Self‐Healable and Stretchable PAAc/XG/Bi2Se0.3Te2.7 Hybrid Hydrogel Thermoelectric Materials

Thermoelectric power generators have attracted increasing interest in recent years owing to their great potential in wearable electronics power supply. It is noted that thermoelectric power generators are easy to damage in the dynamic service process, resulting in the formation of microcracks and performance degradation. Herein, we prepare a new hybrid hydrogel thermoelectric material PAAc/XG/Bi2Se0.3Te2.7 by an in situ polymerization method, which shows a high stretchable and self‐healable performance, as well as a good thermoelectric performance. For the sample with Bi2Se0.3Te2.7 content of 1.5 wt% (i.e., PAAc/XG/Bi2Se0.3Te2.7 (1.5 wt%)), which has a room temperature Seebeck coefficient of −0.45 mV K−1, and exhibits an open‐circuit voltage of −17.91 mV and output power of 38.1 nW at a temperature difference of 40 K. After being completely cut off, the hybrid thermoelectric hydrogel automatically recovers its electrical characteristics within a response time of 2.0 s, and the healed hydrogel remains more than 99% of its initial power output. Such stretchable and self‐healable hybrid hydrogel thermoelectric materials show promising potential for application in dynamic service conditions, such as wearable electronics.


Introduction
A thermoelectric generator (TEG) offers a reliable and sustainable power source for next-generation self-powered microelectronic devices (wireless sensors, wearable electronics, and portable electronic devices [1][2][3][4] ), because of its capacity in converting heat into electricity. [5]However, the widespread application of TEGs in microelectronics has been limited by the intrinsically brittle nature of the commercial TEG, in which vulnerable thermoelectric materials are employed. [6]As known, a TEG includes a plurality of thermoelectric couples comprised of n-type and p-type elements with electrical connection in series and thermal connection in parallel, which means that if one of the elements is damaged, the thermoelectric device fails directly.Such failure occurs frequently when it comes to the unspecified movement of the human body and outdoor equipment. [7]erefore, it is very crucial to improve the long-term reliability of TEGs. [8][11][12] In which, a flexible polymer matrix (e.g., PI and PDMS [13] ), flexible electrodes (e.g., Ag paste, carbon paste, and liquid metals [13,14] ), and thermoelectric materials thin-film, micro thermoelectric legs were demonstrated to achieve the goal of flexible TEGs.Nonetheless, the wearability of TEGs is still in its infancy, as a diversity of challenges (environmental stability and reliability to mechanical fatigue) still need to be addressed to achieve practical and sustainable devices. [15]he existing flexible thermoelectric devices have some disadvantages, such as low strength, large thermal resistance, complex manufacturing processes, and low reliability. [16]The thermoelectric material employed is still easy to be damaged when a flexible TEG meets mechanical resistance (e.g., stretching and bending), thus reducing the output performance and reliability of the thermoelectric power generator. [17]o conquer this challenge, self-healable thermoelectric materials were developed recently, of which the thermoelectric performance can be restored to the state before the damage immediately, thus dramatically increasing the lifetime and reliability of thermoelectric devices. [18,19]For example, TEG based on thermosensitive liquids was developed, in which a soft silicone tube serving as a flexible substrate was filled with ionic liquid of [EMIm][Tf2N] and connected with two electrodes.Soret steady state of the ionic liquid was established with a cycle time of 600 s (10 min). [19]Self-healable and stretchable ternary ionic TE hybrid materials, composed of a conjugated polymer (polyaniline), a non-conjugated anionic polyelectrolyte (poly(2-acrylamide-2methyl-1-propane sulfonic acid)), and phytic acid.The proposed materials exhibit an excellent ionic figure-of-merit (ZT i ) as well as remarkable stretchability (up to 750%) and autonomous self-heal ability without any external stimuli.The electrical conductivity of the ternary hybrids is dominated by ionic conduction revealing a strong humidity dependency. [20]Poly(3-butylthiophene-2,5-diyl) (P3BT) nanowires were p-doped with a Lewis-acid-type molecular dopant and embedded in the thermoplastic elastomer polystyrene-block-polyisoprene-blockpolystyrene (SIS) matrix.The composites exhibited excellent recovery of TE performance after "heat-induced self-healing" and "pressure-induced self-healing". [21]Practically, thermoelectric converters based on ionic materials can accumulate a great number of electric charges in a very short time, but it is challenging for them to generate continuous current and need to be placed in a hot environment intermittently to harvest heat continuously. [18]Since the electrical conductivity is dominated by ionic conduction, it shows a strong humidity dependence, which limits its wide application.How can we develop thermoelectric materials with both self-healing properties and sustainable electrical energy output?
Herein, we report a stretchable and self-healable thermoelectric materials hybrid hydrogel PAAc/XG/Bi 2 Se 0.3 Te 2.7 for continuous power generation.The matrix poly acrylic acid (PAAc) is a hydrophilic and superabsorbent polymer that is frequently used in self-healing applications by exploiting the carboxyl functionality that mediates hydrogen bonding across a damaged hydrogel, [22] which has been widely used for strain sensors, [23] and electronic skins. [24]Xanthan gum (XG), as an edible natural biopolymer, greatly improves its mechanical strength through molecular chain entanglement and hydrogen bonding due to its long molecular chain and multiple side chains. [25]The PAAc/XG/Bi 2 Se 0.3 Te 2.7 was prepared using the in situ polymerization method (see Section 4 for details), it presents ideal mechanical strength and excellent self-healing properties.The damaged PAAc/XG/Bi 2 Se 0.3 Te 2.7 ensures rapid recovery of the electrical property after being rejoined together, exhibits a fast self-healing capability which can restore its initial electrical conductivity within 2 s healing time without external stimuli such as force, light, or heat.Moreover, the PAAc/XG/Bi 2 Se 0.3 Te 2.7 generates a maximum voltage of −17.9 mV and a maximum power of 38.1 nW at a temperature difference of 40 K, respectively.This new thermoelectric material can effectively convert low-grade waste heat from a heat source into electricity and provides new ideas for current thermoelectric materials.

