Structurally stable cellulose nanofiber-based phase change aerogels for thermal management and acoustic insulation

Abstract

Phase change materials (PCMs) hold great potential for thermal regulation in energy storage systems. However, their practical application is often hindered by challenges such as low thermal conductivity, leakage during phase transitions, and environmental concerns. This study introduces the creation of cellulose nanofiber-based phase change composite aerogels (CPCAs), which consist of cellulose nanofibers (CNFs), carbon nanotubes (CNTs), and polyethylene glycol (PEG) as the primary PCM. The aerogels were spray-coated with polylactic acid (PLA) to enhance their hydrophobicity (water contact angle over 100°), structural integrity, and sound absorption. The integration of CNTs markedly enhanced thermal conductivity (up to 0.516 W m⁻¹ K⁻¹) and compressive strength (903.7 kPa at 70% strain). In contrast, the highly porous CNF network facilitated effective PEG encapsulation (∼93%) and a substantial latent heat storage capacity (melting enthalpy ≈161.8 J g⁻¹). The CPCAs had exceptional phase change stability, exhibiting minimal PEG leakage after 180 minutes at 70 °C, and maintained over 90% of their heat storage efficiency following 100 thermal cycles. CPCAs also exhibited significant broadband sound absorption, achieving a noise reduction coefficient (NRC) of around 0.45, surpassing commercial polyurethane foams. These structurally durable and moisture-resistant CPCAs with multifunctional capabilities provide a solution for thermal energy management and noise reduction for energy-efficient building environments.

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1. Introduction

With global development and enhanced need for luxuries, thermal management and acoustic insulation in buildings and automobiles have become a concerning challenge.^1,2 Taking it in context, industries are giving thermal and sound management more and more priority. This creates a great need for multifunctional materials that manage heat and noise simultaneously. In such materials, phase change materials (PCMs) play a vital role due to their heat absorption, release, and storage capabilities during phase changes, which results in a good thermal energy storage mechanism.^3,4 However, low heat conductivity and PCMs' leakage often undermine their effectiveness and stability.⁵ Leakage prevention and performance improvement can be achieved via the encapsulation of PCMs.⁶

The researchers have adopted many approaches to use aerogels as a template for PCMs to address structural integrity and thermal energy storage with the least leakage. Yang et al.⁷ developed graphene oxide-based aerogels for thermal energy storage, while Sun et al.⁸ synthesized carbon aerogels using the chelating effect between Fe³⁺ and gelatin to enhance the carbon skeleton. Similarly, Li et al.⁹ also employed Fe³⁺-gelatin chelation to strengthen carbon aerogels. Lee et al.¹⁰ prepared graphene-based aerogels, and Li et al.¹¹ fabricated polyvinyl alcohol (PVA)-based aerogels. Yan et al.¹² incorporated phase change microcapsules (MPCM), consisting of a polyurethane-acrylate shell and n-tetradecane core, into a cellulose nanofiber/PVA matrix. Wang et al.¹³ embedded silica aerogel particles into a PVA–PEG network to produce composite PCMs. These studies demonstrate notable improvements in thermal conductivity, mechanical strength, and heat resistance. However, many rely on complex chemical syntheses, non-renewable materials, or lack biodegradability, limiting their scalability and sustainability. Cellulose nanofiber (CNF) aerogels are very promising since they are lightweight, highly porous, and made from renewable sources. The porous structure of CNF aerogels is ideal for PCM encapsulation and stores thermal energy during phase transition. A comparison table (Table S1) highlighting the properties of different types of aerogels has been included in the SI of the revised manuscript.

Furthermore, higher thermal conductivity is required to optimize the thermal management ability (heat absorption, release, and energy storage) of PCM-loaded CNF aerogels. Researchers have looked into incorporating conductive additives, such as multiwalled carbon nanotubes (MWCNTs), into the aerogel matrix to enhance its thermal conductivity, which results in improved performance of PCM composites.^14,15 CNF aerogels are ideal for real-time applications because of their porosity, allowing them to be used for acoustic purposes and heat management.^16,17 Notwithstanding their desirable qualities, CNF-based materials are hydrophilic, which leaves them susceptible to moisture and causes structural instability and shape distortion under humid conditions. This makes it evident that hydrophobic elements should be incorporated for the robust functioning of the CNF aerogel-based PCM in the real environment and for maintaining its structural integrity.

Several studies addressed these issues by integrating MWCNTs in the aerogel matrices, which might improve the thermal and mechanical characteristics of CNF aerogels. The pure CNF and pure carbon nanotube (CNT) aerogels exhibit lower overall performance compared to the composites. Pure CNF lacks sufficient thermal conductivity, while pure CNT aerogels do not provide the necessary structural integrity and phase change properties.¹⁸ The synergistic combination of CNF with an optimal amount of CNT balances mechanical strength, thermal conductivity, and latent heat storage, resulting in enhanced overall performance. Mu et al.¹⁹ showed a slight increment in thermal conductivity of 0.049 W m⁻¹ K⁻¹ and enhanced mechanical strength by including MWCNTs in CNF aerogels compared to pristine aerogels. These aerogels were followed by polyacrylonitrile (PAN) encapsulation. Shen et al.¹⁸ used CNF aerogels as paraffin's porous support to avoid leakage, and CNTs to increase heat transfer. CNF treated with methyltrimethoxysilane increased foam-paraffin compatibility. The highly porous (>96%) foams absorbed over 90% of paraffin. The produced PCM composites demonstrated good heat transmission, phase change, and thermal stability. The prepared composite lost 6% enthalpy after 100 melting/freezing cycles. PCM composites have better form stabilities and thermal properties than pristine paraffin, suggesting use in a solar-thermal-electricity harvesting and conversion system. Liu et al.,²⁰ involved multi-step chemical modification (including CNF surface modification and PEG impregnation via vacuum), showing 85% thermal energy storage efficiency of composite aerogels. Even with great potential, hydrophobicity and enhanced acoustic insulation, multifunctional traits are not comprehensively addressed in the aforementioned studies. This positions the aerogels to be studied as advanced materials suitable for real-world applications, particularly in energy-efficient and sound-insulated building materials. Surface modification methods include coating with hydrophobic polymers like polylactic acid (PLA) to enhance cellulose-based aerogels' moisture resistance.²¹ PCMs in aerogel matrices effectively store thermal energy.²² Aerogels' porous structure traps PCMs, reducing leakage and preserving composite material structural integrity during phase transitions. Pore diameter and shape affect a material's acoustics.²³ This gives great potential to these materials for investigation in thermal and acoustic management in real-time applications.

