Aerogels have the characteristics of ultra light, ultra high porosity, ultra-high specific surface area, adjustable thermal/electrical conductivity and mechanical flexibility, and are expected to become the ideal carrier materials for effectively encapsulating phase change materials. The new aerogel composite phase change material has high energy storage density and is considered as an advanced thermal energy storage technology. What is more attractive is that the ingenious combination of phase change materials and aerogels has accelerated the development of advanced multi-functional composite phase change materials, such as mechanical flexibility, shape memory, thermal infrared stealth, flame retardancy, wave absorption and other functions. These advanced functions provide innovative application platforms for phase change materials, such as wearable thermal management, temperature regulating textiles, and intelligent grippers.

Recently, Beijing University of Science and Technology systematically summarized the latest progress of high-performance and multi-functional aerogel based composite phase change materials, with special emphasis on their advanced versatility, such as energy conversion strategies such as photothermal, electrothermal, acoustic thermal and photothermal electric, flexibility, flame retardancy, shape memory, thermal infrared stealth, intelligent gripper and wave absorption. The multifunctional function of different aerogels in composite phase change materials and the relationship between their structures and thermophysical properties were systematically expounded. In addition, the article also shows the interdisciplinary research progress of aerogel and 3D printing technology, which helps to create more advanced functional composite phase change materials. The review was published on ACS Nano under the title of "Aerogels Meet Phase Change Materials: Fundamentals, Advances, and Beyond".

Fig. 1 Various aerogel based composite phase change materials and advanced multi-function applications

Fig. 2 Main progress of aerogel based composite phase change materials in recent years

1. Carbon aerogel based composite phase change material

Carbon aerogels usually have a continuous three-dimensional network structure, consisting of interconnected colloidal carbon particles or polymer carbon chains, which can be controlled and adjusted on a nanoscale. According to the type of precursors, carbon aerogels can be divided into three categories: phenolic resin source carbon aerogels, carbon nanotubes and graphene source carbon aerogels and biomass source carbon aerogels gel, which are produced by pyrolysis of organic aerogels or self-assembly of precursors. Carbon based aerogels have all the structural characteristics of aerogels, and also have the ability to conduct electricity. Because of its unique advantages such as ultra-high porosity, ultra-low density and large specific surface area, it has attracted extensive research interest in the design of packaging support materials for phase change materials. The high porosity of carbon aerogel is conducive to improving the high load of phase change materials, ensuring the high thermal storage capacity of composite phase change materials, and preventing the leakage of melted phase change materials through strong capillary force. At the same time, the highly interconnected three-dimensional thermal conduction framework of carbon aerogels improves the inherent low thermal conductivity of phase change materials and improves the thermal response speed. In addition, carbon aerogel composite phase change material is considered as a promising candidate material for photo thermal and electro thermal energy conversion and storage due to its superior solar capture ability and high conductivity.

2. Silica aerogel based composite phase change material

In recent years, silica aerogels have been used as support materials to maintain the shape stability of phase change materials and prevent leakage during phase change due to their large porosity and specific surface area. Due to the ultra-low thermal conductivity of silica aerogels and the high thermal storage capacity of phase change materials, silica aerogels based composite phase change materials have great potential applications in thermal protection.

3. Polymer aerogel based composite phase change material

Polymer aerogels have become competitive energy storage support materials due to their ultra-low thermal conductivity, ultra-light density, high porosity, excellent mechanical properties and flexible molecular design.

3.1 Cellulose aerogel based composite phase change material

Environmental issues have prompted researchers to develop advanced materials and applications using natural bio based materials. Cellulose aerogels extracted from cellulose nanofibers (CNFs), cellulose nanocrystals (CNCs) or bacterial cellulose (BC) have attracted extensive attention due to their rich resources, good biodegradability, easy degradation and environmental friendliness. Cellulose aerogels not only have the light weight, high porosity and large specific surface area of traditional aerogels, but also have the excellent properties of cellulose itself (good biocompatibility and degradability, high mechanical strength). Most notably, cellulose aerogels have attractive mechanical properties, such as high compressive strength and excellent flexibility. The intelligent integration of cellulose aerogel and phase change materials can produce advanced flexible multi-functional composite phase change materials.

3.2 Synthetic polymer aerogel based composite phase change materials

Synthetic polymer aerogels combine the advantages of aerogels and the convenience of polymer process routes (polyvinyl alcohol, polypropylene, polyimide, etc.), and have good application prospects in developing advanced multi-functional composite phase change materials such as flexibility, infrared stealth, shape memory, and thermal protection.

4. Other aerogel based composite phase change materials

4.1 Metal aerogel based composite phase change materials

Metal aerogels are three-dimensional porous materials made of metal nanomaterials. They combine the porous characteristics of aerogels with the typical characteristics of some metals. One of the key motivations for using metal aerogels to improve the performance of phase change materials is their high intrinsic thermal conductivity and conductivity.

4.2 Boron nitride aerogel based composite phase change materials

Hexagonal boron nitride (h-BN) is similar to graphite in structure, also known as white graphite. It has excellent electrical insulation, high thermal conductivity, high thermal stability and oxidation resistance. Different from the thermal conductivity and conductivity of carbon based nano filler reinforced phase change materials, boron nitride improves the thermal conductivity of phase change materials while giving them higher electrical insulation properties. Therefore, the infiltration of phase change materials into BN aerogels will help to prepare high thermal conductivity but insulating composite phase change materials, which are expected to be used for thermal management of microelectronic devices.

4.3 MXenes aerogel based composite phase change material

MXenes are two-dimensional materials derived from MAX phase or layered hexagonal carbides and nitrides. Since Gogotsi creatively synthesized MXenes in 2011, it has triggered various emerging research hotspots in the field of basic theory and engineering applications. In recent years, MXenes, with its excellent electromagnetic wave absorption, local surface plasmon resonance (LSPR) effect and interlayer structure, has shown outstanding photothermal conversion performance, and has become an excellent support material for phase change materials to effectively use solar energy.

Fig. 3 Advanced multi-function of aerogel composite phase change material

Fig. 4 Advantages and disadvantages, energy conversion strategies and advanced functions of various aerogel based composite phase change materials