Ultra-wide temperature cycle control based on photo-responsive phase change

Abstract

The working principles of temperature control systems differ strikingly under severe temperature conditions. Thus, effective cooling is the main requirement for temperature control at high temperatures, whereas heating is the main concern at low temperatures. Consequently, the simultaneous combination of the heating and cooling functions in the same material at high and low temperatures is a challenging task. In this study, a series of composite aerogels comprising boron nitride–polyvinyl alcohol (BN–PVA) aerogels and Azo-OCn (n = 6, 8, 10 and 12) photo-responsive phase-change materials with energy cycle control capabilities from low to high temperatures (−20 °C to 80 °C) were prepared. The resulting BN–PVA/Azo-OCn composite aerogels achieve high enthalpy energy storage (up to 284.7 J g⁻¹) and tunable photo-responsive response time (half-life from 6.88 min to 175.04 h). Compared with the BN–PVA aerogel, the BN–PVA/Azo-OCn composite aerogels achieve temperature control with low-temperature heat release (an increase of 22.45 °C) and high-temperature heat absorption (a decrease of 11.88 °C) over an ultra-wide temperature range from −20 °C to 80 °C. An unprecedented ultra-wide range of temperature control has been achieved. This study provides new strategies for the future development of intelligent, highly thermally conductive and thermally controllable materials within an ultra-wide temperature range.

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Introduction

Extreme thermal management in harsh environments has become a hot research topic in the aerospace field, particularly regarding the stable operation capabilities of critical components of spacecraft under the severe conditions of alternating day and night in space. In extreme environment thermal control systems, phase-change materials (PCMs) are used to control temperature and store energy on the basis of the utilisation of the absorption and release of a large amount of energy during phase transition.^1–3 Owing to their stable phase-change temperatures, large heat capacity and low cost, PCMs are promising materials for energy storage, temperature regulation and thermal management.^4–7 They are widely used in various fields such as solar energy, construction, industrial waste heat recovery and aerospace.^8–10 Among them, organic solid–liquid PCMs have advantages such as small volume change during phase transition, no phase separation behaviour and high energy storage density, showing great potential in the field of energy storage.^11–15 However, conventional solid–liquid PCMs suffer from three long-standing bottlenecks, i.e. low phase-change enthalpy (<300 J g⁻¹), limited phase change in a wide temperature range and uncontrollable storage time.^16–18 From this perspective, PCMs exhibiting both photo-responsive molecular isomerisation and photo-thermally induced solid–liquid phase transition offer higher energy density across an ultra-wide temperature range by virtue of their sensitive exothermic behaviour at low temperatures and absorption capacity of latent heat at high temperatures.^19,20

However, due to random rotation and molecular chain vibration during the transfer of phonons within molecules, the majority of photo-thermally controlled solid–liquid PCMs have an ultra-low heat conductivity range from 0.15 to 0.2 W mK⁻¹,^21,22 leading to inefficient heat transfer, increased heating and cooling times and heat loss in PCMs as well as reduced energy storage efficiency.^23,24 Thermal management in wide temperature ranges is based on effective heat conduction, and the comprehensive control of heat (storage and conduction) is the key determining factor for differentiated thermal control capability and efficiency. To improve the heat responsiveness and thermal control efficiency, it is essential to reduce thermal accumulation within the material^25–29 and improve the thermal conductivity of photo-responsive PCMs.^30,31 The thermal conductivity of PCMs can be improved using two main methods: adjusting the inherent structure of the materials to elevate their ordered crystalline orientation and fabricating composite materials that incorporate high thermal conductivity substances.^32,33 Therefore, the thermal conductivity of photo-thermal PCMs can be increased by introducing a thermally conductive material skeleton.^34–44

Aerogels stand out as one of the best carriers for phase transition materials because of their continuous skeleton structure and three-dimensional (3D) nano-level interconnecting pores. The nanoscale interconnected confinement space of aerogels can effectively accommodate PCMs, with the continuous skeleton structure acting as the primary support body.^45,46 The nanoporous structure not only endows the aerogels with strong capillary action, effectively entrapping PCMs to prevent leakage during melting, but also facilitates rapid nucleation during the crystallisation process, promoting phase transition. Currently, expanded graphite materials,²⁹ metal foams,^15–17 carbon foams⁴⁰ and two-dimensional thermal conductive materials like boron nitride (BN) and graphene nanosheets are the primary components of high thermal conductivity aerogel skeletons. Among them, defect-free monolayer hexagonal BN (h-BN) with a honeycomb crystal lattice structure exhibits a theoretical thermal conductivity as high as 1700 W mK⁻¹. Moreover, its electrical insulation properties, flexibility and high UV transmittance^47,48 make it an excellent material for manufacturing thermally conductive aerogels.

Several composite macrostructures comprising PCMs and BN aerogels have been reported. High-density phase-change energy storage and high thermal conductivity have been attained by combining different PCMs, such as linear alkanes, fatty acids and polyethylene glycol, with the BN composite aerogel system.^49–51 Zhang et al. prepared a paraffin/BN@chitosan aerogel composite via freeze-drying impregnation. The thermal conductivity reached 1.14 W mK⁻¹, which is 3.07 times higher than that of graphite. When the paraffin content was 41 wt%, the energy storage density was 118.4 J g⁻¹.⁵² However, there are no reports on high thermal conductivity photo-responsive PCMs based on a BN aerogel or its composite aerogels because of the complexity of the aerogel structure and the mutually limiting interaction between thermal conductivity and photo-controlled capacity over a wide temperature range. Therefore, it is necessary to control the micro/nano-level assembly structure within BN nanosheets, while simultaneously enhancing the thermal conduction capability of BN aerogels. As a result, BN aerogels would be a crucial carrier for photo-controlled PCMs, enhancing their thermal response speed and conductivity to achieve a rapid heat release at low temperatures and an efficient absorption of heat at high temperatures, thereby enabling temperature control across an ultra-wide range.

In this study, photo-responsive PCMs (Azo-OCn, n = 6, 8, 10 and 12) were incorporated into a polyvinyl alcohol (PVA)-modified high thermal conductivity BN (BN–PVA) aerogel scaffold using a vacuum impregnation method. The resulting BN–PVA/Azo-OCn photo-thermally controlled phase-change composite aerogels can be used across an ultra-wide temperature range. The heat absorption-release capacity and photo-thermal temperature control effect of the photo-thermally controlled phase-change composite aerogels were studied under low-to-high temperature cycling conditions. The BN–PVA/Azo-OCn composite aerogels not only exhibit excellent thermal conductivity but also utilise photo-responsive PCMs to release heat at low temperatures and absorb latent heat at high temperatures. This capability enables temperature regulation across an ultra-wide temperature range (approximately from −20 °C to 80 °C), which renders the BN–PVA/Azo-OCn composite aerogels potentially applicable in areas such as solar energy storage and release, building energy efficiency, low-temperature thermal protection and temperature regulation for energy storage devices.

Experimental section

Materials

PVA (alcoholysis degree 85–90%) was purchased from Sigma-Aldrich, Shanghai, China, and BN nanoplates were obtained from XF Nano Co., Ltd, Jiangsu, China. All the photo-responsive PCMs were synthesised in advance.¹⁹ All chemicals were used as received.

Synthesis of the BN–PVA aerogel

BN (1 g) and PVA (1 g) were dissolved in 50 mL of deionised water. The mixture was sonicated at 25 °C for 30 min using an ultrasonic cell pulveriser, followed by stirring in a water bath at 70 °C for 6 h to obtain a mixed hydrogel. Subsequently, the hydrogel was subjected to low-temperature freezing and vacuum freeze-drying (72 h) to obtain a nitrogen-doped aerogel with a 3D porous honeycomb structure.