Herein, we present a multifunctional CNF-based phase change aerogel (CPCA) designed to synergistically tackle key issues like thermal energy storage, acoustic insulation, mechanical robustness, PCM leakage, and environmental durability in one place. Efficient thermal buffering is provided by PEG, loaded in the aerogel's hierarchical porous network, which has a high latent heat capacity (161.83 J g⁻¹). The thermal conductivity (0.516 W m⁻¹ K⁻¹) and the CPCA's heat transfer speed were enhanced by incorporating MWCNTs into the matrix. Moreover, the microarchitecture of aerogels featuring linked nanofibers, high porosity, and MWCNT reinforcement enables broadband sound absorption (0.45 NRC), outperforming traditional CNF-based phase change composite. To eliminate PCM leakage, polylactic acid (PLA) coating was employed on the CPCA, which simultaneously enhanced hydrophobicity (contact angle >100°) and compressive strength (903.71 kPa). A prototype incorporating this composite material was prepared for proof of concept, demonstrating the synergistic benefits of thermal energy storage, acoustic damping, hydrophobicity, and mechanical integrity. Our study provides environmentally friendly materials' unique properties and functionalities, and bridges the gap between sustainable material engineering and multifunctional performance for building/transportation applications.

2. Experimental section

2.1 Materials

The wood debris of pigeon peas was obtained from a nearby farm in India. Changshu Hongsheng Fine Chemical Co. Ltd supplied ethanol with a purity of 99.9%. Toluene, sodium chlorite, glacial acetic acid, sodium sulfite, and potassium hydroxide were acquired from Himedia Pvt Ltd, an Indian company. MWCNTs (Type 10, 10–30 μm length) were obtained from Himedia Pvt Ltd, an Indian corporation. Polyethylene glycol (PEG, M. W. 1500) was obtained from LOBA Chemie Pvt Ltd in Mumbai, India. The experiment employed double-distilled water obtained from the laboratory's distillation apparatus. All chemicals utilized in this study were of analytical grade (AR).

2.2 Methods

2.2.1. Fabrication of CNF/CNT/PEG aerogels. The discarded pigeon pea wood sourced from nearby fields underwent a mechanochemical process to extract cellulose nanofibers, employing a methodology similar to our previous research.²² To produce CNF/CNT/PEG aerogels, different concentrations of CNT, between 0.05 and 0.5 weight%, are incorporated into 1.0 weight% CNF suspensions. The mixture is stirred at room temperature for at least 30 minutes to ensure thorough homogenization. PEG (15 weight%) is incrementally introduced to the CNF/CNT suspension prepared before and agitated at 60 °C for 3 hours. The mixture is subjected to sonication using a probe sonicator to achieve improved homogenization. The resulting suspension is frozen below −40 °C overnight. After freezing, the suspension is dehydrated in a freeze dryer (FDU 7006, OPERON International, Korea) under high vacuum conditions (below 2 torr), maintaining the condenser temperature at −60 °C. The drying process continues for at least 48 hours to achieve the final aerogel product. The aerogels were oven-dried at 70–80 °C for one hour to eliminate excess PEG and were then utilized for further investigation. The synthesized cellulose-based phase change composite aerogels were labeled as CPCA-n, with n indicating the concentration of CNTs.

The density of the samples was calculated by dividing the weight of the aerogels by their volume. An electronic digital calliper assessed each sample's dimensions, and M/s Wensar Weighing Scale Limited, India, determined the sample's weight (model no: DAB 200-D), with a readability of 0.0001 g. Aerogels' volume was calculated by assuming a cylindrical shape.

2.2.2. Spray coating of PLA on CNF/CNT/PEG aerogels. Following our prior research, a simplified spray coating technique was employed to fabricate a hydrophobic aerogel.²¹ A 3 weight% PLA solution has been made in chloroform (CHCl₃) solvent. The PLA solution was subsequently sprayed for coating the aerogel using a spray gun. The pressure was set at 1.5–2.3 bar, with an operational distance of 5 cm between the spray gun and the aerogel sample. The synthesised CPCA aerogels underwent spray coating with a formulated PLA solution. The coated aerogel was subsequently dried in a fume hood overnight at room temperature to promote the evaporation of the chloroform solvent; during this process, the PLA adhered to the aerogel surface, altering its wettability. The produced aerogels were stored in a vacuum to inhibit moisture absorption and employed for various characterisations.

2.3 Characterizations

The structural investigation of cellulose after the chemical refining of wood was conducted utilizing Attenuated Total Reflection (ATR) mode on a PerkinElmer Fourier Transform Infrared (FTIR) instrument (USA). The FTIR spectrometer functioned in transmission mode, spanning a spectral range of 4000 to 400 cm⁻¹, with a resolution of 4 cm⁻¹, and 32 scans were conducted.

An X-ray diffraction (XRD) examination was performed at ambient temperature via Japan's Rigaku Ultima IV instrument. The configuration included an X-ray tube functioning at 40 kV and 40 mA, producing CuKα radiation with a wavelength of 1.54 Å, and at a rate of 4° per minute across a 2θ range of 5° to 50°, diffraction patterns were obtained.

The morphological features of the composite CNF aerogels were analyzed using FESEM using a MIRA3 TESCAN (USA) apparatus, functioning at voltages ranging from 2 to 10 kV. Samples were coated with a thin layer of Au–Pd to augment conductivity before imaging.

The specific surface area and pore size distribution of the sample was determined using nitrogen adsorption–desorption isotherms based on the Brunauer–Emmett–Teller (BET) analysis. The isotherm measurements were carried out using a Nova Station A Quantachrome instrument over a relative pressure range of 0.05–0.20 at −196 °C.

