Patent Application
20240266973
The present disclosure relates to an ionic hydrogel-based energy harvester. More specifically, the present disclosure relates to a polymer hydrogel-based ion energy harvester designed to harvest energy from low-frequency mechanical vibrations using slow diffusion of ions as an alternative to existing energy harvesting technologies based on electron transfer or electron conduction.
A polymer hydrogel is a material made of a polymer with a three-dimensional network structure containing a large amount of water. When a salt such as LiCl, NaCl, or KCl is introduced into the polymer hydrogel or an ionic liquid is introduced, an ionic gel capable of ion conduction is formed.
When a polymer molecular structure forming a molecular hydrogel has a charge, the movement of cations or anions can be controlled. For example, when the molecular structure of the polymer forming a hydrogel contains anions, the penetration of anions approaching from the outside is blocked by electrostatic repulsion, but the penetration of cations approaching from the outside proceeds smoothly.
An energy harvester is a device that acquires energy transformed or destroyed in its natural state and then converts the energy into a usable form. The energy harvester is also called an energy scavenger because the energy harvester collects wasted energy.
Disclosed conventional technologies related to the energy harvester include a trioelectric energy harvester based on electrostatic force (Korean Patent Application Publication No. 10-2020-0129064) and a piezoelectric energy harvester based on piezoelectric power (Korean Patent Application Publication No. 10-2017-0075318).
As mentioned, the most representative conventional energy harvesting technologies are trioelectric-based and piezoelectric-based technologies. Since the electrostatic force and piezoelectric devices have a low extracted current density, the formation of materials and devices to overcome the disadvantages is complicated.
The present inventors researched to solve the problems of the conventional arts as described above. The present inventors discovered that an energy harvester that converts vibrational energy from the outside into electrical energy can be manufactured by layering ionic hydrogels with different mechanical properties and hydrogels with cation-permeable characteristics, and completed the present disclosure.
Therefore, one objective of the present disclosure is to provide a polymer hydrogel-based ion energy harvester that harvests energy from low-frequency mechanical vibrations using slow diffusion of ions.
The other objective of the present disclosure is to provide a use of the polymer hydrogel-based ion energy harvester.
To achieve the objectives, the present disclosure provides an ionic hydrogel-based energy harvester including layered hydrogels, the ionic hydrogel-based energy harvester including:
In one embodiment of the present disclosure, the polymer hydrogel layer having the ion selectivity may allow selective movement of cations therethrough.
In another embodiment of the present disclosure, the degree of compression of the soft polymer hydrogel is greater than that of the hard hydrogel when external pressure is applied. This causes the soft hydrogel layer to be selectively compressed, whereby cations and anions dissolved in the soft polymer hydrogel layer move toward the hard polymer hydrogel layer.
In yet another embodiment of the present disclosure, the compressed soft polymer hydrogel layer can be restored to an initial state thereof when the external pressure is removed.
In yet another embodiment of the present disclosure, the soft polymer hydrogel layer of the energy harvester experiences repetitive compression and expansion when an external pressure or external vibration is repeatedly applied to the ionic hydrogel-based energy harvester, and the compression and expansion cause ions to move to the hard polymer hydrogel layer, whereby the ionic hydrogel-based energy harvester supplies electrical energy to an external component.
In yet another embodiment of the present disclosure, the soft polymer hydrogel layer may be a soft polyacrylamide (PAAm) hydrogel layer, the hard polymer hydrogel layer may be a hard polyacrylamide (PAAm) hydrogel layer, and the polymer hydrogel layer having ion selectivity may be a cross-linked polyacrylic acid (PAAc) hydrogel layer.
In yet another embodiment of the present disclosure, the soft polymer hydrogel layer may be a soft polyacrylamide (PAAm) hydrogel layer, the hard polymer hydrogel layer may be a hard polyacrylamide (PAAm) hydrogel layer, and the polymer hydrogel layer having the ion selectivity may be a layer made of a mixture of polyacrylamide and a charged polymer electrolyte.