Phase and Microstructure
The hybrid hydrogel PAAc/XG/Bi 2 Se 0.3 Te 2.7 was synthesized using the in situ polymerization method (Figure 1).Copolymer acrylic acid (AAc), octadecyl methacrylate (SMA), surfactant sodium dodecyl sulfate (SDS), the initiator potassium persulfate (KPS), calcium chloride anhydrous (CaCl 2 ), and Bi 2 Se 0.3 Te 2.7 powder were dissolved/added in a certain amount of XG solution (Figure 1a).The hybrid solution was then placed at 353 K for 4 h.In which, the carboxyl group (COO − ) on AAc and calcium ion (Ca 2+ ) serve as the cross-linking point to form the first network, the hydrophobic monomer SMA and the surfactant SDS form the micellar cross-linking XG to form the second network [25] (Figure 1b).These two networks endow the hydrogel with good mechanical properties and self-healing ability.
The Fourier-transform infrared (FTIR) spectrum of the PAAc/XG/ Bi 2 Se 0.3 Te 2.7 is shown in Figure 2a.The indicative peaks for the PAAc/ XG/Bi 2 Se 0.3 Te 2.7 are detected at 3371, 2918, 1718, 1452, and 1022 cm −1 , which correspond to the stretching vibration peak of the hydroxyl (-OH) group, the C-H bond, the ionic cross-linking between carboxylate groups and (Ca 2+ ) ions, the plane bending of C-H and -CH 2 , and the stretching vibration of C-O-C groups, respectively. [26]he FTIR spectrum confirms the successful synthesis of the PAAc/XG hydrogel.The X-ray diffraction (XRD) pattern of the PAAc/XG/ Bi 2 Se 0.3 Te 2.7 (1.5 wt%, i.e., the amount of Bi 2 Se 0.3 Te 2.7 is 1.5 wt%) is shown in Figure 2b, which can be well indexed into Bi 2 Se 0.3 Te 2.7 (PDF #50-0954) without any detectable impurity, indicating the in situ polymerization process makes no change to the phase of Bi 2 Se 0.3 Te 2.7 .
The field emission scanning electron microscopic (FESEM) images of the freeze-dried PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) are shown in Figure 2c,d.We can well observe the crosslinking PAAc/XG network with a uniformly distributed Bi 2 Se 0.3 Te 2.7 .In which, the PAAc/ XG network provides the stretchability and self-healing of the hybrid hydrogel, while the Bi 2 Se 0.3 Te 2.7 provides the energy harvesting ability from low-grade heat (i.e., thermoelectric power generator), thus it can convert heat into electricity based on the Seebeck effect. [27,28]2.The Thermoelectric Performance of the PAAc/XG/Bi 2 Se 0.3 Te 2.7 Figure S1, Supporting Information shows the thermal conductivities of PAAc/XG/Bi 2 Se 0.3 Te 2.7 .The thermal conductivity of PAAc/XG is ~0.42 W (mÁK) −1 , which slightly increases to 0.45 W (mÁK) −1 for PAAc/XG/Bi 2 Se 0.3 Te 2.7 (2.0 wt%).The thermal conductivity of the polymer material is restricted by the polymer matrix and the thermally conductive filler.Adding Bi 2 Se 0.3 Te 2.7 to the hydrogel affected the thermal conductivity of the composite hydrogel, when the content of Bi 2 Se 0.3 Te 2.7 is low, the distance between the fillers is large, the fillers are divided by the polymer material, phonons cannot propagate continuously and quickly, so the thermal conductivity improvement of hydrogel is very limited.[29] Figure 3b 1): [30] S ¼ ΔV ΔT where Compared with the performance of previously reported hydrogel-based thermoelectric materials, [21] the Seebeck coefficient of PAAc/XG/ Bi 2 Se 0.3 Te 2.7 is higher than that reported in the previous literature (S = 0.18 mV K −1 ). Figure S5, Supporting Information showed PAAc/ XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) operating under a common circumstance (60 °C at the hot end and 25 °C at the cold end).We can see that the material could be working normally for more than 90 min even at a high temperature of 60 °C, proving that  the proposed material has a relatively high operating stability.Figure 3d shows the output powers of PAAc/XG/Bi 2 Se 0.3 Te 2.7 at temperature differences (ΔT) ranging from 5 to 40 K. Herein, the output power was calculated by Equation (2): [31] where We then tested the flexibility and self-healing capacity of PAAc/ XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%), since it shows the highest output powers.The PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) was manually bent on a glass tube, and the flexibility of PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) was analyzed by comparing the conductivity changes of the samples before and after bending.As shown in Figure 4a, after 1000 times of bending, the conductivity decreased to 98.1% of the initial value, indicating that it has better flexibility.In the PAAc/XG matrix, the high molecular weight PAAc and XG have longer molecular chains, which are helpful for the flexibility of the material, making the material less prone to microcracks when it is bent, and ensuring the flexibility of PAAc/XG/Bi 2 Se 0.3 Te 2.7 . [32]The open-circuit voltage and output power of PAAc/XG/Bi  4c-e.We used a razor blade to create a 1 mm-deep crack in the sample and placed it under an optical microscope to observe the crack healing process.It can be seen that the crack width shrinks from 20 to 5 μm with the passing of time, and the crack depth is also gradually decreasing, indicating a good selfhealing capacity without external stimuli such as force, light, or heat.