Preparation of BN–PVA/Azo-OCn composite aerogels

BN–PVA/Azo-OCn composite aerogels were prepared via vacuum impregnation. The BN–PVA aerogel and an excess of Azo-OCn (n = 6, 8, 10 and 12) photo-responsive PCMs were added to a beaker. After vacuum impregnation at 70 °C for 2 h, BN–PVA/Azo-OCn composite aerogels were obtained.

Characterization

The morphology was observed using a scanning electron microscope (SEM; S-4800, Hitachi, Japan). The chemical structure of the BN–PVA and composite aerogels was characterized by using FTIR spectra (Bruker, Germany) based on pure KBr discs. X-ray photoelectron spectroscopy analysis was performed on a surface element analysis system (XPS; Axis Supra, Kratos, Japan). Thermal analysis was performed with a thermogravimetric analyzer (TG/DTA; STA449f3, NETZSCH, Germany) protected by 50 mL min⁻¹ nitrogen purging at a heating rate of 10 °C min⁻¹ from 25 to 800 °C. X-ray diffraction (XRD) patterns were recorded on a D8 Discover diffractometer (XRD; D8 Advanced, Bruker, Germany) using Cu-Kα radiation (λ = 1.5418 Å) with an accelerating voltage and current of 40 kV and 40 mA, respectively. The porous structure of the prepared samples was evaluated by nitrogen adsorption measurement at liquid nitrogen temperature with an automatic surface area analyzer (Micromeritics ASAP 2460, America). Before measurement, the samples were degassed at 300 °C for 3 h under dynamic evacuation. The BET specific surface area was calculated by using the analysis program provided by the manufacturer (Barrett–Joyner–Halenda module). Time-evolved UV-Vis absorption spectra were obtained using a UV-Vis spectrophotometer (UV-3600 Plus, Shimadzu, Japan) in a 10 mm pathlength quartz cuvette. Differential scanning calorimetry (DSC; NETZSCH 214, Germany) tests were performed at −90 °C to 180 °C with heating and cooling rates of 10 °C min⁻¹. The thermal management capabilities of the BN–PVA and composite aerogels were characterized using an infrared thermal imager (Fluke TiX640 Expert HD, America) with a resolution of 640 × 120 (240 fps) and a thermocouple thermometer (Keithley 2400, America).

Results and discussion

Characterization and structure

The high–low temperature control of the BN–PVA/Azo-OCn (n = 6, 8, 10 and 12) composite aerogels is determined by their chemical structure, which includes high energy density photo-responsive PCMs with flexible substituents (chains) and a highly thermally conductive BN–PVA aerogel. The thermogravimetry curves shown in Fig. S1† reveal that all the BN–PVA/Azo-OCn composite aerogels exhibited superior thermal stabilisation.

Fig. 1a shows the schematic diagram of the working principle of BN–PVA/Azo-OCn composite aerogels at −20 °C to 80 °C. The temperature was regulated using light-thermal-responsive phase change materials in composite aerogels, charging with UV light (365 nm) and heat release controlled by sunlight. The composite phase change aerogels can absorb thermal energy at high temperatures (80 °C), convert it into stored latent heat and release it controllably under solar irradiation at low temperatures (−20 °C). This heat storage-release cycle at high and low temperatures allows composite aerogels to maintain a stable temperature even in extremely complex environments. These aerogels possess a thin thickness (∼1.5 mm). The porosity of the BN–PVA aerogel was calculated using eqn S1.† As shown in Fig. 1a and b, the BN–PVA/Azo-OCn composite aerogels were prepared via the vacuum impregnation technique, and the filling degree of the Azo-OCn PCMs was calculated to be up to 83.75% using eqn S2.† These aerogels possess a thin thickness of ca. 1.5 mm, with densities ranging from 0.1 to 0.5 g cm⁻³ (Fig. 1b). The ultra-low density can be attributed to the highly porous structure and porosity (over 90%, Fig. 2 and Table S1†). Photo-responsive PCMs undergo volume changes during phase transition, and the filling degree is regulated to be less than 5%.

Fig. 1 (a) Schematic illustration of the working principle of a BN–PVA/Azo-OCn composite aerogel from −20 °C to 80 °C. (b) An optical image showing the BN–PVA aerogel sample resting on the petal of a flower. The inset shows the optical image of the aerogel before and after filling the light-controlled PCM. (c) The FTIR spectra of BN, BN–PVA, Azo-OC8, and BN–PVA/Azo-OC8 aerogels. (d) XRD comparison of the photo-thermal phase change material Azo-OC10, BN–PVA aerogel, and BN–PVA/Azo-OC10 composite aerogels at 2θ ranging from 5° to 45°. (e) C 1s region spectra of the BN–PVA aerogel. (f) N 1s XPS spectra of BN–PVA/Azo-OC6.

Fig. 2 SEM images of the surface morphology of all the BN–PVA and BN–PVA/Azo-OCn composite aerogels at different magnifications. (a) BN–PVA aerogel, which shows the typical surface optical structure of the aerogel at 20 μm. (b) BN–PVA aerogel morphology at 2 μm. (c) Cross-section of the BN–PVA aerogel (200 μm). (d) Surface morphology of the BN–PVA/Azo-OC10 composite aerogel (20 μm). (e) BN–PVA/Azo-OC10 composite aerogel morphology at 5 μm. (f) Cross-section of the BN–PVA/Azo-OC10 composite aerogel (20 μm). (g) Images of the water contact angles (WCAs) of the BN–PVA aerogel and the filled BN–PVA/Azo-OC8 composite aerogel with water over time (0–100 s). (h) Comparison of the water contact angle curves of the BN–PVA aerogel and BN–PVA/Azo-OC8 composite aerogel with time. (i) The nitrogen adsorption isotherm of the BN–PVA aerogel.

The chemical structure and functional groups of the composite aerogels were determined via Fourier transform infrared (FT-IR) spectroscopy. The spectra of BN, the BN–PVA aerogel, Azo-OC8 and the BN–PVA/Azo-OC8 composite aerogel are shown in Fig. 1c. The distinctive peaks of BN and the BN–PVA aerogel are observed at 1398 and 799 cm⁻¹, respectively, which correspond to the stretching vibration of the B–N bond and the bending vibration of B–N–B. In the FT-IR spectrum of the BN–PVA aerogel, a large peak at 3300–3500 cm⁻¹ corresponding to the stretching vibration of the O–H bond indicates the presence of hydroxyl groups. Furthermore, a peak at 1254 cm⁻¹ attributable to the C–O–C bond demonstrates that PVA was successfully grafted onto BN. The spectrum of Azo-OC8 exhibits the stretching vibrations of the benzene ring skeleton, which are responsible for the absorption peaks at 1594 and 1602 cm⁻¹, and the typical absorption peak of –N=N– at 1501 cm⁻¹. The vibrational absorption peaks of the –C–O–C– bond of the aromatic ether in the Azo-OC8 molecule appear at 1247, 1144 and 1026 cm⁻¹. Meanwhile, the spectrum of the BN–PVA/Azo-OC8 composite aerogel displays peaks of both BN–PVA and Azo-OC8, indicating the formation of the composite aerogel.

