A flexible laminated MEG-FTEG dual-source energy harvesting device and method of manufacture

Abstract

The application provides a flexible laminated MEG-FTEG dual-source energy harvesting device and a preparation method thereof. The device comprises an upper layer of a moisture power generation (MEG) unit and a lower layer of a flexible thermoelectric power generation (FTEG) unit. The MEG unit is composed of an activated carbon electrode, an ion conductive hydrogel, a porous aluminum electrode and an ePTFE diaphragm. The ePTFE diaphragm is used for directional transmission of water vapor and prevention of electrical short circuit. The FTEG unit takes FPCB as a substrate, and P-type and N-type Bi2Te3 thermoelectric arms are alternately arranged and electrically connected through a copper cladding layer and an arch-shaped copper wire. The whole is packaged with a hydrogel. The device realizes independent collection of moisture energy and heat energy through laminated integration, avoids series mismatching problems, has full flexibility and wearable characteristics, and can continuously supply power for health monitoring equipment.

Description

Technical Field

[0001] This invention belongs to the field of flexible electronics and environmental energy harvesting technology, specifically relating to a stacked dual-source energy harvesting device capable of simultaneously harvesting moisture energy and thermal energy due to temperature difference, and its preparation method. Background Technology

[0002] With the rapid popularization of flexible electronics, wearable health monitoring devices, and portable smart terminals, the demand for miniaturization, flexibility, long lifespan, and self-powered capabilities in these devices is becoming increasingly urgent. Traditional energy storage devices such as lithium-ion batteries have inherent drawbacks, including large size, the need for periodic charging, limited cycle life, and the potential for environmental pollution after disposal, making it difficult to meet the long-term stable and maintenance-free operation requirements of next-generation wearable devices. Therefore, harvesting energy from the environment surrounding the human body to achieve self-powered operation has become an important research direction in the field of flexible wearable electronics.

[0003] Low-grade heat and moisture energy are abundant in nature and on the human body. These energy sources are widely distributed, easily accessible, and green and sustainable, making them ideal power sources for wearable systems. Moisture Electric Generators (MEGs) utilize ambient humidity gradients to drive ion migration, generating a potential difference between the upper and lower electrodes, converting moisture energy into electrical energy without the need for external bias voltage. Thermoelectric Generators (TEGs), based on the Seebeck effect, can directly convert the minute temperature difference (typically 1-5°C) between human skin and ambient air into electrical output. However, single energy harvesting methods generally suffer from low output power (MEGs are typically in the sub-microwatt to microwatt range, while TEGs are in the microwatt to tens of microwatt range), high susceptibility to environmental conditions (MEGs depend on humidity, TEGs depend on temperature difference), and insufficient operational stability, making it difficult to independently meet the continuous power supply needs of wearable devices.

[0004] To improve the energy supply capability of wearable devices, researchers have proposed integrating MEGs and TEGs to achieve multi-source energy harvesting. Existing dual-source energy harvesting devices mostly employ a direct series integration method—directly connecting the outputs of the MEG and TEG in series to the same load or energy storage circuit. However, since the internal resistance of MEGs is typically in the megaohm (MΩ) range, while the internal resistance of Bi2Te3-based TEGs is typically in the range of several to tens of ohms, a difference of 3 to 6 orders of magnitude, direct series connection leads to severe impedance mismatch: the high-resistance MEG unit can hardly output power to the low-resistance load, significantly reducing the overall power generation efficiency. Furthermore, key technical challenges such as the flexible integration of brittle inorganic thermoelectric materials (e.g., Bi2Te3), the optimization of the balance between thermal conductivity and mechanical flexibility of encapsulation materials, and the structural compatibility and electrical isolation between the upper and lower layers of the dual-source unit remain unresolved, severely limiting the practical application of dual-source energy harvesting technology in the wearable field. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems existing in the prior art by proposing a flexible stacked MEG–FTEG dual-source energy harvesting device and its fabrication method. By integrating MEG and FTEG units in an upper and lower stacked manner with an ePTFE membrane as a separator, independent energy harvesting and independent electrical output of the two units are achieved on the same flexible platform. This fundamentally solves the output mismatch problem caused by excessive internal resistance differences in traditional series integration schemes. Simultaneously, the use of a fully flexible material system and an arch-shaped interconnect structure improves the device's deformation adaptability and electrical connection reliability.