In yet another embodiment of the present disclosure, the charged polymer electrolyte may be selected from the group consisting of poly(sodium 4-styrenesulfonate), sodium polyacrylic acid, and crystalline nanocellulose.
In yet another embodiment of the present disclosure, the layered hydrogels may have a total thickness in a range of 1 mm to 20 mm.
In yet another embodiment of the present disclosure, the layered hydrogels may be placed between two metal electrodes attached to an acrylic plate.
In the other embodiment of the present disclosure, the open circuit voltage, short circuit current density, and power of the energy harvester may increase as the vibration frequency of the energy harvester decreases.
In addition, to achieve the objectives, the present disclosure provides a self-powered ionotropic device including the ionic hydrogel-based energy harvester.
An ionic hydrogel-based energy harvester of the present disclosure manufactures an energy device by simply layering polymer hydrogel. Thus, the energy harvester is superior to existing methods in terms of extracted current density, manufacturing cost, and manufacturing method.
In addition, the ionic hydrogel-based energy harvester of the present disclosure can be prepared using the knowledge of simple polymer chemistry without complex material synthesis and device design, so the energy harvester can be used as a soft and flexible power source for many self-powering applications in the low-frequency range.
The present disclosure provides an ionic hydrogel-based energy harvester including layered hydrogels, the ionic hydrogel-based energy harvester including:
In the present disclosure, a polymer hydrogel was manufactured through a well-known synthesis method. More specifically, polyamide and poly(acrylic acid) are used in the present disclosure, but the present disclosure is not limited thereto.
The soft polymer may include but is not limited to, polyamide, which has relatively soft physical properties through the addition of a relatively small amount of cross-linking agent during polymer synthesis.
The hard polymer may include but is not limited to, polyamide, which has relatively soft physical properties through the addition of a relatively large amount of cross-linking agent during polymer synthesis.
Even though state-of-the-art piezoelectric, electromagnetic, and triboelectric energy harvesters have reached a high level of development, energy conversion efficiency decreases sharply in the low-frequency range (<1 Hz to 5 Hz), which is a frequency distribution range where vibrations in the natural environment and human movements are detected. This frequency mismatch severely limits the application of these harvesters for extracting electricity from natural vibrations and human movements.
To overcome these limitations, a new type of energy harvester based on “ion conduction” has recently been proposed by several research groups as an alternative to existing energy harvesting technologies.
In the proposed approach, mobile ions in an ionic conductor move in response to applied mechanical stress, and ion transportation gives rise to a conversion into a potential difference through a charge separation process, leading to electron flow through an external circuit.
For example, a hydrogel diode was prepared by layering two ionic hydrogels composed of oppositely charged polyelectrolytes and mobile counter ions. Due to the concentration difference in the bilayer structure, mobile counter ions (both positive and negative ions in separate hydrogels) can diffuse in opposite directions.
This creates a built-in electric potential across the gel interface that opposes the initial ion diffusion to reach equilibrium. When the ionic diode is mechanically compressed, the volume change in the hydrogel redistributed the ionic concentration across the gel interface to increase the potential difference for power generation.
In other examples, polymer-ionic liquid complexes are mechanically bent or twisted to induce ionic movement. Because cations and anions of different sizes move at different rates, each ion accumulates in a separate part of the polymer matrix, resulting in a potential difference under mechanical stress.
Other ionic conductors, such as Li alloy-based composites and 2D black phosphorus nanosheets, have also been used to develop mechanical energy harvesters. Although the detailed expertise used in chemistry and device design differed in each example, those technologies shared a common operating principle of stress-induced ion transportation for power generation.
Given that ionic conductors can exploit the slow diffusion of ions under mechanical stimulation, energy harvesters based on ionic conductors can be expected to produce electricity by harvesting energy from low-frequency natural vibrations and human movements. Although the amount of related literature is quite limited, existing results on hydrogel diodes, polymer ionic liquid composites, lithium alloy-based composites, and 2D black phosphorus nanosheets have partially demonstrated this opportunity.
However, the output performance of those technologies is still not sufficient to meet the requirements for powering small electronic devices, nor can they compete with other energy harvesting technologies such as triboelectric and piezoelectric nanogenerators.