The crystallinities of the Azo-OC10 photo-responsive PCM, the BN–PVA aerogel and the BN–PVA/Azo-OC10 composite aerogel were determined via X-ray diffraction (XRD) in Fig. 1d and S2.† According to Fig. 1d, the BN–PVA aerogel exhibits distinctive h-BN peaks at 2θ values of 26.88° and 41.7°, which correspond to the (002) and (100) crystal planes of h-BN, respectively. The trans-isomer Azo-OC10 shows strong crystallinity, with many sharp peaks in the 2θ range of 5–45° (Fig. 1d). The XRD pattern of the BN–PVA/Azo-OC10 composite aerogel shows characteristic peaks of the BN–PVA aerogel and Azo-OC10 photo-responsive PCM at 26.8° and in the 2θ range of 5°–45°, respectively. Compared with the XRD spectrum of Azo-OC10, that of the BN–PVA/Azo-OC10 composite aerogel exhibits broad peaks in the 2θ range of 16.93–28.54°, indicating the presence of disordered or layered nanostructures and a decrease in the crystallinity of the Azo-OC10 photo-responsive PCM. This suggests the successful incorporation of Azo-OC10 into the BN–PVA aerogel matrix. It is evident from Fig. S2d† that the crystallinity of all the BN–PVA/Azo-OCn composite aerogels is similar, with their XRD patterns displaying simultaneously the distinctive peaks of Azo-OCn and the BN–PVA aerogel (Fig. S2a–c†).

The compositional changes in the BN–PVA/Azo-OCn composite aerogels and the BN–PVA aerogel were more precisely characterised via X-ray photoelectron spectroscopy (XPS), as shown in Fig. 1e–f, S3–S8a and Table S2.† The spectra of the BN–PVA/Azo-OCn composite aerogels show the characteristic peaks of O 1s, N 1s, C 1s and B 1s at binding energies of 531.7, 399.2, 284.2 and 189.9 eV, respectively. The peak positions are virtually the same as those of BN–PVA and Azo-OCn, indicating that Azo-OCn was successfully incorporated into the BN–PVA aerogel (Fig. S4–S7†). It is worth noting that the presence of a peak attributable to a C–N bond at 288.3 eV in the C 1s spectrum of BN–PVA (Fig. 1e) indicates that the BN–PVA aerogel is formed through C–N bonds between PVA and BN rather than simply through intermolecular van der Waals forces, which makes its structure more stable.

The surface functional groups and pore structure of the aerogels are crucial for the solid–liquid interface properties between the photo-responsive PCMs and BN and for confining and uniformly integrating the PCMs within the limited space of BN–PVA, thus achieving a macroscopic composite aerogel suitable for various temperature environments.

Scanning electron microscopy (SEM) images displaying the surface morphology of BN–PVA and the BN–PVA/Azo-OC10 composite aerogel are presented in Fig. 2a–f. The BN–PVA aerogel has a comparatively homogenous 3D network structure at the nanoscale (Fig. 2a–c), which can be ascribed to the strong combination of h-BN nanosheets and water-soluble PVA as a crosslinking agent. This structure provides enhanced mechanical properties and a larger specific surface area over the 3D structure made entirely of h-BN nanosheets. Notably, the pore structure in the BN–PVA aerogel is composed of circular pores with a diameter of roughly 15 μm (Fig. 2b). The BN–PVA aerogel is 1.1 mm thick, as can be seen in the cross-sectional image (Fig. 2c).

The SEM pictures of the BN–PVA/Azo-OC10 composite aerogel are shown in Fig. 2d–f, in which Azo-OC10 is evenly distributed throughout the BN–PVA aerogel. The wrinkled structure of Azo-OC10 is observed on the surface of the BN–PVA/Azo-OC10 composite aerogel, and the layers of the BN–PVA aerogel and Azo-OC10 are closely stacked, forming a full thermal conduction channel. This increases the thermal conductivity of the BN–PVA aerogel and enhances the thermal conductivity coefficient of Azo-OC10, as was confirmed by thermal conductivity measurements. Fig. S8b and c† show an EDS image of the composite aerogel and a comparison of the elemental content before and after filling in the BN–PVA aerogel, demonstrating that Azo-OC10 was filled into the BN–PVA aerogel. The changes in elemental distribution are in line with the XPS results (Fig. S8a†).

The water contact angle (WCA) was measured over time for the BN–PVA aerogel and the BN–PVA/Azo-OC8 composite aerogel (Fig. 2g–h). In the first stage (0 s), the BN–PVA aerogel exhibited a WCA of 101.7° (Fig. 2g), possibly due to its reticulated porous structure increasing the material surface tension and resulting in a hydrophobic state upon initial contact with water droplets (Fig. S9†). However, after 50 s, the WCA between the BN–PVA aerogel and water decreased rapidly, as depicted in Fig. 2g and h, indicating that water penetrated into the aerogel. The WCA values of the composite aerogel were 88.4°, 70.3°, 53.5° and 0° at 3, 5, 10 and 50 s, respectively, which can be attributed to the high concentration of hydrophilic functional groups such as hydroxyl groups in the BN–PVA aerogel. Conversely, the WCA of the BN–PVA/Azo-OC8 composite aerogel was 106.6° (Fig. 2g and h). This strong hydrophobicity, which was observed even upon increasing the test duration (0–100 s), stems from the strong hydrophobicity of Azo-OC8. Additionally, the WCA maintained its value of 106.6°, indicating the hydrophobicity stability of the composite aerogel.

The nitrogen adsorption isotherm of the BN–PVA aerogel (Fig. 2i) showed that the adsorption curve increased slowly at lower relative pressures (P/P₀ = 0.1–0.8) and then rapidly at higher relative pressures (P/P₀ = 0.9–1.0), which was accompanied by a distinct hysteresis loop of a typical H3-type isotherm. This suggests that the BN–PVA aerogel contains abundant mesoporous and microporous structures with large pore diameters. The Barrett–Joyner–Halenda (BJH) model was used to calculate the pore size distribution. The majority of the pores in the BN–PVA aerogel were in the range of 20–50 nm, showing the retention of a multi-level porous structure (Fig. S10†). The specific surface area, micropore specific surface area, pore volume, average pore size, micropore volume and percentage are shown in Table S3.† The results indicate the predominant presence of abundant micropores and mesoporous structures in the BN–PVA aerogel, with micropores accounting for 15.96% of the total number of pores. The higher specific surface area (90.61 m² g⁻¹) of the aerogel provides more space for the filling and adsorption of PCMs, allowing the BN–PVA/Azo-OCn composite aerogels to retain higher energy density. The multi-level porous structure of the BN–PVA aerogel can provide abundant thermal conduction pathways for PCMs, thus offering an efficient thermal conduction network for applications, as will be discussed later.

Isomerization of composite aerogels

Managing the thermal properties of BN–PVA/Azo-OCn in an ultra-wide temperature range poses a considerable challenge, particularly for achieving stable energy storage and rapid energy release regulation based on the photo-isomerisation of the aerogel material (in terms of degree and speed).

Fig. 3a shows an optical microscope (OM) image of the BN–PVA aerogel, displaying a large number of pores on its surface, which provide filling space for photo-responsive PCMs. As displayed in Fig. 3b, the E-BN–PVA/Azo-OC10 isomer appears as bright yellow crystals and melts into a dark orange liquid under 365 nm UV light irradiation. Fig. 3b also illustrates the isomerisation process of E-BN–PVA/Azo-OC10 and Z-BN–PVA/Azo-OC10 upon alternating exposure to UV light (365 nm) and simulated solar light (AM 1.5, 100 mW cm⁻²). Fig. 3c(i–v) illustrate the changes in OM images during this photo-chemical crystal → liquid transition. The well-crystallised morphology of E-BN–PVA/Azo-OC10 gradually disappears under UV light (Fig. 3c(i–iii)) and then recovers when the UV illumination is turned off and simulated sunlight is turned on (Fig. 3c(iv and v)).

