HITEH · Designing Stretchable Hydrogel Electrolytes and 2-Dimensional MXene Electrodes for Energy Harvesting via Ionic Thermoelectrics

Designing Stretchable Hydrogel Electrolytes and 2-Dimensional MXene Electrodes for Energy Harvesting via Ionic Thermoelectrics

- The advancement of wearable electronics requires miniaturized and lightweight components for low power consumption. Furthermore, sustainable wearable self-powered systems should be sufficiently compact and flexible for integration with traditional electronics. Hence, technologies capable of converting energy naturally generated by the human body into electricity have attracted attention because humans continuously generate various types of natural energy sources, such as normal human motion, finger movements, elbow bending, knee bending, skin temperature, and footstep [Figure 1]. Particularly, the human epidermis is a beneficial thermal energy resource that can be converted to electricity by thermoelectric (TE) materials. - The capability of electronic TE (e-TE) materials, in which the thermodiffusion of electrons/holes occurs under a temperature gradient, has been remarkably improved, but their Seebeck coefficients (ΔVoutput/ΔT, Voutput is the output voltage) remain low (100 µV K-1) [Figure 2]. Hence, numerous pairs of p/n type TE legs must be connected to achieve a practical Voutput, which is unsuitable for wearable applications. New strategy for obtaining enhanced Seebeck coefficients (ionic thermoelectric, i-TE) has been demonstrated using thermal diffusion for ions [Figure 2]. Ions can be thermally diffused to the cold side, which is known as Soret effect. It induces differences in ion concentration. Hence, the unbalance in cations and anions concentrations generates enhanced ionic Seebeck coefficient, which is the orders of magnitude larger than those of conventional e-TEs. - When ΔT is set across an electrolyte, the thermodiffusion of mobile ions from the hot to the cold side induces a concentration gradient, as previously explained. As thermo-diffused ions cannot pass through the electrodes, they accumulate at the interface between electrodes and electrolyte, thereby generating Voutput. If an external circuit is connected, the electronic charge from the electrodes compensates for the previously generated Voutput. This behavior results in an electric double layer at the interface between the electrolyte and electrode. A sufficient surface area of electrodes enables the conversion of heat energy to electricity with the ionic Seebeck effect, creating an i-TE supercapacitor (ITESC). The stored energy (E) of an ITESC originating from its ionic Seebeck effect during thermal charging can be indicated as E=(1/2)C(Voutput)^2, where C is the capacitance. - The fellow’s purposes of this MSCA project are 1) design and synthesis of the stretchable hydrogel electrolytes that exhibit intrinsic stretchability (> 300 %) and humidity-independent high ionic Seebeck coefficient (> 50 mV K-1), 2) realizing 2D MXene electrodes capable of high capacitance (> 500 F/g), 3) fabricating energy harvesting and sensing devices based on i-TE. - Publication summary: The fellow published the first paper regarding 1) design and synthesis of stretchable hydrogel and 3) fabricating energy harvesting devices. The second paper was a review article that addressed stretchable i-TE electrolytes for wearable applications in recent literatures.

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