In the literature on ion energy harvesters, Wang et al. recently reported record-high electrical outputs of an output voltage of up to 60 mV and a current density of up to 80 μAcm−2 in mechanically compressed hydrogel diodes at a very low vibration frequency of 0.01 Hz. This value is smaller than the conventional electrical output of high-performance triboelectric and piezoelectric nanogenerators.
However, in this comparison, the electrical output from triboelectric and piezoelectric nanogenerators is generally obtained in the high-frequency range, whereas the electrical output obtained from ionic conductors at lower frequencies should be taken into account. It is also worth noting that triboelectric and piezoelectric nanogenerators typically require a high level of complexity in material synthesis and device design to achieve high output performance.
Defining that improving the output performance of ion energy harvesters in the low-frequency range is one of the most important challenges in this field, the present inventors determined that (a) the number of positive ions transported under a given mechanical stress and (b) the separation efficiency of the charges into positive/negative should be increased.
Based on these assumptions, the present inventors propose an ionic device composed of three hydrogels layered with each hydrogel having different stiffness and ion selectivity. Under mechanical stress, each hydrogel in the layered configuration is compressed to a different extent depending on the stiffness.
As a result, ions dissolved in the hydrogels can be transported from a soft hydrogel to a hard hydrogel by volume change, thereby realizing directional ion transport through the layered hydrogels. At the same time, by inserting the ion-selective hydrogel between the soft hydrogel and the hard hydrogel, the charge separation of cations and anions can be realized.
This simple but innovative design dramatically improved the output performance of the systematically studied ion harvester by adjusting the composition of the layered hydrogels, compressed volume, vibration frequency, and type of hydrogel/electrolyte.
Under optimized conditions, the hydrogel device of the present disclosure produced an open circuit voltage of 12 mV and a short-circuit current density of 72.5 μAcm−2 at a frequency of 0.08 Hz, and the result would be similar to or higher than the record-high performance of existing ion energies.
Additionally, it was found that electrical output could be further improved by increasing ion selectivity by selecting the type of charged polymer. To demonstrate practical aspects, the inventors implanted ionic devices in electrochemical cells and shoes and monitored the self-powered synthesis of Au nanoparticles and energy generation from human walking.
Component hydrogels can be prepared by simple radical polymerization of vinyl monomers with combinations of various commercially available charged polymers. Thus, the inventors' discovery will clearly open up broad opportunities for self-powered ionotronic devices.
The polymer hydrogel layer having the ion selectivity of the present disclosure allows selective movement of cations therethrough when an external pressure is applied, so only the soft polymer hydrogel layer is selectively compressed, whereby cations and anions dissolved inside the soft polymer hydrogel layer move toward the hard polymer hydrogel layer.
The compressed soft polymer hydrogel layer of the ionic hydrogel-based energy harvester can be restored to an initial state thereof when the external pressure is removed.
The soft polymer hydrogel layer of the ionic hydrogel-based energy harvester experiences repetitive compression and expansion when an external pressure or external vibration is repeatedly applied to the ionic hydrogel-based energy harvester, and the compression and expansion cause ions to move to the hard polymer hydrogel layer, whereby the ionic hydrogel-based energy harvester supplies electrical energy to an external component.
The ionic hydrogel-based energy harvester of the present disclosure may include but is not limited to, for example, sequentially layered hydrogels of a soft polyacrylamide (PAAm) hydrogel layer/crosslinked polyacrylic acid (PAAc) hydrogel layer/hard polyacrylamide (PAAm) hydrogel layer.
The ionic hydrogel-based energy harvester of the present disclosure may include but is not limited to, for example, sequentially layered hydrogels of a soft polyacrylamide (PAAm) hydrogel layer mixed with charged polyelectrolyte/crosslinked polyacrylic acid (PAAc) hydrogel layer/hard polyacrylamide (PAAm) hydrogel layer mixed with charged polyelectrolyte.
In the ionic hydrogel-based energy harvester of the present disclosure, the charged polymer electrolyte can be selected from the group consisting of poly(sodium 4-styrenesulfonate) (PSS), sodium polyacrylic acid (PSA), and crystalline nanocellulose (CNC), but is not limited thereto.