Fig. 3 Isomerization of the BN–PVA/Azo-OCn composite aerogel. (a) Optical microscopy (OM) images of the BN–PVA aerogel. (b) Schematic isomerization of E-BN–PVA/Azo-OC10 and Z-BN–PVA/Azo-OC10 composite aerogels. (c) OM images of BN–PVA/Azo-OC10 during one isomerization cycle showing its dynamic change at room temperature (trans → cis, 25.7 °C). (d) Raman spectra of the BN–PVA aerogel, E-Azo-OC10, and Z-Azo-OC10. (e) Raman spectra comparison of the BN–PVA/Azo-OC10 composite aerogel at different cis–trans isomerization degrees. Time-evolved UV-vis absorption spectra of BN–PVA/Azo-OC10 under (f) UV light irradiation, (g) light at RT (25.7 °C), (h) light at low temperature LT (−20 °C), and (i) a dark environment at LT (−20 °C). (j) Z-to-E isomerization rate constants (κ_rev) of BN–PVA/Azo-OCn (n = 6, 8, 10, and 12) composite aerogels under light and at RT (25.7 °C). (k) Comparative histograms of the changes in reversion time against different chain segment lengths (n) under light LT (−20 °C) and light RT (25.7 °C).

To gain further insight into the trans–cis photo-isomerisation of the BN–PVA/Azo-OCn composite aerogels, the Raman spectra of the two isomers were recorded. All the Raman spectra show a characteristic peak near 1361 cm⁻¹ (Fig. 3d), which can be attributed to the B–N high-frequency vibrational mode (E_2g) on the h-BN layers. The Raman spectra of the BN–PVA/Azo-OC10 composite aerogel under UV light irradiation over time are compared in Fig. 3e. The peaks at 1136 cm⁻¹ (1) and 1182 cm⁻¹ (2) are due to the stretching vibration of the cyanide (–CN) group in conjunction with ring breathing modes. The peak at 1408 cm⁻¹ (3) reflects the degree of isomerisation of the azobenzene-based PCM and is mostly associated with its in-plane ring bending mode as well as marginally connected with the N=N stretching. The peak at 1437 cm⁻¹ (4) corresponds to the bending vibration of H–C–H in the alkyl groups. Three moderate intensity modes, including the stretching of N=N bonds, mild in-plane ring bending modes and the bending of H–C–H in alkyl groups, combine into a peak at 1465 cm⁻¹ (5). These assignments are in agreement with previously reported azobenzene spectra.^53,54 Additionally, in the spectra of cis Z₁-BN–PVA/Azo-OC10 and Z-BN–PVA/Azo-OC10, the intensity of peak (3), which represents the in-plane bending mode coupled with the stretching of N=N, decreases considerably compared with that of trans E-BN–PVA/Azo-OC10, whereas the intensities of the other four characteristic peaks remain relatively unchanged. This can be attributed to the conjugation loss of the cis-isomer and the ensuing decline in polarizability and Raman intensity.

The solvent-free photo-isomerisation of BN–PVA/Azo-OCn was monitored using the UV–vis absorption spectra (Fig. 3f–k). The spectrum of the E-BN–PVA/Azo-OCn aerogel shows an intense peak at 348 nm due to the π–π* transition and a weak broad peak generated by electronic n–π* transitions at approximately 441 nm (Fig. 3f and S11–13a†). After UV light irradiation, a continuous and obvious decrease in the π–π* transition peak and a slight increase in the n–π* transition peak were observed owing to E-to-Z photo-isomerisation. Under simulated solar irradiation, the opposite trend was observed for the π–π* and n–π* transition peaks.

The continuous change in absorption originates from the reversible Z-to-E isomerisation, and the time required to achieve the photo-stationary state is influenced by the environmental conditions. Three reversion environments of simulated solar irradiation at room temperature (light at RT), cold-dark (−20 °C, dark at LT) and simulated solar irradiation at low temperature (light at LT) were selected to investigate the effect of the reversion conditions on the reversibility of BN–PVA/Azo-OCn. The UV–vis absorption spectra shown in Fig. 3f–i and S11–13b–d† indicate that sunlight exposure can effectively accelerate the isomerisation and the simultaneous release of phase-transition enthalpy (Fig. 3g, h, k, S11–13b and c†). Compared with blue light,¹⁹ sunlight can accelerate considerably the synchronised release of energy. This is due to the stronger optical power density and broader wavelength range of simulated solar light (AM 1.5), which facilitate the efficient recovery of photo-responsive PCMs. Conversely, dark environments are conducive to long-term energy storage (Fig. 3i and S11–13d†). Meanwhile, the release of both isomerisation and phase-change enthalpies (Fig. 3h, i, S11–13c and d†) is effectively prolonged at low temperature (−20 °C), enabling stable long-term storage. These results were verified by determining the first-order kinetic constant (κ_rev; Fig. 3j, S14, Table S4 and eqn S3†) and half-lives (t_1/2; Fig. S15, Table S4 and eqn S4†). Taking BN–PVA/Azo-OC10 as an example, the sunlight-induced isomerisation rate constant (κ_rev-sunlight = 11.09 × 10⁻⁴ s⁻¹) at RT was the highest; compared with the dark- and cold-induced isomerisation (κ_rev-DC = 14.25 × 10⁻⁶ s⁻¹), the sunlight irradiation in cold environments effectively improved the isomerisation response efficiency (κ_rev-SC = 5.9 × 10⁻⁴ s⁻¹). In addition, dark- and cold-induced isomerisation exhibited the longest half-life (t_1/2 = 135.12 h), enabling stable long-term energy storage. Sunlight-induced isomerisation showed the shortest half-life (t_1/2 = 10.42 min). Similarly, sunlight-and-cold-induced isomerisation (t_1/2 = 19.6 min) was adjustable (Fig. S15†). However, compared with Azo-OC10, the recovery time was substantially extended for BN–PVA/Azo-OC10. This is primarily because the aerogel prevents part of the light from reaching the surface of the photo-responsive PCMs. Therefore, the thermal release of isomerisation must be effectively controlled to achieve long-term energy storage and controlled release in an ultra-wide temperature range.

Energy densities and thermal conductivity in an ultra-wide temperature range

The energy density (ΔH_total) is an important criterion for measuring the thermal storage capacity of materials. Unfortunately, the low thermal conductivity of photo-responsive PCMs seriously limits their practical application. In our composite of photo-responsive PCMs and BN aerogels as the core material for thermal control, using the BN–PVA nanosheets to construct continuous heat conduction pathways and enhance phonon transfer efficiency increases the thermal conductivity efficiency, achieving a synergistic enhancement of latent heat and heat transfer.

