of 48.25 • C. The maximum surface temperature was not as high as that of the PPy@Sponge sample, which could be attributed to the lower thermal conductivity of DA/PPy@Sponge, as previously discussed.It's worth highlighting that the dramatic increase of 15 • C in the maximum temperature after PPy coating indicates an enhancement in the photothermal characteristic.Similar to what was observed for DA/Sponge, when the light source was turned off, the temperature dropped sharply and then stabilized due to the phase change of the trapped DA from liquid to solid.The release process of latent heat took much longer than the thermal energy storage process, revealing the remarkable energy storage and release capacity of DA/PPy@Sponge [21].Overall, these results demonstrate that the conductive PCM sample possesses the capability to efficiently capture and store solar energy, positioning them as promising candidates for applications in energy storage and management systems.

Conclusions
In this study, a bio-based solution was implemented to stabilize organic PCMs, specifically decanoic acid (DA) and palmitic acid (PA), in order to develop efficient energy storage and conversion systems.A porous sponge was fabricated using sodium alginate and flax fiber through the freeze-drying method.To enhance the thermal and electrical conductivity of the fabricated sponge, polypyrrole coating was designed via in situ polymerization, resulting in electrical and thermal conductivities of 1.52 ± 0.07 S/m and 0.453 ± 0.02 W/mK, respectively.Although the porosity slightly decreased from 90 % to 85 % after polymerization, it remained sufficiently high for loading a significant amount of the PCM.Subsequently, the successful integration of DA and PA, 60 % and 74 %, respectively, into the conductive sponge's pores led to a high phase change enthalpy of 100.98 J/g and 154.52 J/g in the conductive sponges containing DA and PA.The conductive sample containing DA exhibited excellent durability, as the phase change properties remained relatively unchanged even after undergoing 100 melting-solidification cycles, indicating a long service life.Furthermore, the DA-incorporated conductive PCM demonstrated remarkable electricity and light-to-heat characteristics, benefiting from the synergistic conductivity and phase change performance it offered.These newly developed PCMs have the potential to pave the way for innovative energy storage and conversion materials, finding applications in various fields such as electronic protection, energy-saving buildings, military stealth, transportation gears, and solar thermal energy utilization.

Fig. 1 .
Fig. 1.The digital photograph of the fabricated samples as well as DA, DA/Sponge, and DA/PPy@Sponge before and after heat treatment at 50 • C.

Fig. 2 .
Fig. 2. a) and b) FTIR spectra of the plain components.c) FTIR spectra of the Sponge, conductive Sponge, and PCM-incorporated Sponge.d) FTIR spectra of the individual components and PA/PPy@Sponge.

Fig. 5 .
Fig. 5. a) TGA and b) DTG plots of fabricated samples.c) TGA and d) DTG plots of the raw components.

Fig. 6 .
Fig. 6.Digital photograph showing light-emitting behavior of diodes connected to a) DA/PPy@Sponge and b) PA/PPy@Sponge demonstrating electrical conductivity behavior of the developed sponge materials.

Fig. 7 .
Fig. 7. a) and b) DSC thermograms of the plain components, Sponge, PPy@Sponge, and PCM-incorporated samples.The curves represent the second heating/cooling cycle performed under a nitrogen atmosphere.Thermal cycling results for DA/PPy@Sponge: c) the entire 100 cycles and d) the 1st and 100th cycles.

Fig. 8 .
Fig. 8. a) Temperature changes over time in the developed samples after applying electricity.b) to e) Temperature change over time in the developed samples after exposure to light.The plots display temperature changes for both sides of the samples.f) Comparison of the surface temperature changes among different samples.The images below show the thermal camera images captured from Sponge, DA/Sponge, and DA/PPy@Sponge samples (g) after 600 s of exposure to the light source (heating step) and (h) 600 s after switching off the light source (cooling step).

Table 2
Physical properties, residual material at 800 • C, and electrical and thermal conductivity of the samples.