Enhanced Ion‐Selective Diffusion Achieved by Supramolecular Interaction for High Thermovoltage and Thermal Stability

Thermoelectric (TE) generators capable of converting thermal energy into applicable electricity have gained great popularity among emerging energy conversion technologies. Biopolymer‐based ionic thermoelectric (i‐TE) materials are promising candidates for energy conversion systems because of their wide sources, innocuity, and low manufacturing cost. However, common physically crosslinked biopolymer gels induced by single hydrogen bonding or hydrophobic interaction suffer from low differential thermal voltage and poor thermodynamic stability. Here, we develop a novel i‐TE gel with supramolecular structures through multiple noncovalent interactions between ionic liquids (ILs) and gelatin molecular chains. The thermopower and thermoelectric power factor of the ionic gels are as high as 2.83 mV K−1 and 18.33 μW m−1 K−2, respectively. The quasi‐solid‐state gelatin–[EMIM]DCA i‐TE cells achieve ultrahigh 2 h output energy density (E2h = 9.9 mJ m−2) under an optimal temperature range. Meanwhile, the remarkable stability of the supramolecular structure provides the i‐TE hydrogels with a thermal stability of up to 80 °C. It breaks the limitation that biopolymer‐based i‐TE gels can only be applied in the low temperature range and enables biopolymer‐based i‐TE materials to pursue better performance in a higher temperature range.


Introduction
The sustainable development of world energy in the future depends on efficient utilization of green energy.Therefore, the pursuit of meeting the growing energy demand of mankind through some benign means has stimulated rapid development of advanced technologies facilitating efficient use of dissipated waste heat. [1]Given that more than 70% of primary energy input is wasted as heat energy, [2,3] efficient low-grade heat recovery helps to reduce greenhouse gas emissions, but current technologies to realize heat energy conversion are still not in the best state. [4]Current methods of utilizing waste heat mainly include the Kalina cycle, [5] organic Rankine cycle, [6] and thermoelectric effect. [7]The former two methods of harvesting waste heat by means of evaporating liquids with low boiling points are generally appropriate for high-temperature environments.In contrast, thermoelectric conversion based on the Seebeck effect allows for heat harvesting in a wider temperature range, making it more suitable for recycling low-grade heat. [8][11][12][13] Research and calculations show that the increase in conductivity of traditional electronic thermoelectric (e-TE) materials such as metals and semiconductors always leads to decreased Seebeck coefficient and improved thermal conductivity.Consequently, the final thermoelectric priority value ZT (ZT = σS 2 T/κ, where σ is conductivity, S is the Seebeck coefficient, κ is the thermal conductivity and T is the absolute temperature) is greatly limited. [14,15]Although semiconductor-based thermoelectric materials exhibit better thermoelectric performance than metals, their Seebeck coefficients with only a few hundred microvolts per Kelvin are still far from satisfactory. [16]][19] The thermopower of i-TE materials is derived from the mobility difference of cations and anions driven by the temperature gradient or the redox reactions of ion pairs (e.g., Fe(CN) 6   4− /Fe(CN) 6   3− ) near electrodes at different temperatures. [20,21]Therefore, there is no obvious coupling relationships among the foregoing parameters of i-TE materials.[24] However, there is only a limited difference in the thermal mobility between anions and cations.How to expand the thermal mobility difference between anions and cations is a difficult problem for i-TE materials.In addition, the conductivity of i-TE materials is completely dependent on ion migration, so the conductivity is generally low, below 1 × 10 −3 S cm −1 . [25,26]-TE gels are considered to be one of the most promising nextgeneration thermoelectric materials and have attracted wide attention due to their high ionic conductivity, safety, flexibility and controllable DOI: 10.1002/eem2.12562Thermoelectric (TE) generators capable of converting thermal energy into applicable electricity have gained great popularity among emerging energy conversion technologies.Biopolymer-based ionic thermoelectric (i-TE) materials are promising candidates for energy conversion systems because of their wide sources, innocuity, and low manufacturing cost.However, common physically crosslinked biopolymer gels induced by single hydrogen bonding or hydrophobic interaction suffer from low differential thermal voltage and poor thermodynamic stability.Here, we develop a novel i-TE gel with supramolecular structures through multiple noncovalent interactions between ionic liquids (ILs) and gelatin molecular chains.The thermopower and thermoelectric power factor of the ionic gels are as high as 2.83 mV K −1 and 18.33 μW m −1 K −2 , respectively.The quasi-solid-state gelatin-[EMIM] DCA i-TE cells achieve ultrahigh 2 h output energy density (E 2h = 9.9 mJ m −2 ) under an optimal temperature range.Meanwhile, the remarkable stability of the supramolecular structure provides the i-TE hydrogels with a thermal stability of up to 80 °C.It breaks the limitation that biopolymer-based i-TE gels can only be applied in the low temperature range and enables biopolymer-based i-TE materials to pursue better performance in a higher temperature range.