The broad exothermic peaks in the heating curves originated from the photo-thermal-induced E-to-Z isomerisation after UV irradiation, from which the isomerisation enthalpies (ΔH_iso) were calculated. The ΔH_iso values of Z-BN–PVA/Azo-OCn were 144.5, 138.8, 129.0 and 112.7 J g⁻¹ (Fig. 4a and Table S5†). As shown in Fig. 4b, S17 and Table S5,† all the E-BN–PVA/Azo-OCn samples exhibited high crystallisation enthalpy (ΔH_c-E) ranging from 94.6 to 129.1 J g⁻¹. The highest enthalpy of 129.1 J g⁻¹ was observed for E-BN–PVA/Azo-OC12. However, compared with Azo-OCn, all the filled aerogels showed an irregular morphology and a marked decrease in energy density. This can be attributed to the incorporation of the BN–PVA aerogel limiting the crystallisation ability and regularity of the flexible chains of Azo-OCn, further affecting its energy storage capacity. Fig. 4c and Table S5† show the total heat released (ΔH_total), which was calculated according to eq S5 and S6.† The highest ΔH_total of BN–PVA/Azo-OC12 reached 284.7 J g⁻¹. Compared with that of conventional organic PCMs, the energy density of BN–PVA/Azo-OCn still increased substantially owing to the combined effect of crystallisation (ΔH_c) and isomerisation enthalpies (ΔH_iso) in terms of energy quantity.

Fig. 4 (a) All DSC exothermic curves of Z-BN–PVA/Azo-OCn (n = 6, 8, 10, and 12) from 50 °C to 160 °C at a heating rate of 10 °C min⁻¹. (b) All DSC exothermic curves of E-BN–PVA/Azo-OCn (n = 6, 8, 10, and 12) from 25 °C to 90 °C at a heating rate of 10 °C min⁻¹. (c) Relationship between the total amount of released heat (ΔH_total, including ΔH_c-E, ΔH_c-Z, and ΔH_iso) and different alkoxy chain lengths n. (d) T_m-E of E-BN–PVA/Azo-OCn (n = 6, 8, pink line) and T_g of Z-BN–PVA/Azo-OCn (n = 6, 8, orange line). (e) T_m-E of E-BN–PVA/Azo-OCn (n = 10, 12, pink line) and T_c-Z of Z-BN–PVA/Azo-OCn (n = 10, 12, blue line). The shaded region represents the temperature range for the optically triggered heat release. (f) Glass transition temperature (T_g) of Z-BN–PVA/Azo-OC12 (temperature range from −80 °C to −40 °C). (g) Comparative compression curves of BN–PVA aerogels at different temperatures (−20 °C to 80 °C) with a thickness of 1.5 mm. (h) Comparative compression curves of the BN–PVA/Azo-OC12 aerogel at different temperatures (−20 °C to 80 °C) with a thickness of 1.5 mm. (i) Thermal conductivity curves of BN–PVA, BN–PVA/Azo-OC6 and BN–PVA/Azo-OC12 aerogels at different temperatures (−20 °C to 150 °C).

The effect of the addition of the BN–PVA aerogel on the crystallisation temperature (T_c), melting temperature (T_m) and isomerisation temperature (T_iso) was negligible for all the BN–PVA/Azo-OCn aerogels (Fig. S17 and Table S5†). Even all Z-BN–PVA/Azo-OCn aerogels showed extremely low T_g values (from −64.27 °C to −44.88 °C; Fig. 4f, S17 and Table S5†) because of the sterically curved Z-isomer structure with large molecular volume and polarity, which disrupts the π–π stacking and impedes crystal formation.⁴ The BN–PVA aerogel and the photo-responsive PCMs simply undergo physical adsorption without forming any new chemical bonds; thus, the inherent properties of the photo-responsive PCMs remain unaltered. Owing to the low T_c-Z or even T_g, the BN–PVA/Azo-OCn aerogels still release heat for energy utilisation at an extremely low temperature. For all the BN–PVA/Azo-OCn composite aerogels, T_m-E and T_c-Z (T_g) are the upper and lower temperature limits for the synchronous heat output, respectively. The photo-controlled PCMs introduced a photo-responsive energy barrier, enabling photo-triggered heat release over an ultra-wide temperature range (Fig. 4d and e).

The representative compressive stress–strain (σ–ε) curves (Fig. 4g–h, S18, 19†) illustrate that the stiffness of the BN–PVA/Azo-OC12 composite aerogel was superior to that of the BN–PVA aerogel, which can be ascribed to the rigidity of the Azo-OC12 PCM. The flexible PVA polymer imparts a certain degree of flexibility to the BN–PVA aerogel framework, whereas Azo-OC12 enhances the stiffness of the BN–PVA/Azo-OC12 composite aerogel.

Compressive stress–strain (σ–ε) curves were also recorded for the same BN–PVA aerogel subjected to the same deformation at different temperatures (−20 °C to 80 °C). As shown in Fig. 4g, at low temperature, the compressive stress of the BN–PVA aerogel increases with increasing temperature, reaching a maximum value of 75.2 kPa at 0 °C. However, upon further increasing the temperature, the BN–PVA aerogels gradually soften and the compressive stress decreases. At 80 °C, the compressive stress drops to the lowest value of 35.9 kPa. The effect of temperature on the stiffness of the BN–PVA aerogel cannot be ignored; therefore, the appropriate temperature must be selected for practical applications. Similar to the BN–PVA aerogel, the BN–PVA/Azo-OC12 composite aerogel reaches a maximum compressive stress of 15.7 kPa at 0 °C. However, the compressive stress decreases with increasing temperature, which is mainly due to Azo-OC12 undergoing a solid–liquid phase transition in the composite aerogel (Fig. 4h). All the aerogels showed relatively stable compression cycling ability after five compression cycles (Fig. S18 and 19†), indicating their stability and durability in long-term thermal energy storage.

Recent work has shown that spatially isotropic heat transfer facilitates heat conduction throughout the whole 3D network,^55,56 which could behave like metals with high isotropic thermal conductivity. In view of its prominent thermal performance, the BN–PVA aerogel could serve as a lightweight heat conductor for thermal management under different temperatures.

Fig. 4i shows the thermal conductivity curves of the BN–PVA aerogel and composite aerogel at different temperatures (−20 °C to 150 °C). The thermal conductivity was calculated using eqn (1) as follows:

λ = α × ρ × c (1)

where α is the thermal diffusion coefficient of the aerogel, λ is the thermal conductivity coefficient of the aerogel, ρ is the density of the aerogel and c is the specific heat capacity of the aerogel.

The BN–PVA aerogel showed high thermal conductivity, which was affected by temperature. At low temperatures (−20 °C to 40 °C), the thermal conductivity of the BN–PVA aerogel increased with increasing temperature, reaching 0.68 W mK⁻¹ at 40 °C. However, as the temperature increased further (40–150 °C), the thermal conductivity decreased to 0.24 W mK⁻¹ at 60 °C. Similarly, the BN–PVA/Azo-OC12 composite aerogel showed the highest thermal conductivity of 1.41 W mK⁻¹ at 40 °C. These results demonstrate that the 3D mesh structure inside the BN–PVA/Azo-OCn composite aerogels can effectively solve the problems of photo-responsive PCMs causing poor heat dissipation, i.e. a single exothermic temperature, effectiveness in a restricted range of temperatures and low thermal conductivity.

Continuous and intelligent thermal management over an ultra-wide temperature range

The isomerisation and phase-change properties of photo-induced and temperature-responsive PCMs were expected to enable the controlled storage and release of photo-thermal energy at different temperatures. To confirm this hypothesis, the temperature response and variation patterns of the thermal management structure were monitored over time and in response to environmental heat flow changes using thermocouples and infrared thermal imaging. This was accomplished by establishing a cold stage for cyclic environmental changes from low to high temperatures and combining it with light exposure to simulate multiple alternating irradiation environments.