The ionic hydrogel-based energy harvester of the present disclosure may have layered hydrogels with a total thickness in a range of 1 mm to 20 mm, preferably 5 mm to 15 mm, and more preferably 10 mm to 15 mm, but is not limited thereto. In addition, the thickness can be changed as needed.
The ionic hydrogel-based energy harvester of the present disclosure may have a structure in which the layered hydrogels are placed between two metal electrodes, for example, copper electrodes, attached to an acrylic plate, but is not limited to the structure.
The open circuit voltage, short circuit current density, and/or power of the ionic hydrogel-based energy harvester may increase as the vibration frequency of the energy harvester decreases, but the energy harvester is not limited to the properties.
For example, the ionic hydrogel-based energy harvester of the present disclosure can generate an open-circuit voltage of 12 mV and a short-circuit current density of 72.5 μAcm−2 at a vibration frequency of 0.08 Hz but is not limited thereto.
Hereinafter, the present disclosure will be described in more detail through examples. However, the following examples are for illustrating the present disclosure, and the scope of the present disclosure is not limited thereto.
Acrylic acid (AAc) and N,N,N′,N′-tetramethylethylenediamine (TEMED) were purchased from Sigma-Aldrich. Acrylamide (AAm>98.0%) was purchased from Duksan Co, Korea. Potassium chloride (KCl) was purchased from Samchum Co, Korea. Sodium chloride (NaCl) and ammonium persulfate (APS) were purchased from Junsei Chemical Co., Japan. N,N′-Methylenebisacrylamide (MBAA) was provided by Tokyo Chemical Industry Co, Japan.
A polyacrylamide (PAAm) hydrogel was synthesized by free radical cross-polymerization from literature methods with minor modifications. During production, the amount of cross-linking agent (MBAA) was adjusted to control the stiffness of the produced hydrogel. For a soft PAAm gel, 3 g of acrylamide (monomer) and 10 mg of MBAA (cross-linking agent) were dissolved in 12 mL of NaCl (or KCl) aqueous solution. The concentration of NaCl (or KCl) was adjusted to the range of 0 M to 3.0 M.
The monomer solution was degassed in a vacuum oven at a temperature of 60° C. for 30 minutes and cooled to room temperature. Then, 100 μL of APS aqueous solution (initiator, 20% by weight) and 10 μL of TEMED (catalyst) were added to the solution. The reaction mixture was quickly cast into a circular glass mold with a 35 mm inner diameter and 4 mm thickness. Polymerization was carried out at room temperature for 30 minutes after covering the mold with a glass slide.
For a hard PAAm gel, the amount of MBAA (crosslinker) was increased to 250 mg while maintaining all other conditions. Similarly, a PAAc hydrogel was prepared. Briefly, 3 g of AAc (monomer) and 20 mg of MBAA (cross-linker) were dissolved in 12 mL of deionized water without dissolved salts. The monomer solution was degassed in a vacuum oven at a temperature of 60° C. for 30 minutes and cooled to room temperature.
Then, 100 μL of APS aqueous solution (initiator, 20% by weight) and 10 μL of TEMED (catalyst) were added to the solution. After casting in the same circular mold, polymerization was performed in an oven at a temperature of 70° C. for 1 hour.
The prepared hydrogels were layered in the order of PAAm (soft), PAAc, and PAAm (hard), and the PAAm (hard) hydrogel was generally located at the bottom. The total thickness of the three-layered hydrogel was 12 mm, and it consisted of 4 mm-thick PAAm(S), PAAc, and PAAm(H) hydrogels. For comparison, various three-layered hydrogels with different sequences were also prepared. The layered hydrogels were aged overnight before device fabrication and electrical measurements.
A device was fabricated by assembling layered hydrogels, two acrylic plates (5 cm×5 cm×0.5 cm), and two copper (Cu) electrodes First, two sheets of Cu foil (5 cm×6 cm) used as electrodes were attached to the surface of the acrylic plate with the use of Kapton films.