[0006] In a first aspect, the present invention provides a flexible stacked MEG–FTEG dual-source energy harvesting device, comprising: an upper layer moisture generator (MEG) unit, a lower layer flexible thermoelectric generator (FTEG) unit, and an ePTFE membrane (4) disposed between the two.

[0007] The MEG unit is arranged from top to bottom as follows: activated carbon electrode (1), ion-conducting hydrogel (2), open-pore aluminum electrode (3), and ePTFE membrane (4). The ePTFE membrane (4) has a pore size of 0.2~1.0μm and a thickness of 25~100μm. It is used to realize the directional transport of water vapor from the upper MEG unit to the lower FTEG unit, and at the same time, it uses its high resistivity characteristics to avoid electrical short circuits between the upper and lower units. The FTEG unit uses FPCB (6) as a flexible substrate. P-type Bi2Te3 thermoelectric arms (7) and N-type Bi2Te3 thermoelectric arms (8) are alternately arranged on the surface of FPCB (6). The spacing between adjacent thermoelectric arms is 0.5~1.5mm. The bottom electrode (9) is connected by the copper layer of FPCB (6). The top electrical connection is completed by the arch-shaped copper wire (10). The entire FTEG unit is wrapped with a hydrogel encapsulation layer (5) to improve the device flexibility and thermal conductivity.

[0008] Preferably, the ion-conductive hydrogel (2) is a CaCl2 crosslinked PVA-sodium alginate composite hydrogel, in which glycerol is added as a plasticizer and water-retaining agent, and the gel thickness is about 2 mm.

[0009] Preferably, the P-type (7) and N-type (8) Bi2Te3 thermocouple arms are 1mm×1mm×1.2mm in size and are soldered to the bottom FPCB (6) copper electrode (9) and the top arch-shaped copper wire (10) respectively to form a series-connected thermocouple array.

[0010] Preferably, the hydrogel encapsulation layer (5) of the FTEG unit and the ion-conducting hydrogel (2) of the MEG unit are prepared using the same formula, which improves the thermal conductivity while ensuring the flexibility of the device. The output power is increased by more than 11.5% compared with the silicone encapsulation at a temperature difference of 4℃.

[0011] Preferably, the device is made of a fully flexible material system, which can withstand large-scale torsion and bending deformation, can closely fit irregular surfaces such as the human arm, and has wearable adaptability.

[0012] Secondly, this invention provides a stacked integrated structure.

[0013] The stacked structure enables the MEG and FTEG to collect moisture energy and thermal energy respectively on the same flexible platform, while maintaining the independent output of the two energy units, avoiding the direct series mismatch problem caused by excessive internal resistance difference.

[0014] Thirdly, the present invention provides a method for preparing the above-mentioned flexible stacked MEG–FTEG dual-source energy harvesting device, comprising the following steps: S1. Preparation of MEG unit: Add 2.5g PVA to 22.5g deionized water, heat and stir at 95℃ until dissolved, add 0.3g sodium alginate and 5g glycerol and stir evenly, pour into mold and freeze at -20℃ for 24h, after thawing, soak in 3wt% CaCl2 aqueous solution for 12h to obtain ion-conductive hydrogel, and attach activated carbon electrode and open-pore aluminum electrode to its upper and lower surfaces respectively; S2. Fabrication of FTEG unit: P-type and N-type Bi2Te3 thermoelectric arms are alternately arranged and welded on FPCB flexible substrate. Bottom electrical connection is achieved through FPCB copper layer, top series connection is completed by arch-shaped copper wires, and then hydrogel is used for overall encapsulation. S3. Stacked Integration: An ePTFE membrane is laid on top of the TEG unit, and then the perforated aluminum lower electrode side of the MEG unit is stacked on the ePTFE membrane with the other side facing down to complete the overall assembly.