[29][30] According to the matrix structure, i-TE gels can be divided into synthetic polymer-based i-TE gels and biopolymer-based i-TE gels.Compared with synthetic polymer-based ionic gels, ionic gels based on biopolymers (e.g., cellulose, [31] gelatin, [32,33] Phytic acid, [34] etc.) have attracted extensive attention due to their wide range of sources, low cost, biodegradability and low environmental pollution.However, most i-TE gels based on biopolymers are physically crosslinked by single hydrogen bonding or hydrophobic interactions, [32] with weakly-selective polymer-ion interactions, poor thermoelectric properties, low thermomechanical stability, and even loss of their properties at high temperatures. [33]In contrast, chemically crosslinked i-TE gels exhibit strong selective interaction and good thermomechanical stability, but usually are cross-linked with specific crosslinking agents, [35- 37] which greatly increases the manufacturing cost and the risk of environmental pollution.Therefore, it is challenging to prepare i-TE gels with excellent thermoelectric properties and thermal mechanical stability by using biopolymer self-assembly technology, which is of great significance for the development of green energy.
In this contribution, ionic liquid (IL), i.e., 1-ethyl-3methylimidazolium dicyandiamide ([EMIM]DCA), is introduced into the gelatin solution to form a supramolecular gel structure through multiple noncovalent interactions between ionic liquid, H 2 O and gelatin molecular chains, as shown in Figure 1a.we focused on the construction of biopolymer-based ion thermoelectric matrix with high thermal stability based on ionic liquid supramolecular self-assembly.Moreover, biopolymer-based i-TE materials with high thermoelectric properties were successfully obtained through supramolecular structure design.The selective interaction between anions and gelatin aqueous solution provides an excellent thermopower of 2.83 mV K −1 for the gel, and the formation of a supramolecular network destroys the shortrange ordered structure of ionic liquid, as shown in Figure 1b.Therefore, a high ionic conductivity of 2.29 S m −1 is obtained, which is 20 times of the order of 1 × 10 −3 S cm −1 required by most electrochemical devices.The higher ion conduction rate and the difference between anion mobility and cation mobility enable the quasi-solid-state gelatin-[EMIM]DCA i-TE cells achieve an instantaneous output power density (0.027 mW m −2 K −2 ) and an ultrahigh 2 h output energy density (E 2h = 9.9 mJ m −2 ) under an optimal temperature range.In addition, the formation of stable supramolecular network structure provides the gel with thermal stability up to 80 °C.This solution blending method is simple and easy to realize large-scale preparation of thermoelectric gels.Therefore, this work breaks the limitation that biopolymer-based i-TE gels can only be applied in the low temperature range, and broadens the application range of biopolymer based i-TE materials.

Preparation and Structure of the i-TE Gels
The i-TE gel consists of gelatin, water and [EMIM]DCA ionic liquid.Gelatin is rich in amino acids, [38,39] which can provide many crosslinking points for multiple noncovalent cross-linking.At the same time, gelatin has the characteristics of nontoxicity, good biocompatibility and easy solution processing, so gelatin was selected as the polymer framework.ILs can form so-called organized "nanostructures" (hydrogenbonded polymeric supramolecules, which are similar to water molecules) with polar and non-polar regions in solid, liquid and solution states or even in the gas phase. [40,41]44] Dupont suggested that the aqueous solution of free enzymes might be embedded in the IL network, which could protect the essential water of proteins and the solvophobic interactions those are critical for maintaining the native structure of proteins. [45]Cations are usually observed to show a less dominant effect relative to anions of the same charge density because anions are more polarizable and hydrate more strongly. [46,47]Here, we chose [EMIM]DCA as the conductive medium because its anions have strong polarity and can form hydrogen bond networks with water more easily. [48,49]At the same time, [EMIM]DCA has high conductivity and thermal stability, which is convenient for improving the thermoelectric properties of materials.The results show that the multiple noncovalent interactions between ionic liquids and gelatin aqueous solutions can form a stable supramolecular polymer network with high thermal stability.At the same time, the difference in anions and cations hydration strength in gelatin aqueous solution gives it excellent thermoelectric properties.
