Liquid-metal-electrode-based compact, flexible, and high-power thermoelectric device

There is an increasing demand for power sources of wearable sensors and personal refrigeration. Flexible thermoelectric device (FTED) can be ideal candidate for such purpose. This paper reports on a compact, high-power and flexible thermoelectric device based on a bulk thermoelectric (TE) material. For thermoelectric device based on bulk TE materials to be flexible, (i) flexible electrodes and (ii) holders to secure TE materials are required. In FTED, we have used liquid-metal encapsulated in polydimethylsiloxane (PDMS) as electrodes on one side above the bending neutral axis and flexible printed circuit boards (FPCB) as electrodes below the bending neutral axis. Hence, the stretchability of the liquid metal electrode and flatness of the FPCB are fully utilized to reduce thermal contact resistance. Additionally, we used PDMS and flexible wires as holders to eliminate filler materials that annihilate thermal bypass, i.e., heat transfer not going through TE materials. For refrigeration using portable batteries as power sources, the refrigerated skin temperature was lowered by 5.4 K which is adequate for humans to perceive coldness, according to theoretical analysis. For human body-heat harvesting, the open-circuit voltage and output power density were 7.38 mV and 8.32 μW/cm² or 123.74 μW, respectively. This implies that the FTED can be used both as a portable refrigerator and a wearable body-heat harvester.