Contact angle measurements were conducted on composite CNF aerogels using an apparatus produced by DSA25 KRÜSS GmbH in Germany to assess their hydrophobicity. During the analysis, a syringe meticulously controls dispensing a 5 μL droplet of water onto the sample's surface. The quantification of several droplets on diverse samples has been ascribed to the aerogel's uneven surface. A series of photos was captured with a temporal interval of 100 ms. The integrated software calculates the contact angle measurements, contingent upon the droplet's morphology on the sample.

A 5 kN load cell was utilized in an Instron Universal Testing Machine (Model 3365) for compression testing. Cylindrical aerogels, with a diameter of 30 mm and a height of 10 mm, were subjected to compression at a rate of 2 mm min⁻¹.

The enthalpy and phase change characteristics of PEG and CPCA aerogels were examined using differential scanning calorimetry (DSC) with a TA Instruments system from the United States. The experiments were executed in an aluminum pan, covering temperatures from 0 to 80 °C, with cooling and heating rates of 10 °C min⁻¹. They were carried out in a nitrogen environment at a 50 mL min⁻¹ flow rate. To evaluate phase change stability, materials underwent 100 phase change cycles within a 5 to 75 °C temperature range. Before testing, a quick thermal cycling technique was conducted to eradicate the samples' thermal history and residual stress.

The TGA was performed utilizing a TA device from the United States. The analysis encompassed a temperature spectrum from 30 to 700 °C in a nitrogen environment, with a heating rate of 10 °C min⁻¹.

Thermal conductivity was assessed utilizing the transient plane source method with a Hot Disc TPS 2500 apparatus (Hot Disc AB, Sweden). The measurement was performed at ambient temperature with an output power of 20 mW. In the 10-second measurement interval, a grey wire positioned two identical samples on opposing sides of a Kapton disc sensor. Thermal conductivity was assessed via the anisotropic approach, with each sample subjected to a minimum of four consecutive tests spaced 20 minutes apart. The aerogel's efficacy under various climatic circumstances was evaluated 24 hours post-drying at 40 °C and conditioning at 27 °C with 65% relative humidity. The specific heat capacity (C_p) required for anisotropic TPS measurements was ascertained utilizing a sapphire standard and differential scanning calorimetry (DSC). Composite aerogels were positioned on a heated plate at 70 °C and a cooled plate at 4 °C to get infrared thermographic images.

The sound absorption coefficients at normal incidence for the samples were measured using a Brüel & Kjaer impedance tube, model 4206, from Denmark. Cylindrical specimens with a diameter of 30 mm were employed to measure sound absorption coefficients.

3. Results and discussion

3.1 Design and fabrication of composite aerogels (CPCAs)

There is growing interest in CNF-based three-dimensional (3D) porous aerogels as support matrices for PCMs to reduce leakage. The interlinked 3D structure of these aerogels efficiently preserves PCMs owing to robust surface tension and capillary forces. Integrating PEG into lightweight CNF aerogels provides a direct method for producing stable PCM composites. The practical use of CNF-based aerogels is constrained by their intrinsically low thermal conductivity and suboptimal heat transfer efficiency.²⁴ To mitigate these constraints, CNTs are proposed as effective additions. Their graphitic structure promotes thermal conductivity and reduces PCM leakage.²⁵ This research additionally emphasizes enhancing moisture resistance, acoustic insulation, and thermal performance. A bio-derived PLA polymer is utilized through spray coating to attain these multifunctional attributes, which improves hydrophobicity, PCM loading capacity, and the aerogel's capacity to dissipate and absorb sound.^26,27Scheme 1 delineates the fabrication procedure of the composite aerogels. Table 1 displays the optimized weight ratios of CNFs, CNTs, PEG, and PLA, ascertained using an extensive series of characterizations elaborated in the subsequent sections. The table links component concentrations with the resultant aerogel density derived from observed mass and dimensions.

Scheme 1 Schematic diagram of the preparation of PLA-coated composite aerogels.

Table 1 Composition and densities of PLA-coated composite aerogels

S. no.	CNF (weight%)	CNT (weight%)	PEG (weight%)	PLA (weight%) for coating	Name of the sample	Density (g cm^−3)
1	1.0	0.05	15	3	CPCA-05	0.193 ± 0.036
2	1.0	0.1	15	3	CPCA-10	0.216 ± 0.015
3	1.0	0.2	15	3	CPCA-20	0.223 ± 0.024
4	1.0	0.5	15	3	CPCA-50	0.241 ± 0.032

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CNFs were effectively extracted from waste biomass by integrated chemical and mechanical treatment techniques. Our prior research comprehensively examined and recorded the structural, morphological, and thermal properties of the extracted CNFs.²⁸ Fig. S1(a–d) displays the FTIR spectra of CNF aerogel (CNFA), CNT, CNF/CNT composite aerogels, PEG, and PLA, respectively, highlighting the key functional groups present in each component. In Fig. S1a, CNFA exhibits characteristic absorption bands at 3329 cm⁻¹ (O–H stretching), 2900 cm⁻¹ (C–H stretching of CH₂/CH₃), 1651 cm⁻¹ (H–O–H bending), 1433 cm⁻¹ (CH₂ bending), 1054 cm⁻¹ (C–O stretching), and a minor peak at 900 cm⁻¹, indicating β-glycosidic linkages typical of cellulose.²² The sharp O–H peak observed for CNFA is due to its highly ordered crystalline or semi-crystalline structure, where extensive intra- and inter-molecular hydrogen bonding between hydroxyl groups results in a well-defined, narrow stretching band. Regarding the role of CNTs, the unfunctionalized CNTs were used, which are inherently chemically inert. As such, their contribution to altering the physical or chemical properties of the composite is minimal. To support this, FTIR analysis was performed on both pristine CNTs and CNF/CNT composite aerogels with varying CNT content, excluding PEG and PLA components. As shown in the FTIR spectra (Fig. S1b), the CNTs exhibit a broad O–H stretching peak at 3339 cm⁻¹ and characteristic peaks at 1651 cm⁻¹ and 1086 cm⁻¹ corresponding to C=C and C–O stretching, respectively.²⁹ Notably, the spectra of composite aerogels indicate that varying the CNT content does not result in the formation or alteration of chemical bonds, as the FTIR profiles closely resemble a combination of the individual CNF and CNT spectra. Fig. S1c shows a prominent peak at 3422 cm⁻¹ in PEG, corresponding to terminal hydroxyl groups. In Fig. S1d, PLA demonstrates distinct stretching vibrations for C=O at 1749 cm⁻¹, asymmetric –CH₃ at 2926 cm⁻¹, symmetric –CH₃ at 2854 cm⁻¹, and C–O at 1086 cm⁻¹. Additionally, PLA exhibits –CH₃ bending modes at 1461 cm⁻¹ (asymmetric) and 1371 cm⁻¹ (symmetric).³⁰Fig. 1a illustrates the FTIR spectra of the CPCAs, which display overlapping peaks corresponding to CNFA, PEG, and PLA. The absence of any new or shifted peaks suggests that no new covalent chemical bonds or interactions have formed among the components, confirming the physical blending of the materials.