In addition, such a self-healing process can be speeded up if external force exists.For example, we cut PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) into two halves using a sharp blade and then contact these two halves together to check its self-heal ability at room temperature, the initial resistance of PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) is 1.20 kΩ (Figure 5a), which becomes infinity when been cut off.Once we contact these two halves together by hand, the resistance of PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) reduces dramatically and returns to the initial value (~1.22 kΩ) in 2.0 s.We then repeat the process five times, and the resistance of the PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) is continuously monitored in real-time during the self-healing process.As shown in Figure 5b, the PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) acted as a conductor connected to a circuit of a light-emitting diode (LED) bulb, the LED bulb immediately went out along with the severance of the hydrogel.Subsequently, the LED bulb could be relighted once the separated gel pieces were recombined, demonstrating the fast electrical self-healing property of the PAAc/XG/Bi 2 Se 0.3 Te 2.7 .
In addition, the PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) also shows good recoverability in mechanical performance.We measured the stress-strain curves of the initial PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) (without cutting) and the self-healed PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) using American Instron Electronic Dynamic and Static Testing Machine (E1000) (see Section 4 for details).Using the tensile test data, the healing properties of the hydrogel can be quantitatively analyzed.The self-healing efficiency can be characterized by the ratio of the tensile strength of the spline healed after fracture to the tensile strength of the original sample. [22]The maximum strain values (ε) can be calculated using Equation (3): [22] ε ¼ where L is the stretched length at break and L 0 is the original length.Equation ( 4) [22] for self-healing efficiency is as follows: where ε 0 is the maximum strain value of the healed hydrogel and ε is the maximum strain value of the initial hydrogel (uncut hydrogel).As shown in Figure 5c, the self-healed PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5wt%) exhibited mechanical strength and stretchability, and could be stretched up to 2.0 times its initial length.Compared to the initial PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) with maximum stress of 0.033 MPa and a maximum strain of 482%, the maximum stress and maximum strain of the self-healed PAAc/XG/ Bi 2 Se 0.3 Te 2.7 (1.5 wt%) can reach 0.018 MPa and 256%, corresponding to 53% recovery.The PAAc/XG/Bi 2 Se 0.3 Te 2.7 was formed by the combination of hydrophobic association and ionic coordination.These two crosslinking points have a reversible change of cross-linking after destruction, after being damaged by force, the cross-linked structure can be regenerated.The coordination of calcium ions with the carboxyl groups on the polyacrylic acid macromolecular chain endowed the hydrogel with good self-healing ability. [22]The associative micelle composed of SDS, SMA, and XG, due to its unique hydrophobic association structure, [25] ensured the mechanical strength of the PAAc/XG/ Bi 2 Se 0.3 Te 2.7 (1.5 wt%).
Figure 5d shows the thermoelectric properties of PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) after self-healing.The open-circuit voltage and output power of PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt %) after healing for 12 h were −17.2 mV and 38.0 nW, at a temperature difference of 40 K. Compared with the initial (before uncut) samples (V oc = −17.9mV, P = 38.1 nW) decreased by 3.9% and 0.3%, respectively.The slight decrease in thermoelectric output may be caused by the partial disconnection of the circuit in the fracture area from before cutting to after healing.It can be seen that PAAc/XG/ Bi 2 Se 0.3 Te 2.7 has both structural self-healing and functional self-healing capabilities.It is noted that there are a large number of carboxyl groups that can form hydrogen bonds in the molecular chain of polyacrylic acid, and hydrogen bonds are an integral part of the adhesion mechanism, which makes PAAc/ XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) have certain adhesion, [33] can adhere to latex gloves and plastic surfaces (Figure 6a,b).It provides favorable conditions for the construction of the temperature difference between the human body and the environment.
To explore the power generation ability of the PAAc/XG/Bi 2 Se 0.3 Te 2.7 for flexible thermoelectric devices, the PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) was connected with copper electrodes to prepare a single-leg device.One end of the PAAc/XG/ Bi 2 Se 0.3 Te 2.7 (1.5 wt%) is in contact with the skin surface as much as possible, and the other end is exposed to the air, as shown in Figure 6c,  which simulated the temperature difference between the human body and environment.As shown in Figure 6c, a temperature difference of 9.7 K can be observed, while a voltage of 4.4 mV is generated.The Seebeck coefficient here could be calculated to be 0.45 mV K −1 , which matches the previous result well.Besides, to exhibit the power generation of PAAc/XG/Bi 2 Se 0.3 Te 2.7 in a vivid and visible way, we took the thermal image of the hydrogel during the experiment.The relevant content can be found in Figure S4, Supporting Information.

