Pyroelectric hydrogel and preparation method and application thereof

Abstract

The present application relates to a kind of thermoelectric water gel and its preparation method and application, preparation method includes the following steps: S1, acrylic acid ethoxy ethoxy ethyl ester, N-hydroxymethyl acrylamide monomer, initiator is added in solvent, under the action of ultraviolet light, free radical copolymerization is carried out, and water gel matrix is obtained;S2, the water gel matrix is soaked in lithium salt solution, and water gel matrix of swelling and lithium ion loading is obtained;S3, the water gel matrix of swelling and lithium ion loading of step S2 is soaked in the solution containing ferricyanide / ferrocyanide redox couple, and thermoelectric water gel is obtained.Compared with prior art, the present application can maintain good mechanical properties and tensile property while synergistically enhancing the soret effect and redox entropy, improving the thermoelectric conversion efficiency and single device output voltage, and then simplifying system integration and expanding its practical application ability.

Description

Technical Field

[0001] This invention relates to the field of thermoelectric materials technology, and in particular to a thermoelectric hydrogel, its preparation method and application. Background Technology

[0002] With the rapid popularization of IoT technology and the booming development of wearable bioelectronic devices, the market has increasingly urgent needs for sustainable, autonomous, distributed power sources. The human body, as a continuous and stable biological heat source, can provide low-grade heat energy (typically below 100°C). Efficiently converting this heat energy into electrical energy has become one of the important strategies for powering next-generation flexible electronic devices.

[0003] Among various energy conversion technologies, thermoelectric converters based on redox electrolytes (also known as thermo-ionized batteries) have attracted widespread attention due to their ability to achieve high thermoelectric conversion efficiency by utilizing the temperature dependence of electrode potential (i.e., the Seebeck effect). However, existing technologies still face several prominent challenges in practical applications. First, traditional thermoelectric converters mostly use liquid electrolytes, which pose a risk of leakage in wearable scenarios, potentially corroding the skin or damaging the circuitry, thus creating physical safety hazards. Second, hydrogel electrolytes introduced to overcome leakage problems, especially in n-type (negative Seebeck coefficient) systems, often struggle to balance mechanical strength and conversion efficiency: increasing polymer concentration or crosslinking density enhances mechanical properties but hinders ion transport, leading to a decrease in power output; while overly loose network structures, although beneficial for ion diffusion, cannot withstand repeated stretching during human movement and are prone to breakage. Furthermore, the Seebeck coefficient of most current n-type hydrogel systems remains at a low level, limiting their energy conversion efficiency, which urgently needs to be improved through mechanisms such as enhancing the Solette effect or adjusting ion entropy change.

[0004] In existing technologies, *Robust, Efficient, and Recoverable Thermocells with Zwitterion-Boosted Hydrogel Electrolytes for Energy-Autonomous and Wearable Sensing* discloses a polymer matrix for a hydrogel composed of polyacrylamide (PAM); the introduction of sulfobetaine methacrylate (SBMA) as a zwitterionic additive to utilize its strong electrostatic interaction to improve material properties; and the use of [Fe(CN)6]... 3- / 4-The redox-active electrolyte enables thermoelectric conversion; LiCl is added to adjust ionic conductivity and antifreeze properties. This scheme uses covalent cross-linking of PAM chains to form the first elastic network, while SBMA zwitterionic monomers are dispersed and attached between the PAM chains. The positively and negatively charged groups on the SBMA side chains bind tightly to the PAM chains and water molecules through strong dipole-dipole interactions and electrostatic interactions, thereby significantly improving the cohesiveness and mechanical strength of the hydrogel. Redox reaction with [Fe(CN)6] 3- / 4- and Li + Cl - Ions are uniformly distributed within the highly hydrated zwitterionic network pores. The zwitterionic groups facilitate the dissociation of salt ions, forming continuous ion transport channels. The signal transmission and energy conversion process is as follows: When the two ends of the device are in a temperature difference environment, a temperature gradient is formed inside the hydrogel; driven by the temperature difference, [Fe(CN)6]... 3- / 4- Electron exchange occurs on the electrode surface, while ions undergo thermal diffusion (Soret effect); the zwitterionic network reduces ion migration resistance and optimizes charge transport efficiency, ultimately converting the temperature difference signal into stable voltage and current output.