Fig. 5a shows simulated temperature variations at different points for the BN–PVA l, Z-BN–PVA/Azo-OC12 and E-BN–PVA/Azo-OC12 aerogels with increasing ambient temperature from −20 °C to 80 °C. The surface temperatures (position 1) were measured using an infrared thermal imager, while the internal temperatures (position 2) were measured using thermocouples placed inside the aerogels. A continuous time–temperature profile was obtained. Fig. 5b illustrates the variation of the exothermic temperature (T) over time for BN–PVA, Z-BN–PVA/Azo-OC12 and E-BN–PVA/Azo-OC12 with increasing temperature from −20 °C to 80 °C. At low temperatures between −20 °C and 25 °C, the maximum temperature of Z-BN–PVA/Azo-OC12 (T_Z-max) consistently exceeded those of E-BN–PVA/Azo-OC12 (T_E-max) and BN–PVA (T_BN-max), demonstrating excellent exothermic capability at low temperatures. However, above 30 °C, the temperature of BN–PVA sharply increased with increasing ambient temperature, indicating poor thermal control capability. Because of the photo-responsive PCM, Z-BN–PVA/Azo-OC12 can absorb a large amount of ambient heat, stabilising at 58.4 °C in 700 s after the ambient temperature reached 80 °C. Subsequently, the heating was stopped and the temperatures of all samples began to decrease. Among them, E-BN–PVA/Azo-OC12 and Z-BN–PVA/Azo-OC12 exhibited slower cooling rates, whereas the temperature of the BN–PVA aerogel decreased rapidly as the surrounding temperature decreased. During the heating process from −20 °C to 80 °C, the total temperature change (ΔT_max–min) was 79.1 °C for BN–PVA, 68.27 °C for E-BN–PVA/Azo-OC12 and 65.5 °C for Z-BN–PVA/Azo-OC12 (Fig. 5c, S21, S22 and Table S7†). Consequently, BN–PVA/Azo-OC12, especially Z-BN–PVA/Azo-OC12, demonstrates superior thermal management and temperature control capabilities.

Fig. 5 (a) Plots of exothermic temperature (T) versus time for BN–PVA, Z-BN–PVA/Azo-OC12 and E-BN–PVA/Azo-OC12 with increasing ambient temperature from −20 °C to 80 °C. The exothermic processes at −20 °C, 0 °C and 80 °C were conducted under simulated solar irradiation. (b) Optical image of BN–PVA, Z-BN–PVA/Azo-OC12 and E-BN–PVA/Azo-OC12 and thermocouple testing states. (c) Relationship between T_max and T_min of ΔT_max–min of BN–PVA, Z-BN–PVA/Azo-OC12 and E-BN–PVA/Azo-OC12. Plot of T versus time measured using thermocouples at (d) low temperature (−20 °C), (e) 0 °C and (f) high temperature (80 °C) for BN–PVA, Z-BN–PVA/Azo-OC12 and E-BN–PVA/Azo-OC12. Temperature difference ΔT versus time plots for BN–PVA, Z-BN–PVA/azo-OC12 (ΔT_Z-BN), and E-BN–PVA/Azo-OC12 (ΔT_E-BN) at (g) low temperature (−20 °C), (h) 0 °C, and (i) high temperature (80 °C). (j) Time-evolved infrared thermal imaging for BN–PVA, Z-BN–PVA/Azo-OC12 and E-BN–PVA/Azo-OC12 at −20 °C for one cycle, including sunlight irradiation (yellow arrow) for 35 min and natural exotherm (gray arrow) for the subsequent 45 min. T_max is the maximum temperature displayed in the infrared thermal imager. (k) Plot of T versus time at low temperature (−20 °C) for BN–PVA, Z-BN–PVA/Azo-OC12 and E-BN–PVA/Azo-OC12. (l) Plot of T versus time at room temperature (25.7 °C) for BN–PVA, Z-BN–PVA/Azo-OC12 and E-BN–PVA/Azo-OC12.

Fig. 5d–i show the results of simultaneously observing the temperature variation over time of the three aerogels using thermocouples at ambient temperatures of −20 °C, 0 °C and 80 °C under solar illumination conditions (AM 1.5, 100 mW cm⁻²). At −20 °C (Fig. 5d, g, S21, S22, and Table S6†), compared with E-BN–PVA/Azo-OC12 (T_E-max = 3.16 °C), Z-BN–PVA/Azo-OC12 (T_Z-max = 12.55 °C) simultaneously releases both isomerisation enthalpy and phase-transition enthalpy under simulated solar irradiation, resulting in the largest temperature difference (ΔT_Z–E) of 9.39 °C, whereas the temperature change in BN–PVA remains relatively small, with a temperature difference (ΔT_Z-BN) of 22.45 °C compared with Z-BN–PVA/Azo-OC12. This phenomenon indicates that the photo-controlled PCMs exhibit excellent low-temperature exothermic capabilities, with the cis-isomer simultaneously releasing both the enthalpies of phase change and isomerisation.

Z-BN–PVA/Azo-OC12 also demonstrated excellent temperature-controlled capacity at 0 °C (Fig. 5e, h, S21 and Table S6†), with ΔT_Z–E and ΔT_Z-BN values of 7.7 °C and 12.75 °C, respectively. When the temperature reached 80 °C (Fig. 5f, i, S21, and Table S6†), the temperature increased sharply due to the poor temperature control ability of BN–PVA, and the T_BN-max reached 70.18 °C. At this time, Z-BN–PVA/Azo-OC12 and E-BN–PVA/Azo-OC12 showed excellent temperature control ability at high temperatures due to the high energy density of the photo-responsive PCMs, thus maintaining low temperatures of −5.1 °C and −11.88 °C for ΔT_Z–E and ΔT_Z-BN, respectively.

The heat release capability of all the aerogels was observed at −20 °C, 25.7 °C and 80 °C, where the latent energy and isomerisation enthalpy can be released through optically and temperature dual-triggered heat output during the heating release process. To monitor the temperature changes at low, room and high temperatures, a high-resolution (±0.03 °C) infrared radiation thermal imaging camera was used. The temperature changes (T) depended on the heat released by Z-BN–PVA/Azo-OCn and E–BN–PVA/Azo-OCn, including sensible latent heat and photo-thermal energy.

Taking BN–PVA/Azo-OC12 as an example, the temperature measurement points are shown in Fig. 5j and k. Under low-temperature conditions (−20 °C), BN–PVA, Z-BN–PVA/Azo-OC12 and E-BN–PVA/Azo-OC12 were first placed on a low-temperature stage. Due to the excellent thermal insulation properties of the BN–PVA aerogel, the surface temperatures of all aerogels were higher than the temperature of the cold stage. Once the temperature stabilised at low temperature, the simulated sunlight was turned on (AM 1.5, 100 m W cm⁻²) and the temperature of all the aerogels began to increase. With increasing exposure time, the maximum exothermic temperature of Z-BN–PVA/Azo-OC12 (T_max = 13.7 °C) and E-BN–PVA/Azo-OC12 (T_max = 10.4 °C) gradually increased. At 35 min, the T_max of Z-BN–PVA/Azo-OC12 reached its highest value of 29.2 °C, while the T_max of E-BN–PVA/Azo-OC12 and BN–PVA was 21.3 °C and 7.3 °C, respectively. The maximum surface temperature difference between the cis-isomer and the trans-isomer was 7.9 °C, and the maximum surface temperature difference between the cis-isomer and BN–PVA reached 11.9 °C (Fig. 5k and Table S8†). These results are consistent with the trend of temperature changes at the bottom of the aerogel. It is well known that pure aerogels with large porosity and low density are thermally insulating. However, the infrared thermograph (Fig. 5j) showed that the BN–PVA/Azo-OC12 composite aerogel still exhibited the best thermal conductive performance at high and low temperatures. This can be attributed to the effect of the high BN–PVA content and the construction of an efficient 3D thermal conductive network overcoming the thermal insulation of air gaps.^57,58

Subsequently, the sunlight was turned off and the exothermic temperature began to decrease. At 60 min, the surface T_max of Z-BN–PVA/Azo-OC12 was still 18.8 °C, and at 80 min, the temperature of all aerogels stabilised and the exothermic process reached completion. Compared with E-BN–PVA/Azo-OC12 and BN–PVA, Z-BN–PVA/Azo-OC12 achieved a high density of heat generation, which is consistent with the differential scanning calorimetry results (ΔH_total = 284.7 J g⁻¹).