I-TE gels were prepared by mixing ionic liquids and gelatin aqueous solutions.Different mass fractions (5, 10, 15 and 20 wt%) of ionic liquids were added into the gelatin solution, which were expressed as G-X%IL@T, where X was the mass fraction of ionic liquid, and T was the treatment temperature of the gel.The interaction between the solid and liquid phases of the ionic gels was investigated by attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy.Amide I (1630 cm −1 ) represents the stretching vibration of -COO − , [50] which is related to the hydrogen bonds formed by -COO − . [51,52]As shown in Figure 2a, these peaks are blue-shifted when [EMIM]DCA forms ionic gels with gelatin.In 2D correlation maps, the 1630 cm −1 band exhibits two lobes at both parts of the diagonal in the asynchronous map (Figure 2b).This indicates that the width of the band remains the same, but a shift to higher wavenumbers occurs. [53]Similar trends for some peak groups are shown in Figure S2, Supporting Information.The shifts become more significant at a higher [EMIM]DCA content, indicating that there is a high proportion of hydrogen bond formation between gelatin and [EMIM]DCA molecules.A consistent blue shift was observed on the C-N-C bending vibration at 1306 cm −1 , [54] indicating that the interaction between anion and gelatin makes the anion structure more stable.Compared with [EMIM]DCA, the anion -C≡N symmetric and asymmetric stretching vibration bands of ionic gels at 2195 and 2128 cm −1 shift obviously to 2205 and 2138 cm −1 (Figure 2a), respectively, indicating that there is a strong interaction between anions and gelatin. [54,55]Raman spectra also confirmed that there is a strong interaction between anions and gelatin (Figure S3, Supporting Information).Solid-state NMR results show that the interaction sites may be concentrated near -CH 2and amide bonds on the peptide chain skeleton (Figure 2c and Figure S1, Supporting Information).
The intensity ratio of the amide I band to the amide III band (I amide I /I amide III ) is usually used to find the loss/gain of the protein secondary structure through the formation of a random curl/triple helix structure. [56,57]For these ionic gels, the intensity ratio of the amide I band to the amide III band decreases with increasing ionic liquid content (Figure 2d), which indicates that the content of triple helix groups in ionic gels decreases with the addition of ionic liquid.Moreover, the I amide I /I amide III of ionic gels treated at 80 °C is less than that of gels treated at 20 °C, indicating that the triple helix structure of ionic gels pretreated at high temperature is less than that pretreated at low temperature. [57,58]This is because the noncovalent crosslinking between ionic liquid and gelatin limits the movement and rearrangement of gelatin molecular chains. [58]n order to further clarify the multiple noncovalent interactions in the system, an all−atom molecular simulation of gelatin, water and [EMIM]DCA system was carried out.The simulation results show that DCA − tends to be "adsorbed" on the hydrated layer on the gelatin surface.The total interaction energy between DCA − and water is 2-6 times that between other substance pairs (Figure 2 e), which confirms that the hydration of anions is stronger than that of cations.Moreover, the number of hydrogen bonds formed between anions and gelatin is much larger than those formed between gelatin and cations (Figure 2f).In addition, the length of the hydrogen bond formed between anions and gelatin molecules is shorter than that between cations and gelatin molecules, indicating that the combination of anions and gelatin is more stable (Figure 2g,h).The network structure formed by these noncovalent interactions endows the ionic thermoelectric resin with excellent thermoelectric properties and outstanding thermal stability.