Then, the layered hydrogels were placed between two acrylic plates attached to Cu electrodes. At this step, each of the surfaces of the layered hydrogels was physically attached to each of the Cu electrodes, and the top and bottom of the acrylic plates were connected with the bent Kapton films to prevent the formation of an air layer between the layered hydrogels and the Cu electrodes during the compression-release cycles. After the assembly step, conductors were additionally connected to the Cu electrodes for electrical circuit measurements.
Field emission scanning electron microscopy (FE-SEM) images were obtained using Hitachi High-Technologies S-4800 equipment. Electrical performance was evaluated by applying cyclic external compression using linear motors of the PMC-1/2HS series at various frequencies.
Open-circuit voltage and short-circuit current were measured with a Keithley 6514 electrometer. To investigate the conductivity of the material, impedance spectroscopy was performed using a Bio-Logic VSP-300 potentiometer in the frequency range of 103 Hz to 107 Hz at an amplitude of 10 mV.
A hydrogel-based ion conductor can be easily prepared by free radical polymerization of vinyl monomers in an aqueous electrolyte solution. In the present disclosure, an ionic PAAm hydrogel was synthesized by dissolving acrylamide monomer, cross-linking agent, and initiator in distilled water containing various electrolytes. Initially, NaCl was chosen as the model electrolyte, but other types of electrolytes such as LiCl and KCl were also tested later in the study.
First, two types of electrolyte-free pure PAAm hydrogels were prepared by adjusting the amount of cross-linker in the monomer solution while maintaining all other conditions. Pure PAAc hydrogels were also prepared in a similar manner (see experimental section for details).
As seen in the compression test in
Considering that modulus is a measure of stiffness, the two PAAm hydrogels are denoted as PAAm(H) and PAAm(S), where H and S represent hard and soft, respectively. Additionally, the compressive modulus of PAAc hydrogel (green) was 3.69 kPa. Due to the different moduli, each hydrogel exhibits different deformation under the same applied stress.
Therefore, when three hydrogels are layered in the order of PAAmn(S)/PAAc/PAAm(H) and compressed by external pressure, the degree of compression of each component hydrogel may be different. As a result, directional ion transport through the layered gels can be induced, which will be discussed later.
As a next step, cyclic compression tests of layered PAAm(S)/PAAc/PAAm(H) hydrogels were performed at a maximum compression strain of 25%. In
As a preliminary study on ion transport, NaCl was added as a model electrolyte to PAAm(S) and PAAm(H) hydrogels, and the resulting ionic conductivity was investigated and is shown in
When two ion-conducting PAAm(S) and PAAm(H) hydrogels (containing [NaCl]=2.0 M) were layered together with NaCl-free pure PAAc hydrogel, the layered PAAm(S)/PAAc/PAAm(H) hydrogels showed a conductivity value of 0.056 S/cm2 (
When the conductivity of the layered gels can be calculated by simply averaging the conductivity of PAAm(S) hydrogel (red in
Ion transport through the hydrogel interface was further confirmed by comparing the conductivities of bilayer hydrogels of PAAmn(S)/PAAm(S), PAAmn(H)/PAAm(H), PAAmn(S)/PAAc, and PAAmn(H)/PAAc (
The energy harvesting performance of layered PAAm(S)/PAAc/PAAm(H) hydrogels was investigated on the basis of the results on mechanical properties and ion transport. For the investigation, PAAm(S), PAAm(H), and PAAc hydrogels were bonded when each hydrogel with the same thickness of 4 mm (diameter=35 mm), and the total thickness of the layered hydrogels was adjusted to 12 nm.
Additionally, for comparison, other hydrogels with a different three-layered structure but the same thickness were also prepared. The layered hydrogels were assembled between two acrylic plates with Cu electrodes and conductive wires (
The PAAm(H) hydrogel was typically placed on the bottom of the acrylic plate. The assembled device was repeatedly compressed and released by a linear motor with a representative frequency of 0.3 Hz. Here, vibrations observed in nature, such as waves, wind, and human walking, generally have low frequencies (<1 Hz). Given this fact, the present experimental conditions could potentially be suitable for extracting electrical energy from environmental vibrations.