[0005] Nevertheless, this existing technology still has significant drawbacks. First, its Seebeck coefficient is relatively low, relying primarily on a basic redox entropy change mechanism and lacking an effective chemical amplification pathway, thus limiting the upper limit of power output. Second, in pursuing higher output power, materials often sacrifice flexibility and ductility, making it difficult to achieve high standard elongation at break and failing to fully adapt to the needs of human movement. Third, due to the low output voltage of a single device, practical applications such as driving commercial LEDs often require multiple units connected in series or external boost circuits, increasing system integration complexity and size. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the existing technology, such as the low Seebeck coefficient leading to limited output power, insufficient flexibility and ductility, and low single-device output voltage requiring complex external circuits. This invention provides a thermoelectric hydrogel, its preparation method, and its application. Based on selective ion-dipole interactions, it synergistically enhances the Solette effect and redox entropy change while maintaining good mechanical properties and tensile strength, thereby improving thermoelectric conversion efficiency and single-device output voltage, simplifying system integration, and expanding its practical application capabilities.

[0007] The objective of this invention can be achieved through the following technical solutions: One of the technical solutions of this invention is to provide a method for preparing a thermoelectric hydrogel, comprising the following steps: S1. Ethoxyethoxyethyl acrylate (EOEOEA), N-hydroxymethylacrylamide monomer (N-MAM), and initiator are added to a solvent and subjected to free radical copolymerization under ultraviolet light to obtain a hydrogel matrix. S2. Immerse the hydrogel matrix in a lithium salt solution to obtain a swollen hydrogel matrix loaded with lithium ions; S3. Immerse the hydrogel matrix swollen and loaded with lithium ions in step S2 in an atmosphere containing ferricyanide / ferrocyanide (Fe(CN)). 3- / Fe(CN) 4- In a solution of redox couples, thermoelectric hydrogels are obtained.

[0008] Further, in step S1, the molar ratio of EOEOEA and N-MAM is 1:1~8; The mass of the initiator is 0.1 wt% to 1 wt% of the sum of the mass of EOEOEA, N-hydroxymethylacrylamide monomer, and solvent. The mass ratio of ethoxyethyl acrylate to the solvent is 1:1~5.

[0009] Further, in step S1, the initiator is dimethyl benzoate (DMPA). The solvent is water and / or ethanol.

[0010] Furthermore, the solvent is composed of water and 75% ethanol in a mass ratio of 1 to 5:1.

[0011] Furthermore, in step S1, the wavelength of the ultraviolet light used in the ultraviolet irradiation is 300~400nm, and the irradiation time is 5~20min.

[0012] Further, in step S2, the lithium salt solution includes a lithium chloride (LiCl) solution; The lithium salt concentration in the lithium salt solution is 3~5M.

[0013] Further, in step S2, the immersion time of the hydrogel matrix in the lithium salt solution is 1~30 min, preferably 2~10 min.

[0014] Further, in step S3, the molar ratio of ferricyanide to ferrousyanide in the solution containing the ferricyanide / ferrousyanide redox couple is 1:0.2~5; The substance containing Fe(CN) 3- / Fe(CN) 4- The concentration of the ferricyanide / ferrocyanide redox couple in the solution is 0.05~0.2 mol / L.

[0015] Furthermore, in step S3, the substance containing Fe(CN) 3- / Fe(CN) 4- The solution of the redox couple is a mixed solution of K3[Fe(CN)6] and K4[Fe(CN)6].

[0016] Further, in step S3, the immersion time of the swollen and lithium-ion-loaded hydrogel matrix in a solution containing a ferricyanide / ferrous cyanide redox couple is 0.1 to 5 min.