FT-IR, Raman, NMR and molecular simulation results clearly show that the interaction between anions and water as well as between anions and gelatin is significantly stronger than that of cations.The difference in the interaction intensity provides excellent thermoelectric properties for ion thermoelectric hydrogels.At the same time, the anionic action sites are concentrated near -CH 2of gelatin molecular chains, which provides a basic framework for the construction of supramolecular network structure.However, further rheological characterization is still needed to confirm the formation of elastic network.

Dynamic Mechanical Performance
The rheological behaviors were examined to show the effect of ionic liquid on the mobility of gelatin chains and the formation of elastic network of gels.Constant temperature experiments were carried by setting the experimental temperature to 80 °C and repeating the frequency sweep of gelatin and G-25% IL samples in the frequency range of 0.05-50 Hz for 80 min.The variation in storage modulus (G 0 ) as functions of frequency of the samples in the initial state and the final state were analyzed.After holding the samples at 80 °C for 80 min, the experimental temperature was gradually decreased from 80 °C to 0 °C, and frequency sweep in the range of 0.05-50 Hz was conducted at each temperature.
The rheological results in Figure 3a,b show that the storage modulus of gelatin increases with the decrease in temperature, meaning that the decrease in temperature restricts the mobility of gelatin chains because of the decrease in the thermal kinetic energy of protein molecular chains.Specifically, some gelatin molecular chains begin to change from random helical conformation to triple helical structure, which indicates the formation of elastic network of gelatin. [59]However, when the temperature is higher than 30 °C, this intermolecular hydrogen bond will break and the elastic network will be destroyed. [51,60]Therefore, the thermoelectric output signal becomes unstable (Figure 3c).
The rheological results in Figure 3d show that the storage modulus of G-25% IL sample increases with the introduction of ionic liquid, meaning that the introduction of ionic liquid can also restricts the mobility of gelatin chains.This is because the multiple noncovalent interactions between ionic liquids and water and gelatin molecular chains formed a supramolecular network structure, which restricts the mobility of gelatin chains. [61]The formation of supramolecular network greatly reduces the effect of temperature on the mobility of gelatin chains (Figure 3e).We also examined the thermoelectric properties of ionic gels at different temperatures.G-15%IL@80 °C gel shows good thermoelectric conversion capacity at 63 °C (Figure 3f).Even when the temperature rises to 83 °C, the G-15%IL@80 °C sample still has a stable thermoelectric response curve (Figure S4, Supporting Information).The above analysis shows that the introduction of IL is conducive to the construction of ionic liquid-water-gelatin supramolecular network structure based on multiple noncovalent interactions in the mixed system.

Ionic Thermoelectric Performance
The thermopower of the Gelatin/[EMIM]DCA ionic gels was measured using the device shown in Figure 1b.An open-circuit voltage can be observed when the Gelatin/[EMIM]DCA ionic gels are subjected to a temperature gradient (Figure 4a).The open-circuit voltage varies linearly with the temperature gradient, as shown in Figure S5, Supporting Information.Therefore, the thermopower was calculated according to the slope (Figure S6, Supporting Information). [62,63]Gelatin/[EMIM] DCA ionic gels exhibit a positive ionic thermopower.This may be related to the interaction between DCA − and gelatin as revealed by FT-IR spectroscopy.This interaction can delay the mobility of anions.Therefore, the thermopower is mainly attributed to the cation migration.
According to the concept of "counterion condensation" proposed by Manning, [64] in the negatively charged gelatin network, a small part of EMIM + tends to "condense" along the negatively charged gelatin molecular chains.These immobilized EMIM + condensed near the gelatin molecular chains will further exert friction resistance on DCA − and reduce the mobility of DCA − .However, the remaining EMIM + which are not condensed around the polymer backbones, are still more mobile Energy Environ.Mater.2024, 7, e12562 than the DCA − dragged by the condensedly fixed EMIM + .The thermopower is observed to be depended on the concentration of ionic liquids (Figure 4b).With increasing ionic liquid concentration, the proportion of mobile cations increases compared with that of condensed cations, which leads to an increase in thermopower at the initial stage.Further increasing the ionic liquid concentration can reduce the Debye length of the electrical double layer and induce a screening effect of the ionic coupling between the ions and gelatin. [32]The thermal mobility of ions tends to converge to the pure ionic liquid solution.Therefore, when the content of ionic liquid is 15 wt%, the ionic gel has the best thermopower of 2.29 mV K −1 .We further increased the thermopower from 2.29 to 2.832 mV K −1 by adjusting the treatment temperature of the gel.Higher temperature is conducive to the stretching of gelatin molecular chains, which is benefit to increasing the interaction differences between anions, cations and gelatin molecular chains.
