Periodic Reporting for period 1 - HITEH (Designing Stretchable Hydrogel Electrolytes and 2-Dimensional MXene Electrodes for Energy Harvesting via Ionic Thermoelectrics)
Reporting period: 2022-09-01 to 2024-08-31
- 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.
- 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.
1) Synthesis of Stable and Stretchable Hydrogel Electrolyte
A stable and stretchable hydrogel was synthesized from 2-hydroxyethyl acrylate (2-HEA), polyethylene glycol diacrylate (PEGDA), and non-volatile ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM+TFSI–) (Figure 3). Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy investigated the changes in the chemical bonds after synthesis. It confirmed the crosslinked PEGDA and PHEA polymer networks and hydrogen bonding between EMIM+ cations and polymer network. The air stability was evaluated using Wt/W0, where Wt was the corresponding weight and W0 was the initial weight of hydrogel, respectively. The weights barely changed at 20 °C and 50% relative humidity (RH). The retention capacity maintained approximately 98.5% of its original weight even after 30 d owing to numerous the hydroxyl groups acting as hygroscopic sites in 2-HEA. Also, the synthesized hydrogel possessed thermal and air stability. The weight changes, investigated using thermogravimetric analysis, were less than 2% of their initial weights up to 200 °C, thereby indicating that hydrogels were quasi-dry and possessed thermal stability. The hydrogels showed stretchability without breaking and their mechanical stretchability can be controlled by varying the initial amounts of PEGDA and EMIM+TFSI–. The maximum stretchability value of hydrogel was larger 1500%, however, it showed a highly viscoelastic deformation because of insufficient crosslinking agent (PEGDA).
2) i-TE Performance of Hydrogels and Water contribution
One strategy to increase i-TE performance is to facilitate a specific interaction between the polymer and ions. Hydrogen bonding between EMIM+ and polymers can promote ionic dissociation as well as facilitate the difference in thermo-diffusion between the anion and cation. Recently, a hydrovoltaic voltage induced by the gradient in water concentration along a temperature gradient has been reported to contribute to i-TE measurement with a lateral geometry (illustrated in Fig. 4a). The evaporation of water on the hot side and condensation on the cold side triggers a concentration difference in water and a large hydrovoltaic voltage. The Seebeck coefficient and ionic conductivity measurements were conducted with or without polyimide encapsulation tape (as depicted in Fig. 4a and b) to distinguish between the hydrovoltaic and Soret effects in the output voltage under a temperature gradient. The fellow investigated the role of water molecules in hydrogels and humidity effect. This synergetic contribution of Soret effect and hydrovoltaic voltage can obtain high Seebeck coefficient of 38.9 mV K–1 at 90% RH. The ionic conductivities increased to 3.764 × 10–1 mS cm–1 at 90% RH. In addition, the output voltage and ionic conductivity were barely altered after consecutive 200% stretching/releasing for 100 cycles.
3) A fully Stretchable ITESC Consisting of Hydrogel and Au-coated TiO2 Nanowires (Au-TiO2 NWs)
The fellow prepared Au-TiO2 NWs electrodes with a length of 3–20 µm on thermoplastic polyurethane (TPU) for stretchable 1-dimensional electrodes. The Au-TiO2 NWs were selected because they possess excellent electrode performance with inert stability, which is suitable for wearable applications. The sheet resistance of the Au-TiO2 NWs encapsulated with the TPU increased from 3.44Ω/sq at 0% to 200.08Ω/sq at 300% due to the disconnection of electrical pathway of Au-TiO2 NWs during mechanical deformation. The Au-TiO2 NWs could be reversibly stretched for consecutive 1000 cycles of 50% strain without significant mechanical failure. Figure 5 displays a sketch of a fully stretchable ITESC, which consisted of Au-TiO2 NWs electrodes on the TPU substrates that are sandwiching and encapsulating the freestanding hydrogel. The stored energy of ITESC during the charging/discharging step was calculated to be 995.44 and 653.61 nJ for ΔT = 8.8 K without strain. When the stretchable ITESC was subjected to approximately 60% strain, the energy during the charging/discharging step reduced to be 720.34 and 530.52 nJ, respectively. It is mainly due to a decrease in output voltage, as the energy is proportional to the square of the output voltage. The stored energy in the stretchable ITESC was decreased with increasing strains; however, the ITESC could harvest and store heat energy.