[0017] The second technical solution of the present invention is to provide a thermoelectric hydrogel, which is prepared by the preparation method described above.

[0018] The third technical solution of the present invention is to provide an application of the thermoelectric hydrogel, wherein the thermoelectric hydrogel is used to prepare thermoelectric conversion devices.

[0019] Furthermore, the thermoelectric conversion device can be further fabricated into a thermoelectric power generation device.

[0020] Furthermore, the thermoelectric power generation device is a flexible thermal energy collection device used to collect the temperature difference between the human body and the environment.

[0021] Compared with the prior art, the present invention has the following advantages: (1) Breaking through the bottleneck of thermoelectric conversion efficiency and realizing high voltage output. This invention introduces N-hydroxymethylacrylamide (N-MAM) functional groups into the polymer network, and utilizes its amide sites to interact with [Fe(CN)6]. 4- Specific ion-dipole interactions form between ions, constructing a chemical amplification mechanism that significantly enhances the thermoelectric response. Compared to existing technologies that primarily rely on the fundamental redox entropy change (whose Seebeck coefficient is typically below -10 mV / K), this invention significantly increases the Seebeck coefficient to -59.6 mV / K. This allows a single device to generate a higher output voltage even under minute temperature differences, fundamentally solving the power output limitation problem caused by the low Seebeck coefficient.

[0022] (2) Overcoming the mutual constraints between strength and efficiency to achieve high ductility and self-healing properties. This invention uses EOEOEA and N-MAM copolymerization to construct a dynamically cross-linked polymer network. The EOEOEA segments provide flexibility, while the N-MAM segments provide rigidity. Their synergistic effect maintains efficient ion transport channels while ensuring excellent mechanical reliability of the material. Existing technologies often sacrifice the mechanical properties of materials in pursuit of high power output, leading to increased brittleness. In contrast, the hydrogel obtained by this invention achieves an elongation at break of 1283% and a tensile strength of 144 kPa, exhibiting extremely high flexibility and ductility. Furthermore, this material possesses room-temperature self-healing capabilities, with the elongation recovering to over 233% after repair, which greatly enhances its durability and practicality in wearable applications.

[0023] (3) Simplifies system integration and enables direct driving under low temperature differences. Benefiting from the ultra-high Seebeck coefficient, a single thermoelectric unit of this invention has a high output voltage gain, reducing the dependence on complex external circuits. Due to the low output voltage of a single device, existing technologies usually require multiple units to be connected in series or external boost circuits to drive common electronic devices, increasing system size and complexity. In contrast, the thermoelectric device prepared by this invention only needs to utilize the natural temperature difference between the human body and the environment of about 10 K, without any boost circuit, to directly drive a commercial LED with an operating voltage of 1.8 V. This significantly simplifies the integrated design of wearable energy harvesting systems and is conducive to achieving device miniaturization and low power consumption. Attached Figure Description

[0024] Figure 1 Fourier transform infrared spectra of EOEOEA, N-MAM, PENM hydrogel matrix and PENM thermoelectric hydrogel as shown in Example 1. Figure 2 The UV-Vis spectra of N-MAM as a special unit with two ions, (a) N+[Fe(CN)6] 4- [Fe(CN)6] 4- (b) N+[Fe(CN)6] 3- [Fe(CN)6] 3- ; Figure 3 (a) Schematic diagram and (b) circuit diagram of a wearable wristband fabricated from PENM thermoelectric hydrogel; Figure 4 The images show the physical images of the wearable wristband prepared from the PENM thermoelectric hydrogel obtained in Example 1, (a) before the bulb is lit, (b) after the bulb is lit, and (c) an enlarged schematic diagram of the bulb being lit. Figure 5(a) Voltages with different numbers of PENM thermoelectric hydrogels and (b) Output power when the number of PENM thermoelectric hydrogels is 5; Figure 6 The tensile test performance diagrams are shown for the thermoelectric hydrogels prepared in Examples 1-3; Figure 7 The following are test images of the self-healing process of the thermoelectric hydrogel prepared in Example 1: (a) after fracture, (b) after repair, and (c) after repair and stretching. Figure 8 Seebeck coefficients of the thermoelectric hydrogels prepared in Examples 1-3 at different monomer molar ratios; Figure 9 The voltage and Seebeck coefficient of the thermoelectric hydrogel prepared in Example 1 under different temperature differences; Figure 10 The conductivity and Seebeck coefficient of the thermoelectric hydrogels prepared in Examples 1, 4, and 5 are given. Figure 11 The conductivity of the thermoelectric hydrogels prepared in Examples 1, 6, and 7; Figure 12 The Seebeck coefficient is the thermoelectric hydrogel of Example 1 and Comparative Example 1. Detailed Implementation