Fig. 1 (a) FTIR analysis. (b) XRD analysis of CPCAs. (c) Schematic representation of hydrogen bonding and the mechanism of shape stabilization in CPCAs.

Further structural analysis is presented in the XRD spectra. The XRD pattern of CNF/CNT composite aerogels without PEG is presented in Fig. S2. The XRD patterns revealed distinct peaks corresponding to both CNF and CNT components. Specifically, the characteristic peaks of CNF were observed at 2θ values of 15.82°, 22.63°, and 34.54°, which are associated with the (110), (200), and (004) crystallographic planes, respectively, and confirm the presence of cellulose I-type crystalline structure.³¹ In addition, peaks at 26.09° and 43.05° correspond to the (002) and (100) planes of CNTs, respectively.³² Following PEG incorporation, the XRD spectra of pure PEG and CPCAs are shown below in Fig. 1b. The spectra display prominent peaks at 2θ = 19.04° and 23.39°, along with additional minor peaks at 15.06°, 26.54°, 36.24°, 39.76°, and 45.41°, which align well with the characteristic crystalline pattern of PEG.³³ These results indicate that PEG retains its crystalline structure within the CPCAs, confirming successful incorporation without disrupting its phase change characteristics. Fig. 1c illustrates a likely mechanism of hydrogen bonding and conformational stabilization. The hydroxyl groups on the surface of CNF aerogels rapidly formed strong hydrogen bonds (O–H⋯O) with the hydroxyl groups of PEG. The broadening of the O–H peak in CPCAs is due to the wider distribution of hydrogen bond environments within the composite matrix, resulting in a broader O–H stretching band. The hydrogen bonding stabilizes the PEG within the CNF matrix and facilitates its uniform dispersion. Concurrently, these hydrogen bonds augment the morphological stability of the CPCAs and facilitate the crystallization behavior of PEG.³³

3.2 Physicochemical analysis of CPCAs

The microstructure, mechanical, thermal properties, and physicochemical parameters of CPCAs were thoroughly examined using several methodologies. Fig. 2 illustrates the morphology of the synthesized aerogel (CPCA-20). Fig. 2a–d presents the SEM images of the cross-sections of CNF/CNT-20 composite aerogels with 0.2 weight% of MWCNT. The synthesized aerogel possesses homogeneous, finely fibrillar, and very porous networks.³⁴ Compared to pure cellulose aerogels, the morphology of aerogels comprising CNTs and cellulose exhibits no significant changes, showcasing numerous well-aligned pore structures that provide ample space for PEG encapsulation. Further, to confirm the aerogel structure by providing insights into surface area, porosity, and overall morphology, BET analysis is performed. The CNF/CNT-20 aerogel exhibited a BET surface area of 87.874 m² g⁻¹. The average pore diameter was approximately 10 nm, with a total pore volume of 0.217 cm³ g⁻¹. The pore size distribution ranged from 30 to 200 Å, indicating the presence of both mesopores and macropores—characteristics that distinguish the material from conventional foams and confirm its aerogel nature. These properties suggest that the aerogels are well-suited for phase change material (PCM) incorporation and are expected to exhibit good structural stability. Fig. 2e–h illustrates the surface morphology of CPCA-20 after PEG incorporation and PLA coating. The aerogel's porous surface is uniformly coated with a continuous PLA sheet featuring specially created microholes. The microstructures' result after chloroform evaporation indicates PLA's successful integration.²¹ The cross-sectional morphology of CPCA-20, illustrated in Fig. 2i–l, demonstrates that spray coating of PLA does not modify the internal structure of the composite aerogels. PEG is prominently dispersed within the interior pores and surfaces, occupying most of the aligned porous structure. This verifies the successful in situ encapsulation of PEG within the aerogel matrix. Furthermore, the unoccupied pores in CPCA-20 are essential for accommodating the volumetric expansion of PEG during phase transitions, hence successfully averting structural damage and leakage, enhancing the aerogel composite's thermal stability and longevity. The FESEM images of aerogels with varying CNT content are provided in the SI, providing clearer visualization and more intuitive comparison of structural differences. Fig. S3a–l presents the surface morphology of the CPCAs at different magnifications, showing a uniformly porous surface coated with a continuous PLA layer featuring distinct microholes. Fig. S4a–l displays the cross-sectional morphology of the CPCAs, also at various magnifications, confirming that the spray coating of PLA does not alter the internal structure of the composite aerogels. Additionally, the PEG appears to be homogeneously distributed throughout the aerogel matrix with different CNT content.

Fig. 2 FESEM images of (a–d) CNF/CNT-20 composite aerogel, (e–h) surface morphology of CPCA-20, (i–l) cross-sectional morphology of CPCA-20 at 500 μm, 20 μm, 2 μm, and 200 nm.