A stable and stretchable hydrogel was synthesized from 2-hydroxyethyl acrylate (2-HEA), polyethylene glycol diacrylate (PEGDA), and non-volatile ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM+TFSI–) (Figure 3). Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy investigated the changes in the chemical bonds after synthesis. It confirmed the crosslinked PEGDA and PHEA polymer networks and hydrogen bonding between EMIM+ cations and polymer network. The air stability was evaluated using Wt/W0, where Wt was the corresponding weight and W0 was the initial weight of hydrogel, respectively. The weights barely changed at 20 °C and 50% relative humidity (RH). The retention capacity maintained approximately 98.5% of its original weight even after 30 d owing to numerous the hydroxyl groups acting as hygroscopic sites in 2-HEA. Also, the synthesized hydrogel possessed thermal and air stability. The weight changes, investigated using thermogravimetric analysis, were less than 2% of their initial weights up to 200 °C, thereby indicating that hydrogels were quasi-dry and possessed thermal stability. The hydrogels showed stretchability without breaking and their mechanical stretchability can be controlled by varying the initial amounts of PEGDA and EMIM+TFSI–. The maximum stretchability value of hydrogel was larger 1500%, however, it showed a highly viscoelastic deformation because of insufficient crosslinking agent (PEGDA).
2) i-TE Performance of Hydrogels and Water contribution
One strategy to increase i-TE performance is to facilitate a specific interaction between the polymer and ions. Hydrogen bonding between EMIM+ and polymers can promote ionic dissociation as well as facilitate the difference in thermo-diffusion between the anion and cation. Recently, a hydrovoltaic voltage induced by the gradient in water concentration along a temperature gradient has been reported to contribute to i-TE measurement with a lateral geometry (illustrated in Fig. 4a). The evaporation of water on the hot side and condensation on the cold side triggers a concentration difference in water and a large hydrovoltaic voltage. The Seebeck coefficient and ionic conductivity measurements were conducted with or without polyimide encapsulation tape (as depicted in Fig. 4a and b) to distinguish between the hydrovoltaic and Soret effects in the output voltage under a temperature gradient. The fellow investigated the role of water molecules in hydrogels and humidity effect. This synergetic contribution of Soret effect and hydrovoltaic voltage can obtain high Seebeck coefficient of 38.9 mV K–1 at 90% RH. The ionic conductivities increased to 3.764 × 10–1 mS cm–1 at 90% RH. In addition, the output voltage and ionic conductivity were barely altered after consecutive 200% stretching/releasing for 100 cycles.
3) A fully Stretchable ITESC Consisting of Hydrogel and Au-coated TiO2 Nanowires (Au-TiO2 NWs)
The fellow prepared Au-TiO2 NWs electrodes with a length of 3–20 µm on thermoplastic polyurethane (TPU) for stretchable 1-dimensional electrodes. The Au-TiO2 NWs were selected because they possess excellent electrode performance with inert stability, which is suitable for wearable applications. The sheet resistance of the Au-TiO2 NWs encapsulated with the TPU increased from 3.44Ω/sq at 0% to 200.08Ω/sq at 300% due to the disconnection of electrical pathway of Au-TiO2 NWs during mechanical deformation. The Au-TiO2 NWs could be reversibly stretched for consecutive 1000 cycles of 50% strain without significant mechanical failure. Figure 5 displays a sketch of a fully stretchable ITESC, which consisted of Au-TiO2 NWs electrodes on the TPU substrates that are sandwiching and encapsulating the freestanding hydrogel. The stored energy of ITESC during the charging/discharging step was calculated to be 995.44 and 653.61 nJ for ΔT = 8.8 K without strain. When the stretchable ITESC was subjected to approximately 60% strain, the energy during the charging/discharging step reduced to be 720.34 and 530.52 nJ, respectively. It is mainly due to a decrease in output voltage, as the energy is proportional to the square of the output voltage. The stored energy in the stretchable ITESC was decreased with increasing strains; however, the ITESC could harvest and store heat energy.