[0025] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments. All other embodiments obtained by those skilled in the art based on the given embodiments without creative effort are within the scope of protection of this application.

[0026] Unless otherwise specified, the reagents, methods, instruments and equipment used in this invention are conventional reagents, methods, instruments and equipment in the art.

[0027] In the following embodiments, the CAS number and manufacturer of each reagent are shown in Table 1.

[0028] Table 1. CAS Numbers and Manufacturers of Each Reagent

[0029] Example 1 One of the technical solutions of this invention is to provide a method for preparing a thermoelectric hydrogel, comprising the following steps: S1. Add EOEOEA and N-MAM to a mixed solvent of water and ethanol (mass ratio 2:1) at a molar ratio of 1:5 (the mass ratio of EOEOEA to solvent is 1:1.5). Add initiator DMPA (mass of 0.5wt% of the sum of the masses of EOEOEA and N-MAM and the solvent). Under ultraviolet light irradiation (wavelength of ultraviolet light irradiation is 365nm, irradiation time is 10min), a free radical copolymerization reaction is carried out to obtain PENM hydrogel matrix.

[0030] S2, the hydrogel matrix (2×1×0.2cm) 3 The PENM hydrogel matrix was soaked in 5 mL of 4 M LiCl solution for 5 min to obtain a swollen and lithium-ion-loaded matrix. LiCl not only provides a supporting electrolyte but also plays a role in maintaining the hydration shell and optimizing the ion transport path.

[0031] S3. The hydrogel matrix swollen and loaded with lithium ions in step S2 is immersed in 5 mL of a solution of 0.1 mol / L K3[Fe(CN)6] and K4[Fe(CN)6] (molar ratio 1:1) redox couple for 1 min. The redox ions are anchored by the amide sites of N-MAM to obtain PENM thermoelectric hydrogel.

[0032] Example 2 The two examples are largely the same as in Example 1, except that the molar ratio of EOEOEA and N-MAM in step S1 is 1:1.

[0033] Example 3 The two examples are largely the same as in Example 1, except that the molar ratio of EOEOEA and N-MAM in step S1 is 1:8.

[0034] Example 4 The majority of the contents are the same as in Example 1, except that the concentration of the K3[Fe(CN)6] and K4[Fe(CN)6] (molar ratio 1:1) redox couple solution in step S3 is 0.05 mol / L.

[0035] Example 5 Most of the components are the same as in Example 1, except that the concentration of the K3[Fe(CN)6] and K4[Fe(CN)6] (molar ratio 1:1) redox couple solution in step S3 is 0.2 mol / L.

[0036] Example 6 Most of the steps are the same as in Example 1, except that the concentration of the LiCl solution in step S2 is 3.0 M.

[0037] Example 7 Most of the steps are the same as in Example 1, except that the concentration of the LiCl solution in step S2 is 5.0 M.

[0038] Comparative Example 1 Compared to Example 2, most aspects are the same, except that in step S1, hydroxyethyl acrylate (HEA) is used to replace N-MAM to prepare a P(EOEOEA-co-HEA) hydrogel matrix, which is used to verify the active sites of specific interactions in this invention. The HEA structure is similar to N-MAM, but it lacks the monomer with an amide group.

