Flexible highly thermally conductive biphasic composite films for multifunctional solar/electro-thermal conversion energy storage and thermal management

Phase change materials (PCMs) have been widely used for thermal energy storage in overcoming the intermittence of renewable energy and passive thermal management. However, low thermal conductivity, leakage, inherent brittleness, and lack of responses under multiple stimuli preclude their widespread applications. Here, we report a flexible and form-stable solid-solid/solid-liquid biphasic phase change composites to achieve efficient solar/electro-thermal energy conversion and storage as well as thermal management of high-power devices simultaneously. Proposed biphasic polyurethane–stearic acid/expanded graphite (PU–SA/EG) composites demonstrate state-of-the-art performances with a high energy storage density of 117.4–137.3 J/g and high thermal conductivity of 10.98–37.80 W/(m⋅K) simultaneously at an EG loading ratio of 10–30 wt %, benefiting from the formation of continuous thermal transport network provided by expanded graphite nanosheets. PU–SA/EG maintains high flexibility and good thermal reliability over more than 500 melting/solidification cycles. High solar- and electro-thermal conversion and storage efficiencies of up to 90.5% and 88.7% are achieved, respectively. When applied in thermal management of flexible electronic devices, PU–SA/EG film demonstrates a superior cooling performance by reducing device temperature prominently by 32 °C under a DC 5A discharging power supply. This work provides an effective way for harvesting and storing multiple energy sources like solar energy and electricity, as well as thermal management of high-power electronic devices.