Further, the mechanical characteristics of the CPCAs are depicted in Fig. 3a and b. The compression stress–strain curve (Fig. 3a) illustrates the deformation characteristics of the aerogels subjected to compressive forces. The curve initially exhibits a sharp ascent in the elastic zone, transitioning smoothly into the plastic collapse phase, and ultimately, densification commences beyond a strain of around 50%. Fig. 3b illustrates the compressive strength of the CPCAs at deformation levels of 20%, 50%, and 70%. At lower strains, all aerogels demonstrate a modest enhancement in compressive strength up to 50% strain. Beyond this junction, the pore walls commence thickening, increasing apparent density and a more compact interior structure.³⁵ This densification improves the material's strength, leading to a more pronounced increase in stress from the collapse and compression of the porous structure.³⁶ An observable enhancement in compressive strength occurs with elevated aerogel density, from 386.55 kPa in CPCA-05 (density: 0.193 g cm⁻³) to 903.71 kPa in CPCA-20 (density: 0.223 g cm⁻³). At 70% compressive strain, the aerogels maintain their structural integrity with only a slight degree of irreversibility. The mechanical boost results from the synergistic reinforcement of CNF and CNT, which markedly enhances the toughness and strength of the aerogel scaffold.³⁷ The enhanced mechanical stability is vital for reducing leakage or structural deformation of the PEG when integrated into the scaffold, particularly at increased temperatures.

Fig. 3 (a) Compressive stress–strain curves at 70% strain. (b) Compressive stress at 20%, 50%, and 70% strain rates of CPCAs. (c) TGA and DTG thermograms of CPCAs. (d) Contact angle analysis of CPCAs.

The TGA and DTG thermograms of pure CNF, PEG, PLA, and CNF/CNT-20 are provided in the Fig. S5. The findings, such as onset temperature (T_on), maximum temperature (T_m), and residual char%, are summarized in Table S2 of the SI of the revised manuscript to support the thermal analysis. The TGA and DTG analysis of pure CNF aerogel (CNFA) shows the most significant reduction in weight between 200 and 400 °C, which was associated with the degradation of cellulosic and noncellulosic components. PEG (MW 1500) exhibits a two-step degradation: the first step occurs below 200 °C, attributed to shorter polymer chains, while the second corresponds to main chain breakdown, leaving negligible char residue.²² PLA, in contrast, undergoes a single-stage degradation centered around 300 °C.³⁸ Additionally, the TGA and DTG profiles of the CNF/CNT-20 composite show thermal behavior consistent with the individual degradation patterns of CNF and CNT.³⁹ Further, the TGA and DTG profiles of CPCAs are shown in Fig. 3c to evaluate the thermal stability. The profiles indicate that CPCAs exhibit a weight loss trend akin to pure CNF, PEG, and PLA, affirming the inclusion of all three components in the composite structure without substantial chemical modification. The thermal deterioration transpires in two separate phases, as outlined in Table S3. The initial phase is chiefly linked to the degradation of PEG, whereas the subsequent phase pertains to the disintegration of stereocrystalline domains within the PLA component.⁴⁰ This multi-phase deterioration behavior suggests that although each component retains its intrinsic thermal properties, their interaction within the composite improves overall performance. The PEG chains are efficiently restricted within the porous CNFA matrix due to surface tension, capillary forces, and hydrogen bonding interactions.⁴¹ These physical restrictions restrict PEG's mobility, decelerating its thermal deterioration and extending its release. The encapsulation of PEG not only maintains its phase transition capabilities but markedly enhances the thermal stability of the CPCAs, rendering them more dependable for thermal energy storage applications.

Water contact angle measurements were performed to investigate the hydrophobic behavior of CPCAs, as illustrated in Fig. 3d. The findings unequivocally demonstrate that the hydrophobic polymer PLA surface coating substantially improves the water-repellent characteristics of the aerogels. In its uncoated state, CPCAs demonstrate hydrophilic properties owing to surface hydroxyl groups from CNF and PEG, which readily engage with water molecules. After surface modification with PLA, the aerogels exhibit pronounced hydrophobicity. This transition is ascribed to the nonpolar characteristics, restricted polar functional groups, and semi-crystalline structure of PLA, which collectively impede efficient interaction with water.⁴² The measured water contact angle values in Table S4 further validate this behavior change. The coated aerogels demonstrate contact angles beyond 100°, signifying a highly hydrophobic surface.

The physicochemical properties, such as porous morphology, increased mechanical strength, thermal stability, and hydrophobicity, render CPCAs exceptionally attractive for various thermal and acoustic insulation applications. The subsequent sections explore their thermal energy storage capabilities and acoustic characteristics.

3.3 Thermal performance of CPCAs

3.3.1. Thermal conductivity analysis. Many organic or bio-based PCMs possess the intrinsic drawback of low thermal conductivity, which considerably impedes their thermal diffusion efficiency. Improving thermal conductivity is essential to enhance effectiveness in energy storage and thermal regulation applications. Prior research has shown that employing numerous fillers can yield a synergistic improvement, resulting in enhanced thermal performance.^43,44 This enhancement is primarily ascribed to the interfacial interactions among thermally conductive fillers, which facilitate the formation of continuous thermal channels and attain elevated conductivity, even at minimal filler loadings. Carbon-based filters are distinguished by their remarkable thermal conductivity among diverse conductive materials, principally attributed to effective heat transmission through lattice vibrations.⁴⁵ Consequently, incorporating a carbon network into composite phase change materials offers a viable approach to enhance thermal performance.⁴⁴ This study inserted MWCNTs into CPCAs at concentrations between 0.05 weight% and 0.5 weight% to establish efficient thermal conduction channels. The thermal conductivity of the CPCAs at different CNT concentrations was assessed, with the results depicted in Fig. 4a and summarised in Table S5. The thermal conductivity of CPCAs significantly rises with increasing CNT content. The thermal conductivity increases from 0.149 W m⁻¹ K⁻¹ to 0.516 W m⁻¹ K⁻¹. This enhancement illustrates that the continuous three-dimensional conductive network of CNTs effectively and reliably promotes thermal conduction in the CPCAs. Fig. 4b illustrates the variations in temperature for the CPCAs. A thermal imaging camera was employed to monitor temperature variations in the materials during the heating phases at various intervals. The results demonstrate that the improved thermal conductivity of the phase change aerogels facilitates uniform thermal energy distribution throughout the sample, thereby reducing undesirable rapid temperature fluctuations in certain regions.⁴⁵

Fig. 4 (a) Thermal conductivity of CPCAs. (b) IR camera images of CPCAs on a heated plate at various time intervals. (c) Liquid leakage test of CPCAs for 180 minutes at 70 °C. (d) The leakage percentage of composite aerogels at different time intervals. (e) Dimension retention of CPCA-20 for 60 minutes after a load of 200 g during the heating process.