The ionic conductivities of Gelatin/[EMIM]DCA ionic gels were analyzed by AC impedance spectroscopy (Figure S7, Supporting Information).As shown in Figure 4c, the ionic conductivity increases with increasing [EMIM]DCA loading.The conductivity of the gelatin hydrogel is only 0.086 S m −1 , while the conductivity of G-20%IL@80 °C gel increases to 2.68 S m −1 , much higher than that of most i-TE materials and ionic gel electrolyte materials at present. [17,18,22,28,65]This can be attributed to the interaction between gelatin and water.The strong hydration and the strong interaction between anions and gelatin make the surface of gelatin molecular chains negatively charged.Such interaction can destroy the short-range ordered structure of ionic liquids, promote ion dissociation, and generate free volumes for ion transport, leading to the increased mobility of ions. [19,28,66]Moreover, the ionic conductivities of gels treated at 80 °C are slightly higher than those of gels treated at 20 °C, which is related to the differences in molecular structure of gels.As shown by the strength ratio of amide I band to the amide III band of gelatin in gels (Figure 2d), the gelatin triple helix structures in the gels treated at 80 °C is less than that in gel treated at 20 °C, so its resistance to ion transport is smaller, which is more conducive to the transport of cations.
The thermal conductivities of the ionic gels were tested by using a modified transient plane source method.As shown in Figure 4d, the thermal conductivities of the ionic gels are between 0.3 and 0.6 W m −1 K −1 without changing significantly with [EMIM]DCA loading.

Ionic Thermoelectric Capacitors
The ionic thermoelectric capacitors (ITECs) were prepared from the ionic thermoelectric gels for the conversion of heat into electricity, and the response curves of ionic gels with different IL contents are shown in Figure S8, Supporting Information.Figure S9, Supporting Information, shows the ITEC mode of operation.Figure 5a shows the voltage curve of an ITEC prepared with a G-15%IL@80 °C gel connected with a 2 K ohms external load.It can be determined that there are four stages in a thermal cycle.In the first stage, the temperature gradient causes the directional migration of EMIM + and DCA − to the cold side, [17] resulting in the accumulation of these ions at the cold and thus producing a thermal voltage.In stage II, the external payload is connected to the ITEC.The external voltage of ITEC is gradually reduced to close to 0 V by the balance of electrons and holes in the external circuit.This stage is called the charging process of ITEC equipment.In stage III, when the heater is turned off and the external load is disconnected, the ions accumulated in the gel are distributed uniformly again due to the removal of the temperature gradient, while the electrodes remain polarized, thus generating a reverse voltage.In the last stage, the external load is reconnected to ITEC.This leads to the discharge of ITEC, which is caused by the retraction of electrons and holes previously accumulated on the electrode.Finally, the absolute voltage on the external load drops to 0 V, and a charge-discharge cycle completes, which successfully converts heat energy into electric energy.
The behavior of stage II is related to the resistance of the external load.As shown in Figure S10, Supporting Information, the output current changes with the change in the external load, and the maximum output power is obtained when the external resistance is close to the gel internal resistance value.After testing, under an external resistance of 2 K ohms, when the temperature difference is 10 K, a single 10 × 10 × 2 mm ionic gel can generate a pulse output power of 0.14 μW and a closed-circuit current of 4.5 μA (Figure 5b,c).The maximum output power is about 45 times higher than that of semiconductive thermoelectric materials. [67]A series of 50 P-type gels is expected to achieve a pulse output power of 7.2 μW and a thermopower close to 1.6 V.It is hoped that low-power humidity and pressure sensors can be driven without a booster low-power humidity sensor (HDC2010, Texas Instruments; operating voltage 1.6-3.6V, 0.3 μA average current at a 1-5 Hz sample rate).