[0039] Figure 1 The Fourier transform infrared spectra of EOEOEA, N-MAM, PENM hydrogel matrix and PENM thermoelectric hydrogel shown in Example 1 are shown. The peaks unique to the N-MAM monomer appeared in the PENM hydrogel matrix and PENM thermoelectric hydrogel, indicating the successful synthesis of PENM thermoelectric hydrogel and the successful introduction of amide groups.

[0040] Figure 2 The UV-Vis spectra of N-MAM as a special unit with two ions, N+[Fe(CN)6] 4- N+[Fe(CN)6] 3- This indicates that [Fe(CN)6] was added to the N-MAM monomer solution. 4- and [Fe(CN)6] 3- The sample contained [Fe(CN)6]. 4- The addition of [Fe(CN)6] caused a shift in the peak position in the sample. 3- The fact that the peak position of the sample did not shift indicates that the N-MAM monomer is effective against [Fe(CN)6]. 4- Selective binding of ions.

[0041] like Figures 3-5 As shown, copper foil of corresponding size is used as electrodes on both sides of the PENM thermoelectric hydrogel of Example 1 and led out with small wires. Different numbers (1~5) of PENM thermoelectric hydrogels are connected in series to form a wearable wristband integrated thermoelectric collector, which is used to power an LED in a human body temperature collection box. The PENM thermoelectric hydrogel of this invention only needs to utilize the natural temperature difference of about 10 K between the human body and the environment to generate sufficient voltage and power to drive the LED light, directly lighting a 1.8 V commercial LED light without the need for an external boost circuit. This simplifies the integration complexity of wearable devices and reduces system power consumption and size.

[0042] Examples 1, 2, and 3 were conducted to investigate the effects of different monomer molar ratios on the mechanical and thermoelectric properties of hydrogels. Examples 1, 4, and 5 were conducted to investigate the effects of Fe(CN)₂. 3- / Fe(CN) 4-The effect of LiCl concentration on the thermoelectric properties of hydrogels. Examples 1, 6, and 7 were conducted to investigate the effect of LiCl concentration on the internal resistance of hydrogels.

[0043] like Figure 6 and 7 As shown, the thermoelectric hydrogels prepared in Examples 1-3 were subjected to tensile tests. They were loaded onto a tensile testing machine and stretched until fracture. The thermoelectric hydrogel of Example 1 not only rapidly recovered its original size under 200% strain (exhibiting excellent elasticity), but also exhibited an elongation at break of approximately 960% and a tensile strength of approximately 330 kPa. With increasing N-MAM content, the elongation at break of the thermoelectric hydrogel of Example 3 decreased; conversely, with decreasing N-MAM content, the elongation at break of the thermoelectric hydrogel of Example 2 increased, reaching a maximum of 1283%. In Example 1, after complete fracture, the two segments of the gel could self-repair upon contact at room temperature, and the repaired material still possessed an elongation at break exceeding 233%.

[0044] The thermoelectric properties and conductivity of the thermoelectric hydrogels prepared in Examples 1-7 were tested. Under a temperature gradient of ΔT = 10 K, the open-circuit voltage and resistance were measured using a multimeter, and the Seebeck coefficient and conductivity were calculated according to the formula.

[0045] like Figure 8 As shown, under a temperature gradient of ΔT = 10 K, the 1:5 monomer molar ratio in Example 1 exhibited the best thermoelectric performance compared to Examples 2 and 3. The Seebeck coefficient of this thermoelectric hydrogel reached approximately -59.6 mV / K. Figure 9 As shown, when the thermoelectric hydrogel of Example 1 is connected to the multimeter test output, its open circuit voltage reaches approximately 0.74 V at ΔT=50 K.

[0046] like Figure 10 As shown, comparing Examples 1, 4, and 5, with [Fe(CN)6] 3- / [Fe(CN)6] 4- Increased concentration in [Fe(CN)6] 3- / [Fe(CN)6] 4- The optimal thermal power was obtained at a concentration of 0.1 mol / L.