[0015] The present invention has the following advantages and beneficial effects: One of the advantages of this invention is that it adopts a stacked dual-source independent output integrated architecture, and realizes water vapor transmission and electrical isolation through an ePTFE membrane (4), which fundamentally solves the internal resistance mismatch problem of traditional series integration, while significantly reducing the device size and improving the integration degree. The second advantage of this invention is that the FTEG unit uses arch-shaped copper wires (10) instead of traditional planar interconnects, which effectively disperses the deformation stress during bending and torsion, and improves the reliability of electrical connection; the innovative use of ion-conductive hydrogel (5) to encapsulate the FTEG, compared with silicone encapsulation and no encapsulation schemes, increases the output power by more than 11.5% at a temperature difference of 4℃, taking into account the device flexibility, structural integrity and thermal conductivity efficiency. The third advantage of this invention is that it constructs a high-performance CaCl2 cross-linked composite ion-conductive hydrogel (2), which combines cryo-crosslinking and ion-crosslinking processes, and introduces glycerol as a plasticizer and water-retaining agent. It also has excellent ion conductivity, flexibility and water retention, which effectively improves the output stability and continuous working time of MEG. The fourth advantage of this invention is that it adopts a fully flexible material system, which can withstand large-scale torsion and bending deformation, can closely fit irregular surfaces such as the human arm, and has excellent deformation adaptability and long-term wearability potential. Attached Figure Description

[0016] Figure 1 This is a three-dimensional structural diagram of the dual-source energy harvesting device of the present invention; Figure 2 This is a three-dimensional structural diagram of the MEG of the present invention; Figure 3 This is a schematic diagram of the FTEG of the present invention; Figure 4This is a schematic diagram of the preparation method of the dual-source energy harvesting device of the present invention; Among them: 1: CaCl2 crosslinked activated carbon electrode; 2: PVA-sodium alginate composite ion-conducting hydrogel; 3: open-pore aluminum electrode; 4: ePTFE membrane; 5: hydrogel encapsulation layer; 6: FPCB flexible substrate; 7: P-type Bi2Te3 thermoelectric arm; 8: N-type Bi2Te3 thermoelectric arm; 9: bottom electrode layer; 10: top arch-shaped copper interconnect layer. Detailed Implementation

[0017] The present invention provides a flexible stacked MEG–FTEG dual-source energy harvesting device, comprising: an upper layer moisture power generation (MEG) unit, a lower layer flexible thermoelectric power generation (FTEG) unit, and an ePTFE membrane (4) sandwiched between the two.

[0018] The MEG unit is arranged from top to bottom as follows: activated carbon electrode (1), ion-conducting hydrogel (2), open-pore aluminum electrode (3), and ePTFE membrane (4). The FTEG unit uses FPCB (6) as a flexible substrate. P-type Bi2Te3 thermoelectric arms (7) and N-type Bi2Te3 thermoelectric arms (8) are alternately arranged on the surface of FPCB. The bottom electrical connection (9) is achieved by the copper-clad layer of FPCB, and the top electrical connection is completed by the arch-shaped copper wire (10). The FTEG unit is encapsulated with hydrogel (5). The ePTFE membrane (4) is set between the MEG unit and the FTEG unit. Its microporous structure allows water vapor to pass through from top to bottom. At the same time, its high resistivity realizes the electrical isolation between the upper and lower units. The MEG unit and the FTEG unit are stacked and integrated on the same flexible platform to form an independent output dual-source energy harvesting device.

[0019] Specifically, Figure 1 A schematic diagram of the layered structure of the flexible stacked MEG–FTEG dual-source energy harvesting device provided by the present invention. The MEG unit is located on the upper layer, and from top to bottom are activated carbon electrode (1), ion-conducting hydrogel (2), and open-pore aluminum electrode (3). The ion-conducting hydrogel is a PVA-sodium alginate composite hydrogel crosslinked with CaCl2, with glycerol added as a plasticizer and water-retaining agent, and the gel thickness is about 2 mm. The FTEG unit is located on the lower layer, with FPCB flexible substrate (6) as the supporting structure. P-type Bi2Te3 thermoelectric arms (7) and N-type Bi2Te3 thermoelectric arms (8) are arranged alternately in an array, with adjacent thermoelectric arms having opposite polarities. The bottom is connected by FPCB copper-clad electrodes (9), and the top is connected in series by arch-shaped copper wires (10) to form a thermocouple array. The ePTFE membrane (4) is tightly attached between the open-pore aluminum lower electrode of the MEG unit and the upper surface of the hydrogel encapsulation layer of the FTEG unit.

[0020] The arch-shaped copper interconnect structure (10) is set on the top of the thermoelectric arm of the FTEG unit, replacing the traditional planar interconnect structure, effectively dispersing the deformation stress during bending and torsion, and improving the reliability of electrode connection; the connection position between the wire and the thermoelectric arm adopts a smooth transition design to avoid stress concentration leading to solder joint detachment, further enhancing the flexibility of the device.