Conclusions
In summary, we demonstrated a stretchable and self-healable hybrid hydrogel thermoelectric materials PAAc/XG/Bi 2 Se 0.3 Te 2.7 , which was prepared by the in situ polymerization method.
PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) produced an open-circuit voltage of −17.9 mV and output power of 38.1 nW at a temperature difference of 40 K, and the conductivity decreased by only 0.3% after 1000 bending cycles.Our tensile and electrical resistance experiments of the hydrogel confirm that it has good self-healing and mechanical properties.The severed PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) were re-spliced together without external stimulation, and the healing efficiency reached 54.5% after being placed at room temperature for 12 h.Equipped with this self-healing hydrogel, we have manufactured a single-leg thermoelectric power generation device that can continue to operate even after cutting, while maintaining a final power output of more than 99%.This work provided a facile and feasible method for preparing self-healing thermoelectric materials to broaden the application prospects of wearable energy harvesting.

Experimental Section
Materials: Acrylic acid (anhydrous, 200 ppm MEHQ, 99.0%), SMA (250 ppm, MEHQ, 96%), SDS (≥99.0%),XG (USP level), KPS purchased from Aladdin, and CaCl 2 (≥99.0%)purchased from Sinopharm Chemical Reagent Co., Ltd.Bi 2 Se 0.3 Te 2.7 commercial ingot purchased from Fuxin Technology Co., Ltd.All reagents mentioned above were not further treated and used as received.Synthesis of the hybrid hydrogel PAAc/XG/Bi 2 Te 3 : Preparation of Bi 2 Se 0.3 Te 2.7 powder-N-type commercial Bi 2 Se 0.3 Te 2.7 ingots were put into an agate mortar and ground for 20 min, then transferred to a planetary ball milling jar, and ballmilled at a rotational speed of 300 rpm for 4 h under a protective atmosphere (N 2 ), finally used 200 Sieve through a mesh sieve to obtain micronscale Bi 2 Se 0.3 Te 2.7 powder.Synthesis of PAAc/XG/Bi 2 Se 0.3 Te 2.7 -The PAAc/XG/Bi 2 Se 0.3 Te 2.7 was synthesized using the in situ polymerization method.First, weigh a certain amount of SDS and dissolve it in deionized water to prepare a 7% SDS aqueous solution.Take 10 mL of this solution, add 0.12 g of XG and stir at room temperature for 4 h to fully dissolve; then add AAc (2.7428 g), SMA (0.2572 g), and continue stirring at room temperature for 1 h.Finally, add the initiator KPS (0.03 g), CaCl 2 (0.01 g), and different contents of Bi 2 Se 0.3 Te 2.7 power (0.5, 1.0, 1.5, and 2.0 wt%), after rapid stirring, go through vacuum degassing, then transfer to the mold, seal, and place on the 353 K heating table for 4 h.The samples were marked as PAAc/XG/ Bi 2 Se 0.3 Te 2.7 (0.5 wt%), PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.0 wt%), PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%), and PAAc/XG/Bi 2 Se 0.3 Te 2 (2.0 wt%).Characterization: The function groups in hydrogel were identified by Fourier infrared absorption spectrum (Nicolet iS50R purchased from America Thermo Scientific Corporation).The samples were characterized by XRD using an XRD-7000 with Cu-Kα radiation apparatus.The morphology images of the hydrogel were obtained by SEM (Gemini SEM300) equipped with energy dispersive X-ray spectroscopy probe.Mechanical testing was performed using the American Instron Electronic Dynamic and Static Testing Machine (E1000), the sensor is 1 kN, the loading speed is 10 mm min −1 , and the test samples size was 10 × 4 × 2.5 mm.The electrical conductivity (σ) was measured on a Namicro-III thermoelectric measurement system.The thermal conductivity (κ) was collected using the transient hot wire method through a XIATECHTC3010 instrument.The thermoelectric properties of the PAAc/XG/Bi 2 Se 0.3 Te 2.7 were measured on a selfmade system equipped with equipment for establishing temperature differences and a digital source meter for measuring voltage and current (Figure S2, Supporting Information).The flexibility of the PAAc/XG/Bi 2 Se 0.3 Te 2.7 was evaluated by manual bending on the glass tube with a radius of 6 mm.