Initially, the assembled device was compressed until the thickness of the layered PAAm(S)/PAAc/PAAm(H) hydrogels decreased from 12 mm to 9 mm. As can be seen in the photograph of
The thickness changes of PAAm(S), PAAc, and PAAm(H) in the layered structure under repeated compression-release cycles were estimated to be 1.46 mm, 0.88 mm, and 0.67 mm, respectively (blue in
On the other hand, when the external pressure is released, the volume of the compressed hydrogel could recover to the initial state thereof, redirecting the ion transport back toward the upward PAAm(S) direction until the initial concentration was again reached. Under repeated compression-release cycles, the upward and downward movement of ions was converted into a measurable electrical signal. In
The average current density and peak-to-peak Voc values reached 31.2 μAcm−2 and 4.6 mV, respectively, at a vibration frequency of 0.3 Hz. The inventors also found that the electrical output of the device was driven by volume change in the hydrogels. Accordingly, the inventors noted that both Isc and Voc could be systematically controlled by the compressed thickness of the layered hydrogels (
For comparison, layered hydrogels, that was, PAAm(S)/PAAc/PAAm(S) and PAAm(H)/PAAc/PAAm(H), were prepared without adjusting the stiffness of the PAAm gel. The Isc and Voc values were significantly reduced in this sample (
Additionally, in the proposed harvester, the presence of PAAc hydrogel in the middle of the layered structure played a critical role. When layered PAAm(S)/PAAm(S)/PAAm(S) hydrogels without the PAAc layer were prepared, the electrical output was almost negligible (
Unfortunately, the layered PAAmn(H)/PAAm(H)/PAAm(H) hydrogels were too brittle and ruptured under the same compression. However, the three-layered hydrogels with only PAAm(H) do not have energy harvesting properties as in the previous example.
The ion selectivity of the PAAc hydrogel varies depending on pH. As the pH value increases, the ion selectivity increases because the deprotonation of the carboxylic acid (—COOH) of the PAAc hydrogel increases, and as a result, both Isc and Voc extracted from the layered PAAm(S)/PAAc/PAAm(H) hydrogels increased (
However, the mechanical properties and chemical stability of PAAc hydrogel deteriorated under high pH conditions, limiting the applicability of pH control. Additionally, even in the case of layered hydrogels such as other types of cation-selective hydrogels (examples: PAAm hydrogels mixed with poly(sodium 4-styrenesulfonate), sodium polyacrylate, or crystalline nanocellulose), it was observed that energy harvesting performance was maintained or improved. Therefore, various ionic hydrogels could replace the intermediate PAAc hydrogel (see detailed results in
The energy harvesting mechanism in
The upper PAAm(S) gel was rich in Cl− and had a negative charge, while the lower PAAm(H) gel was rich in Na+ and had a positive charge. As a result, an electric field could be generated from PAAm(H) to PAAm(S).
Although the electric field formed in this way was temporary, the electric field could redistribute the local concentration of ions within the hydrogels. As shown in
As a result, an electric double layer (EDL) could be created between the upper PAAm(S) gel-electrode and the lower PAAm(H) gel-electrode interface. Therefore, because the bottom electrode had a higher potential than the top electrode, the potential difference could drive current flow with measurable Voc and Isc as shown in
When the external force was removed, all processes could be reversed due to the volume expansion of the component gels. This reverse process could reduce the generated Voc and Isc by eliminating the charge imbalance in the component gels as well as the EDL at the gel-electrode interface.