[0021] The hydrogel encapsulation layer (5) wraps the entire FTEG unit, which improves the heat conduction efficiency without affecting the flexibility of the device, enhances the effect of establishing the temperature difference between the hot and cold ends of the thermoelectric unit, and protects the internal thermoelectric arm and electrode structure, thereby improving the structural stability of the device.

[0022] Based on the above embodiments, this invention provides a method for fabricating a flexible stacked MEG–FTEG dual-source energy harvesting device, specifically including: S1, MEG unit ion-conducting hydrogel was prepared and electrode assembly was completed; S2, fabricate FTEG units, complete thermoelectric arm arrangement, interconnection welding and hydrogel encapsulation; S3, attach the ePTFE diaphragm to the upper surface of the FTEG unit, and then stack the MEG unit on top of the diaphragm to complete the overall stack-integration.

[0023] Specifically, S1 is executed first to prepare MEG cells: Weigh 2.5g of PVA and add it to 22.5g of deionized water. Heat and stir at 95℃ until completely dissolved to form a uniform and transparent PVA solution. Add 0.3g of sodium alginate to the solution and stir continuously until fully dispersed. Then add 5g of glycerol as a plasticizer and water-retaining agent and stir evenly to obtain a precursor solution. Pour the precursor solution into a cuboid mold, control the gel thickness to about 2mm, and freeze at -20℃ for 24h. After thawing, immerse the hydrogel in a 3wt% CaCl2 aqueous solution for 12h to complete Ca²⁺ crosslinking and introduce ion-conducting components to obtain an ion-conducting hydrogel. Attach activated carbon as the upper electrode to the upper surface of the hydrogel and open-pore aluminum as the lower electrode to the lower surface to complete the MEG unit assembly.

[0024] Then, execute S2 to prepare the FTEG unit: An FPCB with dimensions matching that of the MEG hydrogel was selected as a flexible substrate, and a bottom copper-clad electrode was prepared by etching. P-type and N-type Bi2Te3 thermocouple arms were alternately arranged on the FPCB surface, and the bottom of the thermocouple arms was soldered to the bottom copper-clad electrode. The tops of adjacent P-type and N-type thermocouple arms were connected by arc-shaped copper wires to form a series thermocouple array. The FTEG unit was immersed in the hydrogel precursor solution with the same formulation as the MEG unit. After being taken out, it was frozen at -20°C for 24 hours. After thawing, the hydrogel encapsulation was completed.

[0025] Finally, execute S3 for layer integration: The ePTFE membrane is laid flat on the surface of the hydrogel encapsulation layer of the FTEG unit. Then, the perforated aluminum lower electrode side of the MEG unit is placed face down and stably stacked on top of the ePTFE membrane to ensure tight adhesion between the layers, thus completing the overall assembly of the flexible stacked MEG–FTEG dual-source energy harvesting device.

[0026] The preparation process of ion-conducting hydrogels includes: PVA, sodium alginate, glycerol and deionized water are mixed according to the formula, heated and stirred at 95°C until uniform; poured into a mold and frozen to form, and then CaCl2 ion crosslinking is performed after thawing; the freezing process can improve the flexibility and ion conductivity of the hydrogel, Ca²⁺ crosslinking can enhance the structural stability of the hydrogel, and glycerol can improve the water retention capacity and extend the continuous working time of the MEG unit.

[0027] Working principle: The FTEG unit has one side of its FPCB flexible substrate attached to human skin as the hot end (temperature approximately 32~36℃), and the other side of its hydrogel encapsulation layer in contact with ambient air as the cold end (temperature approximately 20~28℃). Based on the Seebeck effect, the 3~8℃ temperature difference between human body temperature and ambient temperature drives the majority carriers (holes in P-type and electrons in N-type) in the Bi2Te3 thermoelectric arm to diffuse directionally from the hot end to the cold end, accumulating charge at the hot and cold ends to form a thermoelectric potential, thus realizing the conversion of thermal energy into electrical energy. The MEG unit relies on the moisture emitted from the surface of human skin (relative humidity can reach 80%~100%) to pass through the ePTFE membrane into the ion-conducting hydrogel, forming a humidity gradient from top to bottom in the thickness direction of the hydrogel. Based on the ion directional diffusion effect, it drives Ca²⁺ and Cl⁻ ions in the hydrogel to migrate directionally along the humidity gradient, forming a potential difference (open circuit voltage can reach 0.5~0.8V) between the activated carbon upper electrode and the open-pore aluminum lower electrode, thus realizing the conversion of moisture energy into electrical energy.