Similarly, the exothermic capacity of the BN–PVA, Z-BN–PVA/Azo-OC12 and E-BN–PVA/Azo-OC12 aerogels was tested at room temperature (25.7 °C, Fig. 5l, Tables S9 and S23†). After the temperature stabilised, the simulated sunlight (AM 1.5, 100 mW cm⁻²) was turned on, and the surface temperature of the aerogels started to increase (0 min). Z-BN–PVA/Azo-OC12 reached a maximum surface temperature of 59.2 °C after 20 min, while those of E-BN–PVA/Azo-OC12 and B–PVA were 47.5 °C and 38.5 °C, respectively. The maximum surface temperature difference between the cis- and trans-isomers was 11.5 °C, and that between the cis-isomer and BN–PVA reached 20.7 °C (Fig. 5l). The prepared composite aerogels also exhibited excellent sustained heat release at room temperature due to the photo-controlled PCMs.

At high temperature (80 °C), the surface of the BN–PVA aerogel demonstrated excellent thermal insulation performance (T_max = 42.9 °C, 0 min), effectively preventing the heat transfer from the base to the surface of the aerogel (Fig. S24, S25† and Table S10†). This endows the composite aerogels with temperature regulation capability at high temperatures. Z-BN–PVA/Azo-OC12 only released isomerisation enthalpy for 25 min. Meanwhile, the surface temperature difference between Z-BN–PVA/Azo-OC12 (T_max = 57.9 °C) and E-BN–PVA/Azo-OC12 (T_max = 55.8 °C) was not significant, which is primarily due to the surface temperature being mainly controlled by light exposure. The regulation and optimisation of the heat output mode can enhance the thermal release capability of the composite aerogels under dual control of light and temperature. This synergistic enhancement of latent heat and heat transfer could enable thermal management control over an ultra-wide temperature range in the future.

Conclusion

By virtue of the high energy density of photo-responsive PCMs and the surface functional groups and porous structure of BN–PVA aerogels, a series of photo-responsive phase-change composite aerogels (BN–PVA/Azo-OCn, n = 6, 8, 10 and 12) were prepared. The temperature regulation capabilities of BN–PVA/Azo-OCn were studied over an incredibly large temperature range, achieving intelligent control based on the absorption of heat at high temperatures and release of heat at low temperatures by the same material. The thermal conductivity of the photo-responsive PCMs was 1.41 W mK⁻¹ after adding the BN–PVA aerogel, resulting in a rapid thermal conductivity response of stored sensible and latent heat. The BN–PVA/Azo-OCn aerogels respond quickly and intelligently over an incredibly wide temperature range (−20 °C to 80 °C), release heat at low temperatures (an increase of 22.45 °C at −20 °C) and absorb latent heat at high temperatures (a decrease of 11.88 °C at 80 °C). The BN–PVA/Azo-OCn aerogels exhibit a distinct capability for thermal management in an ultra-wide temperature range, showing potential for temperature regulation applications in extreme temperature environments.

Data availability

The data supporting this article are available within the article and its ESI.†

Author contributions

Jing Ge: conceptualization, methodology, investigation, data curation, and writing – original draft. Xiaoyu Yang: validation and formal analysis. Zedong Wang: investigation. Yiyu Feng: supervision, writing – review & editing, funding acquisition, and research design. Wei Feng: conceptualization, supervision, writing – review & editing, funding acquisition, and resources. All authors reviewed and approved the manuscript.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 52327802), the State Key Program of National Natural Science Foundation of China (No. 52130303), the National Natural Science Foundation of China (No. 52173078 and No. 51973152), the National Key R&D Program of China (No. 2022YFB3805702), and the Science Foundation for Distinguished Young Scholars in Tianjin (No. 19JCJQJC61700).