In the measurements, the inventors connected the positive probe of an electrometer to the bottom Cu electrode. Therefore, the positive current and voltage in
Interestingly, the amplitudes of Voc and Isc decreased in the initial cycle and reached stable values in sustained compression-release cycles (
Therefore, at relatively high compression-release frequencies, not all ions bound to the electrodes could contribute to the output signal. The stable Voc and Isc values in
Interestingly, by reducing the vibration frequency from 0.3 Hz to 0.06 Hz, both Voc and Isc values gradually increased as shown in
Ultimately, the device assembled at a vibration frequency of 0.08 Hz exhibited constant Voc and Isc peaks without decay behavior with a current density of 72.5 μAcm−2 and an output voltage of 12 mV, respectively (
Since the output current and voltage of a device according to the present disclosure are mainly determined by the number of transported ions, the present inventors studied the concentration-dependent performance of the layered hydrogel device (
PAAm(S)/PAAc/PAAm(H) hydrogels prepared in the absence of NaCl produced negligible output current with a slightly negative voltage (up to −0.85 mV) upon compression. The negative potential appeared to be related to the movement of protons, but the underlying principle was not clearly understood at this step.
With increasing NaCl concentration in both PAAm(S) and PAAm(H) hydrogels, both Voc and Isc values became positive, albeit monotonically, but eventually decreased at higher concentrations ([NaCl]=3.0 M).
Because the number of ions transported under a given compression increases with NaCl concentration, the monotonic increases shown in
Next, the effect of electrolyte type was studied (
In this regard, it should be noted that cations are hydrated in the hydrogel and the hydration radius of cations decreases in the following order: Li+ (3.82 Å)>Na+ (3.58 Å)>K+ (3.31 Å). As the hydrated ionic radius decreases, the ion mobility increases in the following order: Li+<Na+<K+.
Initially, the inventors predicted that cations with higher mobility could generate more electrical output. However, the measured Voc and Isc values increased in a different order: Li+<K+<Na+ at a compression-release frequency of 0.3 Hz (
This result indicated that there might be additional contributions to the observed order. According to recent theoretical calculations, the strength of the bond between cations and metal electrodes varies depending on the type of ions, and the performance observed in the cases of the bond with Na+ and K+ was thought to be related to this bond strength. However, the underlying principle was not clearly understood at this step.
This discussion led the inventors to suggest that the effects of metal salts would be strongly linked to kinetic factors. In this sense, the present inventors lowered the decompression frequency to check whether the influence of dynamic factors could be alleviated, as in the previous results (
Remarkably, at a vibration frequency of 0.08 Hz, the Voc and Isc values of the layered hydrogels with different electrolytes became very similar to each other, with a current density of 25 μAcm−2 and an output voltage of 5 mV, respectively.
Based on concentration and electrolyte dependence studies, the layered hydrogels containing 2.0 M concentration of NaCl provided optimized performance and were selected for the following demonstration. Additionally, under optimized conditions, the electrical output of the layered hydrogels was shown to be maintained without noticeable degradation for 255 cycles (
These results demonstrated the stability and durability of the hydrogel-based device and were in good agreement with the mechanical tests in
As with the single-cell device, several factors could affect the electrical performance of a multi-cell device. Among various factors, the present inventors evaluated the practical aspects of a hydrogel device by controlling the vibration frequency.
In
As can be seen in
In contrast, the maximum power of conventional triboelectric and piezoelectric generators was obtained at much higher resistances (>10 6Ω). The power delivered to an external load could be maximized when the source resistance matched the load resistance. Thus, the hydrogel device of the present disclosure, which had a lower matching resistance compared to triboelectric and piezoelectric generators, might be more suitable for commercial electronic circuits and devices. It was determined that the low matching resistance of the device according to the present disclosure was due to the low internal resistance of the hydrogel.
In
In the case of the friction generator (blue), output performance decreased monotonically as the vibration frequency decreased. The performance of triboelectric generators has been mostly reported at frequencies above 1 Hz, as the generators were inefficient at picking up low-frequency vibrations.
Meanwhile, the performance of recent ion energy harvesters, such as hydrogel diodes (pink), lithium alloy-based composites (green), and 2D black phosphorus nanosheets (green), have been reported at vibration frequencies below 1 Hz. It could be verified that the power density of our hydrogel device (yellow star) in the comparable vibration frequency region (gray) was competitive with or higher than the power density of state-of-the-art high-performance energy harvesters.