[0047] To reduce the battery's internal resistance, the method in step (2) is used to introduce conductive ions to optimize ionic conductivity, such as... Figure 11 As shown, compared with Examples 1, 6, and 7, with the increase of ion concentration and LiCl concentration, the conductivity of the hydrogel first increases and then tends to level off, reaching the optimal value when the LiCl concentration is 4.0 M.

[0048] like Figure 12As shown, the Seebeck coefficients of the thermoelectric hydrogels of Example 2 and Comparative Example 1 were tested, where E:H (1:1) represents the molar ratio of EOEOEA and HEA in Comparative Example 1 (1:1), and E:N (1:1) represents the molar ratio of EOEOEA and N-MAM in Example 2 (1:1). Open-circuit voltages were measured at a temperature difference of 10 K, and the Seebeck coefficients were tested. The Seebeck coefficient of Comparative Example 1 was only ~6.5 mV / K, far lower than the 59.6 mV / K of Example 1 (the comparison was based on absolute values). This demonstrates that the amide group of N-MAM in this invention is anchored to [Fe(CN)6]. 4- And amplify the only active site of the Soret effect.

[0049] Although the present invention has been described in detail above with general descriptions, specific embodiments, and experiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing a thermoelectric hydrogel, characterized in that, Includes the following steps: S1. Ethoxyethyl acrylate, N-hydroxymethylacrylamide monomer, and initiator are added to a solvent and subjected to free radical copolymerization under ultraviolet light to obtain a hydrogel matrix. S2. Immerse the hydrogel matrix in a lithium salt solution to obtain a swollen hydrogel matrix loaded with lithium ions; S3. Immerse the hydrogel matrix swollen and loaded with lithium ions in step S2 in a solution containing a ferricyanide / ferrous ferrocyanide redox couple to obtain a thermoelectric hydrogel. The molar ratio of ethoxyethyl acrylate to N-hydroxymethylacrylamide monomer is 1:1~8.

2. The method for preparing a thermoelectric hydrogel according to claim 1, characterized in that, In step S1, the mass of the initiator is 0.1 wt% to 1 wt% of the sum of the mass of the ethoxyethoxyethyl acrylate, the N-hydroxymethylacrylamide monomer, and the solvent. The mass ratio of ethoxyethyl acrylate to the solvent is 1:1~5.

3. The method for preparing a thermoelectric hydrogel according to claim 1, characterized in that, In step S1, the initiator is benzoin dimethyl ether; The solvent is water and / or ethanol.

4. The method for preparing a thermoelectric hydrogel according to claim 1, characterized in that, In step S2, the lithium salt solution includes a lithium chloride solution; The lithium salt concentration in the lithium salt solution is 3~5M.

5. The method for preparing a thermoelectric hydrogel according to claim 1, characterized in that, In step S2, the hydrogel matrix is ​​immersed in a lithium salt solution for 1 to 30 minutes.

6. The method for preparing a thermoelectric hydrogel according to claim 1, characterized in that, In step S3, the molar ratio of ferricyanide to ferrousyanide in the solution containing the ferricyanide / ferrousyanide redox couple is 1:0.2~5. The concentration of the ferricyanide / ferrous ferrocyanide redox couple in the solution is 0.05~0.2 mol / L.

7. The method for preparing a thermoelectric hydrogel according to claim 1, characterized in that, In step S3, the solution containing the ferricyanide / ferrocyanide redox couple is a mixed solution of K3[Fe(CN)6] and K4[Fe(CN)6].

8. The method for preparing a thermoelectric hydrogel according to claim 1, characterized in that, In step S3, the immersion time of the swollen and lithium-ion-loaded hydrogel matrix in a solution containing a ferricyanide / ferrous cyanide redox couple is 0.1 to 5 min.

9. A thermoelectric hydrogel, characterized in that, It is prepared using any one of the preparation methods described in claims 1 to 8.

10. An application of the thermoelectric hydrogel as described in claim 9, characterized in that, The thermoelectric hydrogel is used to prepare thermoelectric conversion devices.

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