The thermoelectric properties of ion thermoelectric materials can be measured by power factor (PF i ) and output energy density.Based on the ion thermoelectric mechanism, Seebeck coeffi2cient and ionic conductivity are two extremely important parameters for the development of highly efficient ion thermoelectric materials.Seebeck coefficient determines the achievable open circuit voltage.Similarly, ionic conductivity determines the transmission rate of electrolyte.Therefore, based on the thermopower (S), ionic conductivity (σ), the ionic power factor (PF i ) can be calculated by Equation (1): As shown in Figure 5d, the G-15% IL@80 °C gel has the maximum PF i of 18.33 μW m −1 K −2 .
The energy density was determined by integrating the output power to 2 h (E 2h ).Among the investigated external resistors, the highest E 2h value of 0.0099 J m −2 was obtained with 2000 Ω (Figure 5e).We compared the energy density value of our system with other reported i-TE cells (Figure 5f).Zhao et al. designed an i-TE supercapacitor with a high thermopower of 10 mV K −1 and an energy density value of 0.006 J m −2 . [25]The quasi-solid-state gels made of PVDF-HFP can exhibit a high thermopower of 26.1 mV K −1 with a low energy density of about 0.003 J m −2 . [28]Ouyang et al. designed an ionic gel made of elastic waterborne polyurethane and 1-ethyl-3-methylimidazolium dicyandiamide (EMIM: DCA, an ionic liquid), which showed a thermopower of 34.5 mV K −1 , but its energy density was about 0.004 J m −2 . [17]In addition, i-TE battery based on a single thermogalvanic effect showed a poor energy density due to its low thermopower. [68]

Conclusions
The organized "nanostructure" (hydrogen bonded polymer supramolecular) is formed by ionic liquid, gelatin and water so that the aqueous solution of free protein is embedded in the IL network, thus protecting the necessary water of the protein, maintaining the protein structure, and improving the thermal stability of the i-TE gel.Selective adsorption of anions by gelatin molecules and their hydrated shells makes the surface of gelatin molecular chains negatively charged.It accelerates the dissociation of ionic liquids in the substrate, destroys the short-range ordered structure of ionic liquids, and produces free volumes for ion transport, thereby increasing the mobility of ions.At room temperature, the thermopower of the gel is as high as 2.83 mV K −1 , and the ionic power factor can reach 18.33 μW m −1 K −2 .Furthermore, the maximized instantaneous power density normalized by the squared temperature difference (P max /ΔT 2 ) can reach 0.027 mW m −2 K −2 (Figure S11, Supporting Information).This supramolecular selfassembly strategy provides a simple and applicable method for the construction of biopolymer-based thermoelectric gels with high thermoelectric properties and heat resistance.Based on the difference in anions and cations hydration, the thermoelectric properties of materials can be further regulated by selecting different hydrophilic anions and cations.Thus, this method is also suitable for constructing thermoelectric gels based on hydrated ionic liquids and other biopolymers, which opens up a new way for the construction of new green i-TE materials.Although the supramolecular network structure provides the ionic thermoelectric hydrogel with structural stability of up to 80 °C, the evaporation of water is an inevitable loss in the working process of the ionic thermoelectric hydrogel, especially when the ionic thermoelectric hydrogel works at a higher temperature.In our work, it was found through freezing test that G-15IL@80 °C sample can still maintain a non-frozen state for 2-3 h at −20 °C, as shown in Figure S12, Supporting Information.DSC analysis showed that the addition of ionic liquid significantly reduced the melting enthalpy of water in gel, making the melting temperature of water below than 0 °C, as shown in Figure S13, Supporting Information.The melting enthalpy is associated with the amount of freezing water (free water and freezing bound water). [60]hese results show that the addition of ionic liquid greatly increases the content of bound water and reduces the volatilization of water. [69]This problem can be improved by rational solvent design.Based on the polarity difference of ionic liquids, an ionic thermoelectric gel with supramolecular network was designed.By selecting a high boiling organic solvent with similar polarity to water to partially or completely replace the water in the gel, an ion thermoelectric gel with higher thermal stability and no fear of solvent volatilization can be obtained.