[0028] Preferably, to ensure the overall flexibility of the device, the FPCB substrate is made of PI material, which has good bending and mechanical strength. It can be combined with an arch-shaped copper interconnect structure to enable the device to withstand large-scale bending and torsional deformation and closely fit irregular curved surfaces such as the human arm.

[0029] Preferably, the ionic conductivity and water retention of the MEG unit can be changed by adjusting the amount of glycerol added and the concentration of CaCl2 in the hydrogel formulation; the output voltage and internal resistance of the FTEG unit can be changed by adjusting the number of thermoelectric arm arrays and the size of the arch-shaped copper wires, thus adapting to different wearable application scenarios.

Claims

1. A flexible stacked MEG–FTEG dual-source energy harvesting device, characterized in that, include: The upper layer consists of a moisture-generating MEG unit, the lower layer consists of a flexible thermoelectric generator (FTEG) unit, and an ePTFE membrane is placed between the two. The ePTFE diaphragm is used to enable the directional transport of water vapor from the MEG unit to the FTEG unit, while avoiding electrical short circuits between the upper and lower units. The stacked structure allows the MEG and FTEG to independently collect moisture energy and heat energy on the same flexible platform, maintaining the independent output of the two energy units and avoiding the direct series mismatch problem caused by excessive internal resistance differences.

2. The dual-source energy harvesting device according to claim 1, characterized in that, The MEG unit includes a CaCl2 cross-linked activated carbon electrode (1), a PVA-sodium alginate composite ion-conducting hydrogel (2), an open-pore aluminum electrode (3), and an ePTFE membrane (4). The activated carbon electrode (1) is disposed on the upper surface of the composite ion-conducting hydrogel (2), and the open-pore aluminum electrode (3) is disposed on the lower surface of the composite ion-conducting hydrogel (2); glycerol is added to the composite ion-conducting hydrogel (2) as a plasticizer and a water-retaining agent; the ePTFE membrane is disposed below the open-pore aluminum electrode (3).

3. The dual-source energy harvesting device according to claim 2, characterized in that, The FTEG unit includes an FPCB flexible substrate (6), multiple P-type Bi2Te3 thermoelectric arms (7), multiple N-type Bi2Te3 thermoelectric arms (8), a bottom electrode layer (9), and a top arch-shaped copper interconnect layer (10). The P-type Bi2Te3 thermoelectric arm (7) and the N-type Bi2Te3 thermoelectric arm (8) are alternately arranged on the surface of the FPCB flexible substrate (6). The bottom electrode layer (9) is implemented by the FPCB copper cladding layer. The top arch-shaped copper interconnect layer (10) connects the adjacent P-type and N-type thermoelectric arms in series.

4. The dual-source energy harvesting device according to claim 3, characterized in that, The FTEG unit also includes a hydrogel encapsulation layer (5). The hydrogel encapsulation layer (5) completely encapsulates the FTEG unit, improving thermal conductivity while ensuring device flexibility and structural stability.

5. The dual-source energy harvesting device according to claim 1, characterized in that, The device is made of a fully flexible material system, which can withstand large-scale bending and torsional deformation, and can closely fit irregular surfaces such as the human arm, with excellent wearability.

6. The method for preparing the dual-source energy harvesting device according to any one of claims 1-5, characterized in that, Includes the following steps: S1. Preparation of MEG unit: PVA is mixed with deionized water and heated to dissolve. Sodium alginate and glycerol are added and stirred evenly. The mixture is poured into a mold for freezing and cross-linking. After thawing, it is immersed in CaCl2 aqueous solution for ionic cross-linking to obtain an ion-conductive hydrogel. Activated carbon electrodes and open-pore aluminum electrodes are attached to its upper and lower surfaces, respectively. S2. Preparation of FTEG unit: P-type and N-type Bi2Te3 thermoelectric arms are alternately arranged and welded on an FPCB flexible substrate. Bottom electrical connection is achieved through the FPCB copper layer, and top series connection is completed using arch-shaped copper wires. Then, the whole unit is encapsulated with hydrogel. S3. Stacked integration: An ePTFE membrane is laid on top of the FTEG unit. The open-pore aluminum lower electrode side of the MEG unit is then stacked on the ePTFE membrane with the other side facing down to complete the overall assembly.

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