Notes and references

1. E. M. Shchukina, M. Graham, Z. Zheng and D. G. Shchukin, Chem. Soc. Rev., 2018, 47, 4156–4175.
2. Y. Wang, B. Tang and S. Zhang, Adv. Funct. Mater., 2013, 23, 4354–4360.
3. G. G. Han, H. Li and J. C. Grossman, Nat. Commun., 2017, 8(1), 1446.
4. H. Zhou, C. Xue, P. Weis, Y. Suzuki, S. Huang, K. Koynov, G. K. Auernhammer, R. Berger, H. Butt and S. Wu, Nat. Chem., 2017, 9, 145–151.
5. J. Ge, Y. Wang, H. Wang, H. Mao, J. Li and H. Shi, Sol. Energy Mater. Sol. Cells, 2020, 208, 110388.
6. G. Wang, Z. Tang, Y. Gao, P. Liu, Y. Li and A. Li, Chem. Rev., 2023, 123(11), 6953–7024.
7. Z. Shen, M. Qin, F. Xiong, R. Zou and J. Zhang, Energy Environ. Sci., 2023, 16, 830–861.
8. J. Yang, Y. Zhou, L. Yang, C. Feng, L. Bai, M. Yang and W. Yang, Adv. Funct. Mater., 2022, 32(28), 2200792.
9. H. Liu, K. Sun, X. Shi, H. Yang, H. Dong, Y. Kou, P. Das, Z. Wu and Q. Shi, Energy Storage Mater., 2021, 42, 845–870.
10. T. B. Freeman, K. E. Foster, C. J. Troxler, C. W. Irvin, A. Aday, S. K. Boetcher, A. Mahvi, M. K. Smith and A. Odukomaiya, Adv. Energy Mater., 2023, 13(24), 2204208.
11. J. Shi, W. Aftab, Z. Liang, K. Yuan, M. Maqbool, H. Jiang, F. Xiong, M. Qin, S. Gao and R. Zou, J. Mater. Chem. A, 2020, 8(38), 20133–20140.
12. Q. Qiu, Y. Shi and G. G. Han, J. Mater. Chem. C, 2021, 9(35), 11444–11463.
13. R. D. McGillicuddy, S. Thapa, M. B. Wenny, M. I. Gonzalez and J. A. Mason, J. Am. Chem. Soc., 2020, 142(45), 19170–19180.
14. H. Xu, L. Jiang, A. Yuan, Y. Lei, Z. Wei, Y. Wang and J. Lei, Chem. Eng. J., 2021, 421, 129833.
15. D. Wu, W. Wen, S. Chen and H. Zhang, J. Mater. Chem. A, 2015, 3(6), 2589–2600.
16. A. Usman, F. Xiong, W. Aftab, M. Qin and R. Zou, Adv. Mater., 2022, 34(41), 2202457.
17. Z. Y. Zhang, Y. He, Z. Wang, J. Xu, M. Xie, P. Tao, D. Ji, K. Moth-Poulsen and T. Li, J. Am. Chem. Soc., 2020, 142, 12256–12264.
18. Z. Shangguan, W. Sun, Z. Zhang, D. Fang, Z. Wang, S. Wu, C. Deng, X. Huang, Y. He, R. Wang, K. Moth-Poulsen and T. Li, Chem. Sci., 2022, 13(23), 6950–6958.
19. J. Ge, M. Qin, X. Zhang, X. Yang, P. Yang, H. Wang, G. Liu, X. Zhou, B. Zhang, Z. Qu, Y. Feng and W. Feng, SmartMat, 2024, e1300.
20. S. Wu, T. Li, Z. Y. Zhang, T. Li and R. Wang, Matter, 2021, 4(11), 3385–3399.
21. Z. Tang, H. Gao, X. Chen, Y. Zhang, A. Li and G. Wang, Nano Energy, 2021, 80, 105454.
22. Z. Dai, Y. Gao, C. Wang, D. Wu, Z. Jiang, X. She, Y. Ding, X. Zhang and D. Zhao, ACS Appl. Mater. Interfaces, 2023, 15(22), 26863–26871.
23. D. B. Fox, D. Sutter and J. W. Tester, Energy Environ. Sci., 2011, 4(10), 3731–3740.
24. M. A. Gerkman, R. S. L. Gibson, J. Callbo, Y. R. Shi, M. J. Fuchter and G. G. D. Han, J. Am. Chem. Soc., 2020, 142(19), 8688–8695.
25. Z. Kuang, Y. Chen, Y. Lu, L. Liu, S. Hu, S. Wen, Y. Mao and L. Zhang, Small, 2015, 11, 1655–1659.
26. G.-H. Kim, D. Lee, A. Shanker, L. Shao, M. S. Kwon, D. Gidley, J. Kim and K. P. Pipe, Nat. Mater., 2015, 14, 295–300.
27. I. Kholmanov, J. Kim, E. Ou, R. S. Ruoff and L. Shi, ACS Nano, 2015, 9, 11699–11707.
28. I. Ferain, C. A. Colinge and J.-P. Colinge, Nature, 2011, 479, 310–316.
29. J. Shen, Y. Zhu, H. Jiang and C. Li, Nano Today, 2016, 11(4), 483–520.
30. S. A. Mohamed, F. A. Al-Sulaiman, N. I. Ibrahim, M. H. Zahir, A. Al-Ahmed, R. Saidur, B. S. Yilbaş and A. Z. Sahin, Renewable Sustainable Energy Rev., 2017, 70, 1072–1089.
31. R. Hu, Y. Liu, S. Shin, S. Huang, X. Ren, W. Shu, J. Cheng, G. Tao, W. Xu, R. Chen and X. Luo, Adv. Energy Mater., 2020, 10(17), 1903921.
32. S. Malkin and E. Fischer, J. Phys. Chem., 1962, 66(12), 2482–2486.
33. E. Fischer, J. Am. Chem. Soc., 1960, 82(13), 3249–3252.
34. S. H. Song, K. H. Park, B. H. Kim, Y. W. Choi, G. H. Jun, D. J. Lee, B. S. Kong, K. W. Paik and S. Jeon, Adv. Mater., 2013, 25, 732–737.
35. T. Morishita, M. Matsushita, Y. Katagiri and K. Fukumori, J. Mater. Chem., 2011, 21, 5610–5614.
36. T. Morishita, M. Matsushita, Y. Katagiri and K. Fukumori, Carbon, 2010, 48, 2308–2316.
37. K. Hu, D. D. Kulkarni, I. Choi and V. V. Tsukruk, Prog. Polym. Sci., 2014, 39, 1934–1972.
38. A. A. Balandin, Nat. Mater., 2011, 10, 569–581.
39. G. Li, M. Zhu, W. Gong, R. Du, A. Eychmüller, T. Li, W. Lv and X. Zhang, Adv. Funct. Mater., 2019, 29(20), 1900188.
40. V. Guerra, C. Wan and T. McNally, Prog. Mater. Sci., 2019, 100, 170–186.
41. Y. Wu, C. An, Y. Guo, Y. Zong, N. Jiang, Q. Zheng and Z. Yu, Nano-Micro Lett., 2024, 16(1), 118.
42. Y. Hwang, M. Kim and J. Kim, Chem. Eng. J., 2014, 246, 229–237.
43. H. Zhu, Y. Li, Z. Fang, J. Xu, F. Cao, J. Wan, C. Preston, B. Yang and L. Hu, ACS Nano, 2014, 8, 3606–3613.
44. J. Hu, Y. Huang, Y. Yao, G. Pan, J. Sun, X. Zeng, R. Sun, J. B. Xu, B. Song and C. P. Wong, ACS Appl. Mater. Interfaces, 2017, 9, 13544–13553.
45. P. P. Liu, X. Chen, Y. Li, P. Cheng, Z. D. Tang, J. J. Lv, W. Aftab and G. Wang, ACS Nano, 2022, 16(10), 15586–15626.
46. A. Allahbakhsh and M. Arjmand, Carbon, 2019, 148, 441–480.
47. X. Xu, Q. Zhang, M. Hao, Y. Hu, Z. Lin, L. Peng, T. Wang, X. Ren, C. Wang, Z. Zhao, C. Wan, H. Fei, L. Wang, J. Zhu, H. Sun, W. Chen, T. Du, B. Deng, G. Cheng, I. Shakir, C. Dames, T. Fisher, X. Zhang, H. Li, Y. Huang and X. Duan, Science, 2019, 363(6428), 723–727.
48. P. Giusto, D. Cruz, T. Heil, N. Tarakina, M. Patrini and M. Antonietti, Adv. Sci., 2021, 8(17), 2101602.
49. W. Sakuma, S. Tamasaki, S. Fujisawa, T. Kodama, J. Shiomi, K. Kanamori and T. Saito, ACS Nano, 2021, 15(1), 1436–1444.
50. Y. Xue, X. Zhou, T. Zhan, B. Jiang, Q. Guo, X. Fu, K. Shimamura, Y. Xu, T. Mori, P. Dai, Y. Bando, C. Tang and D. Golberg, Adv. Funct. Mater., 2018, 28(29), 1801205.
51. H. He, X. Wei, B. Yang, H. Liu, M. Sun, Y. Li, A. Yan, C. Tang, Y. Lin and L. Xu, Nat. Commun., 2022, 13(1), 4242.
52. G. H. Du, J. F. Hu and Z. G. Zhang, Sol. Energy Mater. Sol. Cells, 2022, 226, 111532.
53. P. Lewis, C. Inman, F. Maya, J. Tour, J. Hutchison and P. Weiss, J. Am. Chem. Soc., 2005, 127(49), 17421–17426.
54. Y. Zheng, J. Payton, C. Chung, R. Liu, S. Cheunkar, B. Pathem, Y. Yang, L. Jensen and P. Weiss, Nano Lett., 2011, 11, 3447–3452.
55. Y. Xue, X. Zhou, T. Zhan, B. Jiang, Q. Guo, X. Fu, K. Shimamura, Y. Xu, T. Mori, P. Dai, Y. Bando, C. Tang and D. Golberg, Adv. Funct. Mater., 2018, 28, 1801205.
56. M. Qin, Y. Xu, R. Cao, W. Feng and L. Chen, Adv. Funct. Mater., 2018, 28, 1805053.
57. Z. Sheng and X. Zhang, Science, 2023, 382(6677), 1358–1359.
58. J. Wang, D. Liu, Q. Li, C. Chen, Z. Chen, P. Song, J. Hao, Y. Li, S. Fakhrhoseini, M. Naebe, X. Wang and W. Lei, ACS Nano, 2019, 13, 7860–7870.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta04540h

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