Preparation of ionic thermoelectric hydrogel: The gelatin was mixed with water at a mass ratio of 3:17 and stirred at 60 °C for 30 min to form a uniformly dispersed solution.Ionic liquids with different mass fractions (5, 10, 15, and 20 wt %) were added to the gelatin aqueous solution, and magnetic stirring was continued for 20 min until the gelatin and ionic liquids were uniformly mixed, and then ultrasonic dispersion was carried out for 3 minutes to eliminate bubbles.The mixed solution was poured into a polytetrafluoroethylene mold, and then treated at 20 °C and 80 °C to form two kinds of ionic thermoelectric hydrogels, respectively.
Selection of treatment temperature: 20 °C: The treatment temperature is between the freezing point of water (0 °C) and the gelling temperature (22 °C-25 °C) of gel at this gelatin concentration, which is conducive to the rapid formation of a stable hydrogen bond network.(In order to avoid the freezing of water in the treatment process, the treatment temperature needs to be higher than the freezing point of water.)80 °C: The treatment temperature is between the melting temperature of gelatin (27 °C-31 °C) and the boiling point of water (100 °C).The treatment temperature higher than the melting temperature of gelatin can ensure that the molecular chains of gelatin are fully extend during the treatment process, so that the ionic liquid can fully interact with gelatin and water to build a supramolecular network structure.At the same time, the treatment temperature is positively related to the reaction rate.Therefore, 80 °C is selected as the treatment temperature.(In order to avoid the boiling of water in the treatment process, the treatment temperature needs to be lower than the boiling point of water.) Nuclear magnetic resonance (NMR): The structure of gelatin/[EMIM]DCA ionic gels was characterized by 500 MHz 1 H NMR spectroscopy (AV III HD 500 MHz, Bruker, Switzerland) at 25 °C.The structure of EMIMDCA was characterized by 400 MHz 1 H NMR spectroscopy (AV III HD 400 MHz, Bruker, Switzerland) at 25 °C.DMSO was used as the deuterated solvent for NMR characterization.
FTIR spectroscopy: Fourier-transform infrared reflectance spectra were recorded on FTIR spectrometer (6700, Thermo Nicolet, USA) in a range of wavenumbers from 4000 to 400 cm −1 with 32 scanning times.The energy density and maximum output current of each cycle.f) Performance comparison of the energy density with that of the reported liquid-state or quasi-solid-state i-TE cells. [17,25,28,68]ergy Environ.Mater.2024, 7, e12562 Raman spectra: Raman spectra were collected on a Renishaw in Via Reflex Raman Microprobe with a 785 nm wavelength incident laser.
Dynamic mechanical property test: The dynamic mechanical measurements of the ionogels were performed using an AR-G2 heometer (TA Instrument) with the plate-to-plate configuration.
Ionic conductivity measurement: The ionic conductivity was measured by AC impedance technique using sp-200 impedance/frequency response analyzer.Use AC voltage with amplitude of 0.1 V and frequency range of 1 Hz-1 × 10 6 Hz.The ionic gels were sandwiched between two stainless Steel meshes.The ionic resistance was obtained by extrapolating the curve with the abscissa based on the equivalent shown in the S7.The impedance spectra of different ionic gels are shown in Figure S7, Supporting Information.The minimum value in the Nyquist diagram of the negative imaginary part of the impedance relative to the real part of the impedance is used as the sampling resistance.The ionic conductivity was then calculated according to Equation (2), where σ i is ionic conductivity, l is gel thickness, R is resistance and A is gel surface area.
Thermal conductivity measurement: The thermal conductivity of the composites was measured by the laser flash technique (Netzsch, LFA 467, USA).
The measurement of differential thermal voltage: The gel was cut into 10*10*2 mm size.Two steel mesh electrodes are bonded on both sides of the gel.A Keithley 2400 in the open circuit mode was used to monitor the electrical signal between the two wires.T-type (Omega) thermocouples were used to monitor the surface temperature.Thermocouples were fixed on both sides of the stencil electrode to reduce thermal resistance.The device diagram is shown in Figure 1b.
Ionic thermoelectric capacitor: The gel was cut into 10*10*2 mm size.Two steel mesh electrodes are bonded on both sides of the gel.The devices were connected to an external load.The voltages were recorded with a Keithley 2400 source/meter.
