cellulose,[69] and GO [66, 104, 105] can be used as supporting materials or binders to help BN build 3D conductive network for polymeric phase change composites. For example, GO/BN porous scaffolds with isotropic and aligned structures have been designed through a facile yet powerful freeze‐casting to fabricate thermally conductive PEG‐based phase change composites,[66, 104, 105] and the final composites with a phase change enthalpy of 143.6 J g−1 exhibit enhanced thermal conductivity as high as 3.18 W m−1 K−1 at BN loading of c. 28.7 wt%.
Furthermore, hybrid fillers with different geometries, such as silver‐graphene[57] and GNP‐BN[106], are capable of forming synergistic network structure to improve the thermal conductivity of PEG‐based phase change composites. Metals, carbon materials, ceramics, and their hybrids can be employed as enhanced component to produce electrically conductive or insulating polymeric phase change composites with high thermal conductivity through facile solution blending, melt blending, and vacuum impregnation methods. More details are summarized in Table 2.4, in which thermal conductivity enhancement (TCE) is defined by Eq. (2.1). Constructing 3D conductive network in phase change composites seems to be an efficient route to achieve a trade‐off among shape stability, thermal conductivity, and energy storage density, attracting ever‐growing interest.
where kc and km represent the thermal conductivities of composites and matrix or matrix with supporting materials, respectively.
2.4 Energy Conversion and Storage Based on Polymeric Phase Change Composites
In recent years, energy conversion, storage, and utilization based on PCMs have become the research frontiers, especially for solar energy, industrial waste heat, and surplus heat. As‐prepared phase change composites with improved comprehensive performance can achieve efficient energy conversion and storage via electro‐to‐heat (Figure 2.7a),[95] photo‐to‐heat (Figure 2.7b),[88, 95, 109] magnetism‐to‐heat (Figure 2.7c),[107] heat‐to‐electricity (Figure 2.7d),[108] and photo‐to‐heat‐to‐electricity (Figure 2.7e)[105] routes, providing a strong support for the popularization of advanced energy‐related devices and systems associated with functional PCMs. Also, both solid–liquid and solid–solid polymeric phase change composites can realize these energy conversion routes.
2.4.1 Electro‐to‐Heat Conversion
Electric thermal storage technique based on PCMs is expected to improve the utilization efficiency of power generation equipment and exhibits broad prospects in the field of off‐peak electricity. Unfortunately, the electrical conductivity of PCMs is inherently low, which makes it impossible to directly employ them for electro‐to‐heat conversion. It is usually necessary to incorporate functional fillers to improve the electrical conductivity of phase change composites, and carbon materials, including biomass carbon,[110] carbon nanofiber (CNF),[111] CNT,[109] and graphene[112], are preferentially considered. Of critical importance is the electro‐to‐heat conversion and storage efficiency (ηe), which can be defined by the ratio of stored heat energy to applied electric energy during phase change process and determined by the electro‐thermal calculation Eq. (2.2).
Table 2.4 Thermal conductivity of polymeric phase change composites.
| Material systems | Processing methods | Thermally conductive filler loading | Melting enthalpy (Jg−1) | Thermal conductivity (W m−1 K−1) | Thermal conductivity enhancement (%) |
|---|---|---|---|---|---|
| PEG/diatomite/silver nanoparticle[84] | Vacuum impregnation | 7.2 wt% | 111.3 | 0.82 | 127 |
| PEG/EVM/silver nanowire[51] | Physical blending and impregnation | 19.3 wt% | 99.1 | 0.68 | 172 |
| PEG/SiO2/cupper[85] | Sol–gel and in‐situ doping method | 2.1 wt% | 110.2 | 0.414 | 15 |
| PW/silver‐PVP nanowire aerogel[87] | Vacuum impregnation | 5.43 wt% | ∼163 | 0.49 | ∼133 |
| PEG‐co‐N,N′‐dihydroxyethyl aniline/single‐walled CNT[88] | Vacuum evaporation | — | 100.5 | 0.334 | 25 |
| PEG/single‐walled CNT[90] | Solution blending | 10 wt% | 165.4 | 3.43 | 1329 |
| PEG/SiO2/CF[91] | Sol–gel and in‐ situ doping method | 3 wt% | 142.6 | 0.45 | 73 |
| PEG/EG[54] | Melt blending | 10 wt% | 161.2 | 1.324 | 344 |
| PEG/GO/GNP[92] | Solution blending | 6 wt% | 167.4 | 1.72 | 493 |
| PEG/unsaturated polyester resin/GNP[93] | Free radical copolymerization and solution blending | 2 wt% | 140.8 | 0.67 | 131 |
| PEG/single‐walled CNT[89] | Vacuum impregnation | 8 wt% | 162.1 | 2.73 | 950 |
| PEG/GNP[89] | Vacuum impregnation | 4 wt% |
169.3
|