3.3.2. Shape stability and thermal reliability. The structural stability of the CPCAs was assessed by exposing the samples to a temperature of 70 °C for 180 minutes and comparing their performance to that of pure PEG. Fig. 4c illustrates that upon heating to 80 °C, PEG progressively melted, transforming into a colorless, transparent liquid that uniformly dispersed across the filter paper, signifying substantial leakage. Fig. S6 displays the leakage test outcomes for uncoated CPCAs, indicating that PEG seepage was also detected in these samples. Conversely, CPCAs coated with PLA exhibited remarkable shape retention, preserving their original configuration after 180 minutes at 70 °C. The improved form stability is due to the robust capillary forces inside the aerogel matrix and the mechanical support from the PLA coating. Nonetheless, negligible PEG leakage was seen in the CPCA-05 and CPCA-10 samples, attributable to their elevated PEG content and comparatively diminished CNT concentration elements that impair the composite's structural reinforcing and containment efficacy.

The findings, as mentioned earlier, were corroborated by evaluating the leakage percentage utilizing eqn (1)

where m₀ represents the initial weight of the filter paper, m denotes the weight of the filter paper post-experiment, and M indicates the weight of the CPCAs.⁴⁶ Fig. S7 demonstrates that the CPCAs had significant mass loss attributable to PEG leakage before applying the PLA coating. The proportion of PEG leakage was markedly diminished after applying the PLA coating, as illustrated in Fig. 4d. This comparison unequivocally demonstrates that PLA coating significantly improves the structural stability of composite aerogels by reducing leakage. The CPCAs were exposed to a 200 g weight to assess their dimensional stability, and a leakage test was performed for a 60-minute duration. Fig. 4e illustrates that the porous architecture and elevated specific surface area of the CPCAs enhance the PEG loading capacity under stress. Moreover, the images in Fig. S8a–d illustrate the dimensional stability of CPCA-20 following exposure to a 200 g load, reinforcing the material's structural robustness. This behavior highlights the function of the three-dimensional interconnected structures within the aerogels, which serve as sturdy scaffolding that contains melted PEG and prevents leaking.⁴⁷

These composite phase change aerogels demonstrate enhanced thermal conductivity, superior form, and dimensional stability, underscoring their significant potential for thermal energy storage applications.

3.3.3. Phase change performance. Thermal energy storage and release by CPCAs occur via phase transitions, and the thermal properties of CPCAs, including thermal conductivity and stability, are essential for the rate and efficiency of energy storage. Consequently, it is necessary to analyze the phase transition and thermal characteristics of the synthesized composite CPCAs. The enthalpies of phase transitions and changes in the temperature of aerogels were ascertained using the DSC method. The DSC thermograms of the CPCAs series are depicted in Fig. 5a and b. Table 2 presents the melting temperatures (T_m), crystallization temperatures (T_c), melting enthalpies (ΔH_m), and crystallization enthalpies (ΔH_c) of PEG and CPCAs. The melting temperature, enthalpy, crystallization temperature, and crystallization enthalpy of pure PEG are 49.93 °C, 172.34 J g⁻¹, 25.36 °C, and 166.47 J g⁻¹, respectively. Unlike pure PCM, the synthesized CPCAs exhibit comparable phase transition behavior, suggesting no chemical interaction between the hybrid aerogel support matrix and PEG.

Fig. 5 DSC thermograms of the (a) melting and (b) crystallization of PEG and CPCAs. (c) The enthalpy of melting and crystallization of aerogels. (d) Thermal storage efficiency and encapsulation capacity of CPCAs. (e) DSC curves of CPCA-20 for 100 thermal cycles. (f) Radar plot of CPCAs in comparison to other documented literature.

Table 2 Thermal characteristics of CPCAs

Name of the samples	T c (°C)	H c (J g^−1)	T m (°C)	H m (J g^−1)	Encapsulation capacity (%)	Thermal storage efficiency (%)
PEG	25.36	166.47	49.93	172.34	100.0	100.0
CPCA-05	27.86	137.09	48.12	142.55	93.45	82.53
CPCA-10	26.09	146.06	46.84	149.35	93.16	87.19
CPCA-20	26.26	155.31	46.68	161.83	92.59	93.60
CPCA-50	25.43	154.23	44.59	157.97	90.91	92.14

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The composite samples demonstrated small differences in melting and crystallization peak temperatures; melting points decreased slightly, whilst crystallization points increased marginally. This phenomenon is probably attributable to the carbon aerogel network's elevated thermal conductivity, which amplifies PEG's thermal response rate. Moreover, the CPCAs possess abundant hydroxyl groups, establishing several locations for hydrogen bonding. The interactions are essential for the efficient loading of PEG within the aerogel matrix. Fig. 5c illustrates that the enthalpy values of the composite aerogels are elevated with the addition of CNTs. This improvement is due to the alteration of the 3D porous structure by CNTs, which enhances PEG encapsulation and latent heat storage capacity. These observations align with findings documented in prior investigations.^45,48 Moreover, the enhanced thermal conductivity of CPCAs relative to pure PEG markedly improves heat transfer efficiency. Consequently, the subcooling effect diminishes, facilitating the crystallization of PEG at higher temperatures, bringing it closer to the melting temperature. This results in accelerated thermal reactivity, a sought-after attribute for several applications, including thermal regulation, energy storage, and heat exchange systems. Incorporating CNTs into CNFA significantly enhances these composite materials' performance and practical applicability.

The encapsulation capacity (E_c) and thermal storage efficiency (E_en) are essential parameters used to assess the phase change properties (Fig. 5d and Table 2). These parameters are calculated using the following formulas: (2) for E_c and (3) for E_en:

where m_PEG and m_CPCAs, respectively, represents the weight of PEG and CPCAs. ΔH_m,CPCAs and ΔH_c,CPCAs represent the melting enthalpy and crystallization enthalpy of CPCAs, respectively; ΔH_m,PEG and ΔH_c,PEG denote the melting enthalpy and crystallization enthalpy of pure PEG, respectively.⁴⁶

The E_c indicates the efficiency with which PCMs are integrated into the 3D structure of the aerogels. The E_c values for the CPCA composite aerogels varied between 90.91% and 93.45%, underscoring aerogels' superior structural integrity and encapsulation effectiveness. Furthermore, the E_en surpassed 90% in the case of CPCA-20 and CPCA-50, demonstrating that the 3D interconnected porous structure of the CPCAs proficiently maintains a significant quantity of PEG within its meso- and microporous network. This elevated efficiency is chiefly ascribed to robust surface tension and capillary forces, which collaboratively inhibit PEG leakage even beyond its melting point.