Differential scanning calorimetry (DSC) measurements: The water state of the hydrogels was evaluated using a DSC Q2000 (TA-Instruments, New Castle, DE) according to the procedure proposed by Ostrowska-Czubenko et al. [69] Each pan was mounted inside the DSC chamber, and cooled down to −40 °C and maintained isothermally for 10 min.DSC curves were obtained in the subsequent heating process from −40 to 40 °C at 10 °C min −1 .An empty aluminum hermetic pan was used as a reference.
Molecular simulation: All the all-atom MD simulations were based on a gro-mos54a7 force field by Automated Topology Builder (ATB) and were carried out using the Gromacs-2020.6software package.The system is a relaxed liquid configuration at 298 K.The total run time was 10 ns NPT for the equilibrium MD simulation.We used the relaxed system as a starting configuration.As it is prior to system relaxation MD, energy minimization was carried out with a composite protocol of steepest descent using termination gradients of 500 kJ/mol⋅nm.The Nose'-Hoover thermostat was used to maintain the equilibrium temperature at 298 K and periodic boundary conditions were imposed on all three dimensions.The Particle Mesh-Ewald method was used to compute long-range electrostatics within a relative tolerance of 1 × 10 −6 .A cut-off distance of 1 nm was applied to real-space Ewald interactions.The same value was used for van der Waals interactions.The LINCS algorithm was applied to constrain bond lengths of hydrogen atoms.A leap-frog algorithm was used with a time step of 1 fs.

Figure 1 .
Figure 1.a) The preparation route and intermolecular interactions of i-TE gels.b) Schematic diagrams of thermopower test device, internal network structures of gels treated at 80 °C (top) and 20 °C (bottom) and cation transport in the gels.

Figure 2 .
Figure 2. a) FT-IR spectra of [EMIM]DCA, gelatin and Gelatin/[EMIM]DCA ionic gels in the range of 3000-1000 cm −1 .b) 2D correlation maps of FT-IR band at 1630 cm −1 (The synchronous maps (left) and the asynchronous maps (right)).c) 1 H-NMR spectra of Gelatin/[EMIM]DCA ionic gels treated at 80 °C in the range of 0-10 ppm (The inset shows an enlarged view of 4.2-3.4ppm (left) and the molecular formula of [EMIM]DCA (right)).d) The strength ratio of the amide I band to the amide III band of gelatin and ionic gels.e) The interaction energy between various substances in the system (The inset shows box diagram of molecular dynamics simulation).f) The number of hydrogen bonds between different substances.(The whole simulation time is 10 ns, and the number of hydrogen bonds between various substances in the system varies with the simulation time by using the equilibrium data of the last 5 ns).Simulation results of hydrogen bonding between gelatin and g) DCA − and h) EMIM + .(Gelatin molecules are displayed in the stick model, and EMIM + , DCA − molecules in the ball-stick model.C atom is yellow, N atom is blue, O atom is red, H atom is white.)

Figure 3 .
Figure 3. Variation in storage modulus (G 0 ) as functions of frequency for the aqueous solution of a) gelatin and d) G-25%IL sample in different periods by rheological measurements.Variation in storage modulus (G 0 ) as functions of frequency for the aqueous solution of b) gelatin and e) G-25%IL sample at different temperatures by rheological measurements.c) The thermoelectric response curve of gelatin at a maximum temperature of 38 °C (the inset shows the photo of melted gelatin sample).f) The thermoelectric response curve of G-15%IL@ 80 °C gel at a maximum temperature of 63 °C.The insets in a-b and d-e show the network structure inside the gel, where the blue lines represent gelatin molecular chains and the yellow balls are anions.

Figure 4 .
Figure 4. a) Thermoelectric response curves of G-15%IL@80 °C ionic gels.The b) thermopower, c) ionic conductivities and d) thermal conductivities of gels treated at different temperatures.

Figure 5 .
Figure 5. ITECs with G-15%IL@80 °C ionic gel.a) Voltage and temperature gradient distribution under 2 K ohms external load.b) Voltage current response curve in stage II under 2 K ohms external load (the inset shows the equivalent circuit diagram).c) Power decay curves on an external load with different resistances connected to the IETC during stage II.d) The power factors of gels treated at different temperatures.e)The energy density and maximum output current of each cycle.f) Performance comparison of the energy density with that of the reported liquid-state or quasi-solid-state i-TE cells.[17,25,28,68]