The human body is an incredible heat engine. It can convert nutrient chemical energy into heat and mechanical energy while also supporting vital energy. To maintain internal thermal balance, the body expends a lot of heat energy into the environment when it works hard. When people are in cold or warm environments, the skin temperatures in the upper and lower extremities are the lowest (around 28-31 °C), and the core body temperature is set at 37 °C.1 The human body mainly releases heat to the external environment by convection, radiation, or evaporation. Moreover, this “heat engine” works stable round the clock and is not limited by climate and location. However, from the perspective of energy recycling or recovery, this power source seems “flexible”, and generally hard to collect.
In terms of this thermal collecting problem, we believe that a flexible thermoelectric device (FTEG) working around room temperature and within a small temperature range can solve this problem very well. Thermoelectric (TE) technology is widely regarded as a green and environmentally friendly method of capturing heat and converting it into electrical power.2The immediate energy conversion from body heat to electrical power without the use of external force is advantageous for self-powering electronics.3 (Figure 1)
Figure 1 Illustration diagram of a thermoelectric application
A traditional thermoelectric generator consists of p-/n-type semiconductor arrays that are connected in series with copper electrodes and sandwiched together by ceramic layers. Generally, there have three important parameters to evaluate the performance of the TE materials: (1) Seebeck Coefficient (S) ( voltage generated under per unit temperature gradient, ). 4 (2) Electrical Conductivity (s), and (3) thermal conductivity (k). An ideal TE material requires high S and s, meanwhile, low k. 5
Traditionally, the materials which are used to fabricate the TE devices, such as Bi2Te3, Sb2Te3, and SnSe, are hard and brittle whereas they have high output performance. Conducting polymers are another category of TE materials, which are flexible, lightweight, and thermal insulated. Thus, inorganic-polymer composites may offer an alternative strategy to replace the rigid, heavy, and sometimes toxic semiconductor TE materials.
Bi2Te3 is one of the most well-studied and popular TE materials which are operational at room temperature range with superior performance.5 Accordingly, some favorable conducting polymers, for example, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and polyaniline (PANI) are regarded as an ideal mediums for flexible and wearable thermoelectric functions.5 Yet, in most cases, these types of materials do not perform well. Therefore, it is necessary to strike a balance between the flexibility and performance of FTEGs.
To achieve functional flexible TE materials, we hereby report a facile one-step method by cooperating n- and p-type Bi2Te3 particles with PEDOT:PSS respectively. These resulted in flexible TE precursor materials taking a solution form, which can be printed, painted, or drop cast on elastic substrates. (Figure 2a) However, not every flexible material can outperform its original rigid counterpart in terms of TE performance. We, too, encountered difficulties at first. The TE performance of the whole flexible composite system is two orders of magnitude lower than the Bi2Te3 after cooperating with PEDOT:PSS when annealed at 423K. Although the resulted lower thermal conductivity is desirable. But yet, a great challenge is that FTEGs derived from such precursors will invariably contain non-conducting binder residuals between the semiconducting particles, lowering the electric conductivity and then the whole TE performance.
Figure 2 (a) Photograph of n-type and p-type Bi2Te3/PEDOT:PSS inks. (b) Measured Seebeck coefficients of n-Bi2Te3/PEDOT:PSS and PEDOT:PSS. (c) Measured Seebeck coefficients of P-Bi2Te3/PEDOT:PSS and PEDOT:PSS. (d) Schematic illustration of FTEG, comprising n- and p-TE composite chip pairs connected by flexible polyimide circuit boards as well as copper fabrics as the heat sink. (e) Photograph of the fabricated FTEG device made from p-/n-composites with (top) and without (middle) the fabric heat sink (Scale bar: 10 mm). (f) specific output power and power density of the composites FTEGs with/ without the heat-sink copper fabric. (g) Demonstration for measuring the VOC of FTEG (32 pairs ) worn on arm at room temperature. (Scale bar: 40mm ), FLIR image: Temperature distribution between arm and surface of FTEG. (h) An FTEG drives an LED fabric skirt via demo board. (See Supplementary Movie-2).
We then further increased the annealing temperature up to 673K gradually based on the thermogravimetric analysis. As expected, we found that the annealing process was attributed to the connection between the Bi2Te3 alloy particles and PEDOT:PSS polymer, resulting the enhanced electrical conductivity. Surprisingly, the optimized TE composite displays a synergistic effect, showing a high room-temperature Seebeck coefficient of -218.0 mV K-1 (n-type), much higher than its constituent bulk alloy and polymer matrix; 273.3 mV K-1 (p-type) slightly higher than the bulk constitutive alloy.(Figure 2b, 2c)
Once the materials are ready, then we began to consider how to realize the route from the materials to the device. The output performance of the FTEG is determined by its functional and structural materials, device structural design, and fabrication process. However, few studies focus on heat sink design in FTEG fabrication. In our device assembly, we tried to reduce the thermal resistance between FTEG and hot or cold sources by using copper fabric.
A sandwich-structured FTEG with heat sink fabric has been designed and fabricated further by employing these high-performance TE composites, producing an output power of 9.0 mW, the specific output power of 2.3 mWg-1 and an areal power density of 0.65 mW cm-2 with a temperature difference of 45K. (Figure 2f) The employment of the heat sink fabrics has yielded a three-fold increment in the output power of the generator since the soft heat sink fills up the air gap between the FTEG and the hot or cold surface of the experimental setup. A more significant result is the high output power at a relatively small temperature difference, say, between the human body and office environment around 15 K. (Figure 2g) Such an FTEG can drive an illuminating fabric with arrays of LEDs in seconds when applying a temperature difference to the FTEG by the lab-made experimental set-up. (Figure 2h) The solid and stationary sandwich-structured FTEG has an areal output power density of 1.07 Wm-2 at 15 K. This value is comparable to the output of an indoor solar cell and more than twice the highest power value from a flexible triboelectric generator reported so far.
In brief, this work highlights the research gap in lack of suitable TE materials, poor understanding of the structural design of the flexible devices, and their fabrication process in FTEGs. The flexible thermoelectric technology also could expand to autonomous robots via the conception of “embodied energy”. We believe that this work will spark further research, eventually leading to the successful use of FTEGs as an alternative power source for wearable microelectronic devices.
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- Zeng, W. et al. Defect-engineered reduced graphene oxide sheets with high electric conductivity and controlled thermal conductivity for soft and flexible wearable thermoelectric generators. Nano Energy54, 163-174 (2018).
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- Zhang, L.S. et al. Fiber-Based Thermoelectric Generators: Materials, Device Structures, Fabrication, Characterization, and Applications. Advanced Energy Materials 8(2018).