Figure
Figure 3a shows the room temperature electrical conductivities of PAAc/XG/Bi 2 Se 0.3 Te 2.7 , in which the content of Bi 2 Se 0.3 Te 2.7 is 0, 0.5, 1.0, 1.5, and 2.0 wt%.The electrical conductivity of the pure hydrogel PAAc/XG is 2.84 S m −1 , which is mainly realized by the ions, such as Ca 2+ , K + , and Cl − .The electrical conductivity of the PAAc/XG/Bi 2 Se 0.3 Te 2.7 increases monotonically with the increasing content of Bi 2 Se 0.3 Te 2.7 , which reaches 4.82 S m −1 when the content of Bi 2 Se 0.3 Te 2.7 reaches Figure 1.a) Schematical illustration for PAAc/XG/Bi 2 Se 0.3 Te 2.7 preparation and b) the reaction in forming a crosslinked hydrogel.

Figure 3 .
Figure 3. a) The room temperature electrical conductivity of PAAc/XG/Bi 2 Se 0.3 Te 2.7 ; b) the current; c) the open-circuit voltages; and d) output powers of PAAc/XG/Bi 2 Se 0.3 Te 2.7 at different temperature difference.

Figure 4 .
Figure 4. a) The electrical conductivity of PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) as a function of bending times; b) thermoelectric performance of PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) after 1000 bending times; and c-e) the crack width shrinks with time and self-healing behavior occurs.

Figure 6 .
Figure 6.PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) adhere to the surface of a) latex gloves and b) plastic product.c) The PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) was placed on the wrist to maintain good thermal contact with the skin.d) Output voltage between two ends of the hydrogel.
and V oc are the current and open-circuit voltage of the sample at different temperature differences.The output powers increase with rising temperature due to the enhanced open-circuit voltages and current of PAAc/XG/Bi 2 Se 0.3 Te 2.7 .In which, the maximum output power of PAAc/XG is 2.48 nW at ΔT = 40 °C, while the maximum output power of PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%) reaches 38.1 nW, an increase of ~1400%.Our thermoelectric power generation measurement results show that Bi 2 Se 0.3 Te 2.7 can largely enhance the thermoelectric performance of PAAc/XG.
2 Se 0.3 Te 2.7 (1.5 wt%) after bending are shown in Figure4b.When the temperature difference is 40 K, the open-circuit voltage is −17.5 mV and the output power is 38.0 nW, respectively.Compared with the pristine PAAc/XG/Bi 2 Se 0.3 Te 2.7 (1.5 wt%), the open-circuit voltage and output power reduce by 2.2% and 0.3% respectively, showing good flexibility.The room temperature self-healing performance of PAAc/XG/ Bi 2 Se 0.3 Te 2.7 (1.5 wt%) is shown in Figure