- Although stretchable i-TE electrolytes with giant Seebeck coefficient have been reported for ITESCs, they have limitations in wearable heat harvesting applications. Their structure of ITESC was planar type in which the rigid electrode was used to characterize a stretchable electrolyte; however, this is not the correct architecture for wearable ITESCs. Since the human skin sustainably generates a vertical thermal power of 20 mW cm-2, the wearable ITESC for the conversion of skin temperature into electricity must be vertically positioned along the z-axis rather than the x–y plane on the human [Figure 6]. Secondly, i-TE electrolytes in some literatures were characterized in a vertical-structured device with rigid or flexible electrode. Since human motions normally induce a mechanical stretching level of 50%, therefore, the integration of stretchable electrodes should be required for the development of wearable ITESC. Additionally, the evaporation of water inside hydrogels at ambient temperature significantly causes low environmental stability, making it difficult to reliably maintain the i-TE performance for a long time without an additional sealing process. Hence, ITESCs for wearable applications require (i) the incorporation of mechanically deformable electrodes, (ii) the long-term stability of i-TE electrolytes, and (iii) vertical assembly of the entire structure of the electrodes and i-TE electrolytes. Such fully stretchable ITESCs have not yet been reported because of the absence of stretchable electrodes and stable i-TE electrolytes with stretchability.
- The fellow proposed a fully stretchable ITESC with a vertical structure that can harvest energy from thermal gradients during deformation (at 60% strain). The fully stretchable ITESC consisted of a hydrogel electrolyte and Au-TiO2 NWs. First, the stretchable electrolyte exhibited excellent air stability for 60 days under ambient conditions (20 °C, 50% RH) without any noticeable degradation owing to the presence of hydroxyl groups (-OH) and nonvolatile EMIM+TFSI-. The ITE exhibited an apparent Seebeck coefficient of 38.9 mV K-1 and an ionic conductivity of 3.76 × 10-1 mS cm-1 at 90% RH.
- The fellow developed an ionic TE device operatable on curved surfaces, in which the electrolyte was vertically located between the Au/Cu foil electrodes. It enabled thermal-to-electrical conversion when the device was mounted on electric appliances or human skin. The compatibility with skin curvature owing to the mechanical advantage ensured that the i-TE devices were conformably attached to human skin. Finally, a fully stretchable ITESC was implemented for the first time in which the ITE was sandwiched between stretchable Au-TiO2 NWs electrodes on TPU. The fully stretchable ITESC could be thermally charged at 60% strain and exhibited reliable mechanical stability for 500 repetitive cycles at 50% strain without noticeable degradation.
- The fellow firmly believes that this results during MSCA project will be of interest to the state of the art because a fully stretchable ITESC with air stability exhibits excellent results and can be applied to thermal-to-electrical conversion even during mechanical deformation. The ITESC can offer tremendous opportunities for wearable and sustainable energy harvesting devices owing to its air stability and outstanding mechanical advantages. Furthermore, this work will be useful for a broad audience in various fields of research, including material science, ionoelastoemr, thermoelectrics, temperature sensors, electrolytes, and hydrovoltaics.
- The fellow proposed a fully stretchable ITESC with a vertical structure that can harvest energy from thermal gradients during deformation (at 60% strain). The fully stretchable ITESC consisted of a hydrogel electrolyte and Au-TiO2 NWs. First, the stretchable electrolyte exhibited excellent air stability for 60 days under ambient conditions (20 °C, 50% RH) without any noticeable degradation owing to the presence of hydroxyl groups (-OH) and nonvolatile EMIM+TFSI-. The ITE exhibited an apparent Seebeck coefficient of 38.9 mV K-1 and an ionic conductivity of 3.76 × 10-1 mS cm-1 at 90% RH.
- The fellow developed an ionic TE device operatable on curved surfaces, in which the electrolyte was vertically located between the Au/Cu foil electrodes. It enabled thermal-to-electrical conversion when the device was mounted on electric appliances or human skin. The compatibility with skin curvature owing to the mechanical advantage ensured that the i-TE devices were conformably attached to human skin. Finally, a fully stretchable ITESC was implemented for the first time in which the ITE was sandwiched between stretchable Au-TiO2 NWs electrodes on TPU. The fully stretchable ITESC could be thermally charged at 60% strain and exhibited reliable mechanical stability for 500 repetitive cycles at 50% strain without noticeable degradation.
- The fellow firmly believes that this results during MSCA project will be of interest to the state of the art because a fully stretchable ITESC with air stability exhibits excellent results and can be applied to thermal-to-electrical conversion even during mechanical deformation. The ITESC can offer tremendous opportunities for wearable and sustainable energy harvesting devices owing to its air stability and outstanding mechanical advantages. Furthermore, this work will be useful for a broad audience in various fields of research, including material science, ionoelastoemr, thermoelectrics, temperature sensors, electrolytes, and hydrovoltaics.