After verifying the low-frequency energy harvesting performance, the inventors attempted to apply a hydrogel device to practical applications. In the first experiment, the present inventors implanted a hydrogel device into the soles of shoes and showed electricity generation during human walking (
For the experiment, a 9 mm thick plastic (poly(lactic acid)) block was installed on the surface of the acrylic plate at the bottom of the hydrogel device (see inset photo). This prevented the layered hydrogels from rupturing when walking. Since the layered hydrogels had a thickness of 12 mm, each walking step could induce compression of 3 mm thickness, which was equivalent to the thickness change in
Since the volume of the layered hydrogels did not change when a person was standing, no noticeable current was generated. On the other hand, executing walking with a step frequency of 1.3 Hz produced electrical output with a current density of up to 7 μA/cm2. The output current increased as the walking speed decreased.
This behavior highlights the hydrogel device's ability to harvest energy from human movement in the low-frequency range. In particular, at a step frequency of 0.2 Hz, the current density reached up to 30 μA/cm2, which was a value quite similar to the current density of a linear motor.
In the following set of experiments, the electrochemical synthesis of Au nanoparticles was demonstrated. For the experiments, the upper and lower Cu electrodes of the hydrogel device were connected to the HAuCl4 aqueous solution using a conductive jumper wire.
Then, a hydrogel device was continuously compressed at a frequency of 0.3 Hz to induce electrochemical reduction of HAuCl4 into Au nanoparticles. As seen in the UV-Vis spectrum (
As can be seen in the transmission electron microscope (TEM) image (
When the hydrogel device was connected to the HAuCl4 solution under the same conditions and without a compression-release procedure, the UV-Vis spectrum of the HAuCl4 solution remained the same with no LSPR peak appearing.
The present inventors demonstrated an ionic device capable of harvesting energy from low-frequency mechanical vibrations based on ion transport in a polymer hydrogel. To optimize the output performance of the ion energy harvester, the hydrogel-based device of the present disclosure focuses on optimizing (a) the amount of ion transport by the stiffness of the PAAm hydrogel and (b) the charge separation of the transported ions by ion selectivity.
There was direction in the experiment using layered hydrogels by adjusting various experimental conditions. However, selective ion transport has been systematically explored to understand and optimize the mechanoelectrical conversion procedure.
In many respects, the experimental results were controlled by the slow diffusion rates of ions and ion-electrode interactions, and the experiments revealed the dynamics of device performance. Interestingly, by lowering the vibration frequency, more ions were transported effectively, leading to continued improvement in device performance.
The retardation effect of ions due to kinetic factors was found to disappear at a vibration frequency of 0.08 Hz. Regardless of the experimental conditions, electrical output was maximized. As practical applications, energy generation from human walking and electrochemical synthesis of Au nanoparticles were further presented.
The presented hydrogel device can be manufactured by simple polymer synthesis without complex material synthesis and device design. Thus, the present disclosure clearly provides a blueprint for the future development of hydrogel-based ion energy harvesters as soft and flexible power sources for self-powered applications in the low-frequency range.
As described above, specific parts of the content of the present disclosure have been described in detail. For those skilled in the art, it will be obvious that these specific techniques are merely preferred examples and do not limit the scope of the present disclosure.
Accordingly, the substantial scope of the present disclosure will be defined by the appended claims and their equivalents. Simple modifications or changes to the present disclosure can be easily used by those skilled in the art, and all such modifications or changes can be considered to be included in the scope of the present disclosure.
By manufacturing an energy device by simply layering polymer hydrogels, an ionic hydrogel-based energy harvester is very superior to existing methods in terms of extracted current density, manufacturing cost, and manufacturing method. Since the ionic hydrogel-based energy harvester can be prepared with simple knowledge of polymer chemistry without complex material synthesis and device design, the ionic hydrogel-based energy harvester is utilized as a soft and flexible power source for many self-powering applications in the low-frequency range.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2021-0143859 | Oct 2021 | KR | national |
This application is a continuation of International Application No. PCT/KR2022/005874 filed on Apr. 25, 2022, which claims priority to Korean Application No. 10-2021-0143859 filed on Oct. 26, 2021, the entire contents of which are herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/KR2022/005874 | Apr 2022 | WO |
| Child | 18639155 | US |