To further validate their performance, CPCA-20 samples underwent 100 thermal cycles. Fig. 5e illustrates that the DSC curves exhibited negligible variations in phase transition enthalpy, affirming the material's exceptional thermal cycle stability. After 100 cycles, CPCA-20 retained more than 90% of its initial thermal storage efficiency, as indicated in Table S6, highlighting the long-term durability and reliability of CPCAs for thermal energy storage applications.

The thermal characteristics of CPCA-20, comprising its melting and crystallization temperatures, enthalpies, encapsulation capacity, thermal storage efficiency, and thermal conductivity, were compared with values documented in prior research in which CNT is used as a filler material to increase the thermal conductivity of cellulose-based aerogels.^{20,45–47,49,50} The comparisons are elaborated in Table S7, accompanied by a visual overview in the radar plot depicted in Fig. 5f, the increased PEG concentration in CPCA-20 results in higher latent heat, improving its energy storage capacity. Moreover, CPCA-20 attains exceptional thermal conductivity while employing a low loading of about 0.2 weight% of CNT. The table demonstrates that CPCA-20 possesses exceptional heat storage capacity alongside good heat transfer performance, underscoring its significant potential for application in battery thermal management systems.

3.4 Solar-to-thermal conversion capability

Thermal energy conversion tests were performed utilizing a prototype apparatus equipped with a temperature sensor to assess the thermal energy storage efficacy of the synthesized CPCAs, as seen in Fig. 6a. Thermocouples linked to a digital thermometer were employed to observe temperature fluctuations in the CPCA-20. The findings indicated that the temperature of CPCA-20 consistently rose to 60 °C; however, in a control box devoid of the phase change aerogel, the temperature abruptly surged from 28 °C to above 65 °C, as depicted in Fig. 6b. This contrast underscores the enhanced efficacy of the phase change aerogel as a proficient thermal energy absorber and heater. The temperature profile of the CPCA-20 illustrates its capacity to absorb solar energy, resulting in a phase transition between solid states, followed by a temperature stabilization before a further increase. Upon the cessation of the heat source, the temperature within the control box decreased abruptly, whereas the box containing CPCA-20 exhibited a steady reduction, signifying the ongoing phase transition process. Further to assess the real-time performance of CPCA-20, the sample was tested under hot and humid environmental conditions, with ambient temperatures exceeding 35 °C and relative humidity above 80%. Even under these challenging conditions, CPCA-20 maintained a temperature difference of at least 10 °C compared to the control setup without the composite, demonstrating its effective energy storage capability (Fig. 6). During sunlight exposure, CPCA-20 successfully absorbed solar energy, triggering a solid–solid phase transition, followed by a temperature stabilization phase and a subsequent increase. Upon removal of the heat source (sunlight), the control box exhibited a rapid temperature drop, while the CPCA-20 sample showed a more gradual decline, indicating the release of stored thermal energy through the ongoing phase transition process. These results confirm that CPCA-20 possesses promising solar-to-thermal energy conversion and storage performance, even under natural, high-temperature, and high-humidity conditions. A cyclic experiment was also conducted to further test its thermal conversion and storage capabilities, with findings presented in Fig. 6d. The temperature profile of CPCA-20 displayed a slight upward trend during the initial 3–4 cycles, likely due to material stabilization and thermal equilibration within the experimental setup. From the 5th cycle onward, the response became consistent, demonstrating a stable and repeatable heating/cooling pattern over the subsequent 10–11 cycles. This indicates excellent thermal cycling stability. Throughout the tests, CPCA-20 consistently reached high peak temperatures and maintained rapid cooling rates, underscoring its strong potential for thermal energy storage and conversion applications.

Fig. 6 (a) Experimental setup diagram for evaluating thermal energy conversion and storage efficacy. (b) The temperature vs. time curve of the empty box and the CPCA-20 during heating and cooling. (c) Temperature vs. time curve of the empty box and the CPCA-20 during natural sunlight exposure. (d.) Thermal energy conversion cycling performance of the CPCA-20.

3.5 Acoustic properties

Besides energy consumption and management, noise pollution is a significant issue that adversely affects human health, contributing to hypertension, elevated stress levels, and auditory impairment.⁵¹ Employing appropriate acoustic insulation materials is the foremost strategy to mitigate the detrimental effects of noise pollution on humans and to foster a healthy atmosphere. Porous materials prevail among noise-absorbing substances due to their interconnected porous architectures, which allow sound waves to propagate and dissipate through thermal and viscous losses.⁵²

Consequently, this study additionally examined the acoustic insulation qualities of the produced CPCAs. The acoustic qualities of CPCAs were assessed by comparing their sound absorption coefficients to those of ordinary polyurethane (PU) foam of identical 30 mm thickness, as seen in Fig. 7a. The CPCAs exhibited superior sound absorption properties compared to commercial PU foam, which is largely attributed to their highly porous architecture. This open-cell structure, formed through the freeze-drying process where solvent sublimation leaves behind interconnected pores, plays a crucial role in trapping and scattering sound waves. The intricate network of pores enhances internal friction and facilitates multiple reflections, thereby improving sound dissipation. Significantly, CNFA samples with elevated CNT content demonstrated improved sound absorption in the higher frequency spectrum (exceeding 2000 Hz), attributable to composite stiffness and density alterations.⁵³ While higher CNT content increases the stiffness of the composite, it can reduce acoustic damping by limiting the material's ability to absorb and dissipate sound energy. However, the highly porous morphology compensates for this effect, maintaining effective sound attenuation.

Fig. 7 (a) Sound absorption characteristics of CPCAs. (b) Comparison of NRC values across different CPCAs. (c) Schematic illustration of the sound-absorption mechanism in CPCAs.

The cumulative effect is seen in the projected Noise Reduction Coefficient (NRC) values in Fig. 7b, where CPCAs demonstrate markedly higher NRCs than PU foam (0.21), signifying enhanced sound absorption proficiency. Moreover, the sound-absorbing efficacy of distinct aerogels fluctuates with their density and thickness, indicating that adjusting these parameters can proficiently influence sound propagation routes.⁵⁴ The highest NRC value is observed in CPCA-05, attributed to its lower density and more open porous structure, which effectively absorbs and dissipates sound waves. However, all composites exhibit high NRC values, regardless of CNT content, and provide good acoustic insulation. As depicted in the schematic in Fig. 7c, sound waves penetrate these porous aerogels and experience successive collisions and frictional interactions with fiber surfaces. These interactions result in internal reflections and a slow attenuation of sound energy within the porous structure, with only negligible remaining sound waves transmitted through the material.

4. Conclusions

This study successfully created structurally robust CPCAs, enhanced with a sparse CNT network and infused with PEG as PCM. The aerogels were spray-coated with PLA to augment their multifunctional performance, enhancing hydrophobicity, mechanical stability, and sound absorption. The resultant CPCAs are lightweight, extremely porous, and adept at combining thermal energy storage with acoustic insulation in a singular, environmentally sustainable material. The incorporation of CNTs, even at minimal concentrations (0.05 to 0.5 wt%), markedly enhanced thermal conductivity, from roughly 0.149 W m⁻¹ K⁻¹ to 0.516 W m⁻¹ K⁻¹ by establishing continuous conductive channels throughout the aerogel matrix. The CPCA-20 concurrently exhibited elevated latent heat enthalpy values (between 137.09 and 161.83 J g⁻¹), attaining a thermal storage efficiency exceeding 93.6%. Remarkably, it preserved over 90% of its phase change enthalpy over 100 heating and cooling cycles, thereby validating its long-term dependability for energy storage applications. The composite demonstrated exceptional mechanical integrity, with the hybrid CNF–CNT network achieving a compressive strength of around 904 kPa at 70% strain. The PLA coating significantly improved the aerogel's functioning by making the surface very hydrophobic (contact angle >100°), effectively limiting PEG leakage during thermal cycling. This was evidenced by negligible mass loss and consistent form retention over prolonged exposure at 70 °C, attributable to robust capillary forces inside the porous structure and the protective PLA barrier. In addition to thermal efficiency, the CPCAs also demonstrated superior acoustic insulation. Their interlinked porous structure, along with the rigidity imparted by CNTs and the viscoelastic damping characteristics of PLA, facilitated effective broadband sound absorption. The aerogels attained an NRC of roughly 0.34–0.45, markedly surpassing that of commercial polyurethane foam of similar thickness, illustrating its efficacy in mitigating noise throughout a broad frequency spectrum. The goal of this study is to demonstrate that the CPCAs can serve as multifunctional materials, offering both thermal energy storage and acoustic insulation. Among all, CPCA-20 demonstrates the most balanced and superior performance across all key properties. Enhancing thermal conductivity, latent heat retention, mechanical strength, moisture resistance, and acoustic absorption render CPCAs viable options for energy-efficient applications in the building, electronics, and transportation industries.

Author contributions

S. B.: conceptualization, methodology, formal analysis, validation, investigation, and writing. S. S.: investigation, validation, formal analysis, and writing. K. D.: formal analysis and validation, and writing. P. K. M.: conceptualization, methodology, supervision, and editing.

Conflicts of interest

The authors declare the absence of any personal or financial conflicts.

Data availability

The data underlying this study's conclusions can be obtained from the corresponding author upon a reasonable request.

Supplementary information is available: Fig. S1: FTIR spectra of (a) CNFA, (b) PEG, and (c) PLA. Fig. S2: XRD analysis of CNF/CNT aerogels without PEG incorporation and PLA coating. Fig. S3: Surface morphology of (a–d) CPCA-05 composite aerogel, (e–h) CPCA-10 composite aerogel, (i–l) CPCA-50 composite aerogel at 500 μm, 20 μm, 2 μm, and 200 nm. Fig. S4: Cross-sectional morphology of (a–d) CPCA-05 composite aerogel, (e–h) CPCA-10 composite aerogel, (i–l) CPCA-50 composite aerogel at 500 μm, 20 μm, 2 μm, and 200 nm. Fig. S5: (a) TGA and (b) DTG thermogram of CNFA, PEG, PLA, and CNF/CNT-20. Fig. S6: Liquid leakage test of CPCA composite aerogels without spray coating with PLA. Fig. S7: Leakage percentage of CPCA composite aerogels without spray coating with PLA at different time intervals. Fig. S8: Photographs of (a) CPCA-20, (b) a 200 g weight over CPCA-20, (c) leakage test of CPCA-20 after 60 min, and (d) stability of CPCA-20 after 60 min of loading test. Table S1: Comparison of melting and crystallization enthalpies, encapsulation capacity, thermal storage efficiency, and thermal conductivity of the current work with other phase change aerogels. Table S2: Thermogravimetric analysis of CNFA, PEG, PLA, and CNF/CNT-20. Table S3: Thermogravimetric analysis of CPCAs. Table S4: Contact angle of CPCAs. Table S5: Thermal conductivity analysis of CPCAs. Table S6: Thermal characteristics of CPCA-20 after 100 heating–cooling cycles. Table S7: Comparison of melting, crystallization enthalpies, encapsulation capacity, thermal storage efficiency, and thermal conductivity of other reported literature with the present work. References. See DOI: https://doi.org/10.1039/d5ta05021a.

Acknowledgements

This project was funded by the National Buildings Construction Corporation (NBCC), Government of India (Grant No. NBC-2293-PPE). The authors, S. B. and S. S., express gratitude for the financial support from the Prime Minister Research Fellowship Programme (PMRF), administered by the Government of India. The authors also extend their sincere thanks to Prof. Bhoje Gowd E. and Mr Aml Raj R. B. from the Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (NIIST), Thiruvananthapuram, Kerala, for their valuable assistance with acoustic insulation analysis. Additionally, the authors are grateful to Mr Vikas, Senior Laboratory Assistant, Dept of Polymer and Process Engineering, IIT Roorkee for his support in the prototype fabrication and integration